Myosin isoenzymes in adult rat skeletal muscles

The myosin isoenzymic content of several adult rat muscles has been analyzed by electrophoresis under non-dissociating conditions. Fast-twitch fibres, whether of the oxidative type, such as the red masseter, or of the glycolytic type, such as the white tensor fasciae latae, are all shown to contain three isomyosins with respective mobilities identical in both types of muscles. These three isoenzymes, which, as in the case of myosins from avian fast muscles, represent alkali light chain hetero-and homodimers with similar large subunits, occur in somewhat variable relative proportions depending on the muscle. No obvious correlation was established between the type of the fast fibres--either oxidative or glycolytic--and the type of the myosins. Besides the three fast isoenzymes, other muscles, such as the predominantly fast latissimus dorsi and the mixed diaphragm, are shown to contain one myosin species of lower electrophoretic mobility; this supplementary isoenzyme comigrates with the major component of the predominantly slow-twich soleus muscle, but differs from the avian slow-tonic isoform. Ca2+ ATPase determinations on gel indicate that the fast isomyosins all display similar activity, which is five to ten times higher, depending on the experimental conditions of the assay, than the activity shown by the slow isoenzymes. Altogether, at least five isoenzymes, corresponding to one "slow-twich", one "slow-tonic", and three "fast-twitch" myosin species, were detected in rat skeletal muscles.U