The ATP-gated P2X(1) ion channel acts as a positive regulator of platelet responses to collagen

ATP is a potent agonist of the P2X(1) ion channel, mediating a rapid, quick ly desensitized influx of Ca2+, In hirudinized PRP, containing apyrase, the two stable selective P2X(1) agonists, alpha,beta -methylene ATP, and L-bet a,gamma -methylene ATP induced extracellular Ca2+-dependent fast and revers ible platelet shape change, leading to desensitization of the PIX, ion chan nel. Preincubation with HPI-C-purified ADP potently antagonized the subsequ ent alpha,beta -methylene ATP- and L-beta,gamma -methylene ATP-evoked plate let shape change. Accordingly, upon heterologous expression of P2X(1) in Xe nopus oocytes, HPLC-purified ADP acted as an antagonist of the ATP-induced current, but was inactive itself. Since ATP and ADP are co-released from de nse granules during platelet activation, we investigated whether the P2X(1) ion channel is involved in the response of platelets to collagen. We found that platelet shape change and aggregation induced by low concentrations o f collagen were strongly inhibited after selective desensitization of P2X(1 ) with its agonists or by pretreating the platelets with a low concentratio n of ADP (0.5 muM), that antagonizes the P2X(1) channel without desensitizi ng the P2Y(1) receptor. Our data suggest that, during collagen -initiated p latelet activation, the early secretion of ATP results in the activation of the P2X(1) ion channel, which plays a role as a positive regulator of furt her platelet responses.