Long-term blood pressure and metabolic effects of vasopressin with servo-controlled fluid volume

Studies were performed in normal mongrel dogs (n = 8) to assess whether changes observed with chronic administration of vasopressin (AVP) were a result of direct actions of AVP or the consequence of changes in body fluid volume. AVP was infused continuously for 2 wk (0.36 ng X kg-1 X min-1 iv), while total body weight and body water (TBW) were maintained constant (+/- 50 g) using a servo-controlled system. A metabolic cage was mounted on sensitive force transducers for continuous monitoring of TBW. The summed voltage output of these transducers was used to servo control an intravenous infusion pump that adjusted the rate of water intake required for maintenance of a constant TBW. AVP infused under these conditions chronically increased plasma AVP levels from 2 to 22 pg/ml but resulted in no change of average 24-h mean arterial pressure, plasma sodium, or osmolality. Urine excretion decreased from 800 to 200 ml/day, whereas urine osmolality increased from 430 to 1,200 mosmol/kg and remained at these levels throughout the 2-wk AVP infusion. A net loss of 20 meq sodium occurred during the 1st day of AVP infusion but thereafter was unchanged. Plasma sodium and osmolality were unchanged from control during AVP infusions. We conclude that AVP-induced changes of arterial pressure, plasma sodium concentration and osmolality, renal escape, suppression of renin activity, and most of the observed natriuresis are events normally dependent on volume expansion.