Nuclear T3 Receptors Research Articles

In vitro effects of sodium selenite on nuclear 3,5,3’-triiodothyronine (T3) receptor gene expression in rat pituitary GH4C1 cells

The present study was undertaken in order to investigate the effects of sodium selenite on: 1. The growth of rat pituitary GH4C1 cells; 2. The nuclear T3 receptor gene expression; 3. The cytoplasmic protein phosphorylation; and 4. The prolactin secretion in rat pituitary GH4C1 cell line. Sodium selenite (up to 2.5 microM) has no inhibitory effect on GH4C1 cell proliferation as well as the prolactin secretion. On the other hand, 0.5 microM sodium selenite significantly decreases the rate of mRNA synthesis and/or degradation of both, the alpha 1 form of the T3 receptor (TR alpha 1) and the alpha 2 isoform of the T3 receptor. At 1 microM of sodium selenite, significant changes in the electrophoretic profile of low molecular mass cytoplasmic proteins were found, moreover, sodium selenite (1 microM) also considerably affects phosphorylation of a higher molecular mass proteins. The results based on the in vitro experiments suggest that sodium selenite may affect specific processes at the pretranslational level as well as it may also take part in processes of posttranslational modification of protein(s), the cell vitality and the cell growth remaining unchanged.

Nuclear and Cytoplasmic Triiodothyronine‐Binding Sites in Primary Sensory Neurons and Schwann Cells: Radioautographic Study During Development

The effects of the thyroid hormones on target cells are mediated through nuclear T3 receptors. In the peripheral nervous system, nuclear T3 receptors were previously detected with the monoclonal antibody 2B3 mAb in all the primary sensory neurons throughout neuronal life and in peripheral glia at the perinatal period only (Eur. J. Neurosci. 5, 319, 1993). To determine whether these nuclear T3 receptors correspond to functional ones able to bind T3, cryostat sections and in vitro cell cultures of dorsal root ganglion (DRG) or sciatic nerve were incubated with 0.1 nM [125I]-labeled T3, either alone to visualize the total T3-binding sites or added with a 10(3) fold excess of unlabeled T3 to estimate the part due to the non-specific T3-binding. After glutaraldehyde fixation, radioautography showed that the specific T3-binding sites were largely prevalent. The T3-binding capacity of peripheral glia in DRG and sciatic nerve was restricted to the perinatal period in vivo and to Schwann cells cultured in vitro. In all the primary sensory neurons, specific T3-binding sites were disclosed in foetal as well as adult rats. The detection of the T3-binding sites in the nucleus indicated that the nuclear T3 receptors are functional. Moreover the concomitant presence of both T3-binding sites and T3 receptors alpha isoforms in the perikaryon of DRG neurons infers that: 1) [125I]-labeled T3 can be retained on the T3-binding 'E' domain of nascent alpha 1 isoform molecules newly-synthesized on the perikaryal ribosomes; 2) the alpha isoforms translocated to the nucleus are modified by posttranslational changes and finally recognized by 2B3 mAb as nuclear T3 receptor. In conclusion, the radioautographic visualization of the T3-binding sites in peripheral neurons and glia confirms that the nuclear T3 receptors are functional and contributes to clarify the discordant intracellular localization provided by the immunocytochemical detection of nuclear T3 receptors and T3 receptor alpha isoforms.

Regulation of porcine skeletal muscle nuclear 3,5,3′-tri-iodothyronine receptor binding capacity by thyroid hormones: modification by energy balance

Thyroid hormones have been implicated in the regulation of nuclear 3,5,3'-tri-iodothyronine (T3) receptor binding capacity (Bmax) but, despite numerous in vivo and in vitro studies, there is considerable controversy regarding their exact role. Since changes in thyroid status alter energy balance and hence may influence T3 receptor numbers, the effects of chronic hypothyroidism and T4 treatment have been studied in young pigs under conditions of controlled energy intake. Four groups of animals comprising a hypothyroid, a euthyroid and a hyperthyroid group, all on the same level of food intake, and a hyperthyroid group on twice the amount of food were used. After 3 weeks on the treatment regimes, both the hypothyroid animals on the same level of food intake and the hyperthyroid animals on twice the amount of food had significantly increased Bmax values (97% and 137% higher respectively) compared with euthyroid controls. However, there was no difference between controls and the hyperthyroid animals on the same level of food intake. In a second study, the effects of short-term treatment of euthyroid animals with T3 was investigated. Results showed that in two groups of controls that received intravenous saline, those on a higher food intake had higher Bmax values (76% increase). Intravenous T3 administration to animals on a low food intake did not change the receptor numbers. In none of the studies was there any change in the dissociation constant of the receptors as a result of different treatments. It is suggested that, at least in postnatal life, thyroid hormones per se have no significant effect on nuclear T3 receptor numbers in skeletal muscle.(ABSTRACT TRUNCATED AT 250 WORDS)

