Comparative metabolism and distribution of rabbit heparin cofactor II and rabbit antithrombin in rabbits

Vascular dermatan sulfate regulates the antithrombotic activity of heparin cofactor II

Heparin cofactor II (HCII)-deficient mice form occlusive thrombi more rapidly than do wild-type mice following injury to the carotid arterial endothelium. Dermatan sulfate (DS) and heparan sulfate (HS) increase the rate of inhibition of thrombin by HCII in vitro, but it is unknown whether vascular glycosaminoglycans play a role in the antithrombotic effect of HCII in vivo. In this study, we found that intravenous injection of either wild-type recombinant HCII or a variant with low affinity for HS (K173H) corrected the abnormally short thrombosis time of HCII-deficient mice, while a variant with low affinity for DS (R189H) had no effect. When HCII was incubated with frozen sections of the mouse carotid artery, it bound specifically to DS in the adventitia. HCII was undetectable in the wall of the uninjured carotid artery, but it became concentrated in the adventitia following endothelial injury. These results support the hypothesis that HCII interacts with DS in the vessel wall after disruption of the endothelium and that this interaction regulates thrombus formation in vivo.

Accelerated atherogenesis and neointima formation in heparin cofactor II deficient mice

Heparin cofactor II (HCII) is a plasma protein that inhibits thrombin when bound to dermatan sulfate or heparin. HCII-deficient mice are viable and fertile but rapidly develop thrombosis of the carotid artery after endothelial injury. We now report the effects of HCII deficiency on atherogenesis and neointima formation. HCII-null or wild-type mice, both on an apolipoprotein E-null background, were fed an atherogenic diet for 12 weeks. HCII-null mice developed plaque areas in the aortic arch approximately 64% larger than wild-type mice despite having similar plasma lipid and glucose levels. Neointima formation was induced by mechanical dilation of the common carotid artery. Thrombin activity, determined by hirudin binding or chromogenic substrate hydrolysis within 1 hour after injury, was higher in the arterial walls of HCII-null mice than in wild-type mice. After 3 weeks, the median neointimal area was 2- to 3-fold greater in HCII-null than in wild-type mice. Dermatan sulfate administered intravenously within 48 hours after injury inhibited neointima formation in wild-type mice but had no effect in HCII-null mice. Heparin did not inhibit neointima formation. We conclude that HCII deficiency promotes atherogenesis and neointima formation and that treatment with dermatan sulfate reduces neointima formation in an HCII-dependent manner.

Metabolism of rabbit plasma-derived factor VII in relation to prothrombin in rabbits

In the human circulation, factor VII is present in relatively low plasma concentration (0.01 microM) and has been reported to have a short half-life (t(1/2); 6 h). In contrast, prothrombin is present in a relatively high plasma concentration (2 microM) and has a relatively long catabolic half-life (t(1/2) = approximately 2-3 days). This report examines the metabolic characteristics of purified rabbit plasma factor VII and prothrombin, radiolabeled with (125)I and (131)I, respectively, in healthy young rabbits. From the plasma clearance curves of protein-bound radioactivities, fractional catabolic rates and compartmental distributions were calculated using a three-compartment model. Turnover of factor VII within the intravascular space (2.95 days) exceeded that of prothrombin (1.9 days). However, the whole body fractional catabolic rate of factor VII (0.34 days(-1); catabolic t(1/2) = 2.04 days) was significantly slower than that of prothrombin (0.53 days(-1); t(1/2) = 1.31 days). Furthermore, the fractional distributions of factor VII in the intravascular (0.14) and extravascular compartments (0.76) differed from those of prothrombin (0.29 and 0.53). Absolute quantities of factor VII and prothrombin catabolized by a 3-kg rabbit amounted to 0.18 and 24.0 mg/day, respectively (molar ratio of prothrombin to factor VII = 100). The molar ratio of catabolism was compared with the release rates of factor VII and prothrombin from rabbit livers perfused ex vivo. After correction for uptake of factor VII and prothrombin by the liver, the molar ratio of released prothrombin to factor VII in the perfusate was approximately 293:1 over a 0.25- to 3-h interval. These results indicate that, compared with prothrombin, factor VII in the healthy rabbit circulates as a relatively long-lived protein. This behavior does not reflect that reported for factor VII in the human circulation.