Extracellular ATP-Induced Alterations in Extracellular H Fluxes From Cultured Cortical and Hippocampal Astrocytes

Small alterations in the level of extracellular H can profoundly alter neuronal activity throughout the nervous system. In this study, self-referencing H-selective microelectrodes were used to examine extracellular H fluxes from individual astrocytes. Activation of astrocytes cultured from mouse hippocampus and rat cortex with extracellular ATP produced a pronounced increase in extracellular H flux. The ATP-elicited increase in H flux appeared to be independent of bicarbonate transport, as ATP increased H flux regardless of whether the primary extracellular pH buffer was 26 mM bicarbonate or 1 mM HEPES, and persisted when atmospheric levels of CO were replaced by oxygen. Adenosine failed to elicit any change in extracellular H fluxes, and ATP-mediated increases in H flux were inhibited by the P2 inhibitors suramin and PPADS suggesting direct activation of ATP receptors. Extracellular ATP also induced an intracellular rise in calcium in cultured astrocytes, and ATP-induced rises in both calcium and H efflux were significantly attenuated when calcium re-loading into the endoplasmic reticulum was inhibited by thapsigargin. Replacement of extracellular sodium with choline did not significantly reduce the size of the ATP-induced increases in H flux, and the increases in H flux were not significantly affected by addition of EIPA, suggesting little involvement of Na/H exchangers in ATP-elicited increases in H flux. Given the high sensitivity of voltage-sensitive calcium channels on neurons to small changes in levels of free H, we hypothesize that the ATP-mediated extrusion of H from astrocytes may play a key role in regulating signaling at synapses within the nervous system.