Separate and combined interactions of fibrinogen-gold and latex with surface-activated platelets

Glycoprotein (GP)IIb-IIIa receptors on human platelets in suspension and after surface activation bind a variety of specific antibodies and ligands and transport them across the outside of the plasma membrane by processes of clustering, patching and capping, and endocytosis to the platelet interior. The present study evaluated the separate and combined interaction of fibrinogen coupled to colloidal gold (Fgn/Au) and small latex particles with fixed and unfixed, surface-activated platelets. Results demonstrate that populations of GPIIb-IIIa and other mobile receptors on fully spread platelets are not exhausted by binding and transporting a single ligand. Addition of a first ligand followed by a second one results in concentration of the initial wave in the platelet center, surrounded by a halo of second ligand. The margin of the double-labeled cell is cleared of both ligands. However, if the double-labeled platelet is fixed briefly and exposed to a third wave of ligand, that also will bind to the cell and cover the peripheral margin. How many waves of ligand can couple to spread platelets and translocate before the ability of the plasma membrane receptors to bind and clear has been depleted remains to be determined. Yet evidence of the interaction with three waves indicates that the surface-activated platelet has a nearly inexhaustible population of mobile receptors to participate in hemostatic events.