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Effects Of Unilateral Ovariectomy On Ovulation And Cycle Length In 4- And 5-day Cycling Rats.
Published 1970 · Biology, Medicine
Cyclic rats were unilaterally ovariectomized on each day of the estrous cycle and the ovulation rate of the remaining ovary was determined at the next expected ovulation by counting tubal ova. Holtzman rats, followed for three successive estrous cycles, were classified as 4- or 5-day cyclic animals. Control (i.e., intact) 4-day animals ovulated (2 ovaries) an average of 9.6 ± 0.6 eggs and 5-day rats ovulated 10.2 ± 0.2 ova. Estrus was designated as day 1 of the cycle. Unilateral ovariectomy on the morning of days 1 to 3 resulted in doubling the number of ovulations (complete compensatory ovulation) by the next estrus in all rats: an average of 10.5 eggs (N = 78 animals). Four-day animals did not compensate when semi-spayed after 8 PM of day 3, whereas, 5-day aniamls continued to double the number of ovulations until 2 AM of day 4. Unilateral ovariectomy of rats with previous 5-day cycles between 8 and 12 AM on day 1 shortened the expected cycle length by one day (60/73 rats). In contrast, only 19.4% of intact 5-day rats, spontaneously shortened their fourth cycle to four days. This effect of unilateral ovariectomy was only apparent in the cycle in which the operation was performed and was preventable by administration of 0.5 to 2.0 mg of progesterone on day 2. This suggests that unilateral ovariectomy on day 1 may have reduced the level of circulating progesterone enough to facilitate the release of LH one day earlier in the estrous cycle.