Identification of the angiotensin II receptor in rat mesenteric artery

Angiotensin II receptors in endothelial cells

1. Angiotensin II (Ang II), the main effector of the renin-angiotensin system, exerts its vasoconstrictory and trophic actions on smooth muscle cells via AT1 receptors. However, Ang II does not act only on smooth muscle cells, as Ang II receptors are also present in endothelial cells. 2. The receptor type on these cells differs depending on the origin of the endothelium and the species. The rat endothelial receptors are mostly of the AT1 type, but AT2 receptors have also been found. The pharmacological characteristics of the AT1 receptors on endothelial cells are similar to those of other cell types. 3. Ang II stimulates phospholipase C and phospholipase A2 activation via the AT1 receptor in endothelial cells. Ang II also stimulates the tyrosine phosphorylation of several proteins in these cells. 4. Some studies suggest that the AT1 receptor mediates the release of vasodilator molecules by endothelial cells and could modulate Ang II effect on smooth muscle cells. Ang II may also inhibit endothelial cell growth via the AT2 receptor. Finally, endothelial Ang II receptors may be implicated in the regulation of fibrinolysis.

Effects of angiotensin II on cardiac function and peripheral vascular structure during compensated heart failure in the rat

The present experiments were designed to test the hypothesis that the activation of the renin-angiotensin system during compensated heart failure may have adverse effects on cardiac function and change the peripheral vascular structure. ANG II (250 ng/kg/min) or saline (0.9% NaCl) were infused in myocardial-infarcted and sham-operated rats. After 2 weeks, cardiac function and peripheral vascular changes were investigated.

RESULTS

ANG II infusion reduced baseline cardiac index in sham rats but did not further reduce this index in ANG II-infused MI rats. Total peripheral resistance was similarly increased in ANG II-infused infarcted and sham rats, and also plasma ANG II concentrations were comparable. ANG II elevated systolic blood pressure by approximately 70 mm Hg in sham rats and increased the medial cross-sectional area of the superior mesenteric artery by 33%. However, ANG II infusions in MI rats resulted in only a minor increase in blood pressure, whereas the cross-sectional area of the superior mesenteric artery did not change. ANG II infusion had no effect on vessel dimensions of the resistance arteries of the pulmonary and mesenteric vascular bed of either group. Calculated ED50 and peak pressor response to acute ANG II injections were comparable in all groups, confirming the presence of functionally intact AT1 receptors. The increases in plasma atrial natriuretic peptide (ANP) and nitric oxide (NO) synthase activity (estimated by aortic cyclic GMP concentrations) were higher in ANG II-infused MI rats than in ANG II-infused sham rats.

CONCLUSION

ANG II infusion in rats with and without MI has comparable negative effects on cardiac function but has different effects on blood pressure and vascular structure. The concomitant increases in plasma ANP and NO synthase activity in ANG II-infused MI rats suggest that the growth stimulatory and hypertensive actions of ANG II in sham rats may be counter-regulated by activation of inhibitory neurohumoral systems such as ANP or NO in MI rats.

Angiotensin II-elicited signal transduction via AT1 receptors in endothelial cells

1. Angiotensin II (AII) actions are mediated by two distinct types of receptors: AT1, which includes two subtypes, AT1A and AT1B, and AT2. AII produces vasoconstriction on the vascular wall acting directly on smooth muscle cells via AT1 receptors. AII receptors have recently been demonstrated on endothelial cells. But the pharmacological characteristics of these receptors and the intracellular signal pathways coupled to them remain unclear. 2. The aim of this work was to characterize the AII receptor subtypes in rat aortic endothelial cells (RAEC) in primary culture and to evaluate the signal pathways coupled to these receptors by measuring the activation of phospholipase C (PLC) and phospholipase A2 (PLA2). 3. Labelled AII bound to RAEC in a specific, saturable manner. Scatchard analysis showed a Kd of 1.87 +/- 0.49 nM and a Bmax of 50.2 +/- 10.9 x 10(3) sites per cell. AII was displaced by the AT1-specific antagonist, DuP753 with a Ki of 17.37 +/- 1.49 nM, but not by the AT2 receptor analogues CGP42771B or PD123177. These data were confirmed by the finding of AT1 mRNA in endothelial cells. Analysis of RNA expression by RT-PCR showed the presence of both subtypes, AT1A and AT1B in endothelial cells, whereas smooth muscle cells express only AT1A. 4. The activation of PLC and PLA2 in response to AII was evaluated by measuring inositol phosphate production and arachidonic acid release, respectively. Both were enhanced by AII in a dose-dependent manner, and inhibited by DuP753, but not by PD123177. 5. We conclude that AT1 receptors are expressed by endothelial cells in primary culture and that phospholipase C and phospholipase A2 activated via this receptor.

