Nuclear thyroid hormone receptor in the rat uterus.

The uterus may be a target organ for T3 action. The present study was done to determine whether uterine nuclei contain receptors for T3 (T3R). Methods were established for the measurement of T3R concentrations in preparations of rat uterine nuclei. The liver was studied concomitantly as a tissue known to contain nuclear T3R. Nuclear fractions were prepared by established procedures and extracted in buffer containing 0.5 M KCl. Specific [125I]T3 binding in the nuclear extract was analyzed by Scatchard plot. In both liver and uterine nuclear extracts, a binding component of high affinity for [125I]T3 (Ka = 1.0-3.0 X 10(9) M-1) was detected. This binding component was lost after heating at 50 C for 10 min. On sucrose density gradients, the principal [125I]T3-binding component in uterine and liver nuclear extracts had a 3.5-4.0 S sedimentation coefficient. Binding specificity was assessed by competition assays for [125I]T3 binding by using stable T3, T4, triiodothyroacetic acid (TRIAC), rT3, and 3,5-diiodothyronine (T2). The order of relative binding affinities (RBAs) in uterine nuclear extracts was T3 greater than TRIAC greater than T4 greater than rT3 greater than T2, and that in liver extracts was TRIAC greater than T3 greater than T4 greater than rT3 greater than T2. In contrast, RBA values derived from dilute serum were distinctly different from both uterine and liver values: T4 greater than T3 greater than rT3 greater than TRIAC greater than T2. The concentrations (femtomoles per mg DNA) of T3R in several tissues were: liver, 595 +/- 162; kidney, 492 +/- 120; uterus, 249 +/- 66; spleen, 48 +/- 10 (mean +/- SE; n = 4). These results provide evidence for the presence of T3R in the nuclear fraction of the rat uterus with properties similar to those of the liver nuclear T3R.