REARRANGEMENTS OF THE FIBRIN NETWORK AND SPATIAL-DISTRIBUTION OF FIBRINOLYTIC COMPONENTS DURING PLASMA CLOT LYSIS - STUDY WITH CONFOCAL MICROSCOPY

Binding of components of the fibrinolytic system to fibrin is importan t for the regulation of fibrinolysis. In this study, decomposition of the fibrin network and binding of plasminogen and plasminogen activato rs (PAs) to fibrin during lysis of a plasma clot were investigated wit h confocal microscopy using fluorescein-labeled preparations of fibrin ogen, plasminogen, tissue-type PA (t-PA), and two chain urokinase type PA (tcu-PA). Lysis induced by PAs present throughout the plasma clot was accompanied by a gradual loss of fibrin content of fibers and by a ccumulation of plasminogen onto the fibers. Two sequential phases coul d be distinguished: a phase of prelysis, during which the fibrin netwo rk remained immobile, and a phase of final lysis, during which fibers moved with a tendency to shrink and even eventually disappeared. The t wo phases occurred simultaneously but in different locations when lysi s was induced by PAs present in the plasma surrounding the clot. The z one of final lysis was located within a 5-8-mu m superficial layer, wh ere fibers were mobile, and surface-associated fibrin agglomerates app eared. Plasminogen accumulated in these agglomerates up to 30-fold as compared with its concentration in the outer plasma, t-PA was also hig hly concentrated in the agglomerates, and tcu-PA bound to them slightl y. The zone of prelysis, where plasminogen was moderately accumulated on the immobile fibers, was located deeper in the clot. This zone was much thinner in the case of t-PA-induced lysis than in the case of tcu -PA-induced lysis, reflecting the difference in penetration of the two PAs into the clot. We conclude that under conditions of diffusional t ransport of fibrinolytic enzymes from outside a plasma clot, extensive lysis is spatially restricted to a zone not exceeding 5-8 mu m from t he dot surface, In this zone the structure of the fibrin network under goes significant changes, and strikingly high accumulation of fibrinol ytic components takes place.