key: cord-0838954-sckq06ku
authors: Jones, Emma S.; Vinh, Antony; McCarthy, Claudia A.; Gaspari, Tracey A.; Widdop, Robert E.
title: AT(2) receptors: Functional relevance in cardiovascular disease
date: 2008-08-31
journal: Pharmacol Ther
DOI: 10.1016/j.pharmthera.2008.08.009
sha: 77acfeb6ca45f717c566cd5dc2a2370c1afe1838
doc_id: 838954
cord_uid: sckq06ku

The renin angiotensin system (RAS) is intricately involved in normal cardiovascular homeostasis. Excessive stimulation by the octapeptide angiotensin II contributes to a range of cardiovascular pathologies and diseases via angiotensin type 1 receptor (AT(1)R) activation. On the other hand, tElsevier Inc.he angiotensin type 2 receptor (AT(2)R) is thought to counter-regulate AT(1)R function. In this review, we describe the enhanced expression and function of AT(2)R in various cardiovascular disease settings. In addition, we illustrate that the RAS consists of a family of angiotensin peptides that exert cardiovascular effects that are often distinct from those of Ang II. During cardiovascular disease, there is likely to be an increased functional importance of AT(2)R, stimulated by Ang II, or even shorter angiotensin peptide fragments, to limit AT(1)R-mediated overactivity and cardiovascular pathologies.

The major effector peptide of the renin angiotensin system (RAS) is the octapeptide angiotensin II (Ang II), which was originally considered a circulating endocrine hormone (Unger et al., 1996; Csikos et al., 1997) , with major effects involving vascular/cardiac contractility and fluid and electrolyte homeostasis. However, identification of components of the RAS in various organs has extended our understanding of the RAS (Zimmerman & Dunham, 1997; Akasu et al., 1998) , and it is now well accepted that in addition to systemic effects, local synthesis of Ang II exerts both autocrine and paracrine functions, influencing cellular growth and regional haemodynamics in a tissue-specific manner (Johnston,1992; Zimmerman & Dunham, 1997; de Gasparo et al., 2000; Paul et al., 2006) .

As has been well documented, the 'classical'or 'renal'-RAS involves the conversion of the hepatic-derived precursor peptide angiotensinogen to angiotensin I (Ang I), which is catalysed by circulating renin released from the kidney. The conversion of Ang I to Ang II is then primarily facilitated by angiotensin converting enzyme (ACE) which is found in endothelial cells, and due to the large surface area, this conversion is prominent in the lungs. Alternatively (but to a much lesser degree), Ang I may be directly converted to Ang (1-7) by tissue endopeptidases (Ferrario et al.,1997) . Ang II is susceptible to degradation by aminopeptidases, resulting in fragments such as Ang III and Ang IV, which may in themselves possess biological activity (Ferrario & Iyer, 1998; de Gasparo et al., 2000) . (see Fig. 1 and Section 2.4).

Based on the differing affinities of several natural and synthetic ligands, two major subtypes of Ang receptors have been identified and cloned, the Ang subtype 1 receptor (AT 1 R) and subtype 2 receptor (AT 2 R) . Both AT 1 and AT 2 R possess similar affinity for Ang II (Timmermans et al., 1991; Griendling et al., 1996) , however, affinity for entities such as the biphenyl tetrazoles, losartan and candesartan, and compounds such as PD123319 and CGP42112, vary significantly. For example, the AT 1 R antagonist, losartan, displays 10 000-fold greater affinity for the AT 1 than the AT 2 R (Timmermans et al., 1991) , and candesartan cilexetil, which in vivo is metabolised to candesartan, has 80 times higher affinity for the AT 1 R than losartan (Shibouta et al., 1993; Morsing, 1999) . On the other hand, the tetrahydroimidazopyridine compounds, PD123177 and PD123319, have been shown to be 3500-fold more selective for the AT 2 R than the AT 1 R (Timmermans et al., 1991; Timmermans et al., 1993) , while CGP42112 exhibits over 1000-fold selectivity for AT 2 R (Whitebread et al., 1989) .

All of the classical actions of Ang II, including vasoconstriction, effects on fluid and electrolyte homeostasis, and influences on cellular growth and differentiation, have been shown to be due to stimulation of AT 1 R located on the plasma membrane of cells. In addition, there has been much recent interest in the cardiovascular effects of other peptides of the RAS, such as Ang 1-7, Ang III and Ang IV, which are formed by various amino-and endopeptidases (see Fig. 1 ), and are believed to influence cardiovascular function both due to modulation of classical Ang II-mediated actions, and by exerting their own specific biological actions via individual cognate receptors.

For example, the G-protein-coupled receptor encoded by the Mas protooncogene was identified as a functional receptor for Ang (1-7) (Santos et al., 2003) . A number of cardiovascular effects have been attributed to Ang (1-7) (Santos et al., 2000) acting via MasR (Ferrario et al., 2005b; Santos & Ferreira, 2007) although there is evidence that Ang (1-7) can act independently of Ang (1-7)/MasR . As will be discussed later, the resurgence of interest in the effects of Ang (1-7) has been buoyed by the discovery of angiotensin converting enzyme 2 (ACE2). There is also evidence for a binding site which recognizes Ang IV, but which has no affinity for either losartan or PD123319 (Wright et al., 1993) , and has been designated the angiotensin II type 4 receptor (AT 4 R) (Swanson et al., 1992; Wright et al., 1995) . However, this site has recently been purified from bovine adrenal membranes, and identified as the enzyme, insulin-regulated aminopeptidase (IRAP) (Albiston et al., 2001) . IRAP has been localised in tissues such as heart, lung, kidney, and brain, and ligand binding results in effects such as enhancement of cognitive function (Wright et al., 1993; Wright et al., 1999) modulation of blood flow (Swanson et al., 1992; Coleman et al., 1998) , increased natriuresis (Ardaillou & Chansel, 1997) and inhibition of cardiomyocyte hypertrophy (Baker & Aceto, 1990; Baker et al., 1992) .

Thus, at present, there are 4 ATR subtypes (see Fig. 1 ). However, for the purpose of this review, the following discussion will predominantly focus on the AT 2 R and its functional relevance in cardiovascular disease.

Like the well characterised AT 1 R, the AT 2 R is a member of the 7 transmembrane spanning receptor family. The AT 2 R shares 34% sequence homology to the AT 1 R (Guthrie, 1995; Unger et al., 1996) , and is expressed abundantly in foetal tissue, but levels decline rapidly after birth. Consequently, it was originally believed that the AT 2 R was primarily involved in cellular growth and differentiation (Capponi, 1996; Akishita et al., 1999; Yamada et al., 1999) . In adults, the AT 2 R has been localised to the heart, kidney, adrenal gland, brain, uterus, pancreas, retina, skin, and both endothelial and vascular smooth muscle cells (VSMCs) of the vasculature Leung et al., 1997; Nora et al., 1998; Allen et al., 1999; Wang et al., 1999; Wheeler-Schilling et al., 1999; Roulston et al., 2003) , and importantly, expression is also increased in numerous cardiovascular pathologies (see Section 3 and Table 2 ).

Signaling mechanisms differ markedly to those associated with AT 1 R (Touyz & Berry, 2002) . The topic of AT 2 R signalling has recently been extensively reviewed (Hannan & Widdop, 2004; Kaschina & Unger, 2003; Steckelings et al., 2005) , and so will only be mentioned briefly. One of the most frequently reported signaling pathways for AT 2 R stimulation in vasculature has been increased production of cyclic guanine 3′,5′-monophosphate (cGMP), nitric oxide (NO) and bradykinin (BK), as was first described by Carey (1996, 1997) : see Widdop et al. (2003) for review. In cultured endothelial (Chaki & Inagami, 1993; Caputo et al., 1995; Saito et al., 1996; Pueyo et al., 1998) , PC12W (Zhao et al., 2003) , and N1E-115 neuroblastoma cells, and in isolated blood vessels from animal Seyedi et al., 1995; Thorup et al., 1998; Thorup et al., 1999; Hannan et al., 2003) , and human (Batenburg et al., 2004) studies, Ang II has been shown to increase NO and/or cGMP levels via AT 2 R and/ or AT 1 R stimulation. Cross talk between AT 2 R and AT 1 R is also implicated by the recent finding that NO/cGMP activates a cGMPdependent protein kinase causing decreased RhoA activity and consequently decreased AT 1 R-mediated vasoconstriction (Savoia et al., 2006a . Involvement of BK and NO/cGMP signalling pathways has also been demonstrated following AT 2 R stimulation in vivo, in the vasculature of rats (Siragy & Carey, 1996 , 1997 Siragy et al., 2000 Siragy et al., , 2001 Walters et al., 2005) , and mice (Tsutsumi Fig. 1 . Summary of the RAS incorporating the Ang peptide family and physiological effects mediated via ATR subtypes. Under the classical RAS schema, Ang II is produced, via renin and ACE, to act with equal affinity on two ATR subtypes, AT 1 R and AT 2 R (large arrows). However, it is now appreciated that a number of breakdown products of Ang II, namely Ang (1-7), Ang III and Ang IV, exert their own unique effects that are distinct (and often opposite) to those of Ang II. Such effects are often mediated via newly recognized receptors such as MasR for Ang (1-7) and AT 4 R (also known as IRAP) for Ang IV, or additionally via AT 2 R stimulation. ACE2 is also a new pathway for the formation of Ang (1-7). Newly identified Ang receptor binding proteins associated with different ATR subtypes may also modify ATR activation. Thus, over-stimulation of AT 1 R (and (P)RR) by Ang II, which can contribute to a plethora of cardiovascular disease processes, may be counter-regulated by a number of non-AT 1 R mechanisms. Most notably, AT 2 R stimulation usually causes opposing effects to AT 1 R, as indicated. It is also likely that the MasR exerts a similar counter-regulatory role, whereas the evidence is more preliminary and speculative for AT 4 R/IRAP. In terms of mediators, Ang II itself stimulates AT 2 R whereas the shorter Ang peptides stimulate their cognate receptors and possibly also AT 2 R. et al., 1999) , and in cardiac growth responses (Liu et al., 1997; Bartunek et al., 1999) .

Several signaling pathways involve activation of protein phosphatases, whose function is to dephosphorylate and thus inactivate MAP kinases such as ERK-1 and ERK-2, resulting in inhibition of both AT 1 Rand other 'classical' growth factor-mediated pathways involved in cellular growth and differentiation (Stoll et al., 1995; Tsuzuki et al., 1996) . Increased PTPase activity following AT 2 R stimulation has been demonstrated in PC12W cells (Bottari et al., 1992; Brechler et al., 1994) , N1E-115 neuroblastoma cells (Nahmias et al., 1995) and R3T3 fibroblasts (Tsuzuki et al., 1996) . In PC12W cells and adult rat ventricular myocytes, induction of the PTPase MKP-1 is G-proteindependent Fischer et al., 1998) . However, studies performed in N1E-115 cells demonstrated that AT 2 R-mediated activation of PTPase did not involve a G-protein, but rather induced the soluble PTPase, SHP-1 (Bedecs et al., 1997) . Thus Ang II stimulation of AT 2 R may result in activation of PTPases via both G-proteindependent and -independent mechanisms. In addition, stimulation of serine/threonine phosphatases, and subsequent inhibition of ERK-1 and -2, appears to be particularly important in neuronal tissue, in which activation of the serine/threonine phosphatase PP2A has been demonstrated to cause opening of a delayed rectifier K + channel (Kang et al., 1994) , upregulation of AT 2 R mRNA, and increased apoptosis (Shenoy et al., 1999) .

AT 2 R stimulation may also activate lipid-signalling pathways. Ang II stimulation of neonatal rat cardiomyocytes (Lokuta et al., 1994) , rabbit proximal tubule epithelia (Jacobs & Douglas, 1996) , and cultured neurons (Zhu et al., 1998) , increased PLA 2 activity and arachadonic acid (AA) release. Long-term stimulation of AT 2 R by Ang II has also been shown to increase synthesis of ceramides, which may then activate stress kinases and caspases involved in the induction of apoptosis Lehtonen et al., 1999) .

It is now recognized that renin exists in two forms, mature renin which can actively cleave angiotensinogen, and the proenzyme, prorenin. Prorenin lacks enzymatic activity, but is transformed into mature renin following cleavage of the 43-amino acid N-terminal propeptide which covers the enzymatic cleft and prevents angiotensinogen access and subsequent cleavage. Interestingly, although synthesized in a limited number of tissues, it has been suggested that prorenin represents up to 90% of total plasma renin in normal subjects, and in certain physiological and pathological conditions, such as pregnancy and diabetes, can circulate at 100-fold higher concentrations than mature renin (Danser et al., 1998) . This excess circulating prorenin cannot be activated in the circulation, which has sparked research into the existence of a renin/prorenin receptor. Indeed it was demonstrated that a major source of renin/prorenin in cardiac tissue is due to sequestration and uptake from the circulation (Danser et al., 1994; Muller et al., 1998) suggesting a functional role for prorenin.

A specific renin/prorenin receptor (P)RR was first identified in cultured human mesangial cells (Nguyen et al., 2002) , and has since been found to be expressed at relatively high levels in rat and human heart, brain, placenta and adipocytes, and at lower levels in kidney and liver (Danser & Deinum, 2005; Nguyen, 2006; Achard et al., 2007) . The (P)RR consists of 350 amino acids, possesses a single transmembrane domain, and exclusively binds renin/prorenin. Binding of renin/ prorenin to (P)RR has been shown to have 2 major consequences: increased catalytic activity of renin/prorenin, and activation of (P)RRmediated signal transduction cascades. Binding of renin to its receptor increases angiotensinogen conversion to Ang I by five-fold (Nguyen et al., 2002) , and prorenin, which is virtually inactive in solution, also displays enzymatic activity following receptor binding. This activation of prorenin is not due to proteolysis of the pro-segment which covers the catalytic site, but rather it has been hypothesized that prorenin undergoes a conformational change when bound to the (P)RR, which unmasks the catalytic site and thus activates the proenzyme without removal of the propeptide (Nguyen et al., 2002) . Importantly, increased renin/prorenin activity at the cell surface may result in greater Ang I and Ang II levels in the immediate vicinity of Ang receptors and thus increase efficiency of Ang II binding. In addition, receptor-bound renin/prorenin appears to induce intracellular signaling via activation of the MAP kinases, ERK1/2, which is distinct from Ang II-mediated effects (Nguyen et al., 2002) . Thus activation and potentiation of renin/prorenin enzymatic activity, together with specific (P)RR-mediated signaling, could conceivably have striking effects on cardiovascular regulation.

In light of these data, recent studies have investigated the role of renin/prorenin and its receptor in physiological and pathophysiological conditions. In particular, a role for the (P)RR in animal models of diabetes and hypertension has recently been identified. Ichihara et al., used a decoy peptide which corresponds with the handle region in the prorenin pro-segment (handle region peptide = HRP), to competitively prevent non-proteolytic activation of prorenin. This group reported that HRP treatment prevented the development of diabetic nephropathy in streptozotocin (STZ)-induced diabetic rats (Ichihara et al., 2004) , and also decreased perivascular fibrosis and left ventricular hypertrophy (LVH) in spontaneously hypertensive stroke prone rats (Ichihara et al., 2006) . However, several groups have since demonstrated that HRP is unable to prevent renin/prorenin binding and subsequent Ang I generation in mouse VSMCs , or to inhibit renin/prorenin-induced ERK1/2 phosphorylation in cultured VSMCs (Feldt et al., 2008b) , or monocytes (Feldt et al., 2008a) , casting doubt on the validity of HRP as a peptide to prevent prorenin activation. Similarly, recent studies have been unable to confirm the beneficial effects of HRP in vivo, and thus saw no improvement in end-organ damage following HRP administration in double-transgenic rats overexpressing renin and angiotensinogen genes (Feldt et al., 2008b) , or in renovascular hypertensive 2-kidney, 1-clip (2K1C) rats .

Nevertheless, over-expression of (P)RR in rats increased systolic blood pressure and heart rate which was shown to gradually worsen with increased aged (Burckle et al., 2006) , lending weight to the notion of (P)RR-mediated cardiovascular regulation. Although the exact mechanisms by which these effects occur require further investigation, it appears that they are not simply due to increased synthesis of Ang II and subsequent potentiation of Ang II-mediated effects. Huang et al. (2006) , demonstrated that treatment of cultured mesangial cells with human and rat recombinant renin increased levels of the pro-fibrotic cytokine transforming growth factor, TGF-β1, and that this effect was not influenced by an inhibitor of the enzymatic activity of renin (RO 42-5892), an AT 1 R antagonist (losartan) or an ACE inhibitor (enalapril), but was significantly inhibited by renin siRNA (Huang et al., 2006) . Furthermore, evidence of a direct interaction between (P)RR and the transcription factor, promyelocytic zinc finger protein (PLZF), has recently identified a novel signal transduction cascade involving renin/(P)RR/PLZF, activation of which results in cellular proliferation via upregulation of PI3K-p85α (Schefe et al., 2006) . Interestingly, PLZF has also been found to associate with AT 2 R in the heart (Senbonmatsu et al., 2003) , and such an interaction has been suggested as explanation for the AT 2 R-mediated cardiac growth-promoting effects deduced from several AT 2 R knockout studies (Senbonmatsu et al., 2000; Ichihara et al., 2001 Ichihara et al., , 2002 ) (see Section 3.2).

Recently the non-peptide renin inhibitor, aliskiren, was approved for the treatment of hypertension. Animal and clinical studies have revealed striking depressor effects of aliskiren (Jensen et al., 2008) . More recently, aliskiren has proven to be beneficial in several pathophysiological settings including hypertension and diabetic nephropathy , cardiac remodelling and fibrosis (Pilz et al., 2005; Whaley-Connell et al., 2008) as well as atherosclerosis (Lu et al., 2008) . Moreover, Nussberger et al. (2008) reported that in comparison with anti-hypertensive treatments including AT 1 R blockade, a β-blocker and calcium channel antagonist, aliskiren was shown to have equally potent blood pressure lowering effects as well as anti-atherosclerotic effects. The history and efficacy of aliskiren in the treatment of hypertension has recently been reviewed by Jensen et al. 2008) .

Therefore, these recent studies support a role for the (P)RR in pathological states and future studies using (P)RR knockout mice may provide more insight into the therapeutic potential of aliskiren and the development of (P)RR inhibitors.

ACE2 is a recently discovered homologue of ACE, with 56% similar homology with the N-terminal domain of ACE (Donoghue et al., 2000; Tipnis et al., 2000; Vickers et al., 2002) . In contrast to the wide distribution of ACE, ACE2 expression was initially thought to be restricted to endothelial and VSMCs of the heart and kidney (Donoghue et al., 2000; Tipnis et al., 2000; Crackower et al., 2002) although a more widespread distribution is emerging (see Hamming et al., 2007) . Unlike the dipeptidyl carboxypeptidase ACE, ACE2 cleaves a single amino acid from the C-termimal of the peptide substrate. Thus, ACE2 cleaves Ang I to the inactive Ang (1-9), which may then be converted to the vasodilator peptide, Ang (1-7), by ACE. More importantly, ACE2 may also directly metabolise Ang II to form Ang (1-7), and this reaction occurs at a faster rate than the formation of Ang (1-9) from Ang I (Carey & Siragy, 2003; Rice et al., 2004) . Thus, ACE2 may counter-regulate ACE activity by simultaneously decreasing Ang II levels and increasing Ang (1-7) formation (Hamming et al., 2007) . While ACE2 shares significant sequence homology with ACE, it is not sensitive to ACE inhibitors.

