Characterization of the Ca 2+ responses evoked by ATP and other nucleotides in mammalian brain astrocytes

1 This study was aimed at characterizing ATP‐induced rises in cytosolic free calcium ion, [Ca 2+ ] i , in a population of rat striatal astrocytes loaded with the fluorescent Ca 2+ probe Fura2, by means of fluorescence spectrometry. 2 ATP triggered a fast and transient elevation of [Ca 2+ ] i in a concentration‐dependent manner. The responses of the purine analogues 2‐methylthio‐ATP (2‐meSATP), adenosine‐5′‐O‐(2‐thiodiphosphate) (ADPβS), as well as uridine‐5′‐triphosphate (UTP) resembled that of ATP, while α,β‐methylene‐ATP (α,β‐meATP) and β,γ‐methylene‐ATP (β,γ‐meATP) were totally ineffective. 3 Suramin (50 μ M ) had only a minor effect on the ATP response, whereas pyridoxal phosphate‐6‐azophenyl‐2′,4′‐disulphonic acid (PPADS) (5 μ M ) significantly depressed the maximum response. 4 Extracellular Ca 2+ did not contribute to the observed [Ca 2+ ] i rise: removing calcium from the extracellular medium (with 1 m M EGTA) or blocking its influx by means of either Ni 2+ (1 m M ) or Mn 2+ (1 m M ) did not modify the nucleotide responses. 5 Furthermore, after preincubation with 10 μ M thapsigargin, the nucleotide‐evoked [Ca 2+ ] i increments were completely abolished. In contrast, 10 m M caffeine did not affect the responses, suggesting that thapsigargin‐, but not caffeine/ryanodine‐sensitive stores are involved. 6 Both application of the G‐protein blocker guanosine‐5′‐O‐(2‐thiodiphosphate) (GDPβS) (1 m M ) and preincubation with pertussis toxin (PTx) (350 ng ml −1 ) partially inhibited the nucleotide‐mediated responses. Moreover, the phospholipase C (PLC) inhibitor U‐73122, but not its inactive stereoisomer U‐73343 (5 μ M ), significantly reduced the ATP‐evoked [Ca 2+ ] i rise. 7 In conclusion, our results suggest that, in rat striatal astrocytes, ATP‐elicited elevation of [Ca 2+ ] i is due solely to release from intracellular stores and is mediated by a G‐protein‐linked P2Y receptor, partially sensitive to PTx and coupled to PLC.British Journal of Pharmacology (1997) 121 , 1700–1706; doi: 10.1038/sj.bjp.0701293