THE P-2Z-PURINOCEPTOR OF HUMAN-LYMPHOCYTES - ACTIONS OF NUCLEOTIDE AGONISTS AND IRREVERSIBLE INHIBITION BY OXIDIZED ATP

1 Extracellular adenosine triphosphate (ATP) is known to open a recept or-operated ion channel (P-2Z class) in human lymphocytes which conduc ts a range of cationic permeants. The activity of a range of different agonists and inhibitors towards the P-2Z-purinoceptor was investigate d by measuring the agonist-induced influx of Ba2+ into fura-2 loaded l ymphocytes. 2 The most potent agonist was 2' and 3'-0-(4-benzoylbenzoy l)-ATP (benzoylbenzoic ATP) which gave 2 fold greater maximum Ba2+ inf lux and had a 10 fold lower EC(50) than for ATP. The rank order of ago nist potency in K+-media was benzoylbenzoic ATP >> ATP = 2-methylthio ATP = 2-chloro ATP > ATP-gamma-S. ADP, UTP and alpha,beta-methylene AT P were unable to stimulate Ba2+ influx. 3 Extracellular Na+ inhibited the increment of Ba2+ influx induced by all concentrations of ATP, 2-m ethylthio ATP, 2-chloroATP and ATP-gamma-S, This inhibitory effect of extracellular Na+ is also reflected in the different EC(50)s for benzo ylbenzoic ATP (8 mu M in K+-media, 18 mu M in Na+-media) but the maxim al response to this agonist was the same in the presence or absence of Na+. 4 Treatment of lymphocytes with 2,3 dialdehyde ATP (oxidized ATP ) at 300 mu M for 60 min gave total and irreversible inhibition of ATP -induced Ba2+ influx. 5'-p-Fluorosulphonyl benzoyiadenosine (FSBA) als o was an irreversible inhibitor but the maximal inhibition achieved wa s 90%. 5 It is concluded that the P-2Z-purinoceptor of human lymphocyt es has a rank order of agonist potency which clearly distinguishes it from other P-2-receptors and that oxidized ATP is a convenient irrever sible inhibitor for the P-2Z-purinoceptor.