Sex differences in thyroid axis function: Relevance to affective disorder and its treatment

AbstractThroughout life the thyroid economy differs for men and women. The “set point” for TSH is higher in women, they experience more goiter and nodule formation, and autoimmune thyroid disease is greater throughout life. Excluding cancer, the lifetime prevalence of thyroid disease for women is variously estimated between 2.25‐4.0%, some 4 to 10 times the estimates for men.Contrary to earlier interpretation, maternal thyroid hormones are now thought to be important in the early life of the fetus and the development of the CNS; however, after the first trimester maternal free thyroid hormone falls to reach a nadir post partum, a fact not to be discounted in postpartum psychosis.Major depression is more common in women and approximately 5‐10% have some metabolic evidence of a diminished thyroid economy. Adjunctive tri‐iodothyronine has been shown to speed recovery from depression in women but not in men.Manic‐Depressive Illness in its classic form (Bipolar I) is approximately equal in prevalence between men and women. However, the malignant variant of treatment resistant rapid cycling (RC) is predominant in women and is significantly associated with Grade I hypothyroidism. Furthermore, the use of high dose thyroxine as an adjunct to anticycling medications such as lithium and the anticonvulsants with frequently mitigate the cycling and induce clinical remission.Studies in premenopausal women (the common sufferers of RC) suggest that this pharmacologic use of thyroxine has few side effects and is without detectable increased risk of osteoporosis.Particularly lithium, but also other clinically employed psychotropic agents, are antithyroid as well as thymoleptic injunction. The study of the interaction of lithium and thyroid metabolism at the cellular and molecular level will help inform these clinical observations. That the c‐erb‐A family of gene products share considerable homology at the DNA binding region across nuclear T3, glucocorticoid, and estrogen receptors may be of significance in this regard.Thyroid screening of women with affective disorder is recommended for those who are aged 45 years or older, have a history of postpartum thyroiditis, or a family history of thyroid disease. Depression 3:33‐42 (1995). © 1995 Wiley‐Liss, Inc.

Insulin regulation of malic enzyme gene expression in rat liver: evidence for nuclear proteins that bind to two putative insulin response elements.

Diabetes in rats is characterized by insulin deficiency accompanied by a decrease in lipogenic enzymes. The malic enzyme (ME) gene, which encodes an important lipogenic enzyme, was used to investigate insulin regulation of gene expression. ME mRNA levels were reduced by more than 90% in the liver of diabetic rats. The administration of insulin (3 U/15 days) to either control or diabetic rats increased ME mRNA by 2- to 10-fold, respectively. Since diabetes reduces circulating T3 and the levels of nuclear T3-receptors, the potential role of thyroid hormone on insulin regulation of ME gene expression was also evaluated in thyroidectomized-diabetic rats. In these animals the levels of ME mRNA were undetectable but were increased by insulin even in the absence of thyroid hormones. These in vivo effects of insulin and T3 were not additive. The transcription rate of the gene was also reduced in the diabetic liver and recovered after insulin therapy. By computer analyses we have identified two different putative insulin response elements (IREs) in the ME gene promoter, hereafter referred to as IRE-I (-683 to -692), which is similar to the phosphoenol pyruvate carboxy kinase promoter IRE and IRE-II (-161 to -170), which is similar to the glyceraldehyde phosphate dehydrogenase gene promoter IRE-A. Results from gel retardation assays suggest that a single nuclear protein binds to IRE-I whereas two different nuclear proteins bind to IRE-II. The protein/IRE-I complex increased in liver nuclear extracts from diabetic rats and decreased after insulin administration.(ABSTRACT TRUNCATED AT 250 WORDS)

