Cortisol mediates the increase in intestinal fluid absorption in Atlantic salmon during parr-smolt transformation

We have previously shown in Atlantic salmon that the rate of fluid absorption by the posterior intestine (Jv) is elevated during the smolt stage in spring as a preadaptive development for osmoregulation in seawater. In the present study, we examined developmental differences in the responsiveness of Jv to cortisol and the corticosteroid antagonist, RU 486, through the parr-smolt transformation. Freshwater, juvenile salmon were administered slow-release implants of cortisol (50 micrograms/g body wt), RU 486 (1 mg/g body wt), or the implant without steroid (controls) at seven times from November 1992 through June 1993. Seven and 8 days after implantation, plasma cortisol concentrations and in vitro Jv were measured. In control salmon, both plasma cortisol and Jv peaked in April and were positively correlated over time. Cortisol implants stimulated Jv of salmon only during the parr and postsmolt stages, when Jv of controls was low. The exogenous cortisol was sufficient to stimulate Jv to a rate comparable to that measured for control salmon in the smolt stage. Conversely, RU 486 implants inhibited Jv only during the peak smolt period, when Jv of controls was elevated. Taken together, these results demonstrate that cortisol is a necessary and sufficient endocrine signal mediating this developmental change in intestinal function during parr-smolt transformation. This is the first report of plasma cortisol levels measured in fish treated with RU 486. Plasma cortisol was elevated by RU 486, suggesting that this corticosteroid antagonist blocked feedback inhibition on the pituitary-interrenal axis.