Effects of thyroid hormone administration on skeletal muscle mitochondria

The effects of thyroid hormone administration on the levels of a number of mitochondrial markers were measured in skeletal muscle and liver of normal rats. Injection of 18 mug of L-thyroxine (T4) per 100 g body wt every 4th day for 3 wk had no effect on the concentrations of cytochrome c, on citrate synthase activity, or on respiratory capacity of skeletal muscle. Injection of 200 mug of L-triiodothyronine (T3) daily for 5 days, or feeding 23 mg T4 and 7 mg T3/kg of diet for 2 wk, resulted in thyrotoxicosis and large increases in the activity of hepatic alpha-glycerophosphate dehydrogenase and other mitochondrial markers; however, the levels of activity of mitochondrial marker enzymes in gastrocnemius and quadriceps muscles were not significantly changed. Only when rats were fed 3 mg T4 and 1 mg T3/kg diet for a 6-wk period did we observe an increase in skeletal muscle mitochondrial markers. Thus, thyroxine treatment must be sufficiently prolonged if it is to be used as a tool for studying skeletal muscle mitochondrial biogenesis.