AT4 receptors: specificity and distribution

The Angiotensin AT2 Receptor: From a Binding Site to a Novel Therapeutic Target

Discovered more than 30 years ago, the angiotensin AT2 receptor (AT2R) has evolved from a binding site with unknown function to a firmly established major effector within the protective arm of the renin-angiotensin system (RAS) and a target for new drugs in development. The AT2R represents an endogenous protective mechanism that can be manipulated in the majority of preclinical models to alleviate lung, renal, cardiovascular, metabolic, cutaneous, and neural diseases as well as cancer. This article is a comprehensive review summarizing our current knowledge of the AT2R, from its discovery to its position within the RAS and its overall functions. This is followed by an in-depth look at the characteristics of the AT2R, including its structure, intracellular signaling, homo- and heterodimerization, and expression. AT2R-selective ligands, from endogenous peptides to synthetic peptides and nonpeptide molecules that are used as research tools, are discussed. Finally, we summarize the known physiological roles of the AT2R and its abundant protective effects in multiple experimental disease models and expound on AT2R ligands that are undergoing development for clinical use. The present review highlights the controversial aspects and gaps in our knowledge of this receptor and illuminates future perspectives for AT2R research. SIGNIFICANCE STATEMENT: The angiotensin AT2 receptor (AT2R) is now regarded as a fully functional and important component of the renin-angiotensin system, with the potential of exerting protective actions in a variety of diseases. This review provides an in-depth view of the AT2R, which has progressed from being an enigma to becoming a therapeutic target.

Angiotensin II and Angiotensin Receptors 1 and 2-Multifunctional System in Cells Biology, What Do We Know?

For years, the renin-angiotensin system (RAS) has been perceived as a system whose role is to primarily modulate the functioning of the cardiovascular system. Years of research into the role of RAS have provided the necessary data to confirm that the role of RAS is very complex and not limited to the cardiovascular system. The presence of individual elements of the renin-angiotensin (RA) system allows to control many processes, ranging from the memorization to pro-cancer processes. Maintaining the proportions between the individual axes of the RA system allows for achieving a balance, often called homeostasis. Thus, any disturbance in the expression or activity of individual RAS elements leads to pathophysiological processes.

International Union of Basic and Clinical Pharmacology. XCIX. Angiotensin Receptors: Interpreters of Pathophysiological Angiotensinergic Stimuli [corrected]

The renin angiotensin system (RAS) produced hormone peptides regulate many vital body functions. Dysfunctional signaling by receptors for RAS peptides leads to pathologic states. Nearly half of humanity today would likely benefit from modern drugs targeting these receptors. The receptors for RAS peptides consist of three G-protein-coupled receptors—the angiotensin II type 1 receptor (AT1 receptor), the angiotensin II type 2 receptor (AT2 receptor), the MAS receptor—and a type II trans-membrane zinc protein—the candidate angiotensin IV receptor (AngIV binding site). The prorenin receptor is a relatively new contender for consideration, but is not included here because the role of prorenin receptor as an independent endocrine mediator is presently unclear. The full spectrum of biologic characteristics of these receptors is still evolving, but there is evidence establishing unique roles of each receptor in cardiovascular, hemodynamic, neurologic, renal, and endothelial functions, as well as in cell proliferation, survival, matrix-cell interaction, and inflammation. Therapeutic agents targeted to these receptors are either in active use in clinical intervention of major common diseases or under evaluation for repurposing in many other disorders. Broad-spectrum influence these receptors produce in complex pathophysiological context in our body highlights their role as precise interpreters of distinctive angiotensinergic peptide cues. This review article summarizes findings published in the last 15 years on the structure, pharmacology, signaling, physiology, and disease states related to angiotensin receptors. We also discuss the challenges the pharmacologist presently faces in formally accepting newer members as established angiotensin receptors and emphasize necessary future developments.

New frontiers in the intrarenal Renin-Angiotensin system: a critical review of classical and new paradigms

The renin-angiotensin system (RAS) is well-recognized as one of the oldest and most important regulators of arterial blood pressure, cardiovascular, and renal function. New frontiers have recently emerged in the RAS research well beyond its classic paradigm as a potent vasoconstrictor, an aldosterone release stimulator, or a sodium-retaining hormone. First, two new members of the RAS have been uncovered, which include the renin/(Pro)renin receptor (PRR) and angiotensin-converting enzyme 2 (ACE2). Recent studies suggest that prorenin may act on the PRR independent of the classical ACE/ANG II/AT1 receptor axis, whereas ACE2 may degrade ANG II to generate ANG (1-7), which activates the Mas receptor. Second, there is increasing evidence that ANG II may function as an intracellular peptide to activate intracellular and/or nuclear receptors. Third, currently there is a debate on the relative contribution of systemic versus intrarenal RAS to the physiological regulation of blood pressure and the development of hypertension. The objectives of this article are to review and discuss the new insights and perspectives derived from recent studies using novel transgenic mice that either overexpress or are deficient of one key enzyme, ANG peptide, or receptor of the RAS. This information may help us better understand how ANG II acts, both independently or through interactions with other members of the system, to regulate the kidney function and blood pressure in health and disease.

Blood pressure and renal hemodynamic effects of angiotensin fragments

Angiotensin (Ang) II, the main effector peptide of the renin-Ang system, increases arterial blood pressure through Ang II type 1A (AT(1a)) receptor-dependent arterial vasoconstriction and by decreasing renal salt and water excretion through extrarenal and intrarenal mechanisms. AT(2) receptors are assumed to oppose these responses mediated by AT(1) receptors, thereby attenuating the pressor effects of Ang II. Nevertheless, a possible role of AT(2) receptors in the regulation of renal hemodynamics and sodium homeostasis remains to be unclear. Several other Ang fragments such as Ang III, Ang IV, Ang-(1-7) and Ang A have also been shown to display biological activity. In this review, we focus on the effects of these Ang on blood pressure, renal hemodynamics and sodium water handling, and discuss the receptors involved in these actions.

Renal vasoconstrictor and pressor responses to angiotensin IV in mice are AT1a-receptor mediated

OBJECTIVES

Angiotensin (Ang) IV was reported to induce renal vasoconstriction or vasodilation in rats via AT1 or AT4 receptors, respectively, whereby the latter one has been identified to be the insulin-regulated aminopeptidase (IRAP). We investigated the effects of Ang IV on mean arterial pressure (MAP) and renal cortical blood flow (CBF) in AT1a, AT1b, AT2 receptor and IRAP knockout (-/-) mice and their corresponding wild-type littermates. Ang II, known as a renal vasoconstrictor in mice, was used as a reference.

