title: Vigorous SO4(2-) influx via the gills is balanced by enhanced SO4(2-) excretion by the kidney in eels after seawater adaptation.
abstract: Sulfate (SO(4)(2-)) is maintained at ∼1 mmol(-1) l(-1) in teleost fishes that are exposed to media of varying SO(4)(2-) concentrations. We first measured plasma SO(4)(2-) concentration in euryhaline fishes that adapt to both SO(4)(2-)-poor freshwater (<0.5 mmol l) and SO(4)(2-)-enriched seawater (30 mmol l(-1)). Unlike Mozambique tilapia and chum salmon, Japanese eels maintained higher plasma SO(4)(2-) concentration in freshwater (6.2±2.3 mmol l(-1)) than in seawater (0.7±0.1 mmol l(-1)). We then analyzed the whole-body SO(4)(2-) budget using (35)SO(4)(2-). (35)SO(4)(2-) influx in seawater-adapted eels occurred by 84.5% via body surfaces and 15.5% via digestive tracts. The SO(4)(2-) influx was higher in seawater eels (1.55 μmol kg(-1) h(-1)) than in freshwater eels (0.09 μmol kg(-1) min(-1)), but it was facilitated in freshwater eels when the difference in SO(4)(2-) concentrations between plasma and environment was taken into account (freshwater eels, 6.2 vs 0.3 mmol l(-1); seawater eels, 0.7 vs 30 mmol l(-1)). One hour after injection of (35)SO(4)(2-) into the blood of seawater eels, the kidney excreted ∼97% of the ionized form, whereas the radioactivity increased gradually in the medium and the rectal fluid more than 3 h after injection. As the radioactivity was poorly adsorbed by anion-exchange resin, (35)SO(4)(2-) in the blood may be incorporated into cells and excreted by the intestine, gills and skin, probably as mucus. These results show that freshwater eels take up SO(4)(2-) actively from the environment, but seawater eels cope with the obligatory influx of SO(4)(2-) through the gills by excreting excess SO(4)(2-) via the kidney and in mucus.
authors.affiliation: Laboratory of Physiology, Department of Marine Bioscience, Atmosphere and Ocean Research Institute, University of Tokyo, Kashiwa, Chiba 277-8564, 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: D000064:Acclimatization; D000222:Adaptation, Physiological; D000818:Animals; D000838:Anions; D002852:Chromatography, Ion Exchange / Q000379:methods; D004524:Eels; D004777:Environment; D005880:Gills / Q000502:physiology*; D007477:Ions / Q000737:chemistry; D007668:Kidney / Q000378:metabolism* / Q000502:physiology; D008954:Models, Biological; D012623:Seawater; D013431:Sulfates / Q000737:chemistry*; D013997:Time Factors
chemical_list: D000838:Anions; D007477:Ions; D013431:Sulfates