Early investigations regarding ACE2 suggested that ACE2 mRNA was increased in both human and animal models of heart failure (Goulter et al., 2004; Burrell et al., 2005) and decreased in genetically hypertensive rats (Crackower et al., 2002) , sparking interest that ACE2 may play an important modulatory role on the RAS in certain cardiovascular pathologies. Furthermore, Ishiyama et al. (2004) have reported that ACE2 mRNA expression was increased by AT 1 R inhibition following myocardial infarction (MI), and spironolactone treatment also increased ACE2 in heart failure patients (Keidar et al., 2005) . Both AT 1 R blockade and ACE inhibition increased cardiac ACE2 expression in Lewis rats, whereas activity of the enzyme was increased by AT 1 R inhibition only (Ferrario et al., 2005a) . However, in direct contrast, Batlle et al. (2006) found no evidence of increased ACE2 activity in biopsies of human heart failure patients. Such inconsistency in results regarding ACE2 expression and activity may be at least partially due to the methods employed, as only poor correlation between ACE2 mRNA and both protein expression and activity has recently been reported in diabetic mice (Wysocki et al., 2006) .

Initial studies regarding ACE2 function performed in ACE2 knockout mice (Crackower et al., 2002) , resulted in severe impairment of cardiac contractility, which was normalized in double ACE/ACE2 knockout mice, suggesting a counter-regulatory function of ACE2 on the RAS (Crackower et al., 2002) . However, several alternative lines of ACE2 knockout mice have since been generated, which exhibit modestly elevated basal blood pressure and normal cardiac phenotype, despite significantly elevated circulating Ang II levels . Furthermore, normal cardiac function has been shown in ACE2 knockout mice (Yamamoto et al., 2006) , although ACE2 deletion did result in a greater hypertrophic response to pressure overload compared to wild type mice. Similarly, in normotensive rats, ACE2 over-expression induced by lentiviral administration of ACE2 mRNA, did not affect basal cardiac function, however, transgenic animals displayed significantly blunted cardiac hypertrophic and fibrotic responses to Ang II infusion compared to control animals (Huentelman et al., 2005) . In an analogous study performed by the same group, ACE2 over-expression in SHR and WKY rats, reduced blood pressure (BP), decreased left ventricular wall thickness, increased left ventricular end diastolic pressure, and attenuated cardiac perivascular fibrosis in hypertensive animals only (Diez-Freire et al., 2006) . These data suggest that cardiac phenotype is not solely dependent on ACE2 expression, but support the notion of a cardioprotective role for ACE2 in situations of cardiac stress.

Interestingly, all strains of ACE2 knockout mice reported to date have increased plasma and tissue levels of Ang II, due to both decreased metabolism of plasma Ang II, and increased tissue synthesis as a result of elevated Ang I, suggesting an important function of ACE2 to regulate Ang II levels (Crackower et al., 2002; Gurley et al., 2006; Yamamoto et al., 2006) . In addition, ACE2 is known to undergo proteolytic shedding of its extracellular ectodomain to release a soluble form of ACE2 in plasma that maintains catalytic activity Warner et al., 2005) . Moreover, ACE2 acts as a receptor for the severe-acute respiratory syndrome (SARS) coronavirus (Li et al., 2003) where it may serve a protective role (Hamming et al., 2007) .

Although the AT 2 R is a member of the G-protein-coupled receptor (GPCR) superfamily, it is well recognized that AT 2 R signal transduction does not always occur via classic G-protein-dependent pathways. Recent studies of GPCR modulation and function, with particular focus on areas such as constitutive activity, formation of homo-and heterooligimers, and interaction with receptor-associated proteins have afforded fresh insights into GPCR signalling, and provide information that may assist in resolving previous controversies regarding AT 2 Rmediated function.

AT 2 R may possess constitutive activity, as several investigators have reported that AT 2 R expression exerts cellular effects without ligand binding. Over-expression of AT 2 R in cultured fibroblasts, CHO cells and VSMCs, caused apoptosis via p38 MAPK and caspase-3 signalling pathways (Miura & Karnik, 2000) . Moreover, the degree of apoptosis was not sensitive to either Ang II or PD123319, but showed significant correlation with the level of AT 2 R protein expression (Miura & Karnik, 2000) . In addition, over-expression of AT 2 R in cultured VSMC has also been shown to downregulate AT 1a R in a ligand-independent manner . This effect on AT 1a R expression was suggested to be due to potentiated BK/NO signaling, as not only was BK and iNOS protein increased by AT 2 R over-expression, but both the B 2 R antagonist, HOE 140, and the NO synthase inhibitor, L-NAME, ameliorated the decrease in AT 1a R expression . The same group also reported a similar downregulation of AT 1a and TGF-β receptor expression in VSMC from WKY, which was associated with reduced basal and Ang II-induced DNA synthesis . Interestingly, AT 1a and TGF-β receptor expression, and both basal and Ang II-stimulated markers of cellular growth, were not altered by AT 2 R over-expression in VSMCs from SHR, suggesting a disturbance of gene regulation in this model of genetic hypertension, which was suggested to contribute to the exaggerated growth of VSMCs from SHR .

A recent study by D' Amore et al. (2005) also showed constitutive activity of AT 2 R in the context of cardiac growth. In this study, transfection of increasing titres of AT 2 R in cultured neonatal cardiomyocytes resulted in cellular hypertrophy, which was not influenced by AT 2 R ligand binding, and also did not affect AT 1 Rmediated hypertrophic signaling, providing evidence for parallel stimulatory roles of AT 1 R and AT 2 R in cardiac hypertrophy. Interestingly, Falcon et al. utilised microarray expression analysis to identify genes whose expression was regulated by AT 2 R expression. This group identified~5224 genes which were regulated independently of AT 2 R-ligand binding, with proposed functions on cell migration, protein processing, intracellular signaling and DNA repair (Falcon et al., 2005) . Moreover, it was found that AT 2 R over-expression inhibited human coronary arterial endothelial cells migration in a ligand-independent manner, albeit in an experimental system in which AT 2 R expression was increased to levels much greater than those which occur in either physiological or pathophysiological settings (Falcon et al., 2005) .

It is well established that GPCRs are susceptible to receptor dimerisation, and that such an interaction between receptors may affect both receptor activation and signaling, as has been well described for AT 1 R (Oro et al., 2007) . Like AT 1 R which are reported to form heterodimers (AbdAlla et al., 2000 (AbdAlla et al., , 2001b , both hetero-and homo-oligomers of AT 2 Rs have been reported. AT 2 R were first shown to directly bind to AT 1 R in PC12W cells and fetal fibroblasts, and subsequent stimulation of cells with Ang II resulted in decreased expression of the of AT 1 R-associated G-proteins, Gα i/o and Gα q/11 , suggesting that such an interaction between receptor subtypes may contribute to AT 2 R-mediated antagonism of AT 1 R (AbdAlla et al., 2001a) . Furthermore, this effect on AT 1 R-mediated signaling was independent of AT 2 R stimulation, as PD123319 had no influence on Gα i/o and Gα q/11 activation (AbdAlla et al., 2001a) . In the same study, AT 2 -AT 1 R heterodimers were decreased in myometrial biopsies from pregnant compared to non-pregnant women, and expression levels paralleled levels of AT 1 R-mediated signaling, demonstrating a functional relevance of heterodimerisation. However, a study in which cultured cardiomyocytes were transfected with AT 1 R and AT 2 R failed to detect any influence of AT 2 R over-expression on AT 1 R signaling pathways (D'Amore et al., 2005) , suggesting that further confirmation of AT 1 /AT 2 R heterodimerisation is required.

AT 2 Rs have also recently been shown to form heterodimers with BK receptors (B 2 R) (Abadir et al., 2006) . Similarly to that seen by AT 1 -AT 2 R dimerisation in PC12W cells, Abadir et al. found that the rate of formation of AT 2 -B 2 R heterodimers was influenced by the level of expression of both receptors, and was not dependent on ligand binding. Furthermore, conditions which maximized AT 2 -B 2 R dimer expression also resulted in maximal NO and cGMP production (Abadir et al., 2006) , demonstrating that heterodimers are indeed functional.

In addition, homooligomerisation due to disulfide bonding between AT 2 R was shown to occur in transfected CHO cells, and was localised to the plasma membrane (Miura et al., 2005) . In this cell line, such an interaction between AT 2 R resulted in apoptosis, as was indicated by increased caspase3-like activity. Interestingly, apoptosis was unaffected by treatment with either Ang II or PD123319, but was prevented by inhibition of disulfide bonding by dithiothreitol, suggesting that not only was AT 2 R-mediated apoptosis ligandindependent, but that homodimerisation of the AT 2 R was essential to the observed effect (Miura et al., 2005) .

GPCRs are now thought to interact with a range of other accessory proteins (see Bockaert et al., 2003) . Recent studies have identified and sequenced 2 distinct AT 1 R-associated binding proteins, ARAP1 and ATRAP, which either promote recycling of the AT 1 R to the plasma membrane (ARAP1), or induce receptor internalization (ATRAP) Guo et al., 2003; Lopez-Ilasaca et al., 2003) . Renal-specific over-expression of ARAP1 in mice resulted in hypertension, decreased urine output, and renal hypertrophy (Guo et al., 2006) , and as these effects were abrogated by AT 1 R inhibition, indicate a functional role for ARAP1 in potentiation of AT 1 R signaling. Conversely, ATRAP has been found to be co-localised with AT 1 R in renal tubules (Tsurumi et al., 2006) , and in vitro studies have demonstrated that over-expression of this protein decreases AT 1 R-mediated signaling and cellular proliferation, thus suggesting that ATRAP may act as a negative regulator of AT 1 R signaling (Cui et al., 2000; Lopez-Ilasaca et al., 2003) . This field has recently been reviewed (Mogi et al., 2007) .

Similar proteins that modulate AT 2 R expression at the cell membrane have also been identified. ATIP1 (AT 2 -interacting protein 1) is the product of the human Mitochondrial Tumour Suppressor gene, MTUSI, and, in contrast to AT 2 R, is widely expressed throughout the body suggesting both AT 2 R-dependent and -independent functions (Nouet et al., 2004) . Alternatively, this mismatch in ATIP/AT 2 R expression could represent an important mechanism by which ATIP modulates AT 2 R function in situations of pathological re-expression of AT 2 R. Binding of endogenous ATIP1 to the C-terminal tail of the AT 2 R inhibited mitogenic pathway signalling, an effect which was potentiated by ligand activation, but was also present in the absence of such stimuli (Nouet et al., 2004 ). An identical protein, designated ATBP50 (AT 2 R-binding protein of 50 kDa), has also been identified in mice. Binding of the Golgi membrane-associated ATBP50 to AT 2 R was shown to promote AT 2 R cell-surface expression, and this effect was prevented by downregulation of ATBP50 by use of siRNA (Wruck et al., 2005) . In mouse neuroblastoma N1E-115 cells, stimulation of AT 2 R inhibited EGF-induced ERK1/2 activation and cellular proliferation, and interestingly, not only was this effect blocked by PD123319, but it was also significantly inhibited by ATBP50 siRNA, indicating a functional modulatory interaction between ATBP50 and AT 2 R-mediated anti-mitogenic signalling (Wruck et al., 2005) .

By contrast, AT 2 R also exert growth-promoting effects when coexpressed with the transcription factor PLZF (Senbonmatsu et al., 2003) . PLZF is highly expressed in cardiac tissue, and was found to colocalise with AT 2 R in the cell membrane upon Ang II stimulation. Within 30 minutes of Ang II administration, AT 2 R and PLZF were shown to translocate to the internal compartment, and nuclear PLZF induced p70 S6 kinase activation (essential for protein synthesis), via increased expression of the phosphatidylinositol-3 kinase p85α subunit (Senbonmatsu et al., 2003) .

Taken collectively, these recent data on interacting proteins demonstrate that AT 2 R-mediated growth effects may vary dramatically depending on the presence and type of AT 2 R-binding proteins, and highlights the importance of future determination of AT 2 Rmodulatory factors, in addition to AT 2 R expression, in the elucidation of AT 2 R function in a given tissue (e.g., see Section 3.2 for differential cardiac modulation).

Another emerging concept of the RAS is the unique roles of shorter Ang II peptide fragments, such as Ang (1-7), Ang III and Ang IV. These peptides were initially thought to be inactive breakdown products of Ang II, however, they are now recognized as active components of the RAS, often with their own unique biological profile.

There has been a resurgence of interest in the actions of the Nterminal heptapeptide Ang (1-7) since the discovery of ACE2 and the realization that this peptide can be efficiently produced by this additional pathway. As already mentioned, Ang (1-7) can be formed directly from Ang I via neutral endopeptidase. Alternatively, ACE2 is a carboxypeptidase that cleaves the C-terminal amino acid from either Ang I or Ang II to form Ang (1-9) or Ang (1-7), respectively. Ang (1-7) evokes a range of acute central and peripheral effects, the most prominent being vasodilatation, inhibition of VSMC proliferation, vasopressin release and fluid and electrolyte homeostasis (Santos et al., 2000) . Interestingly, the mechanism of action of Ang (1-7) is not always straightforward since it can mediate multiple effects via a variety of ATRs including AT 1 R, AT 2 R or an Ang (1-7)-sensitive site that is recognized by the analogue A-779 (for review, see Santos et al., 2000) . More recently, it was postulated that Ang (1-7) is the endogenous ligand for the MasR, mainly on the basis of cardiovascular actions of Ang (1-7) being abolished in MasR-deficient mice (Santos et al., 2003; Pinheiro et al., 2004) . Other studies have suggested that the MasR can heterodimerise with AT 1 R to inhibit the effects of Ang II (Kostenis et al., 2005) . In addition, extracellular matrix (ECM) remodeling in the heart leading to collagen accumulation and impaired heart function was seen in MasR-deficient mice . In the last few years, a number of important chronic effects of Ang (1-7) have been identified as a result of exogenous infusion of the peptide. In particular, Ang (1-7) exerted anti-growth and anti-fibrotic effects in rat models of MI, neointimal formation and fibrosis (Strawn et al., 1999; Loot et al., 2002; Benter et al., 2006; Grobe et al., 2006 Grobe et al., , 2007 . The ATR subtype responsible for these cardiovascular protective effects was examined in only one of the afore-mentioned studies, in which it was only partially mediated by the MasR (Grobe et al., 2007) , and AT 2 R involvement was not examined. By contrast, we found that Ang (1-7) caused vasodepressor effects, during AT 1 R blockade, utilizing BK and NO pathways, that were abolished by AT 2 R blockade but not Ang (1-7) receptor blockade . Analogous findings of specific AT 2 R-but not MasR-sensitivity to Ang (1-7) have recently been reported (Lara Lda et al., 2006) . The likely physiological relevance of these findings is underscored by the fact that the cleavage of Ang (1-7) by ACE2 from Ang II provides a double effect, i.e. the shunting towards Ang (1-7) promotes the formation of a counterregulatory peptide while reducing the levels, and thus action, of proexcitatory Ang II (Hamming et al., 2007) . Updated biochemical and functional aspects of Ang (1-7) have recently been reviewed (Ferrario et al., 2005b; Ferrario, 2006; Reudelhuber, 2006; Santos & Ferreira, 2007) .

The Ang (2-8) fragment, Ang III, is readily cleaved from Ang II via aminopeptidase A. Ang III is usually considered a less potent analogue of Ang II that exerts similar AT 1 R-mediated effects to the parent octapeptide . This lack of potency of Ang III is not usually the case in the central nervous system where it is proposed that Ang III is the main mediator of centrally-mediated pressor effects of Ang II, following conversion from the latter (Reaux et al., 2001; Wright et al., 2003) . However, this view has recently been challenged (Kokje et al., 2007) . Interestingly, large bolus doses of Ang III were reported to exert a biphasic effect on BP in anaesthetized rats consisting of an initial pressor followed by depressor effect; the latter component being AT 2 R-mediated (Scheuer & Perrone, 1993) . More recently, an AT 2 R-mediated vasodepressor effect of Ang III was unmasked during AT 1 R blockade in conscious SHR and involved NO and BK signaling pathways (Walters et al., 2003) , in an identical manner to Ang (1-7) . Similarly, Padia et al.showed that Ang III evoked natriuretic effects in conscious rats via AT 2 R stimulation, and that inhibition of aminopeptidase N (and thus prevention of the conversion of Ang III to Ang IV), potentiated sodium excretion (Padia et al., 2006 (Padia et al., , 2007 . Furthermore, Ang III-mediated natriuresis was shown to involve the NO/BK cascade which this group has championed as being a hallmark of AT 2 R activation (Siragy & Carey, 1996 , 1997 Padia et al., 2006) .

Ang IV is formed by the cleavage of Ang III by aminopeptidase B or N. The role of Ang IV in cardiovascular pathophysiology, like other Ang peptide fragments, is emerging as a possible mediator in cardiovascular disease. Following identification of the AT 4 R as an aminopeptidase (Albiston et al., 2001) , it has been proposed that AT 4 R ligands act by inhibiting IRAP catalytic activity, thereby reducing IRAP cleavage of substrates such as lys-bradykinin and vasopressin and prolonging their biological activity . The precise mechanisms of IRAP modulation by Ang IV is yet to be elucidated, however, the recent identification of a site of Ang IV interaction distinct from the active site of IRAP, suggests that Ang IV may utilise an allosteric mechanism to modulate IRAP activity (Caron et al., 2003) . Currently, available data suggests that Ang IV exerts central effects on learning and memory (Chai et al., 2004) . AT 4 R/IRAP is also upregulated in a rabbit balloon vascular injury model suggesting a possible role in vascular repair or remodeling (Moeller et al., 1999) . On the other hand, Esteban et al. (2005) recently demonstrated that Ang IV activated NFκB and subsequently increased pro-inflammatory mediator expression in cultured VSMCs, via AT 4 R and independently of AT 1 R or AT 2 R. The latter two studies suggest Ang IV may play a role in diseases such as atherosclerosis or neointimal hyperplasia. However, Ang IV also induced vasodilatation, (Kramar et al., 1997; Patel et al., 1998; Chen et al., 2000) and has been shown to increase eNOS activation and subsequent NO release (Patel et al., 1998; Hill-Kapturczak et al., 1999) which represents a protective effect in the vasculature. In this context, we have recently shown that chronic treatment with Ang IV improved endothelial dysfunction in ApoE-deficient mice, and this vasoprotective effect most likely resulted from increased NO bioavailability (Vinh et al., 2008a,b) . Clearly further research is required to elucidate the role of Ang IV in cardiovascular pathology.

Given the contextual actions of the various Ang peptide fragments, it is likely that there will be a greater scrutiny of the relative levels of various peptides in the future. For example, it was recently reported that hypercholesterolemia, evoked by a fat-enriched diet in LDLdeficient mice, stimulated production of many angiotensin peptide fragments, with the greatest increases seen in Ang II and Ang IV plasma concentrations (Ang (1-7) was not analysed) . Whether or not all of these peptide fragments have modulatory roles in the progression of atherosclerosis or other cardiovascular diseases remains to be seen. It is well known that plasma Ang II levels are elevated following AT 1 R blockade. However, in many instances, AT 1 R antagonists as well as ACE inhibitors both elevate plasma Ang (1-7) (Campbell et al., 1991; Campbell et al., 1994) , and a similar finding has been made for plasma Ang IV levels in hypertensive patients (Shibasaki et al., 1999) . Of course, these changes do not necessarily reflect changes in tissue levels which are notoriously difficult to measure. Nevertheless, it is well established that there is differential regulation of Ang II and related peptides in plasma versus tissue (Campbell et al., 1991 (Campbell et al., , 1994 (Campbell et al., , 1995 Campbell, 1996) . Clearly, the plasma and tissue profiles of various Ang peptides are likely to be an important consideration for the full understanding of cardiovascular pathophysiology. New mas spectroscopy techniques (Jankowski et al., 2005) for detection and quantification of plasma and tissue Ang peptides simultaneously should help address this issue.

