SODIUM-BALANCE MODULATES THIRST IN NORMAL MAN

Several lines of evidence suggest that angiotensin II (AII) plays an i mportant physiologic role in the control of thirst in laboratory anima ls but a conflicting one in humans. Sodium (Na+) balance plays a key r ole in the control of the renin-angiotensin system, but studies assess ing the effect of sodium balance on thirst perception in humans are li mited at best. To address this question, we studied the relationship b etween thirst perception and plasma osmolality during 5% saline infusi on (.08 ml/kg/min X 120 min) in 5 healthy volunteers while in metaboli c 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 betw een serum Na+ (a measure of osmolality) and thirst perception was anal yzed 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 f or thirst is lower in LS (high endogenous AII) vs HS (low endogenous A II). 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 s lope or x-intercept, but the pressor response to exogenous AII may hav e inhibited its dipsogenic effect (as has been shown in animal studies ). These data suggest a physiologic role of sodium balance (possibly m ediated via endogenous AII) in the control of thirst in normal humans.