Local versus systemic effects of exogenous estradiol-17 beta on ovarian follicular dynamics in heifers with progestogen implants

Two experiments were designed to determine if the suppressive effect of est radiol treatment on ovarian follicles in progestogen-implanted heifers is m ediated directly at the ovary or systemically, at a higher level, The purpo se of Experiment 1 was to determine a minimal effective dose of estradiol-1 7 beta (E-17 beta) that would induce follicle regression in progestogen-imp lanted heifers. Beef heifers were implanted with progestogen on Day 2. (Day 0 = ovulation) and were assigned randomly to five groups: control (sesame seed oil, n = 9); 0.1 mg of E-17 beta (II = 8); 0.5 mg of E-17 beta (n = 8) ; 1 mg of E-17 beta in = 8); or 5 mg of E-17 beta (n = 8) by intramuscular (im) injection on Day 3. Treatment with 5 and I mg of E-17 beta resulted in smaller (P < 0.05) day-to-day diameter profiles of the dominant follicle c ompared with controls, whereas 0.1 mg of E-17 beta did not have an apparent effect on follicle growth. The effect of a dose of 0.5 mg was intermediate and tended (P < 0.06) to result in a smaller diameter profile of the domin ant follicle compared with control heifers. Experiment 3 was designed to ut ilize a subminimal dose of E-17 beta (0.1 mg), locally, to determine whethe r estradiol treatment induces follicle regression through a direct action o n the ovary. Beef heifers received a progestogen ear implant on Day 2 and w ere assigned randomly to five groups on Day 3: control (sesame seed oil, n = 8); 5 mg of E-17 beta im ((n = 8); 0.1 mg of E-17 beta im (n = 8); 0.1 mg of E-17 beta given into the wall of the uterus, near the tip of the horn i psilateral to the dominant follicle (intrauterine (iu), II = 8); or 0.1 mg of E-17 beta given into the stroma of the ovary, immediately adjacent to th e dominant follicle (intraovarian (io), n = 6). Local (iu and io) treatment s were given via a transvaginal ultrasound-guided needle injection. Treatme nt with 5 mg of E-17 beta im resulted in suppression of the dominant follic le of the first follicular wave and early emergence of the second follicula r wave (P < 0.05). Diameter profiles of the dominant follicle in heifers tr eated with 0.1 mg im or 0.1 mg iu differed from those of control heifers on Day 5, whereas diameter profiles of the dominant follicle in heifers treat ed with 0.1 mg io did not differ from the controls. Daily changes in diamet er of the dominant follicle did not differ among the three groups treated w ith 0.1 mg of E-17 beta (im, iu and io). Hourly changes in circulating conc entrations of FSH and LH were not detected following estradiol treatment ei ther before or after the results were combined for all estradiol-treated gr oups. Results are supportive of the hypothesis that the suppressive effect of estradiol in cattle is exerted indirectly through a systemic route rathe r than directly at the ovary. Although low plasma concentrations of FSH and LH were not detected, systemic treatments with high E-17 beta dosages resu lted in follicular suppression whereas local treatments with subminimal dos ages, within the ovary bearing the dominant follicle, were without effect. (C) 2000 Elsevier Science B.V. All rights reserved.