Immunogold ultrastructural localization of calpastatin, the calpain inhibitor, in rabbit skeletal muscle

The ultrastructural localization of calpastatin, the endogenous inhibitor of the neutral calcium-dependent proteases (calpains), was investigated in rabbit skeletal muscle fibers using a polyclonal antibody against the 34 kDa form of the inhibitor isolated from rabbit. Quantitative studies by pre- and postembedding immunogold techniques revealed that the distribution pattern of the specific immunoreactivity included: 1) the sarcolemma with the adjacent cytoplasm (about 1 μm wide); 2) the myofibrils; 3) the mitochondria and 4) the nuclei (condensed as well as extended chromatin). Other cell substructures, such as lysosomes and the intermyofibrillar cytoplasm, were substantially devoid of immunoreactivity