METHODS

MAP was recorded via a femoral catheter and CBF was measured using a light amplification by stimulated emission of radiation (LASER) Doppler probe; cortical vascular resistance (CVR) was calculated as MAP divided by CBF.

RESULTS

Baseline MAP, CBF and CVR in AT1a (-/-) mice were significantly lower than wild-type mice. AT2 (-/-) mice had a significantly higher baseline MAP, but similar CBF. In wild-type mice, Ang IV and Ang II induced dose-dependent pressor and renal vasoconstrictor responses, which were antagonized by the AT1 receptor blocker candesartan. These responses were almost completely absent in AT1a (-/-) mice, but were enhanced in AT2 (-/-) mice; responses in AT1b (-/-) and IRAP (-/-) mice were comparable to those in corresponding wild-type mice.

CONCLUSION

Ang IV mediates pressure and renal vasoconstrictor effects in mice via AT1a receptors, whereas IRAP/AT4 is not involved.

Selective labeling of IRAP by the tritiated AT(4) receptor ligand [3H]Angiotensin IV and its stable analog [3H]AL-11

'AT(4) receptors' through which Angiotensin IV (Ang IV) improves memory acquisition, were recently identified as insulin regulated aminopeptidase (IRAP). Radioligand binding studies have hitherto been performed with iodinated Ang IV in the presence of divalent cation chelators EDTA and 1,10-phenanthrolin. Hence, they referred to the apo-form of IRAP. Presently, binding of [(3)H]Ang IV and [(3)H]AL-11, a stable Ang IV analog, was compared on Chinese hamster ovary (CHO-K1) and mouse hippocampal (P40H1) cell membranes. With chelators, their high affinity sites showed the same pharmacological profile as for [(125)I]Ang IV binding. Without chelators, only high affinity binding was perceived for [(3)H]AL-11. The same pharmacological profile was recorded in both membrane preparations; it was different from the one in the presence of chelators and corresponded to catalytically active IRAP (despite the concurrent presence of aminopeptidase N (APN) in P40H1 cell membranes). This confirms that the active and apo-forms of IRAP have a distinct pharmacological profile.

From angiotensin IV binding site to AT4 receptor

One of the fragments of the cardiovascular hormone Angiotensin II incited the interest of several research groups. This 3-8 fragment, denoted as Angiotensin IV (Ang IV) causes a number of distinct biological effects (see Introduction), unlikely to be explained by its weak binding to AT(1) and/or AT(2) receptors. Moreover the discovery of high affinity [(125)I]-Ang IV binding sites and their particular tissue distribution led to the concept of the AT(4) receptor. An important breakthrough was achieved by defining the AT(4) receptor as the membrane-bound insulin-regulated aminopeptidase (IRAP). Crucial for the definition as a receptor the binding of the endogenous ligand(s) should be linked to particular cellular and/or biochemical processes. With this respect, cultured cells offer the possibility to study the presence of binding sites in conjunction with ligand induced signaling. This link is discussed for the AT(4) receptor by providing an overview of the cellular effects by AT(4) ligands.

Regulation of K+ transport in the rat distal colon via angiotensin II subtype receptors and K+ -pathways

The role of angiotensin subtype-1 (AT1) and -2 (AT2) receptors in mediating the effects of angiotensin II (ANG II) on several K+ transporters was studied in rat distal colon using an Ussing chamber. Angiotensin II induced K+ secretion at two different doses. Secretion occurred at 10-(8) and 10-(4) M, as a result of an increase in serosal-to-mucosal flux (Js-m). The ANG II-induced stimulation of Js-m at a low dose (10-(8) M) was abolished by PD123319 while losartan did not alter the low-dose ANG II-dependent increase in Js-m. In contrast, the increase in Js-m induced by a high-dose of ANG II (10-(4) M) was blocked by losartan, whereas PD123319 partially reduced the stimulatory effect. In the presence of both blockers, high-dose ANG II induced an inhibition of basal Js-m. Low-dose ANG II activated the barium-sensitive K+ channels, whereas the Na+, K+, 2Cl- cotransporter and the Na+, K+ -ATPase pump were unchanged. At the high dose, ANG II activated the barium-sensitive K+ channels and the Na+, K+, 2Cl- cotransporter and inhibited the Na+, K+ -ATPase pump. These data indicate that ANG II stimulates serosal-to-mucosal K+ flux in the rat distal colon at high and low doses via different receptors and K+ transporters.

Structural modification of the P2' position of 2,7-dialkyl-substituted 5(S)-amino-4(S)-hydroxy-8-phenyl-octanecarboxamides: the discovery of aliskiren, a potent nonpeptide human renin inhibitor active after once daily dosing in marmosets

Due to its function in the rate limiting initial step of the renin-angiotensin system, renin is a particularly promising target for drugs designed to control hypertension, a growing risk to health worldwide. Despite vast efforts over more than two decades, no orally efficacious renin inhibitor had reached the market. As a result of a structure-based topological design approach, we have identified a novel class of small-molecule inhibitors with good oral blood-pressure lowering effects in primates. Further lead optimization aimed for improvement of in vivo potency and duration of action, mainly by P2' modifications at the hydroxyethylene transition-state isostere. These efforts resulted in the discovery of aliskiren (46, CGP060536B, SPP100), a highly potent, selective inhibitor of renin, demonstrating excellent efficacy in sodium-depleted marmosets after oral administration, with sustained duration of action in reducing dose-dependently mean arterial blood pressure. Aliskiren has recently received regulatory approval by the U.S. Food and Drug Administration for the treatment of hypertension.

Angiotensin II and III upregulate body fluid volume of the clam worm Perinereis sp. via angiotensin II receptors in different manners

Angiotensin III (Ang III) as well as angiotensin II (Ang II) suppressed body weight loss of the clam worm Perinereis sp. under a hyper-osmotic condition, and enhanced body weight gain under a hypo-osmotic condition. Under a drying condition where the water inflow from outside the body was eliminated, Ang II suppressed body weight loss, but Ang III did not. Under these conditions, angiotensins I, IV, and (1-7) had no effect, and saralasin blocked the effects of Ang II and Ang III. It is concluded that Ang II and Ang III upregulate body fluid volume of the clam worm via Ang II receptors in different ways.

