Effect of Estradiol on Secretion of Luteinizing Hormone during the Follicular Phase of the Bovine Estrous Cyclel

Mean concentrations of luteinizing hormone (LII) increase during the follicular phase of the estrous cycle in cows. The working hypotheses in the present study were (1) that increasing concentrations of l73-estradiol (E2) during the follicular phase of the estrous cycle cause an increase in mean concentration of LII by increasing amplitude of pulses of LII, and (2) that increasing E2 concentrations during this stage of the estrous cycle decrease frequency of pulses of LII in bovine females. Day of estrus was synchronized in seventeen mature cows. Treatments were initiated on Day 16 of the experimental estrous cycle (Day 0 = estrus). At Hour 0 (on Day 16), 4 cows were lutectomized. Lutectomy of these cows (EE; n = 4) allowed for endogenous secretion of E2. The remaining cows were ovariectomized at Hour 0 and were assigned to one of three E2 treatments: luteal phase E2 (LE, n = 5), increasing then decreasing E2 (DE, n = 5), and no E2 (NE, n = 3). Cows in the group that received LE were administered one E2 implant at Hour 0, which provided low circulating concentrations of E2 similar to those observed during the luteal phase of the estrous cycle. Cows in the group that received DE were administered one E2 implant at Hour 0, and additional implants were administered at 8-h intervals through Hour 40; then, two implants were removed at Hours 48 and 56, and one implant was removed at Hour 64. This protocol mimicked the normal preovulatory rise and fall o