Human Platelet Activation Is Inhibited by the Occupancy of Glycoprotein IIb/IIIa Receptor

We studied the effect of glycoprotein GPIIb/IIIa (integrin αIIbβ3) receptor occupancy by adenosine 5′,1-thiotriphosphate (ATPαS), a competitive inhibitor of the ADP receptor, by fibrinogen, and by peptides containing the RGD (Arg-Gly-Asp) sequence as RGDW (Arg-Gly-Asp-Trp), RGDS (Arg-Gly-Asp-Ser), or the negative control RGGW (Arg-Gly-Gly-Trp) on human platelet physiological functions: aggregation, ATP secretion, and [Ca2+]in. As the presence of a nucleotide binding site on GPIIbαhas been demonstrated in platelets [N. J. Greco, N. Yamamoto, B. W. Jackson, N. N. Tandon, M. Moos, and G. A. Jamieson (1991)J. Biol. Chem.266, 13627–13633], we studied the effect of ATPαS, which specifically binds to this site, on platelet activation. We observed that ATPαS inhibited aggregation by thrombin, ADP, PMA, and ionophore A23187. Moreover, ATPαS dose dependently inhibited ATP secretion by ionophore A23187 and Ca2+transients by thrombin and vasopressin in both the presence and absence of external Ca2+. Fibrinogen, although induced by a potentiation of platelet aggregation, inhibited ATP secretion and [Ca2+]inelevation induced by low thrombin concentrations or by vasopressin, interfering with both Ca2+entry and Ca2+release by the intracellular stores. RGD peptides, which specifically bind to GPIIb/IIIa, inhibited aggregation, secretion, and Ca2+transients by thrombin, whereas the negative control RGGW did not exert any effect. We conclude that the occupancy of the GPIIb/IIIa receptor binding sites modulates platelet function by giving an inhibitory outside-in signal in platelets, particularly effective in platelets stimulated with low agonist doses. We suggest that ATPαS, fibrinogen, or RGD compounds, by interacting with GPIIb/IIIa receptor, prime some intracellular negative feedback mechanisms, which prevent further activation of circulating platelets by low-intensity stimuli and intravascular aggregation.