Functional state of the thyroid gland following exposure to ultrasound

Changes are described in the content of butanol-extracted iodine in the blood serum and total iodine in thyroid tissue of white rat males under ultrasonic effect of various intensity (0.2, 0.6, 1.0 Watt/cm2) and analysed at different time intervals after ultrasound application. A decrease in total iodine content in thyroid glands (at initial level 0.315 mg%) and an increase of butanol-extracted iodine content in the blood serum were observed in all the experimental groups. The data obtained compared with the earlier found increase of the thyroid hormone content in the serum and thyreotropic hormone at a decreased content of thyroxins in the thyroid gland seem to point to the activation of the thyroid function after the application of ultrasound.     

The effect of a low dose of ionizing radiation and of gangliosides on the adenylate cyclase activity of the chicken thymus and thyroid in ontogeny

Adenylate cyclase (AC) activity was studied in thymus and thyroid gland of intact chick embryos and those irradiated with a dose of 0.029 Gy prior to incubation, and newly hatched chicks in the presence of total ganglioside fractions extracted from the same organs. Gangliosides were shown to increase the enzyme activity of thymocytes and thyrocytes during the postnatal development. It is suggested that small radiation doses potentiate the stimulatory effect of ganglioside fractions on AC.     

Iodine-Induced Thyroid Blockade: Role of Selenium and Iodine in the Thyroid and Pituitary Glands

The purpose of this study was to determine the content of iodine and selenium in the thyroid and pituitary glands of rats under iodine-induced blockade of the thyroid gland. Electron probe microanalysis, wavelength-dispersive spectrometry, and point analysis were used in this investigation. We also determined the expression of sodium iodide symporter and caspase 32 in the thyroid and pituitary glands and the expression of thyroid-stimulating hormone in the pituitary. The samples for iodine analysis must be thoroughly dehydrated, and for this purpose, we developed a method that produced samples of constant mass with minimal loss of substrate (human thyroid gland was used for the investigation). Normal levels of iodine and selenium were found in the thyroid, pituitary, ovaries, testes hypothalamus, and pancreas of healthy rats. The levels of iodine and selenium in I- or Se-positive points and the percentage of positive points in most of these organs were similar to those of controls (basal level), except for the level of iodine in the thyroid gland and testes. Blockade of the thyroid gland changed the iodine level in iodine-positive points of the thyroid and the pituitary glands. On the sixth day of blockage, the iodine level in iodine-positive points of the thyroid gradually decreased to the basal level followed by an abrupt increase on the seventh day, implying a rebound effect. The opposite was found in the pituitary, in which the level of iodine in iodine-positive points increased during the first 6 days and then abruptly decreased on the seventh day. Expression of the thyroid-stimulating hormone in the pituitary decreased during the first 5 days but sharply increased on the sixth day, with a minimum level of iodine in the thyroid and maximum in the pituitary, before normalization of the iodine level in both glands preceding the rebound effect. The expression of sodium iodide symporter increased during the first 4 days of blockage and then decreased in both glands. The fluctuations of the thyroid-stimulating hormone in the pituitary gland reflected the changes of iodine in the thyroid gland more precisely than the changes of sodium iodide symporter. The selenium level in the selenium-positive points changed only in the pituitary, dropping to zero on the second and fifth day of the blockade. Simultaneously, the maximum induction of caspase 32 was observed in the pituitary gland. We believe that these results may help to clarify a role of the pituitary gland in the thyroid blockade.     

The trace element selenium and the thyroid gland.

Apart from the essential trace element iodine, which is the central constituent of thyroid hormones, a second essential trace element, selenium, is required for appropriate thyroid hormone synthesis, activation and metabolism. The human thyroid gland has the highest selenium content per gram of tissue among all organs. Several selenocysteine-containing proteins respectively enzymes are functionally expressed in the thyroid, mainly in thyrocytes themselves: three forms of glutathione peroxidases (cGPx, pGPx, and PH-GPx), the type I 5-deiodinase, thioredoxin reductase and selenoprotein P. The thyroidal expression of type II 5-deiodinase still is controversial. As thyrocytes produce H2O2 continuously throughout life an effective cell defense system against H2O2 and reactive oxygen intermediates derived thereof is essential for maintenance of normal thyroid function and protection of the gland. In experimental animal models long-term and strong selenium deficiency leads to necrosis and fibrosis after high iodide loads. Combined iodide and selenium deficiency such as in central Zaire is thought to cause the myxedematous form of endemic cretinism. Inadequate selenium supply and prediagnostically low serum selenium levels are significantly correlated with the development of thyroid carcinoma and other tumors. Though selenium supply controls expression and translation of selenocysteine-containing proteins no direct correlation is found between selenium tissue content and expression of various thyroidal selenoproteins, indicating that other regulatory factors contribute to or override selenium-dependent expression control, e.g., in thyroid adenoma, carcinoma or autoimmune disease. As both trace elements, iodine and selenium, were washed out from the upper layers of the soil during and after the ice ages in many regions of the world adequate supply with these essential compounds needs to be provided either by a balanced diet or supplementation.     

