Fig. 4.

High-affinity competitive antagonism of the human P2Y₁₄-R by PPTN. (A) Concentration effect curves for UDP-glucose (UDPG) for inhibition of forskolin (30 μM)–stimulated cyclic AMP accumulation were generated in P2Y₁₄-C6 glioma cells in the presence of the indicated concentrations of PPTN. The data are the mean of triplicate determinations, and the results are similar to those obtained in four separate experiments. Typical values for basal and forskolin-stimulated [³H]cyclic AMP accumulation were approximately 1000 and 9000 cpm, respectively. (B) A Schild plot was generated by determining the dose ratio (concentration of agonist necessary to produce a chosen level of effect in the presence of the indicated concentration of PPTN divided by the concentration of agonist that produces the same effect in the absence of antagonist) for each concentration of antagonist and then plotting log (dose ratio -1) on the Y-axis versus concentration of antagonist presented on the X-axis. The K_B determined from the intercept of the regressed line on the X- axis was 1150 pM, and a slope of 0.91 was observed. The results are similar to those obtained in four separate experiments.