Abstract
Introduction Dehydration attenuates autonomic thermoregulation in the heat such as sweating and skin blood flow in human. Hyperosmolality in the extracellular fluid is thought to be involved in this mechanism. In contrast, behavioral thermoregulation (i.e. heat escape behavior) is augmented after hypertonic saline injection in rats (Nagashima et al., Am J Physiol 2001). However, it remains unknown if human activate behavioral thermoregulation during dehydration. Methods Ten healthy young men participated in the present study. Body and skin temperature were monitored. Rating for thermal sensation and comfort were conducted. After 45 min rest at 27°C, they did 50 min ergometer exercise, which was at the level of 40% peak VO2 or 15% peak VO2 at an ambient temperature of 35°C or 24°C, respectively. Subjects returned to the rest at 27°C until body and skin temperature being restored. Ambient temperature was then changed from 22°C to 38°C. Results Body weight decreased by 1.1 ± 0.1% after the exercise at 35°C and 0.2 ± 0.1% at 24°C. Plasma osmolality increased by 6 ± 1 mosmol/kg H2O at 35°C, but did not change at ∼24°C. Thermal sensation and comfort in the heat were augmented (i.e. feeling hotter and more uncomfortable) after the exercise at 35°C. Conclusion Dehydration increases thermal sensation and comfort in the heat in human, which are closely related to behavioral thermoregulation. [J Physiol Sci. 2008;58 Suppl:S103]
