Lymph flow rate response to angiotensin II is decreased in pregnant sheep

Abstract

Pregnancy in humans is associated with a number of physiologic changes including interstitial fluid retention (edema) and a decrease in the systemic vascular response to infused angiotensin II. In nonpregnant sheep angiotensin II increases the lymph flow rate by what appears to be a direct effect on the lymphatic vessels. The purpose of this study was to test the hypothesis that during pregnancy the lymph flow rate response to angiotensin II infusion is decreased in relation to that of the nonpregnant state. We speculate that a decrease in lymph flow may explain the interstitial fluid retention observed during human pregnancy. In nine nonpregnant and five pregnant chronically catheterized ewes, we infused angiotensin II at rates of 0.1, 10, and 1000 ng/kg/min during a 5-minute period, with intervals of at least 15 minutes between doses. At the highest angiotensin II dose, peak lymph flow rate increased 286% in pregnant ewes compared with an increase of 344% in the nonpregnant sheep (p less than 0.05). No changes occurred in the intravascular volume, plasma or lymph protein concentration, or venous pressure. The arterial pressure responses to angiotensin II were decreased in pregnant sheep (p less than 0.05). These results are compatible with a model for fluid retention in pregnancy in which a decreased lymph flow rate plays a significant role in interstitial fluid retention.