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

Effect of angiotensin AT2 receptor stimulation on vascular cyclic GMP production in normotensive Wistar Kyoto rats

In the present study in normotensive Wistar Kyoto rats (WKY), we investigated whether any angiotensin II (ANG II) increases in vascular cyclic GMP production were via stimulation of AT(2) receptors. Adult WKY were infused for 4h with ANG II (30 ng/kg per min, i.v.) or vehicle (0.9% NaCl, i.v.) after pretreatment with (1) vehicle, (2) losartan (100 mg/kg p.o.), (3) PD 123319 (30 mg/kg i.v.), (4) losartan+PD 123319, (5) icatibant (500 microg/kg i.v.), (6) L-NAME (1 mg/kg i.v.), (7) minoxidil (3 mg/kg i.v.). Mean arterial blood pressure (MAP) was continuously monitored, and plasma ANG II and aortic cyclic GMP were measured at the end of the study. ANG II infusion over 4h raised MAP by a mean of 13 mmHg. This effect was completely prevented by AT(1) receptor blockade. PD 123319 slightly attenuated the pressor effect induced by ANG II alone (123.4+/-0.8 versus 130.6+/-0.6) but did not alter MAP in rats treated simultaneously with ANG II + losartan (113+/-0.6 versus 114.3+/-0.8). Plasma levels of ANG II were increased 2.2-3.7-fold by ANG II infusion alone or ANG II in combination with the various drugs. The increase in plasma ANG II levels was most pronounced after ANG II+losartan treatment but absent in rats treated with losartan alone. Aortic cyclic GMP levels were not significantly changed by either treatment. Our results demonstrate that the AT(2) receptor did not contribute to the cyclic GMP production in the vascular wall of normotensive WKY.

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

We investigated the effect of angiotensin II on intracellular cyclic GMP content and neurite outgrowth as an indicator of cell differentiation in PC12 W cells. Neurite outgrowth was examined by phase-contrast microscopy. Outgrown neurites were classified as small, medium and large, and were expressed as neurites per 100 cells. Angiotensin II (10-7 m) increased the outgrowth of medium and large neurites by mean +/- SEM 20.2 +/- 2.3 and 6.6 +/- 1.4 compared with 1.66 +/- 0.5 and 0.1 +/- 0.06 neurites per 100 cells in control. Cellular cyclic GMP content increased by 50-250% with angiotensin II at concentrations of 10-6-10-4 m. Both blockade of AT2 receptors and of nitric oxide synthase markedly reduced angiotensin II-induced neurite outgrowth and cyclic GMP production. In contrast, B2 receptor blockade had no effect or even increased these angiotensin II effects. Sodium nitroprusside and 8-bromo-cyclic GMP both mimicked the effects of angiotensin II on cell differentiation. The protein kinase G inhibitor KT-5823 inhibited the neurite outgrowth induced by both angiotensin II and 8-bromo-cyclic GMP. Our results demonstrate that angiotensin II can stimulate cell differentiation in PC12 W cells by nitric oxide-related and cyclic GMP-dependent mechanisms. The effects of angiotensin II on cell differentiation and cyclic GMP production were mediated via the AT2 receptor and further enhanced by bradykinin B2 receptor blockade.

Angiotensin II-induced modulation of endothelium-dependent relaxation in rabbit mesenteric resistance arteries

The role of local endogenous angiotensin II (Ang II) in endothelial function in resistance arteries was investigated using rabbit mesenteric resistance arteries. First, the presence of immunoreactive Ang II together with Ang II type-1 receptor (AT1R) and angiotensin converting enzyme (ACE) was confirmed in these arteries. In endothelium-intact strips, the AT1R-blocker olmesartan (1 microM) and the ACE-inhibitor temocaprilat (1 microM) each enhanced the ACh (0.03 microM)-induced relaxation during the contraction induced by noradrenaline (NA, 10 microM). Similar effects were obtained using CV-11974 (another AT1R blocker) and enalaprilat (another ACE inhibitor). The nitric-oxide-synthase inhibitor NG-nitro-L-arginine (L-NNA) abolished the above effect of olmesartan. In endothelium-denuded strips, olmesartan enhanced the relaxation induced by the NO donor NOC-7 (10 nM). Olmesartan had no effect on cGMP production (1) in endothelium-intact strips (in the absence or presence of ACh) or (2) in endothelium-denuded strips (in the absence or presence of NOC-7). In beta-escin-skinned strips, 8-bromoguanosine 3',5' cyclic monophosphate (8-Br-cGMP, 0.01-1 microM) concentration dependently inhibited the contractions induced (a) by 0.3 microM Ca2+ in the presence of NA+GTP and (b) by 0.2 microM Ca2++GTPgammaS. Olmesartan significantly enhanced, while Ang II (0.1 nM) significantly inhibited, the 8-Br-cGMP-induced relaxation. We propose the novel hypothesis that in these arteries, Ang II localized within smooth muscle cells activates AT1Rs and inhibits ACh-induced, endothelium-dependent relaxation at least partly by inhibiting the action of cGMP on these cells.

Angiotensin-converting enzyme inhibition but not angiotensin II type 1 receptor antagonism augments coronary release of tissue plasminogen activator in hypertensive patients

OBJECTIVES

We compared the effects of perindopril and losartan on endothelium-dependent coronary vasomotor and fibrinolytic function.

BACKGROUND

The renin-angiotensin system regulates the vascular fibrinolytic balance. However, the effects of angiotensin-converting enzyme inhibitors and angiotensin II type 1 receptor antagonists on coronary fibrinolytic function have not been compared in hypertensive patients.

METHODS

Forty-five patients with hypertension were randomly assigned to three groups: 16 patients were treated with perindopril (4 mg/day) for four weeks; 15 were treated with losartan (50 mg/day) for four weeks; and 14 were not treated with either perindopril or losartan (control group). Graded doses of bradykinin (BK) (0.2, 0.6, and 2.0 microg/min) were administered into the left coronary artery. Coronary blood flow (CBF) was evaluated by Doppler flow velocity measurement.

RESULTS

Bradykinin induced dose-dependent increases in CBF in all groups. The increases in CBF induced by BK in the perindopril and losartan groups were significantly greater than those in the control group. Net coronary tissue-type plasminogen activator (t-PA) release was enhanced by BK in all groups, and the increase in the perindopril group was greater than that in the losartan and control groups. Bradykinin did not alter plasminogen activator inhibitor type 1 levels in any of the groups.

CONCLUSIONS

Perindopril and losartan similarly augment BK-induced coronary vasodilation. Perindopril may have a greater potential to enhance the BK-induced coronary release of t-PA than losartan.

AT2 receptor activation regulates myocardial eNOS expression via the calcineurin-NF-AT pathway

The role of AT2-receptors has recently been subject of considerable debate. We investigated the influence of AT2-stimulation/inhibition on myocardial endothelial NO-synthase (eNOS, NOS-III) promoter activity and eNOS protein expression. Stimulation of rat cardiomyocytes with angiotensin II (AngII) increased eNOS protein expression 3.3-fold. This was blocked by Cyclosporin A (CsA). Inhibition of the AT1-receptor did not reduce AngII-mediated eNOS protein expression, whereas AT2 stimulation increased it 2.4-fold and AT2 inhibition suppressed it. The modulatory effects of the AT2-receptor on eNOS expression was confirmed in mice with a genetic deletion of the AT2-receptor (AT2-KO). In gel shift assays two putative NF-AT sites in a 1.6 kb eNOS promoter fragment showed NF-AT binding and a supershift by NF-AT2(-c1)-specific antibodies. Stimulation of transfected cells with AngII or specific AT2-receptor agonists resulted in a significant increase in eNOS promoter activity, which was blocked by CsA, MCIP1, and mutation of an upstream NF-AT site.

