Role of volume in the regulation of vasopressin secretion during pregnancy in the rat.

We previously observed that osmoregulation and the osmotic threshold for antidiuretic hormone secretion were altered during pregnancy in Sprague-Dawley rats and the present study evaluated the influence of volume on arginine vasopressin (AVP) release during gestation in this species. Basal plasma osmolality (Posm) and intravascular volume were 297 +/- 3 mosmol/kg and 16.2 +/- 1.2 ml in virgin animals compared with 290 +/- 2 mosmol/kg and 20.2 +/- 2.3 ml in 14-d pregnant rats and 287 +/- 3 mosmol/kg and 25.2 +/- 2.3 ml in 21-d (near-term) pregnant rats (P less than 0.001, each pregnant group vs. virgin). Isosmotic volume depletion was produced by intraperitoneal polyethylene glycol. Volume decreased from 1 to 26% and blood pressure remained stable during decrements as high as 16%. Plasma AVP (PAVP) did not rise significantly in either group of pregnant animals or virgin controls until blood volume depletion reached 6-7%, after which levels rose in a similar exponential manner in virgin, 14-d, and 21-d pregnant animals. In terms of absolute changes, however, PAVP in gravid rats started to increase when intravascular volume was still considerably greater than basal blood volume in the nonpregnant controls. Other experiments, where Posm was increased by intraperitoneal hypertonic saline, reconfirmed that the osmotic threshold for AVP secretion was reduced congruent to 10 mosmol/kg during pregnancy and that AVP release was stimulated by increments in body tonicity as small as 1-2%. In parallel studies, blood volume contraction and increases in Posm were evoked by intraperitoneal polyethylene glycol dissolved in hypertonic saline and results compared with animals receiving intraperitoneal saline alone. Decrements in volume (congruent to 7%), which alone would increase PAVP minimally, increased the sensitivity of the secretory response to changes in osmolality two- to three-fold, an effect which was similar in virgin and gravid animals. Finally, restricting water intake of pregnant rats to that of virgins on days 16-20 of gestation led to suboptimal volume expansion, hypertonicity, and an exaggerated increase in PAVP. These results demonstrate that despite an intravascular space which at term is nearly twice that of virgin rats, pregnant animals secrete AVP in response to fractional volume depletion in a manner similar to nonpregnant controls; that is, the relationship between total blood volume and AVP secretion is altered during gestation such that the expanded blood volume is recognized as normal.