Opposite effects of 17 beta-estradiol and combined growth hormone-cortisol treatment on hypo-osmoregulatory performance in sea trout presmolts, Salmo trutta.

In March, sexually immature sea trout presmolts (Salmo trutta trutta) were injected every second day with saline, 2 micrograms 17 beta-estradiol (E2)/g, 2 micrograms ovine growth hormone (GH) + 6 micrograms cortisol (F)/g, or all three hormones (E2-GH-F) simultaneously. A SW-challenge test was performed after six injections. At the time of SW-transfer, high total plasma calcium levels in E2- and E2-GH-F-treated fish indicated activated vitellogenesis in these groups. All control, GH-F, and E2-GH-F-treated fish survived SW-transfer, whereas 43% of the E2-treated fish died after transfer. On Day 2 after transfer, there were marked differences among groups in their osmoregulatory response. Changes in ion-osmotic parameters (plasma Na+, Cl-, Mg2+, and total calcium and muscle water) indicated the following degree of osmotic stress: E2 greater than control greater than E2-GH-F greater than GH-F, which was inversely correlated with pretransfer gill Na+/K(+)-ATPase activity: GH-F greater than E2-GH-F greater than control greater than E2. On Day 7 after transfer there were no major differences among the groups with regard to plasma ions and muscle water content. The detrimental influence of elevated plasma E2 levels on hypo-osmo-regulatory physiology may indicate an important role of E2 during development.