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

The aim of the present study was to explore the mechanisms underlying angiotensin II AT2 receptor modulation of AT1 receptor-mediated vasoconstriction in the rat isolated uterine artery, since previous studies have suggested that AT2 receptors may oppose AT1 receptor-mediated effects. Segments of uterine artery were obtained from Sprague-Dawley rats and mounted in small vessel myographs. Concentration-response (CR) curves to angiotensin II (0.1 nm-0.1 microM) were constructed in the absence and presence of PD 123319 (AT2 antagonist; 1 microM), HOE 140 (bradykinin B2 antagonist; 0.1 microM), Nomega-nitro-l-arginine (NOLA) (NOS inhibitor; 30 microM), as well as combinations of these inhibitors. Contractile responses to angiotensin II were expressed as a percent of the response to a K+ depolarizing solution. PD 123319 (1 microM) potentiated angiotensin II-induced contractions; reflected by a significant four-fold leftward shift of the angiotensin II CR curve. HOE 140 (0.1 microM) significantly increased the pEC50 of the angiotensin II CR curve. The combination of HOE 140 plus PD 123319 did not produce additive potentiation. NOLA (30 microM) significantly enhanced sensitivity to angiotensin II, seen as a five-fold leftward shift of the curve, and an augmented maximum contractile response. Combinations of PD 123319 (1 microM) plus NOLA, and of HOE 140 (0.1 microM) plus NOLA, both induced a similar magnitude of potentiation. Cyclic GMP measurements confirmed angiotensin II-induced activation of the nitric oxide (NO) pathway. In conclusion, AT2 receptor-mediated inhibition of angiotensin II-induced contraction of the rat uterine artery involves NO production; a component of which occurs through a bradykinin B2 receptor pathway.     

Effect of cortisol on fetal ovine vascular angiotensin II receptors and contractility

The renin angiotensin system is important in the regulation of fetal blood pressure. This study investigated the expression of angiotensin AT(1) and AT(2) receptors in the ovine fetal heart, aorta and umbilical artery, and how these receptors are affected by cortisol. Cortisol infusion into the fetus has previously been shown to cause an increase in fetal blood pressure. We hypothesised that this effect of cortisol is mediated by upregulation of the angiotensin AT(1) receptor. Binding studies performed on tissues with intact endothelium demonstrated both receptor subtypes in the fetal aorta and right ventricle, although the latter contained mainly angiotensin AT(2) receptors. In contrast, only angiotensin AT(1) receptors were found in the umbilical artery. Cortisol infusion into fetuses (3 mg/day for 3-5 days) caused a physiological increase in plasma cortisol levels to 29+/-4 nM. This was associated with an increase in systolic pressure (57.8+/-1.7 vs. 52.2+/-1.5 mm Hg, P<0.05), but cortisol had no effect on the density or affinity of angiotensin receptors, nor on the in vitro contractile responses of carotid and umbilical arterial rings to 5-microM angiotensin II. In conclusion, this study has demonstrated differential expression of angiotensin AT(1) and AT(2) receptors in the different regions of the ovine fetal cardiovascular system and that the angiotensin AT(1) receptor is functional. The lack of any effect of low doses of cortisol on these receptors and on the contractility of isolated fetal vessels to angiotensin II suggests cortisol acts by other mechanisms to raise fetal arterial pressure.     

