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Changes in the platelet membrane glycoprotein IIb.IIIa complex during platelet activation.

1985; Elsevier BV; Volume: 260; Issue: 20 Linguagem: Inglês

10.1016/s0021-9258(17)39154-8

1083-351X

Sanford J. Shattil, James A. Hoxie, Michael Cunningham, L F Brass,

Cell Adhesion Molecules Research

Platelet activation is accompanied by the appearance on the platelet surface of approximately 45,000 receptor sites for fibrinogen.The binding of fibrinogen to these receptors is required for platelet aggregation.Although it is established that the fibrinogen receptor is localized to a heterodimer complex of the membrane glycoproteins, 1% and IIIa, little is known about the changes in this complex during platelet activation that result in the expression of the receptor.In the present studies, we have developed and characterized a murine monoclonal anti-platelet antibody, designated PAC-1, that binds to activated platelets, but not to unstimulated platelets.PAC-1 is a pentameric IgM that binds to agonist-stimulated platelets with an apparent ICd of 5 nM.Binding to platelets is dependent on extracellular Ca2+ (Kc.= 0.4 PM) but is not dependent on platelet secretion.Platelets stimulated with ADP or epinephrine bind 10,000-15,000 1261-PAC-l molecules/platelet while platelets stimulated with thrombin bind 20,000-25,000 molecules/platelet.Several lines of evidence indicate that PAC-1 is specific for the glycoprotein IIb*IIIa complex.First, PAC-l binds specifically to the IIbnIIIa complex on Western blots.Second, PAC-1 does not bind to thrombasthenic platelets or to platelets preincubated with ethylene glycol bis(@-aminoethyl ether)-N,N,N',N'-tetraacetic acid at 37 "C, both of which lack the intact IIb-IIIa complex.Third, PAC-1 competitively inhibits the binding of 1261-A2Ae, an IgG monoclonal antibody that is specific for the IIb*IIIa complex.Fourth, the antibody inhibits fibrinogen-mediated platelet aggregation.These data demonstrate that PAC-1 recognizes an epitope on the IIb-IIIa complex that is located near the platelet fibrinogen receptor.Platelet activation appears to cause a Ca2+-dependent change involving the glycoprotein IIb-IIIa complex that exposes the fibrinogen receptor and, at the same time, the epitope for PAC-1.Platelet activation is an ordered sequence of events that begins with the binding of an agonist to its receptor and ends with platelet shape change, aggregation, and secretion.Platelet aggregation requires the expression of receptors for fibrinogen on the platelet surface (1-3).While nonactivated platelets are unable to bind fibrinogen, activated platelets bind approximately 45,000 fibrinogen molecules/cell.Although the mechanisms that control the expression of fibrinogen receptors are unknown, the available evidence suggests that the