Relationship Between Plasma Renin, Renin-Substrate, Angiotensin II, Aldosterone and Electrolytes in Normal Pregnancy

Abstract

Plasma concentrations of renin, reninsubstrate, angiotensin II and aldosterone were measured in normal pregnant women at various stages of gestation and postpartum. The mean circulating levels of renin-substrate, angiotensin II and aldosterone increased significantly from the first to the third trimester. Mean plasma renin concentration was highest in the first trimester and fell significantly to the third trimester, although it remained above the normal nonpregnant range throughout pregnancy. Unlike many other physiological and pathological situations in man, no significant relationship was found between concurrent plasma concentrations of renin and angiotensin II, renin and aldosterone or angiotensin II and aldosterone. Plasma renin-substrate concentration did not correlate with plasma angiotensin II concentration, but it did have a positive correlation with plasma aldosterone concentration. The plasma concentration of aldosterone was positively related to plasma sodium concentration, which was below the normal nonpregnant range in most women throughout gestation. The reason for this dissociation between the major components of the renin-angiotensin-aldosterone system in normal pregnancy remains unclear. In pregnant women it appears that, although angiotensin II may be one of the factors affecting aldosterone secretion, it is a relatively minor one and other influences are of greater importance.