J. Anim Sci.
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J. Anim Sci. 1988. 66:735-742.
© 1988 American Society of Animal Science
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Effect of an Estrogen Antagonist (Tamoxifen) on Cloprostenol-Induced Luteolysis in Heifers1

A. L. Jacobs2, L. A. Edgerton, W. J. Silvia and K. K. Schillo

University of Kentucky, Lexington 40546-0215

Abstract

Experiments were conducted to determine the role of estrogens on endogenous PGF2{alpha} secretion and luteolysis following injection of cloprostenol in heifers. In Exp. 1, eight luteal-phase heifers were used to evaluate tamoxifen (T) as an estrogen antagonist. Heifers received T (35 mg i.v.) or ethanol:saline vehicle (ES) every 4 h for 44 h. All received cloprostenol (500 µg i.m.) immediately after the start of T or ES, and received estradiol-1ß (500 µg i.m.) 12 h later. Each ES heifer had a surge of luteinizing hormone (LH) within 48 h of estradiol injection, whereas T-treated heifers did not. Estrus was observed in three ES-treated heifers, but not in T-treated heifers. In Exp. 2, 10 heifers received T (35 mg i.v.) or ES every 4 h for 64 h beginning on d 15 postestrus. Cloprostenol (500 µg i.m.) was injected 16 h after the start of treatment. Concentrations of LH were similar (P > .05) in both groups. In ES heifers, concentrations of 13,14-dihydro-15-keto-prostaglandin F2{alpha} (PGFM) increased; in T-treated heifers, PGFM remained at pre-cloprostenol levels. Luteolysis was induced in all heifers. Progesterone (P4) decreased to <= 1 ng/ml at similar (P > .05) rates in ES-treated and T-treated heifers. Mean concentration of P4 288 h post-cloprostenol was greater (P < .05) in ES-treated than in T-treated heifers. Three ES-treated heifers, but no T-treated heifers, were in standing estrus. We conclude that T effectively antagonizes estrogen in cattle. Cloprostenol treatment is followed by an endogenous, estrogen-dependent secretion of PGF2{alpha}. However, this secretion does not appear to be required for cloprostenol-induced luteolysis.

Footnotes

1 Journal paper no. 87-5-142 of the Univ. of Kentucky Agric. Exp. Sta., Dept. of Anim. Sci. Anti-sera to PGFM and progesterone were generously supplied by Kenneth Kirton, Upjohn Co., Kalamazoo, MI and Robert Loy, Univ. of Kentucky, Lexington, respectively.

2 Present address: Dept. of Biochem. and Mol. Biol., M.D. Anderson Hospital and Tumor Inst., Houston, TX.






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Copyright © 1988 by the American Society of Animal Science.