Kinetic Parameters of Plasma Thyroid Hormone and Thyroid Hormone Receptors in a Dwarf and Control Line of Chicken

Sex-linked dwarf chickens have in plasma low triiodothyronine (T3) levels and slightly raised thyroxine (T4) concentrations and are functional hypothyroid. The kinetic parameters of T4 and T3 were investigated using 125I-labeled hormones. In addition the nuclear T3-receptors in the liver were examined using a radioreceptorassay and Scatchard analysis. Four-week-old dwarf (dw) and normal (Dw) chickens were injected with 125I-labeled T3 or T4 and blood samples taken 60, 120, 180, and 300 min after 125I-T3 injection and 120, 240, 360, and 480 minutes after 125I-T4 treatment. Labeled T4 and T3 and the degradation products were separated by paper chromatography. After the paper strips were dried, the iodinated compounds were visualized and counted in the gamma counter. The kinetic parameters, the half-life time (T1/2), the apparent distribution volume (Vd) and the metabolic clearance rate (MCR) were calculated using the natural base logarithm values of the measured radioactivity plotted against time and used for linear regression. T4 was cleared from circulation more slowly in dwarf than in control chicks and reflected a longer T1/2 (21.8%) and a reduced MCR (45%). The Vd tended to be lower (34.7%) in dwarfs. While the T1/2 of T3 was longer (28.1%) in dwarf chickens than in control animals, the MCR for T3 was considerably increased (31.8%). This results from an increased Vd (63.1%) in the dwarf chicks. The T3-receptor study in the liver of dwarf and non-dwarf chickens from Week 1 to Week 4 posthatching revealed that the total capacity and the affinity constant of the binding sites were comparable in dwarf and normal chickens. However, the occupancy of the receptors was higher in the dwarf animals.

Effect of selenite and selenate on rat liver nuclear 3,5,3'-triiodothyronine (T3) receptor.

The present study was undertaken to investigate the effects of selenite (SeIV) or selenate (SeVI) on nuclear T3 receptors of rat liver. Selenite at 0.1 microM (p < 0.01) inhibited the T3 specific binding to rat liver nuclear receptors. The specific binding of the T3 receptor was fully restored when even 1.0 microM selenite was separated from the T3 receptor by gel filtration. No inhibitory effect of selenite (up to 100 microM) on the T3 binding to nuclear receptor was found in the presence of 1.0 mM dithiothreitol. The rate of dissociation of the T3-nuclear receptor complex was effectively increased by 0.1 microM selenite. Selenate up to 1 mM as well as sulfite or sulfate up to 0.1 mM did not exert an inhibitory effect on T3 receptors. The results based on the in vitro experiments suggest that the selenium in the form of selenite may reversibly affect the T3 binding on the receptor molecule.

Inhibition of aromatase activity in rat Sertoli cells by thyroid hormone

Basal and FSH-induced aromatase activity in prepubertal rat Sertoli cells was inhibited by L-triiodothyronine (T3) in a time- and dose-dependent manner. The effect was evident only after 6 h of preincubation with T3 (10(-7) M) and the half-maximal dose was 0.5 +/- 0.2 nM, which correlated with the Kd of the nuclear T3 receptor of rat Sertoli cells (Kd = 1-2 nM). The effect was specific as judged by the lack of effect of the T3 analogue 3-iodo-L-tyrosine. The inhibitory effect of T3 was present over the entire range of FSH concentrations used (0.001-100 ng/ml). In T3-treated Sertoli cells, aromatase activity induced by 8-bromo-cyclic AMP was inhibited by the same order of magnitude as that of FSH, thus suggesting that the inhibitory effect of T3 was downstream from cyclic AMP formation. Furthermore, pretreatment of Sertoli cells cultures with T3 (24 h, 10(-7) M) did not affect basal or FSH-induced extracellular cyclic AMP accumulation. This effect of T3 on rat Sertoli cell aromatase activity may be regarded as a part of the integrated mechanism by which thyroid hormone modulates the functions of the seminiferous epithelium.

Nuclear triiodothyronine receptor expression is regulated by axon-Schwann cell contact.

