Effects of alpha(2)-macroglobulin and antithrombin on thrombin generation and inhibition in cord and adult plasma

Thromboembolic complications rarely occur during infancy and childhood. It has been reported that increased capacity of cord plasma to inhibit thrombin due to elevated alpha(2)-macroglobulin (alpha(2)-M) levels may in part provide protection from thrombosis. In antithrombin (AT)-deficient plasma, alpha(2)-M exhibits anticoagulant action by complexing substantial amounts of generated free thrombin. It has been suggested that alpha(2)-M has the same impact on thrombin inhibition as AT, the most important thrombin inhibitor in adult plasma. The aim of our study was to examine this assumption by determining time-courses of free thrombin generation and prothrombin activation. Additionally, the amount of thrombin complexed to alpha(2)-M was assessed by comparing the heights of the end-level of amidolytic activity curves (AACs) after extrinsic activation of platelet poor plasma in the presence of different concentrations of AT or alpha(2)-M. Increasing the AT content by 30% resulted in significantly suppressed generation of free thrombin and prothrombin fragment 1+2 (F1+2) in cord and adult plasma. In contrast, increasing the alpha(2)-M content in plasma containing physiologic amounts of AT by the same percentage had no effect on free thrombin generation and on F1+2 generation in both cord and adult plasma. In addition, the effect of AT supplementation on the end-level of the AACs was significantly higher compared to the effect of alpha(2)-M supplementation. Since alpha(2)-M, in contrast to AT, had no effect on free thrombin generation and prothrombin activation, our study suggests that the action between alpha(2)-M and thrombin might not be fast enough to prevent thrombin from its feedback activation in both cord and adult plasma and, therefore, in cord and adult plasma containing physiological amounts of AT alterations of the alpha(2)-M content had no effect on thrombin generation and inhibition.

Alpha-2-macroglobulin inhibits the anticoagulant action of activated protein C in cord and adult plasma

Healthy newborns have a very low risk of thrombosis. It has been suggested that this is partly due to the anticoagulant effect of alpha-2-macroglobulin (a2-M). This broad-spectrum protease binding glycoprotein is physiologically elevated in newborns over adult values and has been shown to complex generated alpha-thrombin. In our present study, we point out that a2-M also acts as a procoagulant by inhibiting activated protein C (APC). In all experiments performed in cord and adult plasma the anticoagulant action of APC was diminished in a dose-dependent manner when a2-M levels were successively elevated, reflected in increased thrombin potential (TP), and enhanced at low a2-M levels, reflected in decreased TP.

Effects of the glycoprotein IIb/IIIa receptor antagonist c7E3 Fab and anticoagulants on platelet aggregation and thrombin potential under high coagulant challenge in vitro

The present study was performed to investigate the combined effects of the platelet glycoprotein IIb/IIIa receptor antagonist c7E3 Fab (abciximab) and the anticoagulants unfractionated heparin (UH), low molecular weight heparin (LMWH), and recombinant hirudin (rH) on platelet aggregation and thrombin generation under high coagulant challenge by extrinsic activation of platelet-rich plasma. Platelet aggregation and thrombin generation were assessed simultaneously in the presence of different concentrations of abciximab and anticoagulants. Increasing concentrations of abciximab resulted in a dose-dependent anti-aggregating effect with a maximum at 20 microg/ml. Doses of 5, 10, and 20 microg/ml abciximab prolonged the lag phase until the onset of platelet aggregation, but this effect was independent of the dosage used. Abciximab had no influence on the thrombin potential under our high coagulant challenge. UH, LMWH, and rH showed a dose-dependent prolongation of the lag phase until the onset of platelet aggregation and decreased the thrombin potential. Addition of anticoagulants did not contribute to further inhibition of platelet aggregation in the presence of abciximab, but the combination of abciximab and anticoagulants exhibited an additive effect on prolongation of the lag phase until the onset of platelet aggregation. Addition of abciximab to anticoagulants did not result in further decrease of the thrombin potential. Our study demonstrates the respective specific effects of abciximab and anticoagulants on platelet aggregation and thrombin potential under high coagulant challenge, and also an additive effect of abciximab and the anticoagulants UH, LMWH, and rH on the lag phase until the onset of platelet aggregation.

Effects of antithrombin and protein C on thrombin generation in newborn and adult plasma

It has been suggested that antithrombin (AT) and protein C (PC) might be less important in newborn plasma than in adult plasma for the inhibition of clotting activation. To assess the importance of AT and PC for the regulation of thrombin generation in newborn as compared with adult plasma, thrombin generation was determined in the presence of different concentrations of AT and PC. In both, newborn and adult plasmas, reduction of AT and PC resulted in an increase of thrombin generation after intrinsic activation. On the other hand, supplementation of AT and PC decreased thrombin generation in a dose-dependent manner. Clotting times were influenced by different PC concentrations, whereas clotting times were almost not affected by different amounts of AT. Our study suggests that the significance of AT and PC in the regulation of thrombin generation is the same in newborn and adult plasma. Our experimental results also support the notion that administration of AT or protein C concentrates may be beneficial in some clinical situations leading to an increased clotting activation in newborns.

Recombinant factor VIIa does not induce hypercoagulability in vitro

Recombinant factor VIIa (rVIIa) has been reported to be clinically effective and safe in haemophilic patients with inhibitor antibodies. Compared to activated prothrombin complex concentrates the risk of thrombotic complications seems to be very low after rVIIa administration. Determination of free thrombin generation has been shown to identify hypercoagulability. Therefore, free thrombin and prothrombinase activity (Xa generation) were assessed after extrinsic activation of rVIIa supplemented factor VIII and factor IX deficient plasma. Free thrombin generation was also determined after supplementation of (activated) prothrombin complex concentrates. Addition of 150 U rVIIa/ml shortened the clotting times markedly in control, factor VIII, and factor IX deficient plasma. In contrast, free thrombin and Xa generation were not different in the absence or presence of 150 U rVIIa/ml. Addition of (activated) prothrombin complex concentrates resulted in a marked increase of free thrombin generation in all investigated plasmas. Although in vitro studies cannot reflect specific clinical circumstances our results support the notion that rVIIa does not induce a hypercoagulable state as sporadically observed after administration of (activated) prothrombin complex concentrates.