Effect of dibutyryl cyclic adenosine 3′5′-monophosphate on mitotic activity in the thyroid of hypophysectomized rats

Summary Effect of dibutyryl cyclic adenosine 35-monophosphate (dbcAMP) on mitotic activity in the thyroid of hypophysectomized rats has been examined. It has been demonstrated that dbcAMP stimulates the incidence of mitoses in the thyroid follicular cells. It is therefore suggested that cAMP may be a mediator of the proliferogenic effect of TSH on the thyroid in vivo. Cyclic AMP could also release some unidentified growth-promoting factors for the thyroid. A direct stimulating effect of dbcAMP on the proliferation of the thyroid follicular cells is assumed to be possible as well.     

Induction of refractoriness to thyrotropin stimulation in cultured thyroid cells. Dependence on new protein synthesis.

Cultured dog thyroid cells were used to investigate the mechanism by which previous exposure to thyrotropin (TSH) induces refractoriness to further TSH stimulation of cellular adenosine 3'-5'-monophosphate (cAMP). Refractoriness of the cAMP response to TSH could not be overcome by exposure of the cells to supramaximal stimulatory concentrations of TSH. Although an unknown factor present in human and fetal calf serum was found to inhibit the thyroid cell cAMP response to TSH, this factor could not account for refractoriness because refractoriness could be induced in the absence of serum. Induction of thyroid refractoriness did not appear to be related to cellular concentrations of cyclic AMP, because equal refractoriness was produced by TSH alone or TSH plus the phosphodiesterase inhibitor, 3-isobutyl-1-methyl xanthine. In addition, preincubation of thyroid cells in 10(-4) M cAMP did not result in subsequent refractoriness. Recovery from the refractory process required almost 24 h. Short term (15 min) stimulation with TSH did not produce thyroid cell refractoriness, and reversal of the stimulation was obtained by thorough washing of the cells. Long term TSH stimulation (16 h), however, resulted in both supramaximal cAMP response to TSH, and inclusion of TSH together with cycloheximide did not produce refractoriness. Cyclic AMP phosphodiesterase activity in thyroid cell homogenate was unaltered by TSH or dibutyryl cyclic AMP pretreatment of the cells for up to 24 h, or cycloheximide for up to 4 h. In contrast, TSH-stimulated, but not F--stimulated, adenylate cyclase activity was reduced in thyroid cell homogenates after preincubation of the cells in TSH. Refractoriness to TSH stimulation was not associated with an alteration in the binding of 125I-TSH to cultured thyroid cells. These studies suggest that the thyroid cAMP response to TSH is modulated by an inhibitory mechanism dependent upon new protein synthesis. TSH stimulation itself increases the degree of this inhibition through a mechanism not involving cAMP.     

Effect of the exposure to chronic-intermittent cold on the thyrotropin and thyroid hormones in the rat

Rats exposed to acute cold (4 degrees C for 2 h), chronic cold (4 degrees C), and chronic-intermittent cold (4 degrees C for 2 h daily) were killed after 1, 2, 3, 4, and 10 days of cold exposure. The control group was maintained at 25 degrees C. In each animal, the plasma concentration of thyrotropine (THS), triiodothyronine (T3), and thyroxine (T4) was determined by radioimmunoassay. At the initial time of exposure, elevations in TSH, T3, and T4 were observed in the rats in each experimental group. However, on the 10th day, in rats exposed to chronic-intermittent cold, TSH, T3, and T4 decreased to values lower than the control values. In animals exposed to acute cold as well as to chronic cold no differences were found, with respect to the controls, in TSH and T4. In rats exposed to acute cold for 10 days, the T3 value was lower than the control value; however, in animals exposed to chronic cold, T3 was same as that in the controls. The results indicate that, in the rat, exposure to chronic-intermittent cold produces an inhibition in the secretion of TSH and thyroid hormones.     

Effects of calcitonin and calcitonin gene-related peptide on 3H-thymidine incorporation into DNA of rat thyroid lobes in vitro.

