ACTIVATION OF P-2Z PURINOCEPTORS DIMINISHES THE MUSCARINIC CHOLINERGIC-INDUCED RELEASE OF INOSITOL 1,4,5-TRISPHOSPHATE AND STORED CALCIUM IN RAT PAROTID ACINI - ATP AS A COTRANSMITTER IN THE STIMULUS-SECRETION COUPLING

The effect of extracellular ATP on the intracellular free Ca2+ concentration ([Ca2+](i)) and inositol phosphate production following stimulation with the muscarinic cholinergic agonist acetylcholine (ACh) was investigated in isolated rat parotid acinar cells. Stimulation of rat parotid acinar cells with ATP(4-) results in a rise in [Ca2+](i) that is due to influx of extracellular Ca2+ and mobilization of Ca2+ from intracellular stores. Stimulation with purinergic agonists revealed that both influx as well as Ca2+ release from intracellular stores was mediated through activation of P-2z receptors. The Ca2+ mobilization from intracellular stores was due to production of Ins(1,4,5)P-3 and was inhibited by U73122, an inhibitor of phospholipase C-coupled processes. Under Ca2+-free conditions ATP(4-) caused a dose-dependent inhibition (IC50 = 8 mu M) of the ACh-evoked Ca2+ release. The inhibitory effect of ATP(4-) is due to activation of the P-2z purinoceptors, which results in a strong reduction in the ACh-induced inositol phosphate production. Prestimulation with 100 mu M ATP(4-) reduced the amount of Ins(1,4,5)P-3 formed after maximal ACh stimulation by 91%. In conclusion, the inhibitory effect of ATP(4-) on the ACh-mediated response is due to interactions of the activated P-2z receptor with the phospholipase C-coupled processes underlying the muscarinic cholinergic response.