The Binding of Unfractionated Heparin and Low Molecular Weight Heparin to Thrombin-Activated Human Endothelial Cells

The binding of unfractionated heparin to endothelium is thought to be responsible for the rapid and saturable phase of unfractionated heparin clearance. Thrombin can induce endothelial cells to express and/or secrete a number of heparin binding proteins that have the potential to increase the binding of unfractionated heparin and to a lesser extent the binding of low molecular weight heparin. To explore this possibility, we examined the binding of unfractionated heparin and low molecular weight heparin to thrombin-activated endothelial cells. Cultured human umbilical vein endothelial cells were used to determine the binding of 125I-labeled unfractionated heparin and low molecular weight heparin to untreated and to thrombin-activated cells. After thrombin treatment, we obtained a time-dependent increase in the binding of radio-labeled unfractionated heparin. In contrast, there was much less binding of low molecular weight heparin, and a time-dependent increase was not apparent. After 30, 45, and 60 minutes of thrombin treatment, the binding of unfractionated heparin was significantly higher than that of low molecular weight heparin. The increase in binding of unfractionated heparin to thrombin-activated cells also was demonstrated using fluorescently labeled unfractionated heparin followed by fluorescence microscopy. The average fluorescence intensity of thrombin-treated cells increased by 44% when compared with resting cells. The present results indicate that thrombin can increase the binding of unfractionated heparin to human umbilical vein endothelial cells. Thus, an activated endothelium may contribute to the variability of the anticoagulant response to unfractionated heparin. In contrast, the binding of low molecular weight heparin is much less affected, which may account for its better bioavailability and longer half-life.