Release of activation markers of HUVEC-bound platelets. To determine whether platelets were activated through the contact with apoptotic HUVEC, the supernatants were assayed for the presence of platelet-specific activation markers. After incubation of the platelets with HUVEC for 20 minutes, the supernatants were tested for β-thromboglobulin and platelet factor 4 using an ELISA assay. Apoptosis of HUVEC was induced by treatment with staurosporine for 6 hours. Results are expressed as the mean ± SD of at least three experiments. *P < .01 by Student’s t-test, compared with the release of activation markers by nonactivated platelets incubated with normal HUVEC.