Relationship of osmotic inhibition in thermoregulatory responses and sweat sodium concentration in humans

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