Na/Bicarbonate Cotransporter NBCn1 is Upregulated under Acidic Conditions and Enhances Ammonium Transport in the Kidney Medullary Thick Ascending Limb Cell Line

Abstract

The sodium/bicarbonate cotransporter NBCn1 regulates intracellular pH (pHi) and transepithelial HCO–3 movement. We examined NBCn1 expression and activity under acidic conditions and its contribution to ammonium movement in the mouse medullary thick ascending limb (MTAL) cell line ST‐1. Incubating cells at pH 6.8 for 24 h lowered the mean steady‐state pHi from 7.10 to 6.56. Immunoblot showed NBCn1 upregulation after acidic incubation. The Cl/HCO3 exchanger AE2 was present but its expression remained unaffected by acidic incubation. Acidic incubation decreased 14C‐methylammonium/ammonia (MA) accumulation by 22%. The 14C‐MA accumulation after acidic incubation was unaffected by 0.5 mM DIDS, whereas the accumulation in controls was reduced by DIDS. In CO2/HCO–3‐free solution, cells at normal pH accumulated 14C‐MA over 7 min and reached a plateau. In CO2/HCO–3 solution, however, cells markedly accumulated 14C‐MA over 30 min. This CO2/HCO–3‐dependent accumulation was reduced by DIDS. Cl– removal reduced the accumulation and this reduction was more substantial with DIDS. Expressed in Xenopus oocytes, NBCn1 increased CO2/HCO–3‐dependent 14C‐MA accumulation, which was abolished by replacing Na+. Two‐electrode voltage clamp exhibited negligible current upon NH4Cl application. We conclude that NBCn1 in the MTAL is upregulated under acidic conditions and the upregulation enhances ammonium uptake.