Effects of dantrolene on force development in slow- and fast-twitch muscle of euthyroid, hypothyroid, and hyperthyroid rats.

1 The effects of dantrolene on twitch and tetanic force development were determined in soleus and gastrocnemius muscle of euthyroid, hypothyroid, and hyperthyroid rats. 2 Maximum twitch force of the gastrocnemius muscle was significantly more depressed by dantrolene than that of the soleus muscle in euthyroid and hyperthyroid rats. In hypothyroid rats, the effect of dantrolene on maximum twitch force was similar in soleus and gastrocnemius muscle. 3 Maximum tetanic force in soleus and gastrocnemius muscle was less depressed by dantrolene than the twitch force in either thyroid state. The effect of dantrolene on maximum tetanic force increased in both muscles in the direction hypothyroid----euthyroid----hyperthryoid. 4 The results are discussed in terms of an effect of thyroid hormones on Ca2+ -cycling during force development, as a result of thyroid hormone-induced proliferation of the sarcoplasmic reticulum.