THE EFFECT OF THROMBIN ON THE DYNAMIC EXCHANGE BETWEEN INTRAPLATELET AND EXTRAPLATELET FIBRINOGEN

We examined the distribution of platelet fibrinogen and the exchange b etween intra-and extra-platelet fibrinogen in unstimulated and thrombi n-stimulated platelets. In unstimulated platelets 60% of platelet fibr inogen was found in the soluble platelet fraction and 40% in the insol uble one. In platelets activated with thrombin, changes took place in the distribution of intraplatelet fibrinogen but not in the total fibr inogen content. At greater than or equal to 0.5 U/ml of thrombin the f ibrin(ogen) content of the insoluble and soluble fractions was approxi mately 80% and 20%, respectively When we evaluated how extraplatelet f ibrinogen affects the content and distribution of intraplatelet fibrin ogen, we found that when unlabelled fibrinogen was added to unstimulat ed and thrombin-stimulated platelets the content and distribution of i ntraplatelet fibrinogen remained unaltered. However, when I-125-fibrin ogen was added, it was incorporated into unstimulated and thrombin-sti mulated platelets, In unstimulated platelets, 70% of the incorporated I-125-fibrinogen was in the soluble fraction and 30% in the insoluble, In thrombin-stimulated platelets the distribution of the incorporated I-125-fibrinogen was 62% and 39% in soluble and insoluble fractions r espectively, MoAb to GPIIb-IIIa produced 80% and 60% inhibition of I-1 25-fibrinogen incorporation by unstimulated and thrombin-stimulated pl atelets. Our data showed dynamic exchange between intraplatelet and ex traplatelet fibrinogen both in unstimulated and thrombin-stimulated pl atelets mediated mainly by GPIIb-IIIa.