Abstract

The question of whether thyroxine (T₄) and TRH have a mitogenic effect on pituitary thyrotrophs and somatotrophs in thyroidectomized rats was investigated. Mitoses were counted in hematoxylin–eosin-stained or periodic acid–Schiff–hematoxylin-stained pituitary slides or immunostained for TSH or GH using male rats thyroidectomized for 5 months. Ten days before they were killed groups of rats were injected with different doses of T₄ (0·5, 3 or 10 μg i.m. every second day for 10 days), TRH alone (100 ng s.c. three times a day for 10 days), or T₄ plus TRH (same doses as above). Mitoses (stopped with colchicine) were counted in 1 mm² areas at a magnification of × 1000. In thyroidectomized rats, mitoses were not significantly increased and treatment with TRH or 0·5 μg T₄ alone in thyroidectomized rats did not affect mitotic counts. In thyroidectomized rats treated with higher doses of T₄, mitoses were increased in a dose-dependent fashion. Simultaneous administration of TRH and T₄ had a significant synergistic effect on pituitary mitoses in a T₄ dose-dependent manner. The treatments also had differential effects on the relative percentages of cellular types in mitosis. Thus, 60% somatotrophs and 12·5% thyrotrophs were found in the euthyroid group. In thyroidectomized and thyroidectomized plus TRH groups, no somatotrophs in mitosis were seen, while thyrotrophs were 28·5% and 33·3% respectively. In thyroidectomized rats treated with low doses of T₄, somatotrophs and thyrotrophs in mitosis increased to 38·4% and 80% respectively and, with simultaneous administration of a low dose of T₄ plus TRH, although less effective than T₄ alone, mitosis increased in somatotrophs and thyrotrophs to 11·1% and 54·5% respectively. A high dose of T₄ alone did not increase the mitotic figures in somatotrophs (38·8%), while it diminished the percentage of thyrotrophs to 25%. The administration of high doses of T₄ plus TRH had an opposite effect on the mitotic figures of somatotrophs and thyrotrophs and thus the percentage of somatotrophs increased to 50% while thyrotrophs decreased to 5·5%. Ten days of treatment with T₄ were insufficient to reverse the histology to euthyroidism. It can be concluded that in long-standing hypothyroidism: (1) thyroid hormone replacement elicits a dose-dependent and differential proliferative response on pituitary thyrotrophs and somatotrophs, (2) TRH is devoid of mitogenic effects when administered alone and (3) the proliferative response of somatotrophs to T₄ is enhanced by its co-administration with TRH, suggesting a permissive and/or synergistic effect of the thyroid hormone and TRH.

Journal of Endocrinology (1997) 154, 149–153

The present study has demonstrated the presence of circulating reverse tri-iodothyronine (rT3) levels and its main generating pathway in rainbow trout. A specific rT3, RIA using thyronine-stripped sera in the standard curve was standardised, allowing precise and accurate quantification of radioimmunoassayable rT3. We also demonstrated that trout skin is an important source of rT3 production. T3 or thyroxine trout skin inner-ring deiodination (IRD) activity was assessed by using rT3 RIA and/or paper chromatography. The kinetic characterisation of this deiodinative pathway disclosed a typical deiodinase type III (DIII) enzyme, except for its conspicuous thermodependency which attained its maximal catalytic efficiency at 15 degrees C. This finding suggested the expression of enzymatic variants, which is a common functional array in teleosts. Both circulating rT3 and DIII activity were present in juvenile and adult individuals and were inversely correlated with age, weight and length. In conclusion, this study demonstrated that (1) skin IRD activity and its product rT3 are present throughout the development of rainbow trout, and (2) trout skin DIII activity attains its higher catalytic efficiency in the physiological range of temperature for this species, thus suggesting the expression of enzymatic variants.

The initial characterization of a thyroid iodotyrosine dehalogenase (tDh), which deiodinates mono-iodotyrosine and di-iodotyrosine, was made almost 50 years ago, but little is known about its catalytic and kinetic properties. A distinct group of dehalogenases, the so-called iodothyronine deiodinases (IDs), that specifically remove iodine atoms from iodothyronines were subsequently discovered and have been extensively characterized. Iodothyronine deiodinase type 1 (ID1) is highly expressed in the rat thyroid gland, but the co-expression in this tissue of the two different dehalogenating enzymes has not yet been clearly defined. This work compares and contrasts the kinetic properties of tDh and ID1 in the rat thyroid gland. Differential affinities for substrates, cofactors and inhibitors distinguish the two activities, and a reaction mechanism for tDh is proposed. The results reported here support the view that the rat thyroid gland has a distinctive set of dehalogenases specialized in iodine metabolism.

Iodide is a trace element and a key component of thyroid hormones (TH). The availability of this halogen is the rate-limiting step for TH synthesis; therefore, thyroidal iodide uptake and recycling during TH synthesis are of major importance in maintaining an adequate supply. In the rat, the thyroid gland co-expresses a distinctive pair of intrathyroidal deiodinating enzymes: the thyroid iodotyrosine dehalogenase (tDh) and the iodothyronine deiodinase type 1 (ID1). In the present work, we studied the activity of these two dehalogenases in conditions of hypo- and hyperthyroidism as well as during acute and chronic iodide administration in both intact and hypophysectomized (HPX) rats. In order to confirm our observations, we also measured the mRNA levels for both dehalogenases and for the sodium/iodide symporter, the protein responsible for thyroidal iodide uptake. Our results show that triiodothyronine differentially regulates tDh and ID1 enzymatic activities, and that both acute and chronic iodide administration significantly decreases rat tDh and ID1 activities and mRNA levels. Conversely, both enzymatic activities increase when intrathyroidal iodide is pharmacologically depleted in TSH-replaced HPX rats. These results show a regulatory effect by iodide on the intrathyroidal dehalogenating enzymes and suggest that they contribute to the iodide-induced autoregulatory processes involved in the Wolff–Chaikoff effect.