Sodium balance modulates thirst in normal man

Several lines of evidence suggest that angiotensin II (AII) plays an important physiologic role in the control of thirst in laboratory animals but a conflicting one in humans. Sodium (Na+) balance plays a key role in the control of the renin-angiotensin system, but studies assessing the effect of sodium balance on thirst perception in humans are limited at best. To address this question, we studied the relationship between thirst perception and plasma osmolality during 5% saline infusion (.08 ml/kg/min x 120 min) in 5 healthy volunteers while in metabolic balance on both a 10 mEq. sodium (LS) and 200 mEq. sodium (HS) diet with and without infusion of AII (5 ng/kg/min). Thirst perception was quantified using a linear visual analogue scale. The relationship between serum Na+ (a measure of osmolality) and thirst perception was analyzed using linear regression. The mean x-intercept ([Na+] mEq/l) which represents the osmotic threshold to thirst was 138.2 +/- 0.5 in LS vs 140.7 +/- 0.8 in HS, p < 0.05. We conclude that the osmotic threshold for thirst is lower in LS (high endogenous AII) vs HS (low endogenous AII). There was no evidence for substantial extracellular volume change in HS vs LS with no significant differences in weight, hematocrit or total serum protein. Acute AII infusion did not result in changes in slope or x-intercept, but the pressor response to exogenous AII may have inhibited its dipsogenic effect (as has been shown in animal studies). These data suggest a physiologic role of sodium balance (possibly mediated via endogenous AII) in the control of thirst in normal humans.