Regulation and function of arginine vasopressin in pregnant sheep.

The aim of this study was to investigate the regulation and function of arginine vasopressin (AVP) in pregnant sheep. The mean plasma osmolality of nonpregnant ewes (298 +/- 1.0 mosmol/kg, n = 8) was not significantly different from that of late pregnant ewes (295 +/- 1.1 mosmol/kg, n = 21). The mean resting plasma [AVP] of nonpregnant ewes (4.1 +/- 0.6 pg/ml,n = 8) was not significantly different from that of pregnant ewes (3.3 +/- 0.3 pg/ml,n = 21). In a series of dehydration experiments it was established that the slope of the function relating log [AVP] to plasma osmolality for pregnant ewes (n = 13) was not significantly different from the slope of the function relating log [AVP] to plasma osmolality for nonpregnant ewes (n = 4). When AVP was infused into water-loaded ewes, a significant decrease in urinary flow rate and free water clearance occurred at an infusion rate of 0.003 microgram/h in both the pregnant (n = 4) and nonpregnant (n = 4) animals. Both groups achieved negative free water clearance at an infusion rate of 0.01 microgram/h. These findings suggest that pregnancy does not alter the relationship between plasma osmolality and plasma [AVP] or the renal responsiveness to AVP in sheep.