Acute effects of ovariectomy on pituitary LH, uterine weight, and vaginal cornification

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Abstract
In the cyclic rat, uterine weight increases at proestrus, pituitary LH content drops from a maximum at proestrus to a minimum at estrus, and ovulation and vaginal cornification then occur. To investigate the timing of the ovarian secretion presumably responsible for these changes, ovariectomy or sham ovariectomy was performed at increasing intervals before the day of estrus. Results in both 4- and 5-day cyclers indicate that the ovary must be in situ between diestrus 4 pm (day before proestrus) and proestrus 10 am in order for estrous vaginal cornification to occur; it must be in situ between diestrus 10 am and 4 pm (day before proestrus) for the uterine weight increase and pituitary LH discharge to occur. In some rats sham ovariectomy, when performed at diestrus 10 am, delayed the proestrous-estrous changes by at least 24 hr. The "extra" day which occurs in normal 5-day cycles results at least partly from a delay in the steroid discharge necessary to cause release of the ovulating surge of LH.

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