Role of the hypophysis in thyroid regeneration after partial thyroidectomy.

The role of the hypophysis in thyroid regeneration was investigated by measuring the mitotic activity of the thyroid remnant in hemithyroidectomized rats as well as the blood levels of thyroid hormone at various time-intervals after hemithyroidectomy. Mitotic activity underwent a significant increase to reach a peak (a 5- to 8- fold increase) 2 days after hemithyroidectomy. The thyroid hormone level in blood was lower than in controls. Histologically, the thyroid gland showed signs of an elevated rate of functional activity, as indicated by losses of colloid and cell hypertrophy. In a second approach, the mitotic activity of the thyroid remnant was estimated in hypophysectomized and in thyroxine treated rats. Both hypophysectomy and thyroxine injection prevented occurrence of the mitotic peak at 2 days. The regeneration of the thyroid after hemithyroidectomy, as it occurred in the present work, may be explained by a release of thyroid stimulating hormone from the pituitary, brought about by the low level of circulating thyroid hormone, itself resulting from a loss of thyroid tissue.     

Cholinergic and VIPergic effects on thyroid hormone secretion in the mouse

The thyroid gland is known to harbor cholinergic and VIPergic nerves. In the present study, the influences of cholinergic stimulation by carbachol, cholinergic blockade by methylatropine and stimulation with various VIP sequences on basal, TSH-induced and VIP-induced thyroid hormone section were investigated in vivo in mice. The mice were pretreated with 125I and thyroxine; the subsequent release of 125I is an estimation of thyroid hormone secretion. It was found that basal radioiodine secretion was inhibited by both carbachol and methylatropine. Furthermore, TSH-induced radioiodine secretion was inhibited already by a low dose of carbachol. Moreover, a high dose of carbachol could inhibit VIP-induced radioiodine secretion. Methylatropine did not influence TSH- or VIP-stimulated radioiodine secretion, but counteracted the inhibitory action of carbachol on TSH- and VIP-induced radioiodine release. In addition, contrary to VIP, six various synthesized VIP fragments had no effect on basal or stimulated radioiodine release. It is concluded that basal thyroid hormone secretion is inhibited by both cholinergic activation and blockade. Furthermore, TSH-induced thyroid hormone secretion is more sensitive to inhibition with cholinergic stimulation than is VIP-induced thyroid hormone secretion. In addition, the VIP stimulation of thyroid hormone secretion seems to require the full VIP sequence.     

The participation of thyroid hormones in modifying the mutagenic effect of microwaves

The mutagenic effect of microwaves (2,450 or 2,750 MHz, 500 microW/cm2, 30 days, 7 h a day) increases with both low and high thyroid hormone content in rats. This indicates that normal functioning of the thyroid gland is an important condition for the stabilization of chromosome integrity under the effect of nonionizing radiation of microwaves.     

Effects of thyroid hormone deficiency and replacement on rat hypothalamic growth hormone (GH)-releasing hormone gene expression in vivo are mediated by GH

The role of thyroid hormone and GH in the regulation of hypothalamic GH-releasing hormone (GRH) gene expression in the rat was examined after the induction of thyroid hormone deficiency by thyroidectomy. Thyroidectomy resulted in a time-dependent decrease in hypothalamic GRH content, which was significant by 2 weeks postoperatively, and a reduction in pituitary GH content to 1% of the control level by 4 weeks. In contrast, GRH secretion by incubated hypothalami under both basal and K(+)-stimulated conditions was increased after thyroidectomy. Hypothalamic GRH mRNA levels also exhibited a time-dependent increase, which was significant at 1 week and maximal by 2 weeks after thyroidectomy. Administration of antirat GH serum to thyroidectomized rats resulted in a further increase in GRH mRNA levels. T4 treatment of thyroidectomized rats for 5 days, which also partially restored pituitary GH content, lowered the elevated GRH mRNA levels. However, comparable effects on GRH mRNA levels were observed by rat GH treatment alone. These results suggest that the changes in hypothalamic GRH gene expression after thyroidectomy in the rat are due to the GH deficiency caused by thyroidectomy, rather than a direct effect of thyroid hormone on the hypothalamus, since the changes were reversible by GH alone despite persistent thyroid hormone deficiency. In addition, they further support the role of GH as a physiological negative feedback regulator of GRH gene expression.     

