During studies of renal tumorigenesis induced by estrogen in Syrian hamsters, we have observed that about 15 to 20% of animals develop bladder lesions with an increased wet weight of tissue from 0.2 g to 0.4 to 1.7 g. Histological examination of the lesions showed a spectrum of changes from inflammatory reactions to squamous metaplasia and intense hyperplasia of the transitional epithelium. The concentration of progesterone-binding sites was increased in the bladders with lesions. No specific progesterone-binding sites could be detected in the cytosol of bladders from hamsters not treated with estrogen. The affinity constant for the progesterone-binding sites in cytosol from bladders with lesions was 109m−1, the same as that reported for progesterone receptors in other target tissues for estrogen. The binding sites are specific for progesterone and are not competed for by 17β-estradiol, 5α-dihydrotestosterone, or aldosterone.

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Supported by Grant CA 16854 from the National Cancer Institute, NIH.

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