Thyroid hormone stimulates myoglobin expression in soleus and extensorum digitalis longus muscles of rats: concomitant alterations in the activities of Krebs cycle oxidative enzymes.

Myoglobin (Mb) gene expression, Citrate Synthase (CS) and Succinate Dehydrogenase (SDH) activities of Soleus (S) and Extensorum Digitalis Longus (EDL) muscles were studied in intact, thyroidectomized and T3-treated (25 microg/100g, BW, ip, 15 days) rats. The fiber type composition of S muscle was also evaluated and used as control of the T3-induced effects. In the S muscle, the T3 treatment increased the Mb mRNA and protein expression, as well as the CS and SDH activity. These changes occurred parallel to the expected increase in type II (fast) and decrease in type I (slow)-fibers in S muscle. In the hypothyroid state, the Mb mRNA was decreased, while the Mb expression and CS activity tended to decrease. In contrast the SDH activity was increased, probably due to the enhanced motor activity that occurs as a short-term response to the hypothermia induced by hypothyroidism. In the EDL, the alterations were milder than those in S muscle in both thyroid states. These findings show that Mb gene expression is induced by T3. This is concomitant with the enhancement of Krebs Cycle enzyme activities and provides additional evidence that thyroid hormone increases the aerobic potential of skeletal muscles, as well as the speed of muscle contraction.