Abstract
Platelet membrane glycoprotein Ib (GPIb), a receptor for von Willebrand factor and thrombin, is present on the platelet surface membrane, in intraplatelet stores, and in plasma (as the proteolytic fragment glycocalicin). We examined the hypothesis that after plasmin-mediated cleavage of platelet surface GPIb, platelets can replenish their surface GPIb pool. Incubation of washed platelets with plasmin (1 hour, 22 degrees C) resulted in loss of platelet surface GPIb, but further incubation (3 hours, 37 degrees C) in autologous plasma resulted in restoration of platelet surface GPIb, as determined by ristocetin- induced platelet agglutination and a flow cytometric assay of platelet binding of three GPIb-specific monoclonal antibodies. Despite the restoration of platelet surface GPIb after the 3-hour incubation of plasmin-treated platelets in autologous plasma, the whole platelet GPIb content (measured by enzyme-linked immunosorbent assay [ELISA], sodium dodecyl sulfate-polyacrylamide gel electrophoresis, and flow cytometry) remained reduced, quantitatively corresponding to an increase in plasma glycocalicin concentration (measured by ELISA). The loss and restoration of platelet surface GPIb occurred on all platelets and, as evidenced by lack of inhibition by prostaglandin E1, EDTA, and cytochalasins, was not mediated by cyclic AMP, extracellular Ca2+, or the platelet microfilament system. In summary, this study shows that after plasmin-mediated cleavage of platelet surface GPIb, platelets can replenish their surface GPIb pool by recruitment of GPIb molecules from the intraplatelet pool (or from a sequestered surface site).
