SLC4A11 function: evidence for H+(OH−) and NH3-H+ transport

Abstract

Whether SLC4A11 transports ammonia and its potential mode of ammonia transport ([Formula: see text], NH3, or NH3-2H+ transport have been proposed) are controversial. In the absence of ammonia, whether SLC4A11 mediates significant conductive H+(OH−) transport is also controversial. The present study was performed to determine the mechanism of human SLC4A11 ammonia transport and whether the transporter mediates conductive H+(OH−) transport in the absence of ammonia. We quantitated H+ flux by monitoring changes in intracellular pH (pHi) and measured whole cell currents in patch-clamp studies of HEK293 cells expressing the transporter in the absence and presence of NH4Cl. Our results demonstrate that SLC4A11 mediated conductive H+(OH−) transport that was stimulated by raising the extracellular pH (pHe). Ammonia-induced HEK293 whole cell currents were also stimulated by an increase in pHe. In studies using increasing NH4Cl concentrations with equal [Formula: see text] extracellular and intracellular concentrations, the shift in the reversal potential ( Erev) due to the addition of ammonia was compatible with NH3-H+ transport competing with H+(OH−) rather than NH3-nH+ ( n ≥ 2) transport. The increase in equivalent H+(OH−) flux observed in the presence of a transcellular H+ gradient was also compatible with SLC4A11-mediated NH3-H+ flux. The NH3 versus Erev data fit a theoretical model suggesting that NH3-H+ and H+(OH−) competitively interact with the transporter. Studies of mutant SLC4A11 constructs in the putative SLC4A11 ion coordination site showed that both H+(OH−) transport and ammonia-induced whole cell currents were blocked suggesting that the H+(OH−) and NH3-H+ transport processes share common features involving the SLC4A11 transport mechanism.