Evidence for a direct action of thyroid hormone in specifying muscle properties

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Abstract
Thyroidal trophic effects on slow-twitch skeletal muscle properties were compared in normally innervated and denervated soleus of rats maintained at different thyroid states. Hypothyroidism caused fast to slow changes in fiber type composition (99% decrease in proportion of type II fibers), ATPase activities (down 20-30%), myosin light chain pattern (54% less fast light chains), Ca uptake by sarcoplasmic reticulum (down 60%), lactate dehydrogenase activity (down 11%) and isozyme pattern (9% decrease in M-subunits). Changes of similar magnitude but opposite in direction were induced by thyrotoxicosis. Denervation reversed, to varying degrees, the fast to slow transformations observed in hypothyroidism. The slow to fast changes found in hyperthyroidism were facilitated rather than inhibited by denervation. The hormone effect evidently can be elicited in the absence of motor innervation. Denervation alone caused slow to fast changes in euthyroid muscles. Denervation and dysthyreosis alter muscle properties by independent mechanisms. The data favor a direct action of thyroid hormone over a neurally mediated mechanism.