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1.
Extrathyroidal T4 5′-monodeiodination, demonstrated in several teleost species, generates T3 which binds more effectively than T4 to putative nuclear receptors and is probably the active thyroid hormone. T4 to T3 conversion is sensitive to the physiological state and provides a pivotal regulatory link between the environment and thyroid hormone action. T3 generation is enhanced in anabolic states (positive energy balance or conditions favoring somatic growth; food intake or treatment with androgens or growth hormone) and is suppressed in catabolic states (negative energy balance or conditions not favoring somatic growth; starvation, stress, or high estradiol levels associated with vitellogenesis). In fish, as in mammals, thyroidal status may be finely tuned to energy balance and through T3 production regulate energy-demanding processes, which in fish include somatic growth, development and early gonadal maturation.  相似文献   

2.
The diurnal rhythms of plasma glucose, cortisol, growth hormone (GH) and thyroid hormone (T4, T3) concentrations and hepatic glycogen content were measured in rainbow trout that had been entrained to a specific time of daily feeding (post-dawn, midday, pre-dusk); the purpose of the study was to investigate the significance of feeding time on hormones and metabolite patterns. Plasma GH, cortisol and T4 concentrations all showed evidence of a diurnal rhythm in some treatment groups. There was a significant interaction between the time of feeding and plasma GH and cortisol concentration rhythms; for GH, this appeared to be related to the phase-shifting of the post-prandial increases in plasma GH concentrations, and for cortisol, the rhythms were only evident in fish fed in the post-dawn period [diurnal rhythms were not evident in treatment groups fed in at midday or pre-dusk]. Peak plasma T4 concentrations were evident during the photophase in all three treatment groups; however, the time of feeding had a negligible effect on the timing of those peaks. There were no apparent diurnal rhythms of plasma T3 and glucose concentrations, hepatic glycogen content or hepatosomatic index in any of the three treatment groups. To whom correspondence should be addressed  相似文献   

3.
Fasting and refeeding have considerable effects on thyroid hormone metabolism. In tilapia (Oreochromis niloticus), fasting results in lower plasma T3 and T4 concentrations when compared to the ad libitum fed animals. This is accompanied by a decrease in hepatic type II (D2) and in brain and gill type III (D3) activity. No changes in kidney type I (D1) activity are observed. Refeeding results in a rapid restoration of plasma T4 values but not of plasma T3. Plasma T3 remains low for two days of refeeding before increasing to normal levels. Liver D2 and gill D3 also do not increase until two days after refeeding. Brain D3, on the other hand, rises immediately upon refeeding. These results suggest that the change in hepatic D2 activity is one of the main factors responsible for the changes in plasma T3 observed during starvation and refeeding in tilapia. This finding supports the hypothesis that, in contrast to mammals and birds, liver D2 is the primary source of plasma T3 in fish. Although the deiodinases important for the gross regulation of plasma T3 during fasting/refeeding differ (mammals: D1 and D3, birds: D3, fish: D2), they all occur in the liver, suggesting that the organ itself may play a crucial role. In addition, the changes in brain and gill D3 suggest that these enzymes constitute a fine tuning mechanism for regulation of T3 availability at the cellular or plasma levels, respectively.  相似文献   

4.
Plasma levels of L-thyroxine (T4) and 3,5,3-triiodo-L-thyronine (T3) and the percentage of plasma T4 and T3 present in the free (dialyzable) form (%FT4 and %FT3) were measured in 16 species (11 families) of tropical marine teleosts from an inshore Barbados reef. Mean plasma T4 varied from 0.2 ng/ml to 42 ng/ml; mean plasma T3 varied from < 0.2 ng/ml to 50 ng/ml. The highest T4 and T3 levels were recorded in parrot-fish and the lowest levels in filefish. The %oFT4 and %FT3 varied from 0.05–3.41%. Estimated levels of plasma free T4 and free T3 levels ranged from 0.4–466 pg/ml. The extremely wide inter- and intra-species ranges in levels of free T4 and T3 do not support a previous suggestion, based on temperate freshwater salmonid species, that free T4 and T3 levels in fish may fall within a relatively range narrow comparable to that of homeothermic vertebrates.  相似文献   

