Injection Of T3 Research Articles

Differential effect of hyperthyroidism on rat epididymal glycosidases

The impact of hyperthyroidism on epididymal glycosidases was studied in albino rats. Hyperthyroidism was induced in Wistar rats aged 30 days by daily injection of T4 (25 microg/100 g body weight/day intramuscularly) for 30 or 60 days; control rats were injected with vehicle (alkaline saline, pH 7.8). One set of hyperthyroid rats was reverted to euthyroid status by withdrawing T4 treatment after 30 days of hyperthyroidism. To asses the direct effect of thyroid hormone on epididymal hexosaminidases, caput, corpus and cauda tissues were stimulated with 25, 50 or 100 ng/mL T3 for 24 h, after an initial culture of 24 h. The activity of beta-glucosidase decreased in caput, corpus and cauda epididymis of hyperthyroid rats. beta-Galactosidase activity increased in the caput epididymis irrespective of the duration of hyperthyroidism. While a similar decrease occurred in the corpus and cauda epididymis in the 30 day hyperthyroid group, an opposite trend was observed in 60 day hyperthyroid rats. Caput beta-N-acetylglucosaminidase activities increased at both time points, whereas activity decreased in the corpus and cauda in 30 day, but increased in 60 day hyperthyroid rats. Hyperthyroidism consistently increased caput and corpus beta-N-acetylgalactosaminidase activity irrespective of the duration. Cauda epididymal beta-N-acetylgalactosaminidase activity was decreased in 30 day and increased in 60 day hyperthyroid rats. Hyperthyroidism induced changes in caput beta-galactosidase, beta-N-acetylgalactosaminidases, corpus beta-N-acetylglucosaminidase and cauda beta-N-acetylgalactosaminidase which were irreversible while the remaining actvities were brought back to normal when T4 treatment was withdrawn. In vitro studies showed that T3 stimulates epididymal hexosaminidases (beta-N-acetylglucosaminidase and beta-N-acetylgalactosaminidase) irrespective of the dose. These data suggest that thyroid hormones have a specific and direct influence on glycosidases in specific regions of the epididymis.

Uncoupling Protein-3 Is a Molecular Determinant for the Regulation of Resting Metabolic Rate by Thyroid Hormone

Thyroid hormones increase energy expenditure, partly by reducing metabolic efficiency. The control of specific genes at the transcriptional level is thought to be the major molecular mechanism. However, both the number and the identity of the thyroid hormone-controlled genes remain unknown, as do their relative contributions. Uncoupling protein-3, a recently identified member of the mitochondrial transporter superfamily and one that is predominantly expressed in skeletal muscle, has the potential to be a molecular determinant for thyroid thermogenesis. However, changes in mitochondrial proton conductance and resting metabolic rate after physiologically mediated changes in uncoupling protein-3 levels have not been described. Here, in a study on hypothyroid rats given a single injection of T3, we describe a strict correlation in terms of time course between the induced increase in uncoupling protein-3 expression (at mRNA and protein levels) and decrease in mitochondrial respiratory efficiency, on the one hand, and the increase in resting metabolic rate, on the other. First, we describe our finding that uncoupling protein-3 is present and regulated by T3 only in metabolically relevant tissues (such as skeletal muscle and heart). Second, we follow the time course (at 0, 6, 12, 24, 48, 65, 96, and 144 h) of both uncoupling protein-3 mRNA levels and mitochondrial uncoupling protein-3 density in gastrocnemius muscle and heart. In both tissues, the maximal (12-fold) increase in uncoupling protein-3 density was reached at 65 h. The resting metabolic rate[ lO2(kg0.75)−1h−1] showed the same time course, and at 65 h the increase vs. time zero was 45% (1.316 ± 0.026 vs. 0.940 ± 0.007; P < 0.001). At the same time point, gastrocnemius muscle mitochondria showed a significantly higher nonphosphorylating respiration rate (nanoatoms of oxygen per min/mg protein; increase vs. time zero, 40%; 118 ± 4 vs. 85 ± 9; P < 0.05), whereas the membrane potential decreased by 8% (168 ± 2 vs. 182 ± 4; P < 0.05). These data are diagnostic of mitochondrial uncoupling. The results reported here provide the first direct in vivo evidence that uncoupling protein-3 has the potential to act as a molecular determinant in the regulation of resting metabolic rate by T3.