CONCLUSION

1) AngII-stimulation of the myocardium, both in vivo and in vitro, is accompanied by increased expression of eNOS. 2) This effect is mediated by the calcineurin pathway and is induced by the AT2-receptor. 3) These results define a calcineurin/NF-AT/eNOS pathway as downstream effector of AT2-receptor activation in the myocardium.

Possible involvement of the adipose tissue renin-angiotensin system in the pathophysiology of obesity and obesity-related disorders

Angiotensin II (Ang II), acting on the AT1 and AT2 receptors in mammalian cells, is the vasoactive component of the renin-angiotensin system (RAS). Several components of the RAS have been demonstrated in different tissues, including adipose tissue. Although the effects of Ang II on metabolism have not been studied widely, it is intriguing to assume that components of the RAS produced by adipocytes may play an autocrine, a paracrine and/or an endocrine role in the pathophysiology of obesity and provide a potential pathway through which obesity leads to hypertension and type 2 diabetes mellitus. In the first part of this review, we will describe the production of Ang II, the different receptors through which Ang II exerts its effects and summarize the concomitant intracellular signalling cascades. Thereafter, potential Ang II-induced mechanisms, which may be associated with obesity and obesity-related disorders, will be considered. Finally, we will focus on the different pharmaceutical agents that interfere with the RAS and highlight the possible implications of these drugs in the treatment of obesity-related disorders.

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.

NAD(P)H oxidase-derived hydrogen peroxide mediates endothelial nitric oxide production in response to angiotensin II

Recently, it has been shown that the exogenous addition of hydrogen peroxide (H(2)O(2)) increases endothelial nitric oxide (NO(.)) production. The current study is designed to determine whether endogenous levels of H(2)O(2) are ever sufficient to stimulate NO(.) production in intact endothelial cells. NO(.) production was detected by a NO(.)-specific microelectrode or by an electron spin resonance spectroscopy using Fe(2+)-(DETC)(2) as a NO(.)-specific spin trap. The addition of H(2)O(2) to bovine aortic endothelial cells caused a potent and dose-dependent increase in NO(.) release. Incubation with angiotensin II (10(-7) mol) elevated intracellular H(2)O(2) levels, which were attenuated with PEG-catalase. Angiotensin II increased NO(.) production by 2-fold, and this was prevented by Losartan and by PEG-catalase, suggesting a critical role of AT1 receptor and H(2)O(2) in this response(.) In contrast, NO(.) production evoked by either bradykinin or calcium ionophore was unaffected by PEG-catalase. As in bovine aortic endothelial cells, angiotensin II doubled NO(.) production in aortic endothelial cells from C57BL/6 mice but had no effect on NO(.) production in endothelial cells from p47(phox-/-) mice. In contrast, stimulated NO(.) production to a similar extent in endothelial cells from wild-type and p47(phox-/-) mice. In summary, the present study provides direct evidence that endogenous H(2)O(2), derived from the NAD(P)H oxidase, mediates endothelial NO(.) production in response to angiotensin II. Under disease conditions associated with elevated levels of angiotensin II, this response may represent a compensatory mechanism. Because angiotensin II also stimulates O(2)() production from the NAD(P)H oxidase, the H(2)O(2) stimulation of NO(.) may facilitate peroxynitrite formation in response to this octapeptide.

Local cardiac renin-angiotensin system: hypertension and cardiac failure

In addition to the effect on arterial pressure, angiotensin II, the effector peptide of the renin-angiotensin system (RAS), exerts mitogenic and growth promoting effects on cardiac myocytes and non-myocytic elements; and both of these effects significantly contribute to the development and progression of hypertensive heart disease (HHD). The traditional concept of the RAS as a systemic, endocrine system has been expanded and the identification of its components in many organs and tissue has been amassed. This paper reviews evidence that supports the concept that the cardiac RAS participate importantly in the development and risk of HHD.

Safety and efficacy of valsartan versus enalapril in heart failure patients

Although a cornerstone in the treatment of heart failure, angiotensin-converting enzyme inhibitors are under-used, partly due to side effects. If proven at least similarly efficacious to angiotensin-converting enzyme inhibitors, angiotensin-receptor blockers may replace them due to their superior tolerability. We aimed to compare the efficacy and safety of valsartan and enalapril in heart failure patients stabilised on an angiotensin-converting enzyme inhibitor. We randomised 141 patients (mean 68 years, 74% males) with stable mild/moderate heart failure and left ventricular ejection fraction 0.45 or less, to valsartan 160 mg q.d. (n=70) or enalapril 10 mg b.i.d. (n=71) for 12 weeks. Changes in 6-min-walk test (primary efficacy variable), patients' wellbeing and left ventricular size and function did not differ significantly between the treatment groups. Valsartan was significantly non-inferior to enalapril in walk test distance change: least-square means treatment difference +1.12 m (95% confidence interval -21.9 to 24.1), non-inferiority P<0.001. Left ventricular size (P<0.001) and function (P=0.048) improved significantly only in the valsartan group. Fewer patients experienced adverse events in the valsartan group (50%) than in the enalapril group (63%), although statistically non-significant. Valsartan is similarly efficacious and safe to enalapril in patients with stable, mild/moderate heart failure, previously stabilised on an angiotensin-converting enzyme inhibitor and directly switched to study medication.

Chronic angiotensin-converting enzyme inhibition and angiotensin II antagonism in rats with chronic renal failure

The current study was undertaken to compare the organ protective effects of an angiotensin-converting enzyme inhibitor, temocapril, with those of an angiotensin II type 1 receptor antagonist, CS-866 (olmesartan medoxomil), alone or combined, in the remnant kidney model of rats. Eight-week-old spontaneously hypertensive male rats were subjected to five-sixths nephrectomy. At the age of 10 weeks, the rats were randomly allocated to groups that received two doses of CS-866 (CS-L, 3 mg/kg/day; CS-H, 10 mg/kg/day), temocapril (TEM, 10 mg/kg/day), CS-866 (3 mg/kg/day) plus temocapril (10 mg/kg/day), or a vehicle alone (untreated control group). Systolic blood pressure (SBP) and urinary protein excretion (UprotV) were measured every 2 weeks. When the rats were 18 weeks old, biochemical measurement and histologic examination were performed. All the drug treatments significantly reduced SBP, UprotV, glomerular sclerosis index (GSI), relative interstitial volume (RIV), and heart weight. The hypotensive effects were on the order of combination therapy > CS-H = TEM > CS-L. Correlational analysis was based on the values for SBP and UprotV derived from the average of values obtained when the rats were 12 to 18 weeks of age. UprotV, GSI, and RIV were found to be highly correlated with SBP among the individual rats pooled from all the groups, and the correlation was maintained among the group means. A similar correlation was found between heart weight and SBP. The results suggest that the organ protective effects of temocapril, CS-866, and combination therapy are closely related to the magnitude of their antihypertensive effects.

Cross-talk between AT(1) and AT(2) angiotensin receptors in rat anococcygeus smooth muscle

Schild regressions for the selective AT(1) and AT(2) receptor antagonists, losartan and PD123319 (S-[+]-1-[(4-dimethylamino]-3-methylphenyl)methyl]-5-[diphenylacetyl]-4,5,6,7-tetrahydro-1H-imidazol[4,5-c]pyridine-6-carboxilic acid), respectively, were calculated to analyze the heterogeneity of receptor populations in the rat anococcygeus muscle. For a one-receptor system, the Schild regression has a slope of unity and an intercept of K(B) for competitive antagonists. However, in a two-receptor system, a deviation from the single-receptor plot will occur. This is predicated on the assumption that the secondary receptor is less sensitive to the antagonist than the primary receptor. Results showed that the Schild regression for losartan did not produce a slope of unity, and PD123319 did not produce any effect. However, tissue incubation with losartan plus PD123319 resulted in a Schild regression that has a slope of unity and a pK(B) of 9.32. In the presence of prazosin, an alpha(1)-adrenoceptor antagonist, losartan did not produce any effect. Conversely, PD123319 enhanced the angiotensin II (Ang II)-induced contraction in a concentration-dependent fashion, suggesting an inhibitory AT(2)-mediated effect. This effect was confirmed with assays that showed a relaxant response induced by Ang II on precontracted tissues incubated with prazosin. PD123319 and N(G)-nitro-L-arginine methyl ester [nitric-oxide (NO) synthase inhibitor)] markedly inhibited the relaxant response of Ang II. In contrast, losartan did not produce any significant effect. Consequently, results show that the mechanism underlying the AT(2)-mediated effect is highly dependent on NO generation. Results indicate the presence of a heterogeneous angiotensin receptor population in the rat anococcygeus muscle following a negative cross-talk relationship between the AT(1) and AT(2) subtypes.

