ATP‐Evoked Ca²⁺ Mobilisation and Prostanoid Release from Astrocytes: P₂‐Purinergic Receptors Linked to Phosphoinositide Hydrolysis

Abstract: Astrocyte cultures prelabelled with either [³H]‐inositol or ⁴⁵Ca²⁺ were exposed to ATP and its hydrolysis products. ATP and ADP, but not AMP and adenosine, produced increases in the accumulation of intracellular ³H‐labelled inositol phosphates (IP), efflux of ⁴⁵Ca²⁺, and release of thromboxane A₂ (TXA₂). Whereas ATP‐stimulated ³H‐IP accumulation was unaffected, its ability to promote TXA₂ release was markedly reduced by mepacrine, an inhibitor of phospholipase A₂ (PLA₂). ATP‐evoked ³H‐IP production was also spared following treatment with the cyclooxygenase inhibitor, indomethacin. We conclude that ATP‐induced phosphoinositide (PPI) breakdown and ⁴⁵Ca²⁺ mobilisation occurred in parallel with, if not preceded, the release of TXA₂. Following depletion of intracellular Ca²⁺ with a brief preexposure to ATP in the absence of extracellular Ca²⁺, the release of TXA₂ in response to a subsequent ATP challenge was greatly reduced when compared with control. These results suggest that mobilisation of cytosolic Ca²⁺ may be the stimulus for PLA₂ activation and, thus, TXA₂ release. Stimulation of α‐adrenoceptors also caused PPI breakdown and ⁴⁵Ca²⁺ efflux but not TXA₂ release. The effects of ATP and noradrenaline (NA) on ³H‐IP accumulation were additive, but their combined ability to increase ⁴⁵Ca²⁺ efflux was not. Interestingly, in the presence of NA, ATP‐stimulated TXA₂ release was reduced. Our data provide evidence that functional P₂‐purinergic receptors are present on astrocytes and that ATP is the first physiologically relevant stimulus found to initiate prostanoid release from these cells.