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.
Brodie
C
C
Health Sciences Research Center, Bar-Ilan University, Ramat-Gan, Israel.
Sampson
S R
SR
D000818:Animals; D002118:Calcium / Q000378:metabolism / Q000502:physiology*; D002478:Cells, Cultured; D004305:Dose-Response Relationship, Drug; D009132:Muscles / Q000166:cytology* / Q000378:metabolism / Q000648:ultrastructure; D051381:Rats; D012530:Saxitoxin / Q000378:metabolism; D015222:Sodium Channels / Q000378:metabolism*; D013779:Tetrodotoxin / Q000494:pharmacology; D013963:Thyroid Hormones / Q000494:pharmacology*; D013974:Thyroxine / Q000494:pharmacology; D014284:Triiodothyronine / Q000494:pharmacology
D015222:Sodium Channels; D013963:Thyroid Hormones; D014284:Triiodothyronine; D012530:Saxitoxin; D013779:Tetrodotoxin; D013974:Thyroxine; D002118:Calcium