Reciprocal modulation of the binding of angiotensin agonists and antagonists to angiotensin receptors in smooth muscle

1. Direct ligand binding studies have shown that the agonist 125I-[Sar1]Ang II and the antagonist 125I-[Sar1Ile8]Ang II bind to bovine uterus smooth muscle membranes in a time-dependent, reversible and saturable manner; both ligands had the same number of high affinity sites. 2. [Sar1Ile8]Ang II inhibited the binding of 125I-[Sar1]Ang II in a non-competitive manner by decreasing the number of high affinity sites without changing the binding affinity of the radioligand. 3. [Sar1]Ang II also inhibited the binding of 125I-[Sar1Ile8]Ang II in a non-competitive manner. 4. Dissociation of both radioligands from their receptor sites was fast enough that pseudo irreversible occupancy of the binding sites could not account for the observed non-competitive inhibition. 5. Displacement studies using 125I-[Sar1Ile8]Ang II as the radioligand provided evidence for the existence of two binding sites when the displacing ligand was [Sar1]Ang II but not when the displacing ligand was [Sar1Ile8]Ang II. 6. GTPS gamma S had no discernible effect on the binding of either 125I-[Sar1]Ang II or 125I-[Sar1Ile8]Ang II to bovine uterine membranes. 7. The present findings are consistent with an allosteric mechanism of antagonism for [Sar1Ile8]Ang II. The data are also consistent with a mechanism wherein agonist and antagonist ligands occupy different binding modes at the same receptor site and induce long-term conformational changes in the receptor which are idiosyncratic with respect to the nature of the ligand. An emerging relationship between the actions of angiotensin peptides and non-peptide mimetics of angiotensin is presented.

Effects of angiotensins on cellular hypertrophy and c-fos expression in cultured arterial smooth muscle cells

An increase in cell size and protein content was observed when quiescent arterial smooth muscle cells in culture were incubated with either angiotensin II or III. These effects were inhibited by the specific angiotensin type-1 receptor antagonist losartan (DuP753) but not by CGP42112A. In parallel, a transient and dose-dependent induction of c-fos was demonstrated not only with angiotensins II and III but also with angiotensin I. Both angiotensins II and III exerted their maximal effect at 1 microM, while angiotensin I needed a tenfold-higher concentration to exert an identical effect. As for hypertrophy, losartan also inhibits angiotensin-induced c-fos expression, suggesting that this gene may be involved into the hypertrophic process. Angiotensin-I-mediated c-fos induction is partially inhibited by the angiotensin-converting enzyme inhibitors captopril and trandolaprilate; given that an angiotensin-converting enzyme activity was detected in these smooth muscle cell cultures, these results suggest that angiotensin-I-induced c-fos expression is mediated in part via angiotensin-I conversion to angiotensin II, but also by other unidentified pathway(s). Angiotensin I could essentially induce smooth muscle cell hypertrophy by indirect mechanisms, while angiotensins II and III act directly on smooth muscle cells.

Radioiodinated CGP 42112A: a novel high affinity and highly selective ligand for the characterization of angiotensin AT2 receptors

CGP 42112A, a potent angiotensin AT2 receptor selective ligand, was radio-iodinated and its binding characteristics compared with those of [125I]angiotensin II. In human myometrium (only AT2 expressed), binding was saturable (Kd 1.03 x 10(-10) M; Bmax 807 fmol/mg) and reversible (K+1 1.89 x 10(8) M-1.min-1; K-1 3.77 x 10(-3) min-1). The order of potency of a number of peptides and non-peptides was the same as when [125I] angiotensin II was used as tracer. No specific binding could be detected on membranes from vascular smooth muscle cells (only AT1 expressed). In rat adrenal glomerulosa membranes (mixed AT1/AT2), [125I]CGP 42112A bound only to AT2. [125I]CGP 42112A can therefore be used as a specific probe for AT2 receptors and will be especially useful in tissues where other subtypes are also present.

