title: Effect of an estrogen antagonist (tamoxifen) on cloprostenol-induced luteolysis in heifers.
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 micrograms i.m.) immediately after the start of T or ES, and received estradiol-17 beta (500 micrograms 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 micrograms i.m.) was injected 16 h after the start of treatment. Concentrations of LH were similar (P greater than .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 less than or equal to 1 ng/ml at similar (P greater than .05) rates in ES-treated and T-treated heifers. Mean concentration of P4 288 h post-cloprostenol was greater (P less than .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.(ABSTRACT TRUNCATED AT 250 WORDS)
authors.affiliation: University of Kentucky, Lexington 40546-0215.
authors.forename: A L
authors.initials: AL
authors.lastname: Jacobs
authors.forename: L A
authors.initials: LA
authors.lastname: Edgerton
authors.forename: W J
authors.initials: WJ
authors.lastname: Silvia
authors.forename: K K
authors.initials: KK
authors.lastname: Schillo
mesh_terms: D000818:Animals; D002417:Cattle / Q000502:physiology*; D003008:Cloprostenol / Q000494:pharmacology*; D003338:Corpus Luteum / Q000187:drug effects*; D015237:Dinoprost; D005260:Female; D008186:Luteolytic Agents / Q000494:pharmacology*; D011460:Prostaglandins F / Q000378:metabolism*; D011461:Prostaglandins F, Synthetic / Q000494:pharmacology*; D013629:Tamoxifen / Q000494:pharmacology*
chemical_list: D008186:Luteolytic Agents; D011460:Prostaglandins F; D011461:Prostaglandins F, Synthetic; D013629:Tamoxifen; D003008:Cloprostenol; D015237:Dinoprost