THE ACTION OF HEPARIN IN PLASMA

Abstract

Much is known about the influence of heparin on the interaction between coagulation proteases and antithrombin III. In the complex milieu of clotting plasma and a fortiori in platelet rich plasma many proteins compete for the heparin, the proteases are partly protected from AT III (-heparin) action and several antiproteases compete for one protease. Also the concentrations of the reactant vary in the course of the clotting process. This makes it difficult to obtain a clear insight in the action of heparin in plasma from studies on purified system only. We therefore developed a method that allows us a) to measure the breakdown constant of endogenously generated thrombin in plasma and to determine what part of it is due to bindingα2 macroglobulin, b) derive the time course of prothrombinase activity from the thrombin generation curve. This method allows us to study separately the action of heparin on prothrombinase and on thrombin.Using this method we could show that: a) Classical heparin acts primarily on thrombin. Its action on other activated clotting factors is of secondary importance,b) The product activation of thrombin isespecially important in the intrinsic pathway, the thrombin dependent activation of factor VIII is the process that determines the onset of explosive thrombin formation. This explains why the APTT is much more sensitive to heparin than the PT is. c) The low molecular weight heparins that we. studied can be divided into two classes: the S type heparins, that, like unfractionated heparin, act practically only on thrombin and the P type heparins that do not act on thrombin but do inhibit prothrombinase. The synthetic pentasaccharide of Choay et al. is a typical example ofthis last group, d) We demonstrated a cooperative effect between thromboplastin and platelets that is mediated via thrombin.It is expressed as a shortening of the agphase of thrombin formation in plateletrich plasma, e) Heparin, but also other antithrombotic drugs inhibit the cooperative effect mentioned above. This suggests that this effect is a good candidate for the screening of antithrombotic drugs, f) The amount of thrombin formed in platelet rich plasma is hardly influenced by concentration of heparin that completely inhibits the formation of thrombin in platelet poor plasma. This is caused by the release of platelet factor 4. g) S type low molecular weight heparins can be conveniently standardized with a standard of classical heparin. P type low molecular weight heparins require a P type standard preparation.