Oxidative metabolism in a teleost, Anabas testudineus Bloch: effect of thyroid hormones on hepatic enzyme activities

Steroid androgen 17 alpha methyltestosterone used in fish farming induces biochemical alterations in zebrafish adults

The 17 alpha methyltestosterone (MT) hormone is fed to Oreochromis niloticus larvae in fish farms with the purpose of inducing sex reversal. The aim of this study was to evaluate the toxicity and sub-lethality of MT (99.9% purity) and cMT (a commercial MT with 90% purity) in zebrafish (Danio rerio) adults, where the animals were exposed to concentrations of 0, 4, 23, 139, 833 and 5000 µg/L for 96 hours. Genotoxicity was evaluated by micronucleus test (MN), nuclear abnormalities (NA) and comet assay. A low genotoxic potential of MT was showed, inducing micronucleus, nuclear abnormalities and DNA damage in Danio rerio, depending on the use of MT or cMT, gender and tested concentrations. In the sub-lethality trials, there was a basal difference in the activity of the enzymatic biochemical markers for males and females, while the Glutatione S transferase (GST) activity decreased in all analyzed tissues, and for males the enzymatic activity decreased only in the intestine. Results suggest that MT has a toxic potential to fish because it alters enzymatic metabolic pathways and may pose a risk to the ecosystems.

Exogenous thyroid hormones regulate the activity of citrate synthase and cytochrome c oxidase in warm- but not cold-acclimated lake whitefish (Coregonus clupeaformis)

Thermal acclimation is known to elicit metabolic adjustments in ectotherms, but the cellular mechanisms and endocrine control of these shifts have not been fully elucidated. Here we examined the relationship between thermal acclimation, thyroid hormones and oxidative metabolism in juvenile lake whitefish. Impacts of thermal acclimation above (19°C) or below (8°C) the thermal optimum (13°C) and exposure to exogenous thyroid hormone (60µg T₄/g body weight) were assessed by quantifying citrate synthase and cytochrome c oxidase activities in liver, red muscle, white muscle and heart. Warm acclimation decreased citrate synthase activity in liver and elevated both citrate synthase and cytochrome c oxidase activities in red muscle. In contrast, induction of hyperthyroidism in warm-acclimated fish stimulated a significant increase in liver citrate synthase and heart cytochrome c oxidase activities, and a decrease in the activity of both enzymes in red muscle. No change in citrate synthase or cytochrome c oxidase activities was observed following cold acclimation in either the presence or absence of exogenous thyroid hormones. Collectively, our results indicate that thyroid hormones influence the activity of oxidative enzymes more strongly in warm-acclimated than in cold-acclimated lake whitefish, and they may play a role in mediating metabolic adjustments observed during thermal acclimation.

The role of thyroid hormones in stress response of fish

Thyroxine (T(4)) and triiodothyronine (T(3)), the principal thyroid hormones (THs) secreted from the hypothalamic-pituitary-thyroid (HPT) axis, produce a plethora of physiologic actions in fish. The diverse actions of THs in fishes are primarily due to the sensitivity of thyroid axis to many physical, chemical and biological factors of both intrinsic and extrinsic origins. The regulation of THs homeostasis becomes more complex due to extrathyroidal deiodination pathways by which the delivery of biologically active T(3) to target cells has been controlled. As primary stress hormones and the end products of hypothalamic-pituitary-interrenal (HPI) and brain-sympathetic-chromaffin (BSC) axes, cortisol and adrenaline exert its actions on its target tissues where it promote and integrate osmotic and metabolic competence. Despite possessing specific osmoregulatory and metabolic actions at cellular and whole-body levels, THs may fine-tune these processes in accordance with the actions of hormones like cortisol and adrenaline. Evidences are presented that THs can modify the pattern and magnitude of stress response in fishes as it modifies either its own actions or the actions of stress hormones. In addition, multiple lines of evidence indicate that hypothalamic and pituitary hormones of thyroid and interrenal axes can interact with each other which in turn may regulate THs/cortisol-mediated actions. Even though it is hard to define these interactions, the magnitude of stress response in fish has been shown to be modified by the changes in the status of THs, pointing to its functional relationship with endocrine stress axes particularly with the interrenal axis. The fine-tuned mechanism that operates in fish during stressor-challenge drives the THs to play both fundamental and modulator roles in stress response by controlling osmoregulation and metabolic regulation. A major role of THs in stress response is thus evident in fish.

