Thyroid Hormone Induced Changes in the Enzyme Activity Pattern of Energy‐Supplying Metabolism of Fast (Whit) Slow (Red), and Heart Muscle of the Rat

Abstract

Influences of thyroid hormones on the activity levels of glycogen phosphorylase, hexokinase, triosephosphate dehydrogenase, citrate synthase, 3-hydroxyacyl-CoA dehydrogenase, mitochondrial glycerolphosphate dehydrogenase and the content of cytochrome u3 were examined in a "white" (m. rectus fernoris), "red" (m. soleus) and heart muscle of male rats. Of the enzymes examined, glycerolphosphate dehydrogenase and hexokinase, revealed most significant increases, with comparable extent and simultaneous time of onset. The pattern of reaction was, however, different in the three types of muscle. I n white muscle, an increase of hexokinase, but not of glycerolphosphate dehydrogenase was found. I n 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 hexokinase. In skeletal muscles it could be shown that the increase of hexokinase activity was limited to hexokinase-isoenzyme 11, hexokinase I being unaffected.Histochemical staining for glycerolphosphate dehydrogenase revealed that the activity of this enzyme is predominantly demonstrable in the subsarcolemmal mitochondria. The increased activities of hexokinase in the skeletal muscles and of glycerolphosphate dehydrogenase in red and heart muscle are interpreted as a coordinate process of a "long-term metabolic regulation" which is related to an elevated aerobic carbohydrate catabolism of muscle tissue and to an enhanced rate of basal metabolism. studied several glycolytic enzymes and found that hexokinase is one of the more strongly induced enzymes. Krause and Wollenberger [ 101 examined enzymes involved in glycogen metabolism and found increased activities of glycogen phosphorylase, phosphorylase kinase and glycogen synthetase. These various results, however, are not easy to compare, since mode of application and dosage of the hormones were different and the measurements were performed in muscles which were not well defined and may have been heterogeneous in composition of "white" and "red" fibres. As was shown in previous communications (for references see [i l]), marked differences exist between the enzyme activity patterns of energy-supplying metabolism in white and red muscle. It wits therefore the purpose of this study to compare muscles of different metabolic type (white, red and heart muscle [11-16]) with regard to their response to increased levels of thyroid hormones. Activities of enzymes were examined which in previous studies had been shown to represent definite metabolic systems and to indicate by their ratios certain metabolic correlations.

Effects of thyroid hormones on skeletal muscle bioenergetics. In vivo phosphorus-31 magnetic resonance spectroscopy study of humans and rats.

“…The only abnormality seen in both animal and human muscle during the hyperthyroid state was a very rapid postexercise recovery of the PCr/Pi. This in vivo finding is consistent with in vitro studies that have shown increased mitochondrial activity in hyperthyroid rat muscle (8,(14)(15)(16)(17). Rats tested after 6 wk of hyperthyroidism showed no abnormality of PCr/Pi during exercise.…”

“…Increased thyroid hormone levels result in increased mitochondrial enzymes activity in muscle and this is again more pronounced in slow fibers (8,(14)(15)(16)(17). Muscle bioenergetic studies have been less conclusive in hyperthyroidism; ATP turnover was increased in a single case compared with controls (10), while rats showed no significant changes (12).…”

Thyroid thermogenesis. Relationships between Na+-dependent respiration and Na+ + K+-adenosine triphosphatase activity in rat skeletal muscle.

“…In cats, however, administration of thyroxine had no effect on Ca++-ATPase activity of skeletal muscle (39). The possible contributions of alterations in regulatory enzymes and mitochondrial function to thyroid thermogenesis have been considered by a number of investigators; thyroid-dependent changes in the activity ratios of hexokinase/citrate synthase and glycerolphosphate dehydrogenase/triosephosphate dehydrogenase have been posited to participate in the thermogenic response (40). Simi-larly, the finding of enhanced transport of reducing 5Lo, C. S., U.…”

Reducing Equivalent Shuttles in Developing Porcine Myocardium: Enhanced Capacity in the Newborn Heart

“…Flavin-linked ma-GPDH activity was measured by following the reduction of cytochrome c as described previously (23). The assay buffer consisted of (in mM) 100 potassium phosphate, 5 EDTA, 1 potassium cyanide, 0.1 phenazine methosulfate, and 0.09 cytochrome c. 100 p L of particulate mitochondrial fraction were added to 2.3 mL of the buffer in a 3-mL plastic cuvette.…”