Interaction of plasmin with alpha 2-macroglobulin and alpha 2-antiplasmin in the presence and absence of tranexamic acid

The role of insulin as an antithrombotic humoral factor

Insulin is well known for its essential role in carbohydrate metabolism: insulin deficiency results in the development of diabetes mellitus. It has been known for many years that people with diabetes mellitus are predisposed to develop thrombotic diseases including myocardial infarction. It was thought that the thrombus formation was the consequence of impaired carbohydrate metabolism. In recent years, it has become apparent that insulin is capable of ameliorating several pathophysiological events, leading to the inhibition and dissolution of the formed thrombus in the system. These insulin-induced events include inhibition of platelet aggregation by prompting the synthesis of NO in platelet and prostacyclin in endothelial cells. Furthermore, insulin upregulates prostacyclin receptors and downregulates alpha(2) adrenergic receptor in platelets, thereby amplifying the inhibition of platelet aggregation. Insulin also releases tissue plasminogen activator, a potent thrombolytic enzyme, from the platelet membrane which dissolves the formed thrombus leading to the resumption of normal blood circulation. In effect, insulin could be an essential tool in the control of thrombotic disorders.

The fibrinolytic system in man

The fibrinolytic system comprises a proenzyme, plasminogen, which can be activated to the active enzyme plasmin, that will degrade fibrin by different types of plasminogen activators. Inhibition of fibrinolysis may occur at the level of plasmin or at the level of the activators. Fibrinolysis in human blood seems to be regulated by specific molecular interactions between these components. In plasma, normally no systemic plasminogen activation occurs. When fibrin is formed, small amounts of plasminogen activator and plasminogen adsorb to the fibrin, and plasmin is generated in situ. The formed plasmin, which remains transiently complexed to fibrin, is only slowly inactivated by alpha 2-antiplasmin, while plasmin, which is released from digested fibrin, is rapidly and irreversibly neutralized. The fibrinolytic process, thus, seems to be triggered by and confined to fibrin. Thrombus formation may occur as the result of insufficient activation of the fibrinolytic system and (or) the presence of excess inhibitors, while excessive activation and/or deficiency of inhibitors might cause excessive plasmin formation and a bleeding tendency. Evidence obtained in animal models suggests that tissue-type plasminogen activator, obtained by recombinant DNA technology, may constitute a specific clot-selective thrombolytic agent with higher specific activity and fewer side effects than those currently in use.