Abstract
The effects of infusions of angiotensin and vasopressin, in stepwise concentrations, on the cardiac baroreflex and on cardiac output were studied in seven adult unanaesthetized sheep and compared with those obtained with infusions of phenylephrine. Six animals were treated with the beta-adrenoceptor blocking drug, propranolol (in order to inhibit the effects of the sympathetic nervous system on the heart). One animal was not treated with propranolol. In those animals in which arterial pressure increased during infusion of vasopressin, the slope of the systolic pressure-pulse interval relation was greater than that seen when phenylephrine was used to increase arterial pressure. Compared with the cardiac response to pressor doses of phenylephrine, infusions of angiotensin were associated with a lesser degree of cardiac slowing and a lesser reduction in cardiac output. The effects of combined infusions of angiotensin and vasopressin on the cardiac baroreflex were studied. In five sheep which were infused with a pressor dose of angiotensin (1.1 microgram/min), the stimulatory effect of vasopressin (1.0 u./min) on pulse interval and its depressant effect on cardiac output were attenuated. In seven sheep infused with 0.5 u./min of vasopressin, I.V. infusion of angiotensin (0.2-5.0 micrograms), produced a progressive decrease in pulse interval and increase in cardiac output as the dose was increased. Therefore, angiotensin can offset the cardioinhibitory effects of vasopressin. Since cardiac sympathetic activity was blocked and neither drug has any direct chronotropic effect on the heart, it would appear that these interactions between the two drugs affect the cardiac vagus either at a peripheral or central level.