Effects of growth hormone, insulin-like growth factor I, triiodothyronine, thyroxine, and cortisol on gene expression of carbohydrate metabolic enzymes in sea bream hepatocytes

The present study investigated the regulatory effects of growth hormone (GH), human insulin-like growth factor I (hIGF-I), thyroxine (T(4)), triiodothyronine (T(3)) and cortisol, on mRNA expression of key enzymes involved in carbohydrate metabolism, including glucokinase (GK), glucose-6-phosphatase (G6Pase), glycogen synthase (GS), glycogen phosphorylase (GP) and glucose-6-phosphate dehydrogenase (G6PDH) in hepatocytes isolated from silver sea bream. Genes encoding GK, G6Pase, GS and GP were partially cloned and characterized from silver sea bream liver and real-time PCR assays were developed for the quantification of the mRNA expression profiles of these genes in order to evaluate the potential of these carbohydrate metabolic pathways. GK mRNA level was elevated by GH and hIGF-I, implying that GH-induced stimulation of GK expression may be mediated via IGF-I. GH was found to elevate GS and G6Pase expression, but reduce G6PDH mRNA expression. However, hIGF-I did not affect mRNA levels of GS, G6Pase and G6PDH, suggesting that GH-induced modulation of GS, G6Pase and G6PDH expression levels is direct, and occurs independently of the action of IGF-I. T(3) and T(4) directly upregulated transcript abundance of GK, G6Pase, GS and GP. Cortisol significantly increased transcript amounts of G6Pase and GS but markedly decreased transcript abundance of GK and G6PDH. These changes in transcript abundance indicate that (1) the potential of glycolysis is stimulated by GH and thyroid hormones, but attenuated by cortisol, (2) gluconeogenic and glycogenic potential are augmented by GH, thyroid hormones and cortisol, (3) glycogenolytic potential is upregulated by thyroid hormones but not affected by GH or cortisol, and (4) the potential of the pentose phosphate pathway is attenuated by GH and cortisol but unaffected by thyroid hormones.

Acclimation of Solea senegalensis to different ambient temperatures: implications for thyroidal status and osmoregulation

We have investigated the regulation of thyroidal status and osmoregulatory capacities in juveniles from the teleost Solea senegalensis acclimated to different ambient temperatures. Juveniles, raised in seawater at 19°C, were acclimated for 3 weeks to temperatures of 12, 19 and 26°C. Since our preliminary observations showed that at 12°C feed intake was suppressed, our experimental design controlled for this factor. The concentration of branchial Na⁺,K⁺-ATPase, estimated by measurements of enzyme activity at the optimum temperature of this enzyme (37°C), did not change. In contrast, an increase in Na⁺,K⁺-ATPase activity (measured at 37°C), was observed in the kidney of 12°C-acclimated fish. In fish acclimated to 12°C, the hepatosomatic index had increased, which correlated with increased plasma levels of triglycerides and non-esterified fatty acids. Plasma cortisol levels did not differ significantly between the experimental groups. In liver and gills, the amount of iodothyronine deiodinases that exhibit thyroid hormone outer ring deiodination was up-regulated only when fish did not feed. When assayed at the acclimation temperature, kidney deiodinase activities were similar, indicating a temperature-compensation strategy. 3,5,3'-triiodothyronine (T3) tissue concentrations in gills and kidney did not differ significantly between experimental groups. However, at 12°C, lower T3 tissue levels were measured in plasma and liver. We conclude that S. senegalensis adjusts its osmoregulatory system to compensate for the effects of temperature on electrolyte transport capacity. The organ-specific changes in thyroid hormone metabolism at different temperatures indicate the involvement of thyroid hormones in temperature acclimation.

