Reduced recycling of P2Y₁₂ receptors in platelets from a subject with a heterozygous P2Y₁₂ P341A mutation. Platelets from healthy donors (HDs) or from the subject with a heterozygous P2Y₁₂ P341A mutation (P341A) were exposed to ADP (10μM; 10 minutes) to promote receptor internalization (ADP on graph) and then apyrase (0.2 U/mL; 30 minutes) to promote receptor recycling (ADP/apyrase on graph). P2Y₁ and P2Y₁₂ surface receptor levels were subsequently measured in fixed platelets with the use of [³H]-2MeSADP (100nM) in the presence of either the P2Y₁ receptor antagonist A3P5P (1mM) or the P2Y₁₂ receptor antagonist AR-C69931MX (1μM). (A-B) Data are expressed as specific [³H]-2MeSADP binding to the P2Y₁ and P2Y₁₂ receptors, respectively (DPM). (B) #Statistical significance at P < .05 for data compared with basal HD1 control. *Statistical significance at P < .05 for data compared with respective resensitized HD1 control (Mann-Whitney U test). (C) Data are expressed as the percentage of surface receptor and represent means ± SEMs. Statistical significance at P < .05 for data compared with respective resensitized HD1 control (Mann-Whitney U test). (D) Data are expressed as specific P2Y₁₂ receptor binding (DPM) in human platelets and represent data from 6 healthy donors and the patient (P341A) in basal, ADP (10μM; 10 minutes) stimulated to promote receptor internalization, and then apyrase (0.2 U/mL; 30 minutes) to promote receptor recycling (ADP/apyrase on graph) conditions.