Oxytocin in mares: lack of evidence for oxytocin production by or action on preovulatory follicles.

It is not known whether the equine preovulatory follicle produces oxytocin or is a target tissue for oxytocin, as has been reported for other species, especially ruminants. Bovine granulosa cells secrete oxytocin, and oxytocin modulates the production of progesterone by granulosa cells in vitro. We examined whether oxytocin plays a comparable role in the equine preovulatory follicle. To test the hypothesis that the equine preovulatory follicle produces oxytocin during estrus and that its production increases in late estrus, preovulatory follicles were isolated during early (Days 1 to 2; n = 4) and late (Days 4 to 5; n = 4) estrus. Granulosa cells, pieces of theca interna and pieces of follicle wall (theca with attached granulosa cells) were cultured for 3 d with or without equine gonadotropins. Culture media were collected, replaced at 3, 6, 12, 24, 48, and 72 hr of culture, and assayed for oxytocin. Granulosa cells from preovulatory follicles secreted negligible amounts of oxytocin during 3 d of culture, irrespective of gonadotropin treatment or stage of estrus. Likewise, negligible amounts of oxytocin were measured in theca and follicle wall cultures at both developmental stages, in the presence or absence of gonadotropins. Furthermore, follicular fluid from early or late estrous follicles contained only negligible amounts of oxytocin. To determine if oxytocin affects steroidogenesis by equine granulosa cells, granulosa cells from follicles obtained on Day 2 of estrus were cultured with graded doses of oxytocin (0, 1, 10, 100, and 1,000 ng/ml) in defined medium supplemented with testosterone (0.5 microM) and culture media were assayed for estradiol-17 beta and progesterone. Estradiol was secreted throughout the culture period, and its production was not significantly affected by oxytocin treatment (P > 0.05). Progesterone secretion was relatively low during the first 24 hr of culture, increased dramatically on the second day of culture, and remained high through the third day. No dose of oxytocin had a significant effect on progesterone secretion (P > 0.05). In conclusion, the results indicate that equine preovulatory follicles, isolated during early or late estrus, are neither a source of oxytocin nor a target for oxytocin action on steroidogenesis. Although ovarian oxytocin appears to play a role in regulating follicular function in some other mammalian species, our data provide no support for such a role for oxytocin in mares.