Differential effects of oxytocin on steroid production by bovine granulosa cells

Oxytocin (OT) and its mRNA are expressed at very low levels in granulosa cells from bovine preovulatory follicles isolated before the LH/FSH surge and increase dramatically between the surge and ovulation. We have shown previously that OT stimulates progesterone secretion by granulosa cells obtained before, but not after the gonadotropin surge, suggesting that OT may be involved in the follicular/luteal phase shift in steroidogenesis from estradiol/androgen to progesterone. One objective of this study was to determine if OT affects estradiol as well as progesterone production by utilizing culture conditions that maintain estradiol secretion in vitro. A second objective was to determine if OT regulates steroidogenesis by effects on the levels of mRNA for the steroidogenic enzymes involved in progesterone synthesis, cytochrome P450 side-chain cleavage (P450(scc)) and 3 beta-hydroxy-steroid dehydrogenase (3 beta-HSD), or in estradiol production, cytochrome P450 aromatase (P450(arom)). Granulosa cells were isolated from bovine preovulatory follicles and cultured for 3 days with or without OT in medium supplemented with either insulin (1 mu g/ml) + 1% fetal calf serum (FCS), which 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 progesterone (P < 0.01) and inhibited estradiol production (P < 0.01) in both control and FSH-treated cultures. In contrast, OT had only a small stimulatory effect on the levels of mRNA for P450(scc) and 3 beta-HSD and no effect on mRNA for P450(arom). These findings suggest that: (1) OT plays an autocrine role in regulating the follicular luteal phase shift in steroidogenesis by both increasing progesterone and inhibiting estradiol production and (2) the differential effects of OT on steroid production are not mediated primarily by effects on levels of mRNA for steroidogenic enzymes.