Calpain activity in fast, slow, transforming, and regenerating skeletal muscles of rat

Fiber-type transitions in adult skeletal muscle induced by chronic low-freq uency stimulation (CLFS) encompass coordinated exchanges of myofibrillar pr otein isoforms. CLFS-induced elevations in cytosolic Ca2+ could activate pr oteases, especially calpains, the major Ca2+-regulated cytosolic proteases. Calpain activity determined by a fluorogenic substrate in the presence of unaltered endogenous calpastatin activities increased twofold in low-freque ncy-stimulated extensor digitorum longus (EDL) muscle, reaching a level int ermediate between normal fast- and slow-twitch muscles. mu- and m-calpains were delineated by a calpain-specific zymographical assay that assessed tot al activities independent of calpastatin and distinguished between native a nd processed calpains. Contrary to normal EDL, structure-bound, namely myof ibrillar and microsomal calpains, were abundant in soleus muscle. However, the fast-to-slow conversion of EDL was accompanied by an early translocatio n of cytosolic mu-calpain, suggesting that myofibrillar and microsomal mu-c alpain was responsible for the twofold increase in activity and thus involv ed in controlled proteolysis during fiber transformation. This is in contra st to muscle regeneration where m-calpain translocation predominated. Taken together, we suggest that translocation is an important step in the contro l of calpain activity in skeletal muscle in vivo.