Spatial and temporal aspects of Ca2+ signaling mediated by P2Y receptors in cultured rat hippocampal astrocytes

ATP produces a variety of Ca2+ responses in astrocytes. To address the complex spatio-temporal Ca2+ signals, we analyzed the ATP-evoked increase in intracellular Ca2+. concentration ([Ca2+]i)in cultured rat hippocampal astrocytes using fura-2 or fluo-3 based Ca2+ imaging techniques. ATP at less than 10 nM produced elementary Ca2+ a release event "puffs" in a manner independent of extracellular Ca2+. Stimulation with higher ATP concentrations (3 or 10 muM) resulted in global Ca2+ responses such as intercellular Ca2+ wave. These Ca2+ responses were mainly mediated by metabotropic P2Y receptors.. ATP acting on both P2Y1 and P2Y2 receptors produced a transient Ca2+ release by inositol 1,4,5-trisphosphate (InsP(3)). When cells were stimulated with ATP much longer, the transient [Ca2+]i elevation was followed by sustained Ca2+ entry from the extracellular space. This sustained rise in [Ca2+]i was inhibited by Zn2+ (< 10 muM), an inhibitor of capacitative Ca2+ entry (CCE). CCE induced by cyclopiazonic acid or thapsigargin and Ca2+ entry evoked by ATP share the same pharmacological profile in astrocytes. Taken together, the hierarchical Ca2+ responses to ATP were observed in hippocampal. astrocytes, i.e., puffs, global Ca2+ release by InsP(3), and CCE in response to depletion of InsP(3)-sensitive Ca2+ stores. It should be noted that these Ca2+ signals and their modulation by Zn2+ could occur in the hippocampus in situ since both ATP and Zn2+ are rich in the hippocampus and could be released by excitatory stimulation. (C) 2002 Elsevier Science Inc. All rights reserved.