5.
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.  相似文献   

6.
Tissue T3 (3,5,3′-triiodo-L-thyronine) concentrations were measured in rainbow trout, Salmo gairdneri, after digestion by Pronase or collagenase and extraction with ethanolic ammonia (99:1, v/v) followed by 2N NH4OH and chloroform. Recoveries of [125I]T3 administered in vivo or in vitro were high and consistent and there was close parallelism between sample dilutions and the radioimmunoassay curve, but recoveries of unlabeled T3 administered in vitro were low and variable. Alternatively, trout were brought to isotopic equilibrium by [125I]T3 infusion for 96 h, the extracted [125I]T3 determined by gel filtration and the tissue T3 content calculated from the specific activity of plasma [125I]T3. By the latter method, tissue T3 concentrations were: intestine (4.2 ng/g), kidney (2.5), liver (2.8), stomach (1.5), heart (1.0), muscle (0.7), gill (0.6) and skin (0.3). Muscle (67% of body weight) comprised the largest tissue T3 pool (82% of all tissues examined). Seven days exposure of trout to water acidified with H2SO4 (pH 4.8) or acidified water containing aluminum (21.6 mM), decreased tissue T3 content generally and particularly in muscle (14% of controls). In conclusion, skeletal muscle is the largest T3 tissue pool and seems highly responsive to altered physiologic state.  相似文献   

7.
The circadian patterns of several tissue and plasma metabolites, and several plasma hormone concentrations are described in rainbow trout (Oncorhynchus mykiss) that were held in groups under three different photoperiod regimes, and given free access to a demand-feeder. Regardless of photoperiod regime, all the measured parameters showed significant diel rhythms that appeared to be synchronized by dawn; dawn was represented by the concomitant onset of both light and feeding. The diel increases in hepatic glycogen content, and plasma T4 and cortisol concentrations were in phase with the main period of feeding activity, whereas the peaks in plasma T3 and glucose concentrations that may also be triggered by feeding activity, were delayed by several hours. The peaks in hepatosomatic index, plasma non-esterified fatty acids and plasma growth hormone concentrations were 180° out of phase with the main period of feeding activity, and associated with periods of hypophagia and low activity.  相似文献   

8.
Blood and ovarian samples were collected at intervals of 4h prior to spawning time from medaka (Oryzias latipes) that were maturationally synchronized with artificial photoperiod (14h light: 10h dark). Plasma estradiol-17β (E2) levels increased rapidly from 16h before spawning and peaked at 8h before spawning. Follicle-enclosed oocytes (ovarian follicles) at different stages of development were isolated from the ovaries and used to study the in vitro effects of thyroid hormone (triiodothyronine; T3) on pregnant mare serum gonadotropin (GTH)-induced E2 production. GTH at a concentration of 100 IU/ml stimulated E2 production by ovarian follicles collected between 32 and 16h before spawning. At 32h before spawning, T3 (5 ng/ml) administered along with GTH (100 IU/ml) resulted in a 3.5 fold increase in E2 production, compared with GTH administered alone. These results suggest that T3 can act on ovarian follicles directly to modulate GTH-stimulated E2 production in the medaka.  相似文献   

9.
Studies were conducted to explore the effect of cohort sampling and stocking density on the interactions between plasma growth hormone (GH), thyroid hormone and cortisol concentrations in rainbow trout. Depending on the experimental design, plasma GH concentrations were either suppressed, or elevated by sequential removal of cohorts from the holding aquarium. Since plasma cortisol concentrations consistently increased during cohort sampling, regardless of experimental design, it would appear that the apparent correlation (direct or inverse) is the result of concomitant changes, i.e. not necessarily a cause-effect relationship. Plasma GH concentrations of rainbow trout were not correlated with eviscerated body weight.Trout stocked at 150 kg m–3 exhibited a significantly lower mean growth rate, hepatosomatic index, hepatic lipid reserve, plasma triiodo-l-thyronine (T3) concentration andin vitro hepatic T3 production than trout stocked at 60 kg m–3. These observations are consistent with the former group being food deprived or ration restricted. Stocking density appeared to have no effect on plasma GH or cortisol concentrations, or on the pituitary-interrenal axis response to stressor challenge, or the thyroid tissue response to exogenous TSH challenge.  相似文献   

