A nucleotide receptor in vascular endothelial cells is specifically activated by the fully ionized forms of ATP and UTP

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Abstract
Extracellular ATP causes an increase in the concentration of cytoplasmic free calcium ([Ca2+]i) in bovine pulmonary-artery endothelial (BPAE) cells that results in the synthesis and release of prostacyclin (PGI2), a potent vasodilator and inhibitor of platelet aggregation. We show here that PGI2 release in BPAE cells correlates with the concentration of the fully ionized form of extracellular ATP (ATP4-) and not with the concentration of other ionic forms of ATP. Concentrations as low as 10 nM-ATP4- elicited an increase in PGI2 release [EC50 (concn. giving half-maximal stimulation) 3 microM] in BPAE cells incubated in an iso-osmotic medium, pH 7.4, lacking Ca2+ and Mg2+. When the pH or the Mg2+ concentration of the medium was varied so as to maintain a constant level of ATP4-, while varying the concentration of proton-ATP (HATP3-) or MgATP2- respectively, PGI2 release remained constant. An inhibitory effect of extracellular Mg2+ on PGI2 release could be attributed solely to a decrease in the concentration of ATP4-. In contrast with Mg2+, extracellular Ca2+ stimulated PGI2 release induced by ATP. Several results suggest that extracellular Ca2+ modulates PGI2 release by increasing Ca2+ uptake through an ATP(4-)-activated plasma-membrane channel. In BPAE cells incubated in Ca(2+)-free medium, ATP elicited a transient increase in [Ca2+]i that declined to the basal level within 60 s. In cells incubated in Ca(2+)-containing medium, ATP caused an increase in [Ca2+]i that had two components: a transient peak in [Ca2+]i (0-60 s) and a sustained increase in [Ca2+]i that was maintained for several minutes after ATP addition. Increasing the concentration of extracellular calcium from 0.25 mM to 10 mM had no effect on the transient rise in [Ca2+]i induced by ATP, but significantly enhanced the magnitude of the sustained increase in [Ca2+]i. Alterations in the magnitude of the sustained increase in [Ca2+]i would likely modulate PGI2 release, which was not complete until 2 min after ATP addition. Extracellular Ca2+ also stimulated PGI2 release induced by bradykinin. Bradykinin caused a sustained increase in [Ca2+]i in BPAE cells in the presence of extracellular Ca2+. Finally, the magnitude of PGI2 release induced by UTP, a more potent agonist than ATP, correlated with the concentration of extracellular fully ionized UTP (UTP4-). These findings support the hypothesis that nucleotide receptors in BPAE cells recognize the fully ionized form of ATP and UTP and are coupled to signal-transduction pathways involving the mobilization of intracellular Ca2+, the influx of extracellular Ca2+ and the subsequent release of PGI2.