Induction of GPIb/IX-vWF receptor-ligand translocation on surface-activated platelets.

Multimers of von Willebrand factor (vWF) readily bind to glycoprotein (GP) Ib/IX receptors on spread human platelets and cover the cell from edge to edge. Addition of anti-vWF antibody to spread platelets covered with vWF caused the multimers to move from peripheral margins into caps over platelet centers. Despite almost complete centralization of receptor-ligand complexes, a significant number of GPIb/IX receptors capable of binding multimers remained available on the peripheral zone. Fixation followed by a second incubation with vWF, anti-vWF, and staph protein A coupled to 5-nm gold particles (PAG5) revealed multimers extending from the centrally concentrated cap of vWF to cell margins. If spread platelets with central caps of vWF were exposed a second time to multimers and anti-vWF antibody before fixation and stained with PAG5 after, the residual GPIb/IX receptors and second wave of vWF formed a ring around the cap, leaving a clear margin. If after fixation and staining with PAG5 the grids with caps and rings of vWF were washed, exposed a third time to vWF, refixed, and then incubated with anti-vWF and PAG10, the clear margin was covered with multimers of vWF forming a second ring around the first circle of receptor-ligand complexes. Thin sections of spread platelets with central caps of GPIb/IX-vWF complexes revealed only rare examples of uptake by the open canalicular system. The interaction of GPIb/IX with vWF multimers observed in the present study suggests a mechanism by which platelets under high shear forces may adhere and attach firmly to a denuded vascular surface.