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

1. The aim of this study was to characterize the angiotensin II receptors in isolated uterine arteries from non pregnant and pregnant rats, since it has been reported from binding studies that ovine uterine arteries contain AT₂ receptors.
2. Uterine arterial segments were obtained from virgin, non-pregnant and late pregnant (18–21 days) Sprague-Dawley rats and mounted in small vessel myographs. Concentration-response curves were constructed to angiotensin II (1 nM–10 μM) in the absence and presence of various angiotensin II receptor subtype selective compounds. These included losartan (AT₁ antagonist; 1, 10 and 100 nM), PD 123319 (AT₂ antagonist; 1 μM) and CGP 42112 (AT₂ agonist; 1 μM). Responses to angiotensin II were measured as increases in force (mN) and expressed as a per cent of the response to a K⁺ depolarizing solution.
3. Losartan (1, 10 and 100 nM) caused significant concentration-dependent rightward shifts of the angiotensin II concentration-response curve in uterine arteries from non-pregnant and pregnant rats. The pA₂ values calculated from these data were 9.8 and 9.2, respectively, although the slope of the Schild plot in the non-pregnant group was less than unity.
4. PD 123319 (1 μM) caused significant 6- and 3 fold leftward shifts of the angiotensin II concentration-response curve in uterine arteries from non-pregnant and pregnant rats, respectively. In vessels from pregnant rats, PD 123319 also significantly increased the maximum response to angiotensin II.
5. CGP 42112 (1 μM) attenuated the response to angiotensin II of uterine arteries from non-pregnant rats. This was reflected by a 14 fold rightward shift of the angiotensin II concentration-response curve and a decrease in the maximum response. In uterine arteries from pregnant rats, CGP 42112 (1 μM) caused a 3 fold rightward shift of the angiotensin II concentration-response curve, but had no effect on the maximum response.
6. PD 123319 (1 μM) and CGP 42112 (1 μM) had no effect on the concentration-response curves to phenylephrine (PE) of uterine arteries from non-pregnant or pregnant rats. In addition, CGP 42112 (1 nM–1 mM) had no vasodilator effect on tissues precontracted with phenylephrine.
7. These results suggest that the contractile responses of the rat uterine artery are mediated by the AT₁ receptor. Furthermore, in this vascular preparation, the AT₂ receptor appears to inhibit the response mediated by the AT₁ receptor, although, this is not uniform between the non-pregnant and pregnant states.

     

Angiotensin receptor subtypes in the uterine artery during ovine pregnancy

This study was undertaken to determine if changes in receptor density or affinity could account for the reduced vascular sensitivity to angiotensin II seen during pregnancy. Angiotensin receptor subtypes in the uterine arteries of non-pregnant, pregnant and postpartum ewes were investigated using saturation and competition receptor binding techniques with the specific receptor antagonists, losartan (DuP-753) and PD-123319 (S)1-[[4-(dimethylamino)-3-methylphenyl]-methyl]-5-(diphenylacetyl)-4,5,6,7-tetrahydro-1H-imidazo(4,5-c)pyridine-6-carboxylic acid, ditrifluoroacetate, monohydrate). Receptor density and affinity of total angiotensin receptors, as well as the angiotensin AT₁ and AT₂ receptor subtypes in uterine arteries were compared with those in the mesenteric artery and aorta. The uterine artery contains both AT₁ and AT₂ receptor subtypes, whereas the mesenteric artery and aorta contain primarily the AT₁ receptor subtype. In uterine arteries from pregnant sheep, angiotensin receptor density was increased because AT₂ receptors were increased. AT₁ receptor density was not altered. This change was not seen in the aorta. In the uterine artery, receptor affinity for [Sar¹,Ile⁸]angiotensin II decreased in mid-gestation (IC₅₀ 7.7±1.2×10⁻⁹ M) compared with non-pregnant ewes (IC₅₀ 3.0±0.6×10⁻⁹ M, P=0.006), and there was decreased affinity of angiotensin AT₁ receptors for losartan during pregnancy (IC₅₀ 2.8±1.0×10⁻⁴ M) compared with non-pregnant ewes (IC₅₀ 2.2±1.3×10⁻⁶ M, P=0.025). Our results show changes in the density and affinity of the angiotensin receptor subtypes in the uterine artery which could explain its reduced responsiveness to circulating angiotensin II during pregnancy.     

