HSP70 induction during exercise and heat stress in rats: Role of internal temperature

The purpose of this study was to determine if the accumulation of the 72- kDa heat shock protein (HSP70) is elevated in response to a prolonged bout of submaximal exercise in which colonic temperature (T(co)) remained at control levels. Sprague-Dawley rats were randomly assigned to one of four testing groups [n = 8 per group; ambient temperatures (T(a)) for each condition are included]: 1) control (cool/rest; T(a) = 24°C); 2) cool and exercise (cool/ exercise; T(a) = 14°C); 3) nonexertional heating (heat/rest; T(a) = 42°C); 4) heat and exercise (heat/exercise; T(a) = 32°C). All interventions were ~60 min in duration. An exercise bout consisted of treadmill running at 17 m/min and 0% grade, while the heat/rest and heat/exercise experiments consisted of heat exposure that was terminated when T(co) reached 41°C. Baseline T(co) was similar for all four groups. In the cool/rest and cool/exercise groups, final T(co) was not different from the baseline values, nor was it different between these two groups. In the heat/rest and heat/exercise groups, heating rates were similar. Tissue samples were obtained from the gastrocnemius, soleus, and extensor digitorum longus (EDL) muscles of the left hindlimb and the left ventricle 30 min after a trial was completed. An enzyme-linked immunosorbent assay specific for HSP70 was used to directly quantitate absolute levels of HSP70 in tissues. There were significant main effects of both heating and exercise for HSP70 levels in the gastrocnemius, soleus, and left ventricle (P < 0.05). However, in the EDL, neither the heating nor exercise conditions alone produced elevated concentrations of HSP70, although there was a significant interaction of heating and exercise for HSP70 levels in this muscle. In all four tissues sampled, the greatest accumulation of HSP70 was observed in the heat/exercise group. These data indicate that prolonged, submaximal exercise increases HSP70 accumulation in locomotor muscles such as the gastrocnemius and soleus muscles and cardiac tissue of rats in a tissue-specific manner. In addition, the observed increases in HSP70 levels during exercise were independent of core body temperature, suggesting that factors other than heat stress may contribute to the expression of HSP70 during exercise.