ENDURANCE TRAINING AFFECTS MYOSIN HEAVY-CHAIN PHENOTYPE IN REGENERATING FAST-TWITCH MUSCLE

The aim of this study was to analyze the effects of treadmill training (2 h/day, 5 days/wk, 30 m/min, 7% grade for 5 wk) on the expression o f myosin heavy chain (MHC) isoforms during and after regeneration of a fast-twitch white muscle [extensor digitorum longus (EDL)]. Male Wist ar rats were randomly assigned to a sedentary (n = 10) or an endurance -trained (ET; n = 10) group. EDL muscle degeneration and regeneration were induced by two subcutaneous injections of a snake toxin. Five day s after induction of muscle injury, animals were trained over a 5-wk p eriod. It was verified that similar to 40 days after venom treatment, central nuclei were present in the treated EDL muscles from sedentary and ET rats. The changes in the expression of MHCs in EDL muscles were detected by using a combination of biochemical and immunocytochemical approaches. Compared with contralateral nondegenerated muscles, relat ive concentrations of types I, IIa, and IIx MHC isoforms in ET rats we re greater in regenerated EDL muscles (146%, P < 0.05; 76%, P < 0.01; 87%, P < 0.01, respectively). Their elevation corresponded to a decrea se in the relative concentration of type IIb MHc (-36%, P < 0.01). Alt hough type I accounted for only 3.2% of total myosin in regenerated mu scles from the ET group, the cytochemical analysis showed that the pro portion of positive staining with the slow MHC antibody was markedly g reater in regenerated muscles than in contralateral ones. Collectively , these results demonstrate that the regenerated EDL muscle is sensiti ve to endurance training and suggest that the training-induced shift i n MHC isoforms observed in these muscles resulted from an additive eff ect of regeneration and repeated exercise.