As is clearly evident from the preceding discussion, there are an increasing number of reports that Ang peptides other than Ang II can cause a range of cardiovascular effects via non-AT 1 R (see Table 1 ). However, there are often mismatches when comparing between functional and binding studies. For example, Ang (1-7) has relatively low binding affinity for AT 2 R relative to Ang II and Ang III (Rowe et al., 1995; Bouley et al., 1998) , and a low affinity for AT 1 R (Rowe et al., 1995; Santos et al., 2003 ) and yet many of its reported physiological effects occur via interaction with these sites (see Santos et al., 2000) . Indeed, we have reported that Ang (1-7) can lower BP via functional stimulation of AT 2 R . Moreover there are other functional and molecular data to support this claim that Ang (1-7) can stimulate AT 2 R (Jaiswal et al., 1993; Muthalif et al., 1998; Hansen et al., 2000; Heitsch et al., 2001; De Souza et al., 2004; Castro et al., 2005; Lara Lda et al., 2006) often despite low potency (Hansen et al., 2000) . Much less data are available pertaining to Ang IV in this respect. This hexapeptide has a much lower binding affinity than Ang II for AT 2 R (Bouley et al., 1998; Hansen et al., 2000) , unlike AT 4 R; nevertheless there are functional data indicating that Ang IV exerts vascular effects via AT 2 R Faure et al., 2006) . Indeed, our own recent data has demonstrated that chronic Ang IV treatment resulted in vasoprotective effects on endothelial function via both AT 4 R and AT 2 R stimulation (Vinh et al., 2008a,b) , consistent with findings in a stroke model (Faure et al., 2006) .

On the other hand, there are substantial binding data suggesting that Ang III has 5-10 times higher affinity for AT 2 R over AT 1 R (Dudley et al., 1990; Timmermans et al., 1991; Rosenstrom et al., 2004) , and indeed possesses higher affinity than Ang II itself at AT 2 R (Mukoyama et al., 1995; Bouley et al., 1998; Hansen et al., 2000) . These binding data fit with the relatively few functional studies that have examined ATR subtype effects of Ang III, since both in vivo (Walters et al., 2003; Padia et al., 2006 Padia et al., , 2007 Padia et al., , 2008 and cellular signaling (Lorenzo et al., 2002) studies have implicated an AT 2 R-selective effect of this peptide.

Intriguingly, Ang (1-7) and Ang III both exerted AT 2 R-mediated effects (Walters et al., 2003 Padia et al., 2006) under conditions in which Ang II itself was ineffective (Gohlke et al., 1998) , which raises the distinct possibility that these smaller Ang peptide fragments are endogenous AT 2 R ligands. Likewise, the effects of chronic Ang (1-7) or Ang IV infusions are strikingly different to those of Ang II itself (Grobe et al., , 2007 Vinh et al., 2008a,b) . Therefore, at the very least, we should consider that Ang peptides other than Ang II have a major role in the cardiovascular system as endogenous non-AT 1 R ligands stimulating multiple ATR subtypes (see Table 1 ).

Of particular note, AT 2 R does not desensitize since concentrationresponse curves to AT 2 R-mediated vasorelaxation are highly reproducible, unlike AT 1 R-mediated contractile effects (Widdop et al., 2002) . The reproducible nature of AT 2 R function is consistent with a lack of AT 2 R internalization previously reported (Mukoyama et al., 1995; Hein et al., 1997) and hence lack of desensitization. Cellular trafficking of both AT 1 and AT 2 R expressed in human embryonic kidney 293 cells indicated that AT 2 R cell-surface binding was not altered after prolonged exposure to Ang II (Mukoyama et al., 1995) , and fluorescently labeled AT 2 R were also not internalized after agonist exposure (Hein et al., 1997) . By contrast, AT 1 R were rapidly internalized (Hein et al., 1997; Thomas, 1999) , which is consistent with functional data (Widdop et al., 2002) . Thus it is possible that, in the event of raised circulating or tissue levels of Ang peptides, such fragments may maintain efficacy at least at the AT 2 R. Indeed, the ability of failing human hearts to produce Ang (1-7) via ACE2 was directly correlated with AT 2 R, but not AT 1 R, density (Zisman et al., 2003) , further supporting the concept of endogenous AT 2 R ligands modulating the effects of Ang II.

The Ang II derived peptide CGP42112 has long been the gold standard for determining functional and selective AT 2 R activity (Whitebread et al., 1989; de Gasparo et al., 2000) . Recently, nonpeptide selective AT 2 R agonists have been developed. Initially, a nonpeptide agonist for both AT 1 R and AT 2 R was identified (Wan et al., 2004a) and this was soon followed by the first selective non-peptide AT 2 R agonist, Compound 21, which was active in 2 in vivo bioassays where it enhanced alkaline secretion from rat intestine and lowered BP in anaesthetized SHR (Wan et al., 2004b) . In addition, a number of other peptide-based AT 2 R mimetics have been made by the same research group following extensive structure-activity relationships using Ang II or analogues (Johannesson et al., 2004; Georgsson et al., 2005; Rosenstrom et al., 2005; Georgsson et al., 2006) . More recently, new peptide-based ligands have been designed that may become lead compounds for future drug development (Georgsson et al., 2007) .

AVE0991 is a non-peptide compound that was first described as a Ang (1-7) mimetic, as it competed for Ang (1-7) binding in bovine aortic endothelial cells, and increased NO release in a similar manner to Ang (1-7) (Wiemer et al., 2002) . The functional antidiuretic and vasodilator effects of AVE0991 are absent in MasR-deficient mice (Pinheiro et al., 2004; Lemos et al., 2005) , although in some instances either MasR blockade (with A-779) or AT 2 R blockade (with PD123319) can markedly attenuate the effects of AVE0991 (Wiemer et al., 2002; Pinheiro et al., 2004) . More recently, chronic AVE0991 administration was reported to attenuate heart failure induced by MI although the ATR subtype mediating this effect was not investigated.

Ang IV mimetics also exist. Modifications to the valine in position 1 of Ang IV lead to the formation of an analogue, norleucine-Ang IV (Nle 1 -Ang IV), which exhibits similar agonistic properties but 100 fold higher affinity for IRAP compared with Ang IV (Sardinia et al., 1994) . In addition, Leu-Val-Val-hemorphin-7 (LVV-hemorphin-7), which was first identified as a ligand based on its ability to displace 125 I-Ang IV (Lee et al., 2003) also mimics the biological effects of Ang IV such as enhanced memory and learning retention. In contrast, divalinal-Ang IV, is used as an antagonist to inhibit Ang IV mediated effects (Wright et al., 1995) .

Having discussed the physiological and pharmacological effects mediated by AT 2 R, it is appropriate to discuss the specific role of AT 2 R in cardiovascular pathologies, given that this ATR subtype is usually upregulated in a range of settings (refer to Table 2 ).

Essential or primary hypertension refers to the condition of elevated arterial BP without known cause, and although usually asymptomatic in its earlier stages, has been shown to be closely correlated with the occurrence of future cardiovascular disorders such as left ventricular hypertrophy (LVH), cardiac failure, arteriosclerosis, and stroke (Unger et al., 1996; Weber, 1997; Simon et al., 1998) . Considering the well documented anti-hypertensive effect of AT 1 R antagonists, and that AT 2 R oppose AT 1 R-mediated actions in many situations, it is tempting to speculate that AT 2 R stimulation may contribute to BP regulation .

Initially, in vivo studies deduced a vasodilator function of the AT 2 R since Ang II evoked either an augmented vasodepressor effect in the presence of AT 1 R blockade or an enhanced vasoconstriction during AT 2 R blockade (Scheuer & Perrone, 1993; Munzenmaier & Greene, 1996) . However, similar studies performed in SHR were unable to demonstrate corresponding Ang II-mediated vasodilatation during AT 1 R blockade (Gohlke et al., 1998) . Such divergence in responses in (1-7) Asp-Arg-Val-Tyr-Ile-His-Pro

Val-Tyr-Ile-His-Pro-Phe + + +++

Sartan compounds

+ indicates relative affinity for receptor based on binding and functional data (+++) indicates compounds that are antagonists.

these experiments may be reconciled by the subtle hypotensive effect of AT 2 R stimulation, which may have been masked by the concomitant dominant AT 1 R-mediated pressor action of Ang II infusion. In order to avoid such confounding influences of AT 1 R stimulation on potential AT 2 R vasodilator function, investigators have assessed the effect of selective AT 2 R agonists and antagonists during AT 1 R blockade (see Fig. 2 ). Using this approach, selective stimulation of AT 2 R by CGP42112 lowered BP, provided that there was a background of AT 1 R blockade in conscious rats (Barber et al., 1999; Carey et al., 2001) . Furthermore, this BP-lowering response to AT 2 R stimulation was shown to be associated with increased blood flow in renal, mesenteric and hindquarter circulations indicating widespread vasodilatation (Li & Widdop, 2004) , which highlights the fact that modest AT 2 R-mediated vasorelaxation observed in isolated blood vessels can translate into significant in vivo haemodynamic effects. A similar acute BP-lowering effect of AT 2 R stimulation was also deduced in a recent study in which the anti-hypertensive action of losartan was potentiated in rats following transient peripheral over-expression of AT 2 R. This hypotensive action was blocked by PD123319, and persisted over the same time frame as enhanced AT 2 R expression (reduced towards basal levels 7 days after viral transduction), lending weight to the importance of relative AT 1 /AT 2 R expression to AT 2 R-mediated functional effects (Li et al., 2006) . These acute in vivo findings are consistent with numerous reports of AT 2 R-mediated vasorelaxation in a wide variety of locations, including mesenteric, renal, coronary, cerebral and uterine vascular beds, which has been shown to be via BK/NO/cGMP signaling pathways. AT 2 R-mediated vasorelaxation has also been indirectly implicated in conduit vessels such as the aorta since aortic banding markedly increased aortic AT 2 R expression as well as activating the eNOS/cGMP axis Yayama et al., 2004 Yayama et al., , 2006 . Consequently, Ang II-mediated contraction via AT 1 R stimulation was reduced in this vessel Recent evidence further suggests that such recruitment of NO/cGMP mechanisms activates a cGMP-dependent protein kinase (cGKI) resulting in downregulation of RhoA activity, which is known to involved in AT 1 R-mediated vasoconstriction (Savoia et al., 2006a) . Interestingly, AT 1 R blockade was shown to increase AT 2 R expression (2-3-fold) and NO production, and to suppress NAD(P)H-driven superoxide generation, in arteries of hypertensive SHRSP, but not normotensive WKY rats (Savoia et al., 2006a) . This potentiated NO signaling decreased RhoA/Rho kinase activation, reduced MLC phosphorylation, and subsequently unmasked AT 2 R-mediated vasorelaxation to Ang II, providing evidence for mechanisms of AT 1 /AT 2 R crosstalk at a signaling level . For further detailed analysis of AT 2 R-mediated relaxation/vasodilatation, see (Hannan & Widdop, 2004; Henrion et al., 2001; Widdop et al., 2008) .

The vast majority of in vitro vascular studies on AT 2 R have used arteries obtained from normotensive animals; interestingly, there is little evidence of AT 2 R-mediated vasorelaxation in arteries taken from untreated hypertensive animals (Matrougui et al., 1999; Matrougui et al., 2000; Cosentino et al., 2005) which contrasts the afore- Fig. 2 . The in vitro and in vivo vasodilator effects of AT 2 R stimulation are often difficult to detect because of the overriding effects of AT 1 R-mediated vasoconstriction. This state can be dramatically changed by performing AT 2 R stimulation against a background of low-dose AT 1 R antagonist (sartan), even using sartan doses that are sub-threshold for BP-lowering (A). Under these circumstances, AT 2 R-mediated vasodilatation can be unmasked and subsequently abolished by concomitant AT 2 R blockade using PD123319 (B). Asano, K., et al. (1997) . Circulation, 95, 1193 -200. 3. Baiardi, G., et al. (2005 . Regul Pept, 124, 7-17. 4. Bautista, R., et al. (2001) . Hypertension, 38, 669-73. 5. Bonnet, F., et al. (2002) . J Hypertens, 20, 1615-24. 6. Brink, M., et al. (1996) . J. Mol. Cell. Cardiol., 28, 1789 -99. 7. Busche, S., et al. (2000 . Am. J. Pathol., 157, 605-11. 8. Cao, Z., et al. (2002) . J Am Soc Nephrol, 13, 1773-87. 9. de P Rodrigues, S. F., et al. (2006) . Life Sci, 78, 2280-5. 10. Hakam, A.C. & Hussain, T. (2005) . Hypertension, 45, 270-5. 11. Hakam, A.C., et al. (2006) . Am J Physiol Renal Physiol, 290, F503-8. 12. Haywood, G.A., et al. (1997) . Circulation, 95, 1201-6. 13. Heymes, C., et al. (1998) . Endocrinology., 139, 2579-87. 14. Hiyoshi, H., et al. (2004) . Hypertension, 43, 1258 -63. 15. Johansson, M.E., et al. (2005 . J Hypertens, 23, 1541-9. 16. Lee, S., et al. (2001) . Cardiovasc Res, 51, 131-9. 17. Li, C., et al. (2005) . Cardiovasc Drugs Ther, 19, 105-12. 18. Lopez, J.J., et al. (1994) . Am J Physiol, 267, H844-52. 19. Lu, Q., et al. (2005) . Neuroreport, 16, 1963 16, -7. 20. Mezzano, S., et al. (2003 . mentioned in vivo data (Barber et al., 1999; Li & Widdop, 2004; Walters et al., 2005) . Moreover, in some cases, Ang II in fact evoked contraction of mesenteric arteries obtained from SHR that involved both AT 1 R and AT 2 R (Touyz et al., 1999) . Pressure overload induced by aortic banding in rats and mice , or 2K1C hypertension (Hiyoshi et al., 2005) , has been shown to upregulate vascular AT 2 R expression and thus attenuate the AT 1 R-mediated contractile response to Ang II. Increased AT 2 R expression in the context of pressure overload was suggested to be due to stimulation of AT 1 R by increased circulating Ang II, as AT 1 R inhibition prevented the upregulation of AT 2 R expression due to aortic banding . On the other hand, vascular AT 2 R was reduced in mesenteric arteries, but increased in aortae, in spontaneously hypertensive rats compared to normotensive controls (Touyz et al., 1999; Widdop et al., 2008) . Thus, sitespecific changes in vascular AT 2 R expression associated with hypertension, and the need to block tonic AT 1 R activity, may account for some of the discrepancies between in vitro and in vivo hypertensive studies.

Indeed, vascular AT 2 R expression is generally increased by chronic AT 1 R blockade You et al., 2005; Savoia et al., 2006a,b) , and such treatment can unmask AT 2 R-mediated vasorelaxation in previously unresponsive aortic vessels from SHR (Cosentino et al., 2005) . In this context, we have recently reported that the AT 2 R vascular phenotype is dependent on basal arterial BP and level of AT 2 R expression, at least in mesenteric arteries (You et al., 2005) . Ang II (in the presence of AT 1 R blockade) evoked AT 2 R-mediated vasorelaxation in mesenteric arteries from WKY rats whereas analogous experiments using mesenteric arteries from SHR resulted in Ang II causing AT 2 Rmediated contraction. At the same time, AT 2 R expression, determined by Western blot analysis, was reduced in mesenteric arteries from SHR compared with WKY rats. Strikingly, when basal BP of SHR was normalized to WKY-like levels with either an ACE inhibitor or an AT 1 R antagonist, the AT 2 R-mediated response was converted from contraction to relaxation. Mesenteric AT 2 R expression was increased with chronic anti-hypertensive treatment, in line with AT 2 R-mediated vasorelaxation (You et al., 2005) . Moreover, this upregulation of AT 2 R expression appeared to be related to the level of basal BP since treatment of SHR with the non-specific anti-hypertensive, hydralazine, also increased AT 2 R expression and converted AT 2 R-mediated vasoconstriction to Ang II in mesenteric vessels into vasorelaxation. Vascular AT 2 R localization was also performed using fluorescentlylabelled Ang II in the presence of AT 1 R blockade. These studies confirmed the upregulation of AT 2 R following anti-hypertensive treatments and identified that AT 2 R were re-expressed at the level of the endothelium (You et al., 2005) . Conversely, it was recently reported that C-reactive protein caused systolic hypertension that was directly related to downregulation of vascular AT 2 R (Vongpatanasin et al., 2007) .

AT 2 R function has also been indirectly examined by determining the effect on BP of AT 2 R blockade during AT 1 R inhibition. In relatively short-term studies performed in rats with either renovascular hypertension or which had been sodium-depleted, the hypotensive effect of AT 1 R blockade was reversed by simultaneous administration of PD123319, suggesting a vasodilator role of the AT 2 R (Gigante et al., 1998; Siragy et al., 2000) . However, in analogous experiments, Duke et al. (2005a) reported that AT 2 R contributed to the BP-lowering and mesenteric vasodilator effect of candesartan in normotensive, but not hypertensive (SHR, 2K1C) rats.

There are a limited number of clinical studies investigating vascular AT 2 R function in vivo, and these studies have not always reported AT 2 R-mediated vasodilator effects, which most likely reflect the different patient populations studied. In healthy male volunteers, intravascular administration of PD123319 had no effect on systemic vascular resistance or arterial stiffness, providing no evidence for acute AT 2 R-mediated haemodynamic effects (Phoon & Howes, 2001; Brillante et al., 2005) . However, in other studies by the same investigators, treatment of adult male subjects (Gilles et al., 2004) or elderly women (Phoon & Howes, 2002) with an AT 1 R antagonist, uncovered acute haemodynamic effects due to AT 2 R, deduced from short-term infusions of PD123319, thus supporting a role for AT 2 R during AT 1 R blockade, albeit with some variation that likely reflects the different patient populations (Phoon & Howes, 2002; Gilles et al., 2004) .

On the other hand, investigations into chronic AT 2 -mediated regulation of BP have yielded less conclusive results. In AT 2 R knockout mice (Hein et al., 1995; Ichiki et al., 1995; Gross et al., 2000a) , and rats administered AT 2 R antisense to 'knockdown' receptor expression (Wang et al., 2004) , both an increase in basal BP and/ or potentiation of the pressor response to exogenous Ang II have been attributed to the absence of AT 2 R modulation of vascular tone. In mice with targeted over-expression of vascular (Tsutsumi et al., 1999) or cardiac ) AT 2 R, there was no change in basal BP, although acute Ang II infusion decreased BP. Stimulation of AT 2 R by chronic Ang II infusion during AT 1 R blockade failed to decrease BP (Cao et al., 1999; Diep et al., 1999) , and similarly, chronic AT 2 R inhibition either had no (Liu et al., 1997; Tea et al., 2000) or minimal (Varagic et al., 2001 ) influence on the anti-hypertensive effect of chronic AT 1 R blockade. Thus in terms of chronic haemodynamic control, and in contrast to the well documented acute vasodilator/relaxation effects of AT 2 R stimulation, AT 2 R activation appears to exert only subtle influences on long-term BP regulation. Further studies using chronic AT 2 R agonist treatment are clearly warranted to address this issue.

Compared to the extensively investigated role of AT 2 R in vascular reactivity, longer term effects on vascular remodeling in hypertension have been less well studied. Vascular remodelling associated with hypertension occurs at all levels of the vascular tree, and increased local expression of RAS components and subsequent enhanced local Ang II production (Shiota et al., 1992; Jandeleit-Dahm et al., 1997; Wang et al., 2003) , implies an important role of the RAS in this process. Furthermore, other factors involved in cellular growth and ECM production which are known to be stimulated by Ang II, such as growth factors (Parker et al., 1998; Su et al., 2002) , factors involved in the inflammatory response (Chou et al., 1998) and reactive oxygen species (ROS) (Virdis et al., 2002; Keidar et al., 2004; Touyz et al., 2003) , are also upregulated in the hypertensive vasculature. The present overview is limited to discussion of vascular remodeling directly related to hypertension, as reports of AT 2 R-mediated vascular effects in the specific contexts of atherosclerosis and diabetes (Savoia et al., 2006b) will be discussed in following sections.

