SLC4A11 is an EIPA-sensitive Na+ permeable pHi regulator

摘要

Slc4a11, a member of the solute linked cotransporter 4 family that is comprised predominantly of bicarbonate transporters, was described as an electrogenic 2Na⁺-B(OH)4⁻ (borate) cotransporter and a Na⁺-2OH⁻ cotransporter. The goal of the current study was to confirm and/or clarify the function of SLC4A11. In HEK293 cells transfected with SLC4A11 we tested if SLC4A11 is a: 1) Na⁺-HCO3⁻ cotransporter, 2) Na⁺-OH⁻(H⁺) transporter, and/or 3) Na⁺-B(OH)4⁻ cotransporter. CO2/HCO3⁻ perfusion yielded no significant differences in rate or extent of pHi changes or Na⁺ flux in SLC4A11-transfected compared with control cells. Similarly, in CO2/HCO3⁻, acidification on removal of Na⁺ and alkalinization on Na⁺ add back were not significantly different between control and transfected indicating that SLC4A11 does not have Na⁺-HCO3⁻ cotransport activity. In the absence of CO2/HCO3⁻, SLC4A11-transfected cells showed higher resting intracelllular Na⁺ concentration ([Na⁺]i; 25 vs. 17 mM), increased NH4⁺-induced acidification and increased acid recovery rate (160%) after an NH4 pulse. Na⁺ efflux and influx were faster (80%) following Na⁺ removal and add back, respectively, indicative of Na⁺-OH⁻(H⁺) transport by SLC4A11. The increased alkalinization recovery was confirmed in NHE-deficient PS120 cells demonstrating that SLC4A11 is a bonafide Na⁺-OH⁻(H⁺) transporter and not an activator of NHEs. SLC4A11-mediated H⁺ efflux is inhibited by 5-(N-ethyl-N-isopropyl) amiloride (EIPA; EC50: 0.1 μM). The presence of 10 mM borate did not alter dpHi/dt or ΔpH during a Na⁺-free pulse in SLC4A11-transfected cells. In summary our results show that SLC4A11 is not a bicarbonate or borate-linked transporter but has significant EIPA-sensitive Na⁺-OH⁻(H⁺) and NH₄⁺ permeability.