Summary

ATP is a potent agonist of the P2X1 ion channel, mediating a rapid,quickly desensitized influx of Ca²⁺. In hirudinized PRP, containingapyrase, the two stable selective P2X₁ agonists, α,β-methylene ATP,and L-β,γ-methylene ATP induced extracellular Ca²⁺-dependent fastand reversible platelet shape change, leading to desensitization of theP2X₁ ion channel. Preincubation with HPLC-purified ADP potentlyantagonized the subsequent α,β-methylene ATP- and L-β,γ-methyleneATP-evoked platelet shape change. Accordingly, upon heterologousexpression of P2X₁ in Xenopus oocytes, HPLC-purified ADP acted asan antagonist of the ATP-induced current, but was inactive itself. SinceATP and ADP are co-released from dense granules during plateletactivation, we investigated whether the P2X₁ ion channel is involved inthe response of platelets to collagen. We found that platelet shapechange and aggregation induced by low concentrations of collagenwere strongly inhibited after selective desensitization of P2X₁ with itsagonists or by pretreating the platelets with a low concentration of ADP(0.5 µM), that antagonizes the P2X₁ channel without desensitizing theP2Y₁ receptor. Our data suggest that, during collagen-initiated plateletactivation, the early secretion of ATP results in the activation of theP2X₁ ion channel, which plays a role as a positive regulator of furtherplatelet responses.