title: Docking of LDCVs is modulated by lower intracellular [Ca2+] than priming.
abstract: Many regulatory steps precede final membrane fusion in neuroendocrine cells. Some parts of this preparatory cascade, including fusion and priming, are dependent on the intracellular Ca(2+) concentration ([Ca(2+)](i)). However, the functional implications of [Ca(2+)](i) in the regulation of docking remain elusive and controversial due to an inability to determine the modulatory effect of [Ca(2+)](i). Using a combination of TIRF-microscopy and electrophysiology we followed the movement of large dense core vesicles (LDCVs) close to the plasma membrane, simultaneously measuring membrane capacitance and [Ca(2+)](i). We found that a free [Ca(2+)](i) of 700 nM maximized the immediately releasable pool and minimized the lateral mobility of vesicles, which is consistent with a maximal increase of the pool size of primed LDCVs. The parameters that reflect docking, i.e. axial mobility and the fraction of LDCVs residing at the plasma membrane for less than 5 seconds, were strongly decreased at a free [Ca(2+)](i) of 500 nM. These results provide the first evidence that docking and priming occur at different free intracellular Ca(2+) concentrations, with docking efficiency being the most robust at 500 nM.
authors.affiliation: Physiologisches Institut, Universität des Saarlandes, Homburg/Saar, Germany.
authors.forename: Mathias
authors.initials: M
authors.lastname: Pasche
authors.forename: Ulf
authors.initials: U
authors.lastname: Matti
authors.forename: Detlef
authors.initials: D
authors.lastname: Hof
authors.forename: Jens
authors.initials: J
authors.lastname: Rettig
authors.forename: Ute
authors.initials: U
authors.lastname: Becherer
mesh_terms: D000818:Animals; D002118:Calcium / Q000378:metabolism*; D002417:Cattle; D002462:Cell Membrane / Q000378:metabolism*; D002478:Cells, Cultured; D008561:Membrane Fusion; D055099:Neuroendocrine Cells / Q000166:cytology / Q000378:metabolism*; D022142:Secretory Vesicles / Q000378:metabolism*
chemical_list: D002118:Calcium