Changes in vasopressin and cortisol secretion during operant drinking in dehydrated pigs.

Two studies were carried out to examine endocrine changes during a 60-min period of operant drinking in 24-h dehydrated pigs. Measurements were made of water intake, osmolality, hematocrit, and plasma concentrations of lysine vasopressin (LVP) and cortisol during rehydration (experiment 1) and of fluid intake, osmolality, and LVP levels during consumption of isotonic saline (experiment 2). Increases in osmolality and LVP produced by dehydration in experiment 1 were rapidly reversed during rehydration with the result that osmolality returned to predeprivation levels after 20 min and LVP after 30 min, but there was no evidence of a decrease in LVP before absorption. Plasma cortisol concentrations were unaffected by dehydration although they declined during the final 40 min of experiment 1. In experiment 2, osmolality remained elevated during saline drinking, but plasma concentrations of LVP declined abruptly. The results suggest that oropharyngeal factors inhibiting vasopressin release, revealed during saline ingestion, are obscured during normal rehydration as a result of the rapid rate at which water is absorbed by the gut in this species.