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PubMed1975

The iodine requirement and influence of iodine intake on iodine metabolism and thyroid function in the adult beagle.

Various aspects of iodine metabolism were studied in adult beagles maintained at iodine intake levels ranging from 480 to 20 mug/day. On the basis of changes in radioiodine metabolism, the minimum daily iodine requirement of the adult beagle was found to be 140 mug. Although striking changes were observed in radioiodine metabolism when iodine intake was reduced to 90mug/day, serum T4 and T3 levels were unaffected. Marked reductions in serum T4 occurred in dogs restricted to an iodine intake of 50 or 20 mug/day, but even at these low levels of iodine intake there were only slight reductions in serum T3 concentration and a eumetabolic state was maintained. Prolonged iodine deficiency (8-12 months) resulted in variable patterns of thyroid histology, which were related to differences in thyroidal 127I content and in the rate of release of radioiodine from the thyroid. The heterogeneity in thyroid morphology and iodine kinetics did not, however, have a significant effect on serum T4 and T3 levels.

Animals↗

PubMed1976

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.

Animals↗

PubMed1976

Metabolism of radioiodinated bovine parathyroid hormone in the rat.

Metabolism of bovine 125I-labeled parathyroid hormone was studied in the rat by gel filtration and by sequence analysis of the iodinated fragments. Analysis of the kinetics of hormone metabolism shows that iodinated intact hormone has a multiexponential disappearance curve with a rapid (3 min) initial and a slower (48 min) second component. Iodinated fragments, which rapidly increase during the first 12 min after injection of the intact hormone, subsequently disappear from the circulation with a t1/2 of no greater than 48 min. Plasma samples collected at various time-intervals after intravenous injection of bovine 125I-labeled parathyroid hormone were gel filtered on Bio-gel P-100. Four radioactive peaks were seen. The first and second peaks eluted, respectively, at the void volume of the column and at the position of intact hormone. The third peak consisted of iodinated fragments, and the last peak eluted at the salt volume of the column. Sequence analysis of the iodinated fragments in the third peak showed that it was heterogeneous, containing several different, but closely related, polypeptides. Before 48 min after injection, the most-abundant fragment is one whose amino-terminal amino acid is residue 34. The amino-terminal residue of the next most-common fragment is the amino acid at position 37. No fragments representing cleavages closer to the amino-terminus than residue 34 were seen. The results of these studies are virtually identical with those previously obtained in the dog. The similarities found in the sites of hormone proteolysis and in the kinetics of hormone metabolism in the rat and dog, coupled with the less direct evidence indicating that similar cleavages are also present in man and bovine, are consistent with the view that proteolysis of parathyroid hormone is peripheral tissues is specific, at least in mammalian species, and may be a critical step in controlling the availability of biologically active hormone.

Animals↗

PubMed1966