PPADS and suramin as antagonists at cloned P-2Y- and P-2U-purinoceptors

1 The effect of suramin and pyridoxalphosphate-6-azophenyl-2',4'-disulphonic acid (PPADS) on the stimulation of phospholipase C in 1321N1 cells transfected with the P-2U-purinoceptor (h-P-2U-1321N1 cells) or with the turkey P-2Y-purinoceptor (t-P-2Y-1321N1 cells) was investigated. 2-methylthioadenosine triphosphate (2MeSATP) was used as the agonist at t-P-2Y-1321N1 cells and uridine triphosphate (UTP) at h-P-2U-1321N1 cells. 2 Suramin caused a parallel shift to the right of the concentration-response curves for 2MeSATP in the t-P-2Y-1321N1 cells, yielding a Schild plot with a slope of 1.16+/-0.08 and a pA(2) value of 5.77+/-0.11. 3 Suramin also caused a shift to the right of concentration-response curves for UTP in the h-P-2U-1321N1 cells, and on Schild plots gave a slope different from unity (1.57+/-0.19) and an apparent pA(2) value of 4.32+/-0.13. Suramin was therefore a less potent antagonist at the P-2U-purinoceptor than the P-2Y-purinoceptor. 4 In the presence of the ectonucleotidase inhibitor, ARL 67156 (6-N,N-diethyl-beta,gamma-dibromomethylene-D-ATP) there was no significant difference in the EC(50) or shapes of curves with either cell type, and no difference in PA(2) values for suramin. 5 PPADS caused an increase in the EC(50) for 2MeSATP in the t-P-2Y-321N1 cells. The Schild plot had a slope different from unity (0.55+/-0.15) and an X-intercept corresponding to an apparent pA(2) of 5.98+/-0.65. 6 PPADS up to 30 mu M had no effect on the concentration-response curve for UTP with the h-P-2U-1321N1 cells. 7 In conclusion, suramin and PPADS show clear differences in their action at the 2 receptor types, in each case being substantially more effective as an antagonist at the P-2Y-purinoceptor than at the P-2U-purinoceptor. Ectonucleotidase breakdown had little influence on the nature of the responses at the two receptor types, or in their differential sensitivity to suramin.