Reversed electrogenic sodium bicarbonate cotransporter 1 is the major acid loader during recovery from cytosolic alkalosis in mouse cortical astrocytes

Key points The regulation of H + i from cytosolic alkalosis has generally been attributed to the activity of Cl – ‐coupled acid loaders/base extruders in most cell types, including brain cells. The present study demonstrates that outwardly‐directed sodium bicarbonate cotransport via electrogenic sodium bicarbonate cotransporter 1 (NBCe1) mediates the major fraction of H + i regulation from cytosolic alkalosis in mouse cortical astrocytes. Cl – ‐coupled acid‐loading transporters play only a minor role in the regulation of H + i from alkalosis in mouse cortical astrocytes. NBCe1‐mediated H + i regulation from alkalosis was dominant, with the support of intracellular carbonic anhydrase II, even when the intra‐ and extracellular [HCO 3 – ] was very low (<1mM), as in nominally CO 2 /HCO 3 – free condition. A reversed NBCe1 in astrocytes may also be significant for stabilizing extracellular pH in brain tissue.Abstract Recovery of intracellular pH from cytosolic alkalosis has been attributed primarily to Cl – coupled acid loaders/base extruders such as Cl – /HCO 3 – or Cl – /OH – exchangers. We have studied this process in cortical astrocytes from wild‐type and transgenic mouse models with gene deletion for the electrogenic sodium bicarbonate cotransporter 1 (NBCe1) and for carbonic anhydrase (CA) isoform II. An acute cytosolic alkalosis was induced by the removal of either CO 2 /HCO 3 – or butyric acid, and the subsequent acid loading was analysed by monitoring changes in cytosolic H + or Na + using ion‐sensitive fluorescent dyes. We have identified that NBCe1 reverses during alkalosis and contributes more than 70% to the rate of recovery from alkalosis by extruding Na + and HCO 3 – . After CA inhibition or in CAII‐knockout (KO) cells, the rate of recovery was reduced by 40%, and even by 70% in the nominal absence of CO 2 /HCO 3 – . Increasing the extracellular K + concentration modulated the rate of acid loading in wild‐type cells, but not in NBCe1‐KO cells. Removing chloride had only a minor effect on the recovery from alkalosis. Reversal of NBCe1 by reducing pH/[HCO 3 – ] was demonstrated in astrocytes and in Xenopus oocytes, in which human NBCe1 was heterologously expressed. The results obtained suggest that reversed NBCe1, supported by CAII activity, plays a major role in acid‐loading cortical astrocytes to support recovery from cytosolic alkalosis.