Difference of [Ca²⁺]_i Movements in Platelets Stimulated by Thrombin and TRAP: the Involvement of α_IIbβ₃-Mediated TXA₂ Synthesis

Summary

This study investigated the difference of [Ca²⁺]_i movement in platelets in response to thrombin and TRAP. The involvement of α_IIbβ₃ in this signaling was also studied. Stimulation of platelets with thrombin at 0.03 U/ml caused platelet aggregation and a two-peak increase in [Ca²⁺]_i. The second peak of [Ca²⁺]_i, but not the first peak was abolished by the inhibition of platelet aggregation with α_IIbβ₃ antagonists or by scavenging endogenous ADP with apyrase. A cyclooxygenase inhibitor, aspirin, and a TXA₂ receptor antagonist, BM13505, also abolished the second peak of [Ca²⁺]_i but not the first peak, although these regents did not inhibit aggregation. Under the same assay conditions, measurement of TXB₂ demonstrated that α_IIbβ₃ antagonists and aspirin almost completely inhibited the production of TXB₂. In contrast to thrombin-stimulation, TRAP caused only a single peak of [Ca²⁺]_i even in the presence of platelet aggregation, and a high level of [Ca²⁺]_i increase was needed for the induction of platelet aggregation. The inhibition of aggregation with α_IIbβ₃ antagonists had no effect on [Ca²⁺]_i change and TXB₂ production induced by TRAP. Inhibition studies using anti-GPIb antibodies suggested that GPIb may be involved in the thrombin response, but not in the TRAP. Our findings suggest that low dose thrombin causes a different [Ca²⁺]_i response and TXA₂ producing signal from TRAP. Endogenous ADP release and fibrinogen binding to α_IIbβ₃ are responsible for the synthesis of TXA₂ which results in the induction of the second peak of [Ca²⁺]_i in low thrombin- but not TRAP-stimulated platelets.

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