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GnRH-FSH and LH Dose-Response Relationships in Anestrous Sheep and Effects of Estradiol-17β and Progesterone Pretreatment¹

University of Minnesota^,4, St. Paul 55108

Abstract

Two studies were conducted in May and June of 1980 using 16- to 17-mo-old anestrous Suffolk ewes. In the first study, 40 ewes were injected iv with 0, 5, 25, 50, 100, 125, 200 and 625 µg gonadotropin-releasing hormone (GnRH), five sheep/dosage. Blood samples were taken pre-and post-GnRH injection and plasma follicle-stimulating hormone (FSH) and luteinizing hormone (LH) concentrations were measured (a heterologous FSH radioimmunoassay was used). Net peak FSH levels (peak post-GnRH value minus the pre-GnRH level) increased at a rate of 7.3 ng/ml for each 10-fold increase in GnRH dosage, compared with 64.8 ng/ml for LH. When expressed in terms of increments over basal concentrations, the rate at which net peak FSH levels increased was l/29th that of LH. In the second study, 20 sheep received estradiol-17β, progesterone, estradiol-17β plus progesterone or control implants in a factorial design 8 d before GnRH injection (50 µg). Estradiol-17β pretreatment selectively increased (P<.01) FSH levels (net peak heights and areas under the response curves). Ratios of the FSH to LH responses were .51 ± .12, .52 ± .08, .24 ± .05 and .17 ± .02 for ewes pretreated with estradiol-17β, estradiol-17β plus progesterone, progesterone or vehicle, respectively. The degree of variation in the FSH and LH responses were similar among the sheep within the treatment groups from both studies (coefficients of variation averaged 40 ± 5% for FSH and 44 ± 7% for LH); the magnitude of the FSH and LH responses were correlated (r = .66, P<.01) in ewes within treatment group. These results demonstrate that GnRH-induced FSH release can be increased relative to LH by using low dosages of GnRH and estradiol-17β pretreatment.

¹ Univ. of Minnesota Agr. Exp. Sta. No. 12,125.

² Present address: Universidad de Concepcion, Casilla 537, Chile.

³ Appreciation is expressed by the authors to Drs. G. D. Niswender and T. M. Nett for providing LH, progesterone and estrogen antisera; to Dr. L. E. Reichert, Jr., for LH and FSH suitable for radioiodination; to Dr. C. Kaltenbach for hypophysectomized sheep plasma, and to the National Pituitary Agency, NIAMDD, for human FSH antiserum and ovine LH and FSH reference material.

⁴ Dept. of Anim. Sci., 495C Anim. Sci./Vet. Med. Bldg.