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

Abstract

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 (V_max) and Mg²⁺activated actomyosin ATPase activity were unaffected in the thyrotoxic soleus. Our findings provide evidence that CT and V_maxcan vary independently and that the histochemical ATPase reaction may not always reflect the biochemical properties that make myosin fast or slow.actomyosin ATPase; histochemical ATPase reaction; isometric contraction time; maximum shortening velocity; one-half relaxation time; sarcoplasmic reticulum; skeletal muscleSubmitted on April 13, 1979Accepted on August 7, 1979