title: Ristocetin-induced platelet agglutination stimulates GPIIb/IIIa-dependent calcium influx.
abstract: We found that intracellular Ca2+ concentration ([Ca2+]i) increased during ristocetin-induced agglutination of aequorin loaded platelets resuspended in plasma. Chelation of extracellular Ca2+ had no effect on platelet clumping, but delayed and greatly reduced Ca2+ increase, indicating that it derived for the most part from Ca2+ influx. Nine monoclonal antibodies (MA) against glycoprotein (GP) Ib largely prevented ristocetin-induced platelet clumping and [Ca2+]i increase, while three anti-GPIb MA with no effect on platelet clumping did not interfere with Ca2+ movement. In unstirred samples platelet agglutination was greatly reduced and [Ca2+]i increase was abolished, suggesting that close platelet-to-platelet contact, in addition to von Willebrand factor (vWF) binding to GPIb, is necessary for Ca2+ transient. Nine MA against GPIIb/IIIa, the gly-arg-gly-asp-ser (GRGDS) peptide and GPIIb/IIIa complex dissociation had no effect on platelet agglutination, but significantly reduced Ca2+ increase. Our results suggest that platelet clumping induced by vWF binding to GPIb is responsible for GPIIb-IIIa dependent Ca2+ influx.
mesh_terms: D001792:Blood Platelets / Q000502:physiology*; D002118:Calcium / Q000378:metabolism*; D015220:Calcium Channels / Q000378:metabolism; D006801:Humans; D010974:Platelet Aggregation / Q000187:drug effects*; D019039:Platelet Glycoprotein GPIIb-IIIa Complex / Q000378:metabolism*; D019038:Platelet Glycoprotein GPIb-IX Complex / Q000378:metabolism*; D012310:Ristocetin / Q000494:pharmacology*
chemical_list: D015220:Calcium Channels; D019039:Platelet Glycoprotein GPIIb-IIIa Complex; D019038:Platelet Glycoprotein GPIb-IX Complex; D012310:Ristocetin; D002118:Calcium