Binding and internalization of soluble fibrin by thrombocytes

Binding of soluble 125I-fibrin to platelets was investigated with thrombocytes separated by gelfiltration or by sedimentation as well as in a thrombocyte concentrate. Gelfiltered platelets failed to retain 125I-fibrin within 16 hours unless they had been pretreated with factor XIIIa. In addition, a 30 kDa-component derived from the N-terminal fibronectin domain was required as a mediator. Platelets isolated by sedimentation bound some 125I-fibrin even in the absence of those cofactors. The 30 kDa-component improved binding and only this increase was sensitive to putrescine inhibition. Evidently, centrifuged platelets unlike gelfiltered ones express two pathways of fibrin binding. In a thrombocyte concentrate with platelets in their plasmatic environment 125I-fibrin was partially internalized. Engulfed radioactivity was detectable only for a limited period between 4-6 hours after substrate application suggesting that 125I-fibrin was intracellularly degraded followed by release of the fragments. The 30 kDa-component promoted internalization, while factor XIIIa improved the capacity. Thrombin inhibitors suppressed the uptake.