Role of angiotensin II and prostaglandins in the regulation of uteroplacental blood flow.

This study was designed to determine the importance of the renin-angiotensin (RAS) and prostaglandin (PG) systems in regulating uteroplacental blood flow (UBF). Our objectives were to determine: 1) whether angiotensin II (ANG II) acts as a vasodilator or purely as a vasoconstrictor in the uteroplacental circulation, and 2) whether this circulation is capable of autoregulation. In chronically instrumented pregnant dogs (41-54 days gestation), ANG II was infused intravenously at increasing doses (8, 16, and 24 ng.kg-1 x min-1). Arterial pressure rose from 108 +/- 6 to 146 +/- 4 mmHg and UBF did not change but uterine vascular resistance (UVR) progressively increased. When the experiment was repeated while servo-controlling uterine arterial pressure, UBF fell at all doses, reaching 62 +/- 7% of control at the highest dose, and UVR increased as before. Meclofenamate (6 mg/kg i.v.) did not alter the dose-response curves. In separate experiments, uterine perfusion pressure was reduced in steps to 55 mmHg. UBF was well autoregulated down to approximately 85 mmHg, and neither captopril (14 micrograms.kg-1 x min-1) nor meclofenamate altered UBF autoregulation. Thus ANG II appears to act as a vasoconstrictor in the uteroplacental circulation and any preservation of UBF during ANG II appears to be due to the increased arterial pressure. Also, in the dog the uteroplacental circulation possesses a mild to moderate degree of autoregulatory capability, which does not appear to be dependent on the RAS or PGs.