Functional Properties of a New Splice Variant of Rat NDCBE

Of the known mammalian SLC4 transporters, human NDCBE (SLC4A8) has been characterized as a Na + ‐driven Cl − /HCO 3 − exchanger. We have cloned and functionally characterized a new NDCBE splice variant (NDCBE‐D) from rat brain. NDCBE‐D encoded a 1092 residue ~ 200 kDa protein that is preferentially expressed in the hippocampus. NDCBE‐D was functionally characterized in HEK‐293 cells using BCECF to assess equivalent base flux (EBF) and MQAE to monitor Cl − flux. In cells expressing NDCBE‐D, Na + ‐driven EBF was significantly greater than in vector transfected cells and was both HCO 3 − and Cl − ‐dependent. If NDCBE‐D exchanges Cl − and Na + ‐(HCO 3 − )n in a given transport cycle, Cl − removal/addition would be predicted to drive net Na + ‐(HCO 3 − )n flux in the opposite direction. Surprisingly, Cl − ‐driven EBF following Cl − removal and re‐addition was not significantly different than in vector transfected cells. Moreover, following prolonged (1.5 h) Cl − depletion, Cl − addition failed to drive EBF at a rate significantly different from vector transfected cells. In cells expressing NDCBE‐D, using identical Cl − removal/addition protocols, the rate of Cl − flux was significantly greater than control suggesting that NDCBE‐D mediates Cl − transport. In addition, in HCO 3 − containing solutions, Cl − flux was significantly greater despite the absence of Cl − ‐driven EBF above control. Finally, although Na + ‐driven EBF was Cl − ‐dependent, Na + ‐driven EBF was not coupled to net Cl − flux in the opposite direction. In conclusion, rat NDCBE‐D mediates both Cl − and Na + ‐(HCO 3 − )n flux, however the transporter does not appear to function as a classic exchanger. Supported by NIH.