Influence of dietary lipid and carbohydrate levels and chronic 3,5,3'-triiodo-L-thyronine treatment on thyroid function in immature rainbow trout, Oncorhynchus mykiss

Influence of dietary carbohydrate level on endocrine status and hepatic carbohydrate metabolism in the marine fish Sparus sarba

Silver sea bream, Sparus sarba, were fed two diets of different carbohydrate levels (2 and 20% dextrin) for 4 weeks, and the effects on organ indices, liver composition, serum metabolite and hormone levels and gene expression profile of key enzymes of carbohydrate metabolism in the liver were investigated. By using real-time PCR, mRNA expression levels of carbohydrate metabolic enzymes including glucokinase (GK, glycolysis), glucose-6-phosphatase (G6Pase, gluconeogenesis), glycogen synthase (GS, glycogenesis), glycogen phosphorylase (GP, glycogenolysis) and glucose-6-phosphate dehydrogenase (G6PDH, pentose phosphate pathway) in liver of sea bream have been examined, and it was found that high dietary carbohydrate level increased mRNA level of GK but decreased mRNA levels of G6Pase and GP. However, mRNA levels of GS and G6PDH were not significantly influenced by dietary carbohydrate. Silver sea bream fed high dietary carbohydrate had higher hepatosomatic index (HSI), liver glycogen and protein, but there were no significant changes in gonadosomatic index (GSI), serum glucose and protein level, as well as liver lipid and moisture level. Pituitary growth hormone (GH) and hepatic insulin-like growth factor I (IGF-I) transcript abundance were assayed by real-time PCR, and it was found that both parameters remained unchanged in fish fed different dietary carbohydrate levels. Serum triiodothyronine (T(3)) and thyroxine (T(4)) were not significantly affected by dietary carbohydrate levels, but lower serum cortisol level was found in fish fed high dietary carbohydrate level. These results suggest that silver sea bream is able to adapt to a diet with high carbohydrate content (up to 20% dextrin), the consumption of which would lead to fundamental re-organization of carbohydrate metabolism resulting in hepatic glycogen deposition.

The interruption of thyroid and interrenal and the inter-hormonal interference in fish: does it promote physiologic adaptation or maladaptation?

Endocrines, the chief components of chemical centers which produce hormones in tune with intrinsic and extrinsic clues, create a chemical bridge between the organism and the environment. In fishes also hormones integrate and modulate many physiologic functions and its synthesis, release, biological actions and metabolic clearance are well regulated. Consequently, thyroid hormones (THs) and cortisol, the products of thyroid and interrenal axes, have been identified for their common integrative actions on metabolic and osmotic functions in fish. On the other hand, many anthropogenic chemical substances, popularly known as endocrine disrupting chemicals, have been shown to disrupt the hormone-receptor signaling pathways in a number fish species. These chemicals which are known for their ability to induce endocrine disruption particularly on thyroid and interrenals can cause malfunction or maladaptation of many vital processes which are involved in the development, growth and reproduction in fish. On the contrary, evidence is presented that the endocrine interrupting agents (EIAs) can cause interruption of thyroid and interrenals, resulting in physiologic compensatory mechanisms which can be adaptive, though such hormonal interactions are less recognized in fishes. The EIAs of physical, chemical and biological origins can specifically interrupt and modify the hormonal interactions between THs and cortisol, resulting in specific patterns of inter-hormonal interference. The physiologic analysis of these inter-hormonal interruptions during acclimation and post-acclimation to intrinsic or extrinsic EIAs reveals that combinations of anti-hormonal, pro-hormonal or stati-hormonal interference may help the fish to fine-tune their metabolic and osmotic performances as part of physiologic adaptation. This novel hypothesis on the phenomenon of inter-hormonal interference and its consequent physiologic interference during thyroid and interrenal interruption thus forms the basis of physiologic acclimation. This interfering action of TH and cortisol during hormonal interruption may subsequently promote ecological adaptation in fish as these physiologic processes ultimately favor them to survive in their hostile environment.

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.

