title: Sub-micromolar increase in [Ca(2+)](i) triggers delayed exocytosis of ATP in cultured astrocytes.
abstract: Astrocytes release a variety of transmitter molecules, which mediate communication between glial cells in the brain and modulate synaptic transmission. ATP is a major glia-derived transmitter, but the mechanisms and kinetics of ATP release from astrocytes remain largely unknown. Here, we combined epifluorescence and total internal reflection fluorescence microscopy to monitor individual quinacrine-loaded ATP-containing vesicles undergoing exocytosis in cultured astrocytes. In resting cells, vesicles exhibited three-dimensional motility, spontaneous docking and release at low rate. Extracellular ATP application induced a Ca(2+)-dependent increase in the rate of exocytosis, which persisted for several minutes. Using UV flash photolysis of caged Ca(2+), the threshold [Ca(2+)](i) for ATP exocytosis was found to be approximately 350 nM. Subthreshold [Ca(2+)](i) transients predominantly induced vesicle docking at plasma membrane without subsequent release. ATP exocytosis triggered either by purinergic stimulation or by Ca(2+) uncaging occurred after a substantial delay ranging from tens to hundreds of seconds, with only approximately 4% of release occurring during the first 30 s. The time course of the cargo release from vesicles had two peaks centered on <or=10 s and 60 s. These results demonstrate that: (1) [Ca(2+)](i) elevations in cultured astrocytes trigger docking and release of ATP-containing vesicles; (2) vesicle docking and release have different Ca(2+) thresholds; (3) ATP exocytosis is delayed by several minutes and highly asynchronous; (4) two populations of ATP-containing vesicles with distinct (fast and slow) time course of cargo release exist in cultured astrocytes.
authors.affiliation: Neuroscience Center, University of Helsinki, P.O. Box 56 (Viikinkaari 4), FIN-00014, Helsinki, Finland.
authors.forename: Evgeny
authors.initials: E
authors.lastname: Pryazhnikov
authors.forename: Leonard
authors.initials: L
authors.lastname: Khiroug
mesh_terms: D000255:Adenosine Triphosphate / Q000502:physiology*; D000818:Animals; D001253:Astrocytes / Q000502:physiology*; D020013:Calcium Signaling / Q000502:physiology*; D002478:Cells, Cultured; D004396:Coloring Agents; D003627:Data Interpretation, Statistical; D004791:Enzyme Inhibitors; D005089:Exocytosis / Q000502:physiology*; D005260:Female; D008247:Lysosomes / Q000187:drug effects / Q000378:metabolism; D008856:Microscopy, Fluorescence; D010782:Photolysis; D011247:Pregnancy; D011796:Quinacrine; D051381:Rats; D013572:Synaptic Vesicles / Q000187:drug effects / Q000378:metabolism
chemical_list: D004396:Coloring Agents; D004791:Enzyme Inhibitors; D000255:Adenosine Triphosphate; D011796:Quinacrine