A transduction pathway associated with receptors coupled to the inhibitory guanine nucleotide binding protein Gi that amplifies ATP-mediated arachidonic acid release.

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Abstract
ATP is copackaged and coreleased with adrenergic, serotonergic, and cholinergic neurotransmitters, suggesting a possible interaction between the signaling pathways for ATP and these coreleased neurotransmitters. Muscarinic m2 and m4, alpha 2-adrenergic, and D2-dopaminergic neurotransmitter receptors, which have in common their ability to inhibit adenylate cyclase through the inhibitory guanine nucleotide binding protein Gi, were transfected and expressed in Chinese hamster ovary (CHO) cells that contain endogenous ATP receptors coupled to the release of arachidonic acid. Normal functional coupling of m2, m4, alpha 2, and D2 receptors was demonstrated by their ability to inhibit forskolin-stimulated cAMP accumulation with dose-response activities consistent with previous reports for these Gi-coupled receptors. Stimulation of m2, m4, alpha 2, and D2 receptors resulted in an augmentation of ATP-stimulated arachidonic acid release. With the exception of the m4 receptor, none of the receptors tested was able to stimulate arachidonic acid release in the absence of ATP. Potentiation of ATP-stimulated arachidonic acid release was independent of changes in cAMP. The augmentation of ATP-stimulated arachidonic acid release and the inhibition of cAMP accumulation were both blocked by pertussis toxin, an inhibitor of Gi, but with different dose-response characteristics. Inhibition of protein kinase C with staurosporine or long-term pretreatment of the cells with the phorbol ester phorbol 12-myristate 13-acetate blocked the augmentation response. This demonstrates that Gi-coupled inhibitory receptors can amplify ATP-receptor-stimulated arachidonic acid release through a pertussis-toxin-sensitive G protein, independent of their ability to inhibit adenylate cyclase activity.