Osmoregulation in pseudopregnant and prolactin-treated rats: comparison with normal gestation

Osmoregulation was studied throughout rodent pregnancy focusing on the importance of the fetoplacental unit and prolactin in the observed alterations. Plasma osmolality (Posmol) and plasma sodium (PNa), similar in 8-day gravid and virgin Sprague-Dawley rats, decreased significantly by gestational day 10, reaching a nadir 8-10 mosmol/kg and 3-5 meq/l, respectively, below virgin levels by day 14 (both P less than 0.001). Despite this, plasma arginine vasopressin (PAVP) was measurable and similar in all pregnant and virgin groups. Osmotic thresholds for arginine vasopressin (AVP) secretion, similar in 8-day gravid and virgin rats, decreased 7.7 and 10.7 mosmol/kg in 12- and 14-day pregnant rats, respectively (both P less than 0.001). In contrast, Posmol decreased less than 2 mosmol/kg in 12- to 14-day pseudopregnant animals. When pseudopregnancy was prolonged to 18 days by prior hysterectomy, Posmol was only 2.6 mosmol/kg below that of cycling, hysterectomized controls. In other studies 14 days of hyperprolactinemia evoked by estradiol or treatment with ovine or rat prolactin had minimal effect on Posmol. We conclude that parallel decrements in Posmol and osmotic thresholds for AVP release occur during early rodent pregnancy, alterations that cannot be explained by gestational increases in circulating prolactin. In addition, the failure of pseudopregnancy. to mimic the hypotonicity of gestation suggests an important role for the fetoplacental unit in the osmoregulatory changes of rat pregnancy.