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

Summary 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.