Role of metabolism and receptor responsiveness in the attenuated responses to Angiotensin II in mice compared to rats

Chronic infusion of Angiotensin II (AngII) to rats is a well-characterized model for determining AngII physiology. Genetic manipulations have strengthened knowledge of AngII; however, they do not permit an increase in AngII to be initiated at a selected age, duration and dose. Therefore, exogenous AngII administration remains an important technique to define its biological effect. We previously noted that infusion of AngII to mice had minimal effects compared to the same dose given to rats. In this study, we compared the effects of chronic infusion of the same dose of AngII to C57BL/6 mice and Sprague-Dawley rats, two commonly used rodent models. Rats administered AngII exhibited reductions (by 22%) in body weight, which were not evident in mice. AngII increased blood pressure by 54 mm Hg in rats, but had no effect in mice. Vascular histology demonstrated that AngII caused medial hypertrophy in rats, with adventitial expansion in mice. Plasma concentrations of AngII and its catabolic fragments were elevated (twofold) in mice compared to rats. Angiotensin receptor affinity, density and distribution were similar in rats and mice. Infusion of AngII decreased AngII receptor density in the kidney (by 78%) and spleen (by 29%) of mice, but had no effect in rats. AngII produced a sustained contractile response in rat aortic strips, but minimal responses in mouse aorta. These results demonstrate that differences in circulating angiotensin peptides, AngII receptor regulation, and vascular reactivity contribute to diminished responses to AngII infusion in mice compared to rats. Results from this study suggest that considerably higher doses of AngII may be required to elicit physiologic effects of AngII in mice.

Endogenous cystinyl aminopeptidase in Chinese hamster ovary cells: characterization by [125I]Ang IV binding and catalytic activity

The angiotensin II C-terminal hexapeptide fragment angiotensin IV (Ang IV) exerts central and cardiovascular effects. Cystinyl aminopeptidase (EC 3.4.11.3), a membrane-associated zinc-dependent metallopeptidase of the M1 family, has recently been found to display high affinity for Ang IV and it was proposed to represent the AT4 receptor. We present evidence for the presence of endogenous cystinyl aminopeptidase in membranes from Chinese hamster ovary (CHO-K1) cells by binding studies with [125I]Ang IV and by measuring the cleavage of L-leucine-p-nitroanilide. The equilibrium dissociation constant of [125I]Ang IV in saturation binding studies (KD= 0.90 nM) was similar to the value (KD= 0.70 nM) calculated from the association and dissociation rates. Binding was displaced with high potency by the "AT4 receptor" ligands (Ang IV > divalinal1-Ang IV approximately LVV-hemorphin-7 approximately LVV-hemorphin-6 > Ang (3-7) > Ang III > Ang (4-8)) but not by AT1/AT2 receptor antagonists. Enzymatic activity in CHO-K1 cell membranes was competitively inhibited upto 94% by Ang IV and other "AT4 receptor" ligands (Ang IV > Ang III approximately divalinal1-Ang IV approximately Ang (3-7) approximately LVV-hemorphin-7 > Ang (4-8) approximately LVV-hemorphin-6). High affinity binding of [125I]Ang IV required the presence of metal chelators and the ligands such as Ang IV and LVV-hemorphin-7 displayed higher potency in the binding studies as in the enzyme assay. This difference in potency varied from one peptide to another. These pharmacological properties match those previously reported for the recombinantly-expressed human cystinyl aminopeptidase in embryonal kidney cells.

Dietary NaCl regulates renal aminopeptidase N: relevance to hypertension in the Dahl rat

Aminopeptidase N (APN) is an abundant metallohydrolase in the brush border of kidney proximal tubule cells that degrades angiotensin III (Ang III) to angiotensin IV (Ang IV) and, along with dipeptidylaminopeptidase, degrades Ang IV. We examined the impact of a high-salt diet on renal APN activity and transcript abundance in the Sprague-Dawley and Dahl salt-sensitive (SS/Jr) rat strains. APN transcript abundance and protein abundance were approximately 2-fold greater (P<0.05; n=6) in the kidneys of Sprague-Dawley and Lewis rats ingesting 8% versus 0.3% salt diets, suggesting that increased aminopeptidase activity may contribute to decreased renal sodium uptake during adaptation to a high-salt diet. In contrast, renal APN transcript abundance and activity were the same in Dahl SS/Jr rats ingesting 8.0% versus 0.3% salt diets. The APN gene was mapped, using a radiation-hybrid panel, to known quantitative loci on chromosome 1 for blood pressure in the Dahl SS/Jr rat. The results suggest that the APN gene is a good candidate for salt-sensitivity in the Dahl SS/Jr rat.

Effects of truncated angiotensins in humans after double blockade of the renin system

Angiotensins different from ANG II exhibit biological activities, possibly mediated via receptors other than ANG II receptors. We studied the effects of 3-h infusions of ANG III, ANG-(1-7), and ANG IV in doses equimolar to physiological amounts of ANG II (3 pmol. kg-1. min-1), in six men on low-sodium diet (30 mmol/day). The subjects were acutely pretreated with canrenoate and captopril to inhibit aldosterone actions and ANG II synthesis, respectively. ANG II infusion increased plasma angiotensin immunoreactivity to 53 +/- 6 pg/ml (+490%), plasma aldosterone to 342 +/- 38 pg/ml (+109%), and blood pressure by 27%. Glomerular filtration rate decreased by 16%. Concomitantly, clearance of endogenous lithium fell by 66%, and fractional proximal reabsorption of sodium increased from 77 to 92%; absolute proximal reabsorption rate of sodium remained constant. ANG II decreased sodium excretion by 70%, potassium excretion by 50%, and urine flow by 80%, whereas urine osmolality increased. ANG III also increased plasma aldosterone markedly (+45%), however, without measurable changes in angiotensin immunoreactivity, glomerular filtration rate, or renal excretion rates. During vehicle infusion, plasma renin activity decreased markedly ( approximately 700 to approximately 200 mIU/l); only ANG II enhanced this decrease. ANG-(1-7) and ANG IV did not change any of the measured variables persistently. It is concluded that 1) ANG III and ANG IV are cleared much faster from plasma than ANG II, 2) ANG II causes hypofiltration, urinary concentration, and sodium and potassium retention at constant plasma concentrations of vasopressin and atrial natriuretic peptide, and 3) a very small increase in the concentration of ANG III, undetectable by usual techniques, may increase aldosterone secretion substantially.

