Relationship of osmotic inhibition in thermoregulatory responses and sweatsodium concentration in humans

Heat acclimatization improves thermoregulatory responses to heat stress and decreases sweat sodium concentration ([Na+](sweat)). The reduced [Na+](swe at) results in a larger increase in plasma osmolality (P-osmol) at a given amount of sweat output. The increase in Posmol inhibits thermoregulatory re sponses to increased body core temperature. Therefore, we hypothesized that the inhibitory effect of plasma hyperosmolality on the thermoregulatory re sponses to heat stress should be attenuated with the reduction of [Na+](swe at) due to heat acclimatization. Eleven subjects (9 male and 2 female) were passively heated by immersing their lower legs into water at 42 degreesC ( room temperature 28 degreesC and relative humidity 30%) for 50 min followin g isotonic or hypertonic saline infusion. We determined the increase in the esophageal temperature (T-es) required to elicit sweating and cutaneous va sodilation (CVD) (DeltaT(es) thresholds for sweating and CVD, respectively) in each condition and calculated the elevation of the T-es thresholds per unit increase in P-osmol as the osmotic inhibition of sweating and CVD. The osmotic shift in the DeltaT(es) thresholds for both sweating and CVD corre lated linearly with [Na+](sweat) (r = 0.858 and r = 0.628, respectively). T hus subjects with a lower [Na+](sweat) showed a smaller osmotic elevation o f the DeltaT(es) thresholds for sweating and CVD. These results suggest the possibility that heat acclimatization attenuates osmotic inhibition of the rmoregulatory responses as well as reducing [Na+](sweat).