The Activation-Induced Decrease in the Platelet Surface Expression

Thrombin decreases the platelet surface expression of the glycoprotein (GP) Ib-IX complex. To determine whether this effect is reversible, flow cytometric studies were performed with GPlb-IX-specific monoclonal antibodies. In both whole blood and washed platelet systems, incubation of platelets with thrombin or a combination of adenosine diphosphate and epinephrine resulted in a maximal decrease of the platelet surface expression of GPlb-IX within 5 minutes, after which there was a time-dependent return of the platelet surface GPlb-IX complex, which was maximal by 60 minutes. Exposure of the same platelets to additional exogenous thrombin resulted in a second decrease in platelet surface GPb-IX, followed by a second reconstitution of platelet surface GPb-IX. Throughout these experiments there was no measurable release from the platelets of glycocalicin (a proteolytic fragment of GPlb). Experiments in which platelets