Watkins, A. M., Cheek, D. J., Harvey, A. E., Willoughby, D. S., Gillam, K. E., & Mitchell, J. B. (2006). The relationship between cellular heat tolerance and whole body heat acclimation in exercising humans. Medicine and Science in Sports and Exercise, 38(5), Supplement abstract 2050.

"Physiological responses to active heat acclimation have been well documented in human subjects; however, levels of HSP-72 as a molecular marker of cellular thermotolerance and its relationship to the typical responses to a heat acclimation protocol in humans has not been established". This study determined the effect of a seven-day heat acclimation protocol on HSP-72 expression in human skeletal muscle. It also examined the relationships between molecular and physiological markers of heat acclimation.

Active, but not specifically trained male subjects (N = 10) completed a 7-day heat acclimation protocol consisting of cycling at 75% of VO2peak in a hot temperature (39.5ºC, 27.3% RH). On days 1 and 7, muscle samples were obtained from the vastus lateralis before exercising, and at 6 and 24 hours post-exercise for the analysis of HSP-72 protein content. Rectal and skin temperatures were recorded every five minutes during exercise. Heart rate and oxygen uptake were recorded every 15 minutes during exercise. Intramuscular temperatures and nude body weights (for sweat rate-SR) were measured before and immediately after exercise. Venous blood was collected before, immediately after, and at 2 and 24 hours after exercise.

Rectal temperature, heart rate, and cortisol showed significant adaptations from day 1 to 7. No significant differences for oxygen uptake, intramuscular temperature, skin temperature, or sweat rate were revealed when days 1 and 7 were compared. No significant time or day x time interactions were detected for HSP-72 expression.

Implication. Evidence of heat acclimation was seen at the physiological level based on adaptations in rectal temperature, heart rate, and cortisol. However, there was no evidence of enhanced thermotolerance at the cellular level as indicated by HSP-72 expression.

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