Evidence for thyroid hormone receptors in uterine nuclei

Nuclear preparations from the rat uterus contain specific, high affinity binding sites for T₃, with the properties expected of a thyroid hormone receptor. The K_d value for the binding of T₃ to these sites is 9.3 ± 0.1 × 10^-10 M and the maximum number of binding sites is 0.11 ± 0.02 pmoles T₃ bound per mg of DNA. Competition experiments with Triac, D-T₃, T₄, and rT₃ compared to L-T₃ revealed relative affinities of 87%, 22%, 15%, and 1.6% respectively. T₃ binding sites solubilized from uterine nuclear preparations by high salt extraction sediment at 3.6 S and are destroyed largely by trypsin treatment. Estradiol treatment of ovariectomized rats for 3 days did not alter the quantity of T₃ binding sites expressed on the basis of DNA content. These results suggest that uterine nuclei contain a specific T₃ receptor and raise the possibility that this hormone may have direct effects on the uterus.