Calpain inhibitor in rabbit skeletal muscle: an immunochemical and histochemical study

Calpastatin, the endogenous inhibitor of calcium-activated neutral proteases (calpains; EC 3.4.22.17), was studied in the rabbit vastus lateralis muscle by means of immunochemical and immunohistochemical techniques. Immunoaffinity chromatography using an antibody raised against the 34-kDa monomer of the 68-kDa dimeric inhibitor allowed us to isolate three main proteins (130-, 100- and 80-kDa). These proteins strongly inhibited calpain activity in muscle homogenate (I50 at about 50 micrograms/ml). Immunohistochemical experiments showed that calpastatin-related immunoreactivity was present in all fibre types (oxidative, glycolytic, oxidative-glycolytic) at both surface and cytoplasmic level. However, a few (20%) of the slow-twitch, oxidative fibres (5% of the total fibres), did not contain the cytoplasmic inhibitor. Specific immunoreactivity for calpastatin was also associated with the interstitial connective tissue. These results suggest that (i) calpastatin in skeletal muscle, as in other tissues, is present as a mixture of proteins of various molecular weights and (ii) the inhibitor may act not only in the cytoplasm but also at the surface or extracellular level.