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Publication : Characterization of ammonia transport by the kidney Rh glycoproteins RhBG and RhCG.

First Author  Mak DO Year  2006
Journal  Am J Physiol Renal Physiol Volume  290
Issue  2 Pages  F297-305
PubMed ID  16131648 Mgi Jnum  J:148068
Mgi Id  MGI:3843434 Doi  10.1152/ajprenal.00147.2005
Citation  Mak DO, et al. (2006) Characterization of ammonia transport by the kidney Rh glycoproteins RhBG and RhCG. Am J Physiol Renal Physiol 290(2):F297-305
abstractText  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 (K(m) = 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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