Quantification of Growth Hormone Receptor Extra- and Intracellular Domain Gene Expression in Chicken Liver by Quantitative Competitive RT–PCR

The very sensitive competitive reverse transcription–polymerase chain reaction (RT–PCR) was used to investigate the expression of the extracellular (GHRe) and intracellular (GHRi) parts of the growth hormone receptor (GHR) in the liver tissue of chickens. Two competitors (internal standards), pGHRi MUT and pGHRe MUT, specific to the GHRi and GHRe genes, respectively, were constructed by site-specific mutagenesis. The internal standards defined PCR products of 394 bp for the pGHRi MUT and 330 bp for the GHRe MUT. These were used as competitors to the wild-type GHRi or GHRe which defined PCR products of 382 and 328 bp, respectively. Coamplification, under standardized conditions, of the native RNA in competition with serial dilutions of the mutant RNA in the same PCR reaction followed by enzymatic digestion produced the expected sizes of internal standard cDNA and predicted target cDNA. Expression levels of GHRe and GHRi were determined from standard curves generated. The method was sensitive enough to detect expressions down to picogram levels. Applying this method, the effect of GH and T3 injection on GHRe and GHRi mRNA expression was determined in the liver of adult female Hisex birds and 1-day-old normal and dwarf chickens. Intravenous GH injection (25 μg/kg body weight) increased plasma levels of GH in Hisex birds after 10 min but rapidly decreased at 60 min followed by an increase in T3. GH injection significantly increased the expression of the GHRe 60 min after injection but not at 10 min, when the GH level in plasma was high. In the liver of saline-treated dwarf (dw) and nondwarf (Dw) chicks, the level of expression of GHRe was similar in both strains despite disparate levels of basal GH and T3. However, the level of GHRi was higher in Dw than in dw chicks. Although GH levels increased in both strains after intravenous GH injection (250 μg/kg body wt), the expression of GHRe in both strains was unaffected. However, the mRNA for the GHRi was significantly depressed by injection in the Dw but unaffected in dw chicks. Intravenous injection of T3 (0.5 and 5 μg/kg body wt) increased plasma levels in both strains but caused depression of GHRi in Dw but not in dw chicks. T3 injections had no effect on GHRe in either Dw or dw chicks. It is concluded that the expression of the GHRe in adult chickens is GH regulated either directly or indirectly. In contrast, in 1-day-old chicks, GH or T3 had no effects on the GHRe but regulated the expression of GHRi in Dw chicks, whereas in dwarf chicks both had no effect on GHRe or GHRi expression. It is postulated that GHRe and GHRi gene expression may be regulated by different agonists/antagonists in different strains and depending on the age of the chicken.

Pituitary nuclear 3,5,3?-triiodothyronine and thyrotropin secretion: An explanation for the effect of thyroxine

An excellent correlation was observed between nuclear triiodothyronine (T3) and the ensuing suppression of thyrotropin (TSH) after a single intravenous injection of T3 to thyroidectomized (hypothyroid) rats. At 1 and 2 hours after injection of thyroxine (T4), in amounts equally potent to the administered T3 in terms of acute suppression of TSH, the same quantities of T3 were found in the pituitary nuclei. Virtually no nuclear T4 was present, and plasma T3 was negligible at these short intervals after T4 injection. These results suggest that suppression of TSH release in hypothyroid rats occurs by interaction of T3 with the nuclear receptor of the thyrotroph. After T4 injection, the T3 found in the nucleus is derived from rapid intrapituitary monodeiodination.

MRNA differential display identification of thyroid hormone-responsive protein (THRP) gene in association with early phase of long-term potentiation.

The process of long-term potentiation (LTP) consists of the early induction and late maintenance phases. Few studies have examined the cellular mechanisms underlying these two phases; their respective mRNA expression profiles have not yet been elucidated. Here we used the technique of PCR differential display to identify genes that are differentially expressed between the early and late phases of LTP in vivo. Our results indicated that the cDNA fragment corresponding to one mRNA with preferentially increased expression during the early, but not late, phase of LTP encodes the rat thyroid hormone-responsive protein (THRP) gene. In situ hybridization analysis confirmed the results obtained from the PCR differential display. Prior NMDA receptor blockade with MK801 prevented induction of LTP and decreased THRP mRNA expression in the dentate gyrus, as assayed by quantitative RT-PCR analysis. THRP antisense oligonucleotide treatment before tetanic stimulation also prevented induction of LTP. However, when THRP antisense oligonucleotide was administered after induction of LTP, it did not affect expression and maintenance of LTP. THRP is known to be responsive to thyroid hormone. Our results indicate that direct thyroid hormone (T3) injection into the dentate gyrus produces a long-lasting enhancement of synaptic efficacy of these neurons. T3 injection also markedly increased THRP mRNA expression in the dentate gyrus. Taken together, our results suggest that THRP mRNA expression plays an important role in the early phase, but not the late phase, of LTP and that both THRP and thyroid hormone are involved in synaptic plasticity in hippocampal neurons.

