Biochemical and biophysical conditions for blood clot lysis

We have studied how pharmacological dissolution of blood clots was affected by clot retraction, the mode of transport of fibrinolytic agents into the clot and the thickness of the composite fibrin fibers. Retracted clots were resistant to fibrinolysis in a milieu without dissolved plasminogen, becau se the amount of fibrin-bound plasminogen in retracted clots was insufficie nt for successful clot lysis. In plasma containing plasminogen, retracted c lots were successfully lysed with fibrin-specific plasminogen activators, b ut not with non-fibrin-specific activators. Preincubation of retracted clot s in plasma increased their plasminogen content as well as their sensitivit y to fibrinolysis. The rate of lysis was increased up to 100-times when pla sminogen activator and plasminogen were introduced into cylindrical clots b y pressure-induced bulk flow in comparison with diffusion alone. The magnit ude of the increase was similar in retracted and nonretracted clots, but th e absolute rate of lysis was faster in non-retracted clots. The influence o f fibrin fiber thickness on fibrinolysis was studied by atomic force micros copy. The time to complete lateral section of fibers did not differ between thick and thin composite fibers, and the rate of diameter reduction was fa ster in thick fibers than in thin ones. Taken together our results suggest that lysis of retracted clots proceeds in circular stages: (a) activation o f bound plasminogen followed by partial degradation of fibrin, (b) opening of new plasminogen-binding sites on partly degraded fibrin, (c) binding of plasminogen to the new binding sites which enhances the susceptibility of c lots to lysis. Lysis is accelerated by bulk flow of plasminogen activator a nd plasminogen into clots in comparison to diffusion alone. Fibrinolysis of thick composite fibrin fibers proceeds more efficiently than lysis of thin fibers.