Morphological and biochemical analysis of regeneration after cardiotoxin injury of Xenopus laevis fast muscles.

The anterior brachial muscle of Xenopus laevis forelimb was characterized as a fast-type muscle composed of type II fibers exclusively. Larval and adult muscles showed three distinct isomyosins composed by two different heavy chains, HCl and HCf, respectively, associated with the same fast light chains. Muscle regeneration was examined after degeneration of the myofibers by injection of cardiotoxin, a snake toxin. 24 h after the injury no myofibers and no myosin were detected. New myosins of larval and adult fast types started to be synthesized two weeks after the injury, during a stage of proliferation of mononucleated cells. 1 month after the injury, the regenerated muscles which showed structural differences with the normal muscle contained only fast isomyosins. The precocious larval to fast heavy chain transition observed in regenerating muscles of the adult X. laevis without any thyroid hormone influence shows that the myogenic program in adult muscle regeneration is regulated by factors that are different from those regulating normal development.