Early direct evidence for a component of the vascular antihypertrophic effect of sartan-treatment being due to AT 2 R stimulation was provided by Tea et al. (2000) , who showed that simultaneous AT 2 R inhibition returned aortic mass, smooth muscle cell number and DNA synthesis back to control levels in SHR. We have similarly shown that chronic PD123319 treatment completely reversed sartanmediated aortic remodeling in both adult and senescent SHR (unpublished data), similarly to aged normotensive rats (Jones et al., 2004) . Importantly, this AT 2 R-mediated vascular anti-hypertrophic effect was not simply a consequence of sartan-induced BP-lowering, as simultaneous PD123319 administration had no further influence on BP in these studies (Tea et al., 2000; Jones et al., 2004) . Similarly, in Ang II-induced hypertension, simultaneous blockade of AT 1 and AT 2 R by Sar-Ile resulted in greater media:lumen ratio than that due to AT 1 R blockade alone in both aortic and mesenteric vessels, suggesting an AT 2 R-mediated anti-hypertrophic effect (Brassard et al., 2005) . Regulation of collagen synthesis also appears to be altered in hypertension, since AT 1 R-mediated Ang II stimulation of collagen production was potentiated in SHR and occurred via both p38 and ERK1/2 signalling pathways, whereas collagen production was reduced, and dependent on only ERK1/2 activation in WKY (Touyz et al., 2001) . In addition, Su et al. (2002) showed that over-expression of AT 2 R in VSMCs from WKY rats decreased AT 1 R and TGF-β receptor expression, but that this response was absent in VSMCs of SHR, suggesting a deficiency in inhibitory mechanisms in this model of genetic hypertension.

Furthermore, both basal media:lumen ratio, and vascular hypertrophy due to pressure overload was significantly elevated in aortic, femoral (Brede et al., 2001) , and coronary (Akishita et al., 2000b; Wu et al., 2002) vessels of AT 2 R knockout mice compared to wild type controls, indicating an AT 2 R-mediated protective effect on vascular hypertrophy. Moreover, consistent with results from pharmacological AT 1 /AT 2 R inhibition in rats, the anti-hypertrophic action of AT 1 R inhibition was reduced in coronary vessels from AT 2 R knockout mice, supporting a role for AT 2 R stimulation in the beneficial remodeling effects of AT 1 R antagonists .

Taken collectively, these experimental results suggest that, although an acute vasodilator role of AT 2 R is well documented, chronic BP regulation seems to be only minimally influenced by AT 2 R stimulation. However, AT 2 R have consistently been shown to be important in the regulation of vascular structure, which may indirectly influence BP maintenance in the longer term. Indeed, restoration of AT 2 R-mediated vasorelaxation in mesenteric arteries from SHR was correlated with anti-hypertensive efficacy (You et al., 2005) .

LVH is considered a major predictor of cardiovascular morbidity and mortality (Levy et al., 1990) , and results in changes in the structural organization of the heart, which necessarily influences cardiac function. When the relative proportions of cell types within the heart remain unchanged, hypertrophy is termed adaptive or physiological, however, by far the most prevalent form of cardiac hypertrophy involves disproportionate changes within the cardiac tissue, which ultimately decrease cardiac function, and results in LVH and subsequent progression to heart failure (Swynghedauw, 1999; Zhu et al., 2003) . Since the cloning of the AT 2 R over a decade ago, and despite considerable investigation into the role of AT 2 R in cardiac hypertrophy, the exact function of AT 2 R in cardiac tissue still remains somewhat controversial (Booz, 2004; Reudelhuber, 2005) .

Importantly, the relatively low expression of the AT 2 R compared with that of the AT 1 R, is upregulated in certain conditions, such that increased AT 2 R expression has been reported in patients with heart failure, ischemic heart disease, and dilated cardiomyopathy (Regitz-Zagrosek et al., 1995; Asano et al., 1997; Haywood et al., 1997; Tsutsumi et al., 1998; Wharton et al., 1998) . Moreover, a correlation between AT 2 R density and severity of heart failure has been reported , raising the obvious question of whether or not such increased AT 2 R expression represents a causative or a reactive consequence of LVH.

In animal models, AT 2 R expression is also increased by cardiac hypertrophy and heart failure, either in terms of absolute numbers, or relative to AT 1 R expression (Suzuki et al., 1993; Lopez et al., 1994; Nio et al., 1995; Ohkubo et al., 1997; Bartunek et al., 1999; Busche et al., 2000) . In vitro studies have demonstrated increased AT 2 R expression due to stabilization of mRNA following cardiomyocyte stretch, providing a potential mechanism for observed enhancement of AT 2 R density during situations of cardiac overload (Kijima et al., 1996) .

In accordance with the notion that the AT 2 R opposes AT 1 R mediated effects, Liu et al. (1997) demonstrated in a rat model of heart failure that the anti-hypertrophic effects of AT 1 R blockade on LV volume and cardiomyocyte size were reversed by concomitant AT 2 R inhibition. Consistent with a protective function of AT 2 R, AT 2 Rdeficient mice showed a greater hypertrophic response to MI than that of wild type controls (Brede et al., 2003; Oishi et al., 2003) , and also attenuated response to AT 1 R blockade (Xu et al., 2002) . In more recent studies in which AT 2 R were over-expressed via a lentiviral vector gene delivery system in cardiomyocytes of 5 day old SHR (Metcalfe et al., 2004) , or Sprague-Dawley rats (Falcon et al., 2004) , AT 2 R over-expression reduced cardiac hypertrophy at 21 weeks of age compared to control SHR, and attenuated Ang II-induced cardiac hypertrophy and fibrosis, respectively. Similarly, in isolated perfused adult hypertrophied hearts (Bartunek et al., 1999) , Ang II-induced production of early response genes were enhanced by AT 2 R inhibition, providing evidence for AT 2 R-mediated regulation of cellular growth at the level of gene expression.

On the other hand, a lack of effect of AT 2 R on indices of cellular growth has also been shown in pharmacological studies. We and others have deduced no influence of AT 2 Rs on cardiac hypertrophy from 'PD-reversal' studies, in which simultaneous PD123319 administration did not modify the cardiac anti-hypertrophic effect of AT 1 R blockade in either aged WKY (Jones et al., 2004) or SHR (Varagic et al., 2001) rats. Similarly, the cardiac growth-inhibitory effect of valsartan, was of similar magnitude in aortic-banded AT 2 R knockout mice to that of animals expressing AT 2 R . Ang II infusion also resulted in a comparable increase in cardiac hypertrophy in mice with cardiac specific over-expression of AT 2 R, to that of wild type mice (Sugino et al., 2001; Kurisu et al., 2003) .

In direct contrast, several in vivo studies have suggested a growthstimulatory role of AT 2 R. Mice in which AT 2 R were over-expressed using the ventricular-specific myosin light-chain promoter, exhibited dilated cardiomyopathy which was associated with increases in both myocyte cross sectional area and fibrosis (Yan et al., 2003) . Similarly, in AT 2 R knockout mice, the cardiac hypertrophic response to aortic banding, Ang II infusion or MI was absent in AT 2 R-deficient animals (Senbonmatsu et al., 2000; Ichihara et al., 2001; Ichihara et al., 2002) , indicating a requisite presence of AT 2 R for cardiomyocyte hypertrophy. Interestingly, D'Amore et al. (2005) recently showed that AT 2 R evoked constitutive growth in cultured neonatal cardiomyocytes. The research groups who reported a growth-promoting function of AT 2 R also found evidence for increased signalling via p70 S6 kinase, which is a well documented hypertrophic mechanism (Senbonmatsu et al., 2000; Yan et al., 2003) . In a subsequent study (Senbonmatsu et al., 2003) , AT 2 R-mediated cellular hypertrophy involving p70 S6 kinase was dependent on PLZF activation as previously discussed (Section 2.3.3). Nevertheless, the cardiac hypertrophic effects of AT 2 R are controversial, with either no effect or hypertrophy being attributed to cardiac AT 2 R in different AT 2 R knockout strains subjected to various cardiac loads (Akishita et al., 2000b; Senbonmatsu et al., 2000; Widdop et al., 2003) . Thus, it is possible that these discrepancies may relate to differential PLZF expression and/ or activity in the 2 AT 2 R knockout strains. In this context, as previously mentioned, a novel signal transduction pathway involving renin/(P)RR/PLZF has been described, that displayed both excitatory and inhibitory actions of PLZF (Schefe et al., 2006) .

Interestingly, a study performed by Lako-Futo et al. may go some way towards explaining such equivocal AT 2 R-mediated effects on cardiac hypertrophy. In acute studies performed in rats, blockade of the AT 2 R with PD123319 during 62 h Ang II infusion, resulted in enhanced expression of genes associated with the promotion of cardiac hypertrophy (c-fos, endothelin-1, and IGF-1), but also increased expression of anti-hypertrophic factors (ANF and BNP), and decreased growth factors (VEGF and FGF-1) (Lako-Futo et al., 2003) . These results suggest that in addition to opposing AT 1 Rmediated effects, AT 2 R stimulation may produce growth-stimulatory actions which partially offset AT 2 R-mediated growth-inhibitory effects. Thus, it is conceivable that even slight alterations in conditions may shift the balance of AT 2 R-mediated actions from a situation in which pro-and anti-hypertrophic effects remain in equilibrium, to that in which either growth-promoting or growth inhibiting-pathways are favoured.

In this context, it is interesting to note that the most consistent reports of a cardioprotective effect of AT 2 R in vivo, have been determined from experiments performed in models of MI-induced heart failure. The vast majority of investigators have deduced AT 2 Rmediated improvement of LV function and remodelling from multiple studies performed in mouse models of either AT 2 R-deletion (Brede et al., 2003; Oishi et al., 2003) or over-expression Bove et al., 2005) . In fact, only 1 study has deduced a negative role for AT 2 R stimulation in this context (Ichihara et al., 2002) , and these discordant results may be due to the alternative strain of knockout mouse used by Ichihara et al., as has been previously discussed (see Widdop et al., 2003) . AT 2 R-mediated cardioprotection has been shown to involve NO signalling, as NO synthase inhibition prevented the beneficial effects of AT 2 R over-expression on LV function (Bove et al., 2004) . BK has also been implicated in cardiac AT 2 R-mediated effects, as pharmacological inhibition of B 2 R in rats (Liu et al., 1997) , or B 2 R deletion in mice (Isbell et al., 2007) , was shown to prevent the anti-fibrotic influence of AT 2 R stimulation. In addition, AT 2 R stimulation has been shown to be involved in the beneficial effects of sartans post-MI, as AT 1 R inhibition was found to be less effective during AT 2 R blockade (Liu et al., 1997) or AT 2 R-deletion (Xu et al., 2002) , and potentiated by AT 2 R-over-expression (Voros et al., 2006) .

Interestingly, genetic influences in humans have also been reported which add another degree of complexity. The AT 2 R gene polymorphism -1332G/A (also known as 1675G/A) has also been linked with LVH. In hypertensive subjects, LVH has been associated with either the A allele (Schmieder et al., 2001) or the G allele (Alfakih et al., 2004) in 2 separate studies, with the method of LVH determination (echocardiography versus magnetic resonance imaging) suggested as explanation for these discrepancies (Alfakih et al., 2004) . Premature coronary artery disease has also been associated with the G allele (Alfakih et al., 2005) . However, the interpretation of these findings is somewhat unclear as the same group has reported that the G allele represents either decreased AT 2 R function (Nishimura et al., 1999; Alfakih et al., 2004; Alfakih et al., 2005) or increased AT 2 R function (Warnecke et al., 2005; Strauss & Hall, 2006) .

Clearly, neither animal nor human studies to date have been able to delineate a consistent influence of AT 2 R stimulation on LVH. Current equivocal results may be partly explained by the range of animal models, indices measured, drugs, doses and treatment times employed by investigators, but most likely reflect the complex and context-specific nature of AT 2 R-mediated cardiac growth effects, probably also involving ligand-independent effects. Given the correlation between LVH and the poor long-term prognosis of patients exhibiting such modification of cardiac structure (Levy et al., 1990; Rihal et al., 1994; Senni et al., 1998) , this area of AT 2 R function undoubtedly requires further research.

In contrast to the range of equivocal results regarding the influence of AT 2 R on LVH, the majority of experimental data relating to cardiac fibrosis report anti-fibrotic effects of AT 2 R stimulation. Indeed, increased AT 2 R expression in the heart has been specifically localised to cardiac fibroblasts, suggesting a link between upregulation of AT 2 R expression and fibrosis present in hypertrophied and failing hearts (Brink et al., 1996; Tsutsumi et al., 1998; Wharton et al., 1998) . Such an association between AT 2 R expression and cardiac disease states, and previous suggestions of AT 2 R-mediated anti-growth effects, has fuelled interest in the AT 2 R as a potential target for cardiac antifibrotic therapies.

It is well accepted that Ang II is an important mediator of ECM homeostasis, with numerous reports of either AT 1 R blockade or ACE inhibitor treatment resulting in decreased cardiac fibrosis in both humans and animal models. Importantly, in both physiological (e.g. heart valve leaflets) and pathophysiological (e.g. MI) sites of repair, AT 2 R are expressed at high levels, along with other RAS components such as angiotensinogen, ACE, and AT 1 R . Moreover, important factors involved in Ang II-mediated connective tissue formation, including TGF-β 1 and its receptors, and type I and III collagens, are also expressed in AT 2 R-rich fibroblasts and myofibroblasts Lijnen et al., 2003) .

Although 2 separate groups have reported a stimulatory effect of AT 2 R on cardiac collagen accumulation (Senbonmatsu et al., 2000; Ichihara et al., 2001; Ichihara et al., 2002; Yan et al., 2003) , the bulk of data from numerous in vivo studies clearly support an inhibitory effect of AT 2 R on cardiac fibrosis. In 'PD123319 reversal' experiments, AT 2 R inhibition reversed the anti-fibrotic effects of AT 1 R blockade both in SHRs (Varagic et al., 2001) , and in aged normotensive WKY (Jones et al., 2004) and aged SHR (unpublished data). Similarly, the antifibrotic effect of AT 1 R inhibition was found to be attenuated in AT 2 Rdeficient mice, since the sartan-mediated reduction in cardiac fibrosis following aortic banding or Ang II infusion, was of smaller magnitude in AT 2 R knockout mice compared to wild type littermates Xu et al., 2002) . Other investigators have also deduced an inhibitory effect of the AT 2 R on cardiac fibrosis, as the promotion of interstitial fibrosis by either aortic banding or L-NAME was augmented in AT 2 R knockout mice (Akishita et al., 2000b; Wu et al., 2002; Gross et al., 2004) and cardiac specific over-expression of AT 2 R diminished the amount of both perivascular and interstitial fibrosis induced by Ang II infusion (Kurisu et al., 2003) .

However, the exact mechanisms by which AT 2 R may modulate collagen deposition are not completely understood. The accumulation of collagen within the heart is a dynamic process, with total collagen content representing the net effect of both collagen synthesis and degradation. In terms of collagen production, a stimulatory role for AT 1 R in terms of collagen synthesis is well established, however, there are contrasting reports of the contribution of AT 2 R to this process. In rat cardiac fibroblasts, collagen production was reduced by inhibition of either AT 1 or AT 2 R (Brilla et al., 1994; Brilla et al., 1995) , and chronic blockade of both AT 1 R and AT 2 R with Sar 1 Ile 8 Ang II in rats also decreased collagen deposition, suggesting a stimulatory role of AT 2 R in collagen synthesis (Brassard et al., 2005) . In addition, Mifune et al. (2000) also reported that AT 2 R stimulation by CGP42112 induced collagen synthesis in VSMCs and mesangial cells, but that CGP42112 treatment had the opposite effect in cultured fibroblasts, indicating heterogeneous AT 2 R-mediated responses in different cell types. In contrast, several other investigators have reported a lack of effect of AT 2 R on collagen secretion, since Ang II stimulation of collagen production in cultured rat and porcine cardiac fibroblasts, and rat mesenteric VSMCs was inhibited by AT 1 R blockade, but unaffected by AT 2 R inhibition (Lijnen et al., 2000; Touyz et al., 2001; Warnecke et al., 2001) . On the other hand, the influence of AT 2 R on collagen degradation has not been extensively investigated. In one of the earliest studies to investigate AT 2 R-specific effects on collagen metabolism, the decrease in collagenase activity due to Ang II stimulation of cultured cardiac fibroblasts was abolished by AT 2 R blockade but not influenced by AT 1 R inhibition (Brilla et al., 1994) . In contrast, Min et al. (2004) demonstrated a diminution of collagenase activity by AT 2 R blockade which was associated with increased tissue inhibitor of matrix metalloproteinase-1 (TIMP-1) expression, and selective AT 2 R stimulation with CGP42112 increased matrix metalloproteinase-1 (MMP-1) production in human monocytes (Kim et al., 2005) .

On the basis of several large-scale clinical trials, it is now accepted that chronic inhibition of the RAS can provide neuroprotection, with reduced occurrence of stroke in high-risk populations (Dahlof et al., 2002; Arnold et al., 2003; Chapman et al., 2004; Schrader et al., 2005) . Neuroprotection has also been demonstrated by candesartan cilexetil in an acute setting (Schrader et al., 2003) . Moreover, drugs that target the renin-angiotensin system offer benefits that extend beyond the control of BP (Arnold et al., 2003; Chapman et al., 2004) .

In accordance with human clinical trials, animal studies have also concluded that inhibition of the RAS protects against neuronal injury following stroke, often in a BP-independent manner (Ito et al., 2002; Groth et al., 2003) . Similarly, AT 1 R knock out mice, exhibited smaller infarct size following temporary middle cerebral artery occlusion (Walther et al., 2002) . Underlying mechanisms may involve the inhibition of AT 1 R-mediated cerebrovascular pathological growth and inflammation, as well as improved cerebrovascular compliance and autoregulation (Nishimura et al., 2000; Saavedra et al., 2006) . In addition, Sugawara et al. (2005) recently demonstrated that a nonhypotensive dose of an AT 1 receptor antagonist reduced superoxide production and improved neurological outcome following global ischemia (Sugawara et al., 2005) .

Not surprisingly considering the contribution of AT 2 R to the effects of AT 1 R inhibition in other settings, attention has recently turned to the potential role of the AT 2 R in neuroprotection, particularly since AT 2 R is reported to be upregulated in stroke (Zhu et al., 2000; Li et al., 2005b; Lu et al., 2005) , specifically in the ischemic area of the brain (Mogi et al., 2006) . Moreover, AT 2 R-deficient mice display a larger ischemic area and a more severe neurological deficit following stroke when compared to wild type (Iwai et al., 2004) , implicating a neuroprotective role for the AT 2 R. In this context, the neuroprotective effect of valsartan was reduced in AT 2 R knockout animals (Iwai et al., 2004; Mogi et al., 2006) , and either peripheral or central (Li et al., 2005b) administration of an AT 2 R antagonist reversed the neuroprotection evoked by AT 1 R blockade, suggesting opposing roles for AT 1 R and AT 2 R during stroke.

