Thyroid hormone induced changes in the enzyme activity pattern of energy-supplying metabolism of fast (white), slow (red), and heart muscle of the rat.

Abstract

Influeces of thyroid hormones onthe activity levels of glyucogen phosphorylase, hexokinase, trisoephosphate dehydrogenease, critrate synthase, 3‐hydroxyacyl‐CoA dehydrogenease, mitochondrial glycerolphosphate dehydrogenase and the contnt of cytochrome a3 were examined in a “white” (m. rectus femoris), “red” (m. soleus) and heart muscle of male rats. Of the enzymes examined, glycerolphosphate dehydrogenase and hexokinase, revaled most significant increases, with comparable extent and simultaneous time of onset. The pattern o reaction was, however, different in the three types of muscle. In white muscle, an increase of hexokinase, but not of glycerolphosphate dehydrogenase was found. In red muscle, the two enzymes increased in parallel and in heart muscle the increase in glycerolphosphate dehydrogenase was the largest and exceeded by far that of hexokinse. In skeletal muscles it could be shown that the increase of hexokinase activity was limited to hexokinase‐isoenzyme II, hexokinase I being unaffected. Histochemical staining for glycerolphosphate dehydrogenase revealed that the activity of this enzyme is predominalty demonstrable in the subsarcolemmal mitochondria. The increased activities of hexokinase in the skeletal muscles and of glycerophosphate dehydrogenase in red and heart muscle are interpreted as a coordinate process of a “long‐term metabolic regulation” which is related to an elvelatd aerobic carbohydrate catabolism of muscle tissue and to an enhanced rate of basal metabolism.