Induction of epidermal growth factor by tri-iodo-L-thyronine in the submandibular glands of mice with testicular feminization

The content of epidermal growth factor (EGF) as a high molecular weight complex (HMW-EGF) in the submandibular glands of mice was measured simply by a single radial immunodiffusion method. In female mice, the amount of HMW-EGF was increased 10-fold by tri-iodo-L-thyronine (T3) and 60-fold by 5 alpha-dihydrotestosterone (5 alpha-DHT). In mice with testicular feminization (Tfm), which are genetically deficient in androgen receptor, T3 but not 5 alpha-DHT increased the HMW-EGF from a non-detectable level to 5.4 +/- 0.94 micrograms/mg protein. It was concluded that EGF is also synthesized under the control of thyroid hormone in vivo, and that androgen was not involved in this induction of EGF by thyroid hormone.     

One of the duplicated matrix metalloproteinase-9 genes is expressed in regressing tail during anuran metamorphosis

The drastic morphological changes of the tadpole are induced during the climax of anuran metamorphosis, when the concentration of endogenous thyroid hormone is maximal. The tadpole tail, which is twice as long as the body, shortens rapidly and disappears completely in several days. We isolated a cDNA clone, designated as Xl MMP-9TH, similar to the previously reported Xenopus laevis MMP-9 gene, and showed that their Xenopus tropicalis counterparts are located tandemly about 9 kb apart from each other in the genome. The Xenopus MMP-9TH gene was expressed in the regressing tail and gills and the remodeling intestine and central nervous system, and induced in thyroid hormone-treated tail-derived myoblastic cultured cells, while MMP-9 mRNA was detected in embryos. Three thyroid hormone response elements in the distal promoter and the first intron were involved in the upregulation of the Xl MMP-9TH gene by thyroid hormone in transient expression assays, and their relative positions are conserved between X. laevis and X. tropicalis promoters. These data strongly suggest that the MMP-9 gene was duplicated, and differentiated into two genes, one of which was specialized in a common ancestor of X. laevis and X. tropicalis to be expressed in degenerating and remodeling organs as a response to thyroid hormone during metamorphosis.     

Enhanced differentiation of sexually dimorphic organs in L-thyroxine treated Tfm mice

Summary In the present study we examined the effects of L-thyroxine on the differentiation of three sexually dimorphic organs in mice affected by the testicular feminization mutation (Tfm): the submaxillary salivary glands, the kidneys, and the adrenal glands. In the salivary gland we compared the effects of L-thyroxine and testosterone in normal and Tfm mice on the morphology of the intercalated tubules and on the NGF content of this gland. In the kidney we examined thyroid hormone effects on the sexually dimorphic proximal convoluted tubules. In the adrenal gland we studied the effects of the same hormone on the weight of the organ and on the structure and ultrastructure of the zona reticularis in untreated and L-thyroxine treated Tfm mice. The results show that the thyroid hormone enhances differentiation of the dimorphic structures in all these organs and increases the secretory activity of the salivary and adrenal glands. The stimulatory effects of L-thyroxine are more pronounced on the weight of the adrenal than of the salivary gland and of the kidney. Our results raise the question whether the enhanced differentiation elicited by L-thyroxine in the three sexually dimorphic organs of Tfm mutants is due to a direct hormonal effect, or whether it is fully, or in part, mediated through the hypothalamo-pituitary-thyroid axis.This investigation was supported by the Progetto Finalizzato Biologia della Riproduzione of the Italian National Council of Research (CNR)     

Tadpole competence and tissue-specific temporal regulation of amphibian metamorphosis: Roles of thyroid hormone and its receptors

