Evidence for new isoform of fast myosin heavy chain in rat skeletal muscle.

The purpose of this study was to investigate whether the previously demonstrated heterogeneity of myosin heavy chain (MHC) in type 2B fibers of the superficial portion of the lateral gastrocnemius (SLG) muscle of the rat may be due to presence of type 2D/X fibers. Immunohistochemical identification of MHC heterogeneity, histochemical identification of appendicular 2D/X muscle fibers, and immunoblotting of myosins were used. It was found that some, but not all, of the heterogeneity of rat SLG is correlated with the presence of type 2D/X fibers. Immunoblots with MHC from several muscles revealed the presence of at least two antigenically distinct isoforms of MHC with the same electrophoretic mobility as the "2B" band. These results show that a previously undetected type of MHC is expressed in rat skeletal muscle and raise the possibility that there may be as yet undetected MHC isoforms in human muscle which may be clinically important.