Effect of Estradiol on Secretion of Luteinizing Hormone during the Follicular Phase of the Bovine Estrous Cycle1

Abstract

Mean concentrations of luteinizing hormone (LH) increase during the follicular phase of the estrous cycle in cows. The working hypotheses in the present study were (1) that increasing concentrations of 17β-estradiol (E2) during the follicular phase of the estrous cycle cause an increase in mean concentration of LH by increasing amplitude of pulses of LH, and (2) that increasing E2 concentrations during this stage of the estrous cycle decrease frequency of pulses of LH 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 of circulating concentrations of E2. Blood samples were collected (at 12-min intervals) from Hour 2 through Hour 80. The response of mean concentration of LH over time was quadratic for cows receiving the DE, LE, and NE treatments. Linear and quadratic responses in the amplitudes of pulses of LH over time were determined in the cows receiving the DE, LE, and NE treatments. Amplitudes of pulses of LH increased at a greater rate through Hour 40 in the cows receiving the DE treatment (time of increasing concentrations of E2 in DE treatment) as compared to those receiving either the LE or NE treatments. The overall quadratic response for frequency of pulses of LH over time was significant for all treatments, with no signUicant interactions between treatments. From these results we conclude that the amplitude of pulses of LH increases at a greater rate due to increasing concentrations of E2 during the follicular phase of the bovine estrous cycle and that frequency of pulses of LH is not influenced by E2 during this stage of the estrous cycle.