The fetal thymus of guinea pig as an estrogen target organ.

[3H]Estradiol ([3H]E2) shows a saturable, high affinity binding (Kd = 0.18 nM) in the cytosol of the guinea pig fetal thymus. The association rate constant at 4 C of [3H]E2 to cytoplasmic receptor is 1.0 X 10(5) M-1 sec-1. The dissociation rate constant of this complex is 4.4 X 10(-6) sec-1 at 4 C and 3.3 X 10(-4) sec-1 at 26 C. Other physicochemical characteristics, such as binding specificity, which is limited to natural and synthetic estrogens, and sedimentation coefficient, which is 8S under low salt conditions and 4S under high salt conditions, are typical for estrogen receptor. The [3H]E2-cytoplasmic receptor complex chromatographed in DEAE-Trisacryl columns is eluted in a continuous 0- to 0.5-M KCl gradient by 0.15- to 0.2-M KCl concentrations. The cytoplasmic [3H]E2-receptor complex can be transferred to the nucleus in vitro by a temperature-dependent process; it can be extracted from the nucleus by 0.4 M KCl as a 5S sedimenting macromolecule. The levels of available cytoplasmic receptors increase during fetal development from 9.7 fmol/mg protein at 36 days of gestation to 31 fmol/mg protein at the end of gestation. Three or six daily injections of E2 or estrone (1 mg/kg BW) to pregnant animals (58-64 days gestation) significantly decreased the weight of the fetal thymus by 50% with respect to values in vehicle-injected animals. Histologically, a reduction in the size of thymic lobules with a decrease in the width of the cortical lymphoid area and an increase in the medulla-cortex ratio was observed. The in vitro incorporation of [3H]thymidine into thymic DNA was reduced by 50% after E2 treatment. The same estrogen treatment also induced a decrease in cytoplasmic estrogen receptors by 63-72% and a concomitant 5- to 6-fold increase in nuclear estrogen receptors, with respect to values in vehicle-injected animals. It is concluded that estrogen receptors are present in the fetal thymus of the guinea pig, and that this fetal organ is estrogen responsive.