The effects of a 4 h incubation of rat thyroid lobes, in the presence of calcitonin (CT) and calcitonin gene-related peptide (CGRP) on the incorporation of 3H-thymidine into DNA, were investigated. In other groups the thyroid lobes were incubated during exposure to CT and thyrotropin (TSH), and to CGRP together with TSH. All concentrations of CT (10(-6)-10(-8) M) revealed a tendency towards lowering 3H-thymidine uptake, but the effect was not statistically significant. The influence of CGRP was dose-dependent; the lowest concentration of CGRP (10(-9) M) significantly enhanced DNA synthesis in the incubated rat thyroids; an intermediate dose of the peptide (10(-8) M) had no effect, while the highest concentration of CGRP (10(-7) M) decreased 3H-thymidine incorporation. Calcitonin (10(-7) M), as well as CGRP (10(-8) M), suppressed the stimulatory effect of TSH on 3H-thymidine incorporation.     

Effect of excitatory amino acids on serum TSH and thyroid hormone levels in freely moving rats

The actions of glutamate (L-Glu), and glutamate receptor agonists on serum thyroid hormones (T4 and T3) and TSH levels have been studied in conscious and freely moving adult male rats. The excitatory amino acids (EAA), L-Glu, N-methyl-D-aspartate (NMDA), kainic acid (KA) and domoic acid (Dom) were administered intraperitoneally. Blood samples were collected through a cannula implanted in the rats jugular 0--60 min after injection. Thyroid hormone concentrations were measured by enzyme immunoassay, and thyrotrophin (TSH) concentrations were determined by radioimmunoassay. The results showed that L-Glu (20 and 25 mg/kg) and NMDA (25 mg/kg) increased serum thyroxine (T4), triiodothyronine (T3) and TSH concentrations. Serum thyroid hormone levels increased 30 min after treatment, while serum TSH levels increased 5 min after i.p. administration, in both cases serum levels remained elevated during one hour. Injection of the non-NMDA glutamatergic agonists KA (30 mg/kg) and Dom (1 mg/kg) produced an increase in serum thyroid hormones and TSH levels. These results suggest the importance of EAAs in the regulation of hormone secretion from the pituitary-thyroid axis, as well as the importance of the NMDA and non-NMDA receptors in this stimulatory effect.     

Thyrotropin and cyclic AMP regulation of ras proto-oncogene expression in cultured thyroid cells

We examined the effect of thyrotropin (TSH) on intracellular levels of c-ras mRNA in a line of differentiated rat thyroid cells obtained from normal Fischer rat thyroids. These cells are totally dependent on TSH for growth. TSH stimulation of quiescent cells increased c-ras mRNA content, with a maximal response (730% of basal) after 6 h, and a decline towards basal levels after 24 h. Dibutyryl cAMP and forskolin mimicked this stimulatory effect of TSH on c-ras, but did not enhance beta-actin mRNA content. This study demonstrates hormonal and cyclic nucleotide control of c-ras expression in a well-differentiated, non-tumorogenic mammalian cell.     

Morphometric studies on the lateral habenular nucleus of Wistar rats after bilateral excision of the superior cervical ganglia and cold exposure with special reference to the pineal gland

To analyse the role of peripheral sympathetic fibres in the regulation of thyroxine (T4), serum thyrotropin (TSH), pituitary TSH, and nuclear size of the lateral habenular nuclei rats were studied 30 d after bilateral cervical ganglionectomy (GX). In order to examine the influence of GX at normal temperature (24 degrees C) and exposure to cold (10 degrees C), rats were subjected to a 72 h exposure to cold before killing. 4 times a day (light-dark cycle: 14L: 10D, light on 05h00) the rats were sacrificed: middle light, middle darkness, 1 h after "light on" and 1 h after "light off". Ganglion removal resulted in a highly significant decrease of serum-T4 and increase of serum- and pituitary-TSH (primary hypothyroidism). Under these circumstances, the karyometric findings are showing a statistically significant magnification of the lateral habenular nuclear volume. In contrast to GX, exposure to cold increased T4- and TSH-levels and reduced the lateral habenular nuclear size. GX diminished the effect of exposure to cold of the T4- and TSH-levels and normalized the habenular nuclear volume. These results indicate that there exists a negative correlation between T4 (but not TSH) and lateral habenular nuclear size. Under consideration of previous investigations of the pineal nuclear size in hypo- and hyperthyroid state, our results are in agreement with the hypothesis of other authors that it is probably an inhibitory feed-back loop between the lateral habenula and the pineal gland (see also the high gamma-aminobutyric acid [GABA] content in the habenular complex). On the other hand, it was possible to confirm that the habenular complex is integrated into the thyroid circuit.     

