Vigorous S042- influx via the gills is balanced by enhanced S042- excretion by the kidney in eels after seawater adaptation.

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]