Osmoregulation during Pregnancy in the Rat

Abstract

AB S T R A C T Osmoregulation was studied in near term and age-matched Sprague-Dawley rats. Basal plasma osmolality (Posm) and plasma sodium (PNa) were 281+3 mosmol/kg and 134+3 meq/liter, respectively, on the 20th gestational day compared with 292+±3 mosmol/kg and 140+1 meq/liter in virgin animals (P < 0.001), whereas Purea and plasma water content were similar in pregnant and control rats. These differences could not be reproduced in animals receiving progesterone, estrone, or a combination of progesterone and estradiol for 2 wk.Pregnant and control rats were deprived of water for changes (A%) were significantly greater in the gravid animals (P < 0.01). Therefore, Posm was increased without concomitant volume depletion by intraperitoneal hypertonic saline. Again PAVP VS. Posm correlated significantly (r > 0.9; P < 0.001) in each group, and the apparent threshold was 14 mosmol lower in pregnant animals. Diluting ability, tested by oral water loading, was not impaired in the pregnant animals which excreted a 30 ml/kg load as well as controls. Also, chronically hydrated virgin animals whose fluid intake was more than twice that of pregnant rats (for 19 d) did not lower their Posm.In separate studies homozygous Brattleboro rats, which produce no endogenous vasopressin, were also shown to have a decreased Posm (pregnant 292 +4 mosmol/kg; virgin 310+6 mosmol/kg P < 0.001), but unchanged Uosm during pregnancy.Data demonstrate a resetting ofthe osmostat in gravid Sprague-Dawley rats as Posm and the threshold for AVP secretion both decrease significantly during gestation in this species. Studies in homozygous Brattleboro animals with hereditary diabetes insipidus suggest that the osmotic threshold for thirst is reset as well. INTRODUCTION Plasma osmolality decreases during normal human pregnancy to values that range 5-10 mosmol/kg below that of nonpregnant women (1-3). This decrement is apparent by the 6th gestational wk and is sustained until term. If such a reduction in osmolality occurred in a nongravid subject she would cease secreting antidiuretic hormone (ADH)l and enter a state of con-

Angiotensin-(1-7) serves as an aquaretic by increasing water intake and diuresis in association with downregulation of aquaporin-1 during pregnancy in rats

“…The present study also confirms previous reports demonstrating increased water intake (1,4,9) and food consumption (16,21) during gestation. In fact, a decrease in the osmotic threshold for drinking as well as for AVP secretion is necessary to maintain the new steady state of fluid homeostasis.…”

“…This reduction in serum Na with pregnancy is confirmed by our study. Also during pregnancy, there is a resetting or lowering of the osmotic threshold for AVP secretion and thirst (4,9,19). In pregnancy, the reduction in osmolality is reported to be accompanied by levels of circulating AVP that are unchanged compared with virgin levels, indicating that the levels of AVP have been reset to a higher than expected level (9).…”

“…Also during pregnancy, there is a resetting or lowering of the osmotic threshold for AVP secretion and thirst (4,9,19). In pregnancy, the reduction in osmolality is reported to be accompanied by levels of circulating AVP that are unchanged compared with virgin levels, indicating that the levels of AVP have been reset to a higher than expected level (9). The current data showing increased circulating AVP above virgin levels are consistent with the leftward shift of higher levels of AVP at every level of osmolality but differ from previous reports in that plasma AVP in our study may be associated with an even greater leftward shift in the relationship.…”

Differential long-term intrarenal and neurohormonal effects of captopril and prazosin in patients with chronic congestive heart failure: importance of initial plasma renin activity.

“…Plasma for osmometry was always collected on heparin and recommendations from the literature were strictly followed. 11 Plasma effective osmolality (Eosm, in mOsm/kg H20) was calculated according to the formula…”

The role of steroid hormones in the regulation of vasopressin and oxytocin release and mRNA expression in hypothalamo neurohypophysial explants from the rat

“…Similarly, in the rat, the osmotic threshold for vasopressin secretion is lower during gestation (Durr et al 1981). Furthermorc, Crowley & Amico (1993) reported that gonadectomy prevented the increases in vasopressin and oxytocin mRNA content observed following chronic saline consumption in males and females, and testosterone replacement in males restored the response.…”