PLATELET ACTIVATION RESULTS IN A REDISTRIBUTION OF GLYCOPROTEIN-IV (CD36)

To investigate the possibility that thrombin and/or other platelet act ivators change the platelet surface expression of glycoprotein IV (GPI V, CD36), we used a panel of five GPIV-specific monoclonal antibodies (OKM5, 5F1, FA6-152, 8A6, and F13) directed against different epitopes . AH these antibodies bound to resting platelets in a concentration-de pendent and saturable manner, as determined by flow cytometry of washe d platelets. Thrombin (1 U/mL) induced an approximately twofold increa se in the platelet surface binding of each of these monoclonal antibod ies. Immunofluorescence microscopy demonstrated an internal pool of GP IV that, after thrombin stimulation, redistributed to the platelet sur face. In a whole-blood flow-cytometric assay, alpha-thrombin and the t hromboxane A(2) analogue U46619 each resulted in an approximately twof old increase in the platelet surface binding of OKM5, whereas ADP had a more modest effect, and collagen and epinephrine had little effect. The activation-induced upregulation of the platelet OKM5 epitope occur red in vivo as demonstrated by flow cytometric analysis of whole blood emerging from a standardized skin puncture site. In summary, both in vitro and in vivo platelet activation results in increased platelet su rface expression of GPIV, as a result of a redistribution of GPIV from an internal pool.