Angiotensin receptors contribute to blood pressure homeostasis in salt-depleted SHR

This study evaluated the contribution of angiotensin peptides acting at various receptor subtypes to the arterial pressure and heart rate of adult 9-wk-old male conscious salt-depleted spontaneously hypertensive rats (SHR). Plasma ANG II and ANG I in salt-depleted SHR were elevated sevenfold compared with peptide levels measured in sodium-replete SHR, whereas plasma ANG-(1-7) was twofold greater in salt-depleted SHR compared with salt-replete SHR. Losartan (32.5 micromol/kg), PD-123319 (0.12 micromol. kg(-1). min(-1)), [d-Ala(7)]ANG-(1-7) (10 and 100 pmol/min), and a polyclonal ANG II antibody (0.08 mg/min) were infused intravenously alone or in combination. Combined blockade of AT(2) and AT((1-7)) receptors significantly increased the blood pressure of losartan-treated SHR (+15 +/- 1 mmHg; P < 0.01); this change did not differ from the blood pressure elevation produced by the sole blockade of AT((1-7)) receptors (15 +/- 4 mmHg). On the other hand, sole blockade of AT(2) receptors in losartan-treated SHR increased mean arterial pressure by 8 +/- 1 mmHg (P < 0.05 vs. 5% dextrose in water as vehicle), and this increase was less than the pressor response produced by blockade of AT((1-7)) receptors alone or combined blockade of AT((1-7)) and AT(2) receptors. The ANG II antibody increased blood pressure to the greatest extent in salt-depleted SHR pretreated with only losartan (+11 +/- 2 mmHg) and to the least extent in salt-depleted SHR previously treated with the combination of losartan, PD-123319, and [d-Ala(7)]ANG-(1-7) (+7 +/- 1 mmHg; P < 0.01). Losartan significantly increased heart rate, whereas other combinations of receptor antagonists or the ANG II antibody did not alter heart rate. Our results demonstrate that ANG II and ANG-(1-7) act through non-AT(1) receptors to oppose the vasoconstrictor actions of ANG II in salt-depleted SHR. Combined blockade of AT(2) and AT((1-7)) receptors and ANG II neutralization by the ANG II antibody reversed as much as 67% of the blood pressure-lowering effect of losartan.

Cellular targets for angiotensin II fragments: pharmacological and molecular evidence

Although angiotensin II has long been considered to represent the end product of the renin-angiotensin system (RAS), there is accumulating evidence that it encompasses additional effector peptides with diverse functions. In this respect, angiotensin IV (Ang IV) formed by deletion of the two N terminal amino acids, has sparked great interest because of its wide range of physiological effects. Among those, its facilitatory role in memory acquisition and retrieval is of special therapeutic relevance. High affinity binding sites for this peptide have been denoted as AT(4)- receptors and, very recently, they have been proposed to correspond to the membrane-associated OTase/ IRAP aminopeptidase. This offers new opportunities for examining physiological roles of Ang IV in the fields of cognition, cardiovascular and renal metabolism and pathophysiological conditions like diabetes and hypertension. Still new recognition sites may be unveiled for this and other angiotensin fragments. Recognition sites for Ang-(1-7) (deletion of the C terminal amino acid) are still elusive and some of the actions of angiotensin III (deletion of the N terminal amino acid) in the CNS are hard to explain on the basis of their interaction with AT(1)-receptors only. A more thorough cross-talk between in vitro investigations on native and transfected cell lines and in vivo investigations on healthy, diseased and transgenic animals may prove to be essential to further unravel the molecular basis of the physiological actions of these small endogenous angiotensin fragments.

Biological properties of the angiotensin peptides other than angiotensin II: implications for hypertension and cardiovascular diseases

Several peptides of the RAS other than angiotensin (1-8) have been identified. They are generally referred as 'angiotensin fragments': Ang (2-8), Ang (3-8) and Ang (1-7) and have been detected in human tissues. There is evidence that they may play a functional role in humans by acting in concert with angiotensin (1-8) and aldosterone. Available knowledge on the pathways leading to synthesis and degradation of angiotensin fragments, as well as on their interactions with receptors and on their possible role in cardiovascular homeostasis and disease are reviewed.

Angiotensin AT(4) receptors in the normal human prostate and benign prostatic hyperplasia

The cellular localisation and expression of angiotensin AT(4) receptors was examined in the normal human prostate and benign prostatic hyperplasia (BPH) by quantitative in vitro autoradiography using [(125)I]-Ang IV. In the normal human prostate, AT(4) receptors were localised to the glandular epithelium. Interestingly, specific AT(4) receptor binding was significantly reduced in BPH compared to the normal prostate, as quantitated macroscopically (normal: 5038+/-476 dpm/mm(2), n=6 vs BPH: 2701+/-176 dpm/mm(2), n=6, P<0.001) and microscopically (normal: 7.28+/-0.36 grains/mm(2), n=6 vs BPH: 2.50+/-0.47 grains/mm(2), n=6, P<0.001). The findings of the present study demonstrate the presence of AT(4) receptors in the human prostate, being localised to the glandular epithelium, which suggest that the Ang IV/AT(4) system may play a role in the regulation of ionic transport and glandular secretion in the human prostate. The observation that AT(4) receptors appear reduced in BPH suggests that the AT(4) receptor may undergo agonist-induced receptor internalisation, possibly due to increased local tissue levels of Ang IV in BPH.

Autoradiographic analysis and regulation of angiotensin receptor subtypes AT(4), AT(1), and AT((1-7)) in the kidney

Receptor autoradiography revealed that angiotensin AT(4) receptors were abundantly expressed in normal mammalian (mouse, rat, gerbil, guinea pig, rabbit) and avian (sparrow, chicken, turkey) kidneys and were more extensively distributed than previously reported (including proximal and distal segments of the nephron, interstitium, renal artery, vein, and ureter). Angiotensin AT(4) receptors were generally found to be more abundant than angiotensin AT(1) receptors in mammalian kidneys, whereas angiotensin AT((1-7)) receptors were not detected in either mammalian or avian kidneys. Rats subjected to various chronic treatments were found to preferentially decrease kidney AT(4) receptor density (furosemide, puromycin aminonucleoside, nitro-L-arginine methyl ester), decrease kidney AT(1) receptor density (bilateral ureteral obstruction), or increase kidney AT(1) receptor distribution in the inner medulla (water diuresis). These results indicate that the AT(4) receptor can be expressed in numerous cell types within the normal kidney of several species. Furthermore, several models of renal dysfunction and injury have been identified that selectively alter kidney AT(4) density and may potentially aid in elucidating the role of this novel angiotensin receptor system in renal function.

A role for the angiotensin IV/AT4 system in mediating natriuresis in the rat

Angiotensin II (AngII) or Angiotensin IV (AngIV) was infused into the renal artery of anesthetized rats while renal cortical blood flow was measured via laser Doppler flowmetry. The infusion of AngII produced a significant elevation in mean arterial pressure (MAP) with an accompanying decrease in cortical blood flow, glomerular filtration rate (GFR), urine volume, and urine sodium excretion. The infusion of AngIV induced significant increases in renal cortical blood flow and urine sodium excretion, without altering MAP, GFR, and urine volume. Pretreatment infusion with a specific AT1 receptor antagonist, DuP 753, blocked or attenuated the subsequent AngII effects, while pretreatment infusion with the specific AT4 receptor antagonist, Divalinal-AngIV, blocked the AngIV effects. These results support distinct and opposite roles for AngII and AngIV, i.e. AngII acts as an anti-natriuretic agent, while AngIV acts as a natriuretic agent.

