Differences in efficacy between P-selectin and VWF release with PAR signaling. Cells were stimulated with indicated concentrations of agonists for 15 minutes, and medium was collected and assayed for VWF while P-selectin was measured on the cell surface. Because of the delayed time course of PAR2-AP–mediated release of VWF, medium was collected and assayed for VWF at 30 minutes. Represented are stimulation with histamine (A), thrombin (B), PAR1-AP (C), and PAR2-AP (D). Data were normalized to maximal does of histamine secondary to variability of fold induction of VWF (3- to 12-fold increase) and P-selectin (2.0- to 3.5-fold increase) observed with different HUVEC preparations. Values represent mean ± SEM of 4 to 6 independent experiments. *Statistically significant difference between release of P-selectin compared with VWF (P < .05 as determined by 2-tailed t test).