Selective localization of calpain I (the low‐Ca2+‐requiring form of Ca2+‐dependent cysteine proteinase) in B‐cells of human pancreatic islets

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Abstract
An immunohistochemical study was performed to localize two distinct Ca2+‐proteases (low‐Ca2+‐requiring calpain I and high‐Ca2+‐requiring calpain II) and their specific inhibitor (calpastatin) in human pancreas using the respective monospecific antibodies. Strongly positive staining by anti‐calpain I antibody was found in pancreatic islets, specifically in B‐cells, whereas the exocrine pancreatic tissue showed essentially no positive immunostaining. No such specific staining was found with anti‐calpain II antibodies or anti‐calpastatin antibodies. The results suggest that the Ca2+‐dependent proteolysis in B‐cells can be triggered by a small rise of the intracellular Ca2+ concentration without serious interference by the endogenous inhibitor.