Fibrinogen receptors do not undergo spontaneous redistribution on surface-activated platelets

Studies with fibrinogen coupled to colloidal fibrinogen-gold (Fgn/Au) have suggested that fibrinogen receptors glycoprotein IIb-IIIa (GPIIb-IIIa) on human platelets may undergo spontaneous reorganization and centralization during surface activation. The present study has examined that hypothesis. Platelets were allowed to spread on grids for 20 minutes, just as in previous studies, but they were fixed before, rather than after, exposure to Fgn/Au or latex spherules. A monoclonal antibody to GPIIb-IIIa was also employed. Control experiments demonstrated that Fgn/Au and latex move toward the central zones and into channels of the open canalicular system on surface-activated platelets treated in the usual manner. However, when surface-activated platelets were fixed after spreading and before exposure to ligand, Fgn/Au particles and latex sperules were evenly dispersed over the entire cell membrane. Immunogold staining of GPIIb-IIIa also revealed edge-to-edge localization of the GPIIb-IIIa receptors on surface-activated cells. The findings are consistent with the concept that fibrinogen receptors do not undergo spontaneous reorganization on surface-activated platelets.