Thyroid hormones, which play an important role in the development and regeneration of the nervous system, require the presence of specific nuclear T3 receptors (NT3R). In this study we provide evidence that NT3R expression by Schwann cells was up-regulated in response to a loss of axonal contact in vitro and in vivo. In dorsal root ganglia explant cultures, Schwann cells which accompanied axons (nerve fibres) were devoid of NT3R. When Schwann cells were orphaned from axon contact by axon transection, all the nuclei of these cells displayed NT3R immunoreactivity. Similar results were obtained in situ; in adult rat sciatic nerve, Schwann cells which ensheathed healthy axons never expressed NT3R immunoreactivity. After sciatic nerve transection in vivo the nuclei of Schwann cells deprived of axonal contact displayed a clear NT3R immunoreaction.

β-Adrenergic receptors, voltage-operated Ca 2+-channels, nuclear triiodothyronine receptors and triiodothyronine concentration in pig myocardium after long-term low-dose amiodarone treatment

Similar features during chronic amiodarone treatment and hypothyroidism suggest that amiodarone induces a state of "triiodothyronine (T3)-resistance" or "cardiac hypothyroidism", which may predispose the heart to pump failure under conditions with severe strain, such as recovery after cardiac surgery. Disagreements exists as to how amiodarone, and possibly its main metabolite desethylamiodarone, act upon the various receptor systems in the heart. The aim of the present study was to elucidate whether chronic amiodarone treatment leads to a functional reduction in the number of myocardial nuclear T3 receptors, the myocardial tissue T3 concentration and the number of beta-receptors and voltage-operated Ca(2+)-channels. Finally, special attention was drawn to any changes that could contribute to explain previous reports on reduced haemodynamic reserve in animals exposed to severe cardiac strain, such as cardiac surgery. Pigs (72 +/- 2 kg) were assigned randomly to amiodarone treatment (20 mg.kg-1.day-1 for 30 +/- 1 days, N = 8); controls received no medical treatment (N = 6). The left ventricle was evaluated for beta-adrenergic receptors, voltage-operated Ca(2+)-channels, T3 nuclear receptors and tissue T3 concentration. Maximum binding capacity for beta-receptors and Ca(2+)-channels was reduced in amiodarone-treated pigs (by 38%, p < 0.05, and by 52%, p < 0.01) and correlated with tissue drug concentrations for both receptor types (p < 0.05). No changes were observed concerning nuclear T3 receptors. In vitro competition studies revealed that amiodarone, but not desethylamiodarone, possessed binding properties to Ca(2+)-channels, whereas neither of the compounds bound to beta-receptors. Desethylamiodarone, but not amiodarone, competitively inhibited T3 binding to its nuclear receptors. Myocardial T3 was undetectable (< 0.05 nmol/kg wet wt) in amiodarone-treated pigs. From our observations we suggest that the active metabolite desethylamiodarone, rather than the parent drug, is mainly responsible for the observed local hypothyroid-like effects during amiodarone treatment. The observed changes after treatment with low-dose amiodarone in pigs are likely to have biological implications. Functionally, the changes may imply reduced cardiac reserve during conditions of extraordinary strain.

Pregnancy and the thyroid

The normal thyroid secretes thyroxine (T4) and, to a lesser extent, triiodothyronine (T3). The T4 is transported in the circulation to peripheral tissues largely bound to thyroxine binding globulin (TBG); T3, which is produced in peripheral tissues by deiodination of T4, also binds to TBG. It is, however, the free thyroid hormone concentration which determines the metabolic state, by its action on T3 nuclear receptors in the cells of kidney, heart, liver, anterior pituitary and other tissues.

Thyroid hormone receptors: multiple forms, multiple possibilities.

I. Introduction THE biological importance of the thyroid gland has been recognized for centuries (1). The realization in the late 19th century that cretinism and myxedema result from loss of thyroid function led to the discovery of T4 (2) and T3 (3) as the predominant and most potent thyroid hormones, respectively. During the ensuing decades the plasma membrane (4), synapse (5), endoplasmic reticulum (6), and mitochondria (7) have all been considered as potential cellular sites of action of the thyroid hormones. In the 1960s, however, it was noted that many of the physiological actions of T3 were preceded by nuclear RNA transcription (8, 9), and high-affinity nuclear receptors for T3 were discovered soon thereafter (10,11). The specificity of the nuclear T3 receptors (TRs) for thyroid hormone analogs closely parallels the biological potency of the compounds (12, 13), and the levels of the nuclear TRs correlate well with the developmental and tissue-specific effects of T3 in most cases (14, 15). Thus, alth...

