Induction of metallothionein synthesis in rainbow trout,Salmo gairdneri, during long-term exposure to waterborne cadmium

Acute Cd Toxicity, Metal Accumulation, and Ion Loss in Southern Catfish (Silurus meridionalis Chen)

The amounts of cadmium in multiple organs and the amounts of Na⁺ and Ca²⁺ in the carcass were measured in dead and surviving southern catfish exposed to different concentrations of Cd. The 96 h median lethal concentration was 6.85 mg/L. The Cd content and Cd accumulation rate were positively correlated with Cd exposure concentrations, and there were significant differences between dead and surviving individuals, indicating that both Cd content in tissues and Cd accumulation rates were correlated with mortality. Cd levels in the liver of dead fish were saturated. A lethal threshold for Cd concentration in the whole fish was obtained. Bioconcentration factors for Cd did not decrease with increasing exposure. Acute exposure to waterborne Cd caused a significant decrease in the ion content of the fish carcass. There was a significant difference between the Na⁺ content of the carcass of dead fish (34.54 μmol/g wet weight) and surviving fish (59.34 μmol/g wet weight), which was not the case with the Ca²⁺ content, indicating that the lethal toxicity of Cd was probably related to the decrease in Na⁺ content. Collectively, these results suggest that whole-fish Cd concentration and carcass Na⁺ content can be useful indicators of fish acutely exposed to Cd.

Involvement of different hemoprotein thiol groups of Oncorhynchus mykiss in cadmium toxicity

BACKGROUND

Cadmium is considered the seventh most toxic heavy metal as per ATSDR ranking but its mechanism of toxicity is debated. Recently, we evaluated the effects of this metal on the erythrocyte of teleost fish (Oncorhynchus mykiss) leading us to hypothesize that the pro-oxidant activity of cadmium is not linked to mitochondria but more likely to haemoglobin. In this context, the main aim of this work was to detect the ability of Cd to induce structural perturbation in haemoproteins that present different structures and thus different functional properties and to identify what sites of interaction are mainly involved.

METHODS

The effect of Cd on the structural destabilization of the different haemoproteins was followed spectrophometrically through their precipitation. In addition, the sites of interaction between the different haemoproteins and bivalent cadmium ions were identified by MIB server followed by molecular docking/molecular dynamics simulations both in the dimeric and tetrameric associations.

RESULTS

Cadmium does not influence the autoxidation rate of Mb, HbA and trout HbI. However, the presence of this metal accelerates the precipitation process in trout HbIV in a dose-dependent manner. Moreover, the presence of 1-10-50-250-500-1000 μM GSH, a chelating agent, reduces the ability of cadmium to accelerate the denaturation process although it is not able to completely prevent it. In order to explain the experimental results, a computational investigations was carried out to identify the cadmium cation affinity for the studied haemoglobins and myoglobin, both in their dimeric and tetrameric forms. As a result, the highest affinity cadmium binding sites for fish HbIV are located at the interface between tetramer-tetramer association, indicating that the cation can assist supramolecular protein aggregations and induce complex precipitation. For mammalian Hb, Mb and fish HbI computational investigation did not detect any site where Cd could to induce such aggregation, in line with the experimental results.

CONCLUSION

The present study provides new information on the mechanisms of toxicity of cadmium by specific interaction with trout O. mykiss haemoglobin component.

Sub-lethal effects of waterborne copper in early developmental stages of rainbow trout (Oncorhynchus mykiss)

