Selective Blockade of Estrogen-Induced Uterine Responses by the Antiestrogen Nafoxidine*

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Abstract
The response of the immature rat uterus to the antiestrogen nafoxidine is maximum at a dose of 5 .mu.g. This dose of nafoxidine sustains the uterine response for at least 72 h. After treatment with 5 .mu.g nafoxidine for 24 h, uterine cytosol contains approximately 1/2 the total number of estrogen receptors originally present in uteri of untreated animals. Uterine nuclei also contain approximately 1/2 the receptors originally present in unstimulated tissue. The total amount of uterine estrogen receptor was not altered 24 h after treatment with nafoxidine relative to saline-treated controls. Estradiol treatment doubled the total amount of receptor relative to controls. Pretreatment with 5 .mu.g nafoxidine for 24 h did not block the uterine response occurring 4 h after the subsequent administration of estradiol, but did block the uterine response occurring 24 h after the subsequent administration of estradiol. Uterine nuclei may contain different acceptor sites for regulating short term and long term uterine responses to estrogen.