Subcellular Localization and Binding of ¹²⁵I-Triiodothyronine in Calf Thyroid

Summary

The subcellular distribution of ¹²⁵I-T₃ was studied in calf thyroid slices, under the same experimental conditions where T₃ inhibits protein and RNA synthesis, labelled hormone was found mainly in the 20,000 × g supernatant. The specificity of each subcellular localization was determined by incubating the slices with 10^-5M T₃. Only in the purified nuclei a significant decrease was found, indicating a specific localization of the labelled hormone. When slices were incubated with ¹²⁵I both labelled T₃ and T₄ were found in purified nuclei, indicating that endogenously synthetized hormones can reach thyroid nuclei.

Purified thyroid nuclei were incubated with labelled T₃ and increasing amounts of cold hormone. Specific binding reached a plateau after 90 min of incubation at 20°C. When the displacement curves were analysed by a Scatchard plot a binding site with a Ka of 5.2 × 10⁷ M^-1 and a capacity of 3.0 × 10^-15 moles/μg DNA was observed. Digestion of nuclei with trypsin and protease abolished completely the binding of ¹²⁵I-T₃ thus indicating the protein nature of the receptor. The hormone-receptor complex could be extracted with 0.4M KCI and eluted in the void volume after Sephadex G-25 column chromatography, similar to peripheral tissues nuclear T₃ receptors.

The present studies provide the first evidence for the existence of nuclear receptors for T₃ in the thyroid, an event probably related to the autoregulatory mechanism.

Key-Words

Thyroid - Triiodothyronine - Receptors - Autoregulation