Sodium appetite, thirst, and body fluid regulation in humans during rehydration without sodium replacement

Abstract

After a 7-h H2O and Na+ depletion period (DP), produced by intermittent light exercise (8 bouts) at 35 degrees C, we examined thirst and taste palatability responses to 10 different NaCl solutions during 23 h of rehydration (RH) at 25 degrees C. During DP, net H2O and Na+ loss were 27.2 +/- 2.9 ml/kg and 3.29 +/- 0.45 meq/kg, respectively. Plasma osmolality (POsm) and plasma Na+ concentration ([Na+]p) increased significantly during DP by 3.4 +/- 1.2 mosmol/kgH2O and 3.0 +/- 1.0 meq/kgH2O, respectively. Plasma volume (PV) decreased by 6.5 +/- 1.9%. Thirst rating, renal fractional reabsorption of H2O, and plasma arginine vasopressin concentration (PAVP) increased as POsm increased. This increased thirst was accompanied by increased palatability ratings to H2O. During RH, subjects drank deionized H2O ad libitum and ate a Na(+)-free diet for 23 h. POsm and [Na+]p returned to control levels within 1 h RH and remained at or below the control thereafter. PV remained reduced by approximately 5% throughout RH. The increased thirst and PAVP returned to their respective control levels within 1 h of RH as POsm decreased, but thirst rating increased against between 17 and 23 h of RH without increase in POsm or PAVP. Palatability ratings to a 1 M NaCl solution at and after 3 h RH and palatability ratings to 0.3 M at 17 and 23 h RH were significantly higher than control. Plasma aldosterone concentration (PAldo) increased after DP, decreased with drinking, and increased again between 6 and 23 h of RH, accompanied by a marked decrease in fractional Na+ excretion to < 0.07%. Thus both Na+ preference and thirst in humans are influenced by body fluid and electrolyte status. The increased Na+ palatability (Na+ appetite) was preceded by osmotically induced thirst, and accompanied by nonosmotically driven thirst [extracellular fluid (ECF) thirst] and increased PAldo. The "Na+ appetite" and "ECF thirst" along with increased renal Na+ retention could contribute to ECF volume regulation after thermally induced H2O and Na+ depletion.