Integrin α₂β₁-dependent activation of integrin α_IIbβ₃. Aspirin-treated human platelets were allowed to adhere to monomeric collagen through integrin α₂β₁, and specific binding of biotinylated fibrinogen to adherent platelets was measured as described in “Materials and methods.” When indicated, platelets were pretreated with 1 mM RGDS, 3 U/mL apyrase, 10 μM Ro31-8220, 30 μM BAPTA-AM, or 10 μM U73122, alone or in combination, before incubation with immobilized monomeric collagen. Specific binding of fibrinogen has been calculated for the same number of adherent platelets untreated or treated with the different inhibitors. Data are reported considering as 100% the specific fibrinogen binding to control platelets (none) and represent the means ± SD of 3 different experiments performed in duplicate.