Regulation of fluid intake in dehydrated humans: Role of oropharyngeal stimulation

We examined the effect of oropharyngeal stimulation on thirst, secretion of arginine vasopressin ([AVP](p)), and fluid intake in six healthy adults after dehydration (28.6 +/- 1.4 ml/kg water loss) induced by mild exercise in the heat (2 h, 38 degrees C, relative humidity <30%). Subjects performed three identical dehydration protocols followed by 75 min of rehydration at 27 degrees C consisting of 1) ad libitum drinking (Con), 2) infusion of a similar volume of water directly into the stomach via a nasogastric tube (Inf) during the first 25 min followed by combined Inf and ad libitum drinking during the remaining 50 min of rehydration; or 3) ad libitum drinking with simultaneous extraction of ingested fluid via a nasogastric tube (Ext). Plasma osmolality (P-osm), [AVP](p), fluid intake, and thirst perceptions were measured throughout. On average, for all three protocols, P-osm increased 7.8 +/- 0.6 mosmol/kgH(2)O and plasma volume decreased 4.7 +/- 1.3%, whereas thirst ratings and [AVP](p) increased 7.6 +/- 1.3 cm and 3.1 +/- 0.4 pg/ml, respectively. Reflex inhibition of [AVP](p) and thirst occurred within 5 min of rehydration in Con and Ext (P < 0.05) but not during Inf, supporting the hypothesis that oropharyngeal reflexes modulate osmotically stimulated thirst and [AVP](p). However, the reduction in [AVP](p) during the first 5 min of Ext (-1.1 +/- 0.3 pg/ml) was less than that seen during Con (-2.1 +/- 0.4 pg/ml), suggesting that oropharyngeal stimulation is not the only factor contributing to the rapid reduction in [AVP](p) during the first 5 min of drinking. During Con, subjects ingested 20.0 +/- 2.0 ml/kg of water but only drank 15% more (31.3 +/- 7.1 ml/kg) during Ext, demonstrating a clear role of oropharyngeal metering in limiting total fluid intake in humans in the presence of a persistently high dipsogenic drive.