Ammonium and methyl ammonium transport by RhAG

The erythroid Rh protein (RhAG) is one component of the “Rh complex” that is known for its immunogenic characteristics. Recently, a role of RhAG in transport of ions has been proposed. In this study, we characterized NH 3 /NH 4 + transport by RhAG expressed in Xenopus oocytes. To do so we used ion‐selective microelectrodes & two‐electrode voltage clamp to measure cellular changes induced by NH 3 /NH 4 + & methyl ammonium (MA/MA + ). In oocytes expressing RhAG, 5mM NH 4 Cl (NH 3 /NH 4 + ) decreased pH i by 0.28±0.04 at a rate of −9.6±0.01 ×10 −4 pH/sec; depolarized the cell by 15±1.3 mV, caused an inward current of −66±8.5 nA & decreased surface pH (pHs) by 0.7±0.04. On the other hand, 5 mM MA/MA + increased pH i by 0.15±0.02 at a rate of 12.6±2 ×10 −4 pH/sec; depolarized the cell by 10.1±1.0 mV; caused an inward current of −49±11.4 nA & decreased pHs by 0.29±0.02. In H 2 O‐injected oocytes the changes were significantly smaller: NH 3 /NH 4 + decreased pH i by 0.25±0.06 at a rate of only −6.0±1 ×10 −4 pH/sec; depolarized the cell by 17±1.9 mV; induced an inward current of ‐ 44±5.7 nA & decreased pHs by 0.4±0.03. Notably, in H 2 O‐injected oocytes MA/MA + did not elicit any change in pH i , V m or current & decreased pHs by 0.11±0.01. These data indicate: 1) RhAG transports NH 3 /NH 4 + & MA/MA + . 2) There is an electrogenic component of transport consistent with influx of NH 4 + & MA + by RhAG. 3) There is also evidence for electroneutral transport of NH 3 & MA.