Receptor-ligand complexes are cleared to the open canalicular system of surface-activated platelets

Human platelets were incubated with gold particles coupled to fibrinogen to label the glycoprotein IIb-IIIa (GPIIb-IIIa) receptor after initial activation of the cells by contact with formvar-coated grid and glass surfaces. Fibrinogen-gold (Fgn-Au) markers were absent on discoid platelets, but diffusely spread over the surface and extended pseudopods of early dendritic cells. Conversion to spread platelets resulted in movement of ligand-receptor complexes away from the cell margin toward cell centres. However, Fgn-Au gold did not concentrate in the central region. Rather, the Fgn-Au, GPIIb-IIIa complexes in the middle of spread platelets appeared to move toward a belt-like, intermediate zone, as did the ligand receptor complexes from the cell margin and pseudopods. The ultimate destination of the mobile receptor-ligand complexes, however, appeared to be channels of the surface-connected open canalicular system (OCS). Fgn-Au was concentrated in OCS channels of most dendritic and a small proportion of spread platelets. The decreased frequency of Fgn-Au filled channels in more transformed platelets may have been due to collapse or evagination of the OCS. Examination of platelets exposed to Fgn-Au after spreading on glass and then prepared for thin sections confirmed that the OCS was the final destination for mobile ligand receptor complexes on surface-activated platelets. Findings of this study are consistent with previous work showing clearance of mobile receptor-ligand complexes to the OCS of platelets activated in suspension.