The aim of this work was to study the impact of copper during a sub-chronic exposure to environmental concentrations in the early life stages of rainbow trout (Oncorhynchus mykiss). Eyed-stage embryos of rainbow trout, at 265 °D, were exposed in semi-static conditions to sub-lethal concentrations of CuSO₄ up to the larval stage (528 °D) under laboratory-controlled conditions. During 3 weeks, they were exposed to the environmentally-realistic concentration of 2 µg/L Cu and to a 10-fold higher concentration, 20 µg/L Cu. Several biological (survival, hatching success, malformation, growth) and behavioral (swimming activity) and molecular endpoints (genotoxicity and gene transcription) were studied. Exposure to 20 µg/L Cu had an inhibitory effect on hatching and increased half-hatched embryos (25%). At the end of the exposure, no significant differences were observed in growth of the larvae exposed to the highest Cu concentration. However, larvae exposed to 2 µg/L Cu exhibited increased growth in comparison with non-exposed larvae. The percentage of malformed larvae was significantly higher for both copper conditions, with skeletal malformations being the most observed. Expression of several genes was evaluated in whole larvae using quantitative real-time PCR. Genes involved in detoxification (gst, mt1 and mt2) and in cell cycle arrest (p53) were significantly repressed in both copper conditions when compared to control. In addition, potential genotoxic effects on larvae were investigated by the comet assay on blood cells, but this test did not demonstrate any significant DNA damage on larvae exposed to copper. This study confirms the adverse effects of copper on early life stages of rainbow trout even at the lowest environmentally relevant tested concentration.

Altered glycometabolism in zebrafish exposed to thifluzamide

Thifluzamide exerts toxic effects to zebrafish and causes liver mitochondrial damage. To better understand the further mechanism, adult zebrafish were exposed to a range of thifluzamide concentrations (0, 0.019, 0.19, and 1.90 mg/L) for 28 days. In response to 1.90 mg/L exposure, liver glycogen significantly increased and blood glucose decreased. The expression of genes related to glycometabolism showed corresponding changes. Genes related to mtDNA replication and transcription and genes participating in mitochondrial complexes showed altered expression, which might lead to the inhibition of the tricarboxylic acid cycle (TCA). Additionally, the activity of glucose-6-phosphate dehydrogenase (G6PDH) was markedly increased at 1.90 mg/L, which might result in the activation of the pentose phosphate pathway. Moreover, the activity of lactate dehydrogenase (LDH) was significantly reduced at 1.90 mg/L, which might indicate that anaerobic glycolysis was inhibited. This study suggests that the altered gene expression and enzyme activities might be responsible for changes in glycometabolism, as evidenced by the altered expression of glycometabolism-related genes, the increased amount of glycogen in the liver and the decreased blood glucose levels. Overall, thifluzamide caused dysfunctional glycometabolism and led to events that might contribute to various thifluzamide-induced abnormalities in zebrafish.

Monitoring and assessment of environmental disturbance on natural Gambusia affinis populations--histopathological analysis

Physiological investigations are often used to assess the toxic effects of aquatic pollutants in fish and to establish criteria for water quality. In the present study, we tend to investigate the growth performance, skeletal abnormalities, and histological alterations noticed in three natural populations of the mosquitofish Gambusia affinis captured from three estuary sites. The analysis of growth performance demonstrated that the studied populations showed different growth patterns. Additionally, various types of skeletal deformities were observed, and the most commonly affected were the hemal and pre-hemal regions of fishes. For the histological study, gills, liver, and kidney tissues were selected for field monitoring. The major alterations observed in gill tissues were partial fusion of adjacent secondary lamellae, hypertrophy of epithelial cells, and disorganization of pillar cells in many areas of the secondary lamellae. In the liver, significant desquamation of tissues, congestion of the central hepatic vein, and hypertrophy of hepatocytes were noticed. For the kidney tissue, the frequencies of histological alterations showed a significant difference between the studied sites. Moreover, the histological aberrations consisted mainly in glomerule alteration and vacuolation of tubular epithelial cells. Taken together, these data support the hypothesis that the physiological alterations noticed in the present investigation are indicators of sensitivity towards environmental disturbance.

