Differential effects of exercise and heat stress on liver HSP70 accumulation with aging.

Previous reports have suggested that the heat shock response to passive heating may be blunted by aging. However, during exertional heating, factors in addition to elevated temperature may amplify the degree of stress compared with hyperthermia alone. The purpose of this study was to compare the pattern of accumulation of the highly inducible 72-kDa heat shock protein (HSP72) in liver tissue of mature (12-mo-old) and senescent (24-mo-old) male Fischer 344 rats after either passive or exertional heat stress. A euthermic control group was exposed to an ambient temperature (Ta) of 25 degrees C for 4.5 h. A passive heating (heat) group was exposed to an Ta of 42 degrees C until colonic temperature (Tco) reached 41 degrees C. An exertional heating (exercise) group performed intermittent moderate-intensity treadmill exercise (similar absolute intensities for the two age groups) at an Ta of 32 degrees C until Tco reached 41 degrees C. Heating rates were similar in the heat and exercise groups (approximately 0.08 degrees C/min). Rats in both the heat and exercise groups were maintained at a Tco of 41 degrees C for an additional 30 min and subsequently returned to an Ta of 25 degrees C for 3 h. Liver HSP72 accumulation was increased in mature rats after both the heat (+192% vs. control) and exercise (+292%) protocols. In contrast, the senescent rats demonstrated no significant increase in inducible HSP70 with heating but a large increase with exercise (+232%; P < 0.01 compared with control and heat groups). These data suggest that the blunted heat shock protein response to heating observed with aging is not a result of the inability to produce inducible HSP72 because older rats had an robust response to exertional hyperthermia.