To gain better insights into the mechanisms of heat acclimation, we examined the relationship between osmotic shift in the body temperature thresholds for thermoregulatory responses, such as sweating and cutaneous vasodilation (CVD), and sweat sodium concentration ([Na⁺]), which decreases in heat acclimated subjects, in six subjects. The esophageal temperature (Tes) thresholds for sweating and cutaneous vasodilation were determined during passive body heating following hypertonic (3% NaCl) or isotonic (0.9% NaCl) saline infusion. The passive body heating was induced by lower leg water immersion at 42 ℃ in a room temperature of 28℃. The rise in Tes during passive heating was much higher following hypertonic saline infusion than isotonic one. The ⊿Tes required to elicit sweating and CVD were also higher following hypertonic saline infusion. Because ⊿Tes during passive body heating linearly correlated with the thresholds for sweating and CVD, plasma hyperosmolality inhibits thermoregulatory responses to increased body temperature by elevating the thresholds for sweating and CVD. The osmotic shifts in the ⊿Tes thresholds for sweating and CVD linearly increased as sweat [Na⁺] increased. Plasma vasopressin concentration increased with plasma osmolality, and the osmosensitivity for vasopressin secretion (⊿［AVp］／⊿Posm) decreased as sweat [Na⁺] increased. These results suggest that heat acclimation reduces osmotic inhibition of thermoregulatory functions but enhances osmoregulatory fluid retention by increasing osmosensitivity for vasopressin release. The osmoregulatory adaptation might be involved in the enhanced thermoregulatory function in heat acclimated subjects during prolonged exercise and/or heat exposure.

DESCENTE SPORTS SCIENCE Vol.19/THE DESCENTE AND ISHIMOTO MEMORIAL FOUNDATION FOR THE PROMOTION SPORTS SCIENCE