OSMOREGULATORY MODULATION OF THERMAL SWEATING IN HUMANS - REFLEX EFFECTS OF DRINKING

To gain better insight into the interaction between thermoregulation a nd osmoregulation, we examined the thermal sweating response to drinki ng in cell-dehydrated humans. Cell dehydration (CDH) was induced by in fusion 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 (euhydrat ed condition, EH). After infusion, subjects were heated by immersion o f 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 (similar to 38 degrees C) at 60 min of heating. The 3% NaCl infusion increased plasma osmolality by 13.6 +/- 0.8 mosmol/kgH(2)O and plasma arginine vasopre ssin concentration ([AVP]) by 3.3 +/- 0.7 pg/ml. Neither variable was altered with 0.9% NaCl infusion. Before drinking, esophageal temperatu re (T-es) 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 (SR(ch)) had incre ased by 0.67 +/- 0.08 and 0.63 +/- 0.07 mg . min(-1) . cm(-2) in CDH a nd EH, respectively. Thus the change in SR(ch) per unit rise in T-es w as much lower in CDH than in EH. Drinking immediately increased SR(ch) and reduced T-es in CDH, with a reduction in plasma [AVP] and thirst rating. Drinking did not change thermoregulatory and osmoregulatory re sponses in EH. These results suggest that the act of drinking itself e liminates, at least partially, an osmotic inhibitory input to the ther moregulatory center, as well as osmotic AVP secretion and thirst.