SMS 201-995 and thyroid function in acromegaly: acute, intermediate and long-term effects.

The acute (TRH-stimulation test), intermediate (0-6 days administration), and long-term (0-30 months administration) effects of SMS 201-995 (octreotide) treatment on thyroid function were studied. Subcutaneous injection of 100 micrograms SMS 201-995 one hour before 200 micrograms TRH intravenously reduced serum TSH response area by more than 50% in 8 healthy volunteers. After 3 days of continuous subcutaneous infusion (CSI) of SMS 201-995 in 9 acromegalic patients (100 micrograms/24 h) a slight but significant decrease in serum total triiodothyronine (TT3) and a concomitant increase in serum TSH were demonstrated, indicating an initial inhibitory effect on peripheral deiodination of thyroxine. After a further 3 days treatment serum T3 and TSH had returned to prevalues. Six of the nine acromegalics were treated with SMS 201-995 (100-1500 micrograms/24 h) and admitted for diurnal hormone profiles on 13 occasions over 30 months. Apart from a barely significant increase in serum TSH, no changes in thyroid function were noted. The study was especially designed to detect minute changes over time in thyroid hormones. The only long-term effect of SMS 201-995 was the barely significant clinically irrelevant increase in serum TSH, possibly caused by a slight inhibition of peripheral deiodination of thyroxine.     

Neuroendocrine integrative mechanisms in mammalian pineal gland: effects of steroid and adenohypophysial hormones on melatonin synthesis in vitro

The time course for the decrease in norepinephrine concentration of rat pineal explants in culture indicated a significant fall starting at the 4th hour and completed after 16-24 h of incubation. Significant decreases of serotonin and 5-hydroxyindoleacetic acid (HIAA) levels in tissue, an increase of HIAA/serotonin ratio, and an increase of melatonin production rate in vitro were also observed as a function of the incubation time. Estradiol (10(-7)-10(-5) M) increased rat pineal melatonin content, testosterone (10(-5) M) decreased it and progesterone was devoid of activity when incubated with explants for up to 6 h. The in vitro stimulatory effect of estradiol on rat pineal methoxyindole synthesis was blocked by propranolol but not by phentolamine; propranolol also blocked the increase of nuclear estradiol-receptor complex produced by estrogen exposure of pineal explants. TSH (1-100 ng/ml), growth hormone (10-100 ng/ml) and LH (10 ng/ml) augmented rat pineal melatonin content while 100 ng/ml of FSH decreased it significantly. Prolactin exerted a biphasic effect on rat pineal explants, the lowest concentration augmenting melatonin content while the high concentration depressed it. Deep, intermediate and superficial segments of guinea-pig pineal glands showed an increase in melatonin concentration after a 6-h incubation in the presence of 10(-7)-10(-5) M estradiol.     

Rapid and transient reduction in circulating thyroid hormones following systemic antigen priming: implications for functional collaboration between dendritic cells and thyroid.

The thyroid hormones T(3) (tri-iodothyronine) and T(4) (thyroxine) are disseminated throughout the body via the circulation and are maintained across a range of physiological concentrations under the control of thyroid-stimulating hormone (TSH). T(3) (and T(4) after conversion to T(3)) influences many biological activities, including gene expression and protein synthesis, though little is known about the nature of pituitary-thyroid immune interactions. In the present study we show that serum T(3) and T(4) levels are sharply but transiently reduced during the first 24 h of systemic antigen exposure and that this is followed by suppressed levels of free T(4), after which there is rapid recovery to normal levels. Splenic dendritic cells, depending upon the stage of maturation/activation, were found to be a rich source of TSH, and CD11c(+) cells with dendritic cell morphology were present in the thyroid 1-3 days after antigen exposure. Moreover, antigen priming of hypophysectomized mice that are unable to make pituitary-derived TSH resulted in significant increases in circulating T(4), implying that compensation in the drop in thyroid hormones can be regulated from extrapituitary sources. These findings thus identify a novel set of immune-endocrine interactions that transpire during the early phase of antigen exposure, and they suggest that under appropriate conditions the immune system directly participates in the process of maintaining physiological homeostasis by contributing to the regulatory control of thyroid hormone activity.     

