Summary
We examined the ability of unfractionated heparin to modulate the procoagulant activities
of stimulated endothelial cells (EC). Confluent human venous umbilical EC were incubated
with heparin before or after stimulation, then rinsed extensively to eliminate any
heparin in the solution. EC, stimulated for 4 h with endotoxin and interleukin 1β,
expressed tissue factor and prothrombinase activities. When EC were treated with heparin
(6 and 60 μg/ml) during the last 10 min of the stimulation period, EC-related procoagulant
activities were inhibited in a dose-dependent manner (80-90% inhibition at 60 μg/ml).
The inhibition was antithrombin-dependent and it disappeared after heparin removal
in less than 15 min at 37° C but persisted at 4° C.
When EC were treated with heparin (60 μg/ml) for 24 h then extensively washed before
stimulation, the anticoagulant effect was more modest (50% inhibition). The effect
was antithrombin-dependent. Inhibition was maximum after 18-24 h of pretreatment of
EC with heparin and was stable for at least 7 h. The cell surface displayed a “heparin-like”
activity: treatment by heparin doubled the rate of thrombin-antithrombin complex formation
and this effect was heparinase sensitive and chondroitinase ABC insensitive.
Thus, heparin modulates the procoagulant properties of stimulated EC according to
two distinct mechanisms. The first one is rapid and transient, probably related to
the presence of heparin molecules bound at the membrane surface. The second is delayed
and persistent, and our results suggest that it is mediated by an increase in the
membrane heparan sulfate molecules.
