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

Abstract

Abstract: Astrocyte cultures prelabelled with either [3 H]‐inositol or 45 Ca 2+ were exposed to ATP and its hydrolysis products. ATP and ADP, but not AMP and adenosine, produced increases in the accumulation of intracellular 3 H‐labelled inositol phosphates (IP), efflux of 45 Ca 2+, and release of thromboxane A 2 (TXA 2). Whereas ATP‐stimulated 3 H‐IP accumulation was unaffected, its ability to promote TXA 2 release was markedly reduced by mepacrine, an inhibitor of phospholipase A 2 (PLA 2). ATP‐evoked 3 H‐IP production was also spared following treatment with the cyclooxygenase inhibitor, indomethacin. We conclude that ATP‐induced phosphoinositide (PPI) breakdown and 45 Ca 2+ mobilisation occurred in parallel with, if not preceded, the release of TXA 2. Following depletion of intracellular Ca 2+ with a brief preexposure to ATP in the absence of extracellular Ca 2+, the release of TXA 2 in response to a subsequent ATP challenge was greatly reduced when compared with control. These results suggest that mobilisation of cytosolic Ca 2+ may be the stimulus for PLA 2 activation and, thus, TXA 2 release. Stimulation of α‐adrenoceptors also caused PPI breakdown and 45 Ca 2+ efflux but not TXA 2 release. The effects of ATP and noradrenaline (NA) on 3 H‐IP accumulation were additive, but their combined ability to increase 45 Ca 2+ efflux was not. Interestingly, in the presence of NA, ATP‐stimulated TXA 2 release was reduced. Our data provide evidence that functional P 2‐purinergic receptors are present on astrocytes and that ATP is the first physiologically relevant stimulus found to initiate prostanoid release from these cells.