Angiotensin II and nitric oxide interaction

Nitric oxide degradation linked to endothelial dysfunction plays a central role in cardiovascular diseases. Superoxide producing enzymes such as NADPH oxidase and xanthine oxidase are responsible for NO degradation as they generate a variety of reactive oxygen species (ROS). Moreover, superoxide is rapidly degraded by superoxide dismutase to produce hydrogen peroxide leading to the uncoupling of NO synthase and production of increased amount of superoxide. Angiotensin II is an important stimulus of NADPH oxidase. Through its AT(1) receptor, Ang II stimulates the long-term increase of several membrane component of NADPH oxidase such as P(22) phox or nox-1 and causes an increased activity of NADPH oxidase with inactivation of NO leading to impaired endothelium-dependent vasorelaxation, vascular smooth muscle cell hypertrophy, proliferation and migration, extracellular matrix formation, thrombosis, cellular infiltration and inflammatory reaction. Several preclinical and clinical studies have now confirmed the involvement of the AT(1) receptor in endothelial dysfunction. It is proposed that the AT(2) receptor counterbalances the deleterious effect of the Ang II-induced AT(1) receptor stimulation through bradykinin and NOS stimulation. This mechanism could be especially relevant in pathological cases when the NADPH oxidase activity is blocked with an AT(1) receptor antagonist.

Differences in AT2 -receptor stimulation between AT1 -receptor blockers valsartan and losartan quantified by renal interstitial fluid cGMP

OBJECTIVE

Angiotensin II-receptor blockers are an established class of antihypertensive agents, but the differences between individual members of the class are largely unknown. The present study employed an animal model to demonstrate angiotensin II-receptor blocker-specific effects and to quantify these differences by comparing two common agents, losartan and valsartan.

METHODS

We measured the effects on angiotensin II AT2-receptor-mediated renal cGMP by microdialysis in the outer renal cortex in conscious normotensive, sodium-depleted, 4-week-old Sprague-Dawley rats. Rats (n = 8) were given equimolar and equidepressor doses of losartan (0.02 mmol/kg) or valsartan (0.02 mmol/kg) either intravenously or orally. Time was allowed for the conversion of losartan into its active metabolite, EXP 3174.

RESULTS

Both drugs had equal effects on blood pressure. There were significantly greater increases in cGMP levels after administration of valsartan than of losartan with both routes of administration. Intravenous administration of valsartan led to a 69.1% increase in cGMP, versus a 10.3% increase with losartan. Five hours after oral administration of valsartan, a 48% increase in cGMP was observed versus a 10.9% increase with losartan. The increase after oral administration of valsartan was sustained 8 h after administration, whereas the effect of losartan was not sustained. The effects of losartan and valsartan on cGMP were completely inhibited by AT2-receptor blockade.

CONCLUSION

The results indicate that AT1-receptor blockade with valsartan influences AT2-receptor-mediated angiotensin II responses to a greater extent than with losartan, as quantified by renal interstitial fluid cGMP.

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.

Angiotensin AT2 receptor ligands: do they have potential as future treatments for neurological disease?

In addition to the systemic renin-angiotensin system (RAS), a local RAS has been identified. Recent research has focused on this latter system and has investigated the effects of locally generated angiotensin II, especially in the kidney, heart and CNS. In the mammalian brain, all components of the RAS are present including angiotensin AT(1) and AT(2) receptor subtypes. While the AT(1) receptor is responsible for the classical effects of angiotensin II, it has been found that the AT(2) receptor displays totally different signalling mechanisms and this has revealed hitherto unknown functions of angiotensin II. AT(2) receptors are expressed at low density in many healthy adult tissues, but are up-regulated in pathological circumstances, e.g. stroke or nerve lesion. Evidence has now emerged that the actions of angiotensin II that are exerted via the AT(2) receptor are directly opposed to those mediated by the AT(1 )receptor. For example, the AT(2) receptor has antiproliferative properties and therefore opposes the growth-promoting effect linked to AT(1) receptor stimulation. It has been reported that the AT(2) receptor regulates several functions of nerve cells, e.g. ionic fluxes, cell differentiation and axonal regeneration, but also modulates programmed cell death. It is possible that a more extensive knowledge of the AT(2) receptor could contribute to the understanding of the clinically beneficial effects of AT(1) receptor antagonists, as this treatment may unmask AT(2) receptor activity. This review presents selected aspects of advances in AT(2) receptor pharmacology, molecular biology and signal transduction with particular reference to possible novel therapeutic options for CNS diseases.

Effect of indomethacin on the antihypertensive efficacy of valsartan and lisinopril: a multicentre study

OBJECTIVE

To compare the effect on antihypertensive efficacy produced by the addition of indomethacin to the angiotensin II (Ang II) antagonist, valsartan, or to the angiotensin-converting enzyme inhibitor, lisinopril, in hypertensive patients with chronic osteoarthritis.

SUBJECTS AND METHODS

One hundred and twenty-eight patients (52 men and 76 women) aged 25-82 years (mean age 55.7 years), with diastolic blood pressure (DBP) > 100 mmHg at the end of a 2-week placebo washout period were allocated randomly to groups to receive valsartan (80-160 mg once daily) or lisinopril (10-20 mg once daily). At the end of 10 weeks of treatment, patients with DBP < 90 mmHg, while continuing to receive valsartan or lisinopril treatment, were allocated randomly to groups to receive either indomethacin (50 mg three times a day) or the corresponding placebo for 2 weeks, with a 1-week washout period between the two treatments, according to a double-blind, crossover design. After the initial washout period, patients were examined at the end of the 4th, 8th and 10th weeks of randomized treatment with valsartan and lisinopril, at the end of the first crossover period and then at the beginning and at the end of the second crossover period. At each visit, sitting and standing blood pressure were measured by standard mercury sphygmomanometer.

RESULTS

The addition of indomethacin blunted the blood pressure-decreasing effect of both antihypertensive drugs. Although indomethacin produced greater increases in both systolic and DBP values in the lisinopril-treated patients (5.45/3.22 mmHg) than in the valsartan-treated ones (2.12/1.87 mmHg), no significant difference between the two drugs was found.

CONCLUSIONS

From a theoretical standpoint, these findings suggest that prostaglandins may play a part in the antihypertensive action of Ang II antagonists. From a practical standpoint, hypertensive patients treated with valsartan or with lisinopril should be monitored to detect changes in blood pressure control while receiving indomethacin.

Pharmacodynamic studies on the angiotensin II type 1 antagonists irbesartan and candesartan based on angiotensin II dose response in humans

The in vivo effects of two unsurmountable angiotensin II type 1 (AT1) antagonists, irbesartan (150 mg) and candesartan (8 mg), were studied in a double-blind, randomized, crossover study in 18 healthy men. The drugs' direct vascular effects were assessed as the rightward shift (dose ratio - 1) of angiotensin dose-effect curves on diastolic blood pressure (DBP). Renal and adrenal effects were assessed by plasma renin activity (PRA), aldosterone concentrations, and antagonistic concentration equivalents (n x Ki) in a radioligand rat lung receptor assay. Both drugs exerted similar substantial (> 30-fold) and long-lasting (> 2-fold 47 h after dosing) rightward shifts of the angiotensin II dose effect declining with half-lives of 15 h irbesartan and 12 h candesartan, respectively. Dose ratio - 1 versus n x Ki showed a linear relationship in Schild regression plots; both drugs increased PRA, decreased DBP, and suppressed aldosterone. The slopes of linear relationship between angiotensin antagonism (dose ratio - 1) and PRA increase were almost threefold steeper (p = 0.005) following irbesartan as compared with candesartan. The findings suggest that for the same degree of angiotensin II antagonism, irbesartan produces a greater increase in PRA than candesartan. These pharmacodynamic differences warrant further investigation and clarification.

