ADP is not an agonist at P2X 1 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, P2X 1 , P2Y 1 and P2T AC . Given the low potency of ADP at P2X 1 receptors and recognized contamination of commercial samples of adenosine nucleotides, we have re‐examined the activation of P2X 1 receptors by ADP following HPLC and enzymatic purification. Native P2X 1 receptor currents in megakaryocytes were activated by α,β‐meATP (10 μ M ) and commercial samples of ADP (10 μ M ), but not by purified ADP (10–100 μ M ). Purified ADP (up to 1 m M ) was also inactive at recombinant human P2X 1 receptors expressed in Xenopus oocytes. Purification did not modify the ability of ADP to activate P2Y receptors coupled to Ca 2+ mobilization in rat megakaryocytes. In human platelets, P2X 1 and P2Y receptor‐mediated [Ca 2+ ] i responses were distinguished by their different kinetics at 13°C. In 1 m M Ca 2+ saline, α,β‐meATP (10 μ M ) and commercial ADP (40 μ M ) activated a rapid [Ca 2+ ] i increase (lag time 0.5 s) through the activation of P2X 1 receptors. Hexokinase treatment of ADP shifted the lag time by ∼2 s, indicating loss of the P2X 1 receptor‐mediated response. A revised scheme is proposed for physiological activation of P2 receptors in human platelets. ATP stimulates P2X 1 receptors, whereas ADP is a selective agonist at metabotropic (P2Y 1 and P2T AC ) receptors.British Journal of Pharmacology (2000) 131 , 108–114; doi: 10.1038/sj.bjp.0703517