Functional state of the thyroid gland during systemic graft vs host reaction

Triiodothyronine (T3), thyroxine (T4) and thyroid stimulating hormone (TSH) serum content was measured in mice during systemic "graft-versus-host" reaction (GVHR), using radioimmunoassay. It was demonstrated that on the 3rd day after GVHR induction the levels of these hormones did not differ from the control values. T3 and T4 concentrations and 125I absorption by thyroid gland diminished by day 10. At the same time TSH level remained unchanged. On day 24 after GVHR induction T3 and T4 content was significantly reduced, although TSH concentration exceeded the control value. 125I absorption was enhanced as compared to the value observed on day 10. The data obtained show the vigorous inhibition of thyroid gland function during systemic GVHR.     

Abnormalities of the hypothalamo-pituitary-thyroid axis in the pro-opiomelanocortin deficient mouse

The melanocortin system is an important regulator of body weight and the hypothalamo-pituitary-thyroid (HPT) axis. The pro-opiomelanocortin (POMC)-null mouse, deficient in all POMC-derived peptides, including alpha-melanocyte stimulating hormone (alpha-MSH), has an obese phenotype. We studied the HPT axis of POMC-null mice, which has not been previously investigated. Because alpha-MSH has a stimulatory effect on the HPT axis, we hypothesised that these mice would have a down-regulated thyroid axis, consistent with a recent study of POMC-null humans. The activity of the HPT axis was studied by collecting blood, pituitaries and hypothalami from ad libitum fed, adult POMC-null, heterozygous and wild-type mice. POMC-null mice had significantly elevated plasma total T(4) (TT(4)) and free T(3) (fT(3)) with reduced plasma thyroid stimulating hormone (TSH), pituitary TSH content and hypothalamic thyrotrophin stimulating hormone (TRH) content compared to wild-type mice. No significant differences between heterozygous and wild-type mice were observed. POMC-null mice have an abnormal HPT axis, which may contribute to their hyperphagia and obesity. These abnormalities are in contrast to those observed in POMC-null humans. These findings support a role for the melanocortin system in the regulation of the HPT axis.     

A Modest Protective Effect of Thyrotropin against Bone Loss Is Associated with Plasma Triiodothyronine Levels

Background

The independent skeletal effect of thyrotropin (thyroid stimulating hormone, TSH) has been suggested in animal studies. However, clinical data on the association between bone loss and variations in TSH levels is inconsistent. This study aimed to investigate the relationship between TSH levels and bone mineral density (BMD).

Methods

We conducted a cross-sectional study with 37,431 subjects (33,052 cases with euthyroidism and 4,379 cases with subclinical thyroid dysfunction) aged over 35 years. We performed thyroid function tests and measured BMD at the lumbar spine, femur neck, and total hip.

Results

Levels of TSH and T3 were positively correlated in women (r = 0.076, P = 0.001) and uncorrelated in men. In both men and women, TSH levels correlated positively and T3 levels correlated negatively with BMD at all skeletal sites in age and body mass index adjusted analyses. BMD increased steadily with TSH levels from the subclinical hyperthyroid to subclinical hypothyroid range in subjects with T3 levels in the highest tertile (119.5–200.0 ng/dL), but was no longer significant in subjects with lower plasma T3 levels.

Conclusions

The variations in TSH levels within the euthyroid and subclinical range were positively correlated with BMD in healthy men and women. The negative effect of T3 on BMD appears to be compensated for by increased TSH in subjects with plasma T3 levels in the upper normal range.     

Regulation of in vivo TSH secretion by leptin

Leptin, the product of the ob gene, is a hormone secreted by adipocytes that regulates food intake and energy expenditure. The hypothalamus-pituitary-thyroid axis is markedly influenced by the metabolic status, being suppressed during food deprivation.The aim of the present study was to assess whether leptin can act as a metabolic signal connecting the adipose tissue with the pituitary-thyroid axis. We studied the effect of leptin administration (10 microg, i.c.v.) on spontaneous TSH secretion and TSH responses to TRH in euthyroid and hypothyroid food-deprived rats. Spontaneous TSH secretion was assessed over 6 h with samples taken every 7 min. Administration of leptin to food-deprived euthyroid rats led to a reversal of the inhibitory effect exerted by fasting on spontaneous TSH secretion. This stimulatory effect of leptin on spontaneous TSH appears to be dependent on the thyroid status since it could not be observed in hypothyroid rats. This data suggests that blunted spontaneous TSH secretion in food-deprived rats is a functional and reversible state, and that the decreased leptin concentrations could be the primary event responsible for the suppression of the hypothalamic-pituitary-thyroid-axis in food-deprived rats.     

