Sodium excretion in human pregnancy: a role for arginine vasopressin.

Plasma arginine vasopressin concentration was measured before and after 1 week's dietary sodium loading (greater than 250 mmol/day) or deprivation (less than 50 mmol/day) in 50 second-trimester and 34 third-trimester primigravidas. A significant rise in plasma arginine vasopressin was observed only in second-trimester subjects following sodium loading (5.6 +/- 0.5 to 6.6 +/- 0.5 pg/ml; p less than 0.05). Plasma sodium concentration and blood pressure did not alter, and plasma volume increased slightly in this group (24.4 +/- 0.5 to 25.9 +/- 0.7 ml/cm height; p less than 0.01). Hence the observed increment in plasma arginine vasopressin concentration in this group could not be explained by alterations to the major factors regulating arginine vasopressin release. It is suggested that arginine vasopressin secretion increases to assist natriuresis following dietary sodium loading in the second trimester of human pregnancy.