AbstractThe effect of adenosine triphosphate (ATP) on the intracellular Ca2+ concentration ([Ca2+]i) of cultured neurohypophysial astrocytes (pituicytes) was studied by fluorescence videomicroscopy. ATP evoked a [Ca2+]i increase, which was dose dependent in the 2.550 M range (EC50=4.3 M). The ATP-evoked [Ca2+]i rise was not modified during the first minute following the removal of external Ca2+. Application of 500 nM thapsigargin inhibited the ATP-dependent [Ca2+]i increase. Caffeine (10 mM) and ryanodine (1 M) did not affect the ATP-induced [Ca2+]i rise. The pituicytes responded to various P2 purinoceptor agonists with the following order of potency: ATP=ATP[-S]=2-MeSATPADP, where ATP[-S] is adenosine 5-O-(3-thiotriphosphate) and 2-MeSATP is 2-methylthio-adenosine-5-triphosphate. Adenosine, AMP, ,-methylene adenosine-5-triphosphate (,-MeATP), , methylene adenosine-5-triphosphate (,-MeATP) and uridine 5-triphosphate (UTP) were ineffective. The P2 purinoceptor antagonists blocked the ATP-evoked [Ca2+]i increase with the following selectivity: RB-2suraminPPADS, where RB-2 is Reactive Blue 2 and PPADS is pyridoxal-phosphate-6-azophenyl-2,4-disulphonic acid. The ATP-evoked [Ca2+]i increase was substantially blocked by pertussis toxin treatment, suggesting that it might be mediated by a pertussis-toxin-sensitive G protein. The phospholipase C (PLC) inhibitor U-73122 (0.5 M) abolished the ATP-evoked [Ca2+]i rise, whereas its inactive stereoisomer U-73343 (0.5 M) remained ineffective. Our results indicate that, in rat cultured pituicytes, ATP stimulation induces an increase in [Ca2+]i due to PLC-mediated release from intracellular stores through activation of a pertussis-toxin-sensitive, G-protein-linked P2Y receptor.