Reversible translocation of glycoprotein Ib in plasmin-treated platelets: consequences for platelet function

Understanding the effect of fibrinolysis on platelet function is of clinical importance. Plasmin is recognized to affect platelet adhesive function by reducing the interaction of platelet glycoprotein (GP) Ib with von Willebrand factor (vWF) bound to the subendothelium. This platelet function is commonly explored in vitro by the ristocetin-induced agglutination test. Our previous study demonstrated a plasmin-induced redistribution of GP Ib molecules from the platelet surface to the linings of the surface-connected canalicular system (SCCS), a critical mechanism for understanding plasmin-induced GP Ib dysfunction. Here, we demonstrate that neutralization of plasmin by its inhibitors, aprotinin or tripeptide Val-Phe-Lys-CH2Cl, permits a time dependent recovery (within 30 min) of ristocetin-induced agglutination in the platelets which were stimulated by plasmin at < 1 CU ml-1. This functional recovery was accompanied with a restoration of a normal amount of GP Ib on the platelet surface, as measured by the binding of both monoclonal anti-GP Ib antibody SZ 2 and 125I-labelled vWF to the platelets. Cytochalasin D did not inhibit this recovery, suggesting that this process may be due to passive actin depolymerization. These findings were further confirmed by immunoelectron microscopic study. Utilizing the platelets pre-labelled with anti-GP Ib antibody prior to plasmin stimulation, it was demonstrated that the observed recovery is due to a reverse translocation from the SCCS to the plasma membrane of the same GP Ib molecules which were present initially at the cell surface.(ABSTRACT TRUNCATED AT 250 WORDS)