Molecular characterisation of pancreatic zymogen granule ion channel and regulator proteins involved in exocytosis

In pancreatic acinar cells Ca2+-dependent secretagogues promote the fusion of zymogen granules (ZG) with the apical plasma membrane (PM) and exocytosi s of digestive enzymes. In addition to exocytotic fusion complexes between SNARE proteins in the ZG membrane (ZGM) and the apical PM, enzyme secretion elicited by Ca2+-dependent secretagogues requires cytosolic Cl- and K+ and is inhibited by blockers of Cl-- and K+-channels. We have identified a Cl- -conductance activated by ATP, and a K+-conductance (with properties simila r to ATP-sensitive K+-channels), regulated by the granule matrix protein Zg -16p in the ZGM. Both conductances are inversely regulated by a 65-kD mdr1 gene product. We have also identified a novel Ca2+-activated anion conducta nce in ZGM, the Ca2+-sensitivity of which increases 50-fold when Cl- is rep laced by I-. This conductance is blocked by micromolar H-2-DIDS or D-TT, re miniscent of a family of epithelial Ca2+-activated Cl--channels (CaCC). Exp ression of a CaCC in exocrine pancreas has been confirmed by RT-PCR analysi s, and by immunoblotting and immunogold labeling of ZG membranes. These dat a suggest that ion channels in the ZGM are essential elements in pancreatic exocytosis.