Characterization of the AT(4) receptor in a human neuroblastoma cell line (SK-N-MC)

Angiotensin IV (Ang IV), the 3-8 fragment of angiotensin II (Ang II), binds to a distinct receptor designated the AT(4) receptor. The peptide elicits a range of vascular and central actions including facilitation of memory retention and retrieval in several learning paradigms. The aim of this study was to characterize the AT(4) receptor in a human cell line of neural origin. Receptor binding studies indicate that the human neuroblastoma cell line SK-N-MC cells express a high-affinity Ang IV binding site with a pharmacological profile similar to the AT(4) receptor: (125)I]-Ang IV and (125)I]-Nle(1)-Ang IV bind specifically to the SK-N-MC cell membranes (K(d) = 0.6 and 0.1 nM) in a saturable manner (B(max) = 1.2 pmol/mg of protein). AT(4) receptor ligands, Nle(1)-Ang IV, Ang IV and LVV-haemorphin 7 (LVV-H7), compete for the binding of [(125)I]-Ang IV or [(125)I]-Nle(1)-Ang IV to the SK-N-MC cell membranes with rank order potencies of Nle(1)-Ang IV > Ang IV > LVV-H7 with IC(50) values of 1.4, 8.7 and 59 nM ([(125)I]-Ang IV) and 1.8, 20 and 168 nM ([(125)I]-Nle(1)-Ang IV), respectively. The binding of [(125)I]-Ang IV or [(125)I]-Nle(1)-Ang IV to SK-N-MC cell membranes was not affected by the presence of GTP gamma S. Both Ang IV and LVV-H7 stimulated DNA synthesis in this cell line up to 72 and 81% above control levels, respectively. The AT(4) receptor in the SK-N-MC cells is a 180-kDa glycoprotein; under non-reducing conditions a 250-kDa band was also observed. In summary, the human neuroblastoma cell line, SK-N-MC, expresses functional AT(4) receptors that are responsive to Ang IV and LVV-H7, as indicated by an increase in DNA synthesis. This is the first human cell line of neural origin shown to express the AT(4) receptor.

Angiotensin receptors--evolutionary overview and perspectives

The structure of the angiotensin molecule has been well preserved throughout the vertebrate scale with some amino acid variations. Specific angiotensin receptors (AT receptors) that mediate important physiological functions have been noted in a variety of tissues and species. Physiological and pharmacological characterization of AT receptors and, more recently, molecular cloning studies have elucidated the presence of AT receptor subtypes. Comparative studies suggest that an AT receptor subtype homologous to the mammalian type 1 receptor subtype (AT(1)), though pharmacologically distinct, is present in amphibians and birds, whereas AT receptors cloned from teleosts show low homology to both AT(1) and AT(2) receptor subtypes. Furthermore, receptors differing from both the AT(1)-homologue receptor and AT(2) receptor exist in some non-mammalian species. This may suggest that the prototype AT receptor evolved in primitive vertebrates and diverged to more than one type of AT receptor subtype during phylogeny. Furthermore, phenotypic modulation of AT receptors appears to occur during individual development/maturation.

The angiotensin type 2 receptor: variations on an enigmatic theme

Since its discovery and molecular characterization, the angiotensin AT2.receptor has been enigmatic with respect to signalling pathways and function. Evidence now emerges that angiotensin II exerts actions through the AT2 receptor which are directly opposed to those mediated by the AT1 receptor. This can be exemplified e.g. by mutually antagonizing effects on cell growth. Upregulated by the endogenous agonist itself, as well as by several growth- and differentiating factors in development and tissue injury, the AT2 receptor appears to act as a modulator of complex biological programmes involved in embryonic development, cell differentiation, tissue protection and regeneration, as well as in programmed cell death. Research on the AT2 receptor has thus unveiled hitherto unknown functions of the renin-angiotensin system extending far beyond the classical role of this old hormonal system in cardiovascular control.

Renal hemodynamic responses to intrarenal infusion of ligands for the putative angiotensin IV receptor in anesthetized rats

Angiotensin IV, a hexapeptide fragment (3-8) of angiotensin II metabolism, has been reported to produce vasodilatation within the renal vasculature by activation of the putative AT4 receptor. However, there are conflicting findings, with previous in vivo studies providing evidence for and against a renal vasodilator action of angiotensin IV. In this study, the renal hemodynamic responses to activation of the putative AT4 receptor were studied in anesthetized rats by left renal arterial infusion of two endogenous ligands, angiotensin IV and LVV-hemorphin-7. Angiotensin IV (10, 100, and 1,000 pmol/min) infusion caused dose-dependent reductions in blood flow to the infused kidney, which were abolished by pretreatment with losartan. In respect to this effect, angiotensin IV was approximately 300-fold less potent than angiotensin II. There were no significant effects of angiotensin IV on mean arterial pressure, heart rate, or blood flow to the noninfused kidney. Intrarenal infusion of LVV-hemorphin-7 (10, 100, and 1,000 pmol/min) had no significant effect on renal blood flow in the infused and noninfused kidneys, or on mean arterial pressure or heart rate. These results provide no evidence for a renal vasodilatory action of angiotensin IV or LVV-hemorphin-7. On the contrary, intrarenal angiotensin IV infusion produced vasoconstriction of the renal vasculature, mediated by activation of AT1 receptors. These observations provide evidence against a vasodilatory role of putative AT4 receptors in the rat kidney.

Angiotensin IV stimulates plasminogen activator inhibitor-1 expression in proximal tubular epithelial cells

BACKGROUND

Angiotensin II (Ang II) has been shown to be implicated in the development of renal fibrosis in several forms of chronic glomerulonephritides, but the precise mechanisms of its effects remain unclear. It has recently been reported that Ang II stimulates the expression of plasminogen activator inhibitor-1 (PAI-1) in several cell lines. PAI-1 is a major physiological inhibitor of the plasminogen activator/plasmin system, a key regulator of fibrinolysis and extracellular matrix (ECM) turnover. PAI-1 induction by Ang II in endothelial cells seems to be mediated by Ang IV via a receptor that is different from Ang II type 1 and 2 receptors (AT1 and AT2).

METHODS

In this study, we sought to evaluate the effects of Ang IV on PAI-1 gene and protein expression in a well-characterized and immortalized human proximal tubular cell line (HK2) by Northern blot and enzyme-linked immunosorbent assay.

