Melengestrol acetate at greater doses than typically used for estrous synchrony in bovine females does not mimic endogenous progesterone in regulation of secretion of luteinizing hormone and 17β-estradiol

Our working hypothesis was that doses of melengestrol acetate (MGA) greater than those typically administered in estrous synchrony regimens would regulate secretion of LH and 17β-estradiol (E₂) as endogenous progesterone (P₄) does during the midluteal phase of the estrous cycle. We also hypothesized that endogenous P₄ from the CL would interact with MGA to further decrease the frequency of LH pulses and E₂. Cows on Day 5 of their estrous cycle (Day 0 = estrus) were randomly assigned to an untreated control group (CONT, n = 5) or to one of six MGA treatment groups (n = 5 per group): 1) MGA administered orally each day via a gelatin capsule at a dose of 0.5 mg MGA/cow with the CL present (0.5 CL); 2) 0.5 mg MGA/cow daily in the absence of CL (0.5 NO); 3) 1.0 mg MGA with CL present (1.0 CL); 4) 1.0 mg MGA without CL (1.0 NO); 5) 1.5 mg MGA with CL present (1.5 CL); 6) 1.5 mg without CL (1.5 NO). MGA was administered for 10 days (Day 5 = initiation of treatment). To regress CL, cows assigned to groups without CL received injections of prostaglandin F(2α) (PGF(2α); 25 mg) on Days 6 and 7 of their estrous cycle. All cows were administered PGF(2α) at the end of the 10-day treatment period. During the treatment period, daily blood samples were collected to determine concentrations of E₂. Serial blood samples were collected at 15- min intervals for 24 h on Days 8, 11, and 14 to determine pattern of LH secretion. Frequency of LH pulses on Days 8, 11, and 14 was greater (p < 0.05) in cows without CL (0.5NO, 1.0NO, and 1.5NO) than in cows with CL (0.5CL, 1.0CL, 1.5CL, and CONT). Mean concentrations of LH were greater (p < 0.05) in cows from the 0.5NO, 1.0NO, and 1.5NO groups on Day 14 as compared to cows with CL. Overall mean concentrations of LH across Days 8, 11, and 14 were greatest (p < 0.05) in cows from the 0.5NO group and were also greater (p < 0.05) in cows from the 0.5NO, 1.0NO, and 1.5NO groups as compared to cows in the 0.5CL, 1.0CL, 1.5CL, and CONT groups. Mean concentrations of E₂ during the treatment period were greater (p < 0.05) in cows from the 0.5NO group than in cows from either the 1.0NO or the 1.5NO group; these values were also greater (p < 0.05) in cows of the 0.5NO, 1.0NO, and 1.5NO groups as compared to cows of the 1.0CL and CONT groups. Therefore, we reject our working hypothesis because doses of MGA greater than those typically used in estrous synchrony protocols did not suppress LH and E₂ to the same extent that endogeneous P₄ does. In addition, MGA treatment when CL were present did not result in a further suppression of LH pulse frequency or of E₂ as compared to the values in control cows with functional CL.