Ca2+ and Na+ currents in developing skeletal myoblasts are expressed in a sequential program: reversible suppression by transforming growth factor beta-1, an inhibitor of the myogenic pathway

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Abstract
We have analyzed the biophysical and developmental properties of Ca2+ and Na+ currents in C2 muscle cells, whose morphological and biochemical phenotype closely resembles differentiated skeletal muscle. Both fused and unfused C2 myocytes possessed: (1) membrane capacitance consistent with the presence of complex sarcotubular invaginations, (2) tetrodotoxin-sensitive Na+ channels, and (3) “fast” and “slow” Ca2+ channels that inactivated at holding potentials of -40 and -20 mV, respectively. Thus, the passive electrical properties, Na+ currents, and Ca2+ currents expressed in C2 cells each differed from those found in the nonfusing muscle cell line, BC3H1, and corresponded more precisely to characteristic findings observed in skeletal muscle fibers. In further contrast to BC3H1 cells, C2 muscle also expressed “transient” Ca2+ channels similar to those reported in embryonic or neonatal skeletal muscle, which were detected within 12–24 hr of mitogen withdrawal, up to 60 hr before appearance of “fast” and “slow” currents. Na+ channels also were induced 12–24 hr after mitogen withdrawal. Unlike the “fast” and “slow” Ca2+ currents, which were maximally expressed at 8–14 d of serum withdrawal, “transient” Ca2+ channels became down-regulated upon prolonged differentiation (as found in postnatal skeletal muscle in vivo) and were no longer expressed at 14 d. Despite their divergent kinetic and developmental properties, all components of Ca2+ and Na+ current in C2 myocytes were suppressed reversibly in the presence of transforming growth factor beta-1, a purified growth factor that inhibits the myogenic phenotype. The results indicate that fusion is not essential for skeletal myoblasts to produce developmentally regulated voltage-gated channels that resemble those of intact muscle and demonstrate that the formation of diverse Ca2+ and Na+ channels can be mediated by a single peptide that affects the myogenic pathway.