Enzyme changes in neonatal skeletal muscle: effect of thyroid deficiency

Abstract

Experiments were carried out to determine: the time course of changes (days 7-42) in marker enzymes of energy metabolism in portions of rodent [rat] neonatal skeletal muscle destined to be fast-oxidative-glycogenolytic (FOG), fast-glycogenolytic (FG), and slow-oxidative (SO); and the effects of thyroid deficiency on the development of these enzymes in mixed fast-twitch muscle. From days 7-28, FOG portions underwent progressive increases in phosphofructokinase (PFK) (300%), citrate synthase (500%), cytochrome c (500%), and myofibril ATPase (400%); whereas FG portions underwent progressive increases only in PFK (300%) and myofibril ATPase (400%). PFK and myofibril ATPase remained low in SO muscle; whereas citrate synthase increased progressively to approximately 70% of the level of FOG muscle. Thyroid deficiency, induced by propylthiouracil (PTU) treatment at birth, markedly impaired body and skeletal muscle mass by approximately 60% compared to normals at day 42; and on the average it caused significant reductions in skeletal muscle protein concentration (-20%), citrate synthase (-50%), cytochrome c (-50%), PFK (-30%), and myofibril ATPase (-30%). Skeletal muscle glycogenolytic enzymes and myofibril ATPase are apparently normally regulated in parallel and by factors different from respiratory enzymes, and thyroxine is necessary for mixed fast skeletal muscle metabolic development to reach full completion.