The involvement of thyroid hormones and cortisol in the osmotic acclimation of Solea senegalensis

The peripheral conversion of the prohormone 3,5,3',5'-tetraiodothyronine (T4) to the biologically active 3,5,3'-triiodothyronine (T3), via enzymatic deiodination by deiodinases, is an important pathway in thyroid hormone metabolism. The aim of this study was to test if thyroid hormones and cortisol, as well as the outer ring deiodination (ORD) metabolic pathway, are involved in the osmoregulatory response of Senegalese sole (Solea senegalensis, Kaup 1858). We measured osmoregulatory and endocrine parameters in immature juveniles S. senegalensis acclimated to seawater (SW, 38 per thousand) and that were transferred and allowed to acclimate to different salinities (5 per thousand, 15 per thousand, 38 per thousand and 55 per thousand) for 17 days. An adjustment and a chronic regulatory period were identified following acclimation. The adjustment period immediately follows the transfer, and is characterized by altered plasma osmolalities. During this period, plasma cortisol levels increased while plasma free T4 (fT4) levels decreased. Both hormones levels returned to normal values on day 3 post-transfer. In the adjustment period, renal and hepatic ORD activities had increased concomitantly with the decrease in plasma fT4 levels in fishes transferred to extreme salinities (5 per thousand and 55 per thousand). In the chronic regulatory period, where plasma osmolality returned to normal values, plasma cortisol had increased, whereas plasma fT4 levels decreased in animals that were transferred to salinities other than SW. No major changes were observed in branchial ORD activity throughout the experiment. The inverse relationship between plasma cortisol and fT4 suggests an interaction between these hormones during both osmoregulatory periods while ORD pathway can be important in the short-term adjustment period.

Metabolic and thyroidal response in air-breathing perch (Anabas testudineus) to water-borne kerosene

To address the physiological compensatory adaptations in air-breathing fish to a toxicant, we studied the metabolite pattern, serum and liver enzymes and thyroidal response in a tropical air-breathing perch, Anabas testudineus (kept at 30 degrees C in a 12-h L:D cycle) after exposing the fish for 48h to the water-soluble fraction of kerosene. The concentrations of serum glucose (P <0.05), triglycerides (P <0.01) and liver total protein (P <0.05) were significantly increased in kerosene-exposed fish. The serum urea level, however, remained unaffected. A significant (P <0.05) increase in liver RNA occurred without changing the liver DNA concentration. Kerosene exposure decreased the level of aspartate aminotransferase activities in serum (P <0.001) and liver (P <0.05) but it increased (P <0.05) the liver alanine aminotransferase activity without changing its activity in serum. The levels of serum (P <0.01) and liver (P <0.001) lactate dehydrogenase activity were declined and the serum (P <0.05) and liver (P <0.05) alkaline phosphatase activity levels were elevated in kerosene-treated fish. The nominated levels (3.33-6.66ml/L) of kerosene significantly (P <0.01) elevated the thyroxine (T(4)) titre, and reduced (P <0.05) the triiodothyronine (T(3)) titre. The fish pretreated with either T(3) or T(4) and exposed to kerosene had a metabolic and thyroidal response that differed from that in control fish treated with kerosene: no rise in serum glucose was observed, nor in triglycerides, total protein and RNA in the liver, whereas declined levels of T(4) and T(3) were observed. The upregulation of the thyroid along with the marked metabolite changes point to a positive involvement of thyroid in energy metabolism during kerosene exposure. This is consistent with the hypothesis that the fish thyroid responds to the action of petroleum products and influences the metabolic homeostasis of this air-breathing fish.

Differential effects of thyroxine on metabolic enzymes and other macromolecules in a freshwater teleost

The effects of thyroxine (T(4)) on citrate synthase (CS), glucose 6-phosphate dehydrogenase (G6-PDH), lactate dehydrogenase (LDH), DNA, RNA, and protein of various tissues were studied to elucidate the hormonal control of metabolism in a freshwater catfish, Clarias batrachus. T(4) did not produce any significant effect on DNA content of the fish. The CS, RNA, and protein contents of brain, liver, and skeletal muscle of the fish exposed to thiourea for 28 days decreased approximately 50-58% as compared to their levels in control individuals. Injection of T(4) to thiourea-exposed fish produced about three-fold increases in CS, RNA, and protein. These macromolecular inductions by T(4) were blocked by actinomycin D or cycloheximide. This suggests T(4)-induced de novo synthesis of macromolecules and stimulation of aerobic capacity. However, the activities of G6-PDH and LDH of brain, liver, and skeletal muscle of the fish exposed to thiourea increased two times that of the activities in control individuals. Administration of T(4) to thiourea-exposed fish reduced LDH and G6-PDH activities by about 64-74%, which reflects T(4)-dependent inhibition in anaerobic power and selective anabolic activities of the HMP pathway. These differential effects of T(4) on some metabolic enzymes and other important macromolecules may be to meet the other T(4)-induced responses in the freshwater catfish.

