Myosin transformation in hypertrophied rat muscle.

Chronic hypertrophy (X = 88%) of the fast-twitch rat plantaris muscle was induced by surgical ablation of the synergistic gastrocnemius. Alterations in the catalytic activity and the light chain composition of myosin extracted from the hypertrophied plantaris muscle were examined. The hypertrophied muscle exhibited a decreased Ca2+-activated myosin ATPase activity and an increase in the content of LCS1 and LCS2 light chains when compared to its contralateral control. These changes, which occur in the absence of a direct manipulation of the muscle's innervation, suggest that the histochemically observed increase in the percent of alkaline labile fibres found in hypertrophied muscle are the result of the expression of a different myosin phenotype.