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Sulfate (S042-) is maintained at ∼1 mmoll-1 in teleost fishes that are exposed to media of varying S042- concentrations. We first measured plasma S042- concentration in euryhaline fishes that adapt to both S042-poor freshwater (<0.5mmoll-1) and S042-enriched seawater (30mmoll-1). Unlike Mozambique tilapia and chum salmon, Japanese eels maintained higher plasma S042-concentration in freshwater (6.2+2.3 mmol I-1) than in seawater (0.7±0.1 mmoll-1). We then analyzed the whole-body S042- budget using 35S042-. 35S042- influx in seawater-adapted eels occurred by 84.5% via body surfaces and 15.5% via digestive tracts. The S042- influx was higher in seawater eels (1.55nmolkg-1 h-1) than in freshwater eels (0.09nmolkg-1 min-1), but it was facilitated in freshwater eels when the difference in S042- concentrations between plasma and environment was taken into account (freshwater eels, 6.2 vs 0.3mmoll-1; seawater eels, 0.7 vs 30mmoll-1). One hour after injection of 35S042- 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 3h after injection. As the radioactivity was poorly adsorbed by anion-exchange resin, 35S042- 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 S042- actively from the environment, but seawater eels cope with the obligatory influx of S042- through the gills by excreting excess S042- via the kidney and in mucus. [ABSTRACT FROM AUTHOR]

Eels are unique in that they maintain lower plasma SO42- concentration in SO42--rich (~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 Na2+ 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 Na2+, 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. [ABSTRACT FROM AUTHOR]

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 (S1c26a1, S1c26a6b and S1c26a6c) are expressed in both SW- and FW-acclimated eels while S1c26a6a and Slcl3al are expressed in SW-acclimated eels and FW-acclimated eels, respectively. We showed that changes in S1c26a6a and SIcl3a1 gene expression occurred 1-3days after SW transfer. In SW eel kidneys, immunohistochemistry using specific antisera against each transporter protein showed that Slc26a6a and S1c26a6c are localized on the apical membrane of the P1 segment of the proximal tubule, while S1c26a6b is localized on the apical membrane and Slc26al 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. [ABSTRACT FROM AUTHOR]