The present experiments were designed to test the hypothesis that the activation of the renin-angiotensin system during compensated heart failure may have adverse effects on cardiac function and change the peripheral vascular structure. ANG II (250 ng/kg/min) or saline (0.9% NaCl) were infused in myocardial-infarcted and sham-operated rats. After 2 weeks, cardiac function and peripheral vascular changes were investigated.
Results: ANG II infusion reduced baseline cardiac index in sham rats but did not further reduce this index in ANG II-infused MI rats. Total peripheral resistance was similarly increased in ANG II-infused infarcted and sham rats, and also plasma ANG II concentrations were comparable. ANG II elevated systolic blood pressure by approximately 70 mm Hg in sham rats and increased the medial cross-sectional area of the superior mesenteric artery by 33%. However, ANG II infusions in MI rats resulted in only a minor increase in blood pressure, whereas the cross-sectional area of the superior mesenteric artery did not change. ANG II infusion had no effect on vessel dimensions of the resistance arteries of the pulmonary and mesenteric vascular bed of either group. Calculated ED50 and peak pressor response to acute ANG II injections were comparable in all groups, confirming the presence of functionally intact AT1 receptors. The increases in plasma atrial natriuretic peptide (ANP) and nitric oxide (NO) synthase activity (estimated by aortic cyclic GMP concentrations) were higher in ANG II-infused MI rats than in ANG II-infused sham rats.
Conclusion: ANG II infusion in rats with and without MI has comparable negative effects on cardiac function but has different effects on blood pressure and vascular structure. The concomitant increases in plasma ANP and NO synthase activity in ANG II-infused MI rats suggest that the growth stimulatory and hypertensive actions of ANG II in sham rats may be counter-regulated by activation of inhibitory neurohumoral systems such as ANP or NO in MI rats.