Effects of angiotensin II on the renal interstitial concentrations of NO2/NO3 and cyclic GMP in anesthetized rats

The present study was conducted to determine whether exogenous angiotensin II (Ang II) may increase the renal interstitial fluid concentrations of NO2/NO3 (NOx) and cyclic guanosine monophosphate (cGMP) concomitantly and which Ang II receptor subtypes may induce these changes in anesthetized rats, using a microdialysis method. Ang II (50 ng/kg per min, i.v.) significantly increased mean blood pressure (MBP), extraction rates of renal interstitial NOx from 23.9+/-1.0 to 31.2+/-1.9 pmol/min, and cGMP from 4.1+/-0.3 to 6.4+/-0.5 fmol/min, and decreased renal blood flow (RBF). The AT1-receptor antagonist CV11974 alone significantly increased RBF, but did not alter MBP, renal interstitial concentrations of NOx and cGMP. A superimposition of Ang II on CV11974 did not affect MBP and RBF, but significantly increased renal interstitial concentrations of NOx and cGMP. The AT2-receptor antagonist PD123319 alone did not change any of the parameters. However, superimposition of Ang II on PD123319 increased MBP and decreased RBF without any effects on renal interstitial concentrations of NOx and cGMP. These results suggest that Ang II stimulates NO production via the AT2-receptor in the kidney.

Effect of angiotensin II on pulse wave velocity in humans is mediated through angiotensin II type 1 (AT(1)) receptors

The objective of this study was to examine the effect of angiotensin II (Ang II) and angiotensin II type 1 (AT(1)) receptor blockade on pulse wave velocity (PWV) in healthy humans. We studied nine young male volunteers in a double-blind randomised crossover design. Carotid-femoral PWV (an index of arterial stiffness) was measured by using a Complior machine. Subjects were previously treated for 3 days with once-daily dose of either a placebo or valsartan 80 mg. On the third day, they were infused with either placebo or 5 ng/kg/min of Ang II over 30 min. Subjects thus received placebo capsule + placebo infusion (P), valsartan + placebo infusion (V), placebo + Ang II infusion (A), and valsartan + Ang II infusion (VA) combinations. Heart rate (HR), blood pressure and PWV were recorded at baseline and then every 10 min during infusion and once after the end of infusion. There were significant increases in systolic blood pressure (SBP), diastolic blood pressure (DBP) and mean arterial pressure (MAP) with A compared with P (P = 0.002, P = 0.002, P = 0.001 respectively). These rises in blood pressure were completely blocked by valsartan. A significant rise in PWV by A was seen compared with P (8.38 +/- 0.24 vs 7.48 +/- 0.24 m/sec, P = 0.013) and was completely blocked by valsartan; VA compared with P (7.27 +/- 0.24 vs 7.48 +/- 0.24 m/sec, P = NS). Multiple linear regression analysis showed that blockade of Ang II induced increase in blood pressure by valsartan contributed to only 30% of the total reduction in Ang II induced rise in PWV (R(2) = 0.306). The conclusions were that valsartan completely blocks the effect of Ang II on PWV. The effect of Ang II on PWV is mediated through AT(1)receptors. Reduction in PWV by Ang II antagonist is not fully explained by its pressure lowering effect of Ang II and may be partially independent of its effect on blood pressure.

Multiple interactions between the renin-angiotensin and the kallikrein-kinin systems: role of ACE inhibition and AT1 receptor blockade

The investigation of therapeutic actions of angiotensin type 1 (AT1) receptor antagonists and ACE inhibitors (ACEI) demonstrated complex interactions between the renin-angiotensin system (RAS) and the kallikrein-kinin system (KKS) in several experimental and clinical studies. They are evidenced by the fact that (1) ACE efficiently catabolizes kinins; (2) angiotensin-derivatives such as ANG-(1-7) exert kininlike effects; and (3) kallikrein probably serves as a prorenin-activating enzyme. (4) Several authors have demonstrated experimentally that the protective effects of ACEI are at least partly mediated by a direct potentiation of kinin receptor response on BK stimulation. (5) Furthermore, studies on AT1 antagonists, which do not directly influence kinin degradation, and studies on angiotensin-receptor transgenic mice have revealed additional interactions between the RAS and the KKS. There is mounting evidence that an autocrine cascade including kinins, nitric oxide, prostaglandins, and cyclic GMP is involved in at least some of the angiotensin type 2 receptor effects. This review discusses multiple possibilities of cross-talks between the RAS and KKS in vascular and cardiac physiology and pathology after ACE inhibition and AT1 receptor blockade.

The role of the renin-angiotensin system in the development of cardiovascular disease

A direct, continuous, and independent relation between blood pressure and the incidence of various cardiovascular events, such as stroke and myocardial infarction, is now well accepted. The increase in risk can be attributed to structural and functional changes in target organs. Central to many of these pathophysiologic processes is the renin-angiotensin system (RAS), specifically, angiotensin II. Binding of angiotensin II to angiotensin II type-1 (AT(1)) receptors produces acute vasoconstriction, leading to an increase in blood pressure. AT(1) receptor activation also contributes independently to chronic disease pathology by promoting vascular growth and proliferation, and endothelial dysfunction. These negative consequences of angiotensin II are partly counteracted by angiotensin II type-2 (AT(2)) receptor stimulation, which has favorable effects on tissue growth and repair processes. Thus, the use of selective AT(1) receptor antagonists in the treatment of hypertension has a 2-fold rationale: (1) selective AT(1) receptor blockade targets the final common pathway for all major detrimental cardiovascular actions of angiotensin II, and (2) circulating angiotensin II levels (which increase during AT(1) receptor antagonist treatment) will be free to act only at unopposed AT(2) receptors, potentially providing additional end-organ protection. Angiotensin-converting enzyme (ACE) inhibitors interrupt the RAS by preventing the conversion of angiotensin I to angiotensin II. They also increase plasma levels of bradykinin, which possesses vasodilatory and tissue-protective properties. The combination of an AT(1) receptor antagonist with an ACE inhibitor represents an appealing therapeutic strategy, because it should produce more complete blockade of the RAS, while preserving the beneficial effects mediated by AT(2) receptor stimulation and increased bradykinin levels.

Angiotensin peptides modulate bradykinin levels in the interstitium of the dog heart in vivo

We previously demonstrated the substantial capacity for angiotensin (ANG) II formation in the interstitium of the dog heart in vivo. The current study tested the hypothesis that interstitial fluid (ISF) bradykinin (BK) is influenced by ANG II formation. Four microdialysis probes were inserted into the left ventricular myocardium of eight open-chest anesthetized dogs. The probe effluent was collected during four stages in each dog. Probes 1 and 3 sequentially delivered: 1) buffer; 2) ANG I (15 microM); 3) ANG II type 1 receptor antagonist (AT(1)-ant; irbesartan, 50 microM) or AT(2)-ant (PD123319, 50 microM); and 4) ANG I + AT(1)-ant or ANG I + AT(2)-ant. Probes 2 and 4 used the same protocol, substituting ANG II for ANG I in a concentration (0.5 microM) equivalent to that achieved during ANG I infusion. ISF BK levels increased 15-fold during ANG I (p < 0.001) but not during ANG II infusion. Co-infusion of selective AT(1)- and AT(2)-ants or nonselective AT-ant did not block the increase in ISF BK. ISF infusions of ANG I also produced a greater than 400-fold rise in ISF ANG-(1-7) over baseline. ISF infusion of ANG-(1-7) (10 microM) produced a 15-fold increase in ISF BK (p < 0.001). The metabolic machinery exists for the formation of BK and ANG-(1-7) in the cardiac ISF space that is not blocked by an AT receptor antagonist. The differential increase in ISF BK during ANG I and ANG-(1-7) but not during ANG II infusions suggests the possibility of decreased catabolism of ISF BK by an angiotensin-converting enzyme due to active site occupation by ANG I and ANG-(1-7).

