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

Whether SLC4A11 transports ammonia and its potential mode of ammonia transport (NH4+, 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 (pH(e)) 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 (pH(e)). Ammonia-induced HEK293 whole cell currents were also stimulated by an increase in pH(e). In studies using increasing NH4Cl concentrations with equal NH4+ extracellular and intracellular concentrations, the shift in the reversal potential (E-rev) 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 E-rev 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.