Putative nuclear triiodothyronine receptors in tadpole liver during metamorphic climax

It has been shown previously that the maximum binding capacity (MBC) of the putative T3 receptors in tadpole red blood cells (RBCs) is increased during development and can be stimulated by treatment with thyroid hormone (TH). The present study was performed to determine if the MBC of tadpole liver nuclei is also increased during development or after treatment with TH. Because of the relatively high levels of endogenous TH in tadpoles during climax, the use of an in vivo saturation assay employing [125I]T3 was not feasible. Thus, MBC was determined by measuring by RIA the amount of T3 bound to the liver nuclei in tadpoles pretreated with sufficient T3 to saturate the receptors. Values were then corrected for the nonsaturable fraction using data obtained in tadpoles given a large dose of T3 (10 nmol). After this dose, essentially all of the T3 in the nucleus was bound to nonsaturable sites. MBC values estimated by this method and by Scatchard analysis were comparable. In contrast to the observations in tadpole RBCs, no significant change in the MBC of liver nuclei occurred as the tadpole progressed from early prometamorphosis to metamorphic climax; in tadpoles at stages XII-XIV and XIX-XXIII, MBC values were 0.308 +/- 0.024 (+/- SE) and 0.260 +/- 0.035 pmol/mg DNA, respectively. Furthermore, treatment of tadpoles with T4 (1 nmol T4; 14 days before study), which resulted in a marked increase in receptor number in RBCs, had no effect on MBC in hepatic nuclei. The amounts of nucleus-bound endogenous T3 in liver and RBCs were also determined. From these data and the MBC values, it was calculated that hepatic and RBC nuclear receptors were, respectively, 80% and more than 90% occupied with T3. These findings indicate that there is tissue specificity in the response of receptor MBC to TH during metamorphosis, and that most of the TH on the receptor during climax is T3.