Regulation and role of calcium in pancreatic exocytosis
Hanc, Stacey Jeanne
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The pancreas is an organ designed to synthesize, package, and release enzymes required by the digestive system. These enzymes are packaged in secretory vesicles and are released by exocytosis following stimulation of the pancreatic acinar cells by gastrointestinal hormones. A recognized aspect of this signal is an increase in the concentration of intracellular Ca2+. Previous work has demonstrated that the increase occurs preferentially in the apical pole of the cell in the region of the secretory vesicles prior to the onset of exocytosis. Secretory vesicles were determined to contain significant amounts of Ca2+ by FURA-2 fluorescence. Purified secretory vesicle membranes were subjected to SDS-PAGE and immunoblotted with antibodies to a consensus region of ryanodine and IP3 receptors. The results show that the pancreatic secretory vesicles express both of these Ca2+ release pathways and is then consistent with the vesicles acting as a physiologically relevant Ca2+ store. Efflux of Ca2+ through these release pathways will require the participation of a counterion. Theoretically, K+ influx or Cl" efflux could serve as the requisite counterion to Ca2+ efflux. Experimental conditions that promote a Cl' flux increased the rate of secretory vesicle swelling. This swelling could be prevented by treatment with the Ca2+ channel blockers NiCl2, LaCU, and ruthenium red. Taken together, the results indicate that secretory vesicles, under the in vitro assay conditions, release Ca2+ preferentially through a ryanodine receptor Ca2* channel and that the rate of release was limited by Cl- transport. The contribution of these pathways to exocytosis was subsequently determined through an analysis of secretory vesicle fusion with apical membranes in vitro. However, blockage of the Ca2+ efflux pathway by ruthenium red or LaCh, significantly reduced the rate of membrane fusion. These results suggest a model whereby Cl' dependent Ca2+ efflux from the luminal vesicle environment is a requirement for exocytosis and membrane fusion.