Induction of a coupling defect in rats during inhibition of tyrosine dehalogenase

We have reported earlier that administration of 3-nitro-L-tyrosine (MNT), 8 mM in drinking water, to rats receiving a low iodine diet (LID) results in greater TSH secretion, larger goiters, and more rapid uptake and release of radioiodine than LID alone, and ultimately may produce hypothyroidism. These findings have been confirmed, and hypothyroidism documented by demonstrating depressed levels of hepatic mitochondrial alpha-glycerophosphate dehydrogenase. Also, prolonged treatment with MNT + LID produced a depression in labeled iodothyronine (ITh) synthesis, as judged by chromatographic analysis of thyroid digests from rats killed 4 or 24 hours after ip injection of radioiodine, or two weeks after adding radioiodine to drinking fluid. Low thyroidal ITh levels were accompanied by low levels of ITh in serum, despite the presence of various other labeled organic iodine compounds. Cessation of MNT treatment, or ip injection of small amounts (0.5-1.0 mug) of Na 127I together with radioiodine 4 h before sacrifice reversed the defect, and large amounts of ITh were found in both thyroid and serum. Labeled thyroprotein from MNT-treated rats showed increased susceptibility to disaggregation during freezing at pH 8.5; this abnormality was also reversed by stable iodine treatment. In glands labeled with radioiodine 24 h before sacrifice, stable iodine injection 20 h later was followed by increased thyroidal ITh. It is concluded that profound iodine deficiency, induced by MNT + LID, can lead to diminished ITh synthesis, or a "coupling defect". The results provide an explanation for the finding of low thyroidal ITh in patients with hereditary deficiency of tyrosine dehalogenase. The findings confirm an important role for iodine supply in ITh synthesis and thyroglobulin stability, and suggest that rats treated with MNT + LID provide a model for study of the effects of extreme iodine deficiency.