Dual regulation of arachidonic acid release by P_2U purinergic receptors in dibutyryl cyclic AMP-differentiated HL60 cells

ATP promoted biphasic effects on both basal and fMLP-stimulated arachidonic acid (AA) release in neutrophil-like HL60 cells: stimulation in the micromolar range (EC₅₀ = 3.2 ± 0.9 μM) and inhibition at higher concentrations (EC₅₀ = 90 ± 11 μM). ATP also inhibited UTP- and platelet activating factor-stimulated AA release. Only stimulatory effects of ATP on basal or fMLP-stimulated phospholipase C were observed. The inhibitory effect of ATP on AA release was not due to reacylation of released AA, chelation of extracellular Ca²⁺, cell permeabilization, or changes in the rise of [Ca²⁺]_i induced by agonist. The inhibition was rapid, being detected within 5-15 s. The inhibitory effect of ATP on fMLP-stimulated AA release could be desensitized by pretreatment of the cells with 2 mM ATP, but not 20 μM ATP, the concentration that resulted in maximal release of AA and inositol phosphates. The inhibition by ATP was neither dependent on generation of adenosine by ATP hydrolysis nor the result of direct interaction of ATP with P₁ purinergic receptors. Among other nucleotides tested (CTP, GTP, ITP, TTP, XTP, adenosine 5′-(β,γ-methylene)triphosphate (AMP-PCP), adenyl-5′-yl imidodiphosphate (AMP-P(NH)P), ADP, adenosine 5′-O-(3-thiotriphosphate) (ATPγS), and UTP), only UTP and ATPγS displayed biphasic effects with potencies and efficacies almost identical to those of ATP. The other nucleotides only exhibited stimulatory effects (EC₅₀ = 60-300 μM). The results are consistent with a model of dual regulation of AA release by two distinct subtypes of P_2U receptors in HL60 cells.