MECHANISM OF DRINKING-INDUCED INHIBITION OF VASOPRESSIN SECRETION IN DEHYDRATED DOGS

Ingestion of water stimulates a powerful inhibitory input to secretion of arginine vasopressin (AVP) in many species. A previous study in dogs has suggested that the stimulus arises from activation of oropharyngeal receptors, but the nature of the stimulus is unknown. The objectives of this study were to determine if the taste, osmolality, or temperature of the solution ingested constituted an important element in the inhibitory mechanism and if these same attributes affected the volume ingested in response to 24 h of water deprivation in conscious dogs. Experiments consisted of a control period, a 6-min period of access to fluid, and a 60-min period after drinking, with blood samples taken frequently to assess changes in plasma AVP. Dogs were placed in a sling that allowed them to stand or lie supported with easy access to a bowl. The solutions were water at 20 and 38-degrees-C; 0.9% NaCl at 20 and 38-degrees-C; 1.8 and 2.7% NaCl at 20-degrees-C; 5% glucose and mannitol and 10% mannitol at 20-degrees-C; and liquified food at 20-degrees-C. In the time-control experiment dogs were allowed to see but not drink water for 6 min. Results suggested that drinking-induced inhibition of AVP secretion consists of two phases: the first, lasting up to 9 min, is consistent with a complete cessation of AVP secretion and appears to be caused by the act of drinking itself because altering the taste, osmolality, or temperature of the solution had no effect on the inhibitory mechanism; in the second AVP secretion may either remain low, e.g., plasma AVP continues to fall after drinking water, or secretion may increase, e.g., plasma AVP increased after drinking solutions isotonic or hypertonic to body fluids. Thus AVP secretion in this phase appears to depend on changes in peripheral plasma osmolality. The mechanism that meters intake of fluid in response to the 24 h of water deprivation also appears to be relatively independent of the taste, osmolality, and temperature of the fluid consumed in the dog because intakes of all the test solutions were similar to the intake of water.