A cDNA for a putative type III deiodinase in the trout (Oncorhynchus mykiss): influence of holding conditions and thyroid hormone treatment on its hepatic expression

A putative rainbow trout type III deiodinase (D3) cDNA was amplified by PCR, using primers to evolutionarily conserved sequences. The RACE-derived complete cDNA was then identified by sequence comparison to that in tilapia and other vertebrates. The cDNA coded for a predicted 31,500 kDa protein of 278 amino acids, with a hydrophobic trans-membrane segment and with 80% similarity to tilapia D3 and 39% similarity to rainbow trout type II deiodinase (D2). It also showed a selenocysteine codon at position 141 and a putative SECIS element in the 3' untranslated end. In the liver, a second form of D3 was found that differed only at this 3' untranslated region; the coding region was identical in both forms. The D3 mRNA, measured by RT-PCR using primers located within the common, translated portion of the cDNA, was expressed in the brain and, depending on thyroidal status, in liver and kidney. Holding trout for 7 days in static water as opposed to flowing water caused increased plasma T4 levels, decreased hepatic D2 mRNA levels and T4 outer-ring deiodination (ORD) activity and increased D3 mRNA levels and T3 inner-ring (IRD) activity. Trout held in flowing water and fed T3 for 7 days showed increased plasma T3 levels and hepatic D3 mRNA levels and T3 IRD activity but decreased D2 mRNA levels and T4ORD activity. Trout held in static water and exposed to ambient T4 for 7 days showed increased plasma T4 levels and hepatic T3IRD activity but with no significant change in D2 or D3 mRNA levels. We conclude that hepatic D3 mRNA levels and T3IRD activity are enhanced and D2 mRNA levels and T4ORD activity are suppressed by adverse holding conditions or T3 treatment suggesting that the putative D3 cDNA and D2 cDNA represent respectively the genes determining T3IRD and T4ORD activities. However, there were changes in the ratios of mRNA levels to enzyme activity, raising the potential for post-transcriptional regulation and showing that mRNA levels alone may be unreliable indices of deiodinase activity. Post-transcriptional regulation of D3 enzyme activity may be influenced by the observed alternative 3' and 5' untranslated regions of the D3 mRNA.

Thyroid function in growth-hormone-transgenic coho salmon (Oncorhynchus kisutch)

We measured growth rate, plasma thyroxine (T4) and 3,5,3'-triiodothyronine (T3) concentrations, and liver and whole-brain T4 and T3 deiodination activities in yearling non-transgenic coho salmon, Oncorhynchus kisutch (Walbaum, 1792), fed a satiation ration (NTS) and in growth-hormone (GH)-transgenic salmon fed for 63 d with either a satiation ration (TS) or pair-fed the satiation ration consumed by NTS fish (TNT). Daily feed intake and specific growth rate for TS fish were significantly enhanced and approximately double those for TNT and NTS fish. There were no differences among groups in plasma T4 concentration or liver T4 outer-ring deiodination activity, but for both TS and TNT fish, plasma T3 concentrations were higher and liver T4 and T3 inner-ring deiodination activities were lower than for NTS fish. Whole-brain deiodination activities did not differ between TS and NTS fish. We conclude that the elevated plasma T3 concentrations of GH-transgenic salmon neither are driven by elevated plasma T4 concentrations nor are they the result of increased hepatic conversion of T4 to T3 by outer-ring deiodination. Instead they can be explained, at least in part, by reduced hepatic degradation of T3 to 3,3'-diiodothyronine by inner-ring deiodination. These changes in T4 and T3 metabolism are tightly linked to the GH-transgenic state and not to food intake or growth rate.

Central regulation of thyroidal status in a teleost fish: nutrient stimulation of T4 secretion and negative feedback of T3

Several experiments were conducted to investigate the dynamics of central regulation of thyroid function in the red drum, Sciaenops ocellatus, by manipulating a well-characterized circadian rhythm of T(4) secretion. In the first experiment, red drum were reared under either a long (16L:8D) or short (8L:16D) photoperiod and fed at the same time relative to dawn. The same feeding time under different photoperiods maintained the same phase relationship between T(4) cycles under each photoperiod. This suggests that the circadian clock that determines when the hypothalamus-pituitary-thyroid (HPT) axis is activated is comprised of a feeding-entrained oscillator and a light-entrained oscillator that interact to determine the phase of the T(4) rhythm. Additionally, the amplitude of the main T(4) peak of the cycle was inversely related to the frequency of feeding, while the duration of the main T(4) peak was directly related to feeding frequency under a long photoperiod. Feeding time appears to modify the diurnal profile of circulating T(4) by stimulating post-prandial T(4) secretion that subsequently results in negative feedback on the HPT axis to regulate thyroidal status. In following experiments, red drum immersed in T(3), in lieu of a meal at a specific time that would diminish the main T(4) peak, exhibited a dose-dependent decline in amplitude of the T(4) cycle. This demonstrates that T(3) can exert negative feedback on the HPT axis of red drum to maintain appropriate thyroid hormone concentrations. These data are consistent with a dynamic and physiologically important central component of the regulation of thyroid function in fish.