10.
With the aim of comparing the effects of oral T3 and NaCl administration on trout hypoosmoregulatory mechanisms, three groups of rainbow trout (Oncorhynchus mykiss Walbaum) held in freshwater (FW) were fed a basal diet (C), the same diet containing 8.83 ppm of 3,5,3-triiodo-L-thyronine (T3) (T) or 10% (w/w) NaCl (N) respectively for 30 d. They were then transferred to brackish water (BW) for 22 d and fed on diet C. Gill (Na++K+)-ATPase activity and its dependence on ATP, Na+ and pH, number of gill chloride cells (CC), serum T3 level as well as fish growth, condition factor (K) and mortality were evaluated. During the FW phase, as compared to C trout, T trout showed a two fold higher serum T3 level, had unchanged gill (Na++K+)-ATPase activity and increased CC number, whereas N trout showed higher gill (Na++K+)-ATPase activity and CC number. At the end of the experiment the enzyme activity was in the order T>N>C groups and all groups showed similar CC number. Both treatments changed the enzyme activation kinetics by ATP and Na+. A transient increase in K value occurred in N group during the period of salt administration. In BW, T and N groups had higher and lower survival than C group respectively. Other parameters were unaffected by the treatments. This trial suggests that T3 administration promotes the development of hypoosmoregulatory mechanisms of trout but it leaves the (Na++K+)-ATPase activity unaltered till the transfer to a hyperosmotic environment.  相似文献   

11.
Rainbow trout fed a 26% canola meal-based (CM) diet for 12 weeks at 15°C exhibited reduced growth, lower feed conversion, enlarged thyroid glands and lower plasma thyroid hormone (TH) levels than comparable fish fed equinitrogenous, equicaloric soybean meal-based (SB) diets. Supplementation of the SB diets with either T4 (20 mg/kg) or T3 (10 or 20 mg/kg) had no effect on the growth rate, feed conversion and thyroid histology of the trout. However, plasma T4 levels weredepressed in trout fed the T4- and high T3-supplemented SB diets. In trout fed T4- and T3-supplemented CM diets the growth rates and feed conversion were not significantly different from those of the SB-fed groups. Moreover, in the T4-supplemented group, plasma T4 levels were in the normal range. However, thyroid enlargement was evident in all the CM-fed fish, and plasma T3 levels were markedly elevated in groups fed the T3-supplemented CM diets. The data suggest that antithyroid components in the CM diets inhibited TH synthesis (but not their release), and impaired TH clearance from the circulation. There were no significant differences in plasma cortisol levels in the 8 treatment groups, nor were there differences in the histological appearance of the interrenal gland. However, when the data from SB- and CM-fed fish were pooled, plasma cortisol levels in the SB-fed fish were significantly lower than in the CM-fed animals. Glucosinolates at a level of 164 mg/kg diet were toxic to young trout, but the effect was ameliorated by dietary TH supplementation.  相似文献   

12.
The effects of different levels of vitamin A (VA) in Senegalese sole larval performance and development were evaluated by means of a dietary dose–response experiment using enriched Artemia metanauplii as a carrier of this micronutrient. Larvae were fed from 6 to 27 days post hatch (dph) with enriched Artemia containing graded levels of total VA (1.3, 2.1, 4.5 and 12.9 µg VA mg− 1 DW). The content of VA in live prey directly affected its accumulation in larvae and early juveniles. Retinyl palmitate accumulated during larval ontogeny, whereas retinol showed the opposite trend, decreasing from hatching until 41 dph and then remaining constant until the end of the study.In metamorphic larvae (10 and 15 dph), VA did not affect the number of thyroid follicles or the intensity of the immunoreactive staining of T3 and T4. However, at older stages of development (post-metamorphic larvae: 20, 30, 41 and 48 dph), VA decreased the number of thyroid follicles but increased their mean size and enhanced T3 and T4 immunoreactive staining. A dietary excess of VA did not affect either larval performance in terms of growth and survival or the maturation of the digestive system. However, the most remarkable impact of this morphogenetic nutrient was detected during skeletal morphogenesis. Dietary VA accelerated the intramembranous ossification of vertebral centrums, which led to the formation of a supranumerary haemal vertebra and a high incidence of fused and compressed vertebrae in fish fed 2.1, 4.5 and 12.9 mg VA mg− 1 DW. In addition, VA also affected those structures from vertebrae and caudal fin formed by chondral ossification, leading to defects in their shape and fusions with adjacent skeletal elements. In particular, the caudal fin was the region most affected by the dietary treatments. In order of importance, the bones with more developmental anomalies were the modified neural and haemal spines, epural, hypurals and parahypural. The impact of systemic factors such as thyroidal hormones in skeletogenesis should not be neglected since present results revealed that an excess of dietary VA affected the levels of T3 and T4, which might have affected bone formation and remodelling, leading to skeletal deformities.  相似文献   

