Long-term regeneration of fast and slow murine skeletal muscles after induced injury by ACL myotoxin isolated from Agkistrodon contortrix laticinctus(broad-banded copperhead) venom

The aim of the present work was to analyze the regenerated muscle types I a nd II fibers of the soleus and gastrocnemius muscles of mice, 8 months afte r damage induced by ACL myotoxin (ACLMT). Animals received 5 mg/kg of ACLMT into the subcutaneous lateral region of the right hind limb, near the Achi lles tendon; contralateral muscles received saline. Longitudinal and cross sections (10 mu m) of frozen muscle tissue were evaluated. Eight months aft er ACLMT injection, both muscle types I and II fibers of soleus and gastroc nemius muscles still showed centralized nuclei and small regenerated fibers . Compared with the left muscle, the incidence of type I fibers increased i n the right muscle (21% +/- 03% versus 12% +/- 06%, P = 0.009), whereas typ e II fibers decreased (78% +/- 02% versus 88% +/- 06%, P = 0.01). The incid ence of type IIC fibers was normal. These results confirm that ACLMT induce d muscle type fiber transformation from type II to type I, through type IIC . The area analysis of types I and II fibers of the gastrocnemius revealed that injured right muscles have a higher percentage of small fibers in both types I and II fibers (0-1,500 mu m(2)) than left muscles, which have larg er normal type I and II fibers (1,500-3,500 mu m(2)). These results indicat e that ACLMT can be used as an excellent model to study the rearrangement o f motor units and the transformation of muscle fiber types during regenerat ion. Anat Rec 254:521-533, 1999. (C) 1999 Wiley-Liss, Inc.