The P2Z‐purinoceptor of human lymphocytes: actions of nucleotide agonists and irreversible inhibition by oxidized ATP

Abstract

1 Extracellular adenosine triphosphate (ATP) is known to open a receptor-operated ion channel (P_2Z class) in human lymphocytes which conducts a range of cationic permeants. The activity of a range of different agonists and inhibitors towards the P_2Z-purinoceptor was investigated by measuring the agonist-induced influx of Ba²⁺ into fura-2 loaded lymphocytes. 2 The most potent agonist was 2′ & 3′-0-(4-benzoylbenzoyl)-ATP (benzoylbenzoic ATP) which gave 2 fold greater maximum Ba²⁺ influx and had a 10 fold lower EC₅₀ than for ATP. The rank order of agonist potency in K⁺-media was benzoylbenzoic ATP>>ATP = 2-methylthio ATP = 2-chloro ATP>ATP-γ-S. ADP, UTP and α,β-methylene ATP were unable to stimulate Ba²⁺ influx. 3 Extracellular Na⁺ inhibited the increment of Ba²⁺ influx induced by all concentrations of ATP, 2-methylthio ATP, 2-chloroATP and ATP-γ-S. This inhibitory effect of extracellular Na⁺ is also reflected in the different EC₅₀s for benzoylbenzoic ATP (8 μm in K⁺-media, 18 μm in Na⁺-media) but the maximal 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 μm for 60 min gave total and irreversible inhibition of ATP-induced Ba²⁺ influx. 5′-p-Fluorosulphonyl benzoyladenosine (FSBA) also was an irreversible inhibitor but the maximal inhibition achieved was 90%. 5 It is concluded that the P_2Z-purinoceptor of human lymphocytes has a rank order of agonist potency which clearly distinguishes it from other P₂-receptors and that oxidized ATP is a convenient irreversible inhibitor for the P_2Z-purinoceptor.