Further evidence of the equivocal effects of cortisol on in vitro steroidogenesis by ovarian follicles of rainbow trout Oncorhynchus mykiss

Previous studies on salmonids have yielded equivocal results as to the role of cortisol in directly inhibiting ovarian steroidogenesis. In an effort to determine why this might be so, isolated ovarian follicles of rainbow trout were incubated with and without cortisol under varying conditions of gonadotropin or steroid precursor stimulation, incubation time and temperature. Cortisol at concentrations of 100-1000 ng ml-1 suppressed basal production of 17β-estradiol (E2) in only 4 out of 20 experiments, had no effect on human chorionic gonadotropin (hCG)-stimulated production, and no suppressive effect on 17-hydroxyprogesterone (17P)-stimulated production in 14 experiments, but increased E2 production in response to 17P in 2 experiments. Cortisone had no effect on basal E2 production, suggesting that cortisol was unlikely to be exerting any effect indirectly after further metabolism. Extended incubation times at 12 °C resulted in overall decreased levels of E2 in incubation media, but this had no obvious effect on patterns generated by treatment with cortisol. Extended incubation at 18 °C did change the pattern of response to treatment with cortisol in 1 out of 3 experiments. All incubations examined produced substantial amounts of E2-glucuronide but this showed no obvious relationship to whether or not inhibition of E2 production by cortisol was observed. Effect of stress history was examined by incubating follicles from stressed or unstressed fish. In follicles from fish nearing the end of vitellogenesis, stress resulted in reduced production of both testosterone and E2 in response to hCG, but increased conversion of 17P to E2. The same effect was not observed in follicles from fish at an earlier stage of vitellogenesis. Measurement of E2 uptake by follicles from selected experiments showed that follicles contained considerable amounts of E2 and were potentially a sink for steroid produced during incubation. The experiments show that a consistent effect of cortisol on ovarian steroidogenesis remains elusive, but that stimulatory effects are as likely to occur as inhibitory effects. All responses are potentially further confounded by loss of free steroid from the medium by conjugation or absorption into the oocytes.