ATP does not affect fibrinogen binding to platelet GPIIbIIIa in systems free of signal transduction

Recent studies have suggested that the platelet fibrinogen (Fg) receptor, p latelet membrane glycoprotein IIbIIIa (GPIIbIIIa, or integrin alpha(IIb)bet a(3)) is also an adenosine triphosphate (ATP) binding site, and that the bi nding of ATP can directly inhibit the Fg-binding function of GPIIbIIIa, How ever, any direct effect of ATP on GPIIbIIIa function in intact fresh platel ets is difficult to distinguish from indirect inhibitory effects via compet ition with ADP or elevation of platelet cyclic AMP levels. We therefore stu died effects of ATP on Fg binding to activated GPIIbIIIa on the following m odel particles: externally activated and fixed platelets, as well as latex particles and liposomes containing functionally competent activated GPIIbII Ia receptors for Fg, These particles have 'normal', activated GPIIbIIIa in terms of: (1) binding affinity, (2) specificity to Fg, and (3) conformation al change(s) after Fg binding. These particles neither require nor respond to further activation in order to bind Fg, With these model particles, we s howed that ATP does not have any direct effect on the binding of Fg to plat elet GPIIbIIIa and platelet aggregation. These simplified model particles a re useful tools in the mechanistic study of platelet GPIIbIIIa function and the interaction between platelet GPIIbIIIa and its ligands.