Comparative behavior of thrombin and an inactive derivative, FPR-thrombin, toward the rabbit vascular endothelium. Heparin liberates FPR-thrombin from the endothelium in vivo.
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Thrombin rapidly binds to and saturates rabbit aorta endothelium in vitro, a process that depends on pericellular glycosaminoglycans and that is inhibited by heparin. To characterize the initial adsorption of thrombin to the endothelium in vivo, an enzymatically inactive derivative, FPR-thrombin (i.e., thrombin inactivated by D-Phe-Pro-Arg-chloromethyl ketone), was prepared. The binding characteristics of thrombin and FPR-thrombin to heparin-Sepharose and to the endothelial surface of rabbit aorta segments in vitro were compared. From these experiments, we concluded that FPR-thrombin mimicked, qualitatively, the binding of thrombin to the endothelium. When injected intravenously, 125I-FPR-thrombin was removed rapidly from the rabbit circulation (T1/2, approximately 1.4 minutes) and simultaneously was adsorbed by the vascular endothelium, particularly in the lung. By injecting heparin (1,000 units/kg i.v.) before 125I-FPR-thrombin, adsorption by the aorta endothelium at 30 minutes after injection was reduced by 90%, and T1/2 was increased to approximately 3.4 minutes. Heparin, administered at various times after 125I-FPR-thrombin, liberated a significant proportion of 125I-FPR-thrombin from the endothelial surface into the plasma compartment as shown by a pronounced "spike" on the plasma curve, a concomitant loss of radioactivity from the lung and from the aorta endothelium, and analysis of the radioactive components of plasma taken before and after heparin injection. Thus, FPR-thrombin was cleared rapidly from the circulation, and endothelium-bound FPR-thrombin was released into the circulation by heparin.
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