Myosin transitions in chronic stimulation do not involve embryonic isozymes

Chronic low‐frequency stimulation of a fast skeletal muscle effects a transition of myosin isozymes from those characteristic of a fast muscle to those characteristic of a slow muscle. This transformation involves changes in both myosin heavy and light chains. During development, muscles usually change directly from embryonic to neonatal to fast or from embryonic to neonatal to slow isozymes. We have questioned whether chronic stimulation of the adult fast muscle results in a direct fast‐to‐slow isozyme shift or whether transformation requires reexpression of the developmental isozymes prior to the synthesis of adult slow isozymes. We have examined these alternatives in the chronically stimulated dog diaphragm using adenosine triphosphatase (ATPase) histochemistry, pyrophosphate gel electrophoresis of native isozymes, peptide mapping of myosin heavy chains, immunoblotting with an antibody specific to embryonic myosin heavy chain, and solid‐phase radioimmunoassay. We have demonstrated that a transition from adult fast to adult slow isozymes during chronic stimulation does not involve a recapitulation of embryonic isozymes.