ATP released from Astrocytes during swelling activates chloride channels

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 (I-Cl,I- swell) through an autocrine pathway involving purinergic receptors. Astrocyte swelling is counterbalanced by a regulatory volume decrease, involving efflux of metabolites and activation of I-Cl,I- swell and K+ currents. We used whole cell patch-clamp recordings in cultured astrocytes to investigate the autocrine role of ATP in the activation of I-Cl,I- swell by hypo-osmotic solution (HOS). Apyrase, an ATP/ADP nucleotidase, inhibited HOS-activated I-Cl,I- swell, whereas ATP and the P2Y agonists, ADPbetaS and ADP, induced Cl- currents similar to I-Cl,I- swell. Neither the P2U agonist, UTP nor the P2X agonist, alpha,beta-methylene ATP, were effective. BzATP was less effective than ATP, suggesting that P2X7 receptors were not involved. P2 purinergic antagonists, suramin, RB2, and pyridoxal-phosphate-6-azophenyl-2',4'-disulfonic acid ( PPADS) reversibly inhibited activation of I-Cl,I- swell, suggesting that ATP-activated P2Y1 receptors. Thus ATP release mediates I-Cl,I- swell in astrocytes through the activation of P2Y1-like receptors. The multidrug resistance protein (MRP) transport inhibitors probenicid, indomethacin, and MK-571 all potently inhibited I-Cl,I- 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 I-Cl,I- swell in astrocytes by HOS-induced swelling.