Effect of plasma osmolality on thermoregulatory responses to passive heating in humans

We quantified the effect of plasma osmolality (Posm) on the body core temperature (Tes) threshold for active cutaneous vasodilation and sweating during passive body heating in six male subjects. To modify Posm, subjects were infused with 0.9 % (0.9% Inf), 2% (2% Inf) or 3 % (3% Inf) NaCl solutions for 90 min. Infusion rate was 0.1, 0.15 and 0.2 ml·kg^-1 body wt. for 0.9% Inf, 2% Inf, and 3% Inf, respectively, which produced similar plasma volume expansion among infusions. Thirty min after the end of infusion, subjects immersed their lower legs in a water bath (42°C; room temperature, 28°C) for 60 min. Passive heating without infusion (NI) was served as time-control to account for the effect of volume expansion. Posm (mosm/kg H₂O) at the onset of passive heating was 289.9 ±1.4, 292.1 ± 0.6, 298.7 ±0.7, and 305.6± 0.6, after NI, 0.9% Inf, 2% Inf, and 3% Inf, respectively. Blood pressure did not change during infusion and passive heating in any conditions. The increase in Tes at equilibrium during passive heating was 0.47 ±0.08 °C, 0.59 ±0.08 °C, 0.85 ±0.13 °C, and 1.09 ± 0.12 °C after NI, 0.9% Inf, 2 % Inf, and 3 % Inf, respectively which indicates that the Tes in the heat increases with the increase of Posm. The Tes thresholds for cutaneous vasodilation and sweating were also increased linearly as Posm increased. The calculated increase in these thresholds per unit increase in Posm were 0.044 °C for vasodilation and 0.034 °C for sweating. Thus, the Tes thresholds for cutaneous vasodilation and sweating are shifted to higher Tes with the increase in Posm, which results in the elevation of Tes during passive heating.