VAMP-3^−/−platelets spread faster on Fg and had enhanced clot retraction. (A) Representative images of WT and VAMP-3^−/− platelets allowed to spread on fibrinogen (50 µg/mL in 1× PBS) for 60 minutes are shown. Images of spread platelets were taken using DIC microscopy as described in supplemental Methods. Exposure times for DIC images were 100 ms. Scale bars, 5 μm. (B) Quantification of platelet surface area from WT and VAMP-3^−/− Fg-spread platelets for indicated times. Data are representative of 2 independent experiments (mean ± SEM). (C) Quantification of static adhesion on 50 μg/mL human Fg and 5% bovine serum albumin-coated surfaces for WT and VAMP-3^−/− calcein-labeled platelets harvested from 3 different VAMP-3^−/− and WT mice. Adherent platelets were measured by fluorescence, using a SpectraMax plate reader. Data were plotted using SigmaPlot software (v13.0). (D) Representative images of thrombin-stimulated (0.05 U/mL; denoted by +) clot retraction in WT and VAMP-3^−/− platelets at increasing times. (E) Clot sizes were measured, and the percentages of clot size relative to initial suspension volume (measured at time 0 and set as 100%) were determined and plotted. Three different VAMP-3^−/− and corresponding WT littermate controls were used in this experiment. Statistical analyses were performed using the Student t test; *P ≤ .05; **P ≤ .01; ***P ≤ .001; n.s., not significant.