In vivo vasodilatory action of atrial natriuretic peptides in canine coronary circulation

Differential modulation of nitric oxide and prostacyclin release in senescent rat heart stimulated by angiotensin II

To elucidate the mechanism of age-related changes in the cardiovascular function stimulated with angiotensin II, we examined the effects of angiotensin II on the coronary flow, production of nitric oxide (NO) and prostacyclin, and on the cardiac function in the Langendorff-perfused young and aged rats' hearts. Angiotensin II decreased coronary flow, left ventricular dP/dt and heart rate. These effects were more pronounced in aged rats. Pretreatment with a NO synthase inhibitor, N(G)-nitro-L-arginine, significantly increased the angiotensin II-induced vasoconstriction in young rats. Angiotensin II increased the concentration of NO in the coronary effluent in young but not in aged rats. In contrast, angiotensin II stimulated the release of prostacyclin to a much greater extent in aged rats than in young rats. These results suggest that impaired production of NO may contribute to the greater constrictor effect of angiotensin II in the aged rat, although aging modulated the production of prostacyclin in a different manner. This age-related endothelial dysfunction may alter the physiological regulation of coronary flow and cardiac function stimulated with angiotensin II.

No cross-tolerance between S-nitrosocaptopril and nitroglycerin in dog coronary arteries in vivo

S-Nitrosocaptopril (S-NO-Cap), a nitrate and an angiotensin-converting enzyme (ACE) inhibitor, may be produced after coadministration of nitroglycerin (NTG) and captopril (CAP). We synthesized S-NO-Cap and investigated its in vivo tolerance. In open-chest dogs, S-NO-Cap [300 microg; intracoronary (i.c.)] and NTG (50 microg, i.c.) increased coronary blood flow (CBF) similarly (8.0 vs. 9.0 ml/min; p = NS; n = 5). After a 2-h i.c. NTG infusion at high dose (1.32 micromol/min), NTG (50 microg, i.c.) had no significant effect on CBF, whereas S-NO-Cap (300 microg, i.c.) still produced an attenuated increase in CBF (4.9 ml/min; p < 0.05 vs. control). On the other hand, after a 2-h i.c. infusion of S-NO-Cap (1.32 micromol/min), the CBF response to S-NO-Cap (300 microg) showed no attenuation, whereas that to NTG (50 microg) was potentiated (8.8 vs. 12.6 ml/min; p < 0.05; n = 6). Under basal conditions, S-NO-Cap (30-300 microg, i.c.) increased CBF dose dependently, whereas CAP (30-300 microg, i.c.) had no effect on CBF, suggesting that S-NO-Cap dilates coronary vessels by a nitrate action but not by an ACE-inhibitory action. In nonsurgical dogs, 2-h intravenous (i.v.) infusion of S-NO-Cap (1.32 micromol/min) had a stable hypotensive effect, whereas that of NTG (1.32 micromol/min) gradually attenuated the effect. Plasma NO3-, an oxidative product of nitric oxide (NO), increased after both infusions, suggesting that S-NO-Cap may act partially as an NO donor, similarly to NTG. Plasma ACE activity was reduced after an S-NO-Cap infusion (5.84 vs. 4.10 IU/L; p < 0.01; n = 5), and plasma aldosterone was markedly increased after NTG infusion relative to that after S-NO-Cap infusion (243.0 vs. 38.6 pg/ml; p < 0.05). Plasma norepinephrine increased after both infusions (393.6 vs. 289.0 pg/ml; p = NS). As judged by the increase in CBF, whereas S-NO-Cap showed partial tolerance with NTG, no tolerance was found with S-NO-Cap itself. The in vivo coronary vascular response to S-NO-Cap may, therefore, be partially reduced by activation of the adrenergic or renin-angiotensin-aldosterone systems or both induced by NTG, because S-NO-Cap showed no cross-tolerance with NTG in our earlier in vitro study.

Effect of endogenous natriuretic peptide system on ventricular and coronary function in failing heart

Ventricular concentrations of atrial, brain (BNP) and C-type natriuretic peptide are enhanced in congestive heart failure (CHF). Natriuretic peptide receptors are present on ventricular myocytes and stimulate guanosine 3',5'-cyclic monophosphate (cGMP) production. cGMP has been demonstrated to affect myocyte function in vitro. Thus we hypothesized that the intracardiac natriuretic peptide system may modulate myocardial and coronary function in CHF. To test this hypothesis, the effects of an intracoronary infusion of the natriuretic peptide receptor antagonist HS-142-1 on ventricular and coronary function were examined in anesthetized dogs with chronic CHF. To determine whether receptor stimulation had contrasting effects to those of receptor blockade, intracoronary BNP was infused in anesthetized normal and CHF dogs. Low-dose HS-142-1 delayed and slowed left ventricular (LV) relaxation and decreased coronary blood flow without changes in LV pressures. Higher doses further impaired LV relaxation without further decreases in coronary blood flow. In normal and CHF dogs, exogenous BNP produced the opposite effect with a quicker onset and faster rate of LV relaxation without effects on LV pressures or coronary blood flow. The endogenous natriuretic peptide system has an autocrine-paracrine role to modulate LV and coronary vascular function in CHF.