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Am J Physiol Regul Integr Comp Physiol 276: R1095-R1101, 1999;
0363-6119/99 $5.00
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Vol. 276, Issue 4, R1095-R1101, April 1999

Core temperature and sweating onset in humans acclimated to heat given at a fixed daily time

Osamu Shido1, Naotoshi Sugimoto1, Minoru Tanabe2, and Sotaro Sakurada1

1 Department of Physiology, School of Medicine, Kanazawa University, Kanazawa 920-8640; and 2 College of Medical Technology, Hokkaido University, Sapporo 060-0812, Japan

The thermoregulatory functions of rats acclimated to heat given daily at a fixed time are altered, especially during the period in which they were previously exposed to heat. In this study, we investigated the existence of similar phenomena in humans. Volunteers were exposed to an ambient temperature (Ta) of 46°C and a relative humidity of 20% for 4 h (1400-1800) for 9-10 consecutive days. In the first experiment, the rectal temperatures (Tre) of six subjects were measured over 24 h at a Ta of 27°C with and without heat acclimation. Heat acclimation significantly lowered Tre only between 1400 and 1800. In the second experiment, six subjects rested in a chair at a Ta of 28°C and a relative humidity of 40% with both legs immersed in warm water (42°C) for 30 min. The Tre and sweating rates at the forearm and chest were measured. Measurements were made in the morning (0900-1100) and afternoon (1500-1700) on the same day before and after heat acclimation. Heat acclimation shortened the sweating latency and decreased the threshold Tre for sweating. However, these changes were significant only in the afternoon. The results suggest that repeated heat exposure in humans, limited to a fixed time daily, alters the core temperature level and thermoregulatory function, especially during the period in which the subjects had previously been exposed to heat.

human heat acclimation; sweating latency; heat stress; circadian rhythm; metabolic hormone


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