Evidence for an Osmoregulatory Role of Thyroid Hormones in the Freshwater Mozambique Tilapia Oreochromis mossambicus

The existing equivocal reports on the osmoregulatory role of triiodothyronine (T3) and thyroxine (T4) in teleosts prompted a reinvestigation of their osmoregulatory function in the euryhaline teleost Oreochromis mossambicus. Evidence is presented for thyroidal involvement in hydromineral balance in freshwater tilapia. Dose- and tissue-related responses to various T3 and T4 concentrations were observed in the branchial and renal tissues. The branchial Na+,K+-ATPase activity, known to reflect sodium pump dynamics, increased significantly after the administration of low doses of T3 (20 and 40 ng · g−1) or T4 (40 and 80 ng · g−1). Higher doses of T3 and T4 (>160 ng · g−1) did not change the enzyme activity, compared to sham-injected fish. Conversely, the specific activity of renal Na+,K+-ATPase decreased significantly at all doses of T3 or T4. Further, immunoreactive Na+,K+-ATPase in T4-treated fish increased in branchial chloride cells and this was coupled with a significant increase in the size of chloride cells. T4 treatment, however, did not change branchial chloride cell density. Plasma osmolality, [Na+], and [Cl−] increased, whereas [K+] decreased following low doses of T3 or T4. As expected, plasma levels of T3 and T4 increased significantly in a dose-dependent manner after a single injection of either T3 or T4. The basal levels of T3 and T4 were 4.45 ± 0.49 and 1.25 ± 0.26 nmol · L−1, respectively. This study shows that physiological concentrations of T3 (<10.57 nmol · L−1) and T4 (<6.64 nmol · L−1) enhance branchial Na+ pump activity and chloride cell morphometric dynamics, favoring hyperosmoregulatory capacity in freshwater tilapia. These data are consistent with the hypothesis that thyroid hormones perform a role in hydromineral regulation in freshwater teleosts.

Expression of rat uncoupling protein family mRNA levels by chronic treatment with thyroid hormone.

The present study was conducted to assess the effects of chronic treatment with triiodothyronine (T3) on mRNA expression of uncoupling protein (UCP) family in male Wistar King A rats. Subcutaneous injection of T3 (37 nmol/body weight 100 g) over 7 d increased mRNA expression of UCP1 in brown adipose tissue (BAT), UCP2 in white adipose tissue (WAT), and UCP3 in skeletal muscle (MSL) mRNA by 1.3, 1.7, and 2.0-fold, respectively. In contrast, the expression of ob gene mRNA in WAT and serum leptin level in the T3-treated rats decreased by 0.5-fold of the controls. These results suggest that T3 may increase UCP family expression independent of leptin action.

3,5-diiodo-L-thyronine regulates glucose-6-phosphate dehydrogenase activity in the rat.

Thyroid hormones influence the activity of lipogenic enzymes such as malic enzyme (ME) and glucose-6-phosphate dehydrogenase (G6PD). The effect of T3 on ME is exerted at the transcriptional level, but it is unclear if its effect on G6PD is also nuclear mediated. Furthermore, other iodothyronines that have been shown to possess biological activity (such as diiodothyronines) could contribute to this enzyme's regulation. In this study the effects of 3,5-diiodothyronine (T2) on the aforementioned enzymes were examined and compared with those of T3. Rats made hypothyroid by propylthiouracil and iopanoic acid treatment were used throughout. Enzyme activities were determined spectrophotometrically, and G6PD messenger RNA (mRNA) expression was analyzed by Northern blotting using a human G6PD complementary DNA probe. Injections of T2 to hypothyroid animals significantly enhanced the activity of both enzymes. The effect of T2 on ME was nuclear mediated and mimicked the effect of T3. The effects of T2 and T3 on G6PD differed. Injection of T3 into hypothyroid rats induced an increase in both enzyme activity and G6PD mRNA expression, indicating a nuclear-mediated effect. The effect of T2 on G6PD activity, on the other hand, was not nuclear mediated. The injection of T2 into hypothyroid animals did not change G6PD mRNA expression, and the strong increase in the enzyme's activity (from +70% to +300%) was unaffected by simultaneous injection of protein synthesis inhibitors. As the lowest dose of 1 microg T2/100 g BW affects G6PD activity 3-5 times more than the same dose of T3, these data provide the first evidence that T2 is a factor capable of regulating G6PD activity.

