NONPARALLEL SECRETION OF GP-2 FROM EXOCRINE PANCREAS IMPLIES LUMINAL COUPLING BETWEEN ACINAR AND DUCT CELLS

The in vivo and in vitro secretion of glycoprotein-2 (GP-2), a glycosy l phosphatidylinositol (GPI)-anchored protein from the rat exocrine pa ncreas, was characterized. GP-2 was secreted in a nonparallel manner c ompared with amylase, a marker of secretory enzymes. Attenuated GP-2 s ecretion correlated with hormones that stimulated exocytosis in acinar cells. Augmented GP-2 secretion correlated with hormones that stimula ted fluid and bicarbonate secretion from ductal elements. Immunofluore scence studies identified an enriched pool of GP-2 tightly bound to th e apical membranes of acinar cells in addition to zymogen granules. Th is non-zymogen granule pool appears to represent the source of GP-2 re leased from acinar cells in a nonparallel manner. With the use of disp ersed pancreatic acini largely devoid of ductal elements, GP-2 release was found to be augmented by alkaline pH. Thus GP-2 secretion appears to be modulated by two discrete cellular processes: 1) delivery of pr ereleased GP-2 within zymogen granules to the ductal lumen by exocytic mechanisms and 2) enzymatic release of GPI-anchored GP-2 from the lum inal membranes, a kinetic process that appears to be regulated by secr etin- or carbachol-induced secretion of bicarbonate.