Enzyme changes in neonatal skeletal muscle: effect of thyroid deficiency

Experiments were carried out to determine 1) the time course of changes (days 742) in marker enzymes of energy metab-olism in portions of rodent neonatal skeletal muscle destined to be fast- oxidative- glycogenolytic (FOG), fast- glycogenoly-tic (FG), and slow-oxidative (SO); and 2) the effects of thyroid deficiency on the development of these enzymes in mixed fast-twitch muscle. From days 7 to 26, 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 approxi-mately 70 % of the level of FOG muscle. Thyroid deficiency, induced by propylthiouracil (PTU) treatment at birth, mark-