L-thyroxine activates the intracellular Ca2+ release channel of skeletal muscle sarcoplasmic reticulum.

L-thyroxine activated the Ca2+ release channel (ryanodine receptor) of skeletal muscle. [3H]ryanodine binding was stimulated by L-thyroxine in a dose dependent manner producing a two-fold increase at 250 microM. The same concentration induced a release of approximately 40% of the 45Ca2+ passively loaded into sarcoplasmic reticulum in 100 msec. Ca2+ release channel activity monitored in planar bilayers increased in the presence of 250 microM L-thyroxine from a control open probability of 0.02 +/- 0.03 to 0.17 +/- 0.12. Thyroid hormones may directly open Ca2+ release channels of skeletal muscle, thus altering intracellular Ca2+ homeostasis.