Progestin-Like Effects of Danazol on Rabbit Uterus*

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Abstract
The action of danazol on cytosol and nuclear progestin and estrogen receptor concentrations and on the induction of uteroglobin synthesis were studied in the rabbit uterus in vivo. In addition, the relative binding affinity of danazol for the uterine progestin receptor was measured in vitro. Administration of increasing doses of danazol (10, 50, or 100 mg/g BW daily) for 5 days to adult rabbits tended to decrease the concentrations of cytosol and nuclear progestin and estrogen receptors, whereas the uterine uteroglobin content increased with increasing doses of danazol. Progesterone (1 mg/kg BW daily) caused changes which were greater than those caused by the largest dose of danazol. Danazol was found to bind to the uterine progestin receptor in vitro with an affinity approximately 3.3% of that of progesterone. These findings suggest that danazol has a profound progestin-like activity in the rabbit uterus which may be mediated through binding to cytosol progestin receptors.