A CIRCADIAN RHYTHM IN OVARIAN CHOLESTEROL

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 [pregnant mares'' serum gonadotropin - human chori-onic gonadotropin] 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 [ovarian cholesterol depletion] 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 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 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. Changes in ovarian cholesterol during the oestrous cycle are of special interest because these changes are generally considered to reflect the cyclic influences of gonadotrophins. Studies on the ovaries of the rat using histochemical methods indicate that ovarian cholesterol increases during dioestrus and reaches a maximum level on the day of prooestrus. A rapid depletion occurs in the latter part of prooestrus followed by a gradual return to high levels during subsequent days of the oestrous cycle (Everett 1945; Beyler and Szego 1954; Franchimont and van Cauwenberge 1962). Claesson and Hillarp (1947) studying the interstitial tissue of the rat ovary reported two peaks in cholesterol concentration, one at prooestrus and one at metoestrus. With the exception of cholesterol determinations made on the day of prooestrus, all previous reports have been concerned with cholesterol determinations made at one time during each day of the cycle. Lawton and Schwartz (1965) reported a diurnal rhythm of ovarian ascorbic acid in PMS-HCG treated immature rats (animals prepared for use in OAAD assay for LH). The present report is concerned with the possiblity that ovarian cholesterol also might exhibit this diurnal rhythm. Both mature cycling rats and PMS-HCG treated immature rats prepared for the ovarian cholesterol depletion (OCD) assay for LH (Bell et al. 1964) were utilized. The OCD assay as originally reported by Bell et al. (1964) has not been useable in this laboratory and it was also hoped that the current study might shed some light on the reasons for its inadequacy. In addition, the effects of androgen sterilization and constant light on ovarian cholesterol levels were studied.