The effects of vasoactive peptides on the carotid cardiac baroreflex.

1. In unanaesthetized sheep, transitory hypertension caused by intravenous angiotensin II and angiotensin III is not associated with a progressive increase in pulse interval, while hypertension caused by intravenous vasopressin produces a progressive increase in pulse interval. 2. Experiments in anaesthetized dogs showed that the three peptides do not alter the sensitivity of the carotid sinus baroreceptors. When arterial pressure was increased by angiotensin II and III, cardiac vagal efferent discharge did not increase as much as it did when blood pressure was raised by intravenous phenylephine or inflation of an intra-aortic balloon--often vagal discharge did not increase at all, or even fell. Vasopressin on the other hand produced a sustained and marked increase in cardiac vagal efferent nerve activity; this was in excess of the increase evoked when other agents were used to raise blood pressure. 3. Thus the three peptides can centrally modulate vagal control of heart rate.