Influence of hyperthyroidism on maximal shortening velocity and myosin isoform distribution in skeletal muscles

The objectives of this study were 1) to examine the effects of hyperthyroidism on the myosin isoform distribution in slow and fast skeletal muscle, 2) to explore how these effects were manifested with respect to the force-velocity relationship and maximal shortening velocity, and 3) to contrast two different techniques of measuring maximal shortening velocity under normal and hyperthyroid conditions. Adult female Sprague-Dawley rats were randomly assigned to one of two groups: control (n = 8) or hyperthyroid (n = 8). Hyperthyroidism was induced by injections of 3,3',5-triiodo-L-thyronine every other day for 20 wk. We found that hyperthyroidism produced a significant shift in the myosin isoform distribution of the soleus but not the plantaris. The relative amount of the slow myosin isoform was reduced from a control value of 93 to 69% in the hyperthyroid condition. In contrast, both the intermediate and fast myosin-3 isoform pools were substantially increased (P less than 0.001) by approximately fourfold. Hyperthyroidism produced an increase in the maximal shortening velocity of the soleus as measured either by the slack test (+57%; P less than 0.001) or by extrapolation of force-velocity data (+33%; P less than 0.001). The hyperthyroid condition did not, however, affect the mechanical properties of the plantaris.