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. 1981 Aug;68(2):337–346. doi: 10.1172/JCI110261

Osmoregulation during Pregnancy in the Rat

EVIDENCE FOR RESETTING OF THE THRESHOLD FOR VASOPRESSIN SECRETION DURING GESTATION

Jacques A Durr 1,2,3, Barbara Stamoutsos 1,2,3, Marshall D Lindheimer 1,2,3
PMCID: PMC370804  PMID: 7263858

Abstract

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 periods ranging from 0 to 48 h. Posm, always lower in gravidity, was 290±3 mosmol/kg after 2 d of water deprivation in pregnant animals compared with 300±2 mosmol/kg in controls (P < 0.001). Thus 48 h of dehydration were required before Posm in gravid rats was similar to basal values in the age-matched virgins.

Despite strikingly lower Posm, plasma arginine vasopressin (PAVP) and urinary osmolality (Uosm) were similar in the basal state averaging 2.16±0.78 pg/ml and 1,652±406 mosmol/kg, respectively, during pregnancy compared with 2.08±2.17 pg/ml and 1,483±203 mosmol/kg in controls (NS). Water deprivation increased PAVP and Uosm similarly in pregnant and virgin rats: these values reached 22.7±3.3 pg/ml and 3,300±123 mosmol/kg at 48 h in gravid compared with 26.0±6.4 pg/ml and 3,342±141 mosmol/kg in the controls (NS). Regression equations for PAVPvs. Posm which were highly significant (P < 0.001) in both groups demonstrated an apparent threshold for AVP secretion approximately 11 mosmol lower in gravid animals.

Intravascular volume decreased, and plasma aldosterone increased during water deprivation, and both changes (Δ%) were significantly greater in the gravid animals (P <0.01). Therefore, Posm was increased without concomitant volume depletion by intraperitoneal hypertonic saline. Again PAVPvs. 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 of the 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.

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Selected References

These references are in PubMed. This may not be the complete list of references from this article.

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