Role of the Renin-Angiotensin System in Maintaining Arterial Pressure in Conscious Pregnant Rats*

The renin-angiotensin system in relation to arterial blood pressure was examined in conscious rats at various stages of pregnancy. Also, the possibility that a site other than the maternal kidney was the source of the increased plasma renin levels in late gestation was tested by examining the reninangiotensin system in nephrectomized term-pregnant rats. Conscious rats, pregnant for 9, 13, 17, and 21 days, had mean arterial pressures that were not different from those of nonbred control rats; however, infusion of the angiotensin II (A-II) antagonist, [Sar1, Ala8]A-II, resulted in significant decreases in mean arterial pressure in the 13-, 17-, and 21-day-pregnant groups. Pregnant rats had significant elevations in PRA at 13, 17, and 21 days of gestation and increases in plasma renin concentrations (PRC) at 13 and 21 days of gestation, while plasma renin substrate concentrations (PRS) were significantly increased in the 9- and 13-day-pregnant groups, but were not increased in the 17- and 21-day-pregnant groups. Nephrectomized nonbred rats had elevations in PRS and undetectably low levels of PRC and PRA. Nephrectomized term-pregnant rats also had increased levels of PRS, and although PRC and PRA were greatly diminished, detectable levels were still present. Infusion of [Sar1, Ala8]A-II did not lower mean arterial pressure in nephrectomized nonbred rats or in nephrectomized termpregnant rats. These studies demonstrated that there is increased activity of the renin-angiotensin system during the latter part of pregnancy in rats, and that this is due to increased plasma levels of renin. In rats in late gestation, A-II has a physiological role in maintaining arterial blood pressure. Although nonrenal sources contributed to the elevation in PRC in rats in advanced gestation, these studies failed to provide evidence that renin from nonrenal sources was involved in maintaining arterial blood pressure in term-pregnant rats.