Platelet reactivity for thrombin-induced surface expression of P-selectin. (A) Representative histograms of thrombin-induced P-selectin expression in platelets from apoE^_/_ (upper panels), apoE^_/_TP^_/_ (middle panels), and apoE^_/_IP^_/_ (lower panels) mice. Platelets were either left unstimulated or were stimulated with 0.2, 0.3, or 0.4 U/ml of thrombin. They were then labeled with FITC-conjugated anti_P-selectin and were analyzed by flow cytometry. Filled histograms indicate background signal. (B) Quantification analysis. Concentration-dependent effect of thrombin for P-selectin expression was determined in platelets from apoE^_/_ (open circles), apoE^_/_TP^_/_ (filled circles), and apoE^_/_IP^_/_ (filled squares) mice. Data are means ± SEM (n = 9 each). *P < 0.05 and **P < 0.01 versus apoE^_/_ mice.