Fibrinogen internalization by ADP-stimulated blood platelets. Ultrastructural studies with fibrinogen-colloidal gold probes.

Interaction of gel filtered, ADP-stimulated human platelets with low (0.05 mg/ml) and high (1 mg/ml) fibrinogen (Fg) was examined by transmission electron microscopy. To visualize exogenous Fg in a course of its interaction with stimulated platelets, Fg coupled to 18-nm colloidal gold (Fg-Au) was employed. In the presence of either Fg or Fg-Au, rapid changes of platelets morphology indicative of resting to activated state transition, were observed. Without external ligands, stimulated platelet suspensions resemble rather control (untreated) samples. Using Fg-Au, it has been found that initial binding of gold labels to platelet surfaces is immediately followed by gold accumulation in plasmalemma pits subjected to further internalization. Serial sections proved that at 1 min, some of the labeled endocytic structures are already isolated in the platelet cytoplasm. After prolonged (20 min) incubations, different platelet subfractions have been found. Many single or loosely aggregated platelets with little or no surface labeling contained abundant stores of internal labels. In these cells, Fg-Au is localized in vacuole-like and/or granule-like structures. Some post-stimulated (discoidal) platelets are likely to release Fg-Au previously internalized. In the centers of platelet aggregates concentrated labels filled intercellular spaces and voluminous intraplatelet cavities, either open or occluded. These results indicate on different ultimate fates of exogenous Fg processed by the ADP-stimulated platelets. The data obtained suggest also that after initial binding, exogenous Fg may be implicated not only in aggregation, but in activation-related cellular responses as well.