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The incorporation of 125I into iodolipids was investigated in calf thyroid slices. Time course studies showed that the iodination of lipid reaches a plateau after 30 min of incubation. A highly significant correlation was found between iodination of lipid and of protein (r = 0.906), suggesting that both reactions may be related. Addition of PTU or MMI caused a significant inhibition of lipid iodination, indicating that a peroxidase could be involved in this reaction. The iodinated lipids are not attached to protein, since they migrated in BEA chromatography and pancreatin digestion of the samples did not modify the percentage radioactivity. The iodolipid fraction observed in BEA chromatography was eluted and analysed by TLC. Iodinated free fatty acids and neutral lipids comprised most of the radioactivity. Although iodolipids are present in every subcellular fraction, the 20 000 X g pellet had the greatest proportion of iodinated fatty acids and neutral lipids.

Previous work from our laboratory has shown that iodothyronines have a direct inhibitory action on the thyroid. In the present studies the uptake, metabolism and subcellular distribution of labelled thyroxine were analyzed. The entry of this hormone reaches a plateau after 15-20 min incubation and is temperature-dependent. The T/M values for T4 were much lower than those of labelled T3 and the curve was different from that obtained with [125I]. The influence of a series of compounds on the T/M values for thyroxine was studied. KI, T3, IOP, PTU and MMI were without effect. Absence of Ca++ in the buffer, or addition of KClO4 or ouabain caused a slight decrease, while TSH produced a stimulation in the T/M ratio. Calf thyroid slices dehalogenated thyroxine, producing both T3 and rT3. TSH increased the generation of these two 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 triiodothyronines in rat liver slices and in rat liver homogenates. When the subcellular distribution of labelled thyroxine was examined most of the radioactivity appeared in the soluble fraction and less than 1% was present in purified nuclei. The addition of unlabelled thyroxine to the slices only caused a significant displacement in the radioactivity of purified nuclei. The presence of labelled thyroxine in the nuclei was confirmed by paper chromatography.

Thyroxine dehalogenation by rat pancreatic islets was studied incubating isolated islets with labelled T4. [125I]T4 added to the incubation medium was deiodinated by the islets with the consequent production of T3, rT3 and iodide. This deiodination process showed a clear glucose-dependence, being significantly increased in the presence of 16.6 mmol/l glucose. The existence of high and low affinity binding sites for T3 was also demonstrated incubating [125I]T3 with islets under different experimental conditions. The properties of these binding sites were greatly influenced by the extracellular concentration of glucose. Addition of T3 to the incubation medium, significantly modified the insulin release, but its effect varied according to the glucose concentration in the medium, i.e. it enhanced the insulin release at a glucose concentration between 2 to 8 mmol/l; it has no effect at 12 mmol/glucose, and significantly inhibited the secretion of insulin in the presence of 16.6 mmol/l glucose. Our results suggest that thyroid hormones might play a direct regulatory effect on insulin secretion, probably mediated by its deiodination and interaction with specific receptors in the islet cell.