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Differential Effects of Various Estradiol-17beta Treatments on Follicle-Stimulating Hormone Peaks, Luteinizing Hormone Pulses, Basal Gonadotropin Concentrations, and Antral Follicle and Luteal Development in Cyclic Ewes¹

Department of Veterinary Biomedical Sciences, Western College of Veterinary Medicine, University of Saskatchewan, Saskatoon, Saskatchewan, Canada S7N 5B4

ABSTRACT

In a previous study, 10-day estradiol implant treatment truncated the FSH peaks that precede follicular waves in sheep, but subsequent ovine FSH (oFSH) injection reinitiated wave emergence. The present study's objectives were to examine the effects of a 20-day estradiol and progesterone treatment on FSH peaks, follicle waves, and responsiveness to oFSH injection. Also, different estradiol doses were given to see whether a model that differentially suppressed FSH peaks, LH pulses, or basal gonadotropin secretion could be produced in order to study effects of these changes on follicular dynamics. Mean estradiol concentrations were 11.8 ± 0.4 pg/ml, FSH peaks were truncated, wave emergence was halted, and the number of small follicles (2–3 mm in diameter) was reduced (P < 0.05) in cyclic ewes given estradiol and progesterone implants (experiment 1). On Day 15 of treatment, oFSH injection failed to induce wave emergence. With three different estradiol implant sizes (experiment 2), estradiol concentrations were 5.2, 19.0, 27.5, and 34.8 (±4.6) pg/ml in control and treated ewes, respectively. All estradiol treatments truncated FSH peaks, except those that created the highest estradiol concentrations. Experiment 2-treated ewes had significantly reduced mean and basal FSH concentrations and LH pulse amplitude and frequency. We concluded that 20-day estradiol treatment truncated FSH peaks, blocking wave emergence, and reduced the small-follicle pool, rendering the ovary unresponsive to oFSH injection in terms of wave emergence. Varying the steroid treatment created differential FSH peak regulation compared with other gonadotropin secretory parameters. This provides a useful model for future studies of the endocrine regulation of ovine antral follicular dynamics.

corpus luteum,, estradiol,, follicle-stimulating hormone,, follicular development,, luteinizing hormone

³Current address: Institut de Génétique et de Biologie Moléculaire et Cellulaire, Illkirch 67404, France.

⁴Current address: Department of Animal and Poultry Science, College of Agriculture, University of Saskatchewan, Saskatoon, Saskatchewan, Canada S7N 5A8.

⁵Current address: Department of Biomedical Sciences, Ontario Veterinary College, University of Guelph, Guelph, Ontario, Canada N1G 2W1.

¹Supported by the Natural Sciences and Engineering Research Council, Canada (N.C.R.). D.M.W.B., K.L.D., and E.T.B. were recipients of University of Saskatchewan Graduate Student Scholarships.

Correspondence: ²FAX: 306 966 7376; e-mail: norman.rawlings{at}usask.ca