Angiotensin II causes vascular hypertrophy in part by a non-pressor mechanism

Angiotensin II, when given in low doses, raises blood pressure slowly. When tested in vitro on vascular smooth muscle cells, it has mitogenic and trophic effects; it is not known if it has these effects in vivo. Our purpose was to determine whether vascular hypertrophy develops during slow pressor infusion of angiotensin II and, if so, whether it is pressure induced. Three experiments were done in rats infused subcutaneously with angiotensin II (200 ng/kg/min) by minipump for 10-12 days. Experiment 1: Angiotensin II gradually raised systolic blood pressure (measured in the tail) from 143 +/- 2 to 208 +/- 8 mm Hg (mean +/- SEM), significantly suppressing plasma renin and increasing threefold (NS) plasma angiotensin II. There was no loss of peptide in the pump infusate when tested at the end of the experiment. Experiment 2: In the perfused mesenteric circulation, vasoconstrictor responses to norepinephrine, vasopressin, and KCl were enhanced in rats given a slow pressor infusion of angiotensin II, but sensitivity of responses was not altered. This combination of changes suggests that vascular hypertrophy develops during slow pressor infusion of angiotensin II. Experiment 3: Vessel myography was done after angiotensin II infusion with and without a pressor response. Angiotensin II raised systolic blood pressure, increased heart weight, and produced myographic changes of vascular hypertrophy in the mesenteric circulation, increasing media width, media cross-sectional area, and media/lumen ratio. Hydralazine given with angiotensin II prevented the rise of pressure and the cardiac effect but not the vascular changes. Two-way analysis of variance showed that angiotensin II significantly increased media width, media cross-sectional area, and media/lumen ratio, all independent of hydralazine. Thus, although hydralazine inhibits the pressor and cardiac effects of angiotensin II, suggesting a pressor mechanism for the cardiac change, it does not inhibit structural vascular change, which suggests that at least part of the effect has a non-pressor mechanism.

The renin-angiotensin system and vascular and dipsogenic regulation in elasmobranchs

The role of a renin-angiotensin-like system (RAS) in the regulation of blood pressure and drinking has been investigated in the elasmobranch, Scyliorhinus canicula. Injection of exogenous angiotensin II produced, as expected, a vasopressor response, though injection of the converting enzyme inhibitor, Captopril, alone produced little change in resting blood pressure. Papaverine, a smooth muscle relaxant, reduced blood pressure which completely recovered within 30 min. A subsequent injection of Captopril produced a rapid vasodepressor response with no recovery over 2 hr. The low basal levels of drinking in dogfish were not altered by Captopril injection but angiotensin II-induced increased drinking and papaverine administration resulted in markedly stimulated water intake, which was inhibited by coadministration with Captopril. Captopril inhibition of the recovery in blood pressure and associated dipsogenic response following the papaverine-induced hypotension is consistent with the activation of a RAS-like system in the dogfish. This and other evidence supporting the presence of a RAS-like system in elasmobranchs are discussed in relation to other vertebrates.

Potassium depletion and regulation of angiotensin II receptors in vascular smooth muscle cells

Mesenteric artery smooth muscle cells were grown in culture media containing high, normal, or low concentrations of potassium to study the effects on angiotensin II (Ang II) receptor regulation. Cell growth was similar among cells grown in the different culture media. Cells grown in high potassium media (K = 5.8 mEq/L) had an equilibrium dissociation constant, Kd, of 1.59 +/- 0.2 nM, whereas those grown in normal potassium media (K = 4.1 mEq/L) had a Kd of 1.79 +/- 0.2 nM and those grown in a low potassium media (K = 2.9 mEq/L) had a Kd of 1.19 +/- 0.12 nM (not significantly different, NS). Binding capacity of smooth muscle cells grown in high potassium media was 81 +/- 16.7 fmol/mg prot, 95.1 +/- 12.4 fmol/mg prot in those grown in normal potassium media and those grown in low potassium media 86.4 +/- 24.1 fmol/mg prot (NS). Binding of radiolabelled Ang II was reduced by approximately 70% in cells exposed to unlabelled Ang II for 30 or 60 minutes. However, this effect of exposure to Ang II to reduce subsequent binding of Ang II was identical in cells grown in high and low potassium medium. Therefore, we were unable to identify a direct effect of low potassium to induce changes in Ang II receptor binding affinity or binding capacity. Previously observed changes in these Ang II binding parameters in potassium-depleted rats was probably a consequence of other factors which were simultaneously altered by potassium deficiency.

Localization and characterization of angiotensin II receptor binding and angiotensin converting enzyme in the human medulla oblongata

Angiotensin II receptor and angiotensin converting enzyme distributions in the human medulla oblongata were localised by quantitative in vitro autoradiography. Angiotensin II receptors were labelled with the antagonist analogue 125I-[Sar1, Ile8] AII while angiotensin converting enzyme was labelled with 125I-351A, a derivative of the specific converting enzyme inhibitor, lisinopril. Angiotensin II receptor binding and angiotensin converting enzyme are present in high concentrations in the nucleus of the solitary tract, the dorsal motor nucleus of vagus, the rostral and caudal ventrolateral reticular nucleus, and in a band connecting the dorsal and ventral regions. In the rostral and caudal ventrolateral reticular nucleus, angiotensin II receptors are distributed in a punctate pattern that registers with neuronal cell bodies. The distribution and density of these cell bodies closely resemble those of catecholamine-containing neurones mapped by others. In view of the known interactions of angiotensin II with both central and peripheral catecholamine-containing neurons of laboratory animals, the current anatomical findings suggest similar interactions between these neuroactive compounds in the human central nervous system. The presence of angiotensin II receptors and angiotensin converting enzyme in the nucleus of the solitary tract, dorsal motor nucleus of vagus, and rostral and caudal ventrolateral reticular nucleus demonstrates sites for central angiotensin II to exert its known actions on vasopressin release and autonomic functions including blood pressure control. These data also suggest a possible interaction between angiotensin II and central catecholeminergic systems.