Effects of thyroxine on L-cysteine desulfuration in mouse liver.

The effect of exogenous thyroxine (T4) administration on the activity of rhodanese, cystathionase, and 3-mercaptopyruvate sulfurtransferase (MPST) in the mitochondrial and cytosolic fractions of mouse liver was investigated. Three groups of mice were treated for 6 consecutive days with subcutaneous injections of T4 (50 micrograms, 100 micrograms, and 250 micrograms per 100 g of body wt, respectively). The other 3 groups were given 100 micrograms of T4 per 100 g of body wt for 1, 2, or 3 days. The dose of 100 micrograms T4 per 100 g of body wt given for 6 days exerted the strongest effect on the activity of all of the investigated enzymes. In comparison to the control, rhodanese activity diminished in the mitochondrial fraction by 40% (P < 0.05), cystathionase activity diminished in the cytosolic fraction by 15% (P < 0.05), and MPST activity in the mitochondrial fraction was reduced by 34% (P < 0.05), whereas cytosolic MPST activity was unaltered. Simultaneously, in the liver homogenate, elevated levels of ATP and sulfate were observed after 6 days of T4 administration. Thus, the present results seem to suggest that in the mouse liver, after 6 days of administration of 100 micrograms T4 per 100 g of body wt, the desulfuration metabolism of L-cysteine is diminished, which is probably accompanied by an increase in oxidative L-cysteine metabolism. The dose of 100 micrograms per 100 g of body wt administered for a shorter period, and the use of a lower dosage (50 micrograms T4 per 100 g of body wt) for 6 days had a stimulatory effect upon MPST activity level, and an increased level of sulfane sulfur was observed.

Effect of Thyroxine and Triiodothyronine on the Amount and Constituents of Milk in the Sarabi Dairy Cows Treated with Growth Hormone

The goal of this study was to determine whether thyroxine (T4) and triiodothyronine (T3) injections increase the amount and constituents of milk in the bovine growth hormone (bGH) treated dairy cows. Each group of 4 Sarabi received daily injections of either saline, 5μg T3/kg BW, 50μg T4, /kg BW, 25μg bGH/kg BW, 5μg T3/kg BW plus 25μg bGH/kg or 50μg T4/kg BW plus 25μg bGH/kg. Blood samples were collected every other day on day 7 till day 15. Plasma T3, T4 and bGH concentrations were measured by double-antibody RIA. Milk samples were daily collected from day 1 till day 20. Injections of T3, T4 and bGH significantly increased the plasma T3, T4 and bGH level and the amount of milk by 7%, 18% and 24% respectively. Injections of bGH plus T4 significantly increased the amount of milk 34%. Protein, fat and lactose concentrations of milk were not changed among all animals in the different groups. The results indicated that T4 increased the amount and constituents of milk in the bGH treated dairy cows.

Role of nitric oxide in the reperfusion induced injury in hyperthyroid rat hearts

