Human and Environmental Physiology Research Unit, School of Human Kinetics, University of Ottawa, Ottawa, Ontario, Canada.
Faculty of Physical Activity Sciences, University of Sherbrooke, Sherbrooke, Québec, Canada.
J Physiol. 2020 Nov;598(22):5133-5148. doi: 10.1113/JP280132. Epub 2020 Sep 29.
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 60 min of cycling at a fixed metabolic heat production of 250 W/m (∼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/m ; P = 0.002) but not older group (229 (16) vs. 227 (20) W/m ; 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.
在人体中，脱水会减弱排汗并减少全身热量损失，可能是为了减轻进一步的液体流失，从而支持血压调节。然而，最近我们证明，与年轻男性相比，在干热环境中进行中等强度运动时，中年男性的脱水介导的全身净热交换（蒸发散热-干热获得）减少；尽管如此，其潜在机制尚无法确定。在这里，我们评估了这样一种假设，即这些发现源于中年至老年男性与年轻男性相比，细胞外高渗度对全身净热交换的影响减弱。与该假设一致，静脉输注高渗盐水（3%NaCl）引起的细胞外高渗度在年轻男性中降低净热交换并增加直肠温度的程度大于中年至老年男性。因此，脱水对中年至老年男性热调节功能的影响似乎归因于随着年龄的增长，对高渗度的敏感性减弱。
我们最近证明，在运动-热应激期间，与年轻男性相比，脱水诱导的出汗会更大程度地减弱全身散热。在这里，我们评估了这种不同的反应是否源于细胞外高渗度对热交换的影响随着年龄的增长而减弱。为了实现这一目标，十名年轻（平均（标准差）：25（5）岁）和十名中年至老年（61（5）岁）男性完成了两项试验，包括 90 分钟的静脉输注等渗盐水（0.9%NaCl；ISO）或高渗盐水（3.0%NaCl；HYP），然后在干热（40°C，相对湿度约 17%）中以固定的代谢产热 250W/m（约 50%最大有氧能力）进行 60 分钟的自行车运动。全身净热交换（蒸发散热-干热获得）通过直接量热法进行测量。直肠温度连续监测。与 ISO 相比，HYP 中的热交换在年轻男性中减少（233（20）与 251（17）W/m；P=0.002），但在老年组中没有减少（229（16）与 227（20）W/m；P=0.621）。此外，与年轻男性相比，中年至老年男性在 ISO 中的热交换较低（P=0.034），但在 HYP 中则没有（P=0.623）。同样，与 ISO 相比，HYP 中的终末运动直肠温度在年轻男性中较高（38.3（0.4）°C 与 37.9（0.3）°C；P=0.015），但在中年至老年男性中则没有（38.3（0.3）°C 与 38.2（0.2）°C；P=0.652）。与年轻男性相比，中年至老年男性在 ISO 期间的直肠温度较高（P=0.035），而在 HYP 中则没有组间差异（P=0.746）。我们的研究结果表明，与年轻男性相比，中年男性在运动-热应激期间，细胞外高渗度对体温调节功能的影响减弱。
