Agonist-induced platelet activation and [³H]2MeS-ADP binding in heterozygous carriers of the P2Y₁₂ K174E mutation. (A) Platelet aggregation in response to the indicated concentrations of ADP in citrated PRP from a healthy volunteer (control) and from PII.2 (patient). (B) Aggregation of platelets in PRP from a healthy volunteer (control) and PI.1 (patient) in response to 10 μM ADP in the absence and presence of the P2Y₁₂ antagonist ARC-67085 (1 μM) as shown. (C) Maximal levels of ATP secretion in PRP from a control subject and PI.1 induced by ADP (100 μM) and a PAR-4 peptide (500 μM) as indicated. The maximal level of secretion in each sample is compared with the maximal levels of ATP secretion in PRP from control subjects studied on separate occasions. (D) P2Y₁ and P2Y₁₂ surface receptor levels were measured in fixed platelets by displacement of [³H]2MeS-ADP (100 nM) by receptor antagonists for P2Y₁ (A3P5P; 1 mM) and P2Y₁₂ (AR-C69931MX; 1 μM), respectively. Data are expressed as [³H]2MeS-ADP binding (DPM) and represent means (± SEM) of 3 independent experiments. *Statistically significant reduction in P2Y₁₂ binding levels at P < .05 for data compared with respective control data (Mann-Whitney U test).