Stimulated platelets do not release substances that activate FXII. (A) Citrated human platelet-rich plasma was either activated with platelet activator Trap6 (30 μM) or by kaolin (5 μg/mL) or treated with platelet inhibitors: aspirin (100 μM), prostaglandin I2 (1 μM), and abciximab (10 μg/mL) (Control). Platelets were removed from platelet-rich plasma by centrifugation at 2500g for 10 minutes, and the spontaneous clotting time after recalcification was measured (n = 3). (B) FXIIa generation in activated human platelet-rich plasma was measured by cleavage of a fluorogenic FXIIa substrate (n = 4). The platelets were either resting (control) or activated with Trap6 (30 µM), PAR4-AP (300 µM), or CRP (2.5 μg/mL). Glass beads in platelet-rich plasma were used as positive control. FXIIa activity was inhibited by addition of CTI (100 µg/mL). Measurements represent the average slope (fluorescence, arbitrary units/min) during 30 minutes and are presented as mean ± SD. Statistical testing was performed with ANOVA and Tukey’s post hoc test.
