SLC4A11 is an EIPA-sensitive Na(+) permeable pHi regulator.

SLC4A11 belongs to the SLC4 family, typically associated with bicarbonate transport. Prior research has shown this family includes proteins like AE1 and NBCe1, which are involved in pH regulation. However, the specific role of SLC4A11 remains unclear. This gap motivated the current study to clarify whether SLC4A11 functions as a bicarbonate transporter or another type of ion channel. No prior work had resolved whether SLC4A11 transports borate, hydroxide, or bicarbonate. Earlier studies proposed SLC4A11 might be a borate cotransporter, but evidence is mixed. The uncertainty around its transport mechanism drove the need for direct experimental testing. The study aimed to distinguish between competing hypotheses about SLC4A11's function. Understanding this protein's role could clarify its physiological relevance in pH regulation.

The study aimed to determine the transport activity of SLC4A11 in HEK293 cells. Researchers sought to test whether SLC4A11 functions as a Na(+)-HCO3(-) cotransporter, a Na(+)-OH(-)(H(+)) transporter, or a Na(+)-B(OH)4(-) cotransporter. The motivation was to resolve conflicting prior findings about SLC4A11's role in pH regulation. This uncertainty limited understanding of its physiological function. The researchers focused on measuring intracellular pH (pHi) and Na(+) flux in transfected versus control cells. They also examined the effects of EIPA and borate on transport activity. The goal was to isolate SLC4A11's specific contribution to pH regulation. This study aimed to provide direct evidence to clarify the protein's transport mechanism.

The study used HEK293 cells transfected with SLC4A11 to assess transport activity. Researchers measured pHi changes under CO2/HCO3(-) conditions and in its absence. They compared SLC4A11-transfected cells to control cells for differences in Na(+) flux and acid-base balance. NH4(+) pulses were used to induce acidification and alkalinization. The rate of acid recovery was measured after Na(+) removal and re-addition. EIPA was applied to test inhibition of H(+) efflux. Borate was added to assess its effect on pHi and ΔpH. Experiments were repeated in NHE-deficient PS120 cells to rule out NHE involvement. The study focused on quantifying transport activity and comparing it to control groups.

SLC4A11-transfected cells showed no significant differences in pHi or Na(+) flux under CO2/HCO3(-) conditions. Acidification and alkalinization rates were similar to control cells in CO2/HCO3(-) perfusion. In the absence of CO2/HCO3(-), transfected cells had higher resting [Na(+)]i (25 vs. 17 mM). NH4(+)-induced acidification was increased in transfected cells. Acid recovery rate was 160% faster after an NH4 pulse. Na(+) efflux and influx were 80% faster following Na(+) removal and add back. EIPA inhibited H(+) efflux with an EC50 of 0.1 μM. Borate had no significant effect on pHi or ΔpH during Na(+)-free pulses. These findings suggest SLC4A11 is not a bicarbonate or borate transporter.

The authors propose that SLC4A11 functions as an EIPA-sensitive Na(+)-OH(-)(H(+)) transporter. The study shows no evidence of bicarbonate or borate transport activity. The increased acid recovery rate supports a role in pH regulation. The EIPA sensitivity confirms the involvement of a Na(+)-dependent H(+) transport mechanism. The findings suggest SLC4A11 is not an NHE activator but a direct transporter. The study clarifies that SLC4A11 has significant NH4(+) permeability. The results align with the hypothesis that SLC4A11 contributes to intracellular pH regulation. These conclusions are based on direct experimental evidence from transfected and control cells.