Thyroid hormones regulate lipid metabolism in a teleost Anabas testudineus (Bloch)

We compared the long-term action of 3,5,3'-triiodo-L-thyronine (T3) and 3,5-diiodo-L-thyronine (T2) on lipid metabolism in a teleost Anabas testudineus. Among the six groups of animals used in this experiment, except for the control group, all received 6-propylthiouracil (6-PTU) to create a hypothyroid state in order to analyse the action of iodothyronines on lipid metabolism. Injections of 6-PTU reduced T3 concentration in the circulation by 79.6% and injections of iodothyronines enhanced the level of T3 in the plasma, and a maximum increase was observed in T3 (500 ng)-treated specimens. Analysis of lipogenic enzymes in liver and heart showed that a tissue-specific variation exists in the action of thyroid hormones and, in many cases, activity is higher in T2-treated groups. Analysis of various lipid classes showed that long-term administration of T2 is also effective in producing a comparable effect with that of T3 on lipid metabolism.

Studies on the effect of growth hormone in vivo and in vitro on lipogenic enzymes and transaminases in a teleost Anabas testudineus (BLOCH)

The specific activities of three lipogenic enzymes, malic enzyme (ME), glucose-6-phosphate dehydrogenase (G6PDH) and isocitrate dehydrogenase (ICDH), in liver and heart and two transaminases (AST & ALT) in liver and muscle, were studied in response to the in vivo and in vitro administration of growth hormone (GH) in a teleost Anabas testudineus. Ovine growth hormone (oGH) in vivo significantly reduced the activities of lipogenic enzymes, except for heart G6PDH, which showed an increase at the highest dose of hormone. Transaminase activity either increased or decreased depending on the dose of GH. The lowest dose of hormone employed (0.1 microg/gm b/w) exhibited a stimulatory effect and the highest dose (0.5 microg/gm. b/w) an inhibitory effect on transaminase activity. Both ovine GH and carp GH (oGH and cGH) in vitro significantly reduced the activities of ME, G6PDH and ICDH. Activities of AST and ALT were increased by oGH and cGH in vitro. The present study reveals that irrespective of origin, GH in vitro has a direct inhibitory effect on lipogenic enzymes ME, G6PDH, ICDH and a stimulatory effect on transaminases AST and ALT in A. testudineus, thus favoring gluconeogenesis.

Short-term effects of 3,5,3'-triiodothyronine on the intermediary metabolism of the dogfish shark Squalus acanthias: evidence from enzyme activities

Plasma 3,5,3'-triiodothyronine (T3) concentration decreased significantly (P < 0.05), during 1-5 days of captivity, from levels in the freshly caught dogfish shark Squalus acanthias. The short-term effects of T3 treatment on the intermediary metabolism of S. acanthias were measured in the gill, kidney, liver, and white muscle. Animals were kept for 1-5 days before experimentation. Three hours after an intraperitoneal injection with either a low T3 dose (8.3 pmol T3/kg fish) or a high T3 dose (830 pmol T3/kg fish), selected enzymes of amino acid metabolism, lipid catabolism, ketone body metabolism, glycolysis, and oxidative metabolism were measured. Activity of enzymes of amino acid metabolism and lipid catabolism increased significantly (P < 0.05) in the liver of fish treated with a low T3 dose. The low dose of T3 apparently influences glycolysis as pyruvate kinase activity significantly increase (P < 0.05) in the kidney and white muscle.