Amphibian metamorphosis is a post-embryonic process that systematically transforms different tissues in a tadpole. Thyroid hormone plays a causative role in this complex process by inducing a cascade of gene regulation. While natural metamorphosis does not occur until endogenous thyroid hormone has been synthesized, tadpoles are competent to respond to exogenous thyroid hormone shortly after hatching. In addition, even though the metamorphic transitions of individual organs are all controlled by thyroid hormone, each occurs at distinct developmental stages. Recent molecular studies suggest that this competence of premetamorphic tadpoles to respond to the hormone and the developmental stage-dependent regulation of tissue-specific transformations are determined in part by the levels of thyroid hormone receptors and the concentrations of cellular free thyroid hormone. In addition, at least two genes, encoding a cytosolic thyroid hormone binding protein and a 5-deiodinase, respectively, are likely to be critical players in regulating cellular free thyroid hormone concentrations. This review discusses how all of these molecuar components coordinate to induce amphibian metamorphosis in a correct spatial and temporal manner. These studies provde us with general clues as to how and why tissues become competent to respond to hormonal signals.     

Thyroid hormones stimulate Na+–Pi transport activity in rat renal brush-border membranes: Role of membrane lipid composition and fluidity

Antecedent studies have suggested that lipid composition and fluidity of cellular membranes of various organs are altered in response to thyroid hormone status. To date, the effects of thyroid hormone status on these parameters have not been examined in rat renal apical membrane in regard to sodium-dependent phosphate transport. In the present study, we determined the potential role of alterations in cortical brush-border membrane lipid composition and fluidity in modulation of Na⁺–P_i transport activity in response to thyroid hormone status. Thyroid hormone status influences the fractional excretion of Pi, which is associated with alteration in renal brush-border membrane phosphate transport. The increment in Na⁺–P_i transport in renal BBMV isolated from Hyper-T rats is manifested as an increase in the maximal velocity (V_max) of Na⁺–P_i transport. Further, the cholesterol content was significantly increased in renal BBM of Hypo-T rats and decreased in Hyper-T rats as compared to the Eu-T rats. The molar ratio of cholesterol/phospholipids was also higher in renal BBM from hypo-T rats. Subsequently, fluorescence anisotropy of diphenyl hexatriene (rDPH) and microviscosity were significantly decreased in the renal BBM of the Hyper-T rats and increased in the Hypo-T rats as compared to Eu-T rats. The result of this study, therefore, suggest that alteration in renal BBM cholesterol, cholesterol/phospholipid molar ratio, and membrane fluidity play an important role in the modulation of renal BBM Na⁺–P_i transport in response to thyroid hormone status of animals. (Mol Cell Biochem 268: 75–82, 2005)     

Effects of thyroid hormones on the receptor level in estrogen target organs

The influence of thyroid hormones on the turnover of cytoplasmic estrogen receptors in the liver, kidney and uterus of intact and ovariectomized female rats was studied under in vivo conditions. Thyroidectomy had no significant effect on the receptor level in the uterus but caused a substantial reduction of the receptor content in the liver and kidney. In livers of intact and ovariectomized animals receptor values were reduced with 70 and 80%, respectively, 30 days after thyroidectomy. Substitution with triiodothyronine (T3) restored the hepatic estrogen receptor concentration in thyroidectomized rats to the preoperative level. If rats that had been both ovariectomized and thyroidectomized were substituted with thyroid hormone for the same time period, the receptor level was increased but did not reach the level seen in animals that had been ovariectomized only. The effects of thyroid hormone substitution was found to be dose dependent and paradoxical. Thus, a high dose of 50 micrograms/day of triiodothyronine given to intact animals for nine days caused a 30% reduction in the hepatic receptor content. The same level of reduction was seen in the ovariectomized rat given a hormone dose of only 1 micrograms/day. When this type of rats was treated with the higher dose of triiodothyronine the reduction in hepatic estrogen receptors was 50%. These results are discussed in relation to existing information concerning the multihormonal regulation of estrogen receptor concentration in the rat liver.     

Dansylated thyrotropin as a probe of hormone-receptor interactions.

