PROGESTERONE BLOCKADE OF THE POSITIVE FEEDBACK EFFECTS OF 17-BETA ESTRADIOL IN THE EWE

Ovariectomized ewes (n = 24) were treated with implants that resulted in circulating concentrations of progesterone and 17beta-oestradiol similar to those seen in intact ewes in the luteal phase of an oestrous cycle. Progesterone implants were left in for 10 days, and 17beta-oestradiol implants for 14 days. Twelve of these ewes received daily injections of 17beta-oestradiol in oil (i.m.) at doses sufficient to cause a surge release of luteinizing hormone (LH) in the absence of progesterone. The other 12 ewes were treated daily with vehicle (oil). Following progesterone withdrawal on Day 10, each group of 12 ewes was divided into three subgroups. Ewes in each subgroup of the groups treated daily with 17beta-oestradiol or vehicle, received an injection of either 17beta-oestradiol (oil i.m.), gonadotrophin-releasing hormone (GnRH) (saline, i.v.) or vehicle, 24 h after progesterone withdrawal. Following progesterone withdrawal, no LH surge was detected in ewes treated with vehicle. Surge secretion of LH was detected in ewes of all other groups. The data suggested that in progesterone-treated ewes, daily exposure to stimulatory doses of 17beta-oestradiol did not desensitize the hypothalamic pituitary axis to the positive feedback effects of 17beta-oestradiol. Daily exposure to 17beta-oestradiol did not suppress pituitary responsiveness to GnRH. It was concluded that circulating concentrations of progesterone, similar to those seen during the luteal phase of an oestrous cycle in intact ewes, may prevent all necessary components of the LH surge secretory mechanism from responding to 17beta-oestradiol.