HUVEC or BMVEC were grown to confluence on glass coverslips and stimulated with histamine 50 μM (H), cocaine 1μg/ml, serum free media (untreated, U), or cocaine metabolites. The coverslips were then placed in a parallel-plate flow chamber and perfused with platelets at 2.5dyne/cm2. Platelet–VWF strings were visualized by phase-contrast videomicroscopy. Arrows indicate platelet-VWF strings. A) Video capture of platelet-VWF strings in a representative 40X field of histamine-stimulated HUVEC cells. Strings per 40X field were counted for HUVEC (B) and BMVEC (C). The results are expressed as means +/− SEM. * Tukey’s HSD test, as compared to untreated cells (p < 0.05). ~ Tukey’s HSD test, indicating no difference as compared to histamine treated cells (p > 0.05). D) GPIb dependence of string formation was assessed in the presence or absence of the GPIbα antibody AK2 after stimulation with no agonist (Untreated, U), cocaine, benzoylecgonine, or cocaethylene at 1μg/ml. The results are expressed as the percent maximal strings formed after histamine stimulation from a representative experiment.