DIFFERENTIAL-EFFECTS OF OXYTOCIN ON STEROID-PRODUCTION BY BOVINE GRANULOSA-CELLS

Oxytocin (OT) and its mRNA are expressed at very low levels in granulo sa cells from bovine preovulatory follicles isolated before the LH/FSH surge and increase dramatically between the surge and ovulation. We h ave shown previously that OT stimulates progesterone secretion by gran ulosa cells obtained before, but not after the gonadotropin surge, sug gesting that OT may be involved in the follicular/luteal phase shift i n steroidogenesis from estradiol/androgen to progesterone. One objecti ve of this study was to determine if OT affects estradiol as well as p rogesterone production by utilizing culture conditions that maintain e stradiol secretion in vitro. A second objective was to determine if OT regulates steroidogenesis by effects on the levels of mRNA for the st eroidogenic enzymes involved in progesterone synthesis, cytochrome P45 0 side-chain cleavage (P450(scc)) and 3 beta-hydroxy-steroid dehydroge nase (3 beta-HSD), or in estradiol production, cytochrome P450 aromata se (P450(arom)). Granulosa cells were isolated from bovine preovulator y follicles and cultured for 3 days with or without OT in medium suppl emented with either insulin (1 mu g/ml) + 1% fetal calf serum (FCS), w hich maintains basal estradiol secretion, or low doses of FSH (1 and 2 ng/ml) + 1% FCS, a culture condition that maximizes effects of FSH on estradiol secretion. After the first day of culture, OT stimulated pr ogesterone (P < 0.01) and inhibited estradiol production (P < 0.01) in both control and FSH-treated cultures. In contrast, OT had only a sma ll stimulatory effect on the levels of mRNA for P450(scc) and 3 beta-H SD and no effect on mRNA for P450(arom). These findings suggest that: (1) OT plays an autocrine role in regulating the follicular luteal pha se shift in steroidogenesis by both increasing progesterone and inhibi ting estradiol production and (2) the differential effects of OT on st eroid production are not mediated primarily by effects on levels of mR NA for steroidogenic enzymes.