Clearance of Thrombin from Circulation in Rabbits by High-affinity Binding Sites on Endothelium: POSSIBLE ROLE IN THE INACTIVATION OF THROMBIN BY ANTITHROMBIN III

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Abstract

The clearance of 125I-thrombin and diisopropylphosphoryl-125I-thrombin (DIP-thrombin) from the circulation in rabbits was studied. When given either intraarterially or intravenously, DIP-thrombin, which is active-site blocked, was ∼90% cleared from the circulation by 1 min, the time of earliest sampling, indicating a large first-pass effect. DIP-thrombin given intravenously is found predominantly in the lungs, whereas DIP-thrombin injected into the aortic arch is distributed diffusely in approximate proportion to the blood supply. Renal artery, femoral artery, ear artery, left atrium, and portal vein infusions demonstrate that kidney, muscle, ear, heart, and liver, respectively, can remove DIP-thrombin from the circulation. These data imply that the clearance of DIP-thrombin is not a function of a specific organ but of the vascular bed per se. The clearance of DIP-thrombin was reversible since injection of 0.5 mg of unlabeled DIP-thrombin 10 min after the injection of a tracer dose of DIP-125I-thrombin resulted in the rapid reappearance of the DIP-125I-thrombin into the circulation. In addition, the clearance of DIP-thrombin was saturable, i.e., clearance of DIP-125I-thrombin was inhibited by unlabeled DIP-thrombin in a dose-dependent fashion. In vivo Scatchard analysis of the saturation of the clearance process demonstrated that DIP-thrombin can be removed by binding to high-affinity binding sites, since dissociation constants (KD) of 10 and 13 nM were obtained for human and bovine DIP-thrombin, respectively.

Authors

Pete Lollar, Whyte G. Owen