Influence of humoral and volume factors on altered osmoregulation of normal human pregnancy.

These studies were designed to characterize mechanisms leading to decreased plasma osmolality (Posmol) and osmotic thresholds (T) for arginine vasopressin (AVP) release (TAVP) and thirst (Tthirst) in pregnancy. First, the influence of the pregnancy hormone, human chorionic gonadotrophin (hCG), was tested in six nonpregnant women who received hypertonic saline during the luteal phase of the menstrual cycle on two occasions (randomly allocated), once after 15,000 IU of hCG when Posmol, TAVP, and Tthirst decreased 6, 5, and 5 mosmol/kgH2O, respectively (P less than 0.01). In contrast, hCG pretreatment of males (n = 6) had no significant effect. Next, the role of decreased effective vascular volume (underfilling) was evaluated in seven women undergoing hypertonic saline infusion in the presence and absence of head-out water immersion (randomly allocated) during early and late pregnancy and postpartum. Posmol, TAVP, and Tthirst were not influenced by immersion and remained 10 mosmol/kgH2O lower in pregnancy (P less than 0.01). Central redistribution of intravascular volume consistently lowered hematocrit and rate of rise of PAVP per unit increment in Posmol (P less than 0.01). Although these data failed to support the hypothesis that the osmoregulatory change in human pregnancy is attributable to decrements in effective central volume (underfill), they do suggest that hCG may play a role.