Reperfusion injury to skeletal muscle affects primarily type II muscle fibers

INTRODUCTION: The injury caused by reperfusion of ischemic skeletal muscle is mediated by the membrane attack complex of complement (C) . This C activation results from local classical pathway activation after deposition of IgM in injured muscle, an event analogous to C deposition in the mucosa of the gut during reperfusion . Our past analysis has indicated that the injury is not uniform even within a single microscopic section. This study was performed to elucidate the exact site of IgM and C deposition on muscle injured by ischemia and reperfusion. MATERIALS AND METHODS: C57Bl/6 mice were subjected to 2 h of tourniquet-induced hindlimb ischemia followed by reperfusion for 0-6 h. Three muscle groups (vastus, gastrocnemius, and soleus) of varying fast-myosin content were compared for muscle fiber damage and C deposition. Adjacent paraffin-embedded cross-sections were immunostained to correlate C3 deposition with muscle fiber type as defined by monoclonal antibodies. RESULTS: Muscle injury after ischemia and reperfusion is not uniform and not all fibers in the same microscopic field are affected. Damaged fibers are also those to which IgM and C bind. Immunostaining for slow-twitch (Type 1) or fast-twitch (Type 2) fibers reveals that injury and C3 deposition is confined to Type 2 fibers with lower myosin content. A correlation of Type 2 fiber content and degree of muscle injury showed that the predominantly fast-twitch vastus muscle had the greatest number of damaged fibers per x10 field (28.2 +/- 12.4) when compared to the mixed fiber-type gastrocnemius muscle (20.5 +/- 5.3) and the mixed, but slow-twitch enriched soleus muscle (17.3 +/- 11.8). CONCLUSION: Complement activation and skeletal muscle reperfusion injury occurs predominantly on Type 2 fibers with low myosin content. This suggests that attempts to control the post-reperfusion inflammation will likely produce substantial muscle recovery. Furthermore, the basis of IgM deposition and complement activation may be revealed in the comparison of the two muscle fiber types.