EXPRESSION OF CARBONIC-ANHYDRASE-V IN PANCREATIC BETA-CELLS SUGGESTS ROLE FOR MITOCHONDRIAL CARBONIC-ANHYDRASE IN INSULIN-SECRETION

Carbonic anhydrase V (CA-V) is a mitochondrial enzyme that provides bi carbonate for pyruvate carboxylase in liver and kidney. In the course of a survey of the tissue distribution of CA-V, we detected intense im munostaining in pancreatic islets when sections from rat and mouse pan creases were reacted with a polyclonal antibody to recombinant mouse C A-V, The distribution and large number of CA-V-positive cells in each islet suggested that they represented beta cells. Double immunofluores cence staining of tissue sections and isolated islet cells showed cell ular colocalization of CA-V and insulin, confirming that beta cells co ntain CA-V, Western blotting of rat islets of Langerhans and primary b eta cells showed 33- and 30-kDa polypeptides of precursor and mature C A-V, respectively. The CA-V expression was beta cell-specific since no CA-V immunoreaction was detected in the primary alpha cells. Immunohi stochemical staining for CA-I, CA-II, CA-TV, CA-VI, and CA-M was negat ive in beta cells, and Western blotting of beta cells also failed to i dentify any CA in beta cells except CA-V, The specific localization of CA-V in beta cells led us to hypothesize that CA-V may be functionall y linked to the regulation of insulin secretion. Consistent with this hypothesis, the CA inhibitor acetazolamide was found to be a strong in hibitor of glucose-stimulated insulin secretion by isolated rat pancre atic islets.