We recently reported that hyperthyroidism affects the heart response to ischemia/reperfusion. A significant tachycardia during reperfusion was associated with an increase in the oxidative stress of hearts from T3-treated animals. In the present study we checked the possible role of nitric oxide (NO) in this major stress induced by the hyperthyroid state. We compared the functional recovery from ischemia/reperfusion of Langendorff preparations from euthyroid (E) and hyperthyroid (H, ten daily intraperitoneal injections of T3, 10 μg/100 g body weight) rats, in the presence and in the absence of 0.2 mM Nω-nitro-L-arginine (L-NNA). At the end of the ischemia/reperfusion protocol (10 min preischemic perfusion, 20 min global ischemia, 30 min reperfusion) lipid peroxidation, antioxidant capacity (CA) and susceptibility to in vitro oxidative stress were determined on heart homogenates. The main effect of hyperthyroidism on the reperfusion functional response was confirmed to be a strong tachycardic response (154% recovery at 25 min reperfusion) accompanied by a low recovery in both left ventricular diastolic pressure (LVDP) and left ventricular dP/dtmax. This functional response was associated with a reduction in CA and an increase in both lipid peroxidation and susceptibility to oxidative stress. Perfusion of hearts with L-NNA per se had small but significant negative chronotropic and positive inotropic effects on preischemic performance of euthyroid rat hearts only. More importantly, L-NNA perfusion completely blocked the reperfusion tachycardic response in the hyperthyroid rats. Concomitantly, myocardium oxidative state (lipid peroxidation, CA and in vitro susceptibility to oxidative stress) of L-NNA perfused hearts was similar to that of E animals. These results suggest that the higher reperfusion-induced injury occurring in hyperthyroid animals is associated with overproduction of nitric oxide.

Involvement of T3Rα- and β-receptor subtypes in mediation of T3 functions during postnatal murine intestinal development

Background & Aims: Thyroid hormones are implicated in intestinal development. Their effects are mediated by nuclear receptors, which are transcriptional regulators activated upon binding of triiodothyronine. The aim of this study was to define the involvement of the receptor subtypes during intestinal development. Methods: We used strains of knockout mice lacking T3Rα, T3Rβ, or both receptors, encoded by T3Rα and T3Rβ genes. Results: Morphological features and expression of digestive enzymes and of two intestinal regulators, Cdx-1 and Cdx-2, were compared in wild-type and T3Rα, T3Rβ, and T3Rαβ knockout animals. T3Rα−/− mice had abnormal intestinal morphology, assessed by a decrease in the number of epithelial cells along the crypt-villus axis and a decrease in proliferating crypt cells. Expression of Cdx-1 and Cdx-2, and of the digestive enzymes, was down-regulated. These parameters can be partially reversed by T3 injection. A similar (jejunum) or more severe (ileum) phenotype was found in T3Rαβ double mutants. In contrast, no changes occurred in T3Rβ mice. Conclusions: These data describe for the first time a direct effect of TH through the T3Rα-receptor subtypes on postnatal intestinal mucosa maturation. They also suggest that T3Rβ receptors are dispensable but can partially substitute for T3Rα. GASTROENTEROLOGY 1999;116:1367-1378

Differential Diagnosis of Thyroid Crisis and Malignant Hyperthermia in an Anesthetized Porcine Model

The intra-operative differential diagnosis between thyroid crisis and malignant hyperthermia can be difficult. Also stress alone can trigger MH. The purposes of this study were: 1) to investigate the metabolic and hemodynamic differences between thyroid crisis and MH, 2) determine how thyroid crisis affects the development of MH, and 3) determine if the stress of thyroid crisis can trigger MH in susceptible individuals. We studied MH susceptible and normal swine. Two groups of animals (MH susceptible and normal) were induced into thyroid crisis (critical core hyperthermia, sustained tachycardia and increase in oxygen consumption) by pretreatment with intraperitoneal triiodothyronine (T3) followed by large hourly intravenous injections of T3. Two similar groups were given intravenous T3 but no pretreatment. These animals did not develop thyroid crisis and served as controls. Thyroid crisis did not result in metabolic changes or rigidity characteristic of an acute episode of MH. When the animals were subsequently challenged with MH triggering agents (halothane plus succinylcholine) dramatic manifestations of fulminant MH episodes (acute serious elevation in exhaled carbon dioxide, arterial CO2, rigidity and acidemia) were noted only in the MH susceptible animals. Although thyroid crisis did not trigger MH in the susceptible animals it did decrease the time to trigger MH (14.1 ± 7.2 minutes versus 47.2 ± 17.7 minutes, p < 0.01) in susceptible animals. Hormone induced elevations in temperature and possibly other unidentified factors during thyroid crisis may facilitate the triggering of MH following halothane and succinylcholine challenge.

Feedback Regulation of Thyrotropin-Releasing Hormone Gene Expression by Thyroid Hormone in the Caudal Raphe Nuclei in Rats**This work was supported by NIH Grants DK-50255 (H.Y.), DK-30110 (Y.T.), and DK-41301, Pilot and Feasibility Award (to H.Y.).

