Postnatal expression of myosin isoforms in the genioglossus and diaphragm muscles.

We studied the expression of myosin heavy chain (MHC) and native myosin isoforms in the genioglossus (GG) and costal diaphragm (DIA) muscles of the rat during postnatal development using both denaturing and nondenaturing gel electrophoresis. Primary myotubes in both fast and slow muscles homogeneously express slow as well as embryonic myosin. Since the adult GG is comprised primarily of fast MHC isoforms, whereas the adult DIA is characterized by a mixture of MHC slow and fast isoforms, we hypothesized that the GG and DIA would be subject to different temporal patterns of MHC isoform expression during postnatal development. Native myosin and MHC gels demonstrated a persistence of neonatal MHC (MHC neo) on day 25 in the GG, whereas this isoform was not detected beyond day 21 in the DIA. The MHC phenotype in GG of the adult demonstrated a predominance of MHC 2X (35% +/- 8) and MHC 2B (45% +/- 10) with a smaller proportion of MHC 2A (19% +/- 5). In contrast, the MHC phenotype in adult DIA was characterized by approximately equal proportions of MHC slow (25% +/- 3), MHC 2A (34% +/- 10), and MHC 2X (31% +/- 12) with a small percentage of MHC 2B (9% +/- 7). These data suggest that postnatal regulation of MHC expression in the GG and DIA is muscle specific.