The potential mechanisms of AT 2 R-mediated neuroprotection in reducing infarct volume following stroke are currently under investigation. AT 2 R-mediated apoptosis (Zhu et al., 2000; Grammatopoulos et al., 2002) is one possibility. Indeed, AT 2 R is capable of inducing apoptosis in PC12W cells through the induction of ceramides and the activation of MKP-1 signalling (Dai et al., 1999) . Furthermore, AT 2 R knockout mice have higher neuronal numbers compared to controls, suggesting that AT 2 R deficit antagonises angiotensin IIinduced apoptosis (Von Bohlen und Halbach et al., 2001) . Another possible AT 2 R-mediated neuroprotective influence involves neuronal regeneration. Earlier studies (Laflamme et al., 1996) demonstrated that in vitro stimulation of AT 2 R was associated with intense neurite outgrowth; an effect that was negated by PD123319. Reinecke et al. (2003) demonstrated that angiotensin II can act similarly to NGF, promoting remyelination, neurite outgrowth and functional recovery following sciatic nerve crush in adult rats via AT 2 R (Reinecke et al., 2003) . In this model of nerve injury, co-administration of losartan and Ang II accelerated regeneration whereas PD123319 alone or in combination with Ang II, prevented recovery. Similar findings were reported using an optic nerve crush model, whereby stimulation of AT 2 R on retinal ganglion cells significantly increased neurite outgrowth (Lucius et al., 1998) . Stroth et al. (1998) identified that the AT 2 R is involved in the regulation of cytoskeleton proteins essential for neurite extension (Stroth et al., 1998) . They examined the expression of several microtubule components in PC12W cells and found Ang II and NGF differentially regulate microtubule proteins and, in the case of Ang II, the upregulation of dendritic proteins was mediated via AT 2 R. Likewise, it was found that selective activation of AT 2 R increased the expression of beta III-tubulin, and the microtubule-associated proteins, tau and MAP2, resulting in increased elongation of neuritis (Cote et al., 1999) . In a recent series of elegant studies from Unger's laboratories, it was found that central administration of PD 123319 abolished the neuroprotective effects of central AT 1 receptor blockade in conscious rats, both in terms of infarct size and neurological outcome following a transient unilateral medial cerebral artery occlusion (Li et al., 2005b) . These authors indicated that AT 2 R was upregulated during stroke and supported neuronal survival and neurite outgrowth in vulnerable peri-ischemic brain areas, as has recently been reviewed (Thone-Reineke et al., 2006) .

At first glance, it seems contradictory that AT 2 R would be involved in both apoptosis and neuronal regeneration, however, during early stages, neuronal injury will initiate a series of events that are identical for apoptosis and regeneration (Lucius et al., 1998) . AT 2 R-mediated activation of the MKP-1 pathway brings the cell to a point where it may enter the repair process, or programmed cell death, depending on the energy state of the cell. Thus AT 2 R contribute to the cellular mechanisms allowing adaptation to ischemic insult, by stimulating both tissue repair and programmed cell death (Lucius et al., 1999) . The apoptotic process can be neuroprotective as it serves to conserve energy by reducing the occurrence of inflammatory reactions, allowing more energy to be directed to the recovery of viable cells. Thus, AT 2 R may play a role in determining the fate of damaged neurons. Indeed, unmasking the apoptotic and regenerative effects of the AT 2 R by AT 1 R blockade could enhance tissue repair following injury and account for the reduction in infarct volume observed in animals treated with AT 1 R antagonists (Lucius et al., 1998) . In very recent studies, Mogi et al. (2006) have proposed that AT 2 R stimulation increases the expression of a neuroprotective factor, methyl methanesulfonate sensitive 2, which is involved in neuronal differentiation (Mogi et al., 2006) .

While most research has focused on the neuronal effects of the AT 2 R in stroke, the vasodilator role of AT 2 R cannot be ignored as a possible contributor to any AT 2 R neuroprotective effect, by maintaining perfusion of penumbral regions following stroke. In accordance with many studies describing AT 2 R-mediated relaxation of peripheral arteries (see Section 3.1), Ang II caused AT 1 R-mediated vasoconstriction in cerebral arteries both in vitro and in vivo; an effect of Ang II that was converted to AT 2 R-mediated vasodilatation in the presence of an AT 1 R antagonist (Vincent et al., 2005) . The fact that a sartan itself leads to an upregulation of AT 2 R in cerebral vessels is consistent with a vascular component to the neuroprotective effect of AT 2 R stimulation. Thus, the neuroprotective role of the AT 2 R appears to be complex, and is likely to involve a delicate interplay with AT 1 R on apoptotic, neuronal regenerative and vascular components in the central nervous system (Saavedra et al., 2006) . Curiously, the direct effect of AT 2 R stimulation in the setting of stroke has not been reported.

AT 2 R are also developmentally regulated in the kidney, with AT 2 R expression higher than that of AT 1 R expression in the foetal kidney (Ciuffo et al., 1993; Shanmugam et al., 1995; Ozono et al., 1997) . However, the relative proportion of these receptors reverses within days of birth, such that AT 1 R expression is higher than AT 2 R in the adult. AT 2 R are detected at relatively low levels in adult kidney, although reports of the extent and location of AT 2 R varies considerably depending on techniques used. Generally, AT 2 R mRNA and protein have been found to be distributed throughout tubular and vascular segments of the renal cortex and medulla, however, the demonstration of glomerular AT 2 R is more variable (Ozono et al., 1997; Miyata et al., 1999; Cao et al., 2000; Ruiz-Ortega et al., 2003) . Discrepancies between studies may involve species differences since renal AT 2 R are more easily detected in mice than rats (Armando et al., 2002; Baiardi et al., 2005) . AT 2 R are detected in human kidneys, primarily in associated vasculature (Zhuo et al., 1996; Matsubara et al., 1998) , but also in tubular and glomerular tissue of patients with glomerular lesions (Mifune et al., 2001) .

In terms of models of heightened RAS activation, there have been variable effects reported for renal AT 2 R expression. Sodium depletion upregulated renal AT 2 R (Ozono et al., 1997) ; AT 2 R was downregulated only in the ischaemic kidney from 2K1C rats (Wang et al., 1999) whereas Ang II infusion per se did not alter renal AT 2 R expression (Wang et al., 1999) . AT 1 R was also downregulated in the former 2 models, while adult Ren-2 gene transgenic rats, TGR(mRen-2)27 exhibited increased AT 1 R but not AT 2 R . However, the functional consequences of such changes in ATR expression were not examined.

On the other hand, in models of overt renal damage, such as induced by renal ablation, subtotal nephrectomy, or kidney damage caused by protein overload, AT 2 R were invariably upregulated (Bautista et al., 2001; Cao et al., 2002; Ruiz-Ortega et al., 2003; Hashimoto et al., 2004; Tejera et al., 2004; Vazquez et al., 2005) . Ex vivo analysis indicated that increased AT 2 R was associated with enhanced renal vasodilatation in perfused kidneys (Bautista et al., 2001) and increased tubular apoptosis following persistent proteinuria (Tejera et al., 2004) . However, Cao et al. (2002) reported that blockade of AT 2 R with PD123319 actually conferred renoprotection, as evidenced by a reduction in albuminuria, as did AT 1 R blockade. These results suggested that both AT 1 R and AT 2 R were renoprotective in the subtotal nephrectomy model (Cao et al., 2002) , although recent data using the same model disputes those findings. Vazquez et al. (2005) found that there was a time-dependent increase in AT 2 R expression over 30 days after renal ablation. Moreover, PD123319 exacerbated renal ischaemia and damage as well as causing a marked increase in telemetered BP . Thus, on balance, AT 2 R appear to be renoprotective, in keeping with other studies in which mice treated with PD123319, or in mice with AT 2 R deleted, exhibit marked increases in renal fibrosis (Ma et al., 1998; Morrissey & Klahr, 1999) . Similarly, vascular over-expression of AT 2 R ameliorated glomerular injury in the mouse remnant kidney model since there were reductions in albuminuria and glomerular expression of platelet-derived growth factor and transforming growth factor (Hashimoto et al., 2004) .

In addition to the well described AT 2 R renal vasodilator function, AT 2 R are thought to exert pronatriuretic effects. Seminal studies by Siragy and Carey (1996 , 1997 Jin et al., 2004) demonstrated that increased renal interstitial levels of cGMP, NO and BK were dependent on AT 2 R activation, and led to natriuresis. Moreover, Carey and colleagues have recently postulated that Ang III may be the endogenous AT 2 R ligand in the kidney since, during AT 1 R blockade, renal interstitial infusion of Ang III, but not Ang II, caused natriuresis. This effect of Ang III was blocked by PD123319, indicative of an AT 2 R mechanism (Padia et al., 2006) , which is consistent with Ang III-mediated depressor effects being AT 2 R-mediated (Walters et al., 2003) . Moreover, stimulation of dopamine D 1 -like receptors with fenoldopam caused natriuresis due to recruitment and activation of AT 2 R in renal proximal tubules (Salomone et al., 2007) . The pressure-natriuresis relationship was also impaired in AT 2 R knockout mice which had elevated basal renal resistance and altered arachidonic acid metabolism (Gross et al., 2000b) . More recent pharmacological studies have directly addressed the role of AT 2 R in natriuresis. In obese Zucker rats, CGP42112 increased urinary and sodium excretion, in line with increased expression of tubular AT 2 R, whereas this effect was absent in lean rats. Moreover, PD123319 reversed the natriuretic and diuretic effects of candesartan, suggesting an involvement of AT 2 R in the effect of the latter compound (Hakam & Hussain, 2005) , consistent with earlier studies . Thus, AT 2 R mediates natriuresis directly, or indirectly after AT 1 R blockade (Hakam & Hussain, 2005; Padia et al., 2006) , thereby opposing the well known antinatriuretic effects of AT 1 R activation. Mechanisms underlying the natriuretic effects of AT 2 R may involve altered proximal tubule bicarbonate reabsorption involving phosholipase A and arachidonic acid (Haithcock et al., 1999) , or by direct inhibition by AT 2 R of the proximal tubule sodium pump via NO/ cGMP signalling as has recently been postulated (Hakam & Hussain, 2006a,b) . Furthermore, renal AT 2 R have recently been shown to inhibit renin biosynthesis and subsequent local Ang II production, thus providing an additional mechanism for modulation of AT 1 Rmediated effects .

The regional haemodynamic effect of Ang II within the rabbit kidney itself has also been investigated by measuring cortical and medullary blood flows. As expected, we found that AT 2 R counteracted AT 1 R-mediated vasoconstriction in the kidney cortex. By contrast, Ang II evoked AT 1 R-mediated medullary vasodilatation that was opposed by activation of AT 2 R (Duke et al., 2003) . Moreover, in 2K1C hypertensive rats, the AT 2 R antagonist increased basal medullary flow, suggestive of AT 2 R-mediated medullary vasoconstriction (Duke et al., 2005b) . Thus, the medullary circulation behaves somewhat differently to other vascular beds in response to Ang II. Likewise, endogenous Ang II acting via both AT 1 R and AT 2 R would appear to enhance renal neurovascular function in rabbit kidneys (Rajapakse et al., 2005 (Rajapakse et al., , 2006 .

Diabetes is one of the major causes of renal failure and blindness and is an important risk factor for the development of cardiovascular disease, where its presence is also associated with a worse prognosis (Gilbert et al., 2003) . Although hyperglycaemia is a major determinant in the development of long-term complications of diabetes there are also glucose-independent mechanisms that contribute to the pathogenesis of organ injury in this disease. Activation of local RAS seems to be involved in the progression of kidney damage, with results of large clinical trials confirming the beneficial effects of ACE inhibitors and AT 1 R antagonists (Brenner et al., 2001; Lewis et al., 2001; Parving et al., 2001) , however as yet the role of the AT 2 R in the pathogenesis of diabetic nephropathy remains to be elucidated.

There are relatively few studies that have examined ATR distribution in this disease. Of these, the majority of studies have focussed on the kidney as diabetic nephropathy is one of the major complications associated with both type 1 and type 2 diabetes. Using the STZ-model of type 1 diabetes, both reductions (Kalinyak et al., 1993; Cheng et al., 1994; Brown et al., 1997; Bonnet et al., 2002) and increases (Brown et al., 1997; Wehbi et al., 2001) in AT 1 R have been reported. On the other hand, AT 2 R expression has been found to be markedly increased in brush-border and basolateral membrane of the kidney , and either not detected in the glomeruli of either diabetic or control rats or found to be significantly decreased (Wehbi et al., 2001) . In a long-term study of type 1 diabetes, both AT 1 R and AT 2 R gene and protein levels were reduced in kidneys of diabetic SHR (both glomeruli and tubulo-interstitial cells) compared with both non-diabetic SHR and a normotensive model in which only the AT 2 R showed a tendency towards a reduced expression . Less information is available regarding type 2 diabetes. Using the obese hyperinsulinaemic Zucker rat as a model of type 2 diabetes, AT 2 R density was also found to be increased in the brush-border and basolateral membrane of the kidney (Hakam & Hussain, 2005) which is consistent with findings from kidney biopsies of patients with type 2 diabetes in which the AT 2 R levels were also found to be increased in tubular cells with a concomitant reduction of AT 1 R expression .

Thus it is difficult to reach consensus on overall receptor changes in the 2 types of diabetes due to differences in methodologies and duration and severity of diabetes. Unlike experimental renal failure, which is fast-onset and exhibits a clear upregulation of AT 2 R, diabetesinduced changes in renal ATR expression reflect the continuum of the diabetic progression. Indeed, it is highly likely that the relative expression of AT 1 and AT 2 R in heterogeneous tissue compartments in the kidney is important in determining the effects of Ang II in progressive diabetic nephropathy.

In this context, in STZ-induced diabetic rats, there are 50-and 80fold increases in AT 2 R in brush-border and basolateral membranes. Diabetes elevated urinary flow and urinary sodium excretion which was blocked by the selective AT 2 R antagonist, PD123319 in diabetic but not control rats . A similar effect of the AT 2 R was observed in the type 2 diabetic model, the obese Zucker rat, with the AT 2 R agonist, CGP42112, promoting an increase in sodium and urine excretion in obese but not lean Zucker rats (Hakam & Hussain, 2005) . On the basis of the effects of PD123319, there was a basal effect of AT 2 R to promote natriuresis in type 1 diabetes but not in type 2 diabetes (Hakam & Hussain, 2005) which most likely reflects the profoundly greater upregulation of tubular AT 2 R in the STZ-model compared with the type 2 model. The increase in natriuresis and diuresis produced by administration of an AT 1 R antagonist in the obese Zucker rat was abolished by an AT 2 R antagonist (Hakam & Hussain, 2005) . The AT 2 R has since been shown to inhibit the Na + -K + -ATPase via an NO/cGMP/protein kinase G pathway (Hakam & Hussain, 2006a,b) , thus providing a possible mechanism responsible for AT 2 R-mediated natriuresis and diuresis in this model.

In addition, in the diabetic kidney, there is significant upregulation of various angiogenic cytokines, such as VEGF, that is mediated via AT 1 R and may thus contribute to the progression of renal injury (Pupilli et al., 1999; Tamarat et al., 2002; Rizkalla et al., 2005) . A link between AT 2 R and VEGF expression, albeit in a non-diabetic context has also been made Sarlos et al., 2003) . In the STZ-induced model of diabetes, there is increased gene and protein expression of VEGF and its receptor (VEGF-R2) in the kidney. Chronic AT 1 R or AT 2 R blockade attenuated expression of cytokines and their receptors (Rizkalla et al., 2005) . It should also be noted that there was increased apoptosis involving tubular and interstitial cells of the cortex and medulla of the kidney in STZ-treated rats (Kumar et al., 2004) . Losartan and PD123319 inhibited apoptosis in the kidney to similar extents and there was no additive effect using both antagonists. Therefore it is possible that AT 1 R and AT 2 R signalling pathways can both lead to apoptosis and induction of angiogenic cytokines in the diabetic kidney.

There is evidence that Ang II and VEGF and their cognate receptors participate in retinal angiogenesis and ischaemic retinopathies such as diabetic retinopathy (see Wilkinson-Berka, 2004) . The AT 2 R has been identified in several ocular tissues including the retina and has been found to predominate in the developing retina (Sarlos et al., 2003) consistent with the view that the AT 2 R influences cell growth and differentiation in organ development. There was an increase in AT 2 R expression and AT 2 R blockade decreased retinal angiogenesis as well as the levels of VEGF, its receptor VEGF-R2 and angiopoietin-2 similarly to that seen in the diabetic kidney (Sarlos et al., 2003) . Zhang et al. (2004) showed that blockade of the AT2R in STZ-treated rats decreased VEGF expression, thereby suggesting that VEGF expression is modulated by both AT 1 and AT 2 receptors and implicating these receptor subtypes in retinal diseases such as diabetic retinopathy. Thus blockade of the AT 2 R as well as the AT 1 R may confer end-organ protection in various retinal diseases.

Limited information is available regarding AT 2 R function in other non-renal tissues from diabetic animals or in the clinical setting. Increased expression of AT 2 R in normal (Sechi et al., 1994) and cardiomyopathic (Li et al., 2005a) hearts from diabetic rats have been reported. AT 2 R-mediated relaxation was also enhanced in aortic tissue obtained from STZ-treated rats compared with controls, and was associated with increased AT 2 R density, but not AT 1 R, in this vessel (Arun et al., 2004) . Recently, it was shown that 1-year treatment of diabetic hypertensive patients with valsartan, but not atenolol, caused an upregulation of AT 2 R in gluteal resistance arteries that was associated with improved AT 2 R-mediated vasorelaxation in these arteries (Savoia et al., 2007) . Furthermore, in line with earlier in vivo vascular AT 2 R function (see Section 3.1), Howes' group has recently examined small-artery vascular stiffness and in fact found that there was increased functional expression of vascular AT 2 R in patients with insulin resistance (Brillante et al., 2008) .

Ang II is recognized as a potent mediator in the pathogenesis of atherosclerosis (Daugherty et al., 2000) , with AT 1 -receptor activation by Ang II promoting vascular inflammation, cellular proliferation and oxidative stress which are considered key stages in the initiation and progression of the disease (Dzau, 2001; . The role of Ang II in atherosclerosis is evident in a number of studies using transgenic animal models and human tissues, that demonstrate an increase in AT 1 -receptor density in hypercholesterolemia (Nickenig & Bohm, 1997; Warnholtz et al., 1999) , as well as increased expression of ACE in atherosclerotic lesions (Diet et al., 1996; Fukuhara et al., 2000) . Furthermore, when the AT 1 R gene was deleted from 2 widely used models of atherosclerosis, i.e. apolipoprotein E (ApoE)-deficient mice or LDL-receptor-deficient mice, both types of double knockouts exhibited markedly reduced atherosclerotic lesions (Wassmann et al., 2004; , suggesting a primary role for AT 1 R in the pathogenesis of atherosclerosis. Indeed, recent studies using pharmacological intervention of the RAS have demonstrated beneficial effects in the progression of atherosclerosis and associated cardiovascular outcomes (see (Dzau, 2001; .

In the same study that used AT 1 R/LDL-receptor double knockouts, AT 2 R/LDL-receptor double knockout mice were also reported as a secondary aim. The authors reported that these mice do not exhibit an increase in atherosclerosis , however, no data was presented to support such a statement. By contrast, the AT 2 R gene was deleted from ApoE-deficient mice independently by 2 groups Sales et al., 2005) , with both studies concluding that AT 2 R do in fact exert an anti-atherosclerotic effect. Iwai et al. (2005) demonstrated that AT2R/ApoE-double knockout (DKO) mice fed a high cholesterol diet display exaggerated atherosclerotic lesion development, together with increased NADPH oxidase activity and superoxide production, when compared with ApoE knockout mice. It is well known that Ang II can induce expression of NADPH oxidase and subsequently increase superoxide production via activation of the AT 1 -receptor (Griendling et al., 1994) . These results suggest inhibitory modulation of oxidative stress via the AT 2 R which is consistent with in vitro data using cultured endothelial cells in which AT 2 R blockade with PD123319 enhanced superoxide formation (Sohn et al., 2000) . These findings also have further implications in terms of the progression of atherosclerosis, as decreased oxidative stress (typically superoxide anion production) increases the bioavailability of the potent anti-atherosclerotic vasodilator NO (Cai & Harrison, 2000) . In addition, the anti-atherosclerotic effect of the AT 1 R antagonist, valsartan was markedly reduced in AT 2 R/ApoE-DKO compared with ApoE-single knockout mice , suggesting that AT 2 R contribute to the beneficial effect of AT 1 R blockade and are counter-regulatory to AT 1 R in the setting of atherosclerosis.

