Differential effects of thyroid hormones on energy metabolism of rat slow‐ and fast‐twitch muscles

Abstract

Thyroid hormone (TH) is an important regulator of mitochondrial content and activity. As mitochondrial content and properties differ depending on muscle‐type, we compared mitochondrial regulation and biogenesis by T3 in slow‐twitch oxidative (soleus) and fast‐twitch mixed muscle (plantaris). Male Wistar rats were treated for 21 to 27 days with T3 (200 μg/kg/day). Oxidative capacity, regulation of mitochondrial respiration by substrates and phosphate acceptors, and transcription factors were studied. In soleus, T3 treatment increased maximal oxygen consumption (V_max) and the activities of citrate synthase (CS) and cytochrome oxidase (COX) by 100%, 45%, and 71%, respectively (P < 0.001), whereas in plantaris only V_max increased, by 39% (P < 0.01). ADP‐independent respiration rate was increased in soleus muscle by 216% suggesting mitochondrial uncoupling. Mitochondrial substrate utilization in soleus was also influenced by T3, as were mitochondrial enzymes. Lactate dehydrogenase (LDH) activity was elevated in soleus and plantaris by 63% and 11%, respectively (P < 0.01), and soleus creatine kinase was increased by 48% (P < 0.001). T3 increased the mRNA content of the transcriptional co‐activator of mitochondrial genes, PGC‐1α, and the I and IV COX subunits in soleus. The muscle specific response to thyroid hormones could be explained by a lower content of TH receptors in plantaris than soleus. Moreover, TRα mRNA level decreased further after T3 treatment. These results demonstrate that TH has a major effect on mitochondrial content, regulation and coupling in slow oxidative muscle, but to a lesser extent in fast muscle, due to the high expression of TH receptors and PGC‐1α transcription factor.