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1 Depart ment of Physiology, University of Pennsylvania School of Medicine, Philadelphia, PA, USA
2 American Red Cross, Philadelphia, PA, USA
3 Division of Nephrology, University of Florida College of Medicine, Gainesville, FL, USA; Nephrologyand Hypertension Section, NF/SGVHS, Gainesville, FL, USA
4 American Red Cross, Philadelphia, PA, USA; Depart ment of Pathology and Laboratory Medicine, University of Pennsylvania School of Medicine, Philadelphia, PA, USA
* To whom correspondence should be addressed. E-mail: westhoff{at}mail.med.upenn.edu.
The erythrocyte Rh-associated glycoprotein (RhAG) was recently found to mediate transport of ammonia/ammonium when expressed in Xenopus oocytes and yeast S. cerevisiae. Non-erythroid 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 analogue 14C-methylamine (MA) uptake and two-electrode voltage clamping (TEV) of Xenopus 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.
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