Differential effects of P2‐purinoceptor antagonists on phospholipase C‐ and adenylyl cyclase‐coupled P2Y‐purinoceptors

Abstract

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 P₂-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 (pK_B = 5.4 ± 0.2 and 7.6 ± 0.1, respectively), whereas PPADS at concentrations up to 100 μm had no effect. 4 In contrast, in the turkey erythrocyte preparation, PPADS at concentrations up to 30 μm was a competitive antagonist of P_2Y-purinoceptor-stimulated phospholipase C activity (pK_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.