The Electrogenic Sodium Bicarbonate Cotransporter NBCe1 Is a High-Affinity Bicarbonate Carrier in Cortical Astrocytes

摘要

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.