Enhanced hypoosmoregulatory response to growth hormone after cortisol treatment in immature rainbow trout, Salmo gairdneri.

The growth-independent effect of ovine growth hormone (oGH) and oGH + cortisol treatment on seawater (SW) adaptation in immature rainbow trout, Salmo gairdneri was investigated. Fish were injected every second day with saline, 2.0 μg oGH/g or 2.0 μg oGH + 8.0 μg cortisol/g for a maximum of 8 injections in freshwater (FW). Subgroups were transferred to 28‰ SW after 4 or 8 injections, and changes in plasma Na(+) and Cl(-), muscle water content and gill Na(+)/K(+)-ATPase activity were measured. In both of the hormone-treated groups retained in FW, gill Na(+)/K(+)-ATPase activity and interlamellar chloride cell density increased. The effects were most pronounced in the oGH + cortisol group after 2 weeks of treatment. After transfer to SW most of the control fish died due to the osmotic stress, whereas in the hormone-treated groups, mortality was low and there was a positive correlation between pretransfer gill Na(+)/K(+)-ATPase and the ability to maintain ionic-osmotic homeostasis after SW transfer. After two weeks of oGH + cortisol treatment, gill Na(+)/K(+)-ATPase activity was maximal. In contrast, after SW transfer, Na(+)/K(+)-ATPase activity increased further in the oGH-treated group. This group regulated ionic-osmotic parameters less effectively than the oGH + cortisol-treated group. The data indicate that GH and cortisol are important hormones in the regulation of hypoosmoregulatory mechanisms in S. gairdneri.