During the day of pro-oestrus in rats showing regular 4-day oestrous cycles, the concentration of progesterone in peripheral plasma increased slowly from 09.30 to 13.30 h and more rapidly during and after the 'critical period' for luteinizing hormone (LH) release to reach a level of 24·4 ng/ml at 21.00 h. Administration of sodium pentobarbitone at 07.00 or 13.30 h (but not at 16.30) on the day of pro-oestrus delayed ovulation and also prevented the rise in progesterone concentration at 21.00 h on that day. Levels were found to be high 24 h later on the evening of the day preceding the delayed ovulation. It is concluded that the major rise during and after the 'critical period' is the result of LH stimulation of ovarian progesterone secretion.
The possibility that the gradual increase in peripheral plasma progesterone concentration that occurred before the 'critical period' was due to adrenal secretion and played some role in facilitating the onset of the ovulatory surge of LH was examined. The rise in plasma corticosterone concentration roughly paralleled that of progesterone up to the 'critical period' but the curves for the two steroids later became divergent. Suppression of adrenal activity during pro-oestrus by the administration of dexamethasone phosphate resulted in a blockade of ovulation which could be reversed by the administration of either progesterone or corticotrophin (ACTH). Conversely, blockade of ovulation followed ACTH administration at metoestrus and approximately 50% of animals adrenalectomized or sham-adrenalectomized at metoestrus or dioestrus failed to ovulate at the expected time. In adrenalectomized rats tested 12–15 days after operation, the period of pro-oestrus during which sodium pentobarbitone administration could block ovulation was more prolonged than in intact rats. The possible roles of progesterone of adrenal origin in facilitating LH release and entraining the LH release mechanism to the light—dark rhythm and of progesterone of ovarian origin in ensuring full sexual receptivity are discussed.
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