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

Abstract

In humans, hypohydration attenuates sweat secretion and attenuates whole-body heat loss, probably to mitigate further fluid losses and thereby support blood pressure regulation. Recently, however, we demonstrated that the hypohydration-mediated reduction in net whole-body heat exchange (evaporative heat loss-dry heat gain) was blunted in middle-aged compared to younger men during moderate exercise in dry heat; albeit, the underpinning mechanisms could not be determined. Here we evaluated the hypothesis that those findings stemmed from a diminished influence of extracellular hyperosmolality on net whole-body heat exchange in middle-aged-to-older compared to young men. Consistent with that hypothesis, extracellular hyperosmolality induced by an intravenous infusion of hypertonic saline (3% NaCl) reduced net heat exchange and augmented rectal temperature to a greater extent in the young compared to middle-aged-to-older men. Thus, age-related differences in the influence of hypohydration on thermoregulatory function appear to be due to blunted sensitivity to hyperosmolality with ageing. 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 60min of cycling at a fixed metabolic heat production of 250W/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.