Contractile, biochemical, and histochemical properties of thyrotoxic rat soleus muscle.

The effects of thyrotoxicosis on the contractile properties of soleus muscle were examined in rats given 3 mg of T4 and 1 mg of T3 per kg of diet for 6--8 wk. Thyrotoxicosis induced significant decreases in isometric twitch contraction time (CT), one-half relaxation time, and peak twitch tension. The Ca2+ uptake activity of the sarcoplasmic reticulum (SR) was increased in the thyrotoxic muscles; this adaptation in the SR provides a possible mechanism for the alterations in isometric contractile properties. Thyrotoxicosis induced a large increase in fibers classified as type 2, on the basis of an alkali-stable histochemical reaction for ATPase, in the soleus. Although this reaction is commonly interpreted as indicating that a muscle is fast, maximum shortening velocity (Vmax) and Mg2+ activated actomyosin ATPase activity were unaffected in the thyrotoxic soleus. Our findings provide evidence that CT and Vmax can vary independently and that the histochemical ATPase reaction may not always reflect the biochemical properties that make myosin fast or slow.