Osmoregulatory modulation of thermal sweating in humans: reflex effects of drinking

To gain better insight into the interaction between thermoregulation and osmoregulation, we examined the thermal sweating response to drinking in cell-dehydrated humans. Cell dehydration (CDH) was induced by infusion of a 3% NaCl solution, at 1.2 ml/kg, for 2 h; infusion of a 0.9% NaCl solution in a separate experiment served as a control (euhydrated condition, EH). After infusion, subjects were heated by immersion of the lower legs in 42 degrees C water at an ambient temperature of 28 degrees C for 90 min. Subjects drank 4.3 ml/kg of H2O (approximately 38 degrees C) at 60 min of heating. The 3% NaCl infusion increased plasma osmolality by 13.6 +/- 0.8 mosmol/kgH2O and plasma arginine vasopressin concentration ([AVP]) by 3.3 +/- 0.7 pg/ml. Neither variable was altered with 0.9% NaCl infusion. Before drinking, esophageal temperature (Tes) had increased by 0.91 +/- 0.08 degrees C in CDH and by 0.40 +/- 0.11 degrees C in EH. Local chest sweating rate (SRch) had increased by 0.67 +/- 0.08 and 0.63 +/- 0.07 mg.min-1.cm-2 in CDH and EH, respectively. Thus the change in SRch per unit rise in Tes was much lower in CDH than in EH. Drinking immediately increased SRch and reduced Tes in CDH, with a reduction in plasma [AVP] and thirst rating. Drinking did not change thermoregulatory and osmoregulatory responses in EH. These results suggest that the act of drinking itself eliminates, at least partially, an osmotic inhibitory input to the thermoregulatory center, as well as osmotic AVP secretion and thirst.