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.