Bicarbonate sensing in mouse cortical astrocytes during extracellular acid/base disturbances.

Extracellular acid/base status of the mammalian brain undergoes dynamic changes during many physiological and pathological events. Although intracellular pH (pHi ) of astrocytes responds to extracellular acid/base changes, the mechanisms mediating these changes have remained unresolved. We have previously shown that the electrogenic sodium-bicarbonate cotransporter, NBCe1, is a high-affinity bicarbonate carrier in cortical astrocytes. In the present study, we investigated whether NBCe1 plays a role in bicarbonate sensing in astrocytes, and in determining the pHi responses to extracellular acid/base challenges. We measured changes in intracellular H+ and Na+ in astrocytes from wild-type (WT) and from NBCe1-knockout (KO) mice, using ion-selective dyes, during isocapnic acidosis, hypercapnic acidosis and hypocapnia. We also analysed NBCe1-mediated membrane currents in Xenopus laevis oocytes under similar conditions. Comparing WT and NBCe1-KO astrocytes, we could dissect the contribution of NBCe1, of diffusion of CO2 across the cell membrane and, after blocking carbonic anhydrase (CA) activity with ethoxyzolamide, of the role of CA, for the amplitude and rate of acid/base fluxes. Our results suggest that NBCe1 transport activity in astrocytes, supported by CA activity, renders astrocytes bicarbonate sensors in the mouse cortex. NBCe1 carried bicarbonate into and out of the cell by sensing the variations of transmembrane [HCO3- ], irrespective of the changes in intra- and extracellular pH, and played a major role in setting pHi responses to the extracellular acid/base challenges. We propose that bicarbonate sensing of astrocytes may have potential functional significance during extracellular acid/base alterations in the brain.