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Am J Physiol Renal Physiol 290: F297-F305, 2006. First published August 30, 2005; doi:10.1152/ajprenal.00147.2005
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Characterization of ammonia transport by the kidney Rh glycoproteins RhBG and RhCG

Don-On Daniel Mak,1 Binh Dang,2 I. David Weiner,3,4 J. Kevin Foskett,1 and Connie M. Westhoff2,5

1Department of Physiology, 5Department of Pathology and Laboratory Medicine, University of Pennsylvania School of Medicine, Philadelphia; 2American Red Cross, Philadelphia, Pennsylvania; 3Division of Nephrology, University of Florida College of Medicine, Gainesville; and 4Nephrology and Hypertension Section, North Florida/South Georgia Veterans Health System, Gainesville, Florida

Submitted 8 April 2005 ; accepted in final form 24 August 2005

The erythrocyte Rh-associated glycoprotein (RhAG) was recently found to mediate transport of ammonia/ammonium when expressed in Xenopus laevis oocytes and yeast Saccharomyces cerevisiae. Nonerythroid homologs, RhBG and RhCG, are expressed in the mammalian kidney connecting segment and the collecting duct, major sites of urinary ammonia secretion. This study characterizes the transport properties of murine RhBG and RhCG by ammonium analog [14C]methylamine (MA) uptake and two-electrode voltage clamping of X. laevis oocytes. Both RhBG and RhCG mediated transport of ammonia, but differed in affinity for substrate (Km = 2.5 and 10 mM, respectively). The rates of RhBG- and RhCG-mediated transport were sensitive to the concentration of the protonated MA species and were stimulated by extracellular alkalosis and inhibited by acidosis, suggesting a role for H+ in the transport process. Whereas expression of RhBG or RhCG caused a small increase in plasma membrane conductance, [14C]MA uptake was not affected by depolarization of oocytes with 100 mM extracellular K+ or by clamping the membrane potential between 0 and –100 mV, indicating that RhBG- and RhCG-mediated transport was independent of the membrane potential. These results strongly suggest that RhBG and RhCG transport ammonia by an electroneutral process that involves NH4+/H+ exchange resulting in net NH3 translocation. The polarized localization of RhBG and RhCG in kidney tubules and the different substrate affinities may enable these proteins to participate in transepithelial ammonia secretion and to therefore play an important role in whole animal acid-base regulation.

Rh proteins; Rh-associated proteins; ammonium


Address for reprint requests and other correspondence: C. M. Westhoff, American Red Cross, 700 Spring Garden St., Philadelphia, PA 19123 or Dept. of Pathology and Laboratory Medicine, Univ. of Pennsylvania, 510 Stellar Chance, Philadelphia, PA 19104 (e-mail: westhoff{at}mail.med.upenn.edu and westhoffc{at}usa.redcross.org)




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