Human Platelets

A B ST R A CT The receptor for Factor VIII/von Willebrand factor (F. VIIIVwF) is not readily available on circulating platelets. We have found that the stimulation of platelets with traces of thrombin at concentrations that are generated physiologically (0.008 U0.05 U/ml) induced concentration-dependent binding of '251-labeled F. VIIIvwF in a steady-state system. The binding induced by thrombin was specific because it was inhibited by a 100-fold molar excess of unlabeled F. VIIIVWF factor, by rabbit monospecific antibody against Factor VIII, and was not inhibited by an excess of fibrinogen or fibronectin. Binding induced by thrombin required metabolically active platelets, in contrast to a system with ristocetin that also prompted binding to glutaraldehyde-treated platelets. The thrombin effects on binding of 125I-F. VIIIvwF was not observed when platelets were washed with EDTA-containing buffers; EDTA and EGTA both inhibited thrombin-induced binding. Platelet membrane glycoproteins were required because enzymatic stripping of them from the platelet surface with chymotrypsin reduced binding 2.5-5.0-fold. Prostacyclin, in the concentration range of 1 to 50 nM, had two distinct effects on the receptor for F. VIIIvwF: (a) it prevented exposure of this receptor when added 10 min before thrombin, and (b) it reversed the binding of 125I-F. VIIIVwF to the platelet receptor when added 30 min after thrombin and the ligand, i.e., when binding was at equilibrium. The dual effect of prostacyclin on the receptor for F. VIIIVwF was reproduced by dibutyryl cyclic AMP.