Time‐course effects of repetitive oestradiol‐17β and thyroxine injections on the natural spring smolting of Atlantic salmon, Salmo salar L.

The impact of thyroxine and oestradiol injections on the smolting process of Salmo salar was investigated over 39 days. The increase in the degree of silvering was advanced by thyroxine but inhibited by oestradiol, as compared to controls. Oestradiol treatment increased the level of total calcium in the plasma, the hepatosomatic index and the concentration of RNA in the liver, indicating the induction of vitellogenin synthesis. The level of plasma chloride and protein in the liver remained unchanged in all experimental groups. Gill Na+/K+-ATPase activity was low initially in the control fish, but increased steadily during the 39 days. The enzyme activity was advanced in the thyroxine-treated fish, but inhibited by oestradiol. Gill chloride cell number increased in control fish and in the thyroxine-treated fish until 25 days after the hormone treatment was initiated. Oestradiol-treatment induced a lower chloride cell number on days 25 and 39. Altogether the present study demonstrates a reciprocal action of exogenous thyroxine and oestradiol during smolting in S. salar.

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