Fate of the GPIb/IX receptor complex following activation of human platelets.

Many excellent workers have concluded that platelet glycoprotein Ib/IX complex, the receptor for von Willebrand factor (vWF) is down-regulated in its ability to bind vWF or monoclonal antibodies and virtually cleared from the exterior surface to internal membranes following exposure of the cells to thrombin in suspension. Studies carried out in our laboratory, however, have not supported the concept of down-regulation or clearance. Experiments employing aggregation-disaggregation-reaggregation, agglutination and flow cytometry have demonstrated that physiologic levels of calcium ions prevent or reverse the phenomenon of down-regulation. GPIb/IX receptors remain edge to edge on surface-activated platelets, whether or not the cells are also exposed to thrombin before or after spreading. Addition of two ligands, fibrinogen labeled gold to mark GPIIb/IIIa and vWF to identify GPIb/IX, to discoid platelets before surface activation demonstrates that GPIIb/IIIa is a mobile receptor that concentrates with its ligand in cell centers while vWF bound to GPIb/IX moves with the membrane as it expands to the lateral edges of spread cell margins. Immunogold studies of thrombin-activated platelets in suspension reveal no clearance of GPIb/IX receptors, bound or not to vWF, from exposed surfaces to internal membranes. The clearance to the OCS reported by others may be due to difficulties encountered in staining GPIb/IX lining OCS channels in resting discoid platelets.