ATP Released From Astrocytes During Swelling Activates Chloride Channels

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Abstract
ATP release from astrocytes contributes to calcium ([Ca2+]) wave propagation and may modulate neuronal excitability. In epithelial cells and hepatocytes, cell swelling causes ATP release, which leads to the activation of a volume-sensitive Cl−current ( ICl,swell) through an autocrine pathway involving purinergic receptors. Astrocyte swelling is counterbalanced by a regulatory volume decrease, involving efflux of metabolites and activation of ICl,swelland K+currents. We used whole cell patch-clamp recordings in cultured astrocytes to investigate the autocrine role of ATP in the activation of ICl,swellby hypo-osmotic solution (HOS). Apyrase, an ATP/ADP nucleotidase, inhibited HOS-activated ICl,swell, whereas ATP and the P2Y agonists, ADPβS and ADP, induced Cl−currents similar to ICl,swell. Neither the P2U agonist, UTP nor the P2X agonist, α,β-methylene ATP, were effective. BzATP was less effective than ATP, suggesting that P2X7 receptors were not involved. P2 purinergic antagonists, suramin, RB2, and pyridoxalphosphate-6-azophenyl-2′,4′-disulfonic acid (PPADS) reversibly inhibited activation of ICl,swell, suggesting that ATP-activated P2Y1 receptors. Thus ATP release mediates ICl,swellin astrocytes through the activation of P2Y1-like receptors. The multidrug resistance protein (MRP) transport inhibitors probenicid, indomethacin, and MK-571 all potently inhibited ICl.swell. ATP release from astrocytes in HOS was observed directly using luciferin-luciferase and MK-571 reversibly depressed this HOS-induced ATP efflux. We conclude that ATP release via MRP and subsequent autocrine activation of purinergic receptors contributes to the activation of ICl,swellin astrocytes by HOS-induced swelling.