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, platelet membrane glycoprotein IIbIIIa (GPIIbIIIa, or integrin alpha(IIb)beta(3)) is also an adenosine triphosphate (ATP) binding site, and that the binding of ATP can directly inhibit the Fg-binding function of GPIIbIIIa, However, any direct effect of ATP on GPIIbIIIa function in intact fresh platelets is difficult to distinguish from indirect inhibitory effects via competition with ADP or elevation of platelet cyclic AMP levels. We therefore studied effects of ATP on Fg binding to activated GPIIbIIIa on the following model particles: externally activated and fixed platelets, as well as latex particles and liposomes containing functionally competent activated GPIIbIIIa receptors for Fg, These particles have 'normal', activated GPIIbIIIa in terms of: (1) binding affinity, (2) specificity to Fg, and (3) conformational change(s) after Fg binding. These particles neither require nor respond to further activation in order to bind Fg, With these model particles, we showed that ATP does not have any direct effect on the binding of Fg to platelet GPIIbIIIa and platelet aggregation. These simplified model particles are useful tools in the mechanistic study of platelet GPIIbIIIa function and the interaction between platelet GPIIbIIIa and its ligands.