Effects of estradiol-17 alpha on nuclear occupancy of the estrogen receptor, stimulation of nuclear type II sites and uterine growth.

Estriol and estradiol-17 alpha (E2-17 alpha) have classically been described as weak or impeded estrogens since they are incapable of stimulating true uterine growth when administered acutely by single injection. We have demonstrated [16] that estriol is capable of stimulating true uterine growth when the hormone is administered by paraffin implant. The possibility that E2-17 alpha is similar to estriol was examined. A single injection of E2-17 alpha causes a rapid accumulation of the estrogen receptor in uterine nuclei and this is correlated with the stimulation of early uterotropic responses. The nuclear receptor content declines rapidly and no stimulation of nuclear type II sites or true uterine growth is observed. E2-17 alpha does however stimulate the replenishment of cytoplasmic estrogen receptor. This receptor-response profile is typical of a short acting estrogen such as estriol. Chronic exposure (96 h) of mature-ovariectomized rats to estradiol-17 alpha (4 mg) by beeswax implant results in continual nuclear occupancy by estrogen receptors, dramatic stimulation of nuclear type II sites and true uterine growth. It is not possible to determine whether the uterotropic stimulation was due to direct effects of E2-17 alpha since this isomer was partially metabolized to E2-17 beta and both isomers were found in uterine nuclei after an implant of E2-17 alpha. We conclude that E2-17 alpha is capable of acting as an estrogen, either by its inherent estrogenicity or by its conversion to E2-17 beta, and that it may be dangerous to consider this steroid to be an ineffective or inadequate estrogen.