Calcium mobilization and glycoprotein IIb-IIIa complex ligands in epinephrine-stimulated platelets

In the presence of extracellular Ca2+, epinephrine induces a rise in cytoplasmic Ca2+ ([Ca2+]i) that is associated with fibrinogen binding to the platelet surface, platelet aggregation, and enhancement of the thrombin-stimulated [Ca2+]i rise and protein phosphorylation. Whether the [Ca2+]i rise induced by epinephrine results from Ca2+ entry associated with fibrinogen binding to its receptor on the platelet surface, the glycoprotein (gp) IIb-IIIa complex, is unknown. To determine the importance of the occupancy of the gp IIb-IIIa receptor on platelet function after epinephrine administration, we studied the effects of two monoclonal antibodies (M-148 and 7E3) and two synthetic peptide analogues to fibrinogen (synthetic tetrapeptides Arg-Gly-Asp-Ser (RGDS) and dodecapeptide His-His-Leu-Gly-Gly-Ala-Lys-Gln-Ala-Gly-Asp-Val [gamma-(400-411)]), all of which bind to gp IIb-IIIa and inhibit fibrinogen binding and platelet aggregation on the epinephrine-induced rise in [Ca2+]i and enhancement of thrombin's phosphorylation of the 47-kDa substrate of protein kinase C (p47). None of the gp IIb-IIIa ligands significantly enhanced or inhibited the epinephrine-induced [Ca2+]i rise or its augmentation of p47 phosphorylation after thrombin administration; however, the synergistic [Ca2+]i rise that follows addition of both epinephrine and thrombin was reduced by both antibodies and both peptides. Thus ligand binding of gp IIb-IIIa does not influence the epinephrine-induced [Ca2+]i rise or its promotion of protein kinase C activation by thrombin; these events can be dissociated from the synergistic [Ca2+]i rise.