Angiotensin II receptors in the fowl aorta

In the domestic fowl, angiotensin II (ANG II) causes an in vivo depressor response and in vitro relaxation of aortic rings which appear to be a direct action of ANG II on the blood vessels. Thus, we determined whether binding sites specific to ANG II exist in the membrane fraction of the fowl aorta. The particulate fraction of aortas from adult female fowl, Gallus gallus, exhibits high specific binding to ANG II ligand. 125I-[Ile5]ANG II (0.5 nM) binding to the above fraction (30 micrograms protein) in 50 mM Tris (pH 7.2), 10 mM MgCl2, and 0.2% bovine serum albumin at 12 degrees (1) is rapid, saturable, and reversible; (2) increases as a function of ligand or membrane concentration, time, and temperature; and (3) optimally fits to a two-site (high and low affinity) model. The equilibrium dissociation constant (0.15 +/- 0.03 nM) and binding site concentration (28.7 +/- 8.1 fmol/mg protein) of the high affinity site as well as association (0.055 nM-1.min-1) and dissociation (0.0122 min-1) rate constants are similar to those of mammalian vascular ANG II receptors. Both 125I-[Ile5]ANG II and 125I-[Val5]ANG II are competitively displaced by unlabeled ANG II. These results suggest that specific ANG II receptors exist in the fowl aorta.

Effect of vasodilator prostaglandins on the vascular renin-angiotensin system

The interaction of prostaglandin (PG) with the vascular renin-angiotensin (R-A) system was examined by studies on the effects of PGI2, PGE2 and the inhibitor of PG synthesis, indomethacin, on the release of angiotensin II (Ang II) from isolated rat mesenteric arteries. The Ang II released from the vasculature was measured after its concentration in a Sep-Pak C18 cartridge connected to the perfusion system. After perfusion with drugs, the specific vascular renin activity inhibited by anti-renin antibody was determined. The basal perfusion pressure was constant (19.6 +/- 1.1 mmHg) at a flow rate of 4.5 ml/min, and was not changed by any of these drugs. The basal levels of Ang II release and vascular renin activity were 44 +/- 5 pg/30 min and 113 +/- 8 pg Ang I/mg protein/hr, respectively. Infusion of PGI2 (10(-6) M) significantly decreased both Ang II release (p less than 0.01) and vascular renin activity (p less than 0.05) as compared with the control levels. Infusion of PGE2 (10(-6) M) decreased Ang II release significantly (p less than 0.05) and vascular renin activity slightly. Infusion of indomethacin (10(-6)M) increased vascular renin activity significantly (p less than 0.01). Pretreatment with indomethacin (10 mg/kg, ip) for 2 days also increased vascular renin activity (p less than 0.01). These results indicate that in contrast to their effects on the renal R-A system, PGs suppress the vascular R-A system and that these two local vasoactive factors interact to regulate vascular tone.

Are angiotensin receptors in vascular smooth muscles a homogeneous population?

The effects of angiotensin II (AII) and angiotensin III (AIII) on mean arterial pressure (MAP) and mean circulatory filling pressure (MCFP), an index of total body venous tone, in the presence and absence of [Sar1,Ile8]AII in conscious rats were examined. The infusion of AII caused dose-dependent increases in MAP and MCFP. The dose-response curves of MAP and MCFP for AII were displaced to the right in the presence of various doses of [Sar1,Ile8]AII. The pA2 values obtained for AII in the presence of the antagonist were 9.2 and 8.4 for the arterioles and veins respectively. The infusion of AIII also caused dose-dependent increases in MAP and MCFP. In the presence of the antagonist the AIII dose-response curves for MAP and MCFP were not displaced to right. The same maximum MAP was obtained for both AII and AIII but the maximum MCFP obtained following the infusion of AIII was smaller than that for AII. It is concluded that AII may act on different sub-classes of angiotensin receptors in arterioles and veins. AIII caused vasoconstriction in arterioles by acting on a sub-class of angiotensin receptors different from the ones activated by AII. AIII may act as a partial agonist on the same types of receptors as AIII in the venous bed.