title: Environmental factors responsible for switching on the SO₄²⁻ excretory system in the kidney of seawater eels.
abstract: Eels are unique in that they maintain lower plasma SO(4)(2-) concentration in SO(4)(2-)-rich (∼30 mM) seawater (SW) than in SO(4)(2-)-poor (<0.3 mM) freshwater (FW), showing drastic changes in SO(4)(2-) regulation between FW and SW. We previously showed that the expression of renal SO(4)(2-) 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 SO(4)(2-) concentration after 3 days in SW. In this study, we attempted to identify the environmental factor(s) that trigger the switching of SO(4)(2-) regulation using changes in plasma and urine SO(4)(2-) concentrations and expression of the transporter genes as markers. Transfer of FW eels to 30 mM SO(4)(2-) or transfer of SW eels to SO(4)(2-)-free SW did not change the SO(4)(2-) regulation. Major divalent cations in SW, Mg(2+) (50 mM) and Ca(2+) (10 mM), were also ineffective, but 50 mM NaCl was effective for switching the SO(4)(2-) 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 SO(4)(2-) 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 SO(4)(2-) excretion by the kidney through downregulation of absorptive Slc13a1 and upregulation of excretory Slc26a6a, resulting in low plasma SO(4)(2-) concentration in SW.
authors.affiliation: Laboratory of Physiology, Department of Marine Bioscience, Atmosphere and Ocean Research Institute, The University of Tokyo, Kashiwa, Chiba, Japan. wataro@aori.u-tokyo.ac.jp
authors.forename: Taro
authors.initials: T
authors.lastname: Watanabe
authors.forename: Yoshio
authors.initials: Y
authors.lastname: Takei
mesh_terms: D000818:Animals; D002118:Calcium / Q000097:blood / Q000378:metabolism; D002352:Carrier Proteins / Q000235:genetics / Q000378:metabolism; D002712:Chlorides / Q000097:blood / Q000378:metabolism; D004524:Eels / Q000502:physiology*; D004777:Environment; D005618:Fresh Water / Q000737:chemistry*; D005786:Gene Expression Regulation / Q000502:physiology; D007668:Kidney / Q000502:physiology*; D008274:Magnesium / Q000097:blood / Q000378:metabolism; D012623:Seawater / Q000737:chemistry*; D012964:Sodium / Q000097:blood / Q000378:metabolism; D013431:Sulfates / Q000097:blood / Q000378:metabolism*; D013997:Time Factors
chemical_list: D002352:Carrier Proteins; D002712:Chlorides; D013431:Sulfates; D012964:Sodium; D008274:Magnesium; D002118:Calcium