Characterization of thyroid hormone effects on na-k pump and membrane potential of cultured rat skeletal myotubes

The purpose of this study was to characterize the effects of thyroid hormone on the Na-K pump and resting membrane potential (E_M) of rat skeletal myotubes in culture. Myotubes were obtained from fetal (19†21 day) or neonatal rats (1†2 day) by serial trypsinization and maintained in culture for up to 10 days. Cells were treated with T₄ or T₃ on day 6 or 7, and measurements were made of EM, [3H]ouabain binding, and ouabain-sensitive ⁸⁶Rb uptake at various times thereafter. Hormone treatment increased the values of all three variables within 24 h, plateau levels being attained by 48†72 h. Cycloheximide and actinomycin D totally blocked the effects of thyroid hormone when added together to the cells, thus suggesting that protein synthesis is necessary for the effects of these hormones. Scatchard analysis showed that the new receptors have lower ouabain affinity than those in control. Blockade of spontaneously occurring action potentials with tetrodotoxin, which blocks voltagedependent Na channels, or Na/H antiporter with amiloride, abolished the hormone effects seen after 24 h and significantly reduced those obtained after 48 h of hormone treatment. The results demonstrate that thyroid hormone-induced increased amount and activity of the electrogenic Na-K pump in cultured myotubes occurs, at least in part, in response to an initial effect to increase Na influx. Moreover, the findings are consistent with the concept that the Na-K pump plays an important role in regulation of E_M in this preparation.