A strongly fluorescent 5-dimethylamino-1-naphthalene sulfonate (dansyl) derivative of bovine thyrotropin has been prepared. The dye-conjugated hormone is bioactive and shares, essentially unchanged, the membrane binding and adenylate cyclase stimulatory activities of the native hormone. Binding of 125I-labeled dansyl-thyrotropin to thyroid plasma membranes is sensitive to inhibition by gangliosides and, as is the case for the binding of 125I-thyrotropin, galactosyl-N-acetylgalactosaminyl[N-acetylneuraminyl-N-acetylneuraminyl]-galactosylglucosylceramide (GDIb) is the most potent binding inhibitor. Gangliosides interact with dansyl-thyrotropin, causing a large increase of the quantum yield and a 5- to 10-nm blue shift of the emission maximum of the hormone-bound naphthalene chromophore; gangliosides cause no change in the fluorescent properties of the free dye. The fluorescence enhancement caused by gangliosides can be specifically reversed by unlabeled thyrotropin. The effect of gangliosides on dansyl-thyrotropin fluorescence is strongly salt-dependent; salts cannot, however, reverse the formation of the dansyl-thyrotropin.ganglioside complex once it has formed. The salt data suggest that the association of the ganglioside with dansyl-thyrotropin is dominated by electrostatic interactions, but that salt-independent, short range interactions, most likely hydrophobic, dominate the dissociation of the dansyl-thyrotropin-ganglioside adduct. Sucrose gradient centrifugation, ultracentrifugation, and fluorescence polarization data indicate that the gangliosides are micellar in nature under the conditions of these experiments. Acid titration of dansyl-thyrotropin causes a marked quenching of dansyl fluorescence which in part reflects dissociation of the hormone into its constituent alpha and beta subunits. In the presence of GDIb, but not N-acetylneuraminylgalactosyl-N-acetylgalactosaminyl-[N-acetylneuraminyl]-galactosylglucosylceramide (GDIa), pH-dependent quenching and subunit dissociation are essentially eliminated. Circular dichroism results and fluorescence polarization studies support the interpretation that the ganglioside interaction causes a conformational change in the thyrotropin molecule. The acid titration data together with differences in the ability of gangliosides to influence the tyrosine fluorescence of the thyrotropin molecule indicate that different gangliosides induce different conformational perturbations in the thyrotropin molecule.     

Circulating thyroid hormone levels and iodothyronine deiodinase activities in Nile tilapia (Oreochromis niloticus) following dietary exposure to Endosulfan and Aroclor 1254

We evaluated the effects of two organochlorinated environmental contaminants, Endosulfan and Aroclor 1254 on peripheral thyroid hormone metabolism and thyroid hormone plasma levels in Nile tilapia (Oreochromis niloticus). Tilapia were exposed through diet to 0.1 and 0.5 microg g(-1) of Endosulfan and 0.5 microg g(-1) of Aroclor 1254 for 21 and 35 days. Decreased plasma T4 and rT3 levels were observed in tilapia exposed to the lower dose of Endosulfan, while treatment with a higher dose and Aroclor 1254 produced no changes. Plasma T3 levels were not affected by these compounds. Hepatic type I deiodinase (D1) activity was depressed by a lower dose of Endosulfan and hepatic type III (D3) activity was increased following 35 days of exposure to the lower dose of Endosulfan and following 21 and 35 days of exposure to Aroclor 1254; while type II (D2) remained unchanged in liver as well as in all other organs analysed. Apart from hepatic D3 activity, Endosulfan and Aroclor 1254 also increased D3 activity in gill, but not in other tested organs. It is concluded that dietary exposure of tilapia to Endosulfan or Aroclor 1254 can lead to changes in circulating thyroid hormone levels and/or in peripheral thyroid hormone metabolism. The changes in hormone metabolism differ between tissues, eventually reflecting tissue-specific differences in adaptation.     

Tissue sensitivity to thyroid hormone in athyroid Xenopus laevis larvae

Tadpoles that spontaneously arrest development and remain as larvae occur occasionally in Xenopus laevis populations. These non-metamorphosing tadpoles continue to grow, and they develop into grossly deformed giant individuals which come as close as any anurans to being truly neotenic. Giant X. laevis tadpoles that fail to metamorphose lack thyroid glands. In this study, the hypothesis that the tissues of these tadpoles nevertheless remain thyroid hormone sensitive was tested, by exposing isolated tadpole tail tips to exogenous thyroid hormone in tissue culture. The tail tips from giant tadpoles significantly shrank in response to the thyroid hormone treatment, showing that their tissue was still capable of metamorphosis. However, the amount of shrinkage was less than that observed in tail tissue from normal tadpoles. It was hypothesized that complete induction of metamorphosis may not be possible in the giant tadpoles due to a disproportionate growth and development of tissues and organs.