The regenerating muscle as an experimental model for the study of factors which affect muscle differentiation or adaptation

Bupivacaine-induced regeneration was studied in the rat soleus muscle in the presence or absence of innervation, in the presence of tetrodotoxin (TTX)-induced block of nerve impulse conduction, and/or in the presence of vinblastine-induced block of nerve axoplasmic flow. Part of experiments were carried out on tenotomized muscles. Regenerated muscles were analysed for myosin heavy chain (MHC) composition 14 days after bupivacaine injection. In TTX-paralysed-regenerated muscles type 1 and type 2A MHC isoforms were not expressed. In denervated-regenerated muscles type 1 isoform was lacking, while all fast isoforms (2A, 2B, 2X) were expressed. Tenotomy alone increased type 2A fibres, but did not modify the effects of surgical or functional denervation. Vinblastine-block caused up-regulation of 2A isoform expression in non-tenotomized muscles. The results confirm the essential role played by neuromotor impulses for type 1 and type 2A isoform expression. They also support the hypothesis that axoplasmic flow carries some chemical factor inhibiting 2A isoform expression.