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 Kd value for the binding of T3 to these sites is 9.3 +/- 0.1 x 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.