Ageing attenuates the effect of extracellular hyperosmolality on whole-body heat exchange during exercise-heat stress.

We recently demonstrated that sweating-induced hypohydration attenuated whole-body heat dissipation to a greater extent in young compared to middle-aged men during exercise-heat stress. Here, we evaluated whether this divergent response stemmed from an attenuated influence of extracellular hyperosmolality on heat exchange with ageing. To achieve this, ten young (mean (SD): 25 (5) years) and ten middle-aged-to-older (61 (5) years) men completed two trials involving a 90-min intravenous infusion of isosmotic saline (0.9% NaCl; ISO) or hyperosmotic saline (3.0% NaCl; HYP) followed by 60 min of cycling at a fixed metabolic heat production of 250 W/m2 (∼50% peak aerobic power) in dry heat (40°C, ∼17% relative humidity). Whole-body net heat exchange (evaporative heat loss - dry heat gain) was measured via direct calorimetry. Rectal temperature was monitored continuously. Heat exchange was attenuated in HYP compared to ISO in the young (233 (20) vs. 251 (17) W/m2 ; P = 0.002) but not older group (229 (16) vs. 227 (20) W/m2 ; P = 0.621). Further, heat exchange was lower in the middle-aged-to-older vs. young men in ISO (P = 0.034) but not in HYP (P = 0.623). Similarly, end-exercise rectal temperature was greater in HYP relative to ISO in the young (38.3 (0.4)°C vs. 37.9 (0.3)°C; P = 0.015) but not the middle-aged-to-older men (38.3 (0.3)°C vs. 38.2 (0.2)°C; P = 0.652). Compared to the young, rectal temperature was greater in the middle-aged-to-older during ISO (P = 0.035) whereas no between-group difference was observed in HYP (P = 0.746). Our findings indicate that ageing blunts the effect of extracellular hyperosmolality on thermoregulatory function during exercise-heat stress.