Medullary TRH regulates autonomic activity, and altered thyroid status is associated with autonomic disorders. We investigated whether thyroid hormone exerts a negative feedback regulation on TRH gene expression in the medullary caudal raphe nuclei. Medullary pro-TRH messenger RNAs (mRNAs) were mainly located in the raphe pallidus and raphe obscurus neurons as shown by in situ hybridization and were significantly increased by 70% and 160-230% by Northern blot analyses in 24 h fasted rats at 1 and 3-5 weeks after thyroidectomy, respectively, when serum T4 levels were reduced by 75-87%. The increased pro-TRH mRNA on the 30th day after thyroidectomy was reversed to euthyroid levels by T4 replacement (2 or 4 microg/100 g x day). T4 injections (10 or 100 microg/100 g x day for 30 days) did not significantly influence medullary pro-TRH mRNA levels in sham-operated rats. Thyroidectomized rats fed normally showed a 500% increase in pro-TRH mRNA levels 30 days after the surgery, while those fasted for 24 h showed only a 180% increase. These data indicate that medullary TRH gene expression is enhanced during hypothyroidism due to the lack of negative feedback regulation by thyroid hormone, and this response is modulated by feeding state. These findings may have important relevance to understanding autonomic-related visceral alterations induced by hypothyroidism.

Development of the adult-type Leydig cell population in the rat is affected by neonatal thyroid hormone levels.

We have investigated the effects of neonatal-prepubertal changes in thyroid hormone levels on the early phases of adult-type Leydig cell development in the rat testis. Hypothyroidism was induced by adding 6-propyl-2-thiouracil (PTU) to the drinking water, while hyperthyroidism was induced by daily injections with triiodothyronine (T3). The proliferative activity of the Leydig cells in PTU-treated animals was not different from that in age-matched controls through the age of 16 days. Nevertheless, the percentage of Leydig cells (i.e., the proportion of Leydig cells among the total interstitial cell population) was approximately 83% and 67% lower at the ages of 12 and 16 days, respectively. The proliferative activity of the Leydig cells in the T3-treated animals compared to the controls was increased approximately 3-fold at the ages of 12 and 16 days. The percentage of Leydig cells in T3-treated animals was also considerably increased at these two ages (400% and 725%, respectively). Concomitantly, the percentage of peritubular cells was decreased, suggesting that the increase in the percentage of Leydig cells may at least partially be the result of differentiation of peritubularly located precursor cells. Plasma testosterone levels fluctuated considerably at these ages. Hence, injection of T3 during the neonatal-prepubertal period not only affects Sertoli cell proliferation and differentiation but also directly or indirectly affects the onset of the formation of the adult-type Leydig cell population and its function.

Effects on temperature on spontaneous and thyroxine‐stimulated locomotor activity of Atlantic cod

Spontaneous locomotor activity of cod Gadus morhua maintained at 6° C tripled from February to May. In contrast, locomotor activity of cod held at 2° C was significantly lower than at 6° C (between 25 and 65% lower) and the seasonal increase was smaller. Plasma levels of both thyroxine (T4) and triiodothyronine (T3) did not differ between 2 and 6° C. T4 injection increased locomotor activity by 10% for both temperature regimes. These data indicate that low water temperature reduces locomotor activity associated with migration in cod and that thyroid hormones are not involved in this decrease. This study provides a possible mechanism through which cold waters may affects migration and distribution of cod via its Effects on locomotor activity and swimming speed.

Direct effects of thyroid hormones on rat coronary artery: nongenomic effects of triiodothyronine and thyroxine.

To determine whether thyroid hormones, triiodothyronine (T3) and thyroxine (T4), have any direct, nongenomic effects on vascular smooth muscle cells, we evaluated the effects of these hormones on rat coronary arteries. Bolus injection of T3 or T4 elicited a transient, dose-dependent decrease in coronary perfusion pressure (CPP), as well as an increase in arterial vasodilation. Vasodilation occurred immediately after injection, peaked at 15 seconds, and lasted 80 seconds. Reverse T3 had no effect on CPP or vasodilation. The rapidity of these effects suggests that they are not mediated by the T3-nuclear receptor, but are direct, nongenomic effects of thyroid hormones. Our results also suggest that thyroid hormones may play a role in preventing myocardial ischemia by inducing coronary artery vasodilation.

