Comparison of thrombin binding in cells expressing the full GP Ib-IX-V complex versus cells expressing only GP Ib-IX. (A) Cells were incubated with thrombin at a concentration of 1 nmol/L, and bound thrombin was detected by flow cytometry after labeling with the FITC-conjugated monoclonal antibody EST-7. Shown is a representative flow cytometry histogram for comparison of thrombin binding at 1 nmol/L in L2H cells (expressing GP Ib-IX), L2H/V cells (expressing the full GP Ib-IX-V complex), and control untransfected parental L cells. (B) Mean fluorescence intensity of the whole cell population from 5 experiments like the 1 depicted in A. *Differences in thrombin binding between L2H/V cells and the other 2 cell lines were statistically significant (Student's t test: t = 10.795, n = 7, P < .001). (C) Thrombin binding to L2H/V cells was evaluated at thrombin concentrations of 0.25 to 8 nmol/L. Specific binding was determined by subtracting the amount of thrombin bound to parental L cells (nonspecific binding) from that bound to L2H/V cells (total binding).