Exocytosis and movement of zymogen granules observed by VEC-DIC microscopyin the pancreatic tissue en bloc

The dynamic aspects of exocytosis, especially in the normal acinar tissue e n bloc, have remained unclear. We visualized exocytosis directly in the tis sue of the exocrine pancreas of rodents by video-enhanced contrast-differen tial interference contrast (VEC-DIC) microscopy to investigate various exoc ytosis-related rates and the relationship between the movement of granules and exocytotic responses. Stimulation of the tissue with bethanechol or cho lecystokinin caused many of the zymogen granules in the apical pole to disa ppear abruptly. The exocytotic transients of individual granules were compl eted in 0.48-0.65 s. Granules destined to participate in the exocytotic res ponse moved randomly at velocities of similar to 0.06 mu m/s or less during stimulation. In the tissue preparation, granules located far from the apic al pole frequently moved back and forth for 1-7 mu m without showing exocyt osis. Colchicine suppressed this movement and the late phase of the secreto ry response. Real-time (VEC-DIC) observation of granule dynamics revealed t hat the initial step of exocytosis was not coupled directly with the microt ubule-dependent translocation but with a continuous, slow Brownian fluctuat ion of granules.