Effects of prostaglandins and nitric oxide on the renal effects of angiotensin II in the anaesthetized rat

Blood pressure and renal hemodynamic effects of angiotensin fragments

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

Interaction of endothelial nitric oxide and angiotensin in the circulation

Discovery of the unexpected intercellular messenger and transmitter nitric oxide (NO) was the highlight of highly competitive investigations to identify the nature of endothelium-derived relaxing factor. This labile, gaseous molecule plays obligatory roles as one of the most promising physiological regulators in cardiovascular function. Its biological effects include vasodilatation, increased regional blood perfusion, lowering of systemic blood pressure, and antithrombosis and anti-atherosclerosis effects, which counteract the vascular actions of endogenous angiotensin (ANG) II. Interactions of these vasodilator and vasoconstrictor substances in the circulation have been a topic that has drawn the special interest of both cardiovascular researchers and clinicians. Therapeutic agents that inhibit the synthesis and action of ANG II are widely accepted to be essential in treating circulatory and metabolic dysfunctions, including hypertension and diabetes mellitus, and increased availability of NO is one of the most important pharmacological mechanisms underlying their beneficial actions. ANG II provokes vascular actions through various receptor subtypes (AT1, AT2, and AT4), which are differently involved in NO synthesis and actions. ANG II and its derivatives, ANG III, ANG IV, and ANG-(1-7), alter vascular contractility with different mechanisms of action in relation to NO. This review article summarizes information concerning advances in research on interactions between NO and ANG in reference to ANG receptor subtypes, radical oxygen species, particularly superoxide anions, ANG-converting enzyme inhibitors, and ANG receptor blockers in patients with cardiovascular disease, healthy individuals, and experimental animals. Interactions of ANG and endothelium-derived relaxing factor other than NO, such as prostaglandin I2 and endothelium-derived hyperpolarizing factor, are also described.

Differential effects of losartan and candesartan on vasoconstrictor responses in the ratThis paper is one of a selection of papers published in this Special Issue, entitled The Cellular and Molecular Basis of Cardiovascular Dysfunction, Dhalla 70th Birthday Tribute

Losartan has been reported to have inhibitory effects on thromboxane (TP) receptor-mediated responses. In the present study, the effects of 2 nonpeptide angiotensin II (AT1) receptor antagonists, losartan and candesartan, on responses to angiotensin II, the thromboxane A2 mimic, U46619, and norepinephrine were investigated and compared in the pulmonary and systemic vascular beds of the intact-chest rat. In this study, intravenous injections of angiotensin II, U46619, and norepinephrine produced dose-related increases in pulmonary and systemic arterial pressure. Losartan and candesartan, in the doses studied, decreased or abolished responses to angiotensin II. Losartan, but not candesartan, and only in a higher dose, produced small, but statistically significant, reductions in pressor responses to U46619 and to norepinephrine in the pulmonary and systemic vascular beds. Furthermore, losartan significantly reduced arachidonic acid-induced platelet aggregation, whereas candesartan had no effect. Pressor responses to angiotensin II were not changed by thromboxane and alpha-adrenergic receptor antagonists, or by cyclooxygenase and NO synthase inhibitors. These results show that losartan and candesartan are potent selective AT1 receptor antagonists in the pulmonary and systemic vascular beds and that losartan can attenuate thromboxane and alpha-adrenergic responses when administered at a high dose, whereas candesartan in the highest dose studied had no effect on responses to U46619 or to norepinephrine. The present data show that the effects of losartan and candesartan on vasoconstrictor responses are different and that pulmonary and systemic pressor responses to angiotensin II are not modulated or mediated by the release of cyclooxygenase products, activation of TP receptors, or the release of NO in the anesthetized rat.

Angiotensin II does not affect endothelial tone in Type 1 diabetes-results of a double-blind placebo controlled trial

AIMS

Previously, we have demonstrated that patients with normoalbuminuric Type 1 diabetes are characterized by impaired nitric oxide bioavailability compensated for by increased vasodilatory prostanoid-mediated vasodilation. Experimental evidence suggests vascular responses to endogenous angiotensin II involve the nitric oxide and prostaglandin pathways. We examined whether selective blockade of angiotensin II influences endothelial tone with particular reference to the nitric oxide/prostaglandin pathways in patients with Type 1 diabetes free from vascular complications.

METHODS

At baseline, we studied changes in forearm blood flow in response to brachial arterial infusions of acetylcholine, l-NMMA, a combination of l-NMMA and the cyclo-oxygenase inhibitor indomethacin and nitroprusside in 30 patients with normoalbuminuric Type 1 diabetes [21 male, 9 female; age 38.5 +/- 1.9 years (mean +/- sem)]. Patients were randomized to 2 weeks' treatment with placebo or the selective angiotensin II receptor blocking agent irbesartan, 300 mg, prior to forearm vasoactive responses being re-examined.

