Serum factor induces selective increase in Na‐channel expression in cultured skeletal muscle

We have examined effects of horse serum (HS) and various fractions (1 million‐1M, 300K, 100K, and 30K nominal molecular weight limit) obtained by ultrafiltration on expression of TTX‐sensitive Na‐channels and on activities of the Na‐K pump and glucose transport systems in cultured myotubes obtained from 1‐2‐day‐old neonatal rat pups. Five‐day‐old cells were transferred to serum‐free medium with no hormone or growth factor supplements (DMEM) for 24 hr and then treated with the various serum fractions for 48 hr. Measurements were made of specific [³H]‐saxitoxin (STX) binding, action potential properties, ⁸⁶Rb‐uptake and 2‐deoxyglucose (2‐DG) uptake. HS significantly increased all parameters compared to DMEM (increases in STX‐binding, 69%; Rb‐uptake, 65%; 2‐DG uptake, 93%). Results of treatment with the separate fractions showed that the 300K fraction caused a significantly greater increase in STX‐binding than either HS or the other fractions. In contrast, the increases in Rb and 2‐DG uptakes induced by the different fractions were not different from that obtained with HS. We conclude that serum contains a factor that selectively increases expression of TTX‐sensitive Na‐channels in skeletal muscle.