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Selective Deiodination of the Oldest Iodinated Molecules in the Hypophysectomized Rat^*

Laboratoire d’Endocrinologie Cellulaire, Universite de Provence 13331 Marseille Cedex 3, France

Address requests for reprints to: Dr. Paul Bastiani, Laboratoire d’Endocrinologie Cellulaire Universite de Provence, Centre St. Charles, 3, Place Victor-Hugo, 13331 Marseille Cedex 3, France.

Abstract

Long term (isotopic equilibration) or short term (label injection) kinetic studies of iodine turnover were carried out on intact or hypophysectomized rats previously adapted to an intake of 5 µg iodide daily.

Long term kinetic studies showed that plasma iodide was entirely renewed in 8 days in both hypophysectomized and control rats. However, in hypophysectomized rats, the turnover rate of thyroid iodide was very much slower than that of plasma PBI, whereas in control rats these two turnover rates were similar. This indicates that a deiodination of the oldest molecules, which are mostly unlabeled, occurs in hypophysectomized rats.

Short term kinetic studies showed that in control rats, the labeled fraction of thyroid iodide was always greater than that of plasma iodide. This difference is caused by a recycling of labeled iodide inside the gland, which begins soon after the labeling. Thus, the iodotyrosine residues of all of the thyroglobulin molecules synthesized between time zero and 96 h after label injection can be deiodinated whatever their labeling time. By contrast, in hypophysectomized rats, the labeled fraction of thyroid iodide was similar to that of plasma iodide until 48 h after injection, becoming slightly greater thereafter. This indicates that in hypophysectomized rats, deiodination of labeled molecules becomes significant only 48 h after injection.

These two different types of experiments led to the conclusion that the oldest molecules are selectively deiodinated in hypophysectomized rats. From comparison with the controls, it was concluded that TSH favors the deiodination of newly synthesized molecules. {Endocrinology 106: 883, 1980)

Footnotes

^* This work was supported by grants from the C.N.R.S. (E.R.A. de Biodynamique thyroidienne) and from the INSERM (Contrat Cooperatif sur Programme 77-1-094).

Received March 26, 1979.