Regeneration after free muscle grafting in normal and dystrophic (mdx) mice.

Soleus muscles from C57BL/10 and mdx mice were isotransplanted to induce a cycle of degeneration/regeneration. Sixty days post-surgery, transplanted and contralateral soleus muscles were removed for mechanical and biochemical analyses. The regeneration which occurs after transplantation, induces in both mdx and C57BL/10 soleus muscles a decrease in maximal isometric force, together with an increase of the velocity of contraction. This increase in velocity is accompanied by the expression of typically fast-type myosin heavy chains. Thus degeneration/regeneration of both mdx and normal mice are very similar, causing a shift towards physiologically 'faster' muscle. Previous physiological and biochemical studies of mdx muscles have shown that mdx muscle is shifted towards 'slower' muscle compared to normal mice. One explanation of these findings was that the degeneration/regeneration cycles inherent in dystrophin-deficient mdx muscle causes a shift towards 'slow'. Our results argue against this hypothesis: degeneration/regeneration in both normal and mdx mice causes a shift towards 'fast'.