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 t o determine why this might be so, isolated ovarian follicles of rainbow tro ut were incubated with and without cortisol under varying conditions of gon adotropin or steroid precursor stimulation, incubation time and temperature . Cortisol at concentrations of 100-1000 ng ml(-1) suppressed basal product ion of 17 beta-estradiol (E-2) in only 4 out of 20 experiments, had no effe ct on human chorionic gonadotropin (hCG)-stimulated production, and no supp ressive effect on 17-hydroxyprogesterone (17P)-stimulated production in 14 experiments, but increased E-2 production in response to 17P in 2 experimen ts. Cortisone had no effect on basal E-2 production, suggesting that cortis ol was unlikely to be exerting any effect indirectly after further metaboli sm. Extended incubation times at 12 degrees C resulted in overall decreased levels of E-2 in incubation media, but this had no obvious effect on patte rns generated by treatment with cortisol. Extended incubation at 18 degrees C did change the pattern of response to treatment with cortisol in 1 out o f 3 experiments. All incubations examined produced substantial amounts of E -2-glucuronide but this showed no obvious relationship to whether or not in hibition of E-2 production by cortisol was observed. Effect of stress histo ry was examined by incubating follicles from stressed or unstressed fish. I n follicles from fish nearing the end of vitellogenesis, stress resulted in reduced production of both testosterone and E-2 in response to hCG, but in creased conversion of 17P to E-2. The same effect was not observed in folli cles from fish at an earlier stage of vitellogenesis. Measurement of E-2 up take by follicles from selected experiments showed that follicles contained considerable amounts of E-2 and were potentially a sink for steroid produc ed during incubation. The experiments show that a consistent effect of cort isol on ovarian steroidogenesis remains elusive, but that stimulatory effec ts are as likely to occur as inhibitory effects. All responses are potentia lly further confounded by loss of free steroid from the medium by conjugati on or absorption into the oocytes.