Concomitant decline in heat-induced hyperthermia and HSP70 mRNA expression in aged rats.

An age-related impairment in the induction of heat-shock proteins (HSPs), which are thought to comprise a protective or adaptive response to cellular stress, may contribute to a reduction in thermal tolerance with age. When 5- and 24-mo-old rats were exposed to ambient temperatures of 23, 35, 37.5, or 40 degrees C for 90 min, a graded increase in the level of HSP70 mRNA expression was observed in brain, lung, and skin of animals from both groups. However, at temperatures above 23 degrees C, HSP70 expression was less in tissues of older rats when compared with those of young animals. This relative decline of heat-induced HSP70 mRNA levels with age correlated with an attenuated increase in colonic temperature (Tc) of old rats resulting from heat exposure. In other experiments, it was determined that the duration of hyperthermia was also an important factor in determining the level of HSP70 mRNA expression in vivo. Thus age-related differences in heat-induced HSP70 mRNA expression appear to result from differences in Tc due to heat stress rather than an impairment in the regulation of HSP70 gene expression.