Elevated carbon dioxide upregulates NBCn1 Na+/HCO3− cotransporter in human embryonic kidney cells

Abstract

The NBCn1 Na+/HCO3− cotransporter catalyzes the electroneutral movement of 1 Na+:1 HCO3− into kidney cells. We characterized the intracellular pH (pHi) regulation in human embryonic kidney cells (HEK) subjected to NH4Cl prepulse acid loading, and we examined the NBCn1 expression and function in HEK cells subjected to 24-h elevated Pco2 (10–15%). After acid loading, in the presence of HCO3−, ∼50% of the pHi recovery phase was blocked by the Na+/H+ exchanger inhibitors EIPA (10–50 μM) and amiloride (1 mM) and was fully cancelled by 30 μM EIPA under nominally HCO3−-free conditions. In addition, in the presence of HCO3−, pHi recovery after acid loading was completely blocked when Na+ was omitted in the buffer. pHi recovery after acidification in HEK cells was repeated in the presence of the NBC inhibitor S0859, and the pHi recovery was inhibited by S0859 in a dose-dependent manner ( Ki = 30 μM, full inhibition at 60 μM), which confirmed NBC Na+/HCO3− cotransporter activation. NBCn1 expression increased threefold after 24-h exposure of cultured HEK cells to 10% CO2 and sevenfold after exposure to 15% CO2, examined by immunoblots. Finally, exposure of HEK cells to high CO2 significantly increased the HCO3−-dependent recovery of pHi after acid loading. We conclude that HEK cells expressed the NBCn1 Na+/HCO3− cotransporter as the only HCO3−-dependent mechanism responsible for cellular alkaline loading. NBCn1, which expresses in different kidney cell types, was upregulated by 24-h high-Pco2 exposure of HEK cells, and this upregulation was accompanied by increased NBCn1-mediated HCO3− transport.