The influence of thrombin and platelets on fibrin clot lysis rates in vitro: A study using a clot lysis system consisting of purified human proteins

An in vitro clot lysis model system was developed in order to study the effects of several components on fibrin formation and fibrin degradation. The system consisted of the following human proteins: (125I)-fibrinogen, plasminogen, F XIII, t-PA, PAI, α2-antiplasmin and platelets. Clot formation was induced with α-thrombin in the presence of Ca2+-ions. Increasing amounts of tissue-type plasminogen activator or plasminogen induced an acceleration of clot lysis, without affecting the clotting time. Prolonged clot lysis times were observed when increasing concentrations of fibrinogen, plasminogen activator inhibitor (PAI) or α-antiplasmin (α2-AP) were added, without affecting the clotting time. F XIII (in the absence of α2-AP) and phospholipid did not influence clot lysis times or clotting times. However, in the presence of α2-AP, F XIII induced a significant prolongation of clot lysis times. The characteristics of this clot lysis system agree with what is known from previously performed fibrinolytic studies. The addition of platelets to the system resulted in a significant decrease in clot lysis rate. At higher α-thrombin concentrations the inhibitory effect of platelets was more pronounced. α-thrombin, in the absence of platelets, did not influence lysis rates. When Reptilase® was used instead of α-thrombin to induce fibrin formation, platelets had no effect on clot lysis, suggesting that the α-thrombin induced platelet release reaction is responsible for the inhibition of clot lysis. Release of PAI from platelets was found to be dependent on the α-thrombin concentration used and is proposed as the explanation for the finding that in the presence of platelets clot lysis is inhibited by α-thrombin in a dose-dependent way.
Chemicals/CAS: plasminogen activator inhibitor, 105844-41-5; thrombin, 9002-04-4; tissue plasminogen activator, 105913-11-9