The thymus of the sexually immature female DDS mouse contains about 2.4 × 1010 estradiol binding sites/thymus which bind with an apparent dissociation constant of about 2.3 × 10-10M. Estrogen binding sites in the glucocorticoid-involuted thymus are similar in number and binding affinity. The putative estrogen receptor sediments on sucrose density gradients with a velocity of about 8S and binding of estradiol can be completely blocked by the addition of a 100 fold molar excess of diethylstilbestrol. Estrogen receptor levels in male mice were about 1 × 109 sites/thymus. Estrogen binding properties were investigated in thymuses of 11 inbred strains of mice, including 6 strains with a high incidence of lymphoma. The number of estradiol binding sites ranged from 0.6-5.5 × 1010 sites/thymus. The mean number of sites in mice with a high incidence of lymphoma was not significantly different from that observed in strains with low incidence. However, with respect to number of binding sites/thymus, there were two discernable groups of mice. The high line group includes strains DBA/2J, SWR/J, C58/J and SJL/J and has a mean binding value of 4.7 × 1010 sites/thymus. The low line group has a mean binding value of 1.7 × 1010 sites/thymus and includes strains A/J, C57BL/6J, DDS, AKR/J, Balb/c Dub, C3H/FeJ and RF/J. When female DDS mice were injected with 1 μg estradiol or 5 μg testosterone/day for 3 days, no decrease in thymic weight was observed. Estradiol, but not testosterone, caused a 3-4 fold increase in thymic glycerol-3-phosphate dehydrogenase. Thymic glycerol-3-phosphate dehydrogenase in male mice did not increase under the influence of estradiol. Estradiol also stimulated the incorporation of [3H]-uridine into RNA in thymuses of 3 wk old female mice. The incorporation of [3H]-thymidine was inhibited about 40%. No effect upon [3H]-leucine incorporation was observed although estradiol did cause a slight, but significant, decrease in total thymic protein content.
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