Environmental factors responsible for switching on the SO42- excretory system in the kidney of seawater eels

Watanabe T, Takei Y. Environmental factors responsible for switching on the SO42- excretory system in the kidney of seawater eels. Am J Physiol Regul Integr Comp Physiol 301: R402-R411, 2011. First published May 4, 2011; doi:10.1152/ajpregu.00624.2010.-Eels are unique in that they maintain lower plasma SO42- concentration in SO42--rich (similar to 30 mM) seawater (SW) than in SO42--poor (< 0.3 mM) freshwater (FW), showing drastic changes in SO42- regulation between FW and SW. We previously showed that the expression of renal SO42- transporter genes, FW-specific Slc13a1 and SW-specific Slc26a6a, changes profoundly after transfer of FW eels to SW, which results in the decrease in plasma SO42- concentration after 3 days in SW. In this study, we attempted to identify the environmental factor(s) that trigger the switching of SO42- regulation using changes in plasma and urine SO42- concentrations and expression of the transporter genes as markers. Transfer of FW eels to 30 mM SO42- or transfer of SW eels to SO42--free SW did not change the SO42- regulation. Major divalent cations in SW, Mg2+ (50 mM) and Ca2+ (10 mM), were also ineffective, but 50 mM NaCl was effective for switching the SO42- regulation. Further analyses using choline-Cl and Na-gluconate showed that Cl- is a primary factor and Na+ is permissive for the Cl- effect. Since plasma SO42- and Cl- concentrations were inversely correlated, we injected various solutions into the blood and found that Cl- alone triggered the switching from FW to SW-type regulation. Furthermore, the inhibitor of Na-Cl cotransporter (NCC) added to media significantly impaired the expression of SW-specific Slc26a6a in 150 mM NaCl. In summary, it appears that Cl- ions in SW are taken up into the circulation via the NCC together with Na+, and the resultant increase in plasma Cl- concentration enhances SO42- excretion by the kidney through downregulation of absorptive Slc13a1 and upregulation of excretory Slc26a6a, resulting in low plasma SO42- concentration in SW.