A CIRCADIAN RHYTHM IN OVARIAN CHOLESTEROL

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Abstract
Total ovarian cholesterol levels in the Wistar and Holtzman rat were found to exhibit a circadian (= diurnal) rhythm in the mature female and in the immature PMS-HCG treated animal although certain differences existed between the two strains. Cholesterol levels were determined at 1:30 p.m. and 8:30 p.m. on each day of the oestrous cycle in mature animals. A 19% depletion occurred at metoestrus in the Wistar rat but no significant change was noted in the Holtzman rat. During dioestrus a significant depletion occurred in both strains, i. e. 15% in the Wistar and 19% in the Holtzman strain. On the day of prooestrus the cholesterol levels in both strains showed a 50% depletion. During oestrus the values for the Wistar remained low while the levels in the Holtzman showed a significant increase of 25%. No diurnal rhythm occurred in mature animals kept in constant light or in animals which had been androgen-sterilized with 1.25 mg of testosterone propionate on day two of age. The cholesterol levels were also measured in immature animals of both strains that had been prepared for use in OCD assay for LH, i. e., 50 IU of PMS on day 22 and 24 and 25 IU of HCG on day 26. Measurements were taken at 8:30 a.m., 1:30 p.m., 8:30 p.m., and 2:30 a.m. on day 36 of age. The immature Wistar rat showed a highly significant diurnal rhythm with the highest value at 1:30 p.m. and the lowest value at 2:30 a.m. The immature rat of the Holtzman strain failed to show any rhythm but the cholesterol levels were twice as high as those noted in the Wistar rat. Androgen sterilization abolished the diurnal rhythm in the Wistar animals and significantly increased the ovarian cholesterol values in both strains. The results indicate that a diurnal rhythm in ovarian cholesterol is present. The cholesterol depletion on the day of dioestrus is discussed with regard to its role in the timing of ovulation.