Interactive effects of high stocking density and triiodothyronine-administration on aspects of the in vivo intermediary metabolism and in vitro hepatic response to catecholamine and pancreatic hormone stimulation in brook charr, Salvelinus fontinalis

Brook charr (Salvelinus fontinalis) were maintained at one of two stocking densities (SD) (30 or 120 kg/m3) and fed either a control or a T3-supplemented (20 mg/kg) diet for 30 days in order to investigate possible interactive effects of SD and T3-administration on growth, feeding rate, food conversion efficiency, and hepatic and dark muscle enzyme activity. In addition, liver slices were incubated in vitro for 6 h with epinephrine, norepinephrine, isoproterenol, propranolol, insulin, glucagon, or somatostatin to evaluate possible SD-T3 interactive effects on hepatic responses to hormonal stimulation. Maintaining the fish at high SD appeared to increase the clearance rate of T3 from the T3-supplemented group. There was no clear evidence of SD-T3 interactive effects on growth rate, feeding rate, or food conversion efficiency, although T3-administration decreased food conversion efficiency, and high SD decreased growth and feeding rates. Of the hepatic enzymes studied, HOAD, malic enzyme, G6PDH, CS, PFK, HK, and GDH activities all showed changes suggestive of interactive SD-T3 effects. Although hepatic FBPase was stimulated by both high SD and T3-administration, there was no evidence of interactive SD-T3 effects. Dark muscle HOAD, CS, and PFK also showed SD-T3-related responses; dark muscle malic enzyme, G6PDH, HK, and GDH were unaffected by either altered SD or T3-administration. Prior treatment of the fish with T3 and high SD had significant effects on free fatty acid (ffa) release to the medium and on hepatic lipid content, but had no effect on the responses to the various endocrine agents used. Glucose release from liver slices of fish stocked at high density (both T3-supplemented and controls) was higher than that of the fish stocked at low density; with the exception of insulin and glucagon, glucose release was similar in all pre-treatment groups. The insulin- and glucagon-stimulated changes in glucose release seen in the fish fed non-supplemented diets were not found in the two groups of fish fed the T3-supplemented diets. High SD and/or T3-administration induced significant lowering of hepatic glycogen content, but there was no effect of pre-treatment on the response to any of the endocrine agents used. The data show a marked effect of SD on energy partitioning processes in brook charr and the animal's ability to respond to T3-stimulation, but provided no evidence of such effects on the liver response to the various agents used.

Role of thyroid hormones in tilapia larvae (Oreochromis mossambicus): II. Changes in the hormones and 5'-monodeiodinase activity during development

Thyroid hormone profiles and 5'-monodeiodinase activity were determined in tilapia at different stages of early development. The results showed that both T4 and T3 were present in significant amounts in fertilized eggs. There was a steady decrease in both T4 and T3 levels during embryonic development. The levels continued to decline after hatching until around 7 days later when most of the yolk had been absorbed. The T4 level started to rise then, suggesting that the larval thyroid had begun to produce T4 at this time, which coincided with the period of faster growth of the larvae. The T3 level remained fairly constant until around 20 days after which it rose significantly. In vitro determination of 5'-monodeiodinase activity (5'-D activity) in the whole-body homogenates of larvae showed that the enzymatic conversion of T4 to T3 was not detectable in eggs and 3-day-old larvae but detected in 5-day-old and older larvae. There was a gradual increase in the Vmax as development proceeded indicating increasing 5'-D activity during larval development. The Km values did not differ significantly in the different stages of development. These results are discussed in relation to the growth and development of the larvae.

The effects of thyroid hormones and starvation on hepatic mitochondrial nucleic acids of rainbow trout (Oncorhynchus mykiss)

Twenty four hours after an intraperitoneal injection of thyroxine (T4; 4.4, 44 ng/g body wt) or triiodothyronine (T3; 3.3, 33 ng/g body wt), DNA and RNA were significantly reduced in isolated liver mitochondria of rainbow trout. Total liver DNA was increased with the higher doses in both T4- and T3-injected specimens while total RNA was significantly reduced with the same doses. Total circulating plasma T4 was reduced with the injections of T3 or starvation, and plasma T3 was increased severalfold with the T3 injections. Plasma T4 also increased fivefold after a higher dose of T4 injection while starvation significantly reduced the concentration, when measured by radioimmunoassay.

