Ristocetin-induced Platelet Agglutination Stimulates GPIIb/IIIa-dependent Calcium Influx

Summary

We found that intracellular Ca²⁺ concentration i[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 plate- let-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.

• References

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