Vesicle dynamics during regulated secretion in a novel pancreatic acinar cell in vitro model

Neonatal rat pancreatic acinar cells were isolated in long-term culture to study vesicular trafficking in a regulated secretory epithelial cell. These cultured cells are of a reduced thickness to facilitate cell biological applications, yet have maintained a differentiated state capable of manufacturing and releasing zymogen from defined cellular locations in response to physiological concentrations of secretagogues. Cells observed with video and electron microscopy display numerous dense zymogen-containing granules which are interconnected by fine filamentous strands. Exposure of 6-day cultured cells to 2 x 10(-9) M of the hormone agonist cholecystokinin octapeptide (CCK-8), or 2 to 5 microM 12-O-tetradecanoylphorbol-13-acetate (TPA), stimulates an increase in directed granule movements to a defined perinuclear region of the cell where they undergo exocytosis. This dramatic exocytic event can be observed readily by video microscopy or measured with enzymatic amylase assays. High voltage electron and scanning electron microscopy reveal that these stimulated cells undergo pronounced changes in shape. Concomitantly, secreting cells insert the integral zymogen granule membrane protein (GP-2) into a defined region of the apical plasmalemma while forming numerous membranous surface projections; both are processes consistent with a regulated exocytic event. These acinar cells provide a novel, fully functional, differentiated exocrine epithelial cell model which possess a receptor signaling cascade capable of stimulating the release of zymogen. To our knowledge, the observations presented here provide the first direct evidence that a) secretory agonists initiate directed transport and polarized delivery of zymogen granules to a defined cellular location for subsequent release, b) zymogen granules are physically interconnected in both resting and activated cells, c) cells undergo active changes in shape and form during secretion, and d) membranous blebs accumulate at a region of the cell surface where exocytosis occurs. The potential contributions of this in vitro cell model to the study of regulated vesicular transport, exocytosis, and membrane retrieval are discussed.