Condensing vacuole conversion and zymogen granule discharge in pancreatic exocrine cells: metabolic studies

Abstract

We have examined, in the pancreatic exocrine cell, the metabolic requirements for the conversion of condensing vacuoles into zymogen granules and for the discharge of the contents of zymogen granules. To study condensing vacuole conversion, we pulse labeled guinea pig pancreatic slices for 4 min with leucine-(3)H and incubated them in chase medium for 20 min to allow labeled proteins to reach condensing vacuoles. Glycolytic and respiratory inhibitors were then added and incubation continued for 60 min to enable labeled proteins to reach granules in control slices. Electron microscope radioautography of cells or of zymogen granule pellets from treated slices showed that a large proportion of prelabeled condensing vacuoles underwent conversion in the presence of the combined inhibitors. Osmotic fragility studies on zymogen granule suspensions suggest that condensation may result from the aggregation of secretory proteins in an osmotically inactive form. Discharge was studied using an in vitro radioassay based on the finding that prelabeled zymogen granules can be induced to release their labeled contents to the incubation medium by carbamylcholine or pancreozymin. Induced discharge is not affected if protein synthesis is blocked by cycloheximide for up to 2 hr, but is strictly dependent on respiration. The data indicate that transport and discharge do not require the pari passu synthesis of secretory or nonsecretory proteins (e.g. membrane proteins), suggesting that the cell may reutilize its membranes during the secretory process. The energy requirements for zymogen discharge may be related to the fusion-fission of the granule membrane with the apical plasmalemma.