Angiotensin II metabolic clearance rate and pressor responses in nonpregnant and pregnant women

Abstract

Normal pregnancy is associated with reduced pressor dose-responses to infused angiotensin II. We tested the hypotheses that alterations in the metabolic clearance rate and the half-life of angiotensin II account for reduced pressor dose-responses during gestation and that angiotensin II increases circulating levels of vasodilatory prostaglandins I2 and E2 relative to thromboxane A2. Eleven nonpregnant and 37 pregnant (30 +/- 0.3 weeks' gestation, mean +/- SE) women were infused with angiotensin II (3.11 to 22.36 ng/min.kg) for 15 minutes, and blood was obtained to evaluate steady-state immunoreactive plasma angiotensin II and eicosanoid concentrations. Angiotensin II pressor responses were dose dependent in all groups and reduced in pregnant women (p < 0.001). Basal immunoreactive plasma angiotensin II concentrations were 2.7-fold greater (p < 0.001) in pregnant versus nonpregnant women. Plasma levels reached steady state by 5 minutes of infusion, and at similar angiotensin II concentrations the increase in blood pressure was greater in nonpregnant versus pregnant women (p < 0.001). The angiotensin II metabolic clearance rate and half-life were similar in nonpregnant and pregnant women: metabolic clearance rate = 85 +/- 10 versus 68 +/- 3 ml/min.kg, respectively (p = 0.130), and half-life = 48 and 49 seconds, respectively. Plasma prostaglandin I2 (6-keto-prostaglandin F1 alpha) prostaglandin E2, and thromboxane B2 levels in pregnant women were unaffected by angiotensin II infusions. Neither changes in angiotensin II metabolism nor angiotensin II-induced increases in plasma levels of prostaglandin I2, prostaglandin E2, or the prostaglandin I2/thromboxane A2 ratio appear responsible for the decreased pressor response sensitivity to infused angiotensin II observed during normal human pregnancy.