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Binding of Thyroid Hormones in vivo by Hepatic Nuclei of Rana catesbeiana Tadpoles^*

VALERIE ANNE GALTON

Department of Physiology, Dartmouth Medical School Hanover, New Hampshire 03755

Abstract

Thyroid hormone is essential for amphibian metamorphosis, and tadpoles develop responsiveness to exogenous T₄ and T₃ during the premetamorphic stage of development. The present studies were performed to investigate the receptors concerned with the initiation of this response. Premetamorphic tadpoles (stages VII-XV of Taylor and Kollros) were injected ip with [¹²⁵I]T:1 or [¹²⁵I]T₄ (0.001–10 nmol/tadpole). Twenty-four hours later, liver and serum were obtained, and organic ¹²⁵I in liver nuclei and serum (shown to be unchanged hormone) was measured. Saturable binding sites for both T₃ and T₄ were present in the liver nuclei. Analysis of binding data indicated for T₃ a mean vjilue for K_d of 1.6 x 10^–12 M (moles of free T₃ per liter plasma) and a mean value for maximum binding capacity of 0.1 ng/mg DNA. For T₄, the mean K_d was 3.9 x 10^–12 M, and the mean maximum binding capacity was 0.5 ng/mg DNA. It was estimated that a significant fraction of these sites was not normally occupied by endogenous hormone.

Properties of the T₃-binding sites were similar in tadpoles at stages x and XV. Stable T₄ and the acetic and propionic acid analogs of T₃ competed with [¹²⁵I]T₃ for the sites almost as readily as did stable T₃. The acetic acid analog of T₄, D-T₄, 3,5- diiodothyronine, and rT₃, less active analogs, were relatively poor competitors. Binding of T₃ to saturable but not to nonsaturable nuclear binding sites was reduced in tadpoles kept at 4 C. On the basis of these findings, it is suggested that these nuclear binding sites are thyroid hormone receptors. (Endocrinology 106: 859, 1980)

Footnotes

^* This work was supported in part by Grant HD-09020 from the NICHHD, Bethesda, MD.

To whom requests for reprints should be addressed.

Received April 3, 1980.

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