Lack of heterologous receptor desensitization induced by angiotensin II type 1 receptor activation in isolated normal rat thoracic aorta

We tested whether heterologous receptor desensitization induced by activation of AT1 receptors may explain the purported relaxation produced by angiotensin II in normal rat aorta. Also, the role for AT2 receptors in the promotion of vasodilation was studied. In endothelium-intact and endothelium-denuded aortic rings, angiotensin II elicited biphasic contractions, which were significantly depressed when repeated in each tissue. Angiotensin II produced biphasic responses on phenylephrine preconstricted endothelium-intact and endothelium-denuded tissues, without reducing precontractile tone. These responses were abolished in the presence of the AT1 receptor antagonist losartan, but no relaxing responses to angiotensin II were uncovered. PD123319 did not influence angiotensin II responses in endothelium-intact tissues precontracted with phenylephrine; thus, under AT2 receptors blockade the contractile effects of angiotensin II were not overexposed. In conclusion, angiotensin II-induced biphasic responses can be attributed to AT1 receptors activation and rapid desensitization with time. Desensitization proved to be homologous in nature, since precontractile tone induced by phenylephrine was not depressed by angiotensin II (i.e., angiotensin II did not induce heterologous α1-adrenergic receptors desensitization). We found no functional evidence of the participation of AT2 receptors in angiotensin II elicited biphasic contractions. Angiotensin II does not exert relaxant effects in normal rat aorta.     

Effects of ACTH-induced hypertension in the pregnant ewe

Adrenocorticotropic hormone (ACTH; 5 microg/kg/ day) infused into 10 pregnant ewes (gestation age, 127-139 days) for 72 h caused an increase in arterial pressure within 1-2 h (p < 0.05), which was sustained for the rest of the experiment. Cardiac output was increased at 24 h (p < 0.05). Total peripheral resistance did not change. There were no changes in four pregnant ewes infused with 0.15 M saline at the same rate for 72 h. In ACTH-treated pregnant ewes, a relation between arterial pressure and plasma renin activity observed in nontreated pregnant ewes (r = 0.71; p = 0.0005) was no longer evident. Compared with nonsurgical pregnant ewes, total angiotensin II (Ang II)-receptor density in the uterine artery was decreased in ewes that had previously had surgery (p = 0.015) and further reduced in ACTH-treated ewes (p < 0.0005). This was due to a reduction in the AT2-receptor density, which was inversely related to plasma cortisol levels (r = 0.73; p < 0.03). AT1-receptor density and the affinities of the AT1 and AT2 receptors were unchanged. The correlation between plasma cortisol and AT2-receptor density in uterine blood vessels may partly explain why these receptors are downregulated after surgery.     

Increased availability of angiotensin AT 1 receptors leads to sustained arterial constriction to angiotensin II in diabetes - role for Rho-kinase activation

BACKGROUND AND PURPOSE

Antagonists of angiotensin AT₁ receptors elicit beneficial vascular effects in diabetes mellitus. We hypothesized that diabetes induces sustained availability of AT₁ receptors, causing enhanced arterial constriction to angiotensin II.

EXPERIMENTAL APPROACH

To assess functional availability of AT₁ receptors, constrictions to successive applications of angiotensin II were measured in isolated skeletal muscle resistance arteries (∼150 µm) of Zucker diabetic fatty (ZDF) rats and of their controls (+/Fa), exposed acutely to high glucose concentrations (HG, 25 mM, 1 h). AT₁ receptors on cell membrane surface were measured by immunofluorescence.

KEY RESULTS

Angiotensin II-induced constrictions to first applications were greater in arteries of ZDF rats (maximum: 82 ± 3% original diameter) than in those from +/Fa rats (61 ± 5%). Constrictions to repeated angiotensin II administration were decreased in +/Fa arteries (20 ± 6%), but were maintained in ZDF arteries (67 ± 4%) and in +/Fa arteries vessels exposed to HG (65 ± 6%). In ZDF arteries and in HG-exposed +/Fa arteries, Rho-kinase activities were enhanced. The Rho-kinase inhibitor, Y27632 inhibited sustained constrictions to angiotensin II in ZDF arteries and in +/Fa arteries exposed to HG. Levels of surface AT₁ receptors on cultured vascular smooth muscle cells (VSMCs) were decreased by angiotensin II but were maintained in VSMCs exposed to HG. In VSMCs exposed to HG and treated with Y27632, angiotensin II decreased surface AT₁ receptors.

