Expression of MyoD in cultured primary myotubes is dependent on contractile activity: correlation with phenotype-specific expression of a sarcoplasmic reticulum Ca(2+)-ATPase isoform.

Myogenic determination factors (MDF) have been implicated in the establishment and maintenance of the fast or slow phenotype in skeletal muscle, with MyoD favoring the fast and myogenin favoring the slow phenotype. Accordingly, contractility-induced changes in muscle phenotype should be accompanied by a change in the MyoD/myogenin ratio. Some reports show such changes, but limitations inherent to in vivo studies complicate interpretation of these data. Here we tested whether a relationship can be found between contractility, MDF expression, and the expression of phenotype-specific muscle proteins in a simple in vitro system of cultured primary myotubes. We show that contractions reduce the MyoD/myogenin ratio by specifically repressing MyoD mRNA expression. This is accompanied by a selective repression at a pretranslational level of the expression of fast-type sarcoplasmic reticulum Ca(2+)-ATPase. These in vitro results support a phenotype-determining role of MDFs as a function of contractile activity and show that cultured myotubes can be a useful model for the analysis of the molecular mechanism of such regulation of muscle phenotype.