Thyroidal regulation of nuclear tri-iodothyronine receptors in the developing rat testis

Previous work has demonstrated that thyroid hormones influence testis development. Specific receptors for tri-iodothyronine (T3) have been demonstrated in Sertoli cells. The aim of the present study was to examine the possible effect of thyroid hormone on its own receptor during pubertal development by evaluating the influence of thyroid status on T3-binding capacity, -binding affinity and receptor occupancy in nuclei isolated from immature rat testes. The binding capacity for T3 of nuclei from rat testis significantly decreased during pubertal development, being 375 +/- 32, 117 +/- 15 and 44 +/- 7 fmol/mg DNA in 7-, 21- and 35-day-old rats respectively, whereas the affinity of binding, as evaluated by the dissociation constant (Kd), did not change. Early induced hypothyroidism significantly affected the time-course of the postnatal decline of nuclear T3 receptors in the testis. At 21 days of age, the binding capacity for T3 in the testis of methimazole-treated rats was significantly higher with respect to euthyroid controls, being 173 +/- 21 and 117 +/- 15 mol/mg DNA respectively, while the Kd was unaffected. T3 replacement therapy completely prevented changes in T3 receptor number induced by hypothyroidism without modifying the Kd. Our results indicate that nuclear T3 receptors in the developing rat testis are modulated by thyroid hormone.

Induction of cytosolic triiodo-L-thyronine (T3) binding protein (CTBP) by T3 in primary cultured rat hepatocytes.

Cytosolic 3, 5, 3'-triiodo-L-thyronine (T3)-binding protein (CTBP) plays an important role in the regulation of intracellular T3 translocation from cytoplasm to the nuclear T3 receptor. We examined whether the CTBP activity could be induced by T3 or not in cultured hepatocytes prepared from thyroidectomized rats. CTBP activity was not detected in primary cultured hepatocytes from thyroidectomized rats. However, the protein was induced by the addition of T3 to the culture medium. The increase in the activity of CTBP was time dependent and the maximal level was obtained by 48 h in the presence of 300 nM T3. CTBP activity was also increased by retinol (35 microM) or by 1,25-(OH)2-vitamin D3 (10 nM). On the other hand, the activity of malic enzyme (ME) was induced by the addition of T3 to the culture medium. The maximal activity of ME was obtained by 48 h in the presence of 300 nM T3. The increase in ME activity was also induced by retinol or 1,25-(OH)2-vitamin D3. These results suggested that not only ME activity but also CTBP activity is induced by T3. Further, retinol and vitamin D3 have similar effects on the induction of CTBP activity and ME activity.

N-Butyrate enhances induction of thyroid hormone-responsive nuclear protein.

Effects of n-butyrate on nuclear thyroid hormone receptors and on thyroid hormone-responsive nuclear protein were investigated by means of a perfusion system in rat liver. Treatment with 5 mM n-butyrate resulted in an increase (150%) in the maximal binding capacity of 3,5,3'-L-triiodothyronine (T3) nuclear receptors without altering the affinity of receptor for T3. However, further perfusion for 4 h decreased the number of the receptors to the control level. n-Butyrate increased the amount of acetylated histone H4. The ability of nuclear T3 receptors to bind to core histones was diminished by acetylation of the core histones. Thyroid hormone-responsive nuclear protein (n protein) was increased by T3. The induction of the n protein by T3 was augmented by n-butyrate. These results suggested that n-butyrate modulates thyroid hormone-responsive gene expression in rat liver via the increased number of nuclear receptors or changes in the chromatin constitution.

Effect of restricted feeding, fasting, and diabetes on the relationship between thyroid hormone receptor occupancy, growth hormone induction, and inhibition of thyrotropin release in thyroidectomized rats.