Thyroid hormones are involved in insulin-like growth factor-I (IGF-I) production by stimulating hepatic growth hormone receptor (GHR) gene expression in the chicken

Effect of thyroid status on IGF-I production in growing chickens was studied. Serum concentrations of GH were not affected by propylthiouracil (PTU) or thyroxine (T4) treatments, whereas serum IGF-I levels were significantly decreased in PTU-treated chickens. The lowered serum IGF-I levels in the PTU-treated group were completely restored to the control levels by T4 injections. In the liver, the messenger RNA (mRNA) expressions both for GH receptor (GHR) and IGF-I were significantly repressed by PTU treatment, and were restored again by T4 replacement. In addition, the results of analysis on radiolabelled GH binding to the liver membrane were consistent with the levels of hepatic GHR mRNA expression. Serum concentrations of IGF-I were positively correlated with hepatic IGF-I mRNA and GHR mRNA expressions. The correlation coefficient between serum T3 levels and hepatic IGF-I mRNA expressions was also significant. These results indicate that thyroid hormones regulate IGF-I production in the chicken by affecting hepatic GHR expression.

Effects of thyroid hormone (thyroxine) and testosterone on hepatic 11beta-hydroxysteroid dehydrogenase mRNA and activity in pubertal hypothyroid male rats.

To investigate the effects of thyroid hormone and testosterone on 11beta-hydroxysteroid dehydrogenase type 1 (11beta-HSD1), we measured changes in hepatic 11beta-dehydrogenase activity and its mRNA levels in pubertal methimazole (MMI)-induced hypothyroid male rats following treatment with thyroxine ([T4] 50 microg/kg/d) or testosterone (250 microg/d) for 14 days. Hypothyroidism in male rats markedly reduced hepatic 11beta-HSD1 mRNA levels and serum testosterone concentrations (P < .01). Subcutaneous injection of T4 in the hypothyroid rats significantly (P < .01) increased hepatic 11beta-HSD1 mRNA to approximately normal levels and simultaneously increased serum testosterone levels. However, the same daily dose of T4 administered to castrated male hypothyroid rats for 14 days did not elevate hepatic 11beta-HSD1 activity. Treatment with testosterone for 14 days in castrated hypothyroid male rats and rats without gonadectomy significantly (P < .01) increased the enzyme activity without administration of T4. Variations in hepatic 11beta-HSD1 activity were demonstrated to be accompanied by changes in serum testosterone levels in the rats following alteration of the thyroid hormone state. These results suggest that the effect of T4 in increasing the subnormal 11beta-HSD1 gene expression in hypothyroid male rats is mediated by its ability to increase testosterone production in these rats, because in castrated hypothyroid rats, T4 does not elevate 11beta-HSD1 gene expression.

Thyroid Hormone-Induced Changes in Cytoplasmic and Mitochondrial Proteins of a Teleost

Abstract The effect of triiodothyronine (T3) on the cytoplasmic and mitochondrial protein contents were studied in the liver and skeletal muscle of a freshwater teleost. The fish exposed to thiouracil for 28 days showed 1.5-2 times reduction in the total protein contents of cytoplasmic and mitochondrial fractions. A single injection of T3 to thiouracil exposed fish caused the earliest induction in the liver and skeletal muscle mitochondrial protein and the skeletal muscle cytoplasmic protein at 12 hr of lapses. However, the initial induction in the cytoplasmic protein of the liver was observed at 3 hr after T3 treatment. The maximum inductions (1.5-3.2 fold) in the cytoplasmic and mitochondrial proteins of the liver and skeletal muscle were obtained at 1 8 -2 4 hr following hormonal administration. Thereafter, the cytoplasmic and mitochondrial protein contents of both the tissues declined to their control levels within 3 6 - 4 8 hr of T3 injection which reflected the half-life and turnover period of the induced proteins. These T3 dependent inductions in the cytoplasmic and mitochondrial proteins of the liver (1 .4 -3 .2 fold) and skeletal muscle (1.8 -2.7 fold) were inhibited by actinomycin D and cycloheximide indicating T3-induced de novo synthesis of the proteins. The induction in the cytoplasmic protein (3 fold) was almost double to that of the mitochondrial protein (1.6 fold) suggesting more synthesis of protein molecules in the cytoplasm for cellular and subcellular activities.