Thyroid of lake sturgeon, Acipenser fulvescens. II. Deiodination properties, distribution, and effects of diet, growth, and a T3 challenge

The authors studied the properties and tissue distribution of thyroid hormone (TH) deiodination activities measured in vitro at subnanomolar substrate levels for cultured 2-year-old lake sturgeon held at 12 to 15 degrees. We also studied the deiodination responses to an exogenous 3,5,3'-triiodothyronine (T3) challenge and to a diet-induced growth suppression. Thyroxine (T4) outer-ring deiodination (T4ORD), T4 inner-ring deiodination (T4IRD), T3IRD, and 3,3',5'-triiodothyronine (rT3)ORD activities were evident in liver and intestine. Their properties resembled those of teleosts. T3IRD and T4IRD activities predominated in brain. Low or negligible deiodination in any form occurred in gill, skeletal muscle, kidney, notochord, or immature gonad. Only T4ORD activity was evident in the thyroid, suggesting that it secretes some T3. T3ORD and rT3IRD activities were undetectable in any tissues. Hepatic T4ORD activity varied during the photophase and was highest during late morning. A dietary T3 challenge that doubled plasma T3 levels decreased hepatic T4ORD activity without altering any other deiodination pathways in liver, intestine, or brain. A diet change from trout pellets to ocean zooplankton reduced somatic growth and plasma T3 levels and increased hepatic and intestinal T3IRD activities and hepatic rT3ORD activity but did not alter hepatic or intestinal T4ORD activity. The authors conclude that plasma T3 in lake sturgeon can be derived both from the thyroid and from hepatic (and intestinal) T4ORD activity, which varies with sampling time and downregulates in response to a T3 challenge. However, a reduction in plasma T3 accompanying a change in diet and reduced growth was not due to a decrease in T4ORD activity; rather, it was due to an increase in hepatic and intestinal T3IRD activities. These results suggest a difference in emphasis in thyroidal regulation between sturgeon and certain teleosts.

Down-regulation of TSH subunit mRNA levels by thyroid hormones in the European eel

The effects of 3,5,3'-triiodothyronine (T3) and thyroxine (T4) on alpha and thyroid-stimulating hormone (TSH) beta subunit mRNA pituitary levels were examined in a teleost, the European silver eel. Northern blot analysis showed that the number and length of mRNAs encoding TSH beta varied among individuals, a variability apparently not related to thyroidal status. When several bands were present, their intensities were summed for quantitative analysis. Increasing circulating thyroid hormones (THs) by implantation of T3 or T4 significantly decreased TSH beta mRNA levels. Depression of circulating THs by thiourea treatment increased alpha and TSH beta mRNA levels. In vitro studies showed that T3 and T4 decrease TSH beta mRNA levels in primary cultures of eel pituitary cells. In conclusion, in vivo and in vitro experiments indicate that T3 and T4 exert a negative feedback action on pituitary TSH beta mRNA level in the European eel and that this effect might be exerted, at least partly, through a direct action on the pituitary.

Individual diurnal plasma profiles of thyroid hormones in rainbow trout (Oncorhynchus mykiss) in relation to cortisol, growth hormone, and growth rate

