The electrogenic sodium bicarbonate cotransporter isoform 1 (NBCe1) has been reported to be an important regulator of intracellular H⁺ in glial cells as in many epithelial cells (Deitmer JW & Chesler M, In: New Encyclopedia of Neuroscience, 2009). We have now employed the mouse model deficient in the NBCe1 gene (Gawenis LR et al., J Biol Chem 282, 2007) to elaborate the role of the NBCe1 in more detail. The intracellular free H⁺ concentration ([H⁺]_i) was monitored in BCECF (2’,7’-bis-(2-carboxyethyl)-5-(and-6)-carboxyfluorescein)-loaded astrocytes in culture and in acute brain slices prepared from cortices of juvenile mice. Experiments were carried out in 5 CO₂/26 mM HCO₃^- and in HEPES-buffered saline (all at pH 7.4) in wild-type (WT) and in NBCe1-K.O. mice. Our results show that NBCe1 significantly contributes to apparent H⁺ buffering in astrocytes, as has been reported for NBCe1 heterologously expressed in Xenopus oocytes (Becker HM & Deitmer JW, J. Biol Chem 279, 2004). Surprisingly, however, also in nominally CO₂/HCO₃^--free saline a striking difference in the rate and amplitude of H⁺ shifts in NBCe1-K.O. as compared to WT mouse astrocytes indicated a role for NBCe1, when the extracellular HCO₃^- concentration was in the submillimolar range. In particular the rate of recovery from alkalinization, usually attributed to the Cl^-/HCO₃^- exchanger, appear to be mediated to a significant part by NBCe1 at low HCO₃^- concentration, i.e. in the nominal absence of CO₂/ HCO₃^-. This is supported by the finding that the rate from alkalinization was not affected by the removal of external Cl^-, whereas inhibiting carbonic anhydrases, which accelerate the reversible conversion of CO₂ to HCO₃^- and H⁺, reduced H⁺ buffering and rate of recovery from alkalinzation. In conclusion, our results suggest that NBCe1 has a high affinity for HCO₃^-, being activated by submillimolar concentrations of HCO₃^-, and contributes to both H⁺ buffering and to the recovery from cytosolic alkalinization in cortical astrocytes, which may play an important role for intracellular H⁺ dynamics at low extracellular HCO₃^- concentrations.

Supported by the Deutsche Forschungsgemeinschaft, DE 231/24.