title: Real-time studies of zymogen granule exocytosis in intact rat pancreatic acinar cells.
abstract: An adequate understanding of secretion requires the measurement of exocytosis on the same time scale as that used for second messenger dynamics. To investigate the kinetics of ACh-evoked secretion in pancreatic acinar cells, exocytosis of zymogen granules was quantified by continuous, time-differential analysis of digital images. The validity of this method was confirmed by simultaneous fluorescence imaging of quinacrine-loaded zymogen granules. Basal rates of exocytosis were low (0.2 events min(-1)). ACh stimulated a biphasic increase in secretory activity, maximal rates exceeding 20 events min(-1) after 10 s of ACh application (10 microM). Over the next 15 s the rate of exocytosis fell to less than 4 events min(-1); then began a second phase of secretion that peaked 15 s later at approximately 11 events min(-1), but subsequently declined in the continued presence of agonist. Measurements of fura-2 fluorescence demonstrated a biphasic increase in intracellular [Ca2+] ([Ca2+]i). Comparison of the [Ca2+]i records and time-differential analysis revealed that the fall in exocytotic rate following the initial burst occurred despite the fact that [Ca2+]i remained high. The second phase of secretion depended on both [Ca2+]i and [ACh]. At 10 microM ACh there was a decrease in the steepness of the relationship between [Ca2+]i and exocytosis that led to an enhancement of the slow secretory phase. We propose that acinar cells contain two pools of secretory vesicles: a small pool of granules that is exocytosed rapidly, but is quickly depleted; and a reserve pool of granules that can be recruited by ACh in a process that is modulated by second messengers other than calcium.
authors.affiliation: Department of Pharmacology, University of Cambridge, Tennis Court Road, Cambridge CB2 1QJ, UK.
authors.forename: M
authors.initials: M
authors.lastname: Campos-Toimil
authors.forename: J M
authors.initials: JM
authors.lastname: Edwardson
authors.forename: P
authors.initials: P
authors.lastname: Thomas
mesh_terms: D000109:Acetylcholine / Q000008:administration & dosage / Q000494:pharmacology; D000818:Animals; D002118:Calcium / Q000378:metabolism; D004305:Dose-Response Relationship, Drug; D005089:Exocytosis / Q000187:drug effects; D008297:Male; D010179:Pancreas / Q000166:cytology / Q000187:drug effects / Q000378:metabolism*; D051381:Rats; D017208:Rats, Wistar; D015290:Second Messenger Systems; D022142:Secretory Vesicles / Q000187:drug effects / Q000378:metabolism*
chemical_list: D000109:Acetylcholine; D002118:Calcium