A new approach to studies of the fibrinolytic enzyme system in man

Assay characteristics and fibrin affinity of plasminogen activators of the intrinsic fibrinolytic system

Plasminogen activators (PA) in the euglobulin fraction of dextran sulfate activated plasma (DS-EF) were assayed on fibrin plates. Activity related to tissue plasminogen activator (t-PA) or urokinase (u-PA) was quantified by antiserum inhibition. The DS-EF contained 30% t-PA, 30% u-PA and 40-50% activity unrelated to t-PA or u-PA. The latter was completely inhibited by 1.7 mumol/1 C1-inhibitor (C1INH), the two former were less sensitive. Addition of flufenamate to the DS-EF (DS-EF/Fluf) from normal and two factor XII (F XII)-deficient plasmas increased their activities to the same high level. More than 50% of the activity was unrelated to t-PA or u-PA, 30-40% was u-PA and 5-10% t-PA related. After addition of fibrinogen to DS-EF/Fluf and clotting with thrombin, the remaining solution contained only about 30% of the total activity, including less than 10% u-PA. The epsilon-aminocaproic acid inhibition pattern obtained with the DS-EF was uniform, and thus different from the biphasic pattern obtained with the low fibrin affinity PA, two-chain urokinase. Thus, both the plasma u-PA and the major unidentified PA in plasma have affinity for fibrin.

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.

Measurement of euglobulin fibrinolysis time in women with retarded fetal development. The effect of the clamping and step tests on plasminogen activator activity

The effects of two experimental stimuli, clamping of the cubital vein and exercise (step test), on the euglobulin lysis time of women with retarded fetal development (92 cases), with physiologic pregnancy (50 cases) and non-pregnant women (20 cases) were studied. Both tests resulted in a pronounced shortening of euglobulin lysis time in the non-pregnant and to a lesser extent in the physiologic pregnancy cases (statistically significant after clamping of the cubital vein, statistically insignificant after the step test). In the etiologically heterogeneous group of pregnancy with retarded fetal development the response to both stimuli was weaker in comparison with the normal pregnancy group, and in many cases (58.7%) even "paradoxical", that is, instead of a rise in fibrinolytic activity, a fall was observed. These results indicate that in women with retarded fetal development fibrinolytic activity of the blood doses not rise as readily as it does in normal pregnancies. This applies both to local response (clamping test) and systemic response (step test), and is probably due to a fall in the plasminogen activator content of the venous vessel walls. As a working hypothesis, it is presumed that insufficiency of the plasminogen activator activity system facilitates the accumulation of fibrin deposits in the uterine--placental blood circulation, and contributes to retardation of intrauterine fetal growth.

Response of fibrinolytic activity to venous occlusion

Resting fibrinolytic activity and the response of the fibrinolytic system to venous occlusion were studied in a group of healthy middle-aged men. All subjects showed increased fibrinolytic activity but of varying degrees. There was a linear relationship between resting and occluded levels of fibrinolytic activity but many subjects with lower levels of fibrinolytic activity showed an anomalous response. Responses over the expected level were more common than unexpectedly low levels of response. Fibrinogen and plasminogen concentrations were inversely correlated with fibrinolytic activity.