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von Willebrand factor bound to glycoprotein Ib is cleared from the platelet
surface after platelet activation by thrombin
P Hourdille, HR Gralnick, E Heilmann, A Derlon, AM Ferrer, G Vezon and AT Nurden
URA 1464 CNRS, Universite de Bordeaux II, Hopital Cardiologique, Pessac,
France.
We recently reported that after activation of human platelets by thrombin,
glycoprotein (GP) Ib-IX complexes are translocated to the surface-connected
canalicular system (SCCS) (Blood 76:1503, 1990). As GPIb is a major
receptor for von Willebrand factor (vWF) in platelet adhesion, we have now
examined the consequences of thrombin activation on the organization of vWF
bound to GPIb on the platelet surface. Studies were performed using
monoclonal or polyclonal antibodies in either immunogold staining and
electron microscopy (Au-EM) or in flow cytometry. When unstirred
platelet-rich plasma was incubated with ristocetin, bound vWF was located
by Au-EM as discrete masses regularly distributed over the cell surface.
Platelets from a patient with Glanzmann's thrombasthenia, lacking
GPIIb-IIIa complexes, gave a similar pattern, confirming that this
represented binding to GPIb. That ristocetin was not precipitating vWF
before their binding to the platelets was shown by the detection of similar
masses on the surface of platelets of a patient with type IIB von
Willebrand disease. Experiments were continued using washed normal
platelets incubated in Tyrode-EDTA, the purpose of the EDTA being to limit
the surface expression of endogenous vWF after platelet stimulation. Under
these conditions, platelets were treated with ristocetin for 5 minutes at
37 degrees C in the presence of increasing amounts of purified vWF. This
was followed by incubation with thrombin (0.5 U/mL) for periods of up to 10
minutes. Flow cytometry showed a time-dependent loss in the surface
expression of vWF bound to GPIb and these changes were confirmed by Au-EM.
In particular, immunogold staining performed on ultrathin sections showed
that the bulk of the vWF was being cleared to internal membrane systems.
Surface clearance of vWF during thrombin- induced platelet activation is a
potential mechanism for regulating platelet adhesivity.
Volume 79,
Issue 8,
pp. 2011-2021,
04/15/1992
Copyright © 1992 by The American Society of Hematology
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