Hormonal Control of the Onset, Magnitude and Duration of Uterine Sensitivity in the Rat by Steroid Hormones of the Ovary

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Abstract
A study of the action of estrogen and progestogen on the onset, magnitude and duration of responsiveness (sensitivity) of the uterus to traumatization in the rat revealed that, in animals ovariectomized after estrus, maximal sensitivity could not be achieved by injection of progesterone alone, but was observed when both estrone and progesterone were administered simultaneously. The timing of the onset, magnitude and duration of sensitivity observed in the intact pseudopregnant rat was reproduced in rats ovariectomized after estrus by daily injections of estrone and progesterone in dosages identical with those necessary for optimal growth and maintenance of the resulting deciduomata. It is suggested that no major alteration in secretory activity of the ovary need occur to account for both the transiency of uterine sensitivity and the resulting decidual growth observed in the pseudopregnant rat. Although estrone potentiated the ability of progesterone to promote the sensitive state in the uterus, it inhibited the ability of progesterone to prolong the period of sensitivity. Evidence is presented to suggest that the appearance of maximal sensitivity in the uterus of the pseudopregnant rat may be related to the period of estrogen secretion associated with the time of last ovulation.