ADP is not an agonist at P2X1 receptors: evidence for separate receptors stimulated by ATP and ADP on human platelets

ADP, an important agonist in thrombosis and haemostasis, has been reported to activate platelets via three receptors, P2X1, P2Y1 and P2TAC. Given the low potency of ADP at P2X1 receptors and recognized contamination of commercial samples of adenosine nucleotides, we have re‐examined the activation of P2X1 receptors by ADP following HPLC and enzymatic purification. Native P2X1 receptor currents in megakaryocytes were activated by α,β‐meATP (10 μM) and commercial samples of ADP (10 μM), but not by purified ADP (10–100 μM). Purified ADP (up to 1 mM) was also inactive at recombinant human P2X1 receptors expressed in Xenopus oocytes. Purification did not modify the ability of ADP to activate P2Y receptors coupled to Ca2+ mobilization in rat megakaryocytes. In human platelets, P2X1 and P2Y receptor‐mediated [Ca2+]i responses were distinguished by their different kinetics at 13°C. In 1 mM Ca2+ saline, α,β‐meATP (10 μM) and commercial ADP (40 μM) activated a rapid [Ca2+]i increase (lag time 0.5 s) through the activation of P2X1 receptors. Hexokinase treatment of ADP shifted the lag time by ∼2 s, indicating loss of the P2X1 receptor‐mediated response. A revised scheme is proposed for physiological activation of P2 receptors in human platelets. ATP stimulates P2X1 receptors, whereas ADP is a selective agonist at metabotropic (P2Y1 and P2TAC) receptors.