[The regulator role of thyroid hormones in myogenesis. Analysis of isoforms of myosin in muscular regeneration].

Fast-twitch muscle regeneration has been studied in experimental hyper- and hypothyroid adult rats. The degeneration of the muscle fibres was achieved through the injection of a snake venom cardiotoxin and the synthesis of new isomyosins was examined 7, 10, 15, and 21 days after the injury. As early as the 7th day after the toxin treatment, that is 3 days after the start of the regeneration, the muscles of hyperthyroid rats do not contain any neonatal myosins and synthesize only adult myosins. In euthyroid rat muscles, neonatal myosins coexist with adult myosins and are no longer present on the 10th day. In hypothyroid rat muscles, both myosin types are still synthesized on the 21st day. Therefore, as for normal myogenesis, hyperthyroidism is shown to favor the synthesis of adult-type myosins and hypothyroidism the synthesis of neonatal-type myosins during regeneration. These results may account, at least in part, for the previously observed differences between the various types of myosins synthesized respectively in postnatal myogenesis and during adult muscle regeneration.