Angiotensin receptors: signaling, vascular pathophysiology, and interactions with ceramide

Angiotensin II (ANG II) is a pleiotropic vasoactive peptide that binds to two distinct receptors: the ANG II type 1 (AT(1)) and type 2 (AT(2)) receptors. Activation of the renin-angiotensin system (RAS) results in vascular hypertrophy, vasoconstriction, salt and water retention, and hypertension. These effects are mediated predominantly by AT(1) receptors. Paradoxically, other ANG II-mediated effects, including cell death, vasodilation, and natriuresis, are mediated by AT(2) receptor activation. Our understanding of ANG II signaling mechanisms remains incomplete. AT(1) receptor activation triggers a variety of intracellular systems, including tyrosine kinase-induced protein phosphorylation, production of arachidonic acid metabolites, alteration of reactive oxidant species activities, and fluxes in intracellular Ca(2+) concentrations. AT(2) receptor activation leads to stimulation of bradykinin, nitric oxide production, and prostaglandin metabolism, which are, in large part, opposite to the effects of the AT(1) receptor. The signaling pathways of ANG II receptor activation are a focus of intense investigative effort. We critically appraise the literature on the signaling mechanisms whereby AT(1) and AT(2) receptors elicit their respective actions. We also consider the recently reported interaction between ANG II and ceramide, a lipid second messenger that mediates cytokine receptor activation. Finally, we discuss the potential physiological cross talk that may be operative between the angiotensin receptor subtypes in relation to health and cardiovascular disease. This may be clinically relevant, inasmuch as inhibitors of the RAS are increasingly used in treatment of hypertension and coronary heart disease, where activation of the RAS is recognized.

Renin-angiotensin blockade improves renal cGMP production via non-AT(2)-receptor mediated mechanisms in hypertension-induced by chronic NOS inhibition in rat

BACKGROUND

To investigate the changes in the angiotensin II (Ang II) receptors and nitric oxide (NO)-cGMP pathway in the rat kidney after nitric oxide synthase (NOS)blockade.

METHODS

Captopril, an angiotensin-converting enzyme (ACE)inhibitor, 20 mg/100 ml; and/or L-158,809 (an Ang II AT1-receptor antagonist, 5 mg/100 ml) and L-NAME (NOS inhibitor, 50 mg/100 ml) were administered orally for 12 weeks. Blood pressure (BP),urinary albumin, urinary cGMP excretion, plasma ANP, and plasma renin activity were measured. In vitro autoradiography was used to locate the Ang II receptors in the kidney.

RESULTS

Captopril and L- 158,809 treatments normalised BP and prevented the appearance of albuminuria in rats receiving L-NAME. Urinary cGMP excretion was significantly increased in L-158,809-treated rats compared with the non-treated group, suggesting that the dysfunctional NO system may be activated by the treatment. AT1-receptor binding in the kidney was inhibited to about 40% of the control value after administration of L- 158,809. The AT2-receptor binding was inhibited to less than 15% of the control value. NOS inhibition had no effect on receptor binding.

CONCLUSION

Blockade of NOS causes hypertension and renal damage. Treatment with an ACE inhibitor and/or Ang II receptor antagonist prevented these changes equally effectively. The stimulatory effect of AT1-receptor antagonism on cGMP production was not mediated by AT2-receptor-dependent mechanisms, since renalAT2-receptor binding density was suppressed following treatment with L-158,809. AT1-receptor blockade per se favours activation of humoral pathways that stimulate cGMP production potentially contributing to renal and vascular protection in hypertension and chronic renal disease.

Irbesartan, an angiotensin type 1 receptor inhibitor, regulates the vascular oxidative state in patients with coronary artery disease

OBJECTIVES

The aim of this study was to determine the effect of angiotensin II type 1 (AT(1)) receptor antagonists on pro-oxidant species observed in the pathogenesis of atherosclerosis. Parameters such as low-density lipoprotein (LDL) susceptibility, monocyte binding capacity, superoxide generation and lipid peroxidation were examined in the presence of the AT(1) receptor antagonist irbesartan.

BACKGROUND

Low-density lipoprotein oxidation is a key component in the process of atherogenesis. This modification may involve various mechanisms, including changes in nitric oxide levels and superoxide levels. Additionally, compounds that suppress these mechanisms may retard or inhibit the pathogenesis of atherosclerosis.

METHODS

Forty-seven patients with documented coronary artery disease were treated with irbesartan for a 12-week period. Patients were randomized to receive irbesartan or placebo. Lipid peroxidation, superoxide levels, monocyte binding and LDL oxidation were measured at 0, 4 and 12 weeks. Findings were statistically evaluated by two-way repeated measures analysis of variance with p < 0.05 being significant.

RESULTS

Treatment with irbesartan significantly decreased the pro-oxidative environment seen in our study population. Lag time for LDL oxidation increased 32% at 12 weeks, suggesting an increased resistance of LDL modification in the serum. Thiobarbituric acid reactive substances activity indicated that lipid peroxidation decreased by 36% in comparison to placebo. In addition, superoxide levels and monocyte-binding capacity were also significantly reduced in coronary artery disease patients receiving irbesartan.

CONCLUSIONS

Our results indicate that irbesartan may suppress the atherosclerotic process by inhibiting the intravascular oxidative state and the production of reactive oxygen species, compounds that may cause damage to the vasculature.

Pharmacological approaches to preserving and restoring coronary endothelial function

There is compelling evidence that the endothelium is critical to normal coronary vascular function and that endothelial dysfunction, generally indicated by an impairment of endothelium-dependent vasodilatation, is an important component of coronary artery disease (CAD). Endothelial cells synthesise and release a number of factors, including prostacyclin, nitric oxide (NO), endothelium-derived hyperpolarising factor (EDHF) and endothelin, which are important in the regulation of vascular tone and the control of platelet and leukocyte adhesion, aggregation and migration. NO appears to be the critical factor in the preservation of normal coronary vascular function and there is a well-established correlation between CAD and an impairment of NO activity. Thus, to preserve endothelial function, drugs have been used to either increase the synthesis of NO, or to decrease its breakdown. Fortuitously, compounds such as the HMG-CoA reductase inhibitors, angiotensin (AT) converting enzyme inhibitors (ACEIs), AT receptor antagonists and oestrogen, which have been introduced into clinical practice because of other beneficial effects, have also been shown to improve coronary endothelial function through a variety of mechanisms. In addition, L -arginine, the substrate for NO synthesis, and the anti-oxidants ascorbate and alpha-tocopherol, are able to increase NO synthesis and bioavailability respectively. Studies in experimental animals strongly support the ability of these agents to enhance the activity of endothelium-derived NO but clinical trials have failed to demonstrate reversal of established CAD. Whether these agents preserve endothelial function and prevent the development of CAD remains to be established.