Using a separately derived AT 2 R/ApoE-DKO, Sales et al. (2005) also published data investigating the effect of AT 2 R on atherosclerotic disease progression. Although this study did not show increases in lesion size in AT 2 R/ApoE-DKO mice, they demonstrated striking differences in plaque composition, cellular proliferation and apoptosis over time. Typically, there is a reduction in macrophage, smooth muscle, lipid and collagen content of plaques with disease progression together with increased apoptosis. However, AT 2 R deletion ameliorated this process suggesting that AT 2 R influences the cellular composition of the atherosclerotic lesion. Importantly, in ApoEdeficient mice, AT 2 R were co-localised with intimal smooth muscle cells and macrophages, and were upregulated following a high cholesterol diet. The authors noted a temporal association of AT 2 R expression and reduced smooth muscle cell content during lesion development, emphasizing an anti-inflammatory and antiproliferative role for AT 2 R (Sales et al., 2005) . Increased AT 2 R expression in atherosclerotic plaques in the same ApoE-deficient mouse model was recently confirmed (Johansson et al., 2005; Vinh et al., 2008b) , and increased AT 2 R and ACE2 immunoreactivity was found in atherosclerotic plaques in rabbits, often co-localised with macrophages (Zulli et al., 2006) . Interestingly, Dandapat et al. (2008) have recently reported that over-expression of the AT 2 R in LDL-receptor knockout mice, abrogated atherosclerotic lesion development, and was associated with a reduction in oxidative stress and other factors involved in the pro-inflammatory cascade. Furthermore, using the same mouse model, Hu et al. (2008) have additionally provided evidence of AT 2 Rmediated regulation of collagen deposition in atherosclerotic plaques which involved reduced MMP expression, decreased macrophage deposition and increased superoxide dismutase. Indeed these studies strongly support previous studies reporting anti-oxidative and antiinflammatory effects mediated via the AT 2 R, both of which are vital processes that contribute to atherosclerotic lesion formation.

Consistent with the anti-atherosclerotic role of AT 2 R deduced from knockout studies, Daugherty et al. (2001) had previously demonstrated that Ang II-induced atherosclerosis and aneurysm formation in male ApoE-KO mice was significantly potentiated with AT 2 R blockade. However, Johansson et al. (2005) were unable to reproduce this potentiation of lesion formation in the same model but using female mice. The authors suggested gender differences may have accounted for the disparity, as it has been demonstrated that male hormones may protect against Ang II-mediated lesion development (Henriques et al., 2004) . Alternatively, the ability of oestrogen to increase AT 2 R expression may play a role (Kintscher & Unger, 2005) , which illustrates important gender differences that are emerging with respect to AT 2 R expression and function (see Section 3.9). On close inspection of the same study, Johansson et al. (2005) reported that the incidence of abdominal aortic aneurysm increased from 11% with Ang II treatment alone, to 50% when treated with both Ang II and PD123319 in female ApoE −/− mice on a standard diet. These results are consistent with the protective effect of the AT 2 R on aneurysm formation (Daugherty et al., 2001) , perhaps indicating different mechanisms involved with atherosclerosis and aneurysm formation.

Recent studies from our own laboratory have revealed striking differences between the effects of chronic administration of either Ang II or Ang IV in ApoE-deficient mice fed a high fat diet. As expected, Ang II increased superoxide production and lesion formation in ApoEdeficient mice and worsened aortic endothelial function in wild type mice. By contrast, Ang IV caused vasoprotection in ApoE-deficient mice since there was a marked improvement in endothelial-mediated vasorelaxation that was accompanied by increased eNOS expression and reduced superoxide activity. Moreover, this action of Ang IV to increase NO biovailability was mediated by both AT 2 R-and AT 4 Rdependent mechanisms (Vinh et al., 2008a,b) .

The anti-growth effects of AT 2 R have also been reported for neointimal formation following vascular injury. The RAS has been implicated in neointimal formation mainly via the activation of the AT 1 R (Kim et al., 1995; . Nakajima et al. (1995) first reported AT 2 R involvement in neointimal formation in rats by transfecting an AT 2 R expression vector into the balloon-injured rat carotid artery and observed that over-expression of AT 2 R attenuated neointimal formation. Similarly, cuff-induced inflammation and subsequent neointimal formation in the femoral artery was substantially enhanced in AT 2 R knockout mice compared with wild type (Akishita et al., 2000a; Wu et al., 2001) , consistent with an increased AT 2 R expression in neointima of wild type mice . It has also been demonstrated that AT 2 R knockout mice show decreased apoptosis in the neointima which is consistent with the increased neointimal formation observed following cuff placement . Furthermore, the protective effects of AT 1 R blockade using valsartan were decreased in AT 2 R knockout mice, suggesting involvement of AT 2 R stimulation in the protection associated with AT 1 R blockade (Wu et al., 2001) . More recently, the effects of a sartan (valsartan) have been compared with an ACE inhibitor (benazepril) on vascular remodeling following carotid artery balloon injury. Interestingly, both RAS inhibitors caused anti-remodelling of balloon-injured rat carotid arteries, but by different molecular mechanisms since only valsartan caused increased AT 2 R expression together with increased cGMP production (Barker et al., 2006) . Thus, with a few minor exceptions Johansson et al., 2005) , AT 2 R exerts vasoprotective effects and substantial anti-remodelling of atherosclerotic lesions and neointimal formation.

Aging is associated with specific cardiovascular changes including cardiac hypertrophy and fibrosis, vascular stiffening and endothelial dysfunction. These cardiovascular adaptations are seen in both elderly normotensive and hypertensive subjects (Lakatta & Levy, 2003) , indicating that modifications are more than a mere consequence of confounding cardiovascular complications such as arterial hypertension, decreased aortic compliance and atherosclerosis, which are particularly prevalent in the elderly population. Circulating levels of Ang II are reduced by aging whereas angiotensinogen, ACE, and AT 1 R are upregulated in both the heart and vasculature (Heymes et al., 1994 (Heymes et al., , 1998 Wang et al., 2003) . AT 1 R blockade and ACE inhibition have been shown to be effective in preventing or regressing age-associated effects on cardiovascular remodelling and function in humans and animals Basso et al., 2005) .

Importantly, although cardiac AT 2 R expression is relatively low in the adult rat heart (Busche et al., 2000) , expression may be upregulated in certain disease states which are particularly common in the elderly, such cardiac fibrosis (Tsutsumi et al., 1998) , hypertrophy (Lopez et al., 1994) , and heart failure (Ohkubo et al., 1997) . Surprisingly, only study has investigated the influence of senescence on AT 2 R expression in the heart. Heymes et al. (1998) found augmented AT 2 R expression in both the left and right ventricles of aged rats, as well as in freshly isolated cardiomyocytes from aged rats. Moreover, there was a correlation between AT 2 R expression and fibrosis in the cardiac tissue. Considering the demonstrated involvement of AT 2 R stimulation to the cardioprotective effects of AT 1 R inhibition in adults, it is tempting to speculate that the AT 2 R may also play an important role in the regulation of cardiovascular aging. Indeed, we have shown that AT 2 R contribute to the effects of AT 1 R inhibition in 20 month old WKY rats, since beneficial actions of candesartan cilexetil administration on both aortic remodeling and cardiac fibrosis were completely abolished by simultaneous PD123319 treatment (Jones et al., 2004) .

In terms of vascular AT 2 R, the vasoconstrictor response of human isolated coronary microarteries to Ang II was potentiated by PD123319, as seen in in vitro animal studies (Zwart et al., 1998; Hannan et al., 2003) , and the magnitude of this effect was positively correlated with the age of the donor (Batenburg et al., 2004) . This finding suggests increased functional effects of vascular AT 2 R with aging, at least in coronary vasculature. More recently, Pinaud et al. (2007) found increased AT 2 R expression in mesenteric arteries that was predominantly localised to the medial smooth muscle layer since confocal imaging of fluorescent Ang II in the presence of candesartan revealed a decrease of AT 2 R in the endothelium. Strikingly, in these vessels, the AT 2 R phenotype was that of superoxide-mediated contraction, as it was blocked by antioxidant treatment (Pinaud et al., 2007) . The effect of aging on aortic AT 2 R localisation has also recently been determined. Like in mesenteric arteries, there was a marked increase in AT 2 R immunofluorescence, predominantly in the media while AT 2 R expression appeared to be preserved in the endothelium .

Gender differences exist in AT 2 R expression in the vasculature and kidney. AT 2 R expression in the kidney was generally higher in female, normotensive mice and rats compared with male counterparts; these changes were abolished by ovariectomy and oestrogen upregulated AT 2 R in various tissue compartments of the kidney in mice (Armando et al., 2002) and rats (Baiardi et al., 2005) . This gender effect was particularly evident in the renal vasculature of the rat since AT 2 R were only detected in arcuate arteries and veins from female rats (Baiardi et al., 2005) . An increased ratio of AT 2 R/AT 1 R was also found in kidneys and vasculature of female SHR compared to male SHR. This effect was due to reciprocal changes in AT 1 R (decreased) and AT 2 R (increased) in the kidneys whereas AT 1 R, but not AT 2 R, was downregulated in aortic and mesenteric vessels of female SHR, in an oestrogen-sensitive manner (Silva-Antonialli et al., 2004) .

Gender-specific modulation of cardiovascular disease, and the treatment thereof, represents an important area for future research (Regitz-Zagrosek, 2006) . Indeed, losartan reduced BP more effectively within 24 h in female SHR than male SHR (Silva-Antonialli et al., 2000) . Moreover, the same group has recently reported that losartan increased vascular AT 2 R expression after either 24 hour-or 15 daytreatment in female, but not male, SHR, which was associated with the restoration of endothelial function (de P Rodrigues et al., 2006) . In addition, (Okumura et al., 2005) reported that the upregulation of vascular AT 2 R, but not AT 1 R, following cuff injury was greater in female mice, and was associated with a greater reduction in neointimal formation as well as in indices of inflammation and oxidative stress in females versus males. Moreover, valsartan caused greater vessel anti-remodelling in wild type females whereas this sex difference was less marked in AT 2 R knockouts (Okumura et al., 2005) . Recently, we have made the remarkable finding that chronic lowdose Ang II infusion lowered BP in conscious normotensive female, but not male, rats. This effect occurred via AT 2 R-dependent mechanisms and involved enhanced renal AT 2 R expression (Sampson et al., 2008) .

There is much less clinical data pertaining to the effects of AT 2 R. Sartans have similar clinical benefit to ACE inhibitors in the majority of cardiovascular diseases. Indeed, large-scale clinical trials have been unable to unequivocally discriminate between these 2 classes of drugs, emphasising the prime importance of disrupting AT 1 R function. However, this does not necessarily rule out subtle effects of AT 2 R contributing to the beneficial effects of AT 1 R antagonists in much the same way that BK may contribute to the effects of ACE inhibition.

One area of uncertainty lies in the use of sartans in high cardiovascular risk patients. The VALUE trial reported a relative increase in first time MI (fatal and non-fatal) in patients on valsartan compared with amlodipine, although there were early differences in BPreduction between treatment arms which may have contributed to differences in MI rates (Julius et al., 2004) . Based on the results of this trial, Verma & Strauss (2004) initiated a debate that is still ongoing, as to the potential harm of sartans for MI in high-risk patients (Verma & Strauss, 2004 ) (for detailed review, see Strauss & Hall, 2006; Tsuyuki & McDonald, 2006) . As explanation for potential differences between sartans and ACE inhibitors, it was argued that the ability of AT 1 R antagonists to indirectly facilitate AT 2 R stimulation may contribute to the risk of MI in high-risk patients (Strauss & Hall, 2006) . On this point, they provided pre-clinical evidence on the potentially deleterious effects of AT 2 R stimulation from limited experimental studies (Strauss & Hall, 2006) , although a much larger body of evidence, as outlined in this review, supporting a cardioprotective role for AT 2 R was overlooked. Indeed, results from the recently completed ONTARGET trial, which evaluated the effects of telmisartan either alone, in combination with ramipril, or ramipril alone, in patients with vascular disease or high-risk diabetes, saw no difference in the incidence of the primary endpoints of cardiovascular death, MI, stroke or heart failure in any treatment arm (Yusuf et al., 2008) .

In the meantime, the AT 2 R gene polymorphism -1332G/A has also been linked with cardiovascular pathologies such as LVH and premature coronary artery disease, although the interpretation of these data is inconsistent, since either beneficial or deleterious effects of AT 2 R have been reported (see Section 3.2). Clinical studies determining forearm vasodilatation or arterial stiffness in small groups of healthy subjects or patients on sartans or with insulin resistance have provided evidence of functional vascular AT 2 R, but with some variation likely reflecting the heterogeneous patient populations examined (see Section 3.1). Finally, as outlined, there is a growing body of literature from ex vivo human data which implicates a cardio-and vaso-protective effect of AT 2 R. For example there is a marked upregulation of AT 2 R that is thought to exert antifibrotic effects in human heart failure (see Section 3.3); and vasorelaxation has been directly demonstrated in human isolated coronary (Batenburg et al., 2004) and gluteal (Savoia et al., 2006b) vasculatures.

There are some limitations in the AT 2 R research just discussed that still confound interpretations and hamper future progress. In particular, there is a dearth of selective AT 2 R agonists and antagonists, with CGP42112 and PD123319, respectively, being the gold standards for comparison. While these compounds are excellent tools, studies using other compounds are needed since AT 2 R knockouts bred on different background strain have most likely contributed to some of the controversial findings regarding AT 2 R function. In the future, data using non-peptide drugs such as Compound 21, under both acute and chronic conditions, are likely to contribute to the understanding of AT 2 R function and its potential therapeutic role. Indeed, to date there are no data reporting chronic effects of direct pharmacological stimulation of AT 2 R to determine definitively any BP and remodelling effects while avoiding potentially confounding issues such as the use of genetically modified animal models, stimulation of AT 2 R by Ang II during AT 1 R inhibition, or inference of AT 2 R function during AT 2 R blockade.

Over the past decade, a variety of effects caused by stimulation of AT 2 R in the cardiovascular system have been reported. In this review, we have highlighted that many such AT 2 R effects, and AT 2 R itself, are upregulated in a range of cardiovascular disease states. Generally, AT 2 R mediates acute haemodynamic effects but do not always translate into marked changes in basal BP inferred from AT 2 R knockout and transgenic studies although further studies with AT 2 R agonists are required. On the other hand, AT 2 R exert substantial antigrowth and anti-remodelling effects that indirectly contributes 'beyond BP control' under chronic conditions. The experimental evidence overwhelmingly indicates that AT 2 R makes a major contribution to the beneficial structural and remodelling effects of 'sartans' although this involvement is difficult to gauge in the clinical context. Often, a pharmacological effect of AT 2 R is only unmasked during AT 1 R blockade suggesting crosstalk between ATR which may take the form of either functional, physical or signalling interactions between AT 2 R and AT 1 R. Clearly, the level of AT 2 R expression, relative to AT 1 R, in different pathological states, together with the relative mixture of angiotensin peptide fragments in the local milieu and the prevailing crosstalk, will determine end-organ response. Thus, there is still much to learn about the AT 2 R context.

Angiotensin II type 2 receptor-bradykinin B2 receptor functional heterodimerization

The angiotensin II AT2 receptor is an AT1 receptor antagonist

Increased AT(1) receptor heterodimers in preeclampsia mediate enhanced angiotensin II responsiveness

AT1-receptor heterodimers show enhanced G-protein activation and altered receptor sequestration

Renin receptor expression in human adipose tissue

Differences in tissue angiotensin II-forming pathways by species and organs in vitro

Inflammation influences vascular remodeling through AT2 receptor expression and signaling

Expression of the AT2 receptor developmentally programs extracellular signalregulated kinase activity and influences fetal vascular growth

Inhibitory effect of angiotensin II type 2 receptor on coronary arterial remodeling after aortic banding in mice

Evidence that the angiotensin IV (AT(4)) receptor is the enzyme insulin-regulated aminopeptidase

The clinical significance of a common, functional, X-linked angiotensin II type 2-receptor gene polymorphism (-1332 G/A) in a cohort of 509 families with premature coronary artery disease

Left ventricle mass index and the common, functional, X-linked angiotensin II type-2 receptor gene polymorphism (-1332 G/A) in patients with systemic hypertension

Localization of angiotensin AT1 and AT2 receptors

Synthesis and effects of active fragments of angiotensin II

Estrogen upregulates renal angiotensin II AT(2) receptors

Prevention of Heart Failure in Patients in the Heart Outcomes Prevention Evaluation (HOPE) Study

Tempol augments angiotensin II-induced AT2 receptor-mediated relaxation in diabetic rat thoracic aorta

Selective down-regulation of the angiotensin II AT1-receptor subtype in human ventricular myocardium

Estrogen upregulates renal angiotensin II AT1 and AT2 receptors in the rat. Regulatory Pept

Angiotensin II stimulation of protein synthesis and cell growth in chick heart cells

Cardiac actions of angiotensin II: role of an intracardiac renin-angiotensin system

AT(2) receptor stimulation enhances antihypertensive effect of AT(1) receptor antagonist in hypertensive rats

Angiotensin II type 2 receptor expression after vascular injury: differing effects of angiotensin-converting enzyme inhibition and angiotensin receptor blockade

Angiotensin II type 2 receptor blockade amplifies the early signals of cardiac growth response to angiotensin II in hypertrophied hearts

Protective effect of the inhibition of the renin-angiotensin system on aging

Aliskiren-binding increases the half life of renin and prorenin in rat aortic vascular smooth muscle cells

Angiotensin II type 2 receptor-mediated vasodilation in human coronary microarteries

Increased expression of the renin-angiotensin system and mast cell density but not of angiotensin-converting enzyme II in late stages of human heart failure

Angiotensin II type AT(2) receptor mRNA expression and renal vasodilatation are increased in renal failure

Angiotensin II type 2 receptors mediate inhibition of mitogen-activated protein kinase cascade and functional activation of SHP-1 tyrosine phosphatase

Angiotensin-(1-7) prevents development of severe hypertension and end-organ damage in spontaneously hypertensive rats treated with L-NAME

The magic tail' of G proteincoupled receptors: an anchorage for functional protein networks

Renal expression of angiotensin receptors in long-term diabetes and the effects of angiotensin type 1 receptor blockade

Cardiac angiotensin AT2 receptor: what exactly does it do? Hypertension 43

The angiotensin AT2 receptor stimulates protein tyrosine phosphatase activity and mediates inhibition of particulate guanylate cyclase

Endothelial AT1-mediated release of nitric oxide decreases angiotensin II contractions in rat carotid artery

N-and C-terminal structure-activity study of angiotensin II on the angiotensin AT2 receptor

The angiotensin II type 2 receptor and improved adjacent region function post-MI

Nitric oxide mediates benefits of angiotensin II type 2 receptor overexpression during post-infarct remodeling

Combined angiotensin II type 1 and type 2 receptor blockade on vascular remodeling and matrix metalloproteinases in resistance arteries

Angiotensin II stimulates protein tyrosine phosphatase activity through a G-protein independent mechanism

Vascular hypertrophy and increased P70S6 kinase in mice lacking the angiotensin II AT(2) receptor

Cardiac hypertrophy is associated with decreased eNOS expression in angiotensin AT2 receptor-deficient mice

Effects of losartan on renal and cardiovascular outcomes in patients with type 2 diabetes and nephropathy

Collagen metabolism in cultured adult rat cardiac fibroblasts: response to angiotensin II and aldosterone

Role of angiotensin II and prostaglandin E2 in regulating cardiac fibroblast collagen turnover

Effects of intravenous PD 123319 on haemodynamic and arterial stiffness indices in healthy volunteers

Arterial stiffness and haemodynamic response to vasoactive medications in patients with insulin resistant syndrome

Localization of the angiotensin II receptor subtypes in the human atrium

Tissue-specific changes in angiotensin II receptors in streptozotocin-diabetic rats

Elevated blood pressure and heart rate in human renin receptor transgenic rats

Angiotensin II receptor antagonists. Potential in elderly patients with cardiovascular disease

