Water homeostasis and vasopressin release during rodent and human gestation.

Body tonicity decreases during both human and rodent gestation, manifested by a decrease in Posm of approximately 10 mosm/kg. In humans, the decrease in the osmotic threshold for thirst may precede decrements in the arginine vasopressin (AVP) secretory threshold by several weeks, whereas the metabolic clearance rate of AVP appears markedly increased in the third trimester. The cause of these osmoregulatory changes are unclear, since through 1986 we have been unable to implicate placental extracts, estrogens, progesterone, prolactin, angiotensin II, and certain endorphins in the decreased Posm observed during rat pregnancy. Changes also occur in volume-AVP secretory relationships in pregnancy. For instance, the volume-sensing AVP release mechanisms are altered during gestation in Sprague-Dawley rats in such a way that the increased intravascular volume is recognized as normal.