Euryhaline pufferfish NBCe1 differs from nonmarine species NBCe1 physiology

Marine fish drink seawater and eliminate excess salt by active salt transport across gill and gut epithelia. Euryhaline pufferfish (Takifugu obscurus, mefugu) forms a CaCO ₃ precipitate on the luminal gut surface after transitioning to seawater. NBCe1 (Slc4a4) at the basolateral membrane of intestinal epithelial cell plays a major role in transepithelial intestinal HCO ₃ ^- secretion and is critical for mefugu acclimation to seawater. We assayed fugu-NBCe1 (fNBCe1) activity in the Xenopus oocyte expression system. Similar to NBCe1 found in other species, fNBCe1 is an electrogenic Na ⁺/HCO ₃ ^- cotransporter and sensitive to the stilbene inhibitor DIDS. However, our experiments revealed several unique and distinguishable fNBCe1 transport characteristics not found in mammalian or other teleost NBCe1-orthologs: electrogenic Li ⁺/nHCO ₃ ^- cotransport; HCO ₃ ^- independent, DIDS-insensitive transport; and increased basal intracellular Na ⁺accumulation. fNBCe1 is a voltage-dependent Na ⁺/nHCO ₃ ^- cotransporter that rectifies, independently from the extracellular Na ⁺ or HCO ₃ ^- concentration, around -60 mV. Na ⁺ removal (0Na ⁺ prepulse) is necessary to produce the true HCO ₃ ^--elicited current. HCO ₃ ^-addition results in huge outward currents with quick current decay. Kinetic analysis of HCO ₃ ^- currents reveals that fNBCe1 has a much higher transport capacity (higher maximum current) and lower affinity (higher Km) than human kidney NBCe1 (hkNBCe1) does in the physiological range (membrane potential = -80 mV; [HCO ₃ ^-] = 10 mM). In this state, fNBCe1 is in favor of operating as transepithelial HCO ₃ ^- secretion, opposite of hkNBCe1, from blood to the luminal side. Thus, fugu-NBCe1 represents the first ortholog-based tool to study amino acid substitutions in NBCe1 and how those change ion and voltage dependence.