Ontogenic relationships between two estrogen binding moieties in the Male rat: Αfetoprotein and the testicular cytoplasmic estrogen receptor

The estrogen binding characteristics of rat αfetoprotein (AFP) and the testicular cytoplasmic estrogen receptor (E_ZR) and the ontogenic relationships between these two estrogen binding moieties were investigated. Sucrose gradient sedimentation analysis revealed that AFP migrated as a single 4.6S peak whereas the receptor migrated as a single peak in the 8 - 9S region. Scatchard analyses of the binding data demonstrated high-affinity (10⁸ M^-1), high-capacity (450 pmol/mg protein) binding sites for AFP-estradiol and high-affinity (10¹⁰ M^-1), low-capacity (16 fmol/mg protein) binding sites for receptor-estradiol. Estradiol binding in serum (AFP) was high at birth, declined during the first 2 weeks and fell to low levels during the fourth week of life. In contrast, the testicular E₂R was not detectable before day 21, rose after day 23, and reached adult concentrations by day 35. The inverse pattern suggested a relationship between the disappearance of AFP and the appearance of the receptor. However, in vivo administration of Dexamethasone, which resulted in a precocious decrease in AFP levels, had no effect on the ontogeny of the receptor. The ontogenic patterns of these two estrogen binding moieties may determine the onset of testicular sensitivity to estrogens.