Myosin isoform transitions and physiological properties of regenerated and re-innervated soleus muscles of the rat.

Soleus muscles in young female rats were destroyed by the local injection of the crude venom of the Australian tiger snake, Notechis scutatus and allowed to regenerate. The regenerated muscles consisted almost exclusively of type I muscle fibres, and histograms of fibre cross-sectional area were unimodal. In contrast the normal contralateral muscles consisted of a mixture of type I and IIa fibres, and histograms of fibre cross-sectional area were bimodal. There was no change in the ability of the regenerated muscles to generate tension in response to indirect stimulation, and muscle fibre number and motor unit index were similar to controls. The regenerated muscles often contained split muscle fibres, but if re-innervation of the regenerated muscles was delayed, splitting did not occur. The principal features of the regenerated muscles (fibre type homogeneity, unimodal histograms of muscle fibre area, normal numbers of muscle fibres and normal motor unit index) were reproduced in soleus muscles that were simply denervated by nerve crush and allowed to re-innervate. We conclude that the phenotypic homogeneity of the regenerated muscles is a consequence of the temporary disconnection of muscle from nerve, rather than of the cycle of degeneration/regeneration, and suggest that this disconnection results in the reprogramming of the soleus motor neurones.