Characterization of thyroid hormone effects on Na channel synthesis in cultured skeletal myotubes: Role of Ca²⁺

Thyroid hormones (TH) cause an increase in spontaneous electrical activity of cultured rat skeletal myotubes. This activity is associated with tetrodotoxin (TTX)-sensitive Na channels. In addition, the initial effect of TH on Na-K pump synthesis has been shown to be TTX dependent. Accordingly, we have studied effects of TH on expression of TTX-sensitive Na channels in cultured skeletal muscle. Expression of Na channels was determined by measurements of the binding of [³H]saxitoxin (STX). The frequency and rate of rise of spontaneously occurring action potentials, which are related to the density of TTX-sensitive Na channels, were also determined. TH caused dose-dependent increases in Na channels as well as in action potential frequency and rate of rise. The increases were detectable as early as 12 h after treatment with TH was begun, and levels reached a maximum plateau after 36–48 h. The effects of TH were blocked by inhibitors of protein synthesis. Scatchard analysis showed the channels in TH-treated myotubes to have lower affinity for STX than those in control cells. The effect of TH to up-regulate Na channels was reduced by growth of the cells in elevated external calcium. In contrast, treatment with TTX or verapamil, which lower cytosolic Ca²⁺, resulted in a marked increase in the effect of TH over that in control myotubes. Thus, TH appears to regulate Na channels in cultured myotubes by two opposing mechanisms; 1) direct stimulation of Na channel synthesis, and 2) indirect decrease in synthesis mediated by an increase in cytosolic Ca²⁺. The results indicate that TH may play an important role in developmental expression of Na channels in excitable tissue.