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 oxyt ocin or is a target tissue for oxytocin, as has been reported for othe r species, especially ruminants. Bovine granulosa cells secrete oxytoc in, 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 e quine preovulatory follicle produces oxytocin during estrus and that i ts production increases in late estrus, preovulatory follicles were is olated 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 follic le wall (theca with attached granulosa cells) were cultured for 3 d wi th or without equine gonadotropins. Culture media were collected, repl aced at 3, 6, 12, 24, 48, and 72 hr of culture, and assayed for oxytoc in. Granulosa cells from preovulatory follicles secreted negligible am ounts 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 development al stages, in the presence or absence of gonadotropins. Furthermore, f ollicular fluid from early or late estrous follicles contained only ne gligible amounts of oxytocin. To determine if oxytocin affects steroid ogenesis by equine granulosa cells, granulosa cells from follicles obt ained 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 te stosterone (0.5 mu M) and culture media were assayed for estradiol-17 beta and progesterone. Estradiol was secreted throughout the culture p eriod, and its production was not significantly affected by oxytocin t reatment (P > 0.05). Progesterone secretion was relatively low during the first 24 hr of culture, increased dramatically on the second day o f culture, and remained high through the third day. No dose of oxytoci n had a significant effect on progesterone secretion (P > 0.05). In co nclusion, the results indicate that equine preovulatory follicles, iso lated during early or late estrus, are neither a source of oxytocin no r a target for oxytocin action on steroidogenesis. Although ovarian ox ytocin appears to play a role in regulating follicular function in som e other mammalian species, our data provide no support for such a role for oxytocin in mares.