Myocardial infarction increases ACE2 expression in rat and humans

Expression of angiotensin AT(1) and AT(2) receptors in adult rat cardiomyocytes after myocardial infarction. A single-cell reverse transcriptase-polymerase chain reaction study

Endothelial dysfunction in cardiovascular diseases: the role of oxidant stress

Endogenous angiotensin II levels and the mechanism of action of angiotensin-converting enzyme inhibitors and angiotensin receptor type 1 antagonists

Effects of converting enzyme inhibitors on angiotensin and bradykinin peptides

Effects of losartan on angiotensin and bradykinin peptides and angiotensin-converting enzyme

Differential regulation of angiotensin peptide levels in plasma and kidney of the rat. Hypertension 18

Angiotensin type 2 receptor antagonism confers renal protection in a rat model of progressive renal injury

Role of angiotensin receptor subtypes in mesenteric vascular proliferation and hypertrophy

Angiotensin type 2 receptor is expressed in the adult rat kidney and promotes cellular proliferation and apoptosis

Distribution and signal transduction of angiotensin II AT1 and AT2 receptors

Angiotensin II increases cGMP content via endothelial angiotensin II AT1 subtype receptors in the rat carotid artery

Newly recognized components of the reninangiotensin system: potential roles in cardiovascular and renal regulation

Angiotensin type 2 receptormediated hypotension in angiotensin type-1 receptor-blocked rats

Angiotensin IV interacts with a juxtamembrane site on AT(4)/IRAP suggesting an allosteric mechanism of enzyme modulation

Evidence for a functional interaction of the angiotensin-(1-7) receptor Mas with AT1 and AT2 receptors in the mouse heart

The angiotensin IV/AT4 receptor

New signaling mechanism of angiotensin II in neuroblastoma neuro-2A cells: activation of soluble guanylyl cyclase via nitric oxide synthesis

Effects of a perindopril-based blood pressure-lowering regimen on the risk of recurrent stroke according to stroke subtype and medical history

Angiotensin IV-mediated pulmonary artery vasorelaxation is due to endothelial intracellular calcium release

Reduced proximal tubule angiotensin II receptor expression in streptozotocin-induced diabetes mellitus

Alterations of nitric oxide synthase expression with aging and hypertension in rats

Glomerular angiotensin II receptor subtypes during development of rat kidney

Autoradiographic identification of kidney angiotensin IV binding sites and angiotensin IV-induced renal cortical blood flow changes in rats

Angiotensin II type 2 receptors contribute to vascular responses in spontaneously hypertensive rats treated with angiotensin II type 1 receptor antagonists

Activation of the AT (2) receptor of angiotensin II induces neurite outgrowth and cell migration in microexplant cultures of the cerebellum

Angiotensin-converting enzyme 2 is an essential regulator of heart function

Extrarenal aspects of angiotensin II function

ATRAP, novel AT1 receptor associated protein, enhances internalization of AT1 receptor and inhibits vascular smooth muscle cell growth

The angiotensin II type 2 receptor causes constitutive growth of cardiomyocytes and does not antagonize angiotensin II type 1 receptor-mediated hypertrophy

Cardiovascular morbidity and mortality in the Losartan Intervention For Endpoint reduction in hypertension study (LIFE): a randomised trial against atenolol

Blockade of central angiotensin AT1 receptors improves neurological outcome and reduces expression of AP-1 transcription factors after focal brain ischemia in rats

Overexpression of angiotensin II type 2 receptor (agtr2) decreases collagen accumulation in atherosclerotic plaque

Renin, prorenin and the putative (pro)renin receptor

Determinants of interindividual variation of renin and prorenin concentrations: evidence for a sexual dimorphism of (pro)renin levels in humans

Cardiac renin and angiotensins. Uptake from plasma versus in situ synthesis

Angiotensin II-mediated development of vascular diseases

Angiotensin II promotes atherosclerotic lesions and aneurysms in apolipoprotein E-deficient mice

Antagonism of AT2 receptors augments angiotensin II-induced abdominal aortic aneurysms and atherosclerosis

Hypercholesterolemia stimulates angiotensin peptide synthesis and contributes to atherosclerosis through the AT1A receptor

Cloning and characterization of ATRAP, a novel protein that interacts with the angiotensin II type 1 receptor

International union of pharmacology. XXIII. The angiotensin II receptors

Differential effect of losartan in female and male spontaneously hypertensive rats

Angiotensin II and angiotensin-(1-7) inhibit the inner cortex Na+-ATPase activity through AT2 receptor

In vivo study of AT(1) and AT(2) angiotensin receptors in apoptosis in rat blood vessels

Increased accumulation of tissue ACE in human atherosclerotic coronary artery disease

ACE2 gene transfer attenuates hypertension-linked pathophysiological changes in the SHR

A novel angiotensin-converting enzyme-related carboxypeptidase (ACE2) converts angiotensin I to angiotensin 1-9

Subclasses of angiotensin II binding sites and their functional significance

Disparate roles of AT2 receptors in the renal cortical and medullary circulations of anesthetized rabbits

AT2 receptors contribute to acute blood pressure-lowering and vasodilator effects of AT1 receptor antagonism in conscious normotensive but not hypertensive rats

AT(2) receptors mediate tonic renal medullary vasoconstriction in renovascular hypertension

Tissue angiotensin and pathobiology of vascular disease: a unifying hypothesis

Angiotensin IV activates the nuclear transcription factor-kappaB and related proinflammatory genes in vascular smooth muscle cells

Angiotensin II type 2 receptor gene transfer elicits cardioprotective effects in an angiotensin II infusion rat model of hypertension

Angiotensin II type 2 receptor-mediated gene expression profiling in human coronary artery endothelial cells

Cerebroprotective effect of angiotensin IV in experimental ischemic stroke in the rat mediated by AT(4) receptors

Effects of aliskiren on blood pressure, albuminuria, and (pro)renin receptor expression in diabetic TG(mRen-2)27 rats

Prorenin and renin-induced extracellular signal-regulated kinase 1/2 activation in monocytes is not blocked by aliskiren or the handle-region peptide

The putative (pro) renin receptor blocker hrp fails to prevent (pro)renin signaling

Angiotensin-converting enzyme 2 and angiotensin-(1-7): an evolving story in cardiovascular regulation

Counterregulatory actions of angiotensin-(1-7)

Angiotensin-(1-7): a bioactive fragment of the reninangiotensin system

Effect of angiotensin-converting enzyme inhibition and angiotensin II receptor blockers on cardiac angiotensin-converting enzyme 2

Advances in biochemical and functional roles of angiotensin-converting enzyme 2 and angiotensin-(1-7) in regulation of cardiovascular function

The nonpeptide angiotensin-(1-7) receptor Mas agonist AVE-0991 attenuates heart failure induced by myocardial infarction

Role of AT1 and AT2 receptors in regulation of MAPKs and MKP-1 by ANG II in adult cardiac myocytes

Angiotensin-converting enzyme expression in human carotid artery atherosclerosis

AT2 receptor stimulation induces generation of ceramides in PC12W cells

Short pseudopeptides containing turn scaffolds with high AT2 receptor affinity

Synthesis of a new class of druglike angiotensin II C-terminal mimics with affinity for the AT2 receptor

Angiotensin II pseudopeptides containing 1,3,5-trisubstituted benzene scaffolds with high AT2 receptor affinity

Role of the angiotensin II AT2-subtype receptors in the blood pressure-lowering effect of losartan in salt-restricted rats

The renin-angiotensin system and the long-term complications of diabetes: pathophysiological and therapeutic considerations

Increase in systemic blood pressure during intra-arterial PD123319 infusion: evidence for functional expression of angiotensin type 2 receptors in normal volunteers

AT2 receptor stimulation increases aortic cyclic GMP in SHRSP by a kinin-dependent mechanism

ACE2 gene expression is upregulated in the human failing heart

Angiotensin protects cortical neurons from hypoxic-induced apoptosis via the angiotensin type 2 receptor

Angiotensin receptors and their therapeutic implications

Angiotensin II stimulates NADH and NADPH oxidase activity in cultured vascular smooth muscle cells

Prevention of angiotensin II-induced cardiac remodeling by angiotensin-(1-7)

Chronic angiotensin-(1-7) prevents cardiac fibrosis in DOCA-salt model of hypertension

Long-term blood pressure telemetry in AT2 receptor-disrupted mice

Inhibition of pressure natriuresis in mice lacking the AT2 receptor

Cardiac hypertrophy and fibrosis in chronic L-NAME-treated AT2 receptordeficient mice

Chronic pretreatment with candersartan improves recovery from focal cerebral ischemia in rats

Type 1 angiotensin II receptor-associated protein ARAP1 binds and recycles the receptor to the plasma membrane

Contribution of reactive oxygen species to the pathogenesis of left ventricular failure in Dahl salt-sensitive hypertensive rats: effects of angiotensin II blockade

Altered blood pressure responses and normal cardiac phenotype in ACE2-null mice

Angiotensin receptors: physiology and pharmacology

Renal proximal tubular AT2 receptor: signaling and transport

Renal angiotensin II type-2 receptors are upregulated and mediate the candesartan-induced natriuresis/diuresis in obese Zucker rats

Angiotensin II AT2 receptors inhibit proximal tubular Na+-K+-ATPase activity via a NO/cGMP-dependent pathway

Angiotensin II type 2 receptor agonist directly inhibits proximal tubule sodium pump activity in obese but not in lean Zucker rats

Renal angiotensin II AT2 receptors promote natriuresis in streptozotocin-induced diabetic rats

The emerging role of ACE2 in physiology and disease

Functional role of angiotensin II AT2 receptor in modulation of AT1 receptor-mediated contraction in rat uterine artery: involvement of bradykinin and nitric oxide

Vascular angiotensin II actions mediated by angiotensin II type 2 receptors

Functional reconstitution of the angiotensin II type 2 receptor and G(i) activation

Overexpression of angiotensin type 2 receptor ameliorates glomerular injury in a mouse remnant kidney model

AT1 and AT2 angiotensin receptor gene expression in human heart failure

Behavioural and cardiovascular effects of disrupting the angiotensin II type-2 receptor in mice

Intracellular trafficking of angiotensin II and its AT1 and AT2 receptors: evidence for selective sorting of receptor and ligand

Angiotensin-(1-7)-stimulated nitric oxide and superoxide release from endothelial cells

Physiological and pathophysiological functions of the AT(2) subtype receptor of angiotensin II: from large arteries to the microcirculation

but not ovariectomy, regulates angiotensin II-induced vascular diseases in apolipoprotein E-deficient mice

Cardiac senescence is associated with enhanced expression of angiotensin II receptor subtypes

Activation of angiotensinogen and angiotensin-converting enzyme gene expression in the left ventricle of senescent rats

Angiotensin II-stimulated nitric oxide release from porcine pulmonary endothelium is mediated by angiotensin IV

Angiotensin type 2 receptormediated phosphorylation of eNOS in the aortas of mice with 2-kidney, 1-clip hypertension

Stimulation of cyclic GMP production via AT2 and B2 receptors in the pressure-overloaded aorta after banding

Angiotensin type 2 receptor dephosphorylates Bcl-2 by activating mitogen-activated protein kinase phosphatase-1 and induces apoptosis

Overexpression of angiotensin II type 2 receptor (agtr2) reduces atherogenesis and modulates LOX-1, endothelial nitric oxide synthase and heme-oxygenase-1 expression

Renin increases mesangial cell transforming growth factor-beta1 and matrix proteins through receptor-mediated, angiotensin II-independent mechanisms

Protection from angiotensin II-induced cardiac hypertrophy and fibrosis by systemic lentiviral delivery of ACE2 in rats

Inhibition of diabetic nephropathy by a decoy peptide corresponding to the "handle" region for nonproteolytic activation of prorenin

Nonproteolytic activation of prorenin contributes to development of cardiac fibrosis in genetic hypertension

Angiotensin II type 2 receptor is essential for left ventricular hypertrophy and cardiac fibrosis in chronic angiotensin II-induced hypertension

Targeted deletion of angiotensin II type 2 receptor caused cardiac rupture after acute myocardial infarction

Effects on blood pressure and exploratory behaviour of mice lacking angiotensin II type-2 receptor

Interaction between bradykinin subtype 2 and angiotensin II type 2 receptors during post-MI left ventricular remodeling

Upregulation of angiotensin-converting enzyme 2 after myocardial infarction by blockade of angiotensin II receptors

Protection against ischemia and improvement of cerebral blood flow in genetically hypertensive rats by chronic pretreatment with an angiotensin II AT1 antagonist

Deletion of angiotensin II type 2 receptor exaggerated atherosclerosis in apolipoprotein E-null mice

Possible inhibition of focal cerebral ischemia by angiotensin II type 2 receptor stimulation

Angiotensin II type 2 receptor subtype mediates phospholipase A2-dependent signaling in rabbit proximal tubular epithelial cells

Differential regulation of prostaglandin synthesis by angiotensin peptides in porcine aortic smooth muscle cells: subtypes of angiotensin receptors involved

Elevated vascular angiotensin converting enzyme mediates increased neointima formation after balloon injury in spontaneously hypertensive rats

Detection of angiotensin II in supernatants of stimulated mononuclear leukocytes by matrix-assisted laser desorption ionization time-of-flight/time-offlight mass analysis

Aliskiren: the first renin inhibitor for clinical treatment

Angiotensin II type 2 receptor gene transfer downregulates angiotensin II type 1a receptor in vascular smooth muscle cells

Renal interstitial guanosine cyclic 3′, 5′-monophosphate mediates pressurenatriuresis via protein kinase G

Renal interstitial cGMP mediates natriuresis by direct tubule mechanism

AT2-selective angiotensin II analogues containing tyrosine-functionalized 5,5-bicyclic thiazabicycloalkane dipeptide mimetics

type 2 receptor is not involved in the angiotensin II-mediated proatherogenic process in ApoE−/− mice

Franz Volhard Lecture. Renin-angiotensin system: a dual tissue and hormonal system for cardiovascular control

Angiotensin AT2 receptor contributes to cardiovascular remodelling of aged rats during chronic AT1 receptor blockade

Outcomes in hypertensive patients at high cardiovascular risk treated with regimens based on valsartan or amlodipine: the VALUE randomised trial

The renin-angiotensin system in streptozotocin-induced diabetes mellitus in the rat

Angiotensin II type 2 receptor stimulation of neuronal K+ currents involves an inhibitory GTP binding protein

Angiotensin AT1/AT2 receptors: regulation, signalling and function

Cultured myofibroblasts generate angiotensin peptides de novo

Mineralocorticoid receptor blocker increases angiotensin-converting enzyme 2 activity in congestive heart failure patients

Aldosterone administration to mice stimulates macrophage NADPH oxidase and increases atherosclerosis development: a possible role for angiotensin-converting enzyme and the receptors for angiotensin II and aldosterone

Mechanical stretch induces enhanced expression of angiotensin II receptor subtypes in neonatal rat cardiac myocytes

Angiotensin II increases human monocyte matrix metalloproteinase-1 through the AT2 receptor and prostaglandin E2: implications for atherosclerotic plaque rupture

Angiotensin II type 1 receptor blockade inhibits the expression of immediate-early genes and fibronectin in rat injured artery

Does the AT2-receptor mediate anti-atherosclerotic actions?

Central pressor actions of aminopeptidase-resistant angiotensin II analogs: challenging the angiotensin III hypothesis

G-protein-coupled receptor Mas is a physiological antagonist of the angiotensin II type 1 receptor

Angiotensin II-and IV-induced changes in cerebral blood flow. Roles of AT1, AT2, and AT4 receptor subtypes. Regulatory Pept 68

Tubular and interstitial cell apoptosis in the streptozotocin-diabetic rat kidney

Cardiac Angiotensin II Type 2 receptor activates the kinin/NO system and inhibits fibrosis

Angiotensin II induction of neurite outgrowth by AT2 receptors in NG108-15 cells

Arterial and cardiac aging: major shareholders in cardiovascular disease enterprises: part II: the aging heart in health: links to heart disease

Evidence for a functional role of angiotensin II type 2 receptor in the cardiac hypertrophic process in vivo in the rat heart

Tumor necrosis factor-alpha convertase (ADAM17) mediates regulated ectodomain shedding of the severe-acute respiratory syndrome-coronavirus (SARS-CoV) receptor, angiotensin-converting enzyme-2 (ACE2)

Involvement of the Gi/o/cGMP/PKG pathway in the AT2-mediated inhibition of outer cortex proximal tubule Na+-ATPase by Ang-(1-7)

Structure-activity study of LVV-hemorphin-7: angiotensin AT4 receptor ligand and inhibitor of insulin-regulated aminopeptidase

Activation of the de novo biosynthesis of sphingolipids mediates angiotensin II type 2 receptorinduced apoptosis

The endothelium-dependent vasodilator effect of the nonpeptide Ang(1-7) mimic AVE 0991 is abolished in the aorta of mas-knockout mice

Localization of angiotensin II receptor subtypes AT1 and AT2 in the pancreas of rodents

Prognostic implications of echocardiographically determined left ventricular mass in the Framingham Heart Study

Angiotensin AT4 ligands are potent, competitive inhibitors of insulin regulated aminopeptidase (IRAP)

Renoprotective effect of the angiotensin-receptor antagonist irbesartan in patients with nephropathy due to type 2 diabetes

Taurine may prevent diabetic rats from developing cardiomyopathy also by downregulating angiotensin II type2 receptor expression

Potentiation of the antihypertensive action of losartan by peripheral over expression of the angiotensin II (Ang II) type 2 receptor

Angiotensin AT2 receptor protects against cerebral ischemia-induced neuronal injury

Angiotensin-converting enzyme 2 is a functional receptor for the SARS coronavirus

AT2 receptor-mediated vasodilatation is unmasked by AT1 receptor blockade in conscious SHR

Transforming growth factor-beta 1 promotes contraction of collagen gel by cardiac fibroblasts through their differentiation into myofibroblasts

Induction of cardiac fibrosis by angiotensin II

Effects of angiotensin-converting enzyme inhibitors and angiotensin II type 1 receptor antagonists in rats with heart failure. Role of kinins and angiotensin II type 2 receptors

Angiotensin II stimulates the release of phospholipid-derived second messengers through multiple receptor subtypes in heart cells

Angiotensin-(1-7) attenuates the development of heart failure after myocardial infarction in rats

The angiotensin II type I receptor-associated protein, ATRAP, is a transmembrane protein and a modulator of angiotensin II signaling

Distribution and function of cardiac angiotensin AT1-and AT2-receptor subtypes in hypertrophied rat hearts

Angiotensin III activates nuclear transcription factor-kappaB in cultured mesangial cells mainly via AT(2) receptors: studies with AT(1) receptor-knockout mice

Functional evidence for an angiotensin IV receptor in rat resistance arteries

Renin inhibition reduces hypercholesterolemia-induced atherosclerosis in mice

Neuroprotective effects of candesartan against cerebral ischemia in spontaneously hypertensive rats

Beyond blood pressure: new roles for angiotensin II

The angiotensin II type 2 (AT2) receptor promotes axonal regeneration in the optic nerve of adult rats

Accelerated fibrosis and collagen deposition develop in the renal interstitium of angiotensin type 2 receptor null mutant mice during ureteral obstruction

Cardiac-specific overexpression of angiotensin II AT2 receptor causes attenuated response to AT1 receptor-mediated pressor and chronotropic effects

Tissue angiotensin II and endothelin-1 modulate differently the response to flow in mesenteric resistance arteries of normotensive and spontaneously hypertensive rats

Activation of AT(2) receptors by endogenous angiotensin II is involved in flow-induced dilation in rat resistance arteries

Tissue-specific expression of human angiotensin II AT1 and AT2 receptors and cellular localization of subtype mRNAs in adult human renal cortex using in situ hybridization

Prevention of cardiac hypertrophy by angiotensin II type-2 receptor gene transfer

