Secretion and gene expression of inhibin, oxytocin and steroid hormones during the in vitro differentiation of bovine granulosa cells

Bovine granulosa cells were cultured under defined conditions to examine (1) their secretion of immunoreactive inhibin, oxytocin, progesterone and oestradiol during differentiation in vitro; (2) their expression, by Northern analysis, of specific mRNAs for inhibin and oxytocin as compared with uncultured cells; (3) possible interrelationships between the four secreted hormones; and (4) the hypothesis that androgens and steroidogenesis influence the secretion of inhibin. The secretion of inhibin and oestradiol fell rapidly over the first few days of culture but remained at detectable levels for at least 7 days. Conversely, the secretion of oxytocin and progesterone rose steadily as culture progressed. These changes occurred spontaneously (i.e. without gonadotrophin treatment) and were not dependent on the addition of serum to the culture medium. Messenger RNAs for the inhibin alpha- and beta A-subunits were present in uncultured cells but barely detectable or undetectable in cells cultured for 4 days. Conversely, the mRNA for oxytocin, which was not detectable in uncultured cells, was present in cultured cells and increased in quantity as culture progressed. Treatment of cells with testosterone (5 nM-5 microM), in the presence or absence of serum (10% FCS), had no effect on the secretion of inhibin but stimulated the declining oestradiol secretion. Treatment with ascorbic acid (0.5 mM) increased the secretion of oxytocin and progesterone, as previously described, but not that of inhibin. Treatment with aminoglutethimide (0.5 mM), an inhibitor of steroidogenesis, substantially inhibited progesterone secretion and the response of oestradiol secretion to testosterone, but had no effect on the secretion of either inhibin or oxytocin. We conclude that bovine granulosa cells differentiate spontaneously in defined culture in a manner that, as defined by the secretion of steroid and peptide hormones, closely resembles their luteinization in vivo. The switch in protein hormone secretion from inhibin to oxytocin is accompanied by a corresponding change in mRNA expression. The changes in steroid and peptide hormone secretions that take place in culture appear to occur independently of one another although their absolute cause remains to be determined. In contrast to previous studies, we could find no evidence for the regulation of inhibin secretion by either androgens or steroidogenesis.