Hyperresponsiveness of residual thyroid lobe of hemithyroidectomised rat to thyrotropin

The effects of hemithyroidectomy and thyrotropin administration on rat thyroid gland function were studied in adult male rats. Immediately after surgery or sham operation rats were treated daily with 0.12 IU of bovine thyrotropin (TSH) for 3 or 5 days. In control rats TSH dose applied resulted in an increase in serum T4 level at day 5 of experiment. Serum thyroxine concentration markedly decreased in sham operated and hemithyroidectomised rats, an effect observed at days 3 and 5 of experiment. TSH administration had no effect on serum T4 concentration in sham operated rats while in hemithyroidectomised animals such a treatment resulted in a marked increase in serum T4 level, a phenomenon observed in both time intervals studied. The reasons for hemithyroidectomy-induced hyperresponsiveness of rat thyroid residual lobe to thyrotropin are unknown.     

Mitotic Rate of Rat Thyroid Follicular Cells In Vivo In Response to A Single Injection of Thyrotropin (Tsh)

The mitotic rate of thyroid follicular cells was assessed by a stathmokinetic method at intervals from 15 min to 24 hr after a single injection of 1 iu/kg of thyrotropin (TSH). the mitotic rate was increased 15 min after TSH and remained elevated for 3 hr. Two further peaks of mitotic activity were present at 9 hr and 24 hr after TSH. Serum TSH concentrations were increased from 5 min to 3 hr with a maximum at 1 hr.     

Influence of somatostatin and epidermal growth factor (EGF) on the proliferation of thyroid follicular cells in organ culture

The aim of the present study has been to examine the effects of various concentrations of somatostatin (SS), epidermal growth factor (EGF), as well as of interactions among SS, EGF and thyrotropin (TSH) in their influence upon the mitotic activity of thyroid follicular cells (TFC) in organ culture. The stathmokinetic method was employed. It was shown that: (1) SS, at the concentration of 10(-7) M, suppressed the mitogenic effect of TSH, as well as of TSH and EGF employed together, on TFC; (2) EGF, at the concentration of 10 and 100 ng/ml, increased the mean mitotic activity rate of TFC; (3) TSH and EGF revealed an additive action on TFC proliferation. The obtained results evidently suggest an antiproliferative effect of SS and mitogenic action of EGF on TFC in organ culture.     

Effects of Combined Exposure to Chronic High-Fat Diet and Arsenic on Thyroid Function and Lipid Profile in Male Mouse

The thyroid is one of the major endocrine glands that contribute to body and fat metabolism. The present study evaluated the effects of combined exposure to chronic high-fat diet (HFD) and arsenic on thyroid function and lipid profile. In this experimental study, 72 male Naval Medical Research Institute mice were divided into six groups and fed HFD or low-fat diet (LFD) while being exposed to 25 or 50 ppm of arsenic in drinking water for 20 weeks. After 24 h of the last experimental day, blood samples were collected for hormonal and biochemical measurements. The data indicated that exposure to HFD alone increased the levels of triiodothyronine (T3), thyroid-stimulating hormone (TSH), leptin, lipid profile, reactive oxygen species (ROS), and malondialdehyde (MDA) and decreased the levels of high-density lipoprotein, albumin, adiponectin, and glutathione sulfhydryl reductase (GSH), whereas exposure to arsenic alone decreased the levels of T3 and GSH and increased the levels of TSH, leptin, ROS, MDA, and T4/T3 ratio compared to those in the control LFD group. Furthermore, concomitant administration of HFD and arsenic decreased the lipid profile and levels of T4, albumin, total protein, T3, and GSH and increased the levels of TSH, adiponectin, leptin, ROS, MDA, and T4/T3 ratio compared to those in the control LFD or HFD group. In conclusion, combined exposure to HFD and arsenic induced hypothyroidism via reduction of thyroid hormones and enhancement of plasma TSH and T3 uptake levels concomitant with hypolipidemia, hyperleptinemia, hyperadiponectinemia, induction of oxidative stress, and reduction of GSH levels.