RESULTS

Ang IV stimulated PAI-1 mRNA expression, whereas it did not induce a significant increase in tritiated thymidine uptake after 24 hours of incubation. This effect was dose and time dependent. Ang IV (10 nM) induced a 7.8 +/- 3.3-fold increase in PAI-1 mRNA expression. The PAI-1 antigen level was significantly higher in conditioned media and the ECM of cells treated with Ang II and Ang IV than in control cells (both P < 0.02). Although Ang II induced a 4.2 +/- 2. 1-fold increase in PAI-1 mRNA expression, its effect underwent a dose-dependent reduction when amastatin, a potent inhibitor of the endopeptidases that catalyzes the conversion of Ang II to Ang IV, was added. In contrast, amastatin was not able to prevent the expression of PAI-1 mRNA induced by Ang IV. Finally, pretreatment of HK2 cells with losartan and N-Nicotinoyl-Tyr-N3-(Nalpha-CBZ-Arg)-Lys-His-Pro-Ile, the specific antagonists of AT1 and AT2 receptors, failed to modify PAI-1 mRNA expression as induced by Ang II.

CONCLUSIONS

Our results demonstrate that Ang II stimulates PAI-1 mRNA expression and the production of its protein in human proximal tubular cells. This is mainly-if not exclusively-due to Ang IV, which acts on a receptor that is different than AT1 or AT2. Therefore, it can be hypothesized that the induction of PAI-1 by Ang IV may be implicated in the pathogenesis of renal interstitial fibrosis in several forms of chronic glomerulonephritides.

High salt intake differentially regulates kidney angiotensin IV AT4 receptors in Wistar-Kyoto and spontaneously hypertensive rats

Functional angiotensin IV (Ang IV) receptors (denoted AT4) are localized to the outer stripe of the medulla in the rat kidney, and may play a critical role in salt homeostasis. The purpose of this study was to determine if AT4 receptor binding in the kidney is differently regulated in the salt-sensitive spontaneously hypertensive (SH) rat compared to Wistar Kyoto (WKY) controls. AT4 receptor binding was determined using in vitro receptor autoradiography. AT4 receptor binding in the outer stripe of the medulla was similar in WKY and SH rats maintained on a 1% salt diet. A high salt diet (8%) resulted in a statistically significant increase (28%) in AT4 receptor binding in kidneys from WKY rats. However, there was no change in AT4 receptor binding in the kidneys of SH rats fed the same diet. The present data indicate that AT4 binding sites are regulated by salt intake. In addition, regulation of this receptor may be impaired in the kidneys of SH rats, explaining in part the salt-sensitivity of this strain.

The renin-angiotensin system and its receptors

The renin-angiotensin system (RAS) plays an important role in blood pressure control and in water and salt homeostasis. It is involved in the pathophysiology of hypertension and structural alterations of the vasculature, kidney, and heart, including neointima formation, nephrosclerosis, postinfarction remodeling, and cardiac left ventricular hypertrophy (LVH). Recently, an increased knowledge of the effector peptides of the RAS, their receptors, and their respective functions has led to a new principle of treatment for hypertension: the inhibition of angiotensin (Ang) II via angiotensin-converting enzyme inhibitors or Ang II-receptor antagonists. In this review, the Ang receptors AT1 and AT2 and the potential roles of shorter angiotensin fragments, including Ang III(2-8), Ang IV(3-8), and Ang(1-7), are discussed.

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

Accumulating evidence suggests that angiotensin-(1-7) [Ang-(1-7)] is an important component of the renin-angiotensin system. As the most pleiotropic metabolite of angiotensin I (Ang I) it manifest actions which are most often the opposite of those described for angiotensin II (Ang II). Ang-(1-7) is produced from Ang I bypassing the prerequisite formation of Ang II. The generation of Ang-(1-7) is under the control of at least three enzymes, which include neprilysin, thimet oligopeptidase, and prolyl oligopeptidase depending on the tissue compartment. Both neprilysin and thimet oligopeptidase are also involved in the metabolism of bradykinin and the atrial natriuretic peptide. Moreover, recent studies suggest that in addition to Ang I and bradykinin, Ang-(1-7) is an endogenous substrate for angiotensin converting enzyme. This suggests that there is a complex relationship between the enzymatic pathways forming angiotensin II and other various vasodepressor peptides from either the renin-angiotensin system or other peptide systems. The antihypertensive actions of angiotensin-(1-7) are mediated by an angiotensin receptor that is distinct from the pharmacologically characterized AT1 or AT2 receptor subtypes. Ang-(1-7) mediates it antihypertensive effects by stimulating synthesis and release of vasodilator prostaglandins, and nitric oxide and potentiating the hypotensive effects of bradykinin.

Characterization of angiotensin IV-degrading enzymes and receptors on rat mesangial cells

Because mesangial cells (MC) are a target and a degradation site for angiotensin II (ANG II), we characterized the degrading enzymes and receptors of ANG IV, a metabolite of ANG II, on these cells. ANG IV was metabolized into its NH2-terminal deleted peptides, ANG II-(4-8), ANG II-(5-8), and ANG II-(6-8) by rat MC. Total protection of ANG IV was obtained only when PC-18, a specific aminopeptidase N (APN) inhibitor, and JFH-27A, a mixed inhibitor of dipeptidylaminopeptidase (DAP) and neutral endopeptidase (NEP), were simultaneously added. In contrast, thiorphan, an NEP inhibitor, was inactive. These results demonstrate the exclusive role of APN and DAP in ANG IV degradation. 125I-labeled ANG IV binding was studied in the presence of PC-18 and JFH-27A to suppress ligand degradation. Under these conditions, ANG IV-specific receptors could be demonstrated with a KD of 1.8 nM and a density of 55 fmol/mg. In contrast with MC, no evidence for ANG IV receptors could be obtained in freshly isolated glomeruli. ANG IV stimulated cytosolic calcium concentration in MC, whereas its NH2-terminal deleted metabolites were inactive. Therefore, ANG IV must be protected from degradation by APN and DAP in studies on the nonimmediate biological effects of this peptide.

Does blockade of angiotensin II receptors offer clinical benefits over inhibition of angiotensin-converting enzyme?

Angiotensin AT1 receptor antagonists represent a new class of drugs for the treatment of hypertension. They are specific for the renin-angiotensin system, selective for the angiotensin AT1 receptor, and act independently of the angiotensin II synthetic pathway. Blockade of the renin-angiotensin system at the receptor level should therefore be more complete. The high circulating levels of angiotensin II following angiotensin AT1 receptor blockade could be beneficial in stimulating other unblocked angiotensin receptors, especially the AT2 receptor. It has been proposed that the angiotensin AT2 receptor, which is re-expressed or up-regulated during pathological circumstances, counterbalances the effect of the stimulation of the angiotensin AT1 receptor. Through this mechanism, angiotensin AT1 antagonists may be superior to ACE inhibitors in cardiac and vascular remodelling as well as in kidney insufficiency. Long-term trials are required to demonstrate the possible clinical superiority of this new class of antihypertensive agents.

