Contributions of glycoprotein Ib and the seven transmembrane domain receptor to increases in platelet cytoplasmic [Ca2+] induced by alpha-thrombin.

The individual contributions of glycoprotein Ib (GPIb) and the seven transmembrane domain receptor (STDR) to increases in platelet [Ca2+]i induced by α-thrombin or the tethered ligand peptide (TLP; SFLLRNPNDKYEPF) have been determined in control platelets, in platelets where the thrombin binding site on GPIb was blocked with the monoclonal antibodies TM60 and LJ-Ib10, in platelets where access of thrombin to the STDR was blocked by polyclonal antipeptide antibodies, and in Bernard-Soulier platelets which constitutively lack GPIb. Curve-fitting analyses (LIGAND) showed that binding of PPACK-thrombin and α-thrombin to the moderate-affinity site was not detected in the best-fit model in the presence of anti-STDR antibodies although with α-thrombin there was also decreased binding at the high-affinity site. Conversely, TM60 blocked binding of α-thrombin to the high-affinity site but also decreased binding at the moderate affinity site. Separately, either TM60 or anti-TNA (150 μg/mL) reduced thrombin (0.5 nM)-...