Therapeutic potential of angiotensin II receptor antagonists

The circulating renin-angiotensin system plays an important role in cardiovascular homeostasis. More importantly, the local tissue renin angiotensin plays a pivotal role in cell growth and remodelling of cardiomyocytes and on the peripheral arterial vasculature. In addition, the renin angiotensin system is related to apoptosis, control of baroreflex and autonomic responses, vascular remodelling and regulation of coagulation, inflammation and oxidation. The cardioprotective and vascular protective effects of the angiotensin receptive blockade appears to be related to selective blockade of the angiotensin II (A-II) Type I (AT(1)) receptors. However, there is now growing evidence showing that some of the effects of AT-II receptor blockers (ARBs) are related to the activation of the kinin pathways. This paper will review some of the recent mechanisms related to the cardiovascular effects of angiotensin and more specifically of ARBs. This paper will present the novel data on the role of ARB in the development of atherosclerosis, vascular remodelling, coagulation balance and autonomic regulation. Finally, the role of ARBs, used alone or in combination with ACE inhibitor in patients with heart failure, will be discussed.

Apstatin, a selective inhibitor of aminopeptidase P, reduces myocardial infarct size by a kinin-dependent pathway

1. Inhibitors of the angiotensin converting enzyme (ACE) have been shown to exert their cardioprotective actions through a kinin-dependent mechanism. ACE is not the only kinin degrading enzyme in the rat heart. 2. Since aminopeptidase P (APP) has been shown to participate in myocardial kinin metabolism to the same extent as ACE, the aims of the present study were to investigate whether (a) inhibition of APP leads to a reduction of myocardial infarct size in a rat model of acute ischaemia and reperfusion, (b) reduction of infarct size is mediated by bradykinin, and (c) a combination of APP and ACE inhibition leads to a more pronounced effect than APP inhibition alone. 3. Pentobarbital-anaesthetized rats were subjected to 30 min left coronary artery occlusion followed by 3 h reperfusion. The APP inhibitor apstatin, the ACE-inhibitor ramiprilat, or their combination were administered 5 min before ischaemia. Rats receiving HOE140, a specific B(2) receptor antagonist, were pretreated 5 min prior to enzyme inhibitors. Myocardial infarct size (IS) was determined by tetrazolium staining and expressed as percentage of the area at risk (AAR). 4. IS/AAR% was significantly reduced in rats that received apstatin (18+/-2%), ramiprilat (18+/-3%), or apstatin plus ramiprilat (20+/-4%) as compared with those receiving saline (40+/-2%), HOE (43+/-3%) or apstatin plus HOE140 (49+/-4%). 5. Apstatin reduces IS in an in vivo model of acute myocardial ischaemia and reperfusion to the same extent than ramiprilat. Cardioprotection achieved by this selective inhibitor of APP is mediated by bradykinin. Combined inhibition of APP and ACE did not result in a more pronounced reduction of IS than APP-inhibition alone.

Protection against ischemia: a physiological function of the renin-angiotensin system

The renin-angiotensin system (RAS) is involved in a complex mechanism that serves to preserve the blood supply to organs so that they can maintain cellular function. Angiotensin II exerts this effect, independently of the blood pressure generated, through two time-related events: a fast opening of the reserve collateral circulation and a much slower response of new vessel formation or angiogenesis. This effect is observed in rats with ligation of the abdominal aorta and in gerbils with abrupt or progressive unilateral carotid artery ligation. Inhibition of the angiotensin-converting enzyme (ACE) or the angiotensin II receptor represses this effect, and it appears that it is mediated through a non-AT1 receptor site of angiotensin II. Many tumors, both benign and malignant, express renin and angiotensin. It seems that the stimulating action of angiotensin II on angiogenesis could also be involved in preserving the blood supply to tumor cells. Administration of converting enzyme inhibitors increases survival and decreases tumor size in tumor-bearing rats. These observations support the hypothesis that the RAS, directly or indirectly, is involved in situations in which the restoration of blood supply is critical for the viability of cells and that it is present not only in normal but also in pathological conditions such as tumors. In view of the ubiquitous presence of renins and angiotensins, it is also likely to be involved in other conditions, such as inflammation, arthritis, diabetic retinopathy, and retrolental fibroplasia, among others in which angiogenesis is prominent. In addition, angiotensin II could be involved, through the counterbalance of the AT1 and AT2 receptors, in the rarefaction of blood vessels as an etiologic component of essential hypertension.

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

BACKGROUND

The roles of angiotensin II (Ang II) in the regulation of heart function under normal and pathological conditions have been well documented. Although 2 types of Ang II receptor (AT(1) and AT(2)) are found in various proportions, most studies have focused on AT(1)-coupled events. In the present study, we examined the hypothesis that signaling by AT(2) is important to the development of left ventricular hypertrophy and cardiac fibrosis by Ang II infusion in mice lacking the AT(2) gene (Agtr2-/Y).

METHODS AND RESULTS

Male Agtr2-/Y and age-matched wild-type (WT) mice were treated long-term with Ang II, infused at a rate of 4.2 ng. kg(-1). min(-1) for 3 weeks. Ang II elevated systolic blood pressure to comparable levels in Agtr2-/Y and WT mice. WT mice developed prominent concentric cardiac hypertrophy, prominent fibrosis, and impaired diastolic relaxation after Ang II infusion. In contrast, there was no cardiac hypertrophy in Agtr2-/Y mice. Agtr2-/Y mice, however, did not show signs of heart failure or impairment of ventricular relaxation and only negligible fibrosis after Ang II infusion. The absence of fibrosis may be a clue to the absence of impairment in ventricular relaxation and account for the normal left ventricular systolic and diastolic performances in Agtr2-/Y mice.

CONCLUSIONS

Chronic loss of AT(2) by gene targeting abolished left ventricular hypertrophy and cardiac fibrosis in mice with Ang II-induced hypertension.

Angiotensin and its AT2 receptor: new insights into an old system

The AT2 receptor represents a true receptor, but signals and functions in unexpected ways compared to the respective features of the 'classical' AT1 receptor. Moreover, some of the actions of the AT2 receptor are even directly opposed to those of the AT1 receptor, especially concerning the growth- and differentiation-modulating actions of ANG II. The regulation of the AT2 receptor itself by its agonist, as well as by growth factors during ontogenesis, and its acknowledged effects on the regulation of cell growth, differentiation and apoptosis, points towards a role of a program modulator in embryonic development and regeneration.

Enhanced regional AT(2)-receptor and PKC(epsilon) expression during cardioprotection induced by AT(1)-receptor blockade after reperfused myocardial infarction

We assessed the effects of the angiotensin II (Ang II) type 1 receptor (AT1-receptor) blocker, candesartan, (CN, 1 mg/kg i.v. over 30 minutes pre-ischaemia) alone or after intracoronary administration of Ang II type 2 receptor (AT2-receptor) blocker (PD 123319), protein kinase C (PKC) inhibitor (chelerythrine), endothelial nitric oxide (NO) synthase inhibitor (N(G)-monomethyl-L-arginine or L-NMMA), and bradykinin (BK) -B2 receptor inhibitor (HOE140) on in vivo left ventricular (LV) function and remodelling (echocardiograms/Doppler) and haemodynamics in 30 dogs with reperfused anterior infarction (90 minutes ischaemia, 120 minutes reperfusion), and ex vivo infarct size, AT1-receptor/AT2-receptor proteins and PKC(epsilon) (immunoblots), and cyclic guanosine 3', 5' monophosphate (cGMP, immunoassay). Compared with controls, CN inhibited the Ang II pressor response, reduced LV preload, improved LV systolic and diastolic function, limited LV remodelling, decreased infarct size, and increased AT2-receptor and PKC(epsilon) proteins in the infarct zone (IZ), and these responses were abrogated by PD 123319, chelerythrine, L-NMMA and HOE140. In addition, the increase in LV cGMP with CN was attenuated by PD 123319, L-NMMA and HOE140. The overall results suggest that AT2-receptor activation and signalling via BK, PKC(epsilon) and cGMP contribute to cardioprotection associated with AT1-receptor blockade during ischaemia-reperfusion injury.

