Altered expression of myosin light-chain isoforms in chronically stimulated fast-twitch muscle of the rat

Fast-twitch tibialis anterior muscle of the rat was chronically stimulated for periods of 18 days, 28 days and 56 days. Changes in the myosin light-chain (LC) pattern consisted in an increase in LC1f, concomitant with a decrease in LC3f. In contrast to previous findings in chronically stimulated fast-twitch tibialis anterior muscle of the rabbit, no substantial increases occurred in the slow myosin light-chain isoforms. In vivo labeling using 35S]methionine incorporation revealed differences in relative turnover between the fast myosin light chains. The relative turnover of the fast myosin light chains appeared to increase in normal muscle in the order LC2f less than LC1f less than LC3f. As judged from 35S]methionine incorporation, the changes in light-chain tissue content mainly resulted from altered synthesis rates. However, in the case of LC3f the decrease in protein content could not only be explained by a reduced synthesis, but, additionally, appeared to be due to enhanced degradation. Parvalbumin, which was included in the present study, was also found to decrease in the stimulated muscle. However, its decrease appeared to result primarily from reduced synthesis.