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 Dehydro genase (SDH) activities of Soleus (S) and Extensorum Digitalis Longus (EDL) muscles were studied in intact, thyroidectomized and T3-treated (25ug/100g , BW, ip, 15 days) rats. The fiber type composition of S muscle was also ev aluated and used as control of the T-3-induced effects. In the S muscle, th e Tg treatment increased the Mb mRNA and protein expression, as well as the CS and SDH activity. These changes occurred parallel to the expected incre ase 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 a nd CS activity tended to decrease. In contrast the SDH activity was increas ed, probably due to the enhanced motor activity that occurs as a short-term response to the hypothermia induced by hypothyroidism. In the EDL, the alt erations were milder than those in S muscle in both thyroid states. These f indings show that Mb gene expression is induced by T-3 This is concomitant with the enhancement of Krebs Cycle enzyme activities and provides addition al evidence that thyroid hormone increases the aerobic potential of skeleta l muscles, as well as the speed of muscle contraction.