Regenerating soleus and extensor digitorum longus muscles of the rat show elevated levels of TNF-alpha and its receptors, TNFR-60 and TNFR-80

We measured the mRNA and protein levels of tumor necrosis factor-alpha (TNF -alpha) and the transcript levels of its receptors (TNFR-60 and TNFR-80) in the rat soleus (slow twitch) and extensor digitorum longus (EDL; fast twit ch) muscles regenerating from notexin-induced necrosis. On the first day af ter administration of the toxin, when most fibers were necrotic and invaded by inflammatory cells/macrophages, dramatic increases of transcript and pr otein levels of TNF-alpha and of the mRNA levels of its receptors were obse rved. The transcript levels of TNF-alpha and TNFR-60, but not of TNFR-80, s howed a second but smaller increase at the time when newly formed muscle fi bers became reinnervated. In situ hybridization showed that on day 1, durin g the phase of extensive necrosis, the transcript of TNF-alpha was abundant ly present and on day 4 of regeneration it was most often seen in areas dev oid of desmin. The mRNA level of TNF-alpha was not detectable in BC(3)H1- a nd C2C12-cultured myoblasts and it was low in freeze-injured muscle, corres ponding to the relatively mild degree of inflammation elicited by freezing. Therefore, our results are most consistent with the view that inflammatory cells/macrophages are the main source of TNF-alpha. (C) 2001 John Wiley 8 Sons, Inc.