Cortisol treatment improves the development of hypoosmoregulatory mechanisms in the euryhaline rainbow trout, Salmo gairdneri.

The effect of cortisol on osmoregulatory parameters was studied in rainbow trout, (Salmo gairdneri), kept in freshwater (FW) and/or transferred to seawater (SW). Repeated injections of 20 µg cortisol/g fish stimulated gill and gut Na(+)/K(+)-ATPase activity and reduced plasma Na(+) and Cl(-) levels after 2 weeks of treatment in FW-adapted fish. Cortisol doses of 0.05 and 1.0 µg/g were without effect. Repeated injections of 10 µg cortisol/g stimulated gill Na(+)/K(+)-ATPase activity and reduced plasma Na+ and Cl(-) levels in fish in FW, and significantly improved ion regulation after their transfer to 28SW. Higher doses of cortisol (10 and 20 µg/g) induced hyperglycemia, whereas low doses (0.05 and 1.0 µg/g were without effect or induced hypoglycemia. Plasma glucose levels decreased in cortisol-treated fish transferred to SW, whereas transient hyperglycemia was seen in the control fish.