Structural and functional alterations in fused membranes of secretory granules during exocytosis in pancreatic islet cells of the mouse

The structure of membrane junctions formed during the attachment of secretory granules to the plasma membrane was examined in pancreatic islet B cells of mice by means of thin section and freeze fracture techniques. In thin sections, pentalaminar fusions were observed to connect plasma and granule membranes. Similar membrane junctions were found between several adjacent secretory granules. From replicas of freeze fractured islet cells it became evident, that the process of membrane fusion probably causes a redistribution and accumulation of membrane associated particles in granule membranes. When islets were exposed to hypertonic media, only a fraction of the cells appeared to be shrunken. A prominent structural alteration of shrunken islet cells is the occurrence of large vesicles in the vicinity of the plasma membrane. As membrane associated particles have the same size in vesicular membranes as in granule membranes and because the number of vesicles increases in glucose stimulated B cells, it is likely that the vesicles in shrunken islet cells are swollen secretory granules. Swelling of adjacent vesicles, which are interconnected only by membrane junctions, demonstrates that such membrane junctions are permeable for all solutes of the hypertonic solution, i.e. for molecules up to the size of sucrose. It is suggested that, as in other cells with exocytotic activity, the formation of permeable membrane junctions precedes solubility and release of insulin from B cells.