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.