In order to characterize the individual diurnal plasma profiles of triiodothyronine (T3) and thyroxine (T4) in rainbow trout (Oncorhynchus mykiss), blood samples from 41 fish were taken every hour during a 24-hr period, through a catheter inserted into the dorsal aorta. The possible influences of day-night alternation, sex, and diet (feed intake, time of meals) on thyroid hormone (TH) profiles were analyzed. The existence of relations between diurnal plasma profiles of T3, T4, T3/T4 ratio, and those of the growth hormone (GH), cortisol (previously described in Gomez et al., J. Exp. Zool. 274, 171-180, 1996), and the growth rate was monitored. Average daily T3 and T4 concentrations were, respectively, 2.6 +/- 0.2 and 5.5 +/- 0.3 ng/ml (n = 41). Our study showed little or no variation in plasma T3 concentrations during one 24-hr period, while those of T4 fluctuated markedly. T4 peaks occurred from a baseline of 4.0 +/- 0.2 ng/ml at a frequency of 2.5 +/- 0.2 peaks/24 hr, with an amplitude of 3.0 +/- 0.4 ng/ml, and a duration of 4.3 +/- 0.4 hr. There was a significant difference between the average circulating T3 level during the day and that at night (2.4 +/- 0.2 vs 2.7 +/- 0.2 ng/ml). No influence of sex or food factors was observed on daily TH concentrations. TH peaks occurred irregularly and asynchronously without apparent influence of day-night alternation, sex, and diet. The growth rate was significantly correlated with the daily T3 concentration (r = 0.77), but not with T4. No significant relationships were found between daily concentrations of T3, T4, GH, and cortisol. The absence of a relationship between TH and GH concentrations suggests that, in salmonids, GH may have no observable short-term action on the conversion of T4 to T3.

Effects of dietary thyroid hormones on the red drum (Sciaenops ocellatus)

Four separate 8-week feeding trials were conducted to assess the effects of supplementing semipurified diets with either triiodothyronine (T3) or thyroxine (T4) at 0, 2, 10, and 50 mg/kg on growth and body composition of juvenile red drum (Sciaenops ocellatus) held in artificial brackish water (6‰) and artificial seawater (32‰). At both levels of salinity, increasing doses of T3 resulted in fish with reduced weight gain, feed efficiency, condition factor (weight × 100/length(3)), and muscle ratio (muscle weight × 100/body weight), as well as a lighter body color. Significant (p < 0.05) effects of T3 on the proximate composition of whole body, liver, and muscle were variable, generally reflecting decreased lipid and protein storage in liver and muscle, respectively. The two highest doses of T3 given to seawater adapted fish increased survival. Dietary T4 supplementation had no distinctive effects on appearance, growth or proximate body composition. These results indicate that whereas T3 may function to regulate protein and lipid metabolism in red drum, dietary supplementation with T3 leads to a hyperthyroidism-induced catabolic state. The elevated endogenous thyroid hormone levels found in fish fed optimal diets may thus adequately supply tissue needs during juvenile growth.

Dietary effects on thyroid hormones in the red drum, Sciaenops ocellatus

Juvenile red drum (Sciaenops ocellatus) were cultured at 25°C on a variety of diets and blood sampled over eight weeks to examine the relationship between growth and plasma thyroid hormone levels. Maximum growth rates were achieved on formulated experimental diets and a simulated natural shrimp diet. Associated with these maximal rates was a significant increase in triidothyronine (T3), but no consistent change in thyroxine (T4). Reduced rations of diets resulted in low growth rates associated with significantly lowered levels of T3 but not T4. To determine whether weight gain could be increased by application of exogeneous hormone, diets were supplemented with T3 or T4 at 2, 10, and 50 mg hormone/kg diet. Significantly elevated T3 was induced by supplementation with 10 and 50 mg T3/kg diet, although there were no indications of an anabolic effect of T3 incorporation, and 50 mg T3/kg diet was in fact associated with decreased weight gain. Incorporation of T4 into diets had no effect on growth or T3, and had effects on T4 which were small and inconsistent, indicating that T4 may not be effectively absorbed from the gut. No difference was found in response to hormone feeding between low (6 ppt) or high (35 ppt) water salinity. T3 levels thus appear to closely parallel growth in fish on unsupplemented diets, whereas T4 which were small and manipulation. Supplementation with T3 is not an effective means of stimulating growth in red drum fed optimum diets. Whereas thyroid hormones may function to regulate intermediary metabolism in red drum, elevated endogenous thyroid hormone levels appear adequate to supply tissue needs during juvenile growth in culture.

