Effects of chronic electrical stimulation on myosin heavy chain expression in satellite cell cultures derived from rat muscles of different fiber-type composition.

Myotube cultures were established from satellite cells of three rat muscles of different fiber-type composition, slow-twitch soleus, diaphragm, and fast-twitch tibialis anterior (TA). Effects of chronic electrical stimulation were studied by exposing these cultures for up to 13 days to a stimulus pattern consisting of 250 ms impulse trains of 40 Hz, repeated every 4 s. Changes in myosin expression were assessed at the mRNA level by Northern blotting and in situ hybridization. Expression of slow myosin at the protein level was analysed by immunoblotting and immunohistochemistry with two antibodies, one specific to adult slow myosin, the other reacting with developmental and adult slow myosin heavy chain (MHCI) isoforms. In all three myotube cultures stimulation enhanced the mRNA and protein expression of a developmental isoform of slow myosin (MHCI). However, the three myotube cultures differed in the extent of the increase in MHCI. It was greatest in soleus-derived myotubes, least in TA-derived myotubes, and intermediate in diaphragm-derived myotubes. In addition to the increase in slow myosin, long-term stimulation led to an isoform switch, as indicated by an increase in myotubes reacting with the antibody specific for the adult MHCI. Our results suggest that enhanced contractile activity promotes the expression of the slow phenotype predetermined in satellite cells of slow-twitch, type I fibers. The different extents of increased slow myosin expression may thus be explained as reflecting different percentages of type I fibers and consequently of slow-type satellite cells in the corresponding donor muscles.