Receptor and biological response to estriol in the fetal uterus of guinea pig

Biochemical and histological effects of vaginal estriol and estradiol applications on the endometrium, myometrium and vagina of postmenopausal women

Estrogen and progesterone receptors in the cytosol (ERc, PRc) and estrogen receptors in the nuclear compartment (ERn) were measured in the endometrium, myometrium and vagina of 29 postmenopausal women who underwent hysterectomy. The effects of vaginal estriol (0.5 mg daily) compared to 17 beta-estradiol (0.05 mg daily) therapy on these receptor levels were studied. In addition, the endometrium was examined by light microscopy for estrogenic stimulation. We found biochemical and histological signs of estrogenic stimulation in all three tissues after estradiol as well as estriol therapy. In the vagina the effect of both estrogens on the ERc concentration was different from that in the endometrium and myometrium. The effects of estradiol and estriol on the ERn were comparable in all three tissues. The PRc levels increased significantly in all tissues after estrogen therapy; in the myometrium it was significantly higher after estriol than after estradiol applications. In conclusion, there were no clear differences between vaginal estradiol and estriol medication with regard to the effects on receptor levels in vaginal and uterine tissues. In the histological studies at the light microscopy level similar signs of estrogen stimulation of the endometrium were found following estradiol and estriol medication.

The agonistic and antagonistic actions of estriol

Estriol is a short acting estrogen, and as such, displays both agonistic and antagonistic properties, when it is injected in saline solution. These are predictable attributes of any short acting agonist if one considers the basic pharmacology of receptor ligand binding interactions. That is, short acting hormones or drugs occupy receptor sites for protracted time periods and, although they stimulate responses, these are of short duration. However, when estriol is administered in continuous fashion the estrogen receptor is occupied for long periods of time and full estrogenic responses are observed, and there is no antagonism of estradiol action. The purpose of this paper is to review the work that leads to the above conclusion and to discuss our more recent work on the relationship between estrogen receptor binding and the stimulation of type II estrogen binding sites.

Biological responses and histological studies of estriol and estriol sulfate in fetal and newborn uteri of guinea pig

The biological responses of estriol (E3) and of estriol-3-sulfate (E3-S) in the fetal and newborn uteri of guinea pig were studied. After a treatment of E3 (1 mg/kg/day) or E3-S (1.4 mg/kg/day) to pregnant guinea pigs (49-60 days of gestation) for 6 days, both estrogens provoke a significant uterotrophic effect in the fetal uterus which increases in weight 1.8-2.5 times in relation to the non-treated animals. The stimulation of progesterone receptor (PR) is also very intense, 7-12 times in relation to the control animals. In another series of experiments newborn guinea pigs (2-day old) were treated with 100 micrograms/animal of E3 or 140 micrograms/animal of E3-S for a short (2 days) or a long (12 days) period. Concerning the uterotrophic effect, the weight of the uterus increases 1.8-2.5 times (in relation to the non-treated animals) after a 2-day treatment and the effect continues to increase up to 4-5 times in the 12-day treated animals. In opposition to the fetal uterus, the effect on PR provoked by E3 or E3-S is very limited (only an increase of 1 time in relation to the non-treated animals); the effect is even less intense after the 12-day treatment. Histological studies show an intense hypertrophic effect particularly in the epithelium of the endometrium in the fetal and newborn uteri for both E3 and E3-S. In the newborns the effect is also intense on the epithelium of the uterine gland. It is concluded that: (1) Estriol and estriol sulfate are very active and with similar intensity on the uterine weight before and after birth; (2) The stimulatory effect on PR decreases very significantly after birth and after a long treatment; (3) E3-S can act as a potent hormonal precursor.