Synthesis, intracellular transport, and discharge of secretory proteins in stimulated pancreatic exocrine cells

Abstract

Our previous observations on the synthesis and transport of secretory proteins in the pancreatic exocrine cell were made on pancreatic slices from starved guinea pigs and accordingly apply to the resting, unstimulated cell. Normally, however, the gland functions in cycles during which zymogen granules accumulate in the cell and are subsequently discharged from it in response to secretogogues. The present experiments were undertaken to determine if secretory stimuli applied in vitro result in adjustments in the rates of protein synthesis and/or of intracellular transport. To this intent pancreatic slices from starved animals were stimulated in vitro for 3 hr with 0.01 mM carbamylcholine. During the first hour of treatment the acinar lumen profile is markedly enlarged due to insertion of zymogen granule membranes into the apical plasmalemma accompanying exocytosis of the granule content. Between 2 and 3 hr of stimulation the luminal profile reverts to unstimulated dimensions while depletion of the granule population nears completion. The acinar cells in 3-hr stimulated slices are characterized by the virtual complete absence of typical condensing vacuoles and zymogen granules, contain a markedly enlarged Golgi complex consisting of numerous stacked cisternae and electron-opaque vesicles, and possess many small pleomorphic storage granules. Slices in this condition were pulse labeled with leucine-(3)H and the route and timetable of intracellular transport assessed during chase incubation by cell fractionation, electron microscope radioautography, and a discharge assay covering the entire secretory pathway. The results showed that the rate of protein synthesis, the rate of drainage of the rough-surfaced endoplasmic reticulum (RER) compartment, and the over-all transit time of secretory proteins through the cells was not accelerated by the secretogogue. Secretory stimulation did not lead to a rerouting of secretory proteins through the cell sap. In the resting cell, the secretory product is concentrated in condensing vacuoles and stored as a relatively homogeneous population of spherical zymogen granules. By contrast, in the stimulated cell, secretory proteins are initially concentrated in the flattened saccules of the enlarged Golgi complex and subsequently stored in numerous small storage granules before release. The results suggest that secretory stimuli applied in vitro primarily affect the discharge of secretory proteins and do not, directly or indirectly, influence their rates of synthesis and intracellular transport.