Influence of overload on phenotypic remodeling in regenerated skeletal muscle.

We studied the effects of 10 wk of functional overload on the expression of myosin heavy chain (MHC), sarcoplasmic reticulum Ca(2+)-ATPase isoforms (SERCA), and the activity of several metabolic enzymes in sham and regenerated plantaris muscles. Overload was accomplished by bilateral surgical ablation of its synergists 4 wk after right plantaris muscles regenerated after myotoxic infiltration. The overload-induced muscle enlargement was slightly less in regenerated than in sham muscles [28% (P < 0.005) and 43% (P < 0.001), respectively]. Overload led to an increase in type I MHC expression (P < 0.01) to a similar extent in sham and regenerated plantaris, while the expected shift from type IIb to type IIa MHC was less marked in regenerated than in sham plantaris. The overload-induced decrease in the expression of the fast SERCA isoform and in the activity of the M subunit of lactate dehydrogenase occurred to a similar extent in sham and regenerated plantaris [66% (P < 0.01) and 27% (P < 0.005), respectively]. In conclusion, the lesser responses of muscle mass and fast MHC composition of regenerated plantaris to mechanical overload suggest an alteration of the transcriptional, translational, and/or posttranslational control of gene expression in regenerated muscle.