Signal-KLGFFKR peptide increases collagen binding to resting platelets independent of platelet activation. (A) Resting human platelets were incubated with 200 μM Sig-α₂ peptide, Sig-KLGFFKR peptide, or Sig-α₂ peptide without GFFKR for 30 minutes at room temperature, fixed, washed, and incubated with FITC-collagen as in Figure 6. Data are shown as average normalized percent fluorescence over basal binding in the absence of peptide (set to zero) ± SEM from 3 to 4 independent experiments. (B) Resting platelets were preincubated with inhibitors (2 U/mL apyrase, 25 μM indomethacin, or 5 ng/mL PGI₂) or vehicle control for indomethacin (ethanol) for 5 to 10 minutes before incubation with 200 μM Sig-KLGFFKR peptide. Data are shown as in panel A, except from 2 to 4 independent experiments. P > .1 for all inhibitors compared with the no inhibitor or vehicle control. (C) Resting live murine platelets were preincubated with hamster antimouse α₂-blocking mAb or control IgG (10 μg/mL final) for 1 hour before incubation with 50 μg/mL FITC-collagen in the presence or absence of 200 μM Sig-KLGFFKR peptide for 8 to 10 minutes at room temperature. Data are shown as in panel A, except from one experiment, representative of 3 separate experiments. P < .05 for the α₂-blocking mAb compared with control IgG.