CONCLUSIONS AND IMPLICATIONS

In diabetes, elevated glucose concentrations activate Rho-kinase which inhibits internalization or facilitates recycling of AT₁ receptors, leading to increased functional availability of AT₁ receptors and sustained angiotensin II-induced arterial constriction.     

ANGIOTENSIN AND THE CARDIAC BAROREFLEX RESPONSE TO PHENYLEPHRINE

1. In adult conscious sheep the pressor actions of infused angiotensin II were prevented by the concomitant intravenous infusions of sodium nitroprusside. The effect of such intravenous infusions of angiotensin II on the cardiac baroreflex response to transient rises in arterial pressure caused by intravenous phenylephrine was studied. 2. Intravenous infusion of angiotensin II caused a reduction in pulse interval in the absence of any change in arterial pressure. It also caused a reduction in baroreflex sensitivity measured by determining the relation between pulse interval and systolic pressure during the rise in pressure caused by injection of phenylephrine. 3. After administration of the converting enzyme inhibitor (captopril) the mean baroreflex sensitivity of 3 of 4 pregnant ewes increased. 4. It is concluded that high levels of angiotensin II can modify the cardiac baroreceptor reflex response so that the heart rate is inappropriately high for a given systolic pressure and that it reduces the sensitivity of the cardiac baroreflex response to transient changes in arterial pressure.     

Characterization of angiotensin II antagonism displayed by Ib, a novel nonpeptide angiotensin AT(1) receptor antagonist

The pharmacologic profile of Ib, 5-n-butyl-4-{4-[2-(1H-tetrazole-5-yl)-1H-pyrrol-1-yl]phenylmethyl}-2,4-dihydro-2-(2,6-dichloridephenyl)-3H-1,2,4-triazol-3-one, a novel nonpeptide angiotensin AT(1) receptor antagonist, was investigated by receptor-binding studies, functional in vitro assays with rabbit and rat aorta, and in vivo experiments in rats. Ib inhibited [(125)I] angiotensin II binding to AT(1) receptors in rat liver membranes (K(i)=2.5+/-0.5 nM) and did not interact with AT(2) receptors in bovine cerebellar membranes. In functional studies with rat and rabbit aorta, Ib inhibited the contractile response to angiotensin II (pD(2)' value: 7.43 and 7.29, respectively) with a significant reduction in the maximum. In pithed rats, Ib inhibited the angiotensin II induced pressor response in a dose-related manner. After intravenous administration, Ib produced a dose-dependent antihypertensive effects in spontaneously hypertensive rats and renal hypertensive rats. These results suggest that Ib is a potent angiotensin AT(1) selective receptor antagonist with a mode of insurmountable antagonism.     

High glucose concentration augments angiotensin II mediated contraction via AT1 receptors in rat thoracic aorta

Elevated glucose concentration is implicated to play major role in development of diabetic associated vascular complications. It was previously reported that angiotensin II (Ang II) induced contractile response is enhanced in thoracic aorta of diabetic rats. In the present study, the effect of high glucose (HG, 25 mM) exposure for 2h on Ang II cumulative concentration response curves recorded isometrically was studied in thoracic aortic rings isolated from male Sprague-Dawley rats pretreated with streptozotocin (STZ, 65 mg kg(-1) i.p.) or vehicle at 8 weeks prior to the study. Ang II induced contraction via AT1 receptor was significantly enhanced (by 60 +/- 2 %) in HG exposed thoracic aortic rings isolated from vehicle treated but not STZ treated rats. However, there was no change in the pD2 of Ang II while potassium chloride (KCl) induced contraction was unaltered. Ang II induced contractile response was blocked by valsartan (100 microM, selective AT1 receptor antagonist) but not PD 123,319 (100 microM, selective and potent AT2 receptor antagonist). Exposure of aortic rings from control rats to 25 mM mannitol or sucrose for 2 h did not have any effect on the Ang II induced contraction. Tempol (100 microM, a cell permeable superoxide dismutase mimetic) partially reduced the augmented Ang II response in HG exposed aortic rings, while it did not affect the Ang II responses in normal glucose (NG 5.5 mM) exposed aortic rings isolated from control rats. [3H] Ang II binding at AT1 receptors was unaltered in vascular smooth muscle membranes prepared from thoracic aorta exposed to HG for 2 h compared to NG exposed aortic rings. From our results, we conclude that high glucose concentration augments Ang II mediated contraction via AT1 receptors and reactive oxygen species partly contribute to this augmented contraction.     

