SUMMARY The concentrations of plasminogen and fast-acting antiplasmin were measured in 65 normal plasmas and matched sera. Concentrations were decreased in serum by 38 % for plasminogen and 32 % for fast-acting antiplasmin. The decrease in plasminogen level was due to both adsorption of plasminogen to fibrin and reaction with antiplasmin.Wiman and Collen' proposed a new model for the mechanism of fibrinolysis which in part relies upon the specific association of plasminogen with fibrin. This feature of the model had been proposed some 20 years earlier2 but had been criticised by some workers who claimed that plasma and serum plasminogen concentiations were identical and that very little plasminogen could be extracted from ex vivo thrombi.3 4 Light was shed on this problem when it was shown that plasminogen gains an increased affinity for fibrin following the loss of an N-terminal peptide.5 It is now clear that plasminogen and plasmin possess specific fibrin binding sites6 which interact with the antifibrinolytic lysine analogues.7The lysine binding sites of plasmin are also important in the reaction between plasmin and the primary plasmin inhibitor, alpha-2-antiplasmin.8 This inhibitor exercises a most effective control over plasmin by inactivating the enzyme at such a fast rate that free plasmin has only a very brief existence in plasma. Plasmin inactivation is one of the fastest known protein-protein interactions and the speed of the reaction is determined by the lysine binding sites of plasmin.8 Since these sites are occupied when plasmin(ogen) is bound to fibrin the bound enzyme is protected from the antiplasmin.'The plasma and serum concentrations of plasminogen and antiplasmin were re-examined in the light of this new model for fibrinolysis which itself is based upon the properties of the newly discovered alpha-2-antiplasmin. Recently developed techniques for the measurement of plasminogen and antiplasmin were also applied in this study to determine whether sufficient quantities of plasminogen naturally bind to fibrin to induce subsequent clot lysis after the exposure to plasminogen activator.
An in vitro model is described which utilizes human umbilical vein endothelial cells cultured on plastic microcarrier spheres and perfused with serum-free medium. This model was used to study the acute release of von Willebrand factor following stimulation of the cells with putative agonists. Thrombin, plasmin and interleukin-1 were found to release von Willebrand factor. Adrenaline and bradykinin also stimulated release but only at high dosage. 1-desamino-8-D-arginine vasopressin (DDAVP) was inactive.
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