Fetal myosin heavy chains in regenerating muscle

There are several lines of evidence for the existence of a distinct class of myosins in developing muscle. Using various biochemical and immunological approaches, Whalen et al. recently suggested that two myosin heavy chain isozymes appear sequentially in rat muscle development, preceding the definitive adult myosins. It is unknown whether these myosins are present only in developing fast muscles or whether they also occur in developing slow muscles. Pyrophosphate of gel electrophoresis studies have suggested that fast-twitch and slow-twitch muscles synthesize the same fetal myosin isozymes early in development. Immunocytochemical studies with antibodies directed against adult fast and slow myosins show differences in myosin composition between fetal muscle fibres but interpretation of these findings is complicated by cross-reactions of these antibodies with fetal isomyosins. We have used a more direct immunocytochemical approach to identify the myosin types present in developing muscle fibres. An antibody specific for bovine fetal myosin and cross-reactive with rat fetal myosin has been prepared. We report here that the fetal myosin heavy chains recognized by this antibody show a heterogeneous fibre distribution in fetal and neonatal rat muscle, disappear progressively during postnatal development and are transiently expressed in regenerating muscle.