Molecular physiology and functional morphology of SO42- excretion by the kidney of seawater-adapted eels

Marine teleosts actively excrete SO42- and keep the plasma concentration of this ion much lower than that of environmental seawater (SW). We used the eel as a model to study the excretory mechanism of SO42-because this euryhaline species changes SO42- regulation drastically after transfer from freshwater (FW) to SW. Time-course studies showed that plasma SO42- concentration decreased 3. days after transfer of eels from FW to SW, while urine SO42-concentration increased on 1. day. Detailed analyses showed that urine SO42- concentration increased linearly from 6h after SW transfer; however, this did not immediately translate to increased SO42- excretion because the volume of urine was decreased. We identified five SO42- transporters in the eel kidney. Three of these (Slc26a1, Slc26a6b and Slc26a6c) are expressed in both SW- and FW-acclimated eels while Slc26a6a and Slc13a1 are expressed in SW-acclimated eels and FW-acclimated eels, respectively. We showed that changes in Slc26a6a and Slc13a1 gene expression occurred 1-3. days after SW transfer. In SW eel kidneys, immunohistochemistry using specific antisera against each transporter protein showed that Slc26a6a and Slc26a6c are localized on the apical membrane of the P1 segment of the proximal tubule, while Slc26a6b is localized on the apical membrane and Slc26a1 on the basolateral membrane of the P2 segment. The current study revealed complex molecular mechanisms of SO42- excretion in the SW eel kidney that involve segment-specific localization of multiple Slc transporters in proximal tubules and modulation of their expression in different SO42- environments. This precise regulatory mechanism may endow the eel with euryhalinity.