13.
Serum thyroid hormone concentrations were measured during the seven stages of metamorphosis (1–7) of the southern hemisphere lamprey, Geotria australis. The respective mean concentrations ± SEM of serum thyroxine (T4) and triiodothyronine (T3) fell from 31.73 ± 4.09 and 5.06 ± 0.70 nM in large ammocoetes sampled in February, at the time when metamorphosis was initiated, to 4.54 ± 0.36 and 1.03 ± 0.12 nM at stage 5. Although there was a small, but significant, recovery of serum T4 concentrations during stages 6 and 7, no such corresponding statistically significant rise occurred in serum T3 concentrations. Serum thyroid hormone concentrations in ammocoetes sampled during the period when metamorphosis was taking place, exhibited a marked seasonal increase between February and May–June (late autumn/early winter); serum T3 and T4 concentrations peaked in May–June and were, respectively, > 2 fold and > 8 fold higher than those recorded for samples in late February (mid summer). By mid-July the serum T4 and T3 levels had declined from the peak values. Ammocoetes taken from streams at 16°C in June and acclimated to aquaria water at 25°C or 6°C had significantly lower serum T3 and T4 concentrations at the higher temperature, and also a lower serum T4, but not T3 concentration, at the lower temperature. Treatment of separate groups of ammocoetes with either propylthiouracil or T3 for 70 days significantly depressed and raised respectively, the serum thyroid hormone and hepatic T3 concentrations and caused significant changes in the body weight, but did not induce the onset of metamorphosis.  相似文献   

14.
In the present study, thiourea-induced thyroid hormone depletion and thyroxine (T4) ‘overdose’ were used as a strategy to understand the influence of thyroid hormones on ovarian recrudescence of juvenile (3-months-old), immature (8-months-old) and adult (1-year-old) air-breathing catfish, Clarias gariepinus. Thiourea-induced thyroid hormone depletion in juvenile catfish impaired ovarian development, but no significant effect was observed in immature catfish and during late stage of ovarian recrudescence of mature catfish. T4 treatment in females undergoing late stages of ovarian recrudescence induced rapid oocyte growth by promoting its early entry into maturational phase as evident from the presence of more number of vitellogenic and post-vitellogenic follicles, decrease in aromatse immunoreactivity and reduced estradiol–17β levels. Hence, thyroid hormones have an important role to play during early stages of ovarian development and vitellogenesis of catfish and also indicating that thyroid has a stage dependent effect on ovary.  相似文献   

15.
A cDNA encoding the subunit of thyrotropin (TSH) was isolated from a goldfish (Carassius auratus) pituitary gland cDNA library. By comparing the sequence with other teleost TSHs, a signal peptide of 19 amino acids and a mature hormone of 131 amino acids were predicted for goldfish TSH subunits. The resulting putative mature hormone of 131 amino acids had well-conserved cysteine positions and a putative N-linked glycosylation site; homology was 51–67% with TSHs from other teleosts, 38–43% with tetrapod TSHs, but only 27 and 29% with goldfish GTH-I and -II, respectively. We also examined the effects of thyroid hormones (TH) and thiourea (TU, an inhibitor of TH production) treatments on TSH and GTH subunit gene expressions in the goldfish pituitary gland. After thyroxine (T4) treatment, circulating T4 concentration increased and TSH mRNA level decreased. Supressing the amount of circulating T4 and triiodothyronine (T3) by TU treatment increased the TSH mRNA level. Moreover, T4 replacement therapy (simultaneous treatment of both TU and T4) caused a high level of circulating T4 and a low level of circulating T3, and a decrease in the TSH mRNA level. Thus, changing levels of circulating TH exert a negative feedback on the level of TSH subunit mRNA in goldfish in vivo. On the other hand, GTH subunit mRNA levels were not affected by changes in the levels of circulating TH.  相似文献   

