Sub-micromolar increase in [Ca2+]i triggers delayed exocytosis of ATP in cultured astrocytes

✍ Scribed by Evgeny Pryazhnikov; Leonard Khiroug

• Publisher: John Wiley and Sons
• Year: 2007
• Tongue: English
• Weight: 375 KB
• Volume: 56
• Category: Article
• ISSN: 0894-1491
• DOI: 10.1002/glia.20590

✦ Synopsis

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 ∼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 ∼4% of release occurring during the first 30 s. The time course of the cargo release from vesicles had two peaks centered on ≤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. © 2007 Wiley‐Liss, Inc.