AT(2) receptor-dependent vasodilation is mediated by activation of vascular kinin generation under flow conditions

Physiological roles of angiotensin II type 2 receptor (AT(2)) are not well defined. This study was designed to investigate the mechanisms of AT(2)-dependent vascular relaxation by studying vasodilation in pressurized and perfused rat mesenteric arterial segments. Perfusion of angiotensin II in the presence of AT(1) antagonist elicited vascular relaxation, which was completely dependent on AT(2) receptors on endothelium. FR173657 (>1 microM), a bradykinin (BK) B(2)-specific antagonist, significantly suppressed AT(2)-dependent vasodilation (maximum inhibition: 68.5% at 10 microM). Kininogen-deficient Brown Norway Katholiek rats showed a significant reduction in AT(2)-mediated vasodilatory response compared with normal wild-type Brown Norway rats. Indomethacin (>1 microM), aprotinin (10 microM) and soybean trypsin inhibitor (10 microM) also reduced AT(2)-dependent vasodilation. Our results demonstrated that stimulation of AT(2) receptors caused a significant vasodilation through local production of BK in resistant arteries of rat mesentery in a flow-dependent manner. Such vasodilation counterbalances AT(1)-dependent vasoconstriction to regulate the vascular tone.     

Pharmacological characterization of KR-30988, a novel non-peptide AT1 receptor antagonist, in rat, rabbit and dog

The pharmacological profile of KR-30988, a non-peptide AT1-selective angiotensin receptor antagonist, has been investigated by use of a variety of experimental models in-vitro and in-vivo. KR-30988 inhibited the specific binding of [125I][Sar1, Ile8]-angiotensin II to the recombinant AT1 receptor from man with a potency similar to that of losartan (IC50 values, the concentrations of drugs displacing 50% of specific binding, 13.6 and 12.3 nM, respectively), but did not inhibit the binding of [125I]CGP 42112A to recombinant AT2 receptor from man (IC50 >10 microM for both drugs). Scatchard analysis showed that KR-30988 interacted competitively with recombinant AT1 receptor from man in the same manner as losartan. In functional studies with rat and rabbit aorta, KR-30988 noncompetitively inhibited the contractile response to angiotensin II (pD2, = -log EC50 (where EC50 is the dose resulting in 50% of a reference contraction), 8.64 and 7.73, respectively) with a 20-85% decrease in the maximum contractile responses, unlike losartan. In pithed rats intravenous KR-30988 resulted in a non-parallel shift to the right of the dose-pressor response curve to angiotensin II (ID50 value, the dose inhibiting the pressor response to angiotensin II by 50%, 0.09 mg kg(-1)) with a dose-dependent reduction in the maximum responses; in this antagonistic effect KR-30988 was 20 times (approx.) more potent than losartan (ID50 1-74 mg kg(-1)). In conscious renal hypertensive rats oral administration of KR-30988 produced a dose-dependent and long-lasting (>24 h) anti-hypertensive effect; the potency was six times that of losartan (ED30 values, the dose reducing mean arterial blood pressure by 30 mmHg, 0.48 and 2.97 mg kg(-1), respectively). In conscious furosemide-treated dogs oral administration of KR-30988 produced a dose-dependent and long-lasting (>8 h) hypotensive effect with a rapid onset of action (time to Emax, the maximum effect, 1-2 h); KR-30988 was eight times more potent than losartan (ED20, the dose reducing mean arterial blood pressure by 20 mm Hg, 1.04 and 7.96 mg kg(-1), respectively). These results suggest that KR-30988 is a potent, orally active selective AT1 receptor antagonist with a mode of insurmountable antagonism.     

