Ontogeny of Steroid Receptors in the Guinea Pig

Abstract

Cytosol and nuclear receptors of various steroid hormones are present during the fetal life of the guinea pig; in particular, estradiol receptors are distributed among several fetal tissues. Although found in such different tissues as uterus, lung, kidney and brain, the physico-chemical properties of the fetal estradiol receptor are similar in all fetal tissues studied. Estradiol binding is of high affinity (Kd = 10−10M), is specific for estrogens, and has a sedimentation co-efficient of 8S in low salt sucrose density gradients and a pi of 6.1 - 6.2. Cytosol and nuclear estradiol receptors are present from mid-gestation (30-34 days) and their numbers increase during development in all fetal tissues studied. They decrease after birth, except in the lung where cytosol receptors continue to increase. There is, however, a vast quantitative difference in the concentrations of estradiol receptors in the various tissues; the concentration in fetal uterus (per mg DNA or per g tissue) is 10 to 100 times higher than in the other fetal tissues. Using the method of cytosol and nuclear exchange, the total number of specific binding sites in fetal uterus has been observed to reach 18-20 pmole per mg DNA of which 13% are occupied by endogenous hormone.