Downregulation of the platelet surface glycoprotein Ib-IX complex in whole blood stimulated by thrombin, adenosine diphosphate, or an in vivo wound

In washed platelet systems, thrombin has been demon-strated to downregulate the platelet surface expression of glycoprotein (GP) Ib and GPIX. In the present study, we addressed the question as to whether, in the more physio-logic milieu of whole blood, downregulation of platelet surface GPlb and GPlX can be induced by thrombin, adeno-sine diphosphate (ADP), and/or by an in vivo wound. Throm-bin-induced downregulation of GPlb and GPlX on the surface of individual platelets in whole blood was demonstrated by the use of flow cytometry, a panel of monoclonal antibodies (MoAbs) and, to inhibit fibrin polymerization, the peptide glycyl-L-prolyl-L-arginyl-L-proline. Platelets were identified in whole blood by a GPIV-specific MoAb and exclusion of monocytes by light scattering properties. Flow cytometric analysis of whole blood emerging from a standardized bleed