Volume homeostasis during pregnancy in the rat

This article reviews alterations in volume and sodium handling during rat pregnancy, noting similarities and contrasts to events in human gestation. Gravid rodents undergo extracellular and plasma volume increases of 50% to 70%, and these changes accompany a marked cumulative sodium retention shared by both dam and fetuses. Pregnancy alters several factors, with opposing effects on renal salt handling; however, mechanisms by which gestational sodium accumulation and volume expansion are achieved remain obscure. Furthermore, despite substantial increases in absolute blood volume, considerable uncertainty exists as to how this volume is sensed, particularly during the final gestational week when a rapid increase in volume is associated with decreases in peripheral resistance and BP. Attempts to assess "effective" intravascular volume by measuring responses to intravenous (IV) or oral sodium loading or to chronic mineralocorticoid administration indicate that pregnant and nonpregnant rats respond similarly, suggesting that such animals sense their volume as normal. In contrast, when salt-restricted, gravid rats fail to expand their plasma volume normally; this relative hypovolemia activates mechanisms leading to free water retention and pathologic hyponatremia, responses not observed in virgin animals.