Angiotensin II type 2 receptor-mediated duodenal mucosal alkaline secretion in the rat

The aims of this study were to elucidate the distribution of angiotensin receptors (AT(1) and AT(2)) in the duodenal wall and to investigate whether AT(2) receptors are involved in the regulation of duodenal mucosal alkaline secretion, which is of importance for the mucosal defense against gastric acid. Immunohistochemistry was used to locate AT(1) and AT(2) receptors in chloralose-anesthetized rats. Duodenal mucosal alkaline output was measured by use of in situ pH-stat titration. Immunohistochemistry demonstrated a distinct staining for both AT(1) and AT(2) receptors in the lamina propria of the villi and also for AT(1) receptors in the muscularis interna. When angiotensin II was infused in the presence of the AT(1) receptor antagonist losartan, mucosal alkaline secretion increased by ~50%. This response was inhibited by the AT(2) receptor antagonist PD-123319. The AT(2) receptor agonist CGP-42112A increased mucosal alkaline secretion by ~50%. This increase was absent in the presence of PD-123319 but not in the presence of losartan or the local anesthetic lidocaine. We conclude that angiotensin II stimulates duodenal mucosal alkaline secretion by activation of AT(2) receptors located in the duodenal mucosa/submucosa.

Left ventricular function in mice lacking the AT2 receptor

OBJECTIVES

The role of the AT2 receptor in the heart is incompletely understood. We investigated left ventricular performance in AT2 receptor knockout mice, with and without deoxycorticosterone acetate (DOCA)-salt treatment. Given the putative opposing functions of the AT1 and AT2 receptor, we also analysed AT1 receptor expression in the left ventricle.

METHODS

We used a miniaturized conductance-manometer system to measure pressure-volume loops for analysing left ventricular performance under baseline conditions and after increasing peripheral vascular resistance. We determined left ventricular AT1-receptor expression by RNase-protection assays.

RESULTS

In AT2 receptor knockout mice, end-systolic and end-diastolic volumes were lower than in wild-type mice, so that pressure-volume loops were shifted leftward. Left ventricular systolic and diastolic kinetics were not different between the groups. AT2 receptor knockout mice and wild-type mice both stabilized their reduced stroke volume after laparatomy as peripheral resistance was increased. DOCA-salt treatment increased elastance in AT2 receptor knockout mice, compared to controls. Furthermore, AT2 receptor knockout mice had a steeper increase in dP/dtmax. Left ventricular AT1 receptor gene expression was increased in AT2 receptor knockout mice and was not down-regulated in response to DOCA-salt treatment. Finally, the hearts of AT2 receptor knockout mice were smaller than controls, but increased in size in response to DOCA-salt treatment.

CONCLUSIONS

AT2 receptor knockout mice displayed no major changes in left ventricular function at baseline or in response to DOCA-salt treatment, compared to wild-type mice. The AT2 receptor may be important to AT1 receptor expression in response to DOCA-salt challenge and may have some influence on cardiac growth responses.

Comparative effects of ACE inhibitors and an angiotensin receptor blocker on atherosclerosis and vascular function

BACKGROUND

Both angiotensin-converting enzyme inhibitors (ACE-I(s)) and angiotensin receptor blockers (ARB(s)) provide vascular protection. This study was designed to compare ACE-I(s) with widely differing tissue affinity (captopril and quinapril) and an ARB (losartan) on vascular protection against the adverse effects of high cholesterol.

METHODS AND RESULTS

Forty-two New Zealand rabbits on a 0.5% cholesterol diet were randomized into control, captopril (10 mg/kg/d), quinapril (0.3 mg/kg/d), and losartan (8 mg/kg/d) groups for 14 weeks. Captopril, quinapril, and losartan significantly attenuated aortic lipid lesions (P=0.001). Captopril and quinapril were more effective than losartan in preserving vascular relaxation.

CONCLUSIONS

Captopril, quinapril, and losartan had similar protective effects against atherogenesis. Captopril and quinapril were more effective than losartan in preserving vascular function. Increased bradykinin by ACE inhibition may be responsible for this improved vascular endothelial function.

At receptor inhibition affects the noradrenaline sensitivity in isolated portal vein of normotensive rat

Short term treatments of normotensive Wistar Kyoto rats with angiotensin II (ANGII) or in combination with the AT1 receptor antagonist, losartan or PD123319, the AT2 receptor antagonist on systemic arterial blood pressure (MABP) and their influence on noradrenaline sensitivity in isolated mesenteric portal vein were evaluated. ANGII increased MABP as well as the contractile response to noradrenaline in vessels from ANGII-treated animals. MABP and the maximal effect of the concentration response curve for noradrenaline were prevented by losartan. However, PD123319 did not influence the blood pressure, but completely removed the vessels sensitivity to noradrenaline. ANGII combined with the AT1 and/or AT2 receptors blockade completely prevented the pressure response to ANGII, but the concentration response curve for noradrenaline did not differ from the vehicle-treated control curve. In conclusion both AT1- and AT2 receptor activation seems to be important in controlling noradrenaline sensitivity of rat portal vein smooth muscle.

Inhibition of platelet activation in stroke-prone spontaneously hypertensive rats: comparison of losartan, candesartan, and valsartan

In vitro studies have suggested that losartan interacts with the thromboxane (TxA2)/ prostaglandin H2 (PGH2) receptor in human platelets, reducing TxA2-dependent platelet activation. The aim of this study was to evaluate the effect of different angiotensin II type 1 receptor antagonists in stroke-prone spontaneously hypertensive rats (SHRSP). The level of platelet activation was assessed by determining P-selectin expression in platelets by flow cytometry. The ex vivo adhesion of platelets was also analyzed. The number of platelets that expressed P-selectin in SPSHR was significantly increased (% P-selectin expression: WKY 4 +/- 0, 4%; SHRSP 15.5 +/- 0, 8% [n = 8], p < 0.05). In SHRSP receiving losartan (20 mg/kg body weight per day) the percentage of platelets expressing P-selectin fell to levels close to that observed in WKY. The number of platelets from SHRSP treated with valsartan and candesartan (20 mg/kg body weight per day for 14 days) that expressed P-selectin was not significantly different from those from untreated SPRHR. Only losartan treatment reduced ex vivo platelet adhesion to a synthetic surface. The antiplatelet effect of losartan does not appear to be related to the level of blood pressure reduction. In ex vivo experiments, losartan significantly reduced the binding of the radiolabeled TxA2 agonist U46619 to platelets obtained from SHRSP in a dose-dependent manner. Treatment with losartan reduced the number of activated platelets in SHRSP independently of its blood pressure effects. TxA2-receptor blockade is proposed as a mechanism by which losartan can prevent platelet activation.

Angiotensin II type 2 receptors: signalling and pathophysiological role

The signalling mechanisms and biological significance of the angiotensin II type 2 receptor have long been unknown. In recent years, studies, first in cell culture models but now increasingly also in vivo, have shed some light on the molecular events occurring after a stimulation of the receptor with its ligand as well as on its physiological effects and its significance for pathophysiological processes. There is increasing evidence that the angiotensin II type 2 receptor is involved in different pathophysiological processes, such as myocardial infarction, heart and kidney failure, and stroke.

