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

The binding of 3H-estriol was examined in the fetal uterus of guinea pig. The physico-chemical characteristics of the binding of 3H-estriol to macromolecules are similar to the typical receptor protein for estrogens. Different estrogens (estriol, estradiol, estrone and diethylstilbestrol) compete with this binding but progesterone and testosterone have no effect. The binding affinity has a Kd of 5.5 +/- 1.6 +/- 10(-10) M. By ultracentrifugation in sucrose gradient, two specific components with sedimentation coefficients of 8 and 4S are found. Competition studies suggest that the same specific binding sites may be present for estriol (E3) and for estradiol. The s.c. administration of E3 to the pregnant guinea pig (1 mg/day per kg body weight for 3 days) provokes two biological responses in the fetal uterus: a uterothopic effect and a significant increase in the progesterone receptor. The increase in the fetal uterine weight is 50-70% in relation to the non-treated animals and the progesterone receptor concentration is 10-14 times higher than in the control animals. These effects are similar (or slightly higher) than in animals primed with equimolecular quantities of estradiol. In contrast, single daily injections of E3 to newborn guinea pig, results only in weak uterotrophic activity. It is concluded that estriol is capable of causing a biological response in the uterus during intra-uterine life.