The effects of a single injection of progesterone on the oestrous cycle, thyroid gland activity and uterus-plasma concentration ratio for radio-iodide in the rat

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Abstract
A single subcutaneous injection of 5 mg, 1.25 mg or 625 [mu]g, but not 312 [mu]g, of progesterone in oil delayed ovulation in the rat by 1 or more days, when injected at the dioestrous stage of a 4-day oestrous cycle. When ovulation was delayed in this way the expected increase in the thyroid-serum concentration ratio for I131 was also delayed but the ratio did increase when delayed ovulation occurred. A single injection of progesterone resulted in an increase in the uterus-plasma and oviduct-plasma concentration ratios for I131; the increase was greatest when steroid was injected at the dioestrous stage of the cycle and was delayed and least when the steroid was given at the pro-oestrous stage. Ovulation was advanced by 1 day when progesterone was injected on the second day of dioestrus in rats showing regular 5-day cycles; this ovulation was not accompanied by an increase in the thyroid-serum concentration ratio. In these experiments a dose of progesterone that failed to advance ovulation produced a rise in uterus-plasma and oviduct-plasma ratio for I131 but no rise was seen when ovulation was induced, suggesting that estrogen secretion had been stimulated. 20[alpha]-Dihydroprogesterone (pregn-4-en-20[alpha]-ol-3-one) was not effective in delaying or advancing ovulation at a dose level of 2.5 mg per rat and had no effect on the uterus-plasma concentration ratio for radio-iodide. These results are discussed in relation to the hypothesis that the increase in thyroid gland activity at the estrous stage of the cycle is related to the neuro-endocrine changes that lead to ovulation.