Sulfate (SO42−) is the second most abundant anion in seawater (SW), and excretion of excess SO42− from ingested SW is essential for marine fish to survive. Marine teleosts excrete SO42− via the urine produced in the kidney. The SO42− transporter that secretes and concentrates SO42− in the urine has not previously been identified. Here, we have identified and characterized candidates for the long-sought transporters. Using sequences from the fugu database, we have cloned cDNA fragments of all transporters belonging to the Slc13 and Slc26 families from mefugu ( Takifugu obscurus ). We compared Slc13 and Slc26 mRNA expression in the kidney between freshwater (FW) and SW mefugu. Among 14 clones examined, the expression of a Slc26a6 paralog (mfSlc26a6A) was the most upregulated (30-fold) in the kidney of SW mefugu. Electrophysiological analyses of Xenopus oocytes expressing mfSlc26a6A, mfSlc26a6B, and mouse Slc26a6 (mSlc26a6) demonstrated that all transporters mediate electrogenic Cl−/SO42−, Cl−/oxalate2−, and Cl−/ nHCO3− exchanges and electroneutral Cl−/formate− exchange. Two-electrode voltage-clamp experiments demonstrated that the SO42−-elicited currents of mfSlc26a6A is quite large (∼35 μA at +60 mV) and 50- to 200-fold higher than those of mfSlc26a6B and mSlc26a6. Conversely, the currents elicited by oxalate and HCO3− are almost identical among mfSlc26a6A, mfSlc26a6B, and mSlc26a6. Kinetic analysis revealed that mfSlc26a6A has the highest SO42− affinity as well as capacity. Immunohistochemical analyses demonstrated that mfSlc26a6A localizes to the apical (brush-border) region of the proximal tubules. Together, these findings suggest that mfSlc26a6A is the most likely candidate for the major apical SO42− transporter that mediates SO42− secretion in the kidney of marine teleosts.
Aflatoxicosis Dysregulates the Physiological Responses to Crowding Densities in the Marine Teleost Gilthead Seabream (Sparus aurata)
