Myosin isoforms in hindlimb muscles of normal and dystrophic (ReJ129 dy/dy) mice

Myosin isoform expression was studied in hindlimb muscles of control (Dy/Dy) and dystrophic (dy/dy) mice of the ReJ129 strain during postnatal development. Three myosin heavy chain isoforms (fast II-B MHC, neonatal MHC, and slow or I MHC) were identified using monoclonal antibodies. Only original fibers, i.e., fibers formed during fetal life, were studied. Necrotic and regenerating fibers were excluded. The disappearance of neonatal MHC was found to be delayed in all muscles of dystrophic mice, except the soleus. The fraction of fibers containing I MHC was similar in control and dystrophic animals at all ages, except during the third postnatal week. The developmental increase in the fraction of fibers expressing II-B MHC was interrupted in dystrophic mice by two marked declines. The first occurred during the second postnatal week at the beginning of the main wave of fiber necrosis, and the second occurred at between 30 and 90 postnatal days.