The cloning of SLC4A11 revealed some homology to the plant borate transporter AtBOR1. Potential borate transport by SLC4A11 was studied in transfected HEK cells. In the absence of borate, SLC4A11 was found to be an electrogenic Na
+ and OH
−(H
+) transporter. In the presence of borate, Na
+B(OH)
4− influx was observed
[19]. The role of borate in mammalian cells is unclear; however, Park et al. suggested that it may have a role in cell growth and differentiation
[18][19]. Using bovine corneal endothelial cells that express physiological levels of SLC4A11, Jalimarada et al.
[10] found that SLC4A11 knockdown reduced Na
+-dependent OH
−(H
+) permeability. However, borate had no effect on Na
+-dependent effects on intracellular pH (pHi) or on intracellular [Na
+] in bovine corneal endothelial cells
[10]. Using SLC4A11 transfected HEK cells, Ogando et al. found that SLC4A11 did not transport HCO
3− or borate, but showed ethyl isopropyl-amiloride (EIPA)-sensitive Na
+-OH
−(H
+) and NH
4+ permeability
[12]. EIPA inhibition of SLC4A11 H
+ flux was also shown by Kao et al.
[3]. As confirmed in two other studies, SLC4A11 does not transport borate
[11][16]. In addition, there is an inconsistent demonstration of Na
+-dependent and -independent electrogenic H
+ flux by SLC4A11 in corneal endothelial cells
[10] and in SLC4A11 transfected cell lines
[3][10][12][16][33][34][35]. Consistent with the inward H
+ flux from SLC4A11, mouse corneal endothelial cell lines from the SLC4A11 KO have a significantly higher pHi
i than WT cells when perfused in a low buffering capacity bicarbonate-free ringer. However, this difference is abolished when cells are perfused with a high buffering capacity bicarbonate ringer, indicating that the absolute inward H
+ flux generated by plasma membrane SLC4A11 is relatively small
[36].
The apparent enhancement of H
+ flux when endothelial cells are perfused with NH
4Cl
[12] led to a series of patch-clamp studies to try to determine the nature of this enhanced flux. Using SLC4A11 transfected PS120 fibroblasts, Zhang et al.
[33] demonstrated inward H
+ currents in response to 10 mM NH
4Cl that increased from pH 6.5 to 7.5 to 8.5. Further analysis indicated that the currents were increasing with increasing [NH
3], but not [NH
4+] or [H
+]. If the [NH
3] was held constant at each pH, the inward current was approximately the same, suggesting that SLC4A11 is activated by increasing [NH
3] and not pH. These inward H
+ currents were not Na
+-dependent and were insensitive to EIPA
[33]. Kao et al.
[3] also showed NH
4Cl-sensitive inward currents in SLC4A11-transfected HEK cells that were, in contrast, mildly inhibited by EIPA. Using two electrode voltage clamps of oocytes transfected with mouse Slc4a11, Loganathan et al.
[11] observed similar inward H
+ currents stimulated by NH
4Cl, but concluded that SLC4A11 transports NH
3 rather than NH
3-H
+. Myers et al.
[34], also using transfected oocytes, showed enhanced Na
+-independent H
+ conduction, but concluded that SLC4A11 was activated by alkaline pH. Moreover, the H
+ conductance of SLC4A11 was found to be steepest at pH 8.5 and it was found that mutants can shift the optimum pKa
[35]. In summary, from these studies, it appears that SLC4A11 provides H
+ conductance that can be either Na
+ dependent or independent, is sensitive to NH
4Cl, and is stimulated by alkaline pH. Zhang et al.
[33] proposed that SLC4A11 is an NH
3/H
+ cotransporter. However, this does not rule out SLC4A11 being an NH
3 activated H
+ transporter, as demonstrating NH
3 net fluxes is difficult. The mechanism of an NH
3 activation is unknown, but could possibly be caused by changing the charge of pore amino acid residues that could also be accomplished by alkaline pH. A more recent study by Kao et al.
[9], using SLC4A11 transfected HEK cells, demonstrated H
+(OH
−) conductive transport stimulated by alkaline pH. Ammonia-stimulated currents were also increased in alkaline pH. The shift in the reversal potential from NH
3 suggested NH
3-H
+ cotransport was competing with H
+(OH
−) and the data, fitting a theoretical model of NH
3-H
+ and H
+(OH
−) interacting competitively within the transporter.
One of the features of the SLC4 family of transporters is their interaction with stilbene derivatives. SLC4A11 binding to stilbenes has been useful in membrane fractionation studies
[37]. Whereas stilbenes generally inhibit transport activity, Kao et al.
[16] showed an enhancement of H
+ flux by the stilbenes, H
2DIDS, SITS, and DIDS, which was also effective in increasing H
+ transport of the R109H mutant. Zhang et al.
[33] found that DIDS had no effect on NH
4Cl-stimulated H
+ currents. Interestingly, SLC4A11 appears to confer a modest amount of membrane water permeability
[15]. Several mutants show diminished water flux
[15][38]. However, in this case, water flux by wild-type SLC4A11 was shown to be inhibited by stilbenes
[15]. Further pharmacological studies examining inhibition or stimulation of SLC4A11 in the context of the different transport modes of SLC4A11 are needed.