TIME-DEPENDENT EFFECTS ON CONTRACTILE PROPERTIES, FIBER POPULATION, MYOSIN LIGHT-CHAINS AND ENZYMES OF ENERGY-METABOLISM IN INTERMITTENTLY AND CONTINUOUSLY STIMULATED FAST TWITCH MUSCLES OF RABBIT

Fast-twitch tibialis anterior and extensor digitorum longus rabbit muscles were subjected to long-term intermittent (8 h daily) or continuous (24 h daily) indirect stimulation with a frequency pattern resembling that of a slow motoneuron. Increases in time to peak of isometric twitch contraction were observed without parallel changes in the pattern of myosin light chains or alterations in the distribution of slow and fast fibers as discernible by the histochemical ATPase reaction. However, changes in the fiber population and in the myosin light chain pattern were observed after intermittent stimulation periods exceeding 40 days or continuous stimulation periods longer than 20 days. Under these conditions even higher instances were found in contraction time. In 1 animal, a complete change in fiber population was observed. In this case myosin light chains of the slow (LCs1), LCs2) and of the fast type (LCf1) were obviously synthesized simultaneously within the same fiber. Early changes in the enzyme activity pattern of energy metabolism indicated a conversion of the fibers including their mitochondrial population. These changes and the earlier reported changes in the sarcoplasmic reticulum are probably responsible for the early changes in contractile properties which occur before the transformation of the myosin.