Role of AT1 and AT2 receptors in the plasma clearance of angiotensin II

This study assessed the role of angiotensin (Ang) AT1 and AT2 receptors as modulators of the plasma clearance of Ang II. Groups of male spontaneously hypertensive rats (SHRs; n = 25) were given an intravenous injection of either saline, losartan, PD123319, losartan in combination with PD123319, or Sar1-Thr8-Ang II. One hour later, Ang II (0.5 microg/kg) was infused for 15 min into a vein. Immediately thereafter, arterial blood samples were collected at regular intervals for the assay of plasma Ang II levels by radioimmunoassay. The infusion of Ang II significantly increased baseline mean arterial pressure (MAP) in rats pretreated with either saline or PD123319 but not in those receiving losartan, losartan combined with PD123319, or Sar1-Thr8-Ang II. The plasma clearance of Ang II was significantly greater in rats injected with either PD123319, losartan combined with PD123319, or Sar1-Thr8-Ang II compared to those injected either saline or losartan. Furthermore, the half-life of Ang II in rats pretreated with saline or losartan was significantly greater than that measured in the other three groups. These results suggest that plasma clearance of Ang II in the SHRs is independent of an AT1 receptor, but plasma levels of the peptide are unexpectedly protected by an AT2 receptor-dependent mechanism.

Angiotensin IV AT4-receptor system in the rat kidney

Angiotensin IV, [[des-Asp1,Arg2]ANG II or ANG-(3-8)], has been shown to preferentially bind to a novel angiotensin binding site (AT4 receptor). The cellular location and function of this receptor in the rat kidney is unknown. Autoradiography localized AT4 receptors to the cell body and apical membrane of convoluted and straight proximal tubules in the cortex and outer stripe of the outer medulla. ANG IV (0.1 pM-1 microM) elicited a concentration-dependent decrease in transcellular Na+ transport (as measured by proximal tubule O2 consumption rates) in fresh suspensions of control or nystatin-stimulated (bypasses rate-limiting step of apical Na+ entry) rat proximal tubules. The inhibitory effect of 1 pM ANG IV was unaltered by either 1 microM losartan (AT1-receptor antagonist) or 1 microM PD-123319 (AT2-receptor antagonist) and yet was abolished by 1 microM divalinal-ANG IV (AT4-receptor antagonist) or ouabain pretreatment. These results demonstrate that the kidney AT4-receptor system is localized to the proximal tubule and suggests that one potential biological role of this system is in the regulation of Na+ transport by inhibiting a ouabain-sensitive component of Na(+)-K(+)-adenosinetriphosphatase activity in the rat.

Characterization of the functional angiotensin II-receptor complex isoform in rat liver plasma membrane

In rat liver plasma membrane a single angiotensin II (Ang II) binding site (Kd of 3.71 +/- 0.33 nM and Bmax of 1143.7 +/- 83.9 fmol/mg protein) was identified using radioligand binding assay. Pharmacologically, this receptor match with the AT1 receptor subtypes in term of affinity for the selective antagonist Losartan, and probably with the AT1A receptor form in term of insensitivity for the antagonist PD123319. Nevertheless, using polyacrylamide gel isoelectric focusing, two 125I-Ang II binding sites migrating to pI 6.8 and 6.5 were found in these membrane preparations. Monophasic displacement of 125I-Ang II bound to isoform migrating at pI 6.8 clearly indicate that this isoform represents a functional Ang II-receptor complex. In contrast, the high concentrations of agonist and peptidic derivates of Ang necessary to displace 125I-Ang II bound to isoform migrating at pI 6.5 indicate that this atypical 125I-Ang II binding site represents a biologically nonfunctional Ang II binding molecule, presumably a nonspecific 125I-Ang II binding site.

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

The present investigation initially determined that specific binding sites for the hexapeptide angiotensin IV (AngIV) are present in the rat kidney cortex and outer medulla but not in the inner medulla, using in vitro autoradiographic techniques. This binding site has been termed AT4, is distinct from the previously characterized AT1 and AT2 sites, and does not bind the specific AT1 receptor antagonist DuP753 or the AT2 receptor antagonist PD123177. Renal artery infusions of AngIV produced a dose-dependent increase in cortical blood flow without altering systemic blood pressure. In contrast, the infusion of angiotensin II (AngII) induced a dramatic decrease in cortical blood flow, accompanied by a significant elevation in systemic blood pressure. The infusion of [D-Val(1)]AngIV, an analog that does not bind at the AT4 receptor site, and the C-terminal truncated analogs AngIV (1-4) and AngIV (1-5) that possess lower affinity for this site, produced no change in cortical blood flow. The infusion of [Nle1]AngIV and [Lys1]AngIV, analogs that bind with high affinity at the AT4 receptor site, produced increases in cortical blood flow with no influence on blood pressure. Pretreatment with a specific AT4 receptor antagonist, Divalinal-AngIV, completely blocked AngIV-induced elevations in blood flow, but failed to influence AngII-induced decreases in blood flow, suggesting that these ligands are acting at different receptor sites. Pretreatment with the nitric oxide synthase inhibitor, NG-Monomethyl-L-Arginine, also blocked subsequent AngIV-induced increases in cortical blood flow. These data support the notion that AngIV exerts a unique influence upon renal hemodynamics via the AT4 receptor subtype, and suggest that AngIV-induced elevations in blood flow may be mediated by nitric oxide.

Angiotensinases restrict locally generated angiotensin II to the blood vessel wall

We tested the hypothesis that angiotensinases limit the spillover of locally formed angiotensin II into the circulation. The release of angiotensin peptides from isolated rat hindquarters perfused with an artificial medium was measured by high-performance liquid chromatography and radioimmunoassay. The spontaneous release of angiotensins was increased by the angiotensinase inhibitors phenanthroline (850+/-195 versus 95+/-33 fmol of angiotensin I per 30 minutes in controls, P<.05, n=5 each) and amastatin (P<.05, n=5 each). Infusion of renin induced sustained local angiotensin I formation, which was also increased by phenanthroline. Stimulation of local angiotensin formation by renin infusion was compared with infusion of exogenous angiotensin II. Renin caused similar increases of perfusion pressure (11.1+/-2.2 versus 7.6+/-1.9 mm Hg after angiotensin II, P>.05) despite lower angiotensin II levels in the venous effluent than during infusion of exogenous angiotensin II (65+/-2 versus 482+/-33 fmol/mL, P<.05, n=7 each). Thus, renin must have caused higher angiotensin II tissue levels than indicated by the measurements in the venous effluent. The pressor response to renin was abolished by the type 1 angiotensin II receptor antagonist losartan. We conclude that the major part of locally generated angiotensins is not released into the circulation but degraded by angiotensinases within the tissue compartment.

