Differences between activation thresholds for platelet P-selectin glycoprotein IIb-IIIa expression and their clinical implications.

Increased platelet reactivity is a descriptor of the risk of cardiovascular events in healthy men and in patients with overt coronary artery disease. We sought to determine if differential thresholds exist for activation of platelets with respect to alpha-granule degranulation and fibrinogen binding in healthy volunteers and in patients with acute coronary syndromes. We also sought to characterize the effect of aspirin on activation. Platelet activation was assessed with flow cytometry in whole blood anticoagulated with corn trypsin inhibitor and incubated with fluorescein isothiocyanate conjugated fibrinogen (to define activation of glycoprotein IIb-IIIa), a phycoerythrin conjugated antibody to P-selectin (a marker of alpha-granule degranulation), and selected concentrations of adenosine diphosphate (ADP) or thrombin receptor agonist peptide. ADP-induced fibrinogen binding was found to be a low threshold activation event (40% of platelets bound fibrinogen in response to 0.2 microM ADP). Alpha-granule degranulation was a higher threshold event (33% of platelets expressed P-selectin in response to 1.0 microM ADP). Intra- and interindividual variability were most apparent with low concentrations of agonist (0.2 microM ADP). Patients with acute coronary syndromes (on aspirin) had significantly increased P-selectin expression in response to ADP compared with healthy subjects (on aspirin), but no difference in ADP-induced fibrinogen binding was observed. Daily ingestion of 325 mg of aspirin had no effect on either P-selectin expression or fibrinogen binding in healthy subjects. Analysis of platelet reactivity with flow cytometry characterizes activation with respect to specific components of the process and should facilitate development and optimal titration of antiplatelet therapy.