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Estriol- and Estradiol-17β-Induced Luteinizing Hormone Release in Ovariectomized Cows and Ewes^1,2,

University of Wyoming⁴, Laramie 82071

Abstract

Two studies were conducted with ovariectomized animals to compare luteinizing hormone (LH) responses during the 24-hr period following an IM injection of either estriol or estradiol-17β. In Exp. 1, six long-term ovariectomized (6 months) beef cows were randomly assigned to be given 1 mg of estriol or estradiol-17β injected in 4 ml of corn oil. Two months later, the experiment was replicated in a switchback design. In Exp. 2, eight ewes that had been ovariectomized for 1 to 4 months were randomly assigned to be given 150 µg of estriol or estradiol-17β injected in 1.5 ml of corn oil. The initial increase in serum LH concentrations occurred earlier in both cows (P<.01) and ewes (P<.05) in response to estriol (8 to 9 hr) than in response to estradiol-17β (12 to 18 hr). In addition, duration of the decrease in LH release that occurred 1 hr after estrogen injection in ewes was 3 hr shorter (P<.05) in response to estriol than in response to estradiol-17β. The total area under the LH response curve was greater for both cows (P<.05) and ewes (P<.01) given estradiol-17β than for those given estriol. Administration of estradiol-17β induced a 10-fold increase in serum estradiol-17β concentrations and a two- to fourfold increase in serum estrone concentrations. Both estrogens returned to basal concentrations by 24 hr after administration. Peak serum estriol exceeded 300 pg/ml within 1 hr of estriol injection, but decreased to nondetectable levels within 12 hours. We concluded that estriol can stimulate LH release from the pituitary. Furthermore, the differential LH response to the two estrogens suggests that the positive effects of estriol are mediated earlier but that estradiol-17β is a more potent stimulus when administered as a single IM injection.

¹ This study was a contribution to Western Regional Project W-112, "Improving Reproductive Performance of Beef Cattle and Sheep." Publication approved by Director, Wyoming Agr. Exp. Sta., Journal Article No. 1043.

² The authors gratefully acknowledge Dr. G. D. Niswender for supplying antisera to the estrogens and LH; Dr. L. E. Reichert for providing bovine and ovine LH for radioiodination; the Hormone Distribution Program of NIH for providing the bovine and ovine LH standards, and Ms. Deana Turner, Ms. Sandra Steckel and Mr. Paul Clemente for their technical assistance.

³ Present address: Dept. of Anim. Sci., Texas A&M Univ., College Station.

⁴ Div. of Anim. Sci.