Ligands to the platelet fibrinogen receptor glycoprotein IIb-IIIa do not affect agonist-induced second messengers Ca2+ or cyclic AMP

Previous studies have suggested that the platelet glycoprotein complex GPIIb-IIIa, which is the putative fibrinogen receptor, regulates Ca2+ influx into platelets, possibly operating as a Ca2+ channel. We have used RGD-peptides (peptides containing the sequence Arg-Gly-Asp; disintegrins), isolated from snake venoms, that have a high affinity and specificity for the fibrinogen-binding site of GPIIb-IIIa to address the question of whether blocking this site inhibits Ca2+ movement from the extracellular medium to the cytosol. Using fura-2-loaded human platelets, we found that neither disintegrins nor a monoclonal antibody (M148) to the GPIIb-IIIa complex altered the level of cytosolic Ca2+ obtained when the cells were stimulated with various agonists in the presence of either nominal or 1 mM extracellular Ca2+. In the presence of Mn2+, an ion that quenches fura-2 fluorescence, fura-2-loaded platelets were stimulated with thrombin or ADP. Neither disintegrins nor the monoclonal antibody altered the kinetics or the amount of quenching of fura-2 fluorescence by Mn2+. These data indicate that the binding of ligands to the fibrinogen receptor is not associated with an inhibition of Ca2+ movement through a receptor-operated channel. Furthermore, the disintegrins have no effect on platelet cyclic AMP metabolism in either the presence or the absence of phosphodiesterase inhibitors.