Expression of P2Y₁ and P2Y₁₂ receptors in platelets from a subject with a heterozygous P2Y₁₂ P341A substitution. (A-B) P2Y₁ and P2Y₁₂ surface receptor levels were measured in fixed platelets from healthy donors (HDs) or from the subject with a heterozygous P2Y₁₂ P341A mutation (P341A) by displacement of [³H]2MeSADP (1nM to 1μM) by receptor antagonists for P2Y₁ (A3P5P; 1mM) and P2Y₁₂ (AR-C69931MX; 1μM), respectively. Data are expressed as [³H]2MeSADP binding (DPM) and represent means ± SEMs. *Statistically significant reduction in P2Y₁₂ binding levels at P < .05 for data compared with respective control data (Mann-Whitney U test). (C) Total P2Y₁₂ receptor expression was assessed by immunoblotting with a P2Y₁₂-specific Ab in cell lysates from HDs or P341A patient platelets and CHO cells stably expressing full-length P2Y₁₂ or pcNEO control. Data shown are representative for 3 independent experiments. Densitometric analysis of 3 experiments was performed and, when data were expressed as fold increase over HD values of 1.1, 0.9 and 1.1 were obtained, indicating no change.