Branchial ATPases and ionic transport in the eel, Anguilla japonica—I. Na+−K+-ATPase

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1.1. Na+−K+-ATPase activity in the gills of both freshwater and seawater-adapted eels (Anguilla japonica) was studied.2.2. The enzyme has the highest specific activity (μmol Pj mg protein−1 hr−1) in the heavy microsomal fraction showing a 5–6 fold increase in activity as compared to whole homogenate.3.3. The enzyme activity doubled in gill preparations prepared from seawater-adapted eels (574.5 ± 26.7, n = 5) as compared to freshwater fish (252.3 ± 30.9, n = 5).4.4. Microsomal Na+−K+-ATPase was maximally activated by 20 mM of K+, 100 mM of Na+ and at pH 7.3. The enzyme was half-maximally inhibited by 2.67 × 10−6 M ouabain.5.5. When freshwater eels were transferred to seawater, gill Na+−K+-ATPase started to increase on day 4 after the transfer and reached maximal level at day 7 representing a three-fold increase.6.6. Serum Na+ rapidly rose to maximal level on day 2 and started to decline before the enzyme level reached a significantly higher level. During the back transfer, Na+−K+-ATPase activity did not return to pre-transfer level even after 21 days back to freshwater.7.7. These data were discussed with regard to the role of the enzyme in ion transport and salinity adaptation.