The kinin B1 receptor antagonist des-Arg9-[Leu8]bradykinin: an antagonist of the angiotensin AT1 receptor which also binds to the AT2 receptor.

1. Agonists and antagonists of kinin B1 and B2 receptors were evaluated in vitro for their effects against angiotensin II (AII)-induced contractile responses in the rabbit aorta and for their binding properties to angiotensin AT1 and AT2 receptors from purified membrane of rat liver and lamb uterus respectively. 2. In aortic rings, the kinin B1 receptor antagonist, des-Arg9-[Leu8]bradykinin (BK) (3-100 microM) caused a concentration-dependent decrease in sensitivity and a depression of the maximum response to AII. Des-Arg10-[Leu9]kallidin (KD), des-Arg9-BK, des-Arg10-KD, BK or KD at 3 microM had no effect against AII-induced contractions. 3. Des-Arg9-[Leu8]BK (3 or 100 microM) did not affect contractions of aortic rings to histamine, potassium chloride, endothelin-1, 5-hydroxytryptamine, noradrenaline and the thromboxane A2-mimetic, U46619. 4. Des-Arg9-[Leu8]BK displaced [125I]-Sar1-AII binding to the AT1 subtype in rat liver membranes with a Ki value of 1.1 +/- 0.4 microM. Values of Ki for des-Arg9-BK and KD were 45 +/- 13 microM and 25 +/- 22 microM, respectively. The other kinin derivatives des-Arg10-KD, BK and des-Arg10-[Leu9]KD at concentrations up to 100 microM did not bind to the AT1 receptor. 5. All the kinin derivatives except BK bound to AT2 receptors in lamb uterus membranes. Values of Ki for des-Arg9-[Leu8]BK, des-Arg10-[Leu9]KD, des-Arg9-BK, des-Arg10-KD and KD were 0.3 +/- 0.1, 0.7 +/- 0.1, 1.2 +/- 0.3, 1.5 +/- 0.3 and 7.0 +/- 1.6 microM, respectively.(ABSTRACT TRUNCATED AT 250 WORDS)     

Chronic hypotensive effects of losartan are not dependent on the actions of angiotensin II at AT 2 receptors.

We have previously demonstrated the chronic hypotensive effects of the AT1 antagonist, losartan, in normotensive, salt-replete rats. One explanation for this response is a reduction in vascular resistance due to blockade of AT1 receptors. Another explanation is that increases in angiotensin II levels during losartan administration can bind to AT2 receptors. Studies suggest that binding of angiotensin II at AT2 receptors lowers arterial pressure by vasodilation. We hypothesized that the chronic effects of losartan are mediated by activation of angiotensin II effects at AT2 receptors. We tested this hypothesis by infusing the AT2 receptor antagonist, PD123319 (74 mg/kg/day), in conjunction with losartan (10 mg/kg/day) for 10 days in rats and compared the hypotensive response in rats treated with losartan alone. After 6 days of treatment, arterial pressure decreased similarly in losartan (-21 +/- 2 mm Hg) and losartan+PD123319 (-23 +/- 2 mm Hg) treated rats. However, by day 10 of the infusion, arterial pressure had decreased to a greater extent (p < 0.05) in rats treated with losartan and PD123319 (-31 +/- 2 mm Hg) compared with rats treated with losartan alone (-22 +/- 2 mm Hg). We conclude that the hypotensive effects of losartan are not dependent on the actions of angiotensin II at AT 2 receptors in normotensive, salt-replete rats.     

Homologous desensitization of the effects of endothelin on rabbit aorta rings and on cultured rat aorta smooth muscle cells

The effects of endothelin-1 on normal and everted rabbit aorta rings and on cultured rat aortic smooth muscle cells were studied. Endothelin-1 (40 nM) contracted both normal and everted rings, and was still able to induce a prolonged contraction in Ca2(+)-free medium. Treatment of cultured cells with 100 nM endothelin-1 caused a sharp transient increase in 45Ca2+ efflux. The contractile and 45Ca2+ responses showed severe and specific desensitization towards endothelin-1; the responses to angiotensin II and adrenaline were not affected. The contractile responses to endothelin-1 were not affected by previous treatment with angiotensin II or with the angiotensin receptor antagonist, [Sar1,Ala8]angiotensin II. It is concluded that endothelin-1 acts on its own specific receptors to elicit Ca2+ mobilization from both intracellular and extracellular sources.     