The present study was undertaken to test the effect of food restriction, fasting, and diabetes on the relationship between thyroid hormone receptor occupancy and two biological end points, GH production and the inhibition of TSH secretion, in thyroidectomized rats. The estimated maximal binding capacity (MBC) in diabetic (D) and fasting (F) rats and in animals limited to 25% (FR25) of the food consumption of normal (C) rats was decreased to 57%, 73%, and 76%, respectively, of C values (P < 0.01-0.001), whereas normal values were found in thyroidectomized (Tx) rats and in animals limited to 50% (FR50) of the food intake of C animals. The nuclear T3 content and T3 receptor occupancy were reduced, respectively, to 25% and 29% in Tx, 77% and 81% in FR50, 52% and 69% in FR25, 49% and 66% in F, and 36% and 64% in D rats of the corresponding C values (P < 0.05-0.001). Pituitaries from Tx, FR50, FR25, F, and D rats contained less GH than C pituitaries (0.14%, 81%, 69%, 88%, and 51%, respectively, of C pituitaries; P < 0.05-0.001). Plasma TSH was lower in FR50, FR25, F, and D rats than in C animals (78%, 57%, 52%, and 48%, respectively (P < 0.01-0.001)), and markedly increased in Tx animals. Administration of a single dose of 2, 5, or 10 micrograms T3/100 g BW to Tx C, Tx FR50, TX FR25, Tx F, and Tx D rats resulted in a similar and progressive increase in nuclear T3 in all groups, except for lower values in Tx D animals. However, receptor occupancy did not differ among the different groups at each T3 dose. This treatment resulted in a progressive increase in pituitary GH in all groups; however, in Tx FR50, Tx FR25, Tx F, and Tx D pituitaries, the GH responses to 10 micrograms T3 were only 77%, 59%, 44%, and 27%, respectively, of that in Tx C rats. (P < 0.05-0.001). Moreover, significant differences in the GH response to 10 micrograms T3 were observed among Tx FR50, Tx FR25, Tx F, and Tx D animals (P < 0.01-0.001). In addition plasma TSH levels in untreated Tx FR50, Tx FR25, Tx F, and Tx D rats were only 88%, 82%, 79%, and 72%, respectively, of that in Tx C animals (P < 0.05-0.01).(ABSTRACT TRUNCATED AT 400 WORDS)

Identification of nuclear triiodothyronine receptors in the thymic epithelium.

Thymic epithelial cell physiology is known to be under neuroendocrine control. In particular, thyroid hormones modulate thymic hormone secretion by thymic epithelial cells in vivo and in vitro, thus suggesting the existence of specific receptors for those hormones in this component of the thymic microenvironment. Yet, thyroid hormone-binding sites have previously been detected only in crude thymus fractions and lymphocytes. We, thus, decided to search for T3 receptors in the thymic epithelium, by using an antinuclear T3 receptor monoclonal antibody. In situ immunohistochemical analysis of thymic frozen sections showed nuclear labeling of both lymphoid and nonlymphoid cells in the cortex and medulla. Moreover, in vitro studies using thymic epithelial cell lines and the so-called thymic nurse cells revealed a positive reaction in the chromatin, with nucleoli remaining negative. Immunoblot data clearly showed a single protein band of 57K reactive with the antinuclear T3 receptor antibody in murine thymus extracts as well as in the thymic epithelial cell lines. Lastly, in vitro treatment of these cells with T3 resulted in a transient, yet profound, down-modulation of the receptor. In conclusion, our findings provide molecular evidence that the action of thyroid hormones on thymic epithelium occurs via the typical 57K nuclear T3 receptors.

3,5,3'-Triiodothyronine positively regulates both MyoD1 gene transcription and terminal differentiation in C2 myoblasts.

