NH3 is involved in the NH4+ transport induced by the functional expression of the human RhC glycoprotein

Renal ammonium (NH3 + NH4+) transport is a key process for body acid-base balance. It is well known that several ionic transport systems allow NH4+ transmembrane translocation without high specificity for NH4+, but it is still debated whether NH3, and more generally, gas, may be transported by transmembrane proteins. The human Rh glycoproteins have been proposed to mediate ammonium transport. Transport of NH4+ and/or NH3 by the epithelial Rh C glycoprotein (RhCG) may be of physiological importance in renal ammonium excretion because RhCG is mainly expressed in the distal nephron. However, RhCG function is not yet established. In the present study, we search for ammonium transport by RhCG. RhCG function was investigated by electrophysiological approaches in RhCG-expressing Xenopus laevis oocytes. In the submillimolar concentration range, NH4Cl exposure induced inward currents (I-AM) in voltage-clamped RhCG-expressing cells, but not in control cells. At physiological extracellular pH (pH(o)) = 7.5, the amplitude of I-AM increased with NH4Cl concentration and membrane hyperpolarization. The amplitude of I-AM was independent of external Na+ or K+ concentrations but was enhanced by alkaline pHo and decreased by acid pHo. The apparent affinity of RhCG for NH4+ was affected by NH3 concentration and by changing pHo, whereas the apparent affinity for NH3 was unchanged by pHo, consistent with direct NH3 involvement in RhCG function. The enhancement of methylammonium-induced current by NH3 further supported this conclusion. Exposure to 500 muM NH4Cl induced a biphasic intracellular pH change in RhCG-expressing oocytes, consistent with both NH3 and NH4+ enhanced influx. Our results support the hypothesis of a specific role for RhCG in NH3 and NH4+ transport.