Withdrawal of magnesium enhances coronary arterial spasms produced by vasoactive agents.

1 The influence of external magnesium ions ([Mg2+]o) on the sensitivity (i.e. EC50) and contractility (maximum response) of isolated large and small coronary arteries of the dog, obtained from different regions of the myocardium, to vasoactive agents was studied. 2 Removal of [Mg2+]o from the physiological salt solution enhanced, while elevation in [Mg2+]o to 4.8 mM, lowered the contractile sensitivity to three different agents, 5-hydroxytryptamine (5-HT), angiotensin II and KCl. 3 Contractility, of both large and small coronary arteries, to 5-HT and angiotensin II was potentiated and depressed, respectively, by withdrawal and elevation of [Mg2+]o; maximum responses to KCl were not altered by 0 or 4.8 mM [Mg2+]o. 4 Cumulative concentration-contractile effect curves to CaCl2 were shifted leftward on removal of [Mg2+]o; elevation of [Mg2+]o to 4.8 mM shifted the CaCl2 concentration-effect curves to the right. Maximal contractile responses to CaCl2 were enhanced by removal of, and reduced by elevation of, [Mg2+]o. 5 The calcium channel blocking agent, verapamil (10(-6)M), inhibited completely contractile responses to KCl; contractile responses elicited by angiotensin II and 5-HT were attenuated by verapamil. 6 A variety of pharmacological antagonists (phentolamine, propranolol, methysergide, atropine, diphenhydramine), as well as use of a prostaglandin cyclo-oxygenase inhibitor, did not modify the altered contractile responses evoked by angiotensin II or KCl in different concentrations of Mg2+. 7 These results suggest: (1) [Mg2+]o may exert considerably greater influence on receptor-operated rather than membrane-potential sensitive channels involved in Ca2+ transport in coronary arterial smooth muscle; (2) Mg2+ interferes with the affinity (binding) of certain agonists (5-HT and angiotensin II) for their respective receptors in coronary vascular muscle; and (3) a functional pool of Ca2+ which is resistant to Ca2+-depletion, but accessible to activation by 5-HT and angiotensin II is present in canine coronary arterial smooth muscle.     

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

1. Renal medullary blood flow is relatively insensitive to angiotensin II (Ang II)-induced vasoconstriction, due partly to AT(1)-mediated release of nitric oxide and/or prostaglandins. AT(2)-receptor activation appears to blunt AT(1)-mediated vasodilatation within the medullary circulation. This could affect long-term efficacy of antihypertensive pharmacotherapies targeting the renin/angiotensin system, particularly in Ang II-dependent forms of hypertension. 2. We tested the effects of AT(1)- and AT(2)-receptor blockade on basal cortical and medullary laser Doppler flux (CLDF and MLDF), and on responses to renal arterial infusion of Ang II, in rats with 2 kidney, 1 clip (2K1C) hypertension and sham-operated controls. Studies were carried out in thiobutabarbital (175 mg kg(-1), i.p.) anaesthetised rats, 4 weeks after clipping, or sham surgery (n=6 in each of eight groups). 3. Candesartan (10 microg kg(-1) h(-1), intravenous (i.v.)) reduced mean arterial pressure ( approximately 17%) and increased CLDF ( approximately 24%), similarly in both sham and 2K1C rats, but did not significantly affect MLDF. PD123319 (1 mg kg(-1) h(-1), i.v.) increased basal MLDF (19%) in 2K1C but not sham rats, without significantly affecting other variables. 4. In sham rats, renal arterial infusion of Ang II (1-100 ng kg(-1) min(-1)) dose dependently decreased CLDF (up to 44%), but did not significantly affect MLDF. These effects were markedly blunted in 2K1C rats. After PD123319, Ang II dose dependently increased MLDF (up to 38%) in sham but not 2K1C rats. Candesartan abolished all effects of Ang II, including those seen after PD123319. 5. Our data indicate that AT(1) receptors mediate medullary vasodilatation, which is opposed by AT(2)-receptor activation. In 2K1C hypertension, AT(2)-receptor activation tonically constricts the medullary circulation.     

