Causal Relationship between Luteinizing Hormone Release and Estrogen Secretion in the Rat

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It was confirmed that estrogen secreted on the day before proestrus (diestrus II) is required for proestrous uterine distension, for ovulatory discharge of luteinizing hormone (LH) and for vaginal cornification in the rat. Further investigation was made to determine the timing of estrogen secretion responsible for LH release at the critical period on proestrus and for uterine and vaginal changes in the rats showing 4-day estrous cycle under the illuminating schedule of 12hrs. light (8a.m. to 8p.m.) a day. The rats were ovariectomized at varying times between diestrus II and proestrus, and autopsied at 7 p.m. on proestrus for the determination of LH concentration in the circulating blood. it was demonstrated that the ovary must be <I>in situ</I> between 4a.m. and 7a.m. at proestrus for the discharge of LH. In the same experiment, a complete uterine distension was observed at 7p.m. on proestrus only in the rats ovariectomized at 4a.m. or later in proestrus, but no uterine distension was obtained in the rats operated at 7p.m. or earlier in diestrus II. In the rats hypophysectomized at 1p.m. or earlier in diestrus II, vaginal cornification did not occur on the expected day of estrus, but did occur in all the rats hypophysectomized at 4p.m. on diestrus II. Anti-estrogen, MER-25, given at diestrus I (metestrus) at the dosage of 10mg per rat inhibited the appearance of proestrous smear, subsequent ovulation and vaginal cornification, while no inhibition of those were obtained when the compound was injected at diestrus II. It is suggested, therefore, that estrogen secreted at diestrus I is responsible for the proestrous change of vaginal smear which is a necessary step to the subsequent cornification; estrogen secreted till 1-4a.m. on proestrus accounts for uterine distension; and estrogen secreted till 4-7a.m. on proestrus is necessary for ovulatory discharge of LH.
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Causal Relationship between Luteinizing Hormone Release and Estrogen Secretion in the Rat

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