Abstract
Mammalian glycosylated rhesus (Rh) proteins include the erythroid RhAG and the nonerythroid RhBG and RhCG. RhBG and RhCG are expressed in multiple tissues, including hepatocytes and the collecting duct (CD) of the kidney. Here, we expressed human RhAG, RhBG and RhCG in Xenopus oocytes (vs. H2O-injected control oocytes) and used microelectrodes to monitor the maximum transient change in surface pH (DpHS) caused by exposing the same oocyte to 5 % CO&sub2;/33 mM HCO&sub3;⁻ (an increase) or 0.5 mM NH&sub3;/NH&sub4;⁺ (a decrease). Subtracting the respective values for day-matched, H&sub2;O-injected control oocytes yielded channel-specific values (*). (ΔpH*(S))(CO&sub2;) and (-ΔpH*(S))(NH&sub3;) were each significantly >0 for all channels, indicating that RhBG and RhCG--like RhAG--can carry CO&sub2; and NH&sub3;. We also investigated the role of a conserved aspartate residue, which was reported to inhibit NH&sub3; transport. However, surface biotinylation experiments indicate the mutants RhBG(D178N) and RhCG(D177N) have at most a very low abundance in the oocyte plasma membrane. We demonstrate for the first time that RhBG and RhCG--like RhAG--have significant CO&sub2; permeability, and we confirm that RhAG, RhBG and RhCG all have significant NH&sub3; permeability. However, as evidenced by (ΔpH*(S))(CO&sub2;)/ (-ΔpH*(S))(NH&sub3;) values, we could not distinguish among the CO&sub2;/ NH&sub3; permeability ratios for RhAG, RhBG and RhCG. Finally, we propose a mechanism whereby RhBG and RhCG contribute to acid secretion in the CD by enhancing the transport of not only NH&sub3; but also CO&sub2; across the membranes of CD cells.