Hyperthermia and dehydration: their independent and combined influences on physiological function during rest and exerci
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ORIGINAL ARTICLE
Hyperthermia and dehydration: their independent and combined influences on physiological function during rest and exercise Anne M. J. van den Heuvel1 · Benjamin J. Haberley1 · David J. R. Hoyle1 · Rodney J. Croft2 · Gregory E. Peoples1 · Nigel A. S. Taylor1,2 Received: 4 March 2020 / Accepted: 3 September 2020 © Springer-Verlag GmbH Germany, part of Springer Nature 2020
Abstract Purpose This experiment was designed to quantify the independent and combined influences of hyperthermia and dehydration on effector control during rest and exercise. Methods To achieve that, whole-body hydration of healthy adults (N = 8) was manipulated into each of three states (euhydrated, 3% and 5% dehydrated), and then clamped within each of two thermal states (normothermia [mean body temperature: 36.1 °C] and moderate hyperthermia [mean body temperature: 38.2 °C]). Those treatment combinations provided six levels of physiological strain, with resting physiological data collected at each level. The effects of isothermal, thermally unclamped and incremental exercise were then investigated in normothermic individuals during each level of hydration. Results At rest, dehydration alone reduced urine flows by 83% (3% dehydrated) and 93% (5% dehydrated), while the reduction accompanying euhydrated hyperthermia was 86%. The sensitivities of renal water conservation to 3% dehydration (−21% mOsm−1 kg H2O−1) and moderate hyperthermia (−40% °C−1) were independent and powerful. Evidence was found for different renal mechanisms governing water conservation between those treatments. Cutaneous vasomotor and central cardiac responses were unresponsive to dehydration, but highly sensitive to passive thermal stress. Dehydration did not impair either whole-body or regional sweating during rest or exercise, and not even during incremental cycling to volitional exhaustion. Conclusion In all instances, the physiological impact of these thermal- and hydration-state stresses was independently expressed, with no evidence of interactive influences. Renal water-conservation was independently and powerfully modified, exposing possible between-treatment differences in sodium reabsorption. Keywords Dehydration · Hyperthermia · Plasma volume · Sweating · Urine production · Vascular conductance Abbreviations SD Standard deviation CVI Intra-individual coefficient of variation Communicated by George Havenith. Electronic supplementary material The online version of this article (https://doi.org/10.1007/s00421-020-04493-4) contains supplementary material, which is available to authorized users. * Nigel A. S. Taylor [email protected] 1
School of Medicine, Centre for Human and Applied Physiology, University of Wollongong, Wollongong, NSW 2522, Australia
Australian Centre for Electromagnetic Bioeffects Research, School of Psychology, Illawarra Health & Medical Research Institute, University of Wollongong, Wollongong, NSW 2522, Australia
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Introduction Humans are constantly exchanging matter and energy with our surroundings. Eve
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