Stimulation of oxidative metabolism by thyroid hormones in an apodan amphibian, Gegenophis carnosus (Beddome)

Administration of different doses of L-thyroxine (T4) and triiodo-L-thyronine (T3) in vivo in G. carnosus stimulated the activities of cytochrome oxidase, alpha-glycerophosphate dehydrogenase (alpha-GPDH), succinate dehydrogenase (SDH), and Mg2+ adenosine triphosphatase (Mg2+ ATPase) and inhibited the activity of malate dehydrogenase (MDH). While a low dose of thiouracil administration produced a stimulatory effect on cytochrome oxidase and alpha-GPDH activities, a higher dose of thiouracil significantly inhibited the activities of cytochrome oxidase, alpha-GPDH, SDH, Mg2+ ATPase, and MDH. Injection of T4 or T3 into thiouracil-treated animals significantly restored the stimulatory effect of thyroid hormones on oxidative enzyme activities. It is suggested that thyroid hormones in vivo increase and that thiouracil decreases the oxidative capacity of hepatic mitochondria of G. carnosus.

Effects of corticosteroids and protein synthesis inhibitors on activities of oxidative enzymes in a bony fish, Anabas testudineus (Bloch)

The activities of oxidative enzymes of hepatic mitochondria of Anabas testudineus increased following exogenous administration of hydrocortisone and corticosterone. The inhibitors of protein synthesis prevented the hormone-mediated increase of hepatic enzymes, suggesting that corticosteroids regulate the oxidative metabolism of this teleostean species through the protein synthetic machinery of liver.

Intermediary metabolism in a lizard, Calotes versicolor: role of thyroid hormones

In Calotes versicolor, thyroidectomy did not alter the blood glucose level, lactate dehydrogenase (LDH liver and heart), acid phosphatase (Ac.Pase liver and kidney), and alkaline phosphatase (Alk.Pase liver and kidney) activities; significantly decreased the activities of glucose-6-phosphatase (G-6-Pase liver and kidney), glutamic oxaloacetic transaminase (GOT liver and heart), glutamic pyruvic transaminase (GPT liver), and urea concentration (liver and kidney); and increased liver cholesterol when compared to sham-operated controls. Administration of L-thyroxine (L-T4) or triiodo-L-thyronine (L-T3) to thyroidectomized lizards significantly stimulated the activities of G-6-Pase, Ac.Pase, GOT and GPT, concentration of glucose and urea, and decreased the cholesterol level. While the activities of all the enzymes studied and cholesterol level remain unchanged, glucose and urea levels decreased and increased, respectively, in thyroidectomized animals treated with actinomycin D. Chloramphenicol treatment did not affect any of the parameters studied. Simultaneous injections of actinomycin D or chloramphenicol with L-T4 prevented the hormone-stimulated activities of Ac.Pase, GOT, and GPT while the activities of LDH, G-6-Pase, Alk.Pase, glucose, urea, and cholesterol levels remain unchanged.

Oxidative metabolism in a teleost,Anabas testudineus Bloch: effect of testosterone and estradiol-17β on hepatic enzyme activities

Testosterone (T) administration to maleAnabas testudineus significantly stimulated the activities of cytochrome oxidase, α-glycerophosphate dehydrogenase (α-GPDH) and Mg(2+) adenosine triphosphatase (Mg(2+) ATPase) and inhibited lactate dehydrogenase (LDH), cytosolic and mitochondrial malate dehydrogenases (MDH). The activities of succinate dehydrogenase (SDH), glucose-6-phosphate dehydrogenase (G-6-PDH) and catalase were unaffected by testosterone treatment. Administration of estradiol-17β (E2) in female fish, significantly stimulated cytochrome oxidase activity, inhibited Mg(2+) ATPase, SDH, catalase and cytosolic and mitochondrial MDH activity, and was without effect on other enzymes studied.The simultaneous injections of actinomycin D or chloramphenicol and T or E2 prevented the hormonal influence on hepatic enzyme activities. The present study demonstrates that inA. testudineus sex steroids influence hepatic oxidative metabolism by a mechanism sensitive to the action of inhibitors of protein synthesis.