Hemorphins inhibit angiotensin IV binding and interact with aminopeptidase N

[125I]-Ang IV binding to rabbit collecting duct cell membranes was inhibited by hemorphins (H), a class of endogenous peptides obtained by hydrolysis of the beta chain of hemoglobin. The most potent competitors were those with a valine in their N-terminal part such as LVV-H7 and VV-H7 (IC50 = 1.3 nM) followed by VV-H8 and K6VV-H7 (5.1 nM). The same H, like Ang IV, interacted with aminopeptidase N (APN) as shown by their inhibitory effect (28-36%) on APN activity. HPLC analysis showed that only H with a N-terminal valine or leucine were hydrolyzed. Since H are detected in the body fluids, they are likely to act as endogenous competitors of Ang IV.

Dose-dependent inhibitory effects of angiotensin II on visual responses of the rat superior colliculus: AT1 and AT2 receptor contributions

Angiotensin II (Ang II) has traditionally been regarded as a peripherally circulating and acting hormone involved in fluid homeostasis and blood pressure regulation. With the rather recent localization of Ang II receptors within the mammalian brain, renewed interest has emerged in the hope of elucidating the central impact and function of this hormone. One region that has been clearly demonstrated to express Ang II receptors is the superior colliculus (SC). This mesencephalic structure plays an important role in sensory visuomotor integration. Receptors for Ang II (of both the AT1 and AT2 subtypes) have been localized within the superficial layers of this structure, i.e. the areas that are visually responsive. In the hopes of characterizing the role of Ang II in the SC, we have attempted to physiologically activate these receptors in vivo and observe the effects of Ang II on visually evoked responses. In the attempt to identify the receptor subtype(s) responsible in mediating these effects, Ang II was injected concomitantly with selective receptor ligands. Experiments were performed on adult rats prepared in classical fashion for electrophysiological studies. Through microinjection of Ang II, and the simultaneous recording of visually evoked potentials to flash stimulation, we have observed that this peptide yields a strong suppressive effect on visual neuronal activity. By injecting Ang II at various concentrations (10(-3)-10(-10) M), we have further observed that the effects of this peptide express a dose-related dependency. Injection of Ang II in progressively more ventral layers yielded less pronounced effects, demonstrating physiologically the discrete localization of these receptors in the stratum griseum superficiale. Coinjection of Ang II with Losartan yielded a near complete blockade of Ang II suppressive effects, suggesting that AT1 receptors play a prominent role in mediating these responses. However, coinjection of Ang II with PD 123,319 yielded a slight, yet significant partial blockade. Coinjection of Ang II with both the AT1 and AT2 receptor antagonists yielded a complete blockade of the Ang II effect. Finally, some of the results suggest that the AT2 receptor ligand CGP 42,112 may possess agonist properties. Taken together, these findings suggest that the AT1 receptor is predominantly involved in mediating Ang II responses in the SC and there also appears to be some indication of AT2 receptor involvement. However, the underlying mechanisms (such as receptor interactions), the exact specificity of the ligands used, and the possibility of other receptor subtype implication have yet to be explored fully.

Evidence for angiotensin IV receptors in human collecting duct cells

Because angiotensin II (Ang II) has been found at high concentrations in the proximal tubule fluid and because tubular brush border membranes exhibit a marked capacity for degrading Ang II, we thought it of interest to examine the binding sites for Ang II (3-8) (referred to as Ang IV), a metabolite of Ang II, downstream in the nephron. We studied the binding of [125I]-Ang IV and also of [125I]-Sar1, Ala8, Ang II to SV-40 transformed human collecting duct cell (HCD) membranes. No specific binding site for [125I]-Sar1, Ala8, Ang II and no Ang II-dependent cytosolic calcium response could be observed. Moreover, no signal for the human type I Ang II receptor (hAT1) mRNA was present in HCD cells. In contrast, [125I]-Ang IV bound specifically to HCD cell membranes. Mean Kd and Bmax values derived from saturation binding studies were 5.6 +/- 2.0 nM and 1007.6 +/- 140.2 fmol/mg protein, respectively. The rank order of affinity for competitive Ang II-related peptides was: Ang IV > Ang III > Ang II > Ang II (4-8) > Ang II (1-7). [125I]-Ang IV binding was not modified by nonpeptide AT1 (losartan) or AT2 (PD123177) antagonists. GTP gamma S and dithiotreitol did not affect [125I]-Ang IV binding. Ang IV stimulated cAMP production by intact HCD cells in the presence of forskolin but did not modify cGMP production or cytosolic calcium concentration. Taken together, these results indicate that HCD cells represent a target site for Ang IV but do not possess Ang II receptors.

Comparative effects of angiotensin II and its degradation products angiotensin III and angiotensin IV in rat aorta

1. In the present study, the contractile effects of angiotensin III (AIII) and angiotensin IV (AIV) compared with those of angiotensin II (AII) were determined in rat aortic ring preparations. 2. All three peptides caused concentration-dependent contractions with similar maximal responses. AIII proved approximately 4 times less potent than AII, whereas AIV was about 1000 times less active than AII. 3. The selective AT1-receptor antagonist, losartan (10-300 nM) caused parallel rightward shifts of the concentration-response curves (CRC) for all three peptides. The Schild plot slopes for the effect of losartan on AIII curves were significantly lower than unity (P < 0.05). The selective AT2-receptor antagonist, PD123177 did not influence the CRCs for AII and AIV. However, the AIII curves were moderately shifted leftward in the presence of PD123177 (0.1 and 1 microM). 4. Destruction of the endothelium or incubation with the NO-synthesis inhibitor NG-monomethyl-L-arginine acetate (L-NMMA) (0.1 mM) significantly enhanced the contractile responses to all three peptides. 5. Tachyphylaxis was investigated by constructing a second CRC for all three peptides, after an interval of 1 h. The presence of endothelium significantly enhanced the development of tachyphylaxis to all three peptides. However, in endothelium-denuded preparations, the Emax value of the second curve elicited by AII was about 50%, compared with the first one, whereas for AIII and AIV Emax values were as high as 90% and 100%, respectively. 6. Our results indicate that both AIII and AIV are less potent but similarly efficacious vasoconstrictor agents compared with AII. Their contractile effects are also mediated by AT1-receptors and probably modulated by endothelium. Tachyphylaxis induced by AIII and AIV proved weaker than that for AII. Tachyphylaxis appears to be enhanced by the presence of an intact endothelium.