Influence of thrombin in suspension, surface activation, and high shear onplatelet 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 cl eared from exposed surfaces to the internal membranes of platelets activate d in suspension by thrombin alone, by interaction with formvar surfaces alo ne, or by a combination of the two modes of stimulation. The present study has examined the influence of three different types of stimulation includin g activation by thrombin in suspension followed by surface activation on fo rmvar, then exposure to high shear stress in a flat chamber. Samples were f ixed for study in the electron microscope after each single stimulus, after the combination of two modes of activation, and after the combination of s uspension, 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 recepto rs remain on activated platelets from edge to edge and that there is no dif ference in the number or distribution of receptor complexes on thrombin-act ivated platelets, surface-activated cells, or platelets exposed to the comb ination of suspension, surface, and shear activation. The findings add addi tional support to the concept that GPIb/IX is not a mobile receptor and is not cleared from exposed surfaces to internal membranes under physiologic c onditions.