Hormonal effect on Na-K-ATPase activity in the gill of Japanese eel, Anguilla japonica, with special reference to seawater adaptation.

Na-K-activated adenosinetriphosphatase (Na-K-ATPase) has been implicated in the active excretion of sodium ions by the gill of teleosts living in sea water. In the eel, Anguilla japonica, the activity of this enzyme increased markedly one week after transfer from fresh water to sea water. Hypophysectomy resulted in a reduction of this adaptive increase and the impairment was restored by cortisol treatment. Cortisol injection into intact freshwater eels induced a rise in the branchial Na-K-ATPase activity. When seawater-adapted eels were returned to fresh water, the enzyme activity decreased gradually. Prolactin injection into hypophysectomized eels promoted this decrease. Injection of prolactin into intact or hypophysectomized seawater eels, however, failed to affect the enzyme activity. The significance of these results was discussed in relation to possible hormonal influences on seawater adaptation of the eel.