Nerve growth factor and fibroblast growth factor influence post-fusion expression of Na-channels in cultured rat skeletal muscle

We have examined effects of nerve growth factor (NGF) and fibroblast growth factor (FGF) on the density of tetrodotoxin (TTX)-sensitive Na-channels in cultured rat skeletal muscle. Measurements were made of specific binding of [3H]saxitoxin (STX) and the frequency and rate of rise of spontaneously occurring action potentials, the physiological expression of Na-channel density. Cells were transferred to various growth conditions at 6 days in vitro, and measurements were made beginning 24 hr later. Both growth factors (GF) caused dose-related increases in Na-channels compared with myotubes maintained in normal, serum-supplemented growth medium. Maximum effects occurred with a concentration of NGF of 50 ng/ml and FGF of 15 ng/ml. Scatchard analysis of specific STX binding showed an increase in Bmax with no significant change in Kd. Similar increases occurred on rate of rise and frequency spontaneous action potential. Treatment of cultures with cycloheximide or actinomycin D, inhibitors of protein and RNA synthesis, completely prevented the increase in STX-binding induced by GF treatment. The results indicate that NGF and FGF have important effects on regulation of excitable cell gene products after differentiation.