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Diego Ogando, Supriya Jalimarada, Eranga Vithana, Joseph Bonanno; SLC4A11 is an EIPA-sensitive Na+-Dependent pHi Regulator. Invest. Ophthalmol. Vis. Sci. 2013;54(15):1675.
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© ARVO (1962-2015); The Authors (2016-present)
Mutations in SLC4A11 are associated with corneal endothelial dystrophies. In a previous report (ARVO 2012 #6005) we observed that SLC4A11 acts as a Na+:OH- cotransporter or a Na+: H+ exchanger but does not transport Na+:HCO3-. In the present study we further tested these two hypotheses and a third one that SLC4A11 acts as a Na+: B(OH)4- (Borate) cotransporter.
HEK293 cells were transfected with phCMV2-HA-SLC4A11 (T) or empty vector (C). Membrane surface expression was confirmed by immunofluorescence and membrane protein isolation. Forty eight hours post-transfection the cells were loaded with BCECF or SBFI dyes to measure pHi and Na+i in CO2/HCO3- -rich (BR) or HCO3- -free (BF) Ringer.
We previously observed that the BF to BR induced pHi change (acidification and slow recovery) was not significantly different between C and T cells. This transition was accompanied by a slow Na+ influx (C: 0.882±0.187 vs T: 0.897±0.152 mM Na+/min, p=0.951). A Na+-free pulse in BR produced a similar rate of acidification in T and C (C: 0.042±0.014 vs T: 0.048±0.006 pHi/min, p=0.741). Further, the rate of alkalinization upon Na+-addition in BR was not significantly different (C: 0.131±0.029 vs T: 0.155±0.015 pHi/min, p=0.477). These results indicate that SLC4A11 does not transport Na+: HCO3-. We previously reported that T cells recover from NH4Cl pulse in BF 2.6 times faster than C cells. This recovery was sensitive to the amiloride analogue EIPA (1 µM). A Na+-free pulse in BF induces a faster acidification in T respect to C (C: 0.048±0.004 vs T: 0.074±0.009 pHi/min, p=0.039). Likewise, the rate of alkalinization recovery upon Na+-addition in BF is higher in T (C: 0.151±0.023 vs T: 0.267±0.044 pHi/min, p=0.036). This recovery is completely inhibited by EIPA 1 µM in C and T cells. These results are consistent with SLC4A11 acting as a Na+: OH- cotransporter or a Na+:H+ exchanger (NHE), however they do not rule out SLC4A11 being an activator of endogenous NHEs as the recovery in presence of EIPA 1 µM is not significantly different between C and T. In T cells the presence of Borate 10 mM did not affect the Na+-free induced acidification rate (T: 0.097±0.018 vs T+Borate: 0.079±0.015 pHi/min, p=0.465); inconsistent with SLC4A11 being a Na+: Borate transporter.
(1) SLC411 does not provide Na+:HCO3- or Na+: Borate transport; (2) SLC4A11 transports Na+:OH-; or (3) it is not an ion transporter per se but an activator of NHEs.
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