Binding by thyroid hormones by nuclei of cells from bullfrog tadpole tail fins

The regression of the tadpole tail is under the direct control of the thyroid hormones and offers a unique system for the study of the action of these hormones. We have examined the binding of L-triiodothyronine (T3) and L-thyroxine (T4) in vitro using tail fin bricks which included epidermal and connective cells. The binding of 125I-labeled hormones was followed in both nuclear and extranuclear fractions. High affinity and limited capacity sites for T3 and T4 were observed only for the nuclear fraction. Scatchard plots gave similar apparent dissociation constants for both hormones, about 10(-10)M. The maximum number of binding sites per nucleus for T3 was about 1500 and for T4 about 800. There was no significant change in the chemical identity of [125I]T3 and [125I]T4 which was associated with binding in the nuclei. Nuclear binding of [125I]T4 was inhibited competitively for both unlabeled T3 and T4, but unlabeled T3 displaced [125I]T3 significantly more than unlabeled T4. Thus, both binding and competition data support the conclusion that tadpole tail nuclei had more T3 than T4 binding sites.