Trophic effect of a sciatic nerve extract on fast and slow myosin heavy chain synthesis.

Myosin heavy chain (MHC) synthesis in cultures from chick pectoralis muscle cells was determined by [35S]methionine incorporation. Two types of MHC, migrating as 200,000-dalton components on sodium dodecyl sulfate polyacrylamide gels, were distinguished with antibodies against adult fast and slow MHC. Their synthesis was revealed by autoradiography. The effect of a sciatic nerve extract on the synthesis of the two types of MHC was also determined. Control experiments show that fast MHC is primarily synthesized in 48-h cultures. At a later stage of development (5- to 7-day cultures), slow MHC is also produced. The nerve extract promotes muscle cell differentiation and stimulates the synthesis of the slow type of MHC at an earlier stage of development (i.e., at 48 h as compared with 5-7 day in controlled cultures). It is concluded therefore that presumptive fast muscle cells in culture synthesize initially fast MHC and later both types of MHC (slow and fast). These results also suggest that the sciatic nerve extract is capable either of activating the transcription of the structural gene for slow MHC or of activating the translation of preexisting messenger RNA coding for this protein.