Binding of thyroid hormones in vivo by hepatic nuclei of Rana catesbeiana tadpoles.

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]T3 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 value for Kd 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 Kd was 3.9 x 10(-15 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 non-saturable 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.