The concentrations of progesterone and 20α-hydroxypregn-4-en-3-one in the systemic circulation of 5-day cyclic rats were estimated by gas—liquid chromatography with electron capture detection. Behavioural receptivity (lordosis) was the criterion taken to mark the start of the oestrous cycle. Progesterone levels were highest during the pre-ovulatory period when lordosis was being displayed (3·11 ± 0·41 μg./100 ml. plasma). Several hours later, at the end of the receptive period, the progesterone concentration had declined to 1·18 ± 0·24 μg./100 ml. plasma. Except for a small mid-cycle increase the concentrations throughout the remainder of the oestrous cycle remained below this level until the morning of expected behavioural receptivity, when they rose to 2·14 ± 1·15 μg./100 ml. plasma. The concentration of 20α-hydroxypregn-4-en-3-one did not vary significantly with the cycle.
After ovariectomy, the 20α-dihydro metabolite disappeared from the circulation within 48 hr. Progesterone, however, was measurable in the plasma even 25 days after ovariectomy. After ovariectomy and adrenalectomy, progesterone was undetectable within 8 hr. It was concluded that under anaesthesia and surgical stress the rat adrenal may release significant amounts of progesterone.
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