Platelet-collagen interaction: inhibition by ristocetin and enhancement by
von Willebrand factor-platelet binding
FM LaDuca, RE Bettigole, WR Bell and EB Robson
The contribution of von Willebrand factor (vWF)-platelet binding to
platelet-collagen interaction was examined in vitro. The binding of vWF to
platelets was mediated and regulated by ristocetin. Subthreshold
concentrations of ristocetin (less than or equal to 1 mg/mL), insufficient
to cause ristocetin-induced platelet aggregation (RIPA), were added to
platelet-rich plasma (PRP) prior to the addition of collagen. The
collagen-induced platelet aggregation (CIPA) was modified by ristocetin and
the degree of alteration was dependent on the ristocetin concentration.
Response as a function of ristocetin concentration was designated the
Collagen-Platelet Aggregation Response (CoI-PAR). In normal PRP the CoI-PAR
was a progressive inhibition followed by decreasing inhibition and then an
enhanced response. The enhanced response occurred over a narrow range of
ristocetin concentrations (0.8 to 1.0 mg/mL). In the absence of vWF (severe
von Willebrand's disease, Type I, vWF less than 1%) the CoI-PAR was a
progressive, eventually complete inhibition with no enhanced response (with
ristocetin concentrations up to 3.0 mg/mL). With addition of vWF to this
PRP an enhanced response was observed at a ristocetin concentration
inversely proportional to the vWF level. PRP from a patient with severe
Hemophilia A showed a response within the normal range. Subthreshold
ristocetin did not cause plasma protein precipitation or platelet release
of 3H-serotonin, nor induce micro platelet aggregate formation. Digestion
of platelet membrane glycoproteins (GP(s] with chymotrypsin demonstrated
that upon removal of GPI, RIPA was absent, CIPA retained and the CoI-PAR
was progressive inhibition, with no enhancement. With removal of GPs I, II,
and III, RIPA, CIPA, and the CoI-PAR were absent. A dose-response 125I-vWF-
platelet binding occurred with increasing ristocetin concentrations which
was unchanged by the addition of collagen. These results demonstrated that
ristocetin-platelet association inhibited CIPA, and vWF-platelet binding
enhanced platelet-collagen adhesion and platelet aggregation. The in
vitro-enhanced CIPA represents a vWF-dependent aggregation of sufficient
magnitude to overcome the inhibitory effect of ristocetin. These studies
demonstrate an influential interaction of ristocetin, vWF, and collagen
with the platelet membrane and imply an important hemostatic contribution
of vWF-platelet binding in platelet- collagen interaction.
Volume 68,
Issue 4,
pp. 927-937,
10/01/1986
Copyright © 1986 by The American Society of Hematology
