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Thrombin decreases von Willebrand factor binding to platelet glycoprotein
Ib
JN George and MM Torres
Department of Medicine, University of Texas Health Science Center, San
Antonio 78284-7880.
Thrombin is a physiological agonist that promotes platelet aggregation and
secretion. In this study we observed that thrombin can also inhibit a
function of platelets related to primary hemostasis. Platelet stimulation
by thrombin decreased the binding of von Willebrand factor (vWF) to
glycoprotein (GP) Ib and decreased ristocetin-induced agglutination, in
vitro reactions that correlate with initial platelet adhesion to the vessel
wall. Binding of the monoclonal antibody API to GP Ib was also decreased.
Cytoskeletal participation in the change of GP Ib was suggested because
pretreatment of platelets with cytochalasin to prevent actin filament
formation prevented the thrombin-induced decreases in vWF binding. API
binding, and ristocetin-induced agglutination. Measurement of GP Ib in
detergent extracts by electroimmunoassay demonstrated no loss after
thrombin stimulation. Electroimmunoassay also demonstrated that the API
epitope of GP Ib on intact thrombin-treated platelets was accessible for
complete digestion by chymotrypsin. Therefore GP Ib was neither released
from the platelet surface nor internalized by thrombin treatment. A
previously recognized effect of thrombin is its induction of receptor sites
on platelet surface GP IIb-IIIa for contact-promoting proteins, including
vWF that are involved in the platelet spreading and aggregation that follow
adhesion. Therefore the action on GP Ib may combine with the effect on GP
IIb-IIIa to shift platelet reactivity from GP Ib-vWF-mediated initial
contact with the vessel wall to GP IIb-IIIa-mediated spreading and
aggregation.
Volume 71,
Issue 5,
pp. 1253-1259,
05/01/1988
Copyright © 1988 by The American Society of Hematology
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