Thyroid hormone deiodinase systems in salmonids, and their involvement in the regulation of thyroidal status

The trout thyroid secretes L-thyroxine (T4) which undergoes enzymatic deiodination in liver and other tissues. Based on mammalian studies, T4 outer-ring deiodination (ORD) or T4 inner-ring deiodination (IRD) could generate respectively 3,5,3'-triiodo-L-thyronine (T3) or 3,3',5'-T3(rT3), while subsequent T3ORD or T3IRD could generate respectively 3,5-diiodo-L-thyronine (T2) or 3,3'-T2, and rT3ORD or rT3IRD could generate respectively 3,3'-T2 or 3',5'-T2. In practice, T4 in trout undergoes hepatic ORD to produce T3 but negligible IRD to produce rT3, and T3 in turn undergoes negligible ORD but modest IRD to produce 3,3'-T2. T4ORD, which is particularly important in converting T4 to the biologically more potent T3, also occurs in gill, muscle and kidney. At least two isozymes are involved: i) a high-affinity, propylthiouracil (PTU)-sensitive T4ORD which displays ping-pong kinetics, requires thiol as a cofactor, and is present in liver, gill and muscle, and ii) a low-affinity, PTU-insensitive T4ORD with sequential kinetics with a thiol cofactor, and is present in liver and kidney. Receptor-bound T3 is derived primarily from the plasma for kidney, mainly from intracellular sources for gill and about equally from both plasma and intracellular sources for liver. Thus, the high-affinity T4ORD may produce T3 for local intracellular use while the low-affinity 5'-monodeiodinase may produce T3 for systemic use. T4ORD activity responds to nutritional factors and the physiologic state of the fish. Furthermore, T3 administered orally for either 6 weeks or 24h reduces the functional level (Vmax) of hepatic T4ORD, and T3 added to isolated hepatocytes also reduces activity, indicating direct T3 autoregulation of T4ORD to maintain hepatocyte T3 homeostasis. However, T3 administration also induces T4IRD to produce biologically inactive rT3 and induces T3IRD to produce 3,3'-T2. Thus, the trout liver has several iodothyronine deiodinase systems which in a coordinated manner regulate tissue T3 homeostasis in the face of a T3 challenge. It does this by decreasing formation of T3 itself, by diverting T4 substrate to biologically inactive rT3 and by increasing the degradation of T3. These deiodinases differ in many respects from any mammalian counterparts.

Comparison of 3,5,3'-triiodo-L-thyronine and L-thyroxine absorption from the intestinal lumen of the fasted rainbow trout, Oncorhynchus mykiss

The absorptions of 3,5,3'-triiodo-L-thyronine (T3) and L-thyroxine (T4) from the intestinal lumen of the rainbow trout were compared in vivo. Tracer doses of [(125)I]T4 ((+)T4) or [(125)I]T3 ((*)T3) were injected through an anal cannula into the duodenum of trout fasted for 3 days at 12°C, and radioactivity was measured in blood and tissues at 4-48 h. (*)T3 was removed more extensively than (*)T4 from the intestinal lumen and more radioactivity was absorbed into the blood and tissues of u+T3-injected trout than (*)T4-injected trout. HPLC analysis showed that a high proportion of the radioactivity in the plasma, liver, kidney and intestinal lumen of (*)T3-injected trout remained as the parent (*)T3. However, in (*)T4-injected trout most plasma radioactivity was in the form of (125)I(-), and by 24 h a high proportion of luminal radioactivity was (125)I(-). By 48 h, over 4% of the injected (*)T3 and 1% of the injected (*)T4 dose resided in the gall bladder, primarily as derivatives of (*)T3 or (*)T4. We conclude that T3 is absorbed more effectively than T4 from the intestinal lumen of fasted trout, indicating the potential for an enterohepatic T3 cycle.

Thyroxine 5'-monodeiodinase activity in microsomes from isolated hepatocytes of rainbow trout: effects of growth hormone and 3,5,3'-triiodo-L-thyronine

