Influence of thrombin in suspension, surface activation, and high shear on platelet surface GPIb/IX distribution

Studies in our laboratory have shown that glycoprotein Ib/IX (GPIb/IX), the receptor for von Willebrand factor (vWf), is not decreased in number or cleared from exposed surfaces to the internal membranes of platelets activated in suspension by thrombin alone, by interaction with formvar surfaces alone, or by a combination of the two modes of stimulation. The present study has examined the influence of three different types of stimulation including activation by thrombin in suspension followed by surface activation on formvar, then exposure to high shear stress in a flat chamber. Samples were fixed for study in the electron microscope after each single stimulus, after the combination of two modes of activation, and after the combination of suspension, surface, and shear activation and were stained by an immunogold procedure using monoclonal antibodies to localize GPIb/IX on singly, doubly, or multiply activated cells. The results demonstrate that GPIb/IX receptors remain on activated platelets from edge to edge and that there is no difference in the number or distribution of receptor complexes on thrombin-activated platelets, surface-activated cells, or platelets exposed to the combination of suspension, surface, and shear activation. The findings add additional support to the concept that GPIb/IX is not a mobile receptor and is not cleared from exposed surfaces to internal membranes under physiologic conditions.