Evidence for ammonium conductance in a mouse thick ascending limb cell line

Lack of Charge Interaction in the Ion Binding Site Determines Anion Selectivity in the Sodium Bicarbonate Cotransporter NBCe1

The Na/HCO₃ cotransporter NBCe1 is a member of SLC4A transporters that move HCO₃⁻ across cell membranes and regulate intracellular pH or transepithelial HCO₃ transport. NBCe1 is highly selective to HCO₃⁻ and does not transport other anions; the molecular mechanism of anion selectivity is presently unclear. We previously reported that replacing Asp⁵⁵⁵ with a Glu (D555E) in NBCe1 induces increased selectivity to other anions, including Cl⁻. This finding is unexpected because all SLC4A transporters contain either Asp or Glu at the corresponding position and maintain a high selectivity to HCO₃⁻. In this study, we tested whether the Cl⁻ transport in D555E is mediated by an interaction between residues in the ion binding site. Human NBCe1 and mutant transporters were expressed in Xenopus oocytes, and their ability to transport Cl⁻ was assessed by two-electrode voltage clamp. The results show that the Cl⁻ transport is induced by a charge interaction between Glu⁵⁵⁵ and Lys⁵⁵⁸. The bond length between the two residues is within the distance for a salt bridge, and the ionic strength experiments confirm an interaction. This finding indicates that the HCO₃⁻ selectivity in NBCe1 is established by avoiding a specific charge interaction in the ion binding site, rather than maintaining such an interaction.

Evidence for ammonium conductance in a mouse thick ascending limb cell line

In this study, we examined an ammonium conductance in the mouse thick ascending limb cell line ST-1. Whole cell patch clamp was performed to measure currents evoked by NH4Cl in the presence of BaCl2, tetraethylammonium, and BAPTA Application of 20 mmol/L NH4Cl induced an inward current (-272 ± 79 pA, n = 9). In current-voltage (I-V) relationships, NH4Cl application caused the I-V curve to shift down in an inward direction. The difference in current before and after NH4Cl application, which corresponds to the current evoked by NH4Cl, was progressively larger at more negative potentials. The reversal potential for NH4Cl was +15 mV, higher than the equilibrium potential for chloride, indicating that the current should be due to NH4+ We then injected ST-1 poly(A) RNA into Xenopus oocytes and performed two-electrode voltage clamp. NH4Cl application in the presence of BaCl2 caused the I-V curve to be steeper. The NH4+ current was retained at pH 6.4, where endogenous oocyte current was abolished. The NH4+ current was unaffected by 10 μmol/L amiloride but abolished after incubation in Na+-free media. These results demonstrate that the renal cell line ST-1 produces an NH4+ conductance.