16.
The 5′-monodeiodinase (5′-MDA) activity was measured in liver slices that were incubated for 3 hours with epinephrine (E) or norepinephrine (NE) in order to examine the influence of these catecholamine hormones on the regulation of hepatic monodeiodination of thyroxine (T4) in rainbow trout. Both E and NE induced a dose-dependent increase in 5′-MDA activity and in addition, E stimulated the release of T3 into the medium. In liver slices taken from trout that had been treated with the β-adrenoceptor inhibitor propranolol, the response to both E and NE was attenuated. The findings provide evidence of an action of these catecholamine hormones on the peripheral regulation of T3 production, and suggest that the control operatesvia the β-adrenoceptors. Corresponding author.  相似文献   

17.
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/length3), 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.  相似文献   

18.
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 [125I]T4 (+T4) or [125I]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 125I–, and by 24 h a high proportion of luminal radioactivity was 125I–. 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.  相似文献   

19.
Developmental profiles of thyroxin (T4), triiodothyronine (T3) and radioactive iodide uptake were established for eggs and T4 and T3 profiles were established for larvae (whole-body, yolk-only and body-only) of coho and chinook salmon. T4 and T3 were consistently present in all samples. In eggs, hormone levels remained fairly constant in all cohorst for at least the first three weeks of incubation, but then fluctuated in both directions in some sample groups. Large increases in T4 (from 9 ng/g to 245 ng/g) were seen in 1985 chinook eggs 28 days after fertilization. Radioactive iodide uptake (which was used as a possible indicator of thyroxinogenesis) increased at least 10-fold in both 1986 coho and chinook eggs from 23–30 days after fertilization. T4 (62 ng/g) and T3 (393 ng/g) were found in the bodies of 28-day-old 1986 chinook embryos. In whole larvae, hormone levels varied depending upon the cohort studied. In general, initial body-only concentrations of both T4 and T3 decreased as body weight increased, but before yolksac resorption was completed, both thyroid hormone content and concentration increased (except for chinook T3). T4 and T3 content in larval yolk stayed constant as yolksac size decreased, resulting in increased thyroid hormone concentration in the yolksac. All of these data suggest that the initial source of thyroid hormones in coho and chinook salmon eggs is maternal, but that by approximately 3–4 weeks after fertilization, the developing embryos begin to produce their own thyroid hormones. After hatching, increases in tissue T4 and T3 concentration coupled with constant T4 and T3 content in diminishing yolksacs suggest that larvae also produce their own thyroid hormones; yolksac content then may reflect both the original maternal hormones and the larva-producted hormones.  相似文献   

20.
Rainbow trout (Salmo gairdneri Richardson) were fed either a soybean mealbased (SM) or canola meal-based (CM) diet for up to 20 weeks. Plasma thyroxine (T4) and triiodothryonine (T3) levels were significantly lower in the CM-fed fish sampled after 12 weeks. However, there appeared to be some compensation after 12 and 20 weeks in that the thyroid hormone levels in trout fed the CM were not significantly different from those of the SM-fed fish. Nevertheless, there was marked thyroid hyperplasia and hypertrophy in the CM-fed fish sampled at 12, 16 and 20 weeks after commencement of the experiment. Moreover, the growth rate was significantly lower in the CM-fed fish in comparison to the SM-fed fish throughout the 20 week study period.Plasma T4 levels were similar in SM-fed fish sampled 12, 16 and 20 weeks after commencement of the experiment, but plasma T3 levels progressively increased over this period, as did the apparent activity of the thyroid tissue based on histological criteria.Fasting for up to 8 weeks resulted in the arrested growth of the SM-fed fish, and a loss in body weight of the CM-fed animals over the 8 week period of the fast. In addition, the plasma thyroid hormone levels in the fasted fish tended to be lower than in fish fed both the SM and CM diets prior to fasting, and there was histological evidence indicating a reduced activity of the pituitary-thyroid axis. However, thyroid hyperplasia and hypertrophy were still evident in the fasted fish previously fed the CM diet indicating that the adverse affects of CM diets are not completely reversible after 8 weeks.In fish fed the CM diet for 12 weeks and then the SM diet for up to a further 8 weeks (diet C-S) there was a compensatory increase in plasma thyroid hormone levels evident within 4 weeks after the change in diet, but no apparent decrease in thyroid hyperplasia or hypertrophy. In addition, in the fish fed the C-S diet there was a marked compensatory growth rate, and an increased feed: gain ratio; body weights of this group of fish were not significantly different from those of the SM-fed animals after 20 weeks of study, indicating a considerably higher growth rate over the last 8 week period.  相似文献   

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