Collagen-stimulated platelets promote FXII binding to HUVEC monolayers.

(A) Washed human platelets (0.1 × 10⁹ to 3.5 × 10⁹/mL) in HCB, pH 7.4, with no added Zn²⁺ were incubated over washed HUVEC monolayers in microtiter plate cuvette wells in the same buffer in the presence of 10 nM biotin-FXII and the presence (●) or absence (▪) of 5 μg/mL collagen for 90 minutes at 37°C on a rotating shaker. Specific biotin-FXII binding (▴) was determined by subtracting the level of binding seen with unactivated platelets from that occurring with collagen-activated platelets. At the completion of the incubation, the microtiter plate cuvette wells were washed and the amount of biotin-FXII bound to the HUVECs was determined as described in the legend to Figure 1. The single symbol ▾ represents 10 nM specific biotin-FXII binding in HCB, pH 7.4, containing 10 μM Zn²⁺in the absence of platelets. (B) The concentration of Zn²⁺was determined in the releasate of collagen-activated platelets (0.1 × 10⁹ to 3.5 × 10⁹/mL) after the level of Zn²⁺ in the suspension media of unactivated platelets was subtracted. The concentration of Zn²⁺ was determined by a colorimetric assay (see “Materials and methods”). The results presented for both panels are the means ± SEM for 5 experiments.