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Release of FSH and LH in Beef Heifers by Synthetic Gonadotrophin Releasing Hormone^{1, 2, 3,}

University of Wyoming, Laramie 82071 and Colorado State University, Fort Collins 80521

Abstract

Hereford, Angus and crossbred heifers were utilized to test the effect of synthetic gonadotrophin releasing hormone (Gn-RH) on serum follicle stimulating hormone (FSH) and luteinizing hormone (LH). Initially, 1 mg Gn-RH was given as a single intracarotid injection to individual heifers on days 16, 18 or 20 of the estrous cycle or on the day of estrus. Increases in serum LH were observed on all days with the greatest response occurring on the day of estrus. Subsequently, 250 µg Gn-RH was given intramuscularly (im) to five heifers 24 hr. after removal of a progestogen implant. Mean duration of the induced LH and FSH surges was approximately 7 hr. with peaks occurring at approximately 100 min. post-injection. All heifers exhibited estrus and ovulated. A similar response was obtained in three of five heifers when Gn-RH was given 36 hr. post-implant removal. Endogenous LH release preceded Gn-RH injections in the remaining two heifers and little or no increase in serum levels of FSH or LH was obtained following Gn-RH. Four heifers received Gn-RH (250 µg, im) 60 hr. after a single subcutaneous injection of 15 mg prostaglandin F₂. The mean LH surge commenced within 15 min., reached a peak at 150 min. and lasted for 6 hours. Estrus and ovulation were observed in only one of these heifers.

¹ Published with approval of Director, Wyoming Agricultural Experiment Station as Journal Article No. 598.

² This research supported in part by Merck & Co., Rahway, N.J., Abbot Laboratories, North Chicago, Illinois and Graduate Research Committee, University of Wyoming, Laramie.

³ The authors thank Dr. L.E. Reichert for the LH and FSH used in the radioimmunoassays.

⁴ Division of Animal Science, University of Wyoming, Laramie.

⁵ Department of Physiology and Biophysics, Colorado State University, Fort Collins.