Thyroid hormones are among the positive regulators of muscle development in vivo, but little is known about the way they work. We demonstrate here that MyoD1, one of the master genes controlling myogenesis, is a target of T3. After proliferating C2 myoblasts have been treated with T3 for 15 h, we observed a rise in MyoD1 expression at both the mRNA and protein levels. This is the first positive hormonal control of MyoD1 gene expression reported so far. We also provide data which suggest that T3 nuclear receptor(s) have a direct role on MyoD1 gene transcription: 1) C2 cells express the alpha 1 form of T3 nuclear receptors; 2) T3 up-regulates MyoD1 gene transcription and does not affect MyoD1 mRNA stability, as demonstrated by run-on and actinomycin D chase experiments, respectively; and 3) this transcriptional activation does not need the synthesis of intermediate protein(s) since it is not abolished by simultaneous treatment with cycloheximide. Moreover, in presence of T3, the increase of MyoD1 transcripts is associated with a faster terminal differentiation. Indeed we observed an earlier expression of various markers of myogenesis including myogenin (a regulatory gene of the MyoD1 family mainly involved in the triggering of terminal differentiation), myosin light chain 1A, and troponin T in T3-treated cells vs. untreated cells. We suggest that the regulation of a pivotal myogenic gene could be an important step in the control exerted by T3 on muscle development in vivo.

Increase in the expression of thyroid hormone receptors in porcine granulosa cells early in follicular maturation.

Thyroid hormone has been demonstrated to synergize with FSH to exert stimulatory effects on the differentiation of porcine granulosa cells. In order to further characterize the nature of thyroid hormone action on granulosa cells, the presence of triiodothyronine (T3) receptors in the nuclei of porcine granulosa cells was examined, and qualitatively and quantitatively compared during follicular maturation. Then, comparative abilities of granulosa cells from varying follicle stages to respond to T3 were assessed in terms of FSH-induced LH/hCG receptor formation and progesterone secretion. Furthermore, the expression of erb-A was analyzed using Northern blot hybridization of porcine granulosa cell RNA with a v-erb-A probe. Binding experiments with [125I]T3 showed that granulosa cell nuclei obtained from small follicles had a greater ability to bind [125I]T3 compared to those from large follicles. Scatchard analysis revealed the presence of nuclear T3 receptors with a single class of binding sites. There was little difference in the affinity of the T3 receptors during follicular maturation. By contrast, the number of the T3 receptors was higher in small follicle granulosa cells compared to that in large follicle granulosa cells. Thus, the increased T3 binding to small follicle granulosa cells relative to large follicle granulosa cells appears to be attributable to the increased number of the nuclear T3 receptors rather than to a change in the affinity. The magnitude of the stimulatory effects of T3 on granulosa cell functions was maximal in small follicle granulosa cells, but negligible in large follicle granulosa cells.(ABSTRACT TRUNCATED AT 250 WORDS)

Sodium ipodate increases triiodothyronine action in vivo.

Sodium ipodate (IPO) has been shown to bind nuclear T3 receptors (NT3R) in vitro, but previous studies have conflicted in regard to demonstration of this interaction in vivo. We sought evidence for IPO-NT3R binding in vivo by giving large doses of IPO to thyroidectomized (TDX) rats replaced with low doses of T3. We predicted that IPO-NT3R binding would inhibit T3 induced increases in mitochondrial alpha glycerophosphate dehydrogenase activity (alpha-GPDH) in kidney, heart and liver. Three groups of ten euthyroid rats each received 13 daily injections of vehicle, or 6 or 12 mg/100 g body weight of IPO, respectively. Both doses of IPO resulted in decreases in serum T3 and increases in serum TSH. Liver and kidney alpha-GPDH, however, were decreased only in the group receiving 6 mg IPO. In addition, three groups of 30 TDX rats were implanted with osmotic minipumps that contained T3 in the following concentrations: 33, 69 and 96 ng/ul. Ten rats in each group received 13 daily injections of vehicle, or IPO (vide supra). The alpha-GPDH responses were complex in that there was significant interaction between T3 and IPO effects in the kidney (AxB F ratio 5.13, p less than 0.001) and liver (AxB F ratio 2.85, p less than 0.05). The major finding, however, was that alpha-GPDH was not significantly reduced by IPO in any T3 replaced group. Rather, in all three organs, alpha GPDH was significantly increased above that produced by T3 alone by at least one combination of IPO and T3. Changes in serum TSH also suggested that IPO could enhance T3 effects. We conclude that IPO-NT3R binding is not a prominent mechanism via which the drug attenuates T3 effects in vivo. The data suggest that IPO may enhance T3 effects at the cellular level and that this enhancement may not be reflected by routinely monitored serum TSH. The latter observation may have clinical importance.