Renin-angiotensin system activation and interstitial inflammation in human diabetic nephropathy

Examination of angiotensin II type 1 and type 2 receptor expression in human kidneys by immunohistochemistry

Angiotensin II type 2 receptors stimulate collagen synthesis in cultured vascular smooth muscle cells

Regulation of collagen synthesis in mouse skin fibroblasts by distinct angiotensin II receptor subtypes

Ligand-independent signals from angiotensin II type 2 receptor induce apoptosis

Constitutively active homo-oligomeric angiotensin II type 2 receptor induces cell signaling independent of receptor conformation and ligand stimulation

Distribution of angiotensin AT1 and AT2 receptor subtypes in the rat kidney

Up regulation of AT4 receptor levels in carotid arteries following balloon injury

Emerging concepts of regulation of angiotensin II receptors. new players and targets for traditional receptors

Angiotensin II type-2 receptor stimulation prevents neural damage by transcriptional activation of methyl methanesulfonate sensitive 2

Effect of AT2 receptor blockade on the pathogenesis of renal fibrosis

Candesartan: a new-generation angiotensin II AT1 receptor blocker: pharmacology, antihypertensive efficacy, renal function, and renoprotection

Characterization of a rat type 2 angiotensin II receptor stably expressed in 293 cells

Local angiotensin II generation in the rat heart: role of renin uptake

Pro)renin receptor peptide inhibitor "handle-region" peptide does not affect hypertensive nephrosclerosis in Goldblatt rats

Opposing actions of angiotensin II on microvascular growth and arterial blood pressure

Signal transduction mechanisms involved in angiotensin-(1-7)-stimulated arachidonic acid release and prostanoid synthesis in rabbit aortic smooth muscle cells

Angiotensin II AT2 receptors are functionally coupled to protein tyrosine dephosphorylation in N1E-115 neuroblastoma cells

The angiotensin II type 2 (AT2) receptor antagonizes the growth effects of the AT1 receptor: gain-of-function study using gene transfer

Renin/prorenin receptors

Pivotal role of the renin/prorenin receptor in angiotensin II production and cellular responses to renin

Regulation of the angiotensin AT1 receptor expression by hypercholesterolemia

Regulation of gene transcription of angiotensin II receptor subtypes in myocardial infarction

Role of the angiotensin type 2 receptor gene in congenital anomalies of the kidney and urinary tract, CAKUT, of mice and men

Angiotensin II AT1 blockade normalizes cerebrovascular autoregulation and reduces cerebral ischemia in spontaneously hypertensive rats

Localization of the ANG II type 2 receptor in the microcirculation of skeletal muscle

Transinactivation of receptor tyrosine kinases by novel angiotensin II AT2 receptorinteracting protein

Renin inhibition by aliskiren prevents atherosclerosis progression: comparison with irbesartan, atenolol, and amlodipine

Angiotensin type 2 receptors are reexpressed by cardiac fibroblasts from failing myopathic hamster hearts and inhibit cell growth and fibrillar collagen metabolism

Cardioprotective role of AT2 receptor in postinfarction left ventricular remodeling

Sex difference in vascular injury and the vasoprotective effect of valsartan are related to differential AT2 receptor expression

Type 1 angiotensin receptor pharmacology: signaling beyond G proteins

Expression of the subtype 2 angiotensin (AT2) receptor protein in rat kidney. Hypertension 30

Renal angiotensin type 2 receptors mediate natriuresis via angiotensin III in the angiotensin II type 1 receptor-blocked rat

Conversion of renal angiotensin II to angiotensin III is critical for AT2 receptor-mediated natriuresis in rats

Intrarenal aminopeptidase N inhibition augments natriuretic responses to angiotensin III in angiotensin type 1 receptor-blocked rats

Pressure mediates angiotensin II-induced arterial hypertrophy and PDGF-A expression

The effect of irbesartan on the development of diabetic nephropathy in patients with type 2 diabetes

Angiotensin IV receptor-mediated activation of lung endothelial NOS is associated with vasorelaxation

Physiology of local renin-angiotensin systems

Role of angiotensin type 2 receptors in human forearm vascular responses of normal volunteers

Forearm vasodilator response to angiotensin II in elderly women receiving candesartan: role of AT(2)-receptors

Aliskiren, a human renin inhibitor, ameliorates cardiac and renal damage in doubletransgenic rats

Paradoxical role of angiotensin II type 2 receptors in resistance arteries of old rats

Nonpeptide AVE 0991 is an angiotensin-(1-7) receptor Mas agonist in the mouse kidney

Angiotensin II stimulates the production of NO and peroxynitrite in endothelial cells

Angiotensin II stimulates the synthesis and secretion of vascular permeability factor/vascular endothelial growth factor in human mesangial cells

ANG II type 2 receptors and neural control of intrarenal blood flow

Angiotensin II and nitric oxide in neural control of intrarenal blood flow

Angiotensin III: a central regulator of vasopressin release and blood pressure

Therapeutic implications of the gender-specific aspects of cardiovascular disease

Regulation, chamber localization, and subtype distribution of angiotensin II receptors in human hearts

Angiotensin II accelerates functional recovery in the rat sciatic nerve in vivo: role of the AT2 receptor and the transcription factor NF-kB

The continuing saga of the AT2 receptor: a case of the good, the bad, and the innocuous

A place in our hearts for the lowly angiotensin 1-7 peptide?

Evaluation of angiotensin-converting enzyme (ACE), its homologue ACE2 and neprilysin in angiotensin peptide metabolism

Systolic and diastolic dysfunction in patients with clinical diagnosis of dilated cardiomyopathy. Relation to symptoms and prognosis

Temporal renal expression of angiogenic growth factors and their receptors in experimental diabetes: role of the renin-angiotensin system

Increased renal vascular endothelial growth factor and angiopoietins by angiotensin II infusion is mediated by both AT1 and AT2 receptors

Angiotensin II-receptor subtypes in human atria and evidence for alterations in patients with cardiac dysfunction

Synthesis and AT2 receptor-binding properties of angiotensin II analogues

New selective AT2 receptor ligands encompassing a gamma-turn mimetic replacing the amino acid residues 4-5 of angiotensin II act as agonists

Localization of AT(2) receptors in the nucleus of the solitary tract of spontaneously hypertensive and Wistar Kyoto rats using [125I] CGP42112: upregulation of a non-angiotensin II binding site following unilateral nodose ganglionectomy

Angiotensin-(1-7) binding at angiotensin II receptors in the rat brain

Renal expression of angiotensin type 2 (AT2) receptors during kidney damage

Mechanisms of the anti-ischemic effect of angiotensin II AT(1) receptor antagonists in the brain

Angiotensin II activates endothelial constitutive nitric oxide synthase via AT1 receptors

Angiotensin type 2 receptor is expressed in murine atherosclerotic lesions and modulates lesion evolution

Intrarenal dopamine D1-like receptor stimulation induces natriuresis via an angiotensin type-2 receptor mechanism

Enhanced AT2R-vasodilator mechanisms mediate Ang II-induced decreases in arterial pressure in female rats

Angiotensin-(1-7) and the renin-angiotensin system

Angiotensin-(1-7): an update

Impairment of in vitro and in vivo heart function in angiotensin-(1-7) receptor MAS knockout mice

Angiotensin-(1-7) is an endogenous ligand for the G protein-coupled receptor Mas

AT4 receptor structure-binding relationship: N-terminal-modified angiotensin IV analogues

Retinal angiogenesis is mediated by an interaction between the angiotensin type 2 receptor, VEGF, and angiopoietin

Angiotensin II/AT2 receptor-induced vasodilation in stroke-prone spontaneously hypertensive rats involves nitric oxide and cGMP-dependent protein kinase

Negative regulation of RhoA/Rho kinase by angiotensin II type 2 receptor in vascular smooth muscle cells: role in angiotensin II-induced vasodilation in stroke-prone spontaneously hypertensive rats

Angiotensin type 2 receptor in resistance arteries of type 2 diabetic hypertensive patients

Angiotensin type 2 receptor in resistance arteries of type 2 diabetic hypertensive patients

A novel signal transduction cascade involving direct physical interaction of the renin/prorenin receptor with the transcription factor promyelocytic zinc finger protein

Angiotensin type 2 receptors mediate depressor phase of biphasic pressure response to angiotensin

Effect of the angiotensin II type 2-receptor gene (+1675 G/A) on left ventricular structure in humans

The ACCESS study. Evaluation of acute candesartan cilexetil therapy in stroke survivors

Morbidity and mortality after stroke, eprosartan compared with nitrendipine for secondary prevention: principal results of a prospective randomized controlled study (MOSES)

The cardiac renin-angiotensin system in STZ-induced diabetes

Evidence for angiotensin II type 2 receptor-mediated cardiac myocyte enlargement during in vivo pressure overload

A novel angiotensin II type 2 receptor signaling pathway: possible role in cardiac hypertrophy

Congestive heart failure in the community: a study of all incident cases in Olmsted County

Coronary kinin generation mediates nitric oxide release after angiotensin receptor stimulation

Expression of angiotensin II AT2 receptor mRNA during development of rat kidney and adrenal gland

Angiotensin II type 2 receptor-mediated apoptosis of cultured neurons from newborn rat brain

Differential kinetics of circulating angiotensin IV and II after treatment with angiotensin II type 1 receptor antagonist and their plasma levels in patients with chronic renal failure

Pharmacological profile of a highly potent and long-acting angiotensin II receptor antagonist

Increase of angiotensin converting enzyme gene expression in the hypertensive aorta

Sexual dimorphism in the response of thoracic aorta from SHRs to losartan

A lower ratio of AT1/AT2 receptors of angiotensin II is found in female than in male spontaneously hypertensive rats

Structural vascular changes in hypertension: role of angiotensin II, dietary sodium supplementation, blood pressure, and time. Hypertension 32

The subtype-2 (AT2) angiotensin receptor regulates renal cyclic guanosine 3′, 5′-monophosphate and AT1 receptor-mediated prostaglandin E2 production in conscious rats

The subtype 2 (AT2) angiotensin receptor mediates renal production of nitric oxide in conscious rats

Protective role of the angiotensin AT2 receptor in a renal wrap hypertension model

Angiotensin type 2 receptor mediates valsartan-induced hypotension in conscious rats

Angiotensin-converting enzyme inhibition potentiates angiotensin II type 1 receptor effects on renal bradykinin and cGMP

NO and cGMP mediate angiotensin AT2 receptor-induced renal renin inhibition in young rats

Sustained hypersensitivity to angiotensin II and its mechanism in mice lacking the subtype-2 (AT2) angiotensin receptor

Differential role of angiotensin II receptor subtypes on endothelial superoxide formation

The AT2 receptor-a matter of love and hate

The angiotensin AT2-receptor mediates inhibition of cell proliferation in coronary endothelial cells

Angiotensin receptor blockers may increase risk of myocardial infarction: unraveling the ARB-MI paradox

Angiotensin-(1-7) reduces smooth muscle growth after vascular injury

Angiotensin II and NGF differentially influence microtubule proteins in PC12W cells: role of the AT2 receptor

Effect of AT2 receptor on expression of AT1 and TGF-beta receptors in VSMCs from SHR. Hypertension 40

Candesartan reduces superoxide production after global cerebral ischemia

Apoptosis is not increased in myocardium overexpressing type 2 angiotensin II receptor in transgenic mice

Role of angiotensin II-regulated apoptosis through distinct AT1 and AT2 receptors in neointimal formation

Rat angiotensin II (type 1A) receptor mRNA regulation and subtype expression in myocardial growth and hypertrophy

Discovery of a distinct binding site for angiotensin II (3-8), a putative angiotensin IV receptor

Molecular mechanisms of myocardial remodeling

Angiotensin II angiogenic effect in vivo involves vascular endothelial growth factor-and inflammation-related pathways

Proapoptotic and growth-inhibitory role of angiotensin II type 2 receptor in vascular smooth muscle cells of spontaneously hypertensive rats in vivo

Persistent proteinuria up-regulates angiotensin II type 2 receptor and induces apoptosis in proximal tubular cells

Regulation of angiotensin II type 1 (AT1) receptor function. Regulatory Pept 79

Angiotensin receptor blockers and cerebral protection in stroke

AT1 receptor inhibition blunts angiotensin II-stimulated nitric oxide release in renal arteries

Angiotensin-II stimulates nitric oxide release in isolated perfused renal resistance arteries

Nonpeptide angiotensin II receptor antagonists

Angiotensin II receptors and angiotensin II receptor antagonists

A human homolog of angiotensin-converting enzyme. Cloning and functional expression as a captopril-insensitive carboxypeptidase

Recent advances in angiotensin II signaling

Role of AT2 receptors in angiotensin II-stimulated contraction of small mesenteric arteries in young SHR

p38 Map kinase regulates vascular smooth muscle cell collagen synthesis by angiotensin II in SHR but not in WKY

Redox-dependent signalling by angiotensin II and vascular remodelling in hypertension

Interacting molecule of AT1 receptor, ATRAP, is colocalized with AT1 receptor in the mouse renal tubules

Angiotensin II type 2 receptor overexpression activates the vascular kinin system and causes vasodilation

Angiotensin II type 2 receptor is upregulated in human heart with interstitial fibrosis, and cardiac fibroblasts are the major cell type for its expression

Angiotensin receptor blockers do not increase risk of myocardial infarction

Angiotensin II type 2 receptor inhibits cell proliferation and activates tyrosine phosphatase

Angiotensin receptors

Coronary hemodynamic and ventricular responses to angiotensin type 1 receptor inhibition in SHR: interaction with angiotensin type 2 receptors

Angiotensin II-dependent induction of AT(2) receptor expression after renal ablation

Angiotensin receptor blockers and myocardial infarction

Hydrolysis of biological peptides by human angiotensin-converting enzyme-related carboxypeptidase

Constrictor and dilator effects of angiotensin II on cerebral arterioles

Chronic angiotensin IV treatment reverses endothelial dysfunction in ApoE-deficient mice

Angiotensin IV-evoked vasoprotection is conserved in advanced atheroma

Spironolactone improves angiotensin-induced vascular changes and oxidative stress

Expression of angiotensin II AT2 receptors in the rat skin during experimental wound healing

Balloon angioplasty enhances the expression of angiotensin II AT1 receptors in neointima of rat aorta

Genetic deletion of angiotensin AT2 receptor leads to increased cell numbers in different brain structures of mice

C-reactive protein causes downregulation of vascular angiotensin subtype 2 receptors and systolic hypertension in mice

Interaction between AT1 and AT2 receptors during postinfarction left ventricular remodeling

Angiotensin III acts as a vasodepressor agent via AT2R

Angiotensin-(1-7) acts as a vasodepressor agent via angiotensin II type 2 receptors in conscious rats

Ischemic injury in experimental stroke depends on angiotensin II

First reported nonpeptide AT1 receptor agonist (L-162,313) acts as an AT2 receptor agonist in vivo

Design, synthesis, and biological evaluation of the first selective nonpeptide AT2 receptor agonist

Elevated blood pressure in normotensive rats produced by knockdown' of the angiotensin type 2 receptor

Aging increases aortic MMP-2 activity and angiotensin II in nonhuman primates

Differential regulation of renal angiotensin subtype AT1A and AT2 receptor protein in rats with angiotensin-dependent hypertension

Adenovirus-mediated overexpression and stimulation of the human angiotensin II type 2 receptor in porcine cardiac fibroblasts does not modulate proliferation, collagen I mRNA expression and ERK1/ERK2 activity, but inhibits protein tyrosine phosphatases

Intronic ANG II type 2 receptor gene polymorphism 1675 G/A modulates receptor protein expression but not mRNA splicing

ACE2), but not ACE, is preferentially localized to the apical surface of polarized kidney cells

Increased NADH-oxidase-mediated superoxide production in the early stages of atherosclerosis: evidence for involvement of the renin-angiotensin system

Inhibition of diet-induced atherosclerosis and endothelial dysfunction in apolipoprotein E/angiotensin II type 1A receptor double-knockout mice

Extracellular matrix remodeling in heart failure: a role for de novo angiotensin II generation

Myofibroblasts and local angiotensin II in rat cardiac tissue repair

Early streptozotocin-diabetes mellitus downregulates rat kidney AT2 receptors

Effect of renin inhibition and AT1R blockade on myocardial remodeling in the transgenic Ren2 rat

Differential distribution of angiotensin AT2 receptors in the normal and failing human heart

Angiotensin II receptor subtype gene expression and cellular localization in the retina and nonneuronal ocular tissues of the rat

Preliminary biochemical characterization of two angiotensin II receptor subtypes

Angiotensin AT2 receptors: cardiovascular hope or hype?

AT2 receptor-mediated relaxation is preserved after long-term AT1 receptor blockade

Vascular angiotensin AT2 receptors in hypertension and ageing

AVE 0991, a nonpeptide mimic of the effects of angiotensin-(1-7) on the endothelium

Diabetes and retinal vascular disorders: role of the reninangiotensin system

The angiotensin IV system: functional implications

Angiotensin II(3-8) (ANG IV) hippocampal binding: potential role in the facilitation of memory

Contributions of the brain angiotensin IV-AT4 receptor subtype system to spatial learning

Conversion of brain angiotensin II to angiotensin III is critical for pressor response in rats

Regulation of transport of the angiotensin AT2 receptor by a novel membraneassociated Golgi protein

Effect of angiotensin II type 1 receptor blockade on cardiac remodeling in angiotensin II type 2 receptor null mice

Roles of angiotensin II type 2 receptor stimulation associated with selective angiotensin II type 1 receptor blockade with valsartan in the improvement of inflammation-induced vascular injury

ACE and ACE2 activity in diabetic mice

Role of AT2 receptors in the cardioprotective effect of AT1 antagonists in mice

AT2 receptor and vascular smooth muscle cell differentiation in vascular development

Deletion of angiotensin-converting enzyme 2 accelerates pressure overloadinduced cardiac dysfunction by increasing local angiotensin II

Ventricular-specific expression of angiotensin II type 2 receptors causes dilated cardiomyopathy and heart failure in transgenic mice

Angiotensin II type 2 receptor overexpression preserves left ventricular function after myocardial infarction

Angiotensin II stimulates endothelial NO synthase phosphorylation in thoracic aorta of mice with abdominal aortic banding via type 2 receptor

Upregulation of angiotensin II type 2 receptor in rat thoracic aorta by pressureoverload

High blood pressure reduction reverses angiotensin II type 2 receptor-mediated vasoconstriction into vasodilation in spontaneously hypertensive rats

or both in patients at high risk for vascular events

Retinal expression of vascular endothelial growth factor is mediated by angiotensin type 1 and type 2 receptors

Contribution of bradykinin and nitric oxide to AT2 receptor-mediated differentiation in PC12 W cells

AT1 receptor blockade regulates the local angiotensin II system in cerebral microvessels from spontaneously hypertensive rats

Angiotensin II type 2 receptor stimulation of neuronal delayed-rectifier potassium current involves phospholipase A2 and arachidonic acid

Role of angiotensin AT1 and AT2 receptors in cardiac hypertrophy and cardiac remodelling

Expression of angiotensin II AT2 receptor in the acute phase of stroke in rats

Presence of angiotensin II AT2 receptor binding sites in the adventitia of human kidney vasculature

Roles of AT1 and AT2 receptors in the hypertensive Ren-2 gene transgenic rat kidney

Tissue renin-angiotensin system: a site of drug action?

Increased angiotensin-(1-7)-forming activity in failing human heart ventricles: evidence for upregulation of the angiotensin-converting enzyme Homologue ACE2

Immunolocalization of ACE2 and AT2 receptors in rabbit atherosclerotic plaques

Modulation of AT1 receptor-mediated contraction of rat uterine artery by AT2 receptors

The original research findings discussed in this review were supported by Project Grants from the National Health and Medical Research Council (NHMRC) of Australia (ID 143564; 384237 and 436823).