Effect of selective ADP receptor antagonists on TRAP-induced platelet aggregation and PtIns(3,4)P₂accumulation. [³²P]-labeled human platelets (7.5 × 10⁸/mL) were stimulated with 40 μmol/L TRAP in the absence or in the presence of the selective P2Y₁ antagonist A2P5P (100 μmol/L), the selective antagonist of the P2 receptor coupled to adenylyl cyclase AR-C66096 (1 μmol/L), or an antagonist of both ADP receptors ATPS (100 μmol/L). (A) Aggregation was measured as described in Materials and Methods. (B) Time course of PtdIns(3,4)P₂ accumulation. Lipids were immediately extracted and [³²P]PtdIns(3,4)P₂ was separated and quantified as in Fig 1. Results are expressed as the percentage of PtdIns(3,4)P₂ produced, with 100% being the maximal production observed upon TRAP stimulation, and are the means ± SD from 2 independent experiments.