Acute haemodynamic effects of the atrial natriuretic hormone in rats

Haemodynamic effects of atriopeptin II (AP II) were determined in conscious and anaesthetized rats chronically instrumented with a Doppler flow probe on the ascending aorta. Intravenous injection of AP II (7 micrograms/kg) produced a decrease in mean arterial pressure within 5 min; however, a biphasic change occurred in total peripheral resistance (TPR). At 1 min in the anaesthetized rats, TPR decreased while cardiac output tended to increase, and similar changes occurred in the conscious rats. By 5-8 min the haemodynamic profile had reversed: cardiac output had decreased in both anaesthetized and conscious rats and TPR had increased in anaesthetized rats. Mean circulatory filling pressure (MCFP) was measured during brief circulatory arrest by inflating an intracardiac balloon. Blood volume was measured with 51Cr-erythrocytes and organ blood volume by whole-animal freezing in liquid nitrogen. There were no changes in MCFP in the conscious and the anaesthetized rats, nor in the blood volume or cardiopulmonary blood volume in the anaesthetized rats, at 5-7 min after AP II. Atriopeptin III (10 micrograms/kg intravenously) had no effects on MCFP and the blood volume in the conscious rats at 5 and 15 min after injection. These results suggest multiple mechanisms of action for the acute haemodynamic effects of atrial peptides.