Transcriptional and translational regulation of calpain in the rat heart after myocardial infarction--effects of AT(1) and AT(2) receptor antagonists and ACE inhibitor

1. Recent studies demonstrated that the cardiac calpain system is activated during ischaemic events and is involved in cardiomyocyte injury. The aim of this study was to investigate the contribution of AT(1) and AT(2) receptors in the regulation of calpain-mediated myocardial damage following myocardial infarction (MI). 2. Infarcted animals were treated either with placebo, the ACE inhibitor ramipril (1 mg kg(-1) d(-1)), the AT(1) receptor antagonist valsartan (10 mg kg(-1) d(-1)) or the AT(2) receptor antagonist PD 123319 (30 mg kg(-1) d(-1)). Treatment was started 7 days prior to surgery. On day 1, 3, 7 and 14 after MI, gene expression and protein levels of calpain I, II and calpastatin were determined in left ventricular free wall (LVFW) and interventricular septum (IS). At day 3 and 14 post MI, morphological investigations were performed. 3. Calpain I mRNA expression and protein levels were increased in IS 14 days post MI, whereas mRNA expression and protein levels of calpain II were maximally increased in LVFW 3 days post MI. Ramipril and valsartan decreased mRNA and protein up-regulation of calpain I and II, and reduced infarct size and interstitial fibrosis. PD 123319 did not affect calpain I or II up-regulation in the infarcted myocardium, but decreased interstitial fibrosis. Calpastatin expression and translation were not affected by AT receptor antagonists or ACE inhibitor. 4. Our data demonstrate a distinct, temporary-spatial up-regulation of calpain I and II following MI confer with the hypothesis of calpain I being involved in cardiac remodelling in the late and calpain II contributing to cardiac tissue damage in the early phase of MI. The up-regulation of calpain I and II is partly mediated via the AT(1) receptor and can be reduced by ACE inhibitors and AT(1) receptor antagonists.

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

OBJECTIVES

Our study goal was to analyze whether gene variants of angiotensin II type 2-receptor (AT2-R) modulate the effects of angiotensin II on the left ventricle (LV).

BACKGROUND

Experimental data suggest that angiotensin II modifies ventricular growth responses via angiotensin II type 1-receptors (AT1-R) and AT2-R.

METHODS

In 120 white, young male subjects with normal or mildly elevated blood pressure, we assessed plasma angiotensin II and aldosterone concentrations (RIA), 24-h urinary sodium excretion, 24-h ambulatory blood pressure and LV structure (two-dimensional guided M-mode echocardiography). The intronic +1675 G/A polymorphism of the X-chromosomal located AT2-R gene was investigated by single-strand conformational polymorphism analysis and DNA-sequencing.

RESULTS

Hypertensive subjects with the A-allele had a greater LV posterior (11.0 +/- 1.3 vs. 9.9 +/- 1.3 mm, p < 0.001), septal (11.8 +/- 1.4 vs. 10.1 +/- 1.2 mm, p < 0.001) and relative wall thickness (0.44 +/- 0.06 vs. 0.39 +/- 0.06, p < 0.01) as well as LV mass index (138 +/- 23 vs. 120 +/- 13 g/m2, p < 0.001) than those with the G-allele. Confounding factors (i.e., body mass index and surface area, plasma angiotensin II, sodium excretion, systolic and diastolic ambulatory blood pressure) were similar between the two genotypes. In normotensive subjects, relative wall thickness (0.36 +/- 0.05 vs. 0.35 +/- 0.05) and LV mass index (115 +/- 21 vs. 112 +/- 17 g/m2) were nearly identical across the two genotypes, with similar confounding variables.

CONCLUSIONS

Our data indicate that the X-chromosomal located +1675 G/A-polymorphism of the AT2-R gene is associated with LV structure in young male humans with early structural changes of the heart due to arterial hypertension.

Acute effects of ACE inhibition on coronary endothelial dysfunction

The prerequisite of atherosclerosis, endothelial dysfunction, is characterised by impaired endothelium-dependent vasodilation caused by the reduced bioavailibility of nitric oxide (NO). In order to assess the role of acute ACE inhibition in this setting, coronary arterial endothelial function was quantified following acute intracoronary administration of the angiotensin-converting enzyme (ACE) inhibitor quinapril. Twenty-one patients with non-limiting coronary artery disease were studied before and after acute intracoronary administration of 10 mg quinapril. Nine patients received pre-treatment with the angiotensin AT(1)-receptor antagonist losartan (2 x 50 mg, p.o.). Coronary cross-sectional diameter was measured via quantitative angiography and microvascular reaction was investigated by intracoronary Doppler flow measurement during intracoronary infusion of 0.1 to 10 micromol/l acetylcholine. Quinapril acutely improved endothelial dysfunction on the macro- as well as the microvascular level. Losartan did not alter macrovascular function but facilitated microvascular endothelial function. Acute quinapril application led to no further improvement of endothelial dysfunction in patients pre-treated with losartan. Acute quinapril infusion improved endothelial function in patients with coronary heart disease. Treatment with the AT(1)-receptor antagonist losartan led to a slight improvement in microvascular endothelial function, but pre-treatment with losartan blunted the vascular effect of quinapril, suggesting that the combination of ACE inhibition and AT(1)-receptor antagonism may not exert a synergistic benefical impact on the coronary vasculature.

Effect of losartan on angiotensin II-mediated endothelin and prostanoid excretion in humans

Angiotensin II (Ang II) stimulates renal prostanoid and vascular endothelin-1 (ET-1) release. Most known Ang II effects are mediated by AT1 receptors. Our aim was to determine whether AT1 receptor activation mediates Ang II-evoked renal prostanoid and ET-1 release. Eleven healthy men were randomized in a crossover, double-blind fashion to receive 100 mg/day of losartan or matching placebo, for 8 days. Blood and urine were sampled before and after a 2-h infusion of Ang II at a rate previously determined to increase mean arterial pressure (MAP) by 25 to 30 mm Hg in each subject. After a 14-day washout, subjects received the alternate treatment. Pretreatment with losartan had little effect on baseline MAP, but increased plasma renin activity, and virtually eliminated the pressor response to Ang II infusion. Angiotensin II significantly increased prostanoid excretion after placebo; the prostanoid response to Ang II was even greater after losartan. Plasma ET-1 was not altered by Ang II infusion, with or without losartan. In contrast, urine ET-1 excretion rate decreased to 40% of baseline after Ang II but not after losartan pretreatment; losartan alone had no effect. We conclude that Ang II decreases renal ET-1 synthesis and release through the AT1 receptor. In contrast, Angiotensin II-mediated renal prostanoid synthesis does not require activation of AT1 receptors. These findings indicate that AT1 receptor antagonists could provide renal protection through indirect mechanisms.

Evolution and pathophysiology of chronic systolic heart failure

Understanding of the pathophysiology of chronic systolic heart failure evolved from a purely mechanical model to one in which a cascade of neurohormones and biologically active molecules are thought to be critical in the development, maintenance, and progression of the disease. Two important neurohormonal systems are the sympathetic nervous and renin-angiotensin-aldosterone systems. Initially, increases in norepinephrine concentrations from the sympathetic nervous system and in angiotensin II and aldosterone are beneficial in the short term to maintain cardiac output after an insult to the myocardium. However, long-term exposure to these neurohormones causes alterations of myocytes and interstitial make-up of the heart. These alterations in myocardium lead to progression of heart failure and, eventually, death.

Homologous and heterologous phosphorylation of the AT(2) angiotensin receptor by protein kinase C

The angiotensin AT(2) receptor is an atypical seven transmembrane domain receptor that is coupled to activation of tyrosine phosphatase and inhibition of MAP kinase, and does not undergo agonist-induced internalization. An investigation of the occurrence and nature of AT(2) receptor phosphorylation revealed that phorbol ester-induced activation of protein kinase C (PKC) in HA-AT(2) receptor-expressing COS-7 cells caused rapid and specific phosphorylation of a single residue (Ser(354)) located in the cytoplasmic tail of the receptor. Agonist activation of AT(2) receptors by angiotensin II (Ang II) also caused rapid PKC-dependent phosphorylation of Ser(354) that was prevented by the AT(2) antagonist, PD123177, and by inhibitors of PKC. In cells coexpressing AT(1) and AT(2) receptors, Ang II-induced phosphorylation of the AT(2) receptor was reduced by either PD123177 or the AT(1) receptor antagonist, DuP753, and was abolished by treatment with both antagonists or with PKC inhibitors. These findings indicate that the AT(2) receptor is rapidly phosphorylated via PKC during homologous activation by Ang II, and also undergoes heterologous PKC-dependent phosphorylation during activation of the AT(1) receptor. The latter process may regulate the counteracting effects of AT(2) receptors on growth responses to AT(1) receptor activation.