Thyroxine and 3,5,3′-Triiodothyronine Bind to the Same Putative Receptor in Hepatic Nuclei ofRana catesbeianaTadpoles*

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Abstract
Previous studies have provided conflicting evidence as to whether tadpole liver nuclei contain the same number of high affinity binding sites for T4 as they do for T3. Inability to prepare stable tadpole liver nuclei may have contributed to these inconsistencies. Thus, a study was carried out in which the in vitro binding of T4 and T3 by tadpole liver nuclei was reexamined with a greatly improved method for isolating liver nuclei. It was found that the maximum binding capacities of the nuclei for T3 and T4 were not significantly different [0.161 .+-. 0.015 (.+-. SEM) vs. 0.185 .+-. 0.046 pmol/mg DNA]. However, the affinity of the binding sites for T3 was 2-3 times their affinity for T4 (Kd, 1.65 .+-. 0.31 vs. 4.32 .+-. 0.92 .times. 10-12 M). Furthermore, the nuclear binding of [125I]T4 or [125I]T3 was decreased to comparable levels by administration of saturating amounts of T3 and T4 given either before or with the 125I-labeled hormone, indicating that both hormones were competing for the same set of sites. It is suggested that at least some of the previous conflicting findings related to the number of putative thyroid hormone receptors in tadpole liver nuclei are attributable to inadequate methodology which resulted in an overestimation of the maximum binding capacity for T4.