The interaction of platelets with polymerizing fibrin was studied in recalcified plasma systems, utilizing rheological and biochemical techniques. The effect of platelet concentration on the viscoelastic properties (storage and loss moduli) of clots during clot structure formation was ascertained. Maximum storage modulus (a measure of clot strength) increased more than one order of magnitude when platelet-rich plasma rather than platelet-free plasma was clotted and a quantitative relationship between maximum storage modulus and platelet concentration was obtained. Results suggested that clot retraction was a major determinant of clot strength. Studies of samples with abnormal platelet function strongly supported this view, since defective platelet aggregation to ADP, collagen and epinephrine did not affect final clot strength unless accompanied by markedly abnormal clot retraction. It is proposed that rheological studies are useful in examining the interaction of platelets with fibrin and that alterations in such an interaction may be responsible for hemostatic defects associated with quantitative or qualitative platelet disorders.
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