title: Estrogen replacement in ovariectomized rats results in physiologically significant levels of circulating progesterone, and co-administration of progesterone markedly reduces the circulating estrogen.
abstract: Estrogen and progesterone replacement in ovariectomized rats in an often-used experimental system for determination of the specific effects of these hormones. In this study, two different delivery systems and two different dosage levels of estrogen, progesterone or a combination of the two have been used. Estrogen and progesterone in the circulation have been measured in response to each treatment. It is reported that estrogen treatment (237.2 +/- 49.2 pg/mL) results in physiologically significant levels of circulating progesterone (11.1 +/- 1.3 ng/mL). Also, co-administration of progesterone (23.7 +/- 2.0 ng/mL) with estrogen decreases the level of estrogen over that seen with estrogen alone (96.7 +/- 19.2 pg/mL with progesterone vs 237.2 +/- 49.2 pg/mL without progesterone). Thus, contrary to expectations, estrogen replacement therapy is not specific to estrogen and some of the antagonistic effects of progesterone are the result of a decrease in circulating estrogen, and not a specific effect on a target tissue. Whereas the mechanism of these effects has not been determined, obvious artifactitous phenomena have been excluded as being their cause. These results could have a major impact on the interpretation of past and future experiments of this kind.
authors.affiliation: Division of Biomedical Sciences, University of California, Riverside 92521-0121, USA.
authors.forename: J W
authors.initials: JW
authors.lastname: Liu
authors.forename: D D
authors.initials: DD
authors.lastname: Dawson
authors.forename: C E
authors.initials: CE
authors.lastname: Peters
authors.forename: M A
authors.initials: MA
authors.lastname: Baker
authors.forename: A M
authors.initials: AM
authors.lastname: Walker
mesh_terms: D000818:Animals; D000918:Antibody Specificity; D004958:Estradiol / Q000097:blood; D004967:Estrogens / Q000097:blood* / Q000502:physiology*; D005260:Female; D010052:Ovariectomy; D011374:Progesterone / Q000097:blood*; D011388:Prolactin / Q000097:blood; D011863:Radioimmunoassay; D051381:Rats; D017207:Rats, Sprague-Dawley
chemical_list: D004967:Estrogens; D011374:Progesterone; D004958:Estradiol; D011388:Prolactin