Response of mitochondria of different types of skeletal muscle to thyrotoxicosis.

To determine the effect of long-term thyrotoxicosis on muscle mitochondria, we measured representative mitochondrial enzymes from three different types of skeletal muscle (fast-twitch red and fast-twitch white from the quadriceps, and slow-twitch red from the soleus) in rats given 3 mg L-thyroxine and 1 mg triiodo-L-thyronine per kilogram of diet for 12 wk. Marker enzymes of the electron transport chain and citric acid cycle (cytochrome oxidase, cytochrome c, and citrate synthase) increase approximately twofold in soleus muscle in response to this treatment. The fast-twitch muscles exhibit no more than 44% increases in these enzymes in response to the same treatment. Relative to initial concentration, 3-hydroxybutyrate dehydrogenase increased to the same extent in fast-twitch red muscle as it did in the soleus (70%). Mitochondrial alpha-glycerophosphate dehydrogenase increased 76% in red quadriceps and 170% in soleus, but did not change in white muscle in the thyrotoxic rats. This differential sensitivity of the three types of muscle provides a tool for studying the mechanisms underlying the action of thyroid hormones on muscle mitochondria.