Uptake, metabolism and subcellular distribution of [125I]T4 in calf thyroid slices

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Abstract
Iodothyronines have a direct inhibitory action on the thyroid. In the present studies the uptake, metabolism and subcellular distribution of labeled thyroxine [T4] were analyzed. The entry of this hormone reached a plateau after 15-20 min incubation and its temperature-dependent. The T/M [thyroid/medium ratio] values for T4 were much lower than those of labeled T3 [triiodothyronine] and the curve was different from that obtained with [125I]. The influence of a series of compounds on the T/M values for T4 was studied. KI, T3, IOP [iopanoic acid], PTU [propylthiouracil] and MMI [methylmercaptoimidazole] were without effect. Absence of Ca2+ in the buffer, or addition of KCIO4 or ouabain caused a slight decrease, while TSH produced a stimulation in the T/M ratio. Calf thyroid slices dehalogenated T4, producing both T3 and r[reverse]T3. TSH increased the generation of these 2 compounds. Neither PTU nor IOP altered the production of T3 and rT3 significantly. The lack of effect of IOP on T3 and rT3 generation was confirmed in calf thyroid homogenates. However, and in agreement with previous reports, IOP significantly decreased production of both T3 and rT3 in rat liver slices and in rat liver homogenates. When the subcellular distribution of labeled T4 was examined most of the radioactivity appeared in the soluble fraction and less than 1% was present in purified nuclei. The addition of unlabeled T4 to the slices only caused a significant displacement in the radioactivity of purified nuclei. The presence of labeled T4 in the nuclei was confirmed by paper chromatography.