Contractile and fatigue properties of thyrotoxic rat skeletal muscle.

The effects of thyrotoxicosis on the contractile properties and development of muscle fatigue in the slow soleus (SOL) and fast extensor digitorum longus (EDL) muscles were examined in rats given 3 mg of L-thyroxine and 1 mg of L-triiodothyronine per kilogram of diet for 6 weeks. The hormone treatment produced significant decreases in the contraction time, one-half relaxation time, and twitch tension in the SOL, while the peak rate of tension development (+ dP/dt) and decline (- dP/dt) in this muscle were elevated. Additionally, the force-frequency curve was shifted to the right and, thus, resembled the curve of a normal fast-twitch muscle. In contrast, the contractile properties of the fast EDL were relatively unaltered by the hormone administration. Thyrotoxicosis also changed the SOL response to contractile activity as twitch tension, + dP/dt, and - dP/dt remained high, and a faster decline in muscle glycogen and an increase in lactate occurred compared to control muscles. These results clearly demonstrate a preferential effect of thyroid hormone on slow compared to fast skeletal muscle.