Drinking, oropharyngeal signals, and inhibition of vasopressin secretion in dogs.

Ingestion of water leads to a rapid fall in plasma levels of vasopressin in 24-h water-deprived dogs. The rapid inhibition of vasopressin secretion is not due to absorption of water and dilution of plasma nor is it due to gastric distension. Blood pressure rises sharply during drinking, and this may provide the afferent signal leading to inhibition of vasopressin secretion. Other possibilities include a generalized increase in oropharyngeal motor activity or simply the sight of water. To test these hypotheses, 24-h water-deprived dogs were given either phenylephrine, to mimic the rise in blood pressure observed during drinking, offered canned meat, to induce oropharyngeal motor activity and swallowing, or presented with a bowl of water that was placed just out of reach. Administration of phenylephrine, ingestion of food, and the presence of water all caused increases in blood pressure, which were similar to the rise in blood pressure during drinking. However, none of these paradigms caused a fall in plasma vasopressin. In contrast, ingestion of water consistently led to a significant fall in plasma vasopressin, which was detectable within 3 min of drinking and well before changes in plasma osmolality. Therefore we conclude that the mechanism by which ingestion of water causes rapid inhibition of vasopressin secretion in dehydrated dogs cannot be dependent on a rise in blood pressure, a nonspecific increase in oropharyngeal motor activity, or the presence of water.