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The role of the antioxidant defense system during endometrial receptivity, a phenomenon crucial for implantation and decidualization, and the effect of ormeloxifene, a selective estrogen receptor modulator, were investigated in the guinea pig, a laboratory mammalian species with interstitial implantation and a long functional luteal phase during each estrous cycle. A sharp rise in the activity of superoxide dismutase (SOD) in both antimesometrial (AM) and mesometrial segments and peroxidase in the AM segment of the uterus was observed on the day of maximal endometrial receptivity. Pretreatment with ormeloxifene resulted in loss of endometrial responsiveness, as evidenced by inhibition of trauma-induced decidualization and the activity of ornithine decarboxylase, a marker of tissue growth and repair. This was associated with a decrease in SOD and estradiol dehydrogenase activities, with corresponding increases in estrone dehydrogenase activity and stimulation of uterine luminal epithelial cell height and a distension of the uterine and glandular lumen. A decrease in peroxidase activity was observed only in the AM segment of the uterus on the imminent day of maximal endometrial receptivity. No effect on peripheral plasma progesterone concentration or surface ultrastructure was evident. These findings demonstrate that SOD plays an important role, with peroxidase having a supplementary role, in the first line of defense against superoxide anion radicals during the period of maximal endometrial receptivity in the guinea pig. Inhibition of endometrial receptivity and decidualization by ormeloxifene administered during the pre-receptive phase appears to be due to a depressed antioxidant defense system via dysregulation of redox-sensitive signaling, resulting in altered cellular toxicity due to increased superoxide radicals, and might contribute to the contraceptive action of ormeloxifene. This might be related to its estrogen antagonistic activity and/or decreased bioavailability of estradiol at a cellular level due to its increased metabolism to biologically less-active estrone via activation of estradiol-17 beta-hydroxysteroid dehydrogenase and suppression of estrone-17 beta-hydroxysteroid dehydrogenase.

Antioxidant defense system during endometrial receptivity in the guinea pig: effect of ormeloxifene, a selective estrogen receptor modulator