Physiological actions of angiotensin II mediated by AT1 AND AT2 receptors in the brain

1. Autoradiographic binding studies have shown that the AT(1) receptor is the predominant angiotensin II (AngII) receptor subtype in the central nervous system (CNS). Major sites of AT(1) receptors are the lamina terminalis, hypothalamic paraventricular nucleus, the lateral parabrachial nucleus, rostral and caudal ventrolateral medulla, nucleus of the solitary tract and the intermediolateral cell column of the thoraco-lumbar spinal cord. 2. While there are differences between species, AT(2) receptors are found mainly in the cerebellum, inferior olive and locus coeruleus of the rat. 3. Circulating AngII acts on AT(1) receptors in the subfornical organ and organum vasculosum of the lamina terminalis (OVLT) to stimulate neurons that may have a role in initiating water drinking. 4. Centrally administered AngII may act on AT(1) receptors in the median preoptic nucleus and elsewhere to induce drinking, sodium appetite, a sympathetic vasoconstrictor response and vasopressin secretion. 5. Recent evidence shows that centrally administered AT(1) antagonists inhibit dipsogenic, natriuretic, pressor and vasopressin secretory responses to intracerebroventricular infusion of hypertonic saline. This suggests that an angiotensinergic neural pathway has a role in osmoregulatory responses. 6. Central angiotensinergic pathways which include neural inputs to the rostral ventrolateral medulla may use AT(1) receptors and play a role in the function of sympathetic pathways maintaining arterial pressure.     

Are multiple angiotensin receptor types involved in angiotensin (1-7) actions on isolated rat portal vein.

Angiotensin (1-7) [Ang (1-7)] is a bioactive component of the renin angiotensin system. Ang (1-7) may interact with angiotensin type 1 (AT1) or type 2 (AT2) receptors and with Ang (1-7) - specific receptors. We examined the interactions between different doses of Ang (1-7) (1 nM-1 microM) and angiotensin II (Ang II) (10 and 100 nM) on isolated rat portal vein. In endothelium-denuded portal vein rings, Ang (1-7) inhibited contractile effects induced by Ang II. The effects of Ang (1-7) were modified by indomethacin, N(G)-nitro-L-arginine methyl ester (L-NAME), (D-Ala7)-Angiotensin (1-7) (H-2888) and losartan. Our results suggest that on rat isolated portal vein rings without endothelium, Ang (1-7) reduces Ang II-induced contractions by acting mostly on Ang (1-7) specific receptors, and this effect is mediated by vasodilatatory prostaglandins. At high concentrations, Ang (1-7) effects are mediated by AT1-receptors, though to a lesser extent than by Ang (1-7) specific receptors.     

Vascular angiotensin AT2 receptors in hypertension and ageing

1. Angiotensin II (Ang II) acts on both subtype 1 receptors (AT1R) and subtype 2 receptors (AT2R). AT1R stimulation mediates all of the classical actions of Ang II including vasoconstriction and cardiovascular hypertrophy. By contrast, AT2R stimulation is thought to exert a counter-regulatory effect on AT1R and exert direct vasodilatation and antigrowth effects. 2. In this brief review, the vascular effects of AT2R are reviewed in the context of recent data relating to hypertension and ageing. 3. In particular the vascular AT2R phenotype in isolated mesenteric arteries may switch from that of relaxation to contraction in hypertension or ageing and this AT2R contractile phenotype can be converted to relaxation, at least with chronic antihypertensive treatment in spontaneously hypertensive rats. 4. In vivo data pertaining to hypertension and ageing are consistent with vasodilator and antiremodelling effects of AT2R.