Endothelium in hemostasis and thrombosis.

Endothelial cells synthesize and secrete PA and PAI, and thus provide anticoagulant and procoagulant regulatory mechanisms, respectively. Both plasminogen and plasminogen activators (t-PA and u-PA) bind to specific cellular receptors; assembly of components of the fibrinolytic system at the endothelial cell surface results in stimulation of fibrinolytic activity. Several mechanisms contribute to this stimulation, eg, enhanced plasminogen activation by t-PA or u-PA, enhanced conversion of scu-PA to tcu-PA, and impaired inhibition of plasmin by alpha2-antiplasmin or of PAs by PAIs. Thus, the endothelial cell surface serves as a focal point for plasmin generation.