Activated human protein C prevents thrombin-induced thromboembolism in mice. Evidence that activated protein c reduces intravascular fibrin accumulation through the inhibition of additional thrombin generation.

Abstract

Activated protein C (APC) is a potent physiologic anticoagulant with profibrinolytic properties, and has been shown to prevent thrombosis in different experimental models. We investigated the effect of human APC on thrombin-induced thromboembolism in mice, a model of acute intravascular fibrin deposition leading to death within minutes. APC given intravenously (i.v.) as a bolus 2 min before thrombin challenge (1,250 U/kg) reduced mortality in a dose-dependent manner despite the lack of thrombin inhibitor activity. Significant inhibition of thrombin-induced death was observed at the dose of 0.05 mg/kg, and maximal protection was obtained with 2 mg/kg (> 85% reduction in mortality rate). Histology of lung tissue revealed that APC treatment (2 mg/kg) reduced significantly vascular occlusion rate (from 89.2 to 46.6%, P < 0.01). The protective effect of APC was due to the inhibition of endogenous thrombin formation as indicated by the fact that (a) the injection of human thrombin caused a marked decrease in the coagulation factors of the intrinsic and common pathways (but not of Factor VII), suggesting the activation of blood clotting via the contact system; (b) APC pretreatment reduced markedly prothrombin consumption; (c) the lethal effect of thrombin was almost abolished when the animals were made deficient in vitamin K-dependent factors by warfarin treatment, and could be restored only by doubling the dose of thrombin, indicating that the generation of endogenous thrombin contributes significantly to death; and (d) APC failed to protect warfarin-treated animals, in which mortality is entirely due to injected thrombin, even after protein S supplementation. Other results suggest that APC protects from thrombin-induced thromboembolism by rendering the formed fibrin more susceptible to plasmin degradation rather than by reducing fibrin formation: in thrombin-treated mice, fibrinogen consumption was not inhibited by APC; and inhibition of endogenous fibrinolysis by epsilon-aminocaproic or tranexamic acid resulted in a significant reduction of the protective effect of APC. Since APC did not enhance plasma fibrinolytic activity, as assessed by the measurement of plasminogen activator (PA) or PA inhibitor (PAI) activities, PAI-1 antigen, or 125I-fibrin degrading activity, we speculate that the inhibition of additional (endogenous) thrombin formation by APC interrupts thrombin-dependent mechanisms that make fibrin clots more resistant to lysis, so that the intravascular deposited fibrin can be removed more rapidly by the endogenous fibrinolytic system.

Mapping of the Discontinuous H-kininogen Binding Site of Plasma Prekallikrein

“…Their studies indicate that FXI apple domain A1 binds to HK via a sequence segment (41) that mirrors the corresponding HK attachment site of PPK A1 (15). In addition FXI apple A1 binds to thrombin (42), an important endogenous activator of FXI in blood coagulation (43,44). FXI domain A3 has been shown to bind to platelets (45) and heparin (46), and domain A4 to FXIIa (47).…”

Oral Antithrombotic Effects of Acylhydrazone Derivatives

“…Pulmonary thromboembolism in mice was induced by a method described previously 16) and was measured in vivo using albino mice of both sexes (20-25 g). Compounds were administered orally (100 μmol/kg = 0.1 mL/20 g) as a suspension in a mixture of 5% (in mass/vol) of arabic gum with a saline vehicle 60 min before the thrombogenic stimulus.…”

“…The doses of collagen plus norepinephrine and human thrombin used were established from dose-response curves (data not shown). The assay induced by collagen plus norepinephrine evaluated thrombotic activity that directly involves platelets, since the collagen injection mimics vascular injury 16) . I.V.…”

The long-term effects of pre-treatment with activated protein C in a rat model of compression-induced spinal cord injury

“…APC was kindly provided by Baxter (Vienna, Austria). 11 All other reagents used were of analytical grade.…”