INTRODUCTION: The 70-kDa heat shock protein (HSP70) is a ubiquitous molecular chaperone which is highly inducible by cellular stress such as exercise. HSP70 seems to be involved both in the acute cellular restoration following damaging exercise, and likely also in the long-term adaptations to training. METHODS: To further understand the role of muscle glycogen content on the expression of HSP70, muscle glycogen was manipulated by consumption of either water (H2O) or a carbohydrate-enriched diet (CHO) during recovery from 4 hours of moderate-intensity glycogen-depleting cycling exercise in fourteen elite endurance athletes (VO2max: 66.2 ± 1.3 ml·kg-1·min-1). Muscle biopsies were obtained from m. vastus lateralis pre- and post-exercise, and after 4 and 24 hours of recovery, and analyzed for HSP70 mRNA expression, as well as HSP70 protein expression and muscle glycogen within the same skeletal muscle fibers using immunohistochemistry. RESULTS: Exercise reduced muscle glycogen by 59 ± 10% (P < 0.0001). After 4hrs of recovery, glycogen approached resting levels in the CHO group (86% of pre, P = 0.28) but remained suppressed in the H2O group (41% of pre, P < 0.001) (group x time interaction: P = 0.002). The expression of HSP70 mRNA (+1.9 ± 0.9-fold, P = 0.03) and protein (+247 ± 99 % of pre, P < 0.0001) was substantially increased during exercise. During recovery, changes in the HSP70 mRNA and protein expressions followed the same pattern in the H2O and CHO group (group x time interactions: P = 0.70 and P = 0.24, respectively). After 4hrs of recovery, the HSP70 mRNA expression remained elevated in comparison to pre-exercise levels (H2O: 1.7 ± 0.8-fold, CHO: 1.4 ± 0.4-fold, P = 0.004). This was also the case with respect to the HSP70 protein expression which remained at post-levels at this time-point (H2O: 100 ± 23 % of post; CHO: 90 ± 16% of post, P = 0.24 in comparison to post). Importantly, the maintenance of the HSP70 protein expression in both groups was evident despite clear differences in muscle glycogen content and accordingly muscle glycogen content did not explain the variation in HSP70 expression at the 4-hour time-point (r2=0.004). After 24hrs of recovery, the HSP70 mRNA expression returned to pre-exercise levels (H2O: 0.96 ± 0.4-fold, CHO: 0.91 ± 0.4-fold, P = 0.55 in comparison to pre), which was also the case for the HSP70 protein expression (H2O: 136 ± 86% of pre; CHO: 108 ± 60% of pre, P = 0.48). CONCLUSION: In conclusion, muscle HSP70 expression remained elevated during recovery from prolonged exercise in highly trained skeletal muscle, irrespective of glycogen availability.
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