The binding of unfractionated heparin and low molecular weigh heparin to thrombin-activated human endothelial cells

The binding of unfractionated heparin to endothelium is thought to be respo nsible for the rapid and saturable:phase of unfractionated heparin clearanc e. Thrombin can induce endothelial cells to express and/or secrete a number of heparin binding proteins that have the potential to increase the bindin g of unfractionated heparin and to a lesser extent the binding of low molec ular weight heparin. To explore this possibility, we examined the binding o f unfractionated heparin and low molecular weight heparin to thrombin-activ ated endothelial cells. Cultured human umbilical vein endothelial cells wer e used to determine the binding of I-125-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 bin ding of radiolabeled unfractionated heparin. In contrast, there was much le ss binding of low molecular weight heparin, and a time-dependent increase w as not apparent. After 30, 45, and 60 minutes of thrombin treatment, the bi nding of unfractionated heparin was significantly higher than that of low m olecular weight heparin. The increase in binding of unfractionated heparin to thrombin-activated cells also was demonstrated using fluorescently label ed unfractionated heparin followed by fluorescence microscopy. The average fluorescence intensity of thrombin-treated cells increased by 44% when comp ared with resting cells. The present results indicate that thrombin can inc rease the binding of unfractionated heparin to human umbilical vein endothe lial cells. Thus, an activated endothelium may contribute to the variabilit y of the anticoagulant response to unfractionated heparin. In contrast, the binding of low molecular weight heparin is much less affected, which may a ccount for its better bioavailability and longer half-life. (C) 1999 Elsevi er Science Ltd. All rights reserved.