Incorporation and metabolism of cortisol in oocytes of Tilapia (Oreochromis mossambicus)

The entry and metabolism of H-3-cortisol in oocytes were investigated using isolated follicles of the tilapia (Oreochromis mossambicus) in order to ex amine the mechanisms of incorporation of maternal hormones into oocytes. Th e composition of H-3-labeled steroids in the oocyte was analyzed by high-pe rformance liquid chromatography. A significant amount of cortisol was conve rted to cortisone and an unidentified molecule by the follicular layer. The contents of H-3-cortisol and H-3-cortisone in the oocyte reached an equili brium level within 12 hr, whereas the content of the unidentified metabolit e continued to increase for 36 hr. The total content of the incorporated co rtisol and its metabolites was proportional to cortisol in the medium over the concentration range of 5 ng/ml to 5 mug/ml. The amounts of cortisone an d the unidentified molecule increased proportionally when the concentration of cortisol in the medium was lower than 500 ng/ml, whereas they reached a plateau when the concentration of cortisol exceeded 500 ng/ml. Cortisol en try was reversible, because 90% of cortisol and cortisone in the oocyte was lost within 18 hr when the medium was changed to that without H-3-cortisol . On the other hand, 50% of the unidentified molecule was preserved at the end of the incubation. In conclusion, the entry of cortisol into the oocyte was considered to be nonspecific and due probably to simple diffusion. How ever, a considerable amount of cortisol (50-70%) was specifically converted to cortisone and another unidentified molecule during passage through the follicular layer. J. Exp. Zool. 287:485-492, 2000. (C) 2000 Wiley-Liss, Inc .