Mechanisms of Action of Free Arachidonic Acid on Ovarian Steroid Production in the Goldfish
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This study explores the mechanisms by which free arachidonic acid (AA) affects ovarian steroidogenesis by full-grown prematurational follicles of the goldfish in vitro. AA (6-400 microM) stimulated testosterone production and this action was mediated by prostaglandin E2 (PGE2). The steroidogenic actions of AA and the corresponding increase in the production of PGE2 were blocked by inhibitors of the cyclooxygenase pathway (indomethacin, ETYA). Exogenous PGE2 (20-2000 ng/ml) also stimulated steroid production. In the presence of human chorionic gonadotropin (hCG), AA had differential effects. AA potentiated the steroidogenic actions of low dosages of hCG (0.1 IU/ml), while with maximal gonadotropin (1-10 IU/ml) stimulation a high concentration of AA (400 microM) attenuated steroid production in spite of elevated PGE2 synthesis, nor did it affect the PGE2 production obtained with AA-treated follicles. The steroidogenic induction by AA via PGE2 was mediated in part by Ca2+ since the calcium channel blocker nifedipine (25 microM) inhibited stimulated steroid production by both AA and PGE2. The conversion of AA to PGE2 does not require Ca2+ since PGE2 production by AA-treated follicles was not affected by nifedipine. However, treatment with the calcium ionophore A23187 (1 microM) potentiated the stimulatory actions of AA on steroid and prostaglandin production. The phosphodiesterase inhibitor 3-isobutyl-1-methylxanthine (1 mM) potentiated the stimulatory actions of AA on testosterone production but had no effect on the conversion of AA to PGE2. The steroidogenic actions of AA and PGE2 involve both transcription and translation since stimulated steroidogenesis was inhibited by actinomycin D and and cycloheximide (1-10 micrograms/ml). The conversion of AA to PGE2 was also blocked by these inhibitors. These results underscore the importance of AA and PGE2 in the regulation of ovarian steroidogenesis in the goldfish.
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