The electrogenic sodium bicarbonate cotransporter NBCe1 is a high-affinity bicarbonate carrier in cortical astrocytes

The electrogenic sodium bicarbonate cotransporter NBCe1 (SLC4A4) is a robust regulator of intracellular H(+) and a significant base carrier in many cell types. Using wild-type (WT) and NBCe1-deficient (NBC-KO) mice, we have studied the role of NBCe1 in cortical astrocytes in culture and in situ by monitoring intracellular H(+) using the H(+)-sensitive dye BCECF [2',7'-bis-(carboxyethyl)-5-(and-6)-carboxyfluorescein] in wide-field and confocal microscopy. Adding 0.1-3 mm HCO3(-) to an O2-gassed, HEPES-buffered saline solution lowered the intracellular H(+) concentration with a Km of 0.65 mm HCO3(-) in WT astrocytes, but slowly raised [H(+)]i in NBCe1-KO astrocytes. Human NBCe1 heterologously expressed in Xenopus oocytes could be activated by adding 1-3 mm HCO3(-), and even by residual HCO3(-) in a nominally CO2/HCO3(-)-free saline solution. Our results demonstrate a surprisingly high apparent bicarbonate sensitivity mediated by NBCe1 in cortical astrocytes, suggesting that NBCe1 may operate over a wide bicarbonate concentration in these cells.