Rainbow trout hepatocytes isolated by collagenase perfusion were suspended in primary culture for up to 72 hr at 11 degrees and then the microsomal L-thyroxine (T4) 5'-monodeiodinase (5'D) activity was evaluated by 125I- generation from [125I]T4. The 5'D activity and Vmax (level of functional enzyme) and Km (Michaelis-Menten constant) values for microsomes obtained from incubated hepatocytes corresponded to those for microsomes obtained directly from intact livers. HPLC analysis revealed 3,5-[125I]3'-triiodo-L-thyronine (T3) as the only significant 125I-labeled organic product. Hepatocyte survival ( > 90%) and 5'D activity were unaltered by insulin (10(-9) M) in the incubate, but 5'D activity was inhibited by 10% fetal calf serum. Human growth hormone (hGH) at concentrations of 5-250 ng/ml did not increase 5'D activity. These results do not support previous in vivo studies demonstrating hGH-enhanced hepatic 5'D function in trout and indicate that either hGH acts indirectly on the liver to enhance 5'D activity or incubated hepatocytes lose GH responsiveness. However, coincubation of hepatocytes with T3 (15 or 30 nM) for 24 hr inhibited 5'D activity in a dose-dependent manner and induced the production of 3-[125I]3',5'-triiodo-L-thyronine (reverse T3). These data support previous in vivo studies in showing that T3 autoregulates its own hepatic production and show that T3 does so by acting directly on the hepatocyte to modify deiodination pathways.

Gill (Na+ + K+)-ATPase involvement and regulation during salmonid adaptation to salt water

1. The involvement of gill (Na+ +K+)-ATPase in salmonid adaptation to salt water (SW) is discussed. 2. Gill (Na+ +K+)-ATPase increase during SW adaptation is mainly related to the increased number and complexity of chloride cells deputed to salt extrusion. 3. The temporal relationships between serum peaks of thyroid hormones, cortisol, growth hormone, prolactin and gill (Na+ +K+)-ATPase rise during salmonid smoltification, suggest a hormonal involvement in the enzyme stimulation and thus in the acquirement of SW tolerance. 4. Literature on gill (Na+ +K+)-ATPase response to hormonal treatment is reviewed. The effects produced on gill (Na+ +K+)-ATPase and chloride cells by exogenous hormones point out a complex inter-relationship between the hormones considered. The mechanisms involved in hormonal regulation of the enzyme remain a matter of debate.

Intra- and extra-cellular sources of T3 binding to putative thyroid hormone receptors in liver, kidney, and gill nuclei of immature rainbow trout, Oncorhynchus mykiss

The sources of extracellular and intracellular 3,5,3'-triiodo-L-thyronine (T3) binding to putative thyroid hormone receptors in liver, kidney, and gill nuclei were determined in vivo for immature rainbow trout at 12 degrees C. Both [131I]T3 and [125I]T4 were injected intraperitoneally, the plasma and tissues were examined at isotopic equilibrium at 20 h, and the proportions of intracellular [125I]T3 and extracellular [131I]T3 saturably bound in the nucleus were determined. Comparable total amounts of T3 were saturably bound in the nuclei of liver (7.2), kidney (8.0), and gill (9.7 moles x 10(-13) .mg DNA-1), but the percentage of nuclear T3 generated within the target cell was greater for gill (76%) than for liver (50%) and kidney (28%). Both gill and liver possess a low Km T4 5'monodeiodinase which could be responsible for the high proportion of the nuclear T3 generated within those tissues.

The acute influence of ingested thyroid hormones on hepatic deiodination pathways in the rainbow trout, Oncorhynchus mykiss

Juvenile rainbow trout were fed once daily with trout pellets supplemented with L-thyroxine (T4) or 3,5,3'-triiodo-L-thyronine (T3) and the effects on plasma T4 and T3 levels and hepatic 5'-monodeiodinase (5'D) activity determined after 1, 2, or 3 days. In all cases T3 (12 ppm) elevated plasma T3 and caused a significant reduction in the functional level (Vmax) of 5'D with no change in enzyme affinity (Km). After 3 daily meals (3 ppm T3), 5'D activity fell to 52% of control levels. In most instances plasma T4 was increased. In contrast, ingestion of T4 for 3 days at levels up to 48 ppm did not modify hepatic 5'D. This may reflect either insensitivity of the 5'D system to T4 or poor T4 uptake from the gut, as plasma T4 levels were influenced to a small extent by T4 ingestion. HPLC analyses showed that dietary T3 supplements (0, 3, 6, or 12 ppm) for 3 days acted in a dose-dependent manner, not only to suppress T3 formation from T4 by outer-ring deiodination, but also to promote inner-ring deiodination of T4 to 3,3',5'-triiodo-L-thyronine (reverse T3) and outer-ring deiodination of T3. In conclusion, the present data indicate that in the face of a T3 challenge there is a rapidly responding hepatic autoregulation of T3 production, achieved by a complex coordinated regulation of several different iodothyronine deiodination pathways.