Identification of aromatizable androgens secreted by isolated thecal cell layers from vitellogenic ovarian follicles of a teleost, amago salmon (Oncorhynchus rhodurus)
Aromatizable androgens, which are converted to estradiol-17.beta. in granulosa cell layers, secreted during vitellogenesis by thecal cell layers of the teleost, amago salmon (Oncorhynchus rhodurus), were purified and chemically identified. Isolated thecal cell layers from vitellogenic ovarian follicles were incubated for 18 h in Ringer medium with chum salmon gonadotropin. Aromatizable androgens in samples (residues) at various steps of purification were detected by an in vitro estradiol-17.beta. production assay using isolated granulosa cell layers from vitellogenic ovarian follicles of amago salmon. After ether extraction from the incubation medium and partition of the extract with equal volumes of 50% methanol and n-hexane, the 50% methanol phase was fractionated (27 separate fractions) by reversed-phase high-performance liquid chromatography and further analyzed by thin-layer chromatography and mass spectrometry. The aromatizable androgen in fractions 12 and 13 was identified as androstenedione, and that in fractions 18 and 19 as testosterone. The amount of testosterone secreted by thecal layers was about three times that of androstenedione. The present study, together with previous reports, indicates that thecal cell layers of vitellogenic ovarian follicles of amago salmon, under the influence of gonadotropin, secrete aromatizable androgens, testosterone as the major precursor steroid and androstenedione as the other effective one, that are converted to estradiol-17.beta. by granulosa cell layers where the aromatase enzyme is localized.