Human endothelial cells maintain anti-aggregatory activity for platelets during apoptosis

Despite evidence of elevated levels of tissue factor and platelet binding b y apoptotic endothelial cells, microthrombi do not appear to be associated with apoptotic endothelium and this suggests maintained anti-aggregatory ac tivity for platelets. We report that anti-aggregatory activity is maintaine d by apoptotic endothelium obtained by serum and or matrix deprivation, whi ch we propose as models for apoptotic endothelial cells released during mic rovascular remodelling and traumatic detachment respectively. Both apoptoti c and non-apoptotic endothelium had strong anti-aggregatory activity for pl atelets stimulated with either ADP or thrombin. Inhibition experiments usin g L-NAME and indomethacin indicated a role for nitric oxide and prostacycli n in this activity. Experiments with latex beads further confirmed that inh ibited platelet aggregation by endothelium was not merely a non-specific ph enomenon. These data support the idea that EC maintain active antithromboti c activity during apoptosis, consistent with maintained urokinase levels an d canalicular fragmentation reported elsewhere.