Vasopressin response to hyperosmolality and hypotension during ovine pregnancy

The arginine vasopressin (AVP) responses to hyperosmolality and to hypotension were compared in pregnant and nonpregnant ewes. When the responses to infusion of normal or hypertonic saline were compared, plasma AVP and Na+ concentrations were lower in pregnant ewes than nonpregnant ewes, but the relation between plasma AVP and Na+ concentrations was not altered in the pregnant state. In a second study the AVP response to hypotension, induced by the infusion of 2.5, 5.0, or 10.0 micrograms nitroprusside.kg-1.min-1, was compared in pregnant and nonpregnant ewes. Despite significantly lower mean arterial blood pressures in the pregnant ewes, the mean plasma AVP concentration after infusion of nitroprusside was not increased during pregnancy. Therefore, the relation between mean arterial pressure and AVP was significantly shifted to the left in the pregnant ewes, indicating lower AVP concentrations for a given level of arterial pressure during pregnancy. The results suggest that pregnancy alters the regulation of AVP by arterial pressure but does not affect the regulation of AVP by plasma sodium concentration in the ewe.