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  • Title: Effect of progesterone on the time of ovulation in immature rats pretreated with pregnant mare's serum gonadotrophin (PMSG).
    Author: Ying SY, Meyer RK.
    Journal: Acta Endocrinol (Copenh); 1973 Jan; 72(1):161-72. PubMed ID: 4346052.
    Abstract:
    21-day-old female rats maintained under 14 hours light and 10 hours darkness were used to determine if there is any effect of progesterone on the time of ovulation, time of release of pituitary gonadotropins (critical period), and the change in plasma LH levels in immature rats pretreated with pregnant mare's serum gonadotropin (PMSG). 3 OR 12 IU PMSG was injected on Day 22, 7:00 - 8:00 A.M. Colony Time. 3 IU PMSG-treated rats were injected at 10:00 A.M. Day 24 with either .5 mg progesterone or 10 mcg estrone. 2 groups of 3 IU PMSG rats were given .5 or 2 mg progesterone at 5:30 A.M., Day 24. Time of ovulation was determined by sacrificing rats every 30 minutes from midnight - 4:00 A.M. and at 8:00 A.M. on Day 25. In 2 experiments rats primed with 3 or 12 IU PMSG were injected with 2 mg progesterone at 5:30 A.M. on Day 24 followed by 4.5 mg phenobarbital from 5:30 - 4:00 P.M. Samples for LH assay were taken at 6:00 A.M., 10:00 A.M., 3:00 P.M., and 8:00 P.M. Day 24 and 9:00 A.M., Day 25. 23-day-old Holtzman rats were used for LH assay. 50 IU PMSG and 25 IU HCG were given between 10:00 and 11:00 A.M., Day 25, or 4:00 and 5:00 P.M., Day 27, respectively, with daily injections of 10 mcg estradiol-3-benzoate Days 30-38. Assay was performed when rats were 39 days old by the method of bogdanove and Gay. The concentration of ascorbin acid was determined by the method of Mindlin and Butler. Duncan's multiple range was used to calculate significance. Ovulation time of 3 IU PMSG-primed rats was between 1:00 and 4:00 A.M. on Day 25 as determined by presence of ova in the oviducts. Time of ovulation was 1:00 - 2:00 A.M., Day 25, when .5 mg progesterone was given at 10:00 A.M., Day 24. Ovulation occurred at 11:00 P.M., Day 24 when .5 or 2 mg progesterone was injected at 5:30 A.M., Day 24. 12 IU PMSG-treated rats had ovulation occur 10:00 P.M., Day 24 - 2:00 A.M., Day 25 when 2 mg progesterone was given 5:30 A.M., Day 24. 4.5 mg phenobarbital given at 10:30 or 11:00 A.M. Day 24 in rats treated with 3 or 12 IU PMSG respectively inhibited ovulation, indicating the critical period had been shifted forward by 3 hours. Highest level of LH was present in 3:00 and 8:00 P.M. plasma, Day 24 in rats primed with 3 IU PMSG alone and at 10:00 A.M. and 3:00 P.M., Day 24 when progesterone was also given. Progesterone advanced the time of ovulation by shifting the critical period and causing an adequate release of ovulatory gonadotropins 10:00 - 3:00 P.M. on the day preceding ovulation.
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