MHC isoform composition and Ca2+- or Sr2+-activation properties of rat skeletal muscle fibers

Chemically skinned single fibers from adult rat skeletal muscles were used to test the hypothesis that, in mammalian muscle fibers, myosin heavy chain (MHC) isoform expression and Ca2+- or Sr2+-activation characteristics are only partly correlated. The fibers were first activated in Ca2+- or Sr2+-buffered solutions under nearphysiological conditions, and then their MHC isoform composition was determined electrophoretically. Fibers expressing only the MHC I isoform could be appropriately identified on the basis of either the Ca2+- or Sr2+-activation characteristics or the MHC isoform composition. Fibers expressing one or a combination of fast MHC isoforms displayed no significant differences in their Ca2+- or Sr2+-activation properties; therefore, their MHC isoform composition could not be predicted from their Ca2+- or Sr2+-activation characteristics. A large proportion of fibers expressing both fast- and slowtwitch MHC isoforms displayed Ca2+- or Sr2+-activation properties that were not consistent with their MHC isoform composition; thus both fiber-typing methods were needed to fully characterize such fibers. These data show that, in rat skeletal muscles, the extent of correlation between MHC isoform expression and Ca2+- or Sr2+-activation characteristics is fiber-type dependent.