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

The dynamicaspects of exocytosis, especially in the normal acinar tissue en bloc,have remained unclear. We visualized exocytosis directly in the tissueof the exocrine pancreas of rodents by video-enhancedcontrast-differential interference contrast (VEC-DIC) microscopy to investigate various exocytosis-related rates and the relationship between the movement of granules and exocytotic responses. Stimulation of the tissue with bethanechol orcholecystokinin caused many of the zymogen granules in the apical poleto disappear abruptly. The exocytotic transients of individual granuleswere completed in 0.48-0.65 s. Granules destined to participate in the exocytotic response moved randomly at velocities of ~0.06 µm/sor less during stimulation. In the tissue preparation, granules locatedfar from the apical pole frequently moved back and forth for 1-7µm without showing exocytosis. Colchicine suppressed this movementand the late phase of the secretory response. Real-time (VEC-DIC)observation of granule dynamics revealed that the initial step ofexocytosis was not coupled directly with the microtubule-dependent translocation but with a continuous, slow Brownian fluctuation of granules.