ABSENCE OF LIGANDS BOUND TO GLYCOPROTEIN IIB-IIIA ON THE EXPOSED SURFACE OF A THROMBUS MAY LIMIT THROMBUS GROWTH IN FLOWING BLOOD

We examined the distribution of glycoprotein IIb-IIIa (GPIIb-IIIa) and its ligands fibrinogen and von Willebrand factor (vWf) on platelets w hich had adhered under flow conditions. Immunoelectron microscopy was performed on whole mounts and frozen thin sections of adhering platele ts. GPIIb-IIIa was homogeneously distributed on dendritic platelets an d on interplatelet membranes of formed thrombi. Fibrinogen and vWf wer e predominantly associated with interplatelet membranes and membranes facing the substrate. On whole mounts, vWf appeared in clumps and line ar arrays, representing the tangled or extended forms of the multimeri c molecule. From semiquantitative analysis, it appeared that fibrinoge n and vWf were, respectively, nine- and fourfold higher on interplatel et membranes than on surface membranes facing the blood stream, while GPIIb-IIIa was evenly distributed over all platelet plasma membranes. Ligand-induced binding sites (LIBS) of GPIIb-IIIa, as measured with co nformation specific monoclonal antibodies RUU 2.41 and LIBS-1, were pr esent on the surface of adhered platelets and thrombi. A redistributio n of LIBS-positive forms of GPIIb-IIIa towards interplatelet membranes was not observed. Our data support the hypothesis that, under flow co nditions, ligands have first bound to activated GPIIb-IIIa but this bi nding is reversed on the upper surface of adhering platelets. This rel ative absence of ligands on the exposed surface of thrombi may play a role in limiting their size.