Effects of angiotensin II on the blood flow and its distribution in fetal lambs

Abstract

We studied the effects of intravenous infusion of angiotensin II on the circulation of the fetus in lambs in utero through chronically maintained intravascular catheters. Angiotensin II infused in doses of 29-280 ng/min per kg fetal weight resulted in an increase in plasma angiotensin II from a control value of 87 ± 17 to 341 ± 129 (mean ± SE) pg/ml; these levels are similar to those observed following hemorrhage in fetal lambs. Fetal mean arterial blood pressure increased from 46 ± 2.0 to 56 ± 2.7 torr and fetal heart rate increased from 172 ± 6 to 189 ± 6 beats/min, an effect which was not altered by β-adrenergic or cholinergic blockade. Fetal cardiac output and its distribution were measured before and during infusion of angiotensin II by the radionuclide-labeled microsphere technique. Combined ventricular output increased significantly from 526 ± 32 to 616 ± 24 ml/min per kg fetal body weight. Angiotensin constricted the umbilical-placental circulation as well as the gastrointestinal, renal, and thyroidal vascular beds. Angiotensin increased blood flow to the myocardium and markedly increased blood flow to the pulmonary circulation. These studies indicate that angiotensin II, at the plasma levels achieved with moderate hemorrhage, has marked influences on the circulation. The renin-angiotensin system may be important in the fetal response to stress.