title: Docked granules, the exocytic burst, and the need for ATP hydrolysis in endocrine cells.
abstract: Ca(2+)-triggered exocytosis was studied in single rat melanotrophs and bovine chromaffin cells by capacitance measurements. Sustained exocytosis required MgATP, but even in the absence of MgATP, Ca2+ could trigger exocytosis of 2700 granules in a typical melanotroph and of 840 granules in a chromaffin cell. Granules undergoing ATP-independent exocytosis were similar in number to those appearing docked to the plasmalemma in quickly frozen unfixed sections (3300 in a melanotroph and 830 in a chromaffin cell). Most exocytosis required tens of seconds, but a small pool of granules was released in tens of milliseconds. Evidently, only a small subset of docked granules is rapidly releasable. We suggest that, temporally, the last ATP-dependent step in exocytosis is closely associated with docking and that docked granules reach fusion competence only after subsequent steps.
authors.affiliation: Abteilung Molekulare Zellforschung, Max-Planck Institut fur Medizinische Forschung, Heidelberg, Federal Republic of Germany.
authors.forename: T D
authors.initials: TD
authors.lastname: Parsons
authors.forename: J R
authors.initials: JR
authors.lastname: Coorssen
authors.forename: H
authors.initials: H
authors.lastname: Horstmann
authors.forename: W
authors.initials: W
authors.lastname: Almers
mesh_terms: D000255:Adenosine Triphosphate / Q000378:metabolism* / Q000494:pharmacology; D000311:Adrenal Glands / Q000378:metabolism / Q000648:ultrastructure; D000818:Animals; D002118:Calcium / Q000494:pharmacology; D002417:Cattle; D002838:Chromaffin System / Q000378:metabolism* / Q000648:ultrastructure; D003594:Cytoplasmic Granules / Q000378:metabolism*; D005089:Exocytosis / Q000187:drug effects; D006868:Hydrolysis; D009074:Melanocyte-Stimulating Hormones / Q000378:metabolism; D010902:Pituitary Gland / Q000378:metabolism* / Q000648:ultrastructure; D051381:Rats
chemical_list: D000255:Adenosine Triphosphate; D009074:Melanocyte-Stimulating Hormones; D002118:Calcium