ATP, a partial agonist for the P2Z receptor of human lymphocytes

1 Although extracellular adenosine 5′‐triphosphate (ATP) is the natural ligand for the P2Z receptor of human lymphocytes it is less potent than 3′‐ O ‐(4‐benzoylbenzoyl)‐ATP (BzATP) in opening the associated ion channel, which conducts a range of permeants including Ba 2+ and ethidium + . We have quantified the influx of ethidium + into lymphocytes produced by BzATP, ATP, 2‐methylthio‐ATP (2MeSATP) and ATPγS, studied competition between ATP and BzATP and investigated the effects of KN‐62, a new and potent inhibitor of the P2Z receptor. 2 BzATP and ATP stimulated ethidium + influx with EC 50 values of 15.4±1.4 μ M ( n =5) and 85.6±8.8 μ M ( n =5), respectively. The maximal response to ATP was only 69.8±1.9% of that for BzATP. Hill analysis gave n H of 3.17±0.24 ( n =3) and 2.09±0.45 ( n =4) for BzATP and ATP, suggesting greater positive cooperativity for BzATP than for ATP in opening the P2Z receptor‐operated ion channel. 3 A rank order of agonist potency of BzATP>ATP=2MeSATP>ATPγS was observed for agonist‐stimulated ethidium + influx, while maximal influxes followed a rank order of BzATP>ATP>2MeSATP>ATPγS. 4 Preincubation with 30–50 μ M oxidized ATP (ox‐ATP), an irreversible P2Z inhibitor, reduced the maximal response but did not change the steepness of the Ba 2+ influx‐response curve produced by BzATP (n H 3.2 and 2.9 for 30 and 50 μ M ox‐ATP, respectively ( n =2)). 5 ATP (300–1000 μ M ) added simultaneously with 30 μ M BzATP (EC 90 ) inhibited both ethidium + and Ba 2+ fluxes to a maximum of 30–40% relative to the values observed with BzATP alone. Moreover, ATP (300 μ M ) shifted the concentration‐response curve to the right for BzATP‐stimulated Ba 2+ influx, confirming competition between ATP and BzATP. 6 KN‐62, a new and powerful inhibitor of the lymphocyte P2Z receptor, showed less potency in antagonizing BzATP‐mediated fluxes than ATP‐induced fluxes when maximal concentrations of both agonists (BzATP, 50 μ M ; ATP, 500 μ M ) were used. 7 These data suggest that the natural ligand, ATP, is a partial agonist for the P2Z receptor while BzATP is a more efficacious agonist. Moreover the competitive studies show that only a single class of P2‐receptor (P2Z class) is expressed on human leukaemic lymphocytes.British Journal of Pharmacology (1997) 122 , 911–917; doi: 10.1038/sj.bjp.0701447