Regulation by thyroid hormones of terminal differentiation in the skeletal dorsal muscle. I. Neonate mouse.

Changes both in the ATPase myofibrillar profile and in the electrophoretic pattern of myosin isoforms were examined in the mouse dorsal skeletal muscle (longissimus) during postnatal development. In the newborn, only type II C and a few type I fibers were present; differentiation into type II A and II B fibers took place during the 3 weeks following birth. During the same period, a transition from three neonatal isomyosins to four adult isoforms was observed. The two phenomena were related to a marked increase in the serum thyroid hormones levels. Hypothyroidism and hyperthyroidism experiments were performed. Hypothyroidism produced by propylthiouracil treatment of pregnant females and thiourea injections of the litters was shown to induce a complete inhibition of postnatal muscular differentiation. Hyperthyroidism produced by triiodothyronine treatment of the neonate mice significantly accelerated the myosin transition and the switch in the myofibrillar pattern. Our results suggest a primordial role for thyroid hormones in directly regulating the appearance of myosin and fiber adult types and in modulating directly or indirectly the disappearance of the neonatal types.