Differential effects of P 2 ‐purinoceptor antagonists on phospholipase C‐ and adenylyl cyclase‐coupled P 2Y ‐purinoceptors

1 Stimulation of P 2Y ‐purinoceptors on turkey erythrocytes and many other cell types results in activation of phospholipase C. In contrast, we have observed recently that P 2Y ‐purinoceptors on C6 rat glioma cells are not coupled to phospholipase C., but rather, inhibit adenylyl cyclase. 2 In this study we investigated the pharmacological selectivity of the P 2 ‐purinoceptor antagonists, suramin, reactive blue 2, and pyridoxal phosphate 6‐azophenyl 2′,4′‐disulphonic acid (PPADS) for phospholipase C‐ and adenylyl cyclase‐coupled P 2Y ‐purinoceptors. 3 In C6 glioma cells, suramin and reactive blue 2 competitively antagonized the inhibitory effect of 2MeSATP on adenylyl cyclase (p K B = 5.4 ± 0.2 and 7.6 ± 0.1, respectively), whereas PPADS at concentrations up to 100 μ m had no effect. 4 In contrast, in the turkey erythrocyte preparation, PPADS at concentrations up to 30 μ m was a competitive antagonist of P 2Y ‐purinoceptor‐stimulated phospholipase C activity (p K B = 5.9 ± 0.1). Suramin and reactive blue 2 produced both a shift to the right of the concentration‐effect of 2MeSATP for the activation of phospholipase C and a significant decrease in the maximal inositol phosphate response. 5 Turkey erythrocytes also express a phospholipase C‐coupled β‐adrenoceptor. Concentrations of PPADS that competitively inhibited the P 2Y ‐purinoceptor‐mediated response had only minimal effects on the activation of phospholipase C by β‐adrenoceptors. In contrast, suramin and reactive blue 2 produced a non‐competitive inhibition, characterized by decreases in the maximal response to isoprenaline with no change in the potency of this β‐adrenoceptor agonist. 6 The differential effect of PPADS on P 2Y ‐purinoceptors of C6 glioma cells and turkey erythrocytes adds further support to the idea that different P 2Y ‐purinoceptor subtypes mediate coupling to adenylyl cyclic and phospholipase C.