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.