Effects of stage of the cycle and estradiol-17 beta on oxytocin synthesis by ovine granulosa and luteal cells.

Individual ovine follicles or corpora lutea (CL) were obtained at different stages of the estrous cycle to compare the pattern of oxytocin synthesis with time in vitro. Granulosa cells from follicles in the early follicular phase produced minimal amounts of oxytocin whereas output from preovulatory (post LH surge) follicles increased to a peak of 540 pg/10(4) cells.24 h on days 4-7 in vitro declining to 180 pg/10(4) cells.24 h by day 11. Production from day 1 CL was also high, peaking at 1639 pg/10(4) cells.24 h. In contrast the capacity for oxytocin synthesis by day 2 CL had already declined, with peak output reaching only 185 pg/10(4) cells.24 h on days 3-4. Day 9 CL produced small amounts of oxytocin (50 pg/10(4) cells in the first 24 h) followed by a low output thereafter. The effect of estradiol-17 beta (E2 beta) on oxytocin synthesis was examined. The results were dependent on the stage of the cycle at which the cells were obtained. Oxytocin production was significantly stimulated in three and inhibited in four out of nine preovulatory follicles by the addition of 50 or 500 ng/ml E2 beta, whereas in days 1 and 2 CL E2 beta consistently inhibited oxytocin synthesis and in day 9 CL no response was found. These data indicate that the ovarian capacity to synthesize oxytocin varies markedly at different stages of the cycle, and that cells obtained close to ovulation do not experience the rapid down-regulation in oxytocin synthesis which occurs in vivo in the early luteal phase. E2 beta may switch from having a stimulatory to an inhibitory action on oxytocin synthesis shortly before ovulation.