FITC-collagen binding is dependent on α₂β₁ integrin and divalent cation in agonist-stimulated live human platelets. (A) Human platelets were preincubated with an α₂β₁ function-blocking mAb (BHA 2.1) or an isotype-matched control IgG (10 μg/mL final) as indicated, for 30 minutes. Platelets were simultaneously exposed to FITC-collagen and TRAP (20 μM) or control buffer. Data shown are the normalized mean fluorescence ± SEM from one experiment, representative at least 5 similar experiments. P < .0001 for TRAP compared with control and for α₂β₁ mAb BHA 2.1 compared with control IgG. (B) Human platelets resuspended in EDTA (5 mM) or control buffer were exposed simultaneously to FITC-collagen and TRAP (50 μM) or ADP (50 μM). Data shown are normalized mean fluorescence ± SEM of one experiment, representative of 3 similar experiments. P < .05 for TRAP plus EDTA compared with TRAP alone.