Binding of Thyroid Hormones in Vivo by Hepatic Nuclei of Rana catesbeiana Tadpoles at Different Stages of Metamorphosis*

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Abstract
Studies in this laboratory have demonstrated, using in vivo techniques, that hepatic nuclei from premetamorphic Rana catesbeiana tadpoles contain high affinity, low capacity binding sites for thyroid hormone (1). It was postulated that these sites are hormone receptors (1). Similar techniques were used in the present study to investigate the number and properties of these putative receptors during prometamorphosis and metamorphic climax. No significant differences were observed in either the dissociation constants (Kd) or the maximum binding capacities (MBC) of the T3-binding sites in hepatic nuclei from pre- and prometamorphic tadpoles, indicating that there was no change in either the number or affinity of the sites during these developmental stages. Furthermore, it was estimated that the sites were only partly occupied with endogenous hormone before developmental stage XX. In contrast, binding of [125I]T3 or T4 to saturable sites decreased to insignificant levels during metamorphic climax. However, it is known that plasma levels of endogenous thyroid hormone increase substantially during climax, reaching concentrations that would result in saturation of the hepatic binding sites in premetamorphic tadpoles. In the present study, binding of [125I]T3 to hepatic nuclei was greatly diminished in premetamorphic tadpoles pretreated with sufficient stable T4 or T3 to cause near or complete saturation of the binding sites. The plasma concentrations of T3 and T4 attained in these experiments were well below the maximum levels observed during metamorphic climax. It was also shown that the rate of dissociation of T3 from the sites was such that significant exchange of bound and unbound hormone would not have occurred. It was concluded that the decrease in binding of 125I-labeled hormone by the hepatic nuclear sites during climax is related to the increased plasma levels of endogenous hormone. The levels are high enough to both saturate the number of hepatic sites present in late premetamorphosis and reduce the specific activity of the injected 125I-labeled hormone. Unless there is a substantial increase in the number of sites, it is likely that both phenomena would have contributed to the diminished binding of labeled hormone in these experiments.

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