Acute reduction of plasma vasopressin levels by rehydration in sheep

Sheep were depleted of water by restricting water intake to 500 ml/day for 7-9 days and were then rehydrated by three treatments: voluntary intake of water, administration of water by tube into the stomach, or voluntary intake of 0.9% NaCl solution (saline). The volumes of fluids drunk within 2-3 min, or administered by tube, were approximately equal to the animal's weight loss. Plasma vasopressin rose from 4.4 +/- 0.6 to 16.8 +/- 1.0 pg/ml during water restriction. After drinking water plasma vasopressin fell from 19.0 +/- 1.9 to 7.5 +/- 0.4 pg/ml (P less than 0.001) in 15 min and gradually fell to 3.2 +/- 0.4 pg/ml over 6 h. Plasma osmolality fell from 302.4 +/- 0.9 to 301.0 +/- 1.1 mosmol/kg (NS) 15 min after water drinking and then gradually fell to subnormal levels. Sheep given water by stomach tube showed a similar decline in plasma osmolality, but the fall in plasma vasopressin was attenuated. The fall in plasma vasopressin in the first 30 min after drinking saline was almost identical with the fall after drinking water, but plasma osmolality was unaltered. Plasma vasopressin fell so rapidly after drinking water or saline as to suggest that the act of drinking caused almost complete inhibition of vasopressin release without a change in plasma osmolality. The results are consistent with earlier evidence that oropharyngeal receptors initiate the inhibition of vasopressin release after drinking.