EFFECT OF ESTROGEN ANTAGONIST (TAMOXIFEN) ON STEROID-HORMONE LEVELS, MATURATION PROCESS OF PLACENTOMES AND COURSE OF LATE PREGNANCY IN COWS

In order to obtain further information on the role of oestrogens in th e initiation of parturition and maturation process of placenta an endo crine, histological and clinical study on the effect of the oestrogen antagonist tamoxifen in cows before parturition was performed. Startin g on day 265 of pregnancy, six cows received 360 mg tamoxifen into the aorta abdominalis daily (in six doses) until parturition. Six other c ows infused with vehicle served as a control. Concentrations of estron e (E(1)), estrone sulphate (E(1)S) and progesterone (P-4) were measure d by RIA, as well as histological investigations of placentomes obtain ed through the vagina upon parturition, were performed. In tamoxifen-t reated cows, the duration of gestation was physiological (276.7 +/- 1. 2 days) and all cows calved normally; however, one cow had retained pl acenta. The concentrations of oestrogens were typical for late pregnan t cows (E(1) 1.0-3 ng/ml and E(1)S 8-11 ng/ml), as well as the rise in estrone accompanied by a drop in progesterone before parturition was found. The level of progesterone was significantly lower in treated an imals than in control cows for the period from days -9 to -2 before pa rturition. In the histological studies, morphological changes of place ntomes (maturation process) were observed. These findings suggest that tamoxifen was not able to anatgonize oestrogens in late pregnancy.