Dosage of the synthetic progestin, norgestomet, influences luteinizing hormone pulse frequency and endogenous secretion of 17β-estradiol in Heifers

The objective of this study was to determine whether there were doses at which the synthetic progestin, norgestomet, could mimic midluteal phase concentrations of progesterone in regulating the secretion of LH and 17̄- estradiol in bovine females. Heifers were randomly assigned to one of five groups to receive: 1) one (1Norg, n = 5), 2) two (2Norg, n = 5), 3) four (4Norg, n = 5), or 4) eight (8Norg, n = 5) norgestomet implants or to serve as untreated control heifers (control, n = 5). On Day 7 (Day 0 = behavioral estrus), implants containing norgestomet were inserted, and they remained in place for 10 days. All heifers implanted with norgestomet were treated with 25 mg prostaglandin F(2α) (PGF(2α) on Days 7 and 8 to lyse the CL. Controls were treated with 25 mg PGF(2α) at the time norgestomet implants were removed from heifers of the other treatment groups. Blood samples were collected every 15 min for 24 h on Days 10 and 16 to determine the frequency of LH pulses. Beginning 24 h after removal of implants, samples of blood were collected at 4-h intervals for 96 h to determine the time of the preovulatory surge of LH. Daily blood samples were collected from Day 2 to Day 48 to determine concentrations of progesterone, and samples collected between Days 2 and 17 were used to determine concentrations of 17̄-estradiol. Ultrasonography was performed daily from Day 2 until Day 23 to evaluate ovarian follicular development. Mean concentrations of LH (Days 10 and 16), frequency of LH pulses (Days 10 and 16), and mean concentrations of 17̄- estradiol during the treatment period were greater (p < 0.05) in heifers from the 1Norg group than in heifers from the other groups. The interval to the preovulatory surge of LH after treatment withdrawal was longer (p < 0.05) in heifers from the 8Norg group than in heifers from the 1Norg, 2Norg, and 4Norg groups. Rate of ovarian follicular growth during the treatment period was greater (p < 0.05) in heifers from the 1Norg group (1.68 ± 0.14 mm/d) than in controls, but heifers among the other groups did not differ from controls. Size of the largest follicle on the day before ovulation was greater (p < 0.05) in heifers treated with 1Norg (18.6 ± 1.2 mm) than in controls (14.8 ± 0.66 mm). In summary, one of the doses (4Norg) of norgestomet used in the present study did mimic midluteal phase concentrations of progesterone with respect to modulation of frequency of LH pulses, rate of ovarian follicular growth, secretion of 17̄-estradiol, time to the preovulatory surge of LH after removal of progestin, and size of the ovulatory follicle.