RESULTS

The forearm responses to nitroprusside and acetylcholine were unchanged by both placebo (P = 0.23 and P = 0.36, respectively) and irbesartan (P = 0.41 and P = 0.36). Similarily, dose-response curves to acetylcholine in the presense of l-NMMA alone (P = 0.42) and a combination of l-NMMA and indomethacin (P = 0.44) were not altered by angiotensin II blockade.

CONCLUSION

This study demonstrated that physiological blockade of endogenous angiotensin II in Type 1 diabetes does not augment agonist-evoked vasodilation or the contribution of nitric oxides and prostanoids to endothelial tone.

Role of nitric oxide and cyclooxygenase-2 in regulating the renal hemodynamic response to norepinephrine

We have reported that the renal hemodynamic effects of norepinephrine (NE) are modulated by cyclooxygenase-2 (COX-2)-derived metabolites. Our main objective was to examine whether there is an interaction between nitric oxide (NO) and COX-2 in modulating the renal hemodynamic effects of NE. NE was infused at three doses to anesthetized dogs pretreated with vehicle (n = 8), a selective COX-2 inhibitor (nimesulide) (n = 6), an NO synthesis inhibitor [NG-nitro-l-arginine methyl ester; l-NAME] (n = 8), or with nimesulide and l-NAME (n = 5). During NE infusion, PGE2 excretion increased (125%) in the control group and did not change in the l-NAME-treated dogs. The simultaneous inhibition of NO and COX-2 potentiated to a greater extent the NE-induced renal vasoconstriction than inhibition of either NO or COX-2. The NE-induced renal vasoconstriction during NO and COX-2 inhibition was reduced (P < 0.05) by infusing an AT1 receptor antagonist (n = 6). These results suggest that there is an interaction between NO and COX-2 in protecting the renal vasculature from the NE effects and that angiotensin II partly mediates the NE-induced renal vasoconstriction when NO synthesis and COX-2 activity are reduced.

Effect of exogenous and endogenous angiotensin II on intrarenal distribution of glomerular filtration rate in rats

Different changes in glomerular filtration rates (GFR) in deep and superficial glomeruli have been suggested to influence renal NaCl excretion and concentrating ability. Angiotensin II (AngII) has been implicated in such changes, but the experimental evidence has been conflicting, probably because of the methodological limitation of just one 'snapshot' measurement of local GFR per kidney. We have therefore studied the effect of AngII and AT(1)-receptor blockade on glomerular filtration in outer, middle and inner cortex (OC, MC and IC, respectively) in pentobarbitone-anaesthetised rats using the aprotinin (Ap) method, providing control and experimental measurements in the same kidney. Glomerular filtration rate per gram cortical tissue was measured based on 'free' glomerular filtration of Ap followed by complete tubular uptake and a 20 min sojourn in the proximal tubular cells before breakdown and incipient return to the plasma.(125)I-labelled Ap was injected I.V. to determine control Ap clearance, followed after 13 min by injection of AngII or the A1 type AngII receptor blocker losartan and 2 min thereafter by (131)I-labelled Ap to determine clearance in the experimental period. Tracer activity in frequent blood samples and in tissue samples allowed calculation of GFR in the two periods. Mean GFR control values were: 1.13 ml min(-1) in whole kidney and 1.44, 1.27 and 0.76 ml min(-1) per gram cortical tissue in OC, MC and IC, respectively. The most sensitive and comprehensive measure of altered GFR distribution is the ratio between the relative filtration change in inner versus that in outer cortex, F = (IC(E)/IC(C))/(OC(E)/OC(C)), where subscripts E and C stand for experimental and control, respectively. F values greater than 1.00 directly indicate and quantify a relatively greater increase of filtration rate in inner than in outer cortex. We found in salt-replete rats that at practically unchanged total GFR, intravenous and intra-arterial infusion of AngII increased F to 1.07 and 1.04 (P < 0.05) whereas losartan reduced F to 0.99. After pretreatment with the inhibitor of nitric oxide production L-NAME, losartan increased total GFR by 8 % and F fell to 0.95 (P < 0.05). In salt-depleted rats losartan reduced F to 0.95 (P < 0.05) at unchanged total GFR. All IC/OC changes induced by losartan were significantly different from that obtained by AngII infusions. We conclude that deep nephrons have higher postglomerular AngII tone and also higher AngII sensitivity than superficial nephrons. The better preserved GFR in deep cortex during AngII action may contribute towards maintaining the renal concentrating ability by providing NaCl for reabsorption by the ascending limb of the loop of Henle.