Heavy metal accumulation and histomorphological alterations in Aphanius fasciatus (Pisces, Cyprinodontidae) from the Gulf of Gabes (Tunisia)

The present study illustrates an analysis of histological changes; cadmium (Cd), copper (Cu) and zinc (Zn) accumulation; and metallothionein (MT) levels in normal and deformed Mediterranean killifish, Aphanius fasciatus (Pisces, Cyprinodontidae), collected from unpolluted (S1) and polluted areas (S2) in the Gulf of Gabes in Tunisia. Metal determination in water and sediment showed that the concentrations were significantly higher (p < 0.0001) in S2 compared to S1. Deformed fish showed a significantly higher accumulation of Cd, Cu, and Zn and high levels of MTs in their tissues compared to normal ones. Histopathological investigations revealed greater changes in gills, kidney, liver, and bone tissues of fish from the polluted area than those recorded in fish from the reference area. In comparison to normal fish of the polluted area (S2), tissue alterations were more developed in deformed specimens of this site. A possible relationship between metallic pollution, incidence of spinal deformities, and histological changes in A. fasciatus in the polluted site was discussed.

Dietary CdSe/ZnS quantum dot exposure in estuarine fish: bioavailability, oxidative stress responses, reproduction, and maternal transfer

Continued development, use, and disposal of quantum dots (QDs) ensure their entrance into aquatic environments where they could pose a risk to biological organisms as whole nanoparticles or as degraded metal constituents. Reproductive Fundulus heteroclitus were fed a control diet with lecithin, diets containing 1 or 10 μg of lecithin-encapsulated CdSe/ZnS QD/day, or a diet containing 5.9 μg CdCl2/day for 85 days. Cadmium concentrations in liver, intestine, and eggs were quantified with inductively coupled plasma mass spectrometry. In fish fed 10 μg QD/day, QDs or their degradation products traversed the intestinal epithelia and accumulated in the liver. Less than 0.01% of the QD's cadmium was retained in the liver or intestinal tissues. This compares to 0.9% and 0.5% of the cadmium in the intestine and liver, respectively of fish fed a CdCl2 diet. Cadmium was also detected in the eggs from parents fed 10 μg QD/day. No significant changes in hepatic total glutathione, lipid peroxidation, or expression of genes involved in metal metabolism or oxidative stress were observed. While QDs in the diet are minimally bioavailable, unusual levels of vitellogenin transcription in male fish as well as declining fecundity require further investigation to determine if endocrine disruption is of environmental concern.

Metallothionein and superoxide dismutase responses to sublethal cadmium exposure in the clam Mactra veneriformis

The objective of this study was to identify the relationship between cadmium (Cd) and stress responses in the clam Mactra veneriformis. Metallothionein (MT) and Cu-Zn superoxide dismutase (SOD) cDNAs from the clam were isolated and characterized. The full-length cDNA of MvMT and MvSOD contained 830 and 689 nucleotides encoding 59 and 159 amino acids, respectively. Multiple alignments indicated that deduced amino acid sequences of MvMT and MvSOD shared high homology with MT and SOD sequences of other mollusks. Clams were exposed to 0, 50, 100, and 200microg/L Cd for 21days. The mRNA transcripts of the two genes, MT protein content and SOD activity in the digestive gland were examined. Cd treatment significantly elevated MvMT and MvSOD mRNA expression in a dose-dependent manner and MT protein level in a dose- and time-dependent manner. SOD activity significantly increased at the start of Cd exposure, then decreased and finally returned to the normal level. These results indicate that MT and SOD play an important role in maintaining cellular metabolism homeostasis and protecting M. veneriformis from Cd toxicity. MT and SOD could be used as biomarkers of Cd pollution in aquatic environment for the studied species.

Can thiol compounds be used as biomarkers of aquatic ecosystem contamination by cadmium?

Due to anthropogenic activities, heavy metals still represent a threat for various trophic levels. If aquatic animals are exposed to heavy metals we can obviously observe considerable toxicity. It is well known that an organism affected by cadmium (Cd) synthesize low molecular mass thiol compounds rich in cysteine (Cys), such as metallothioneins (MT) and glutathione (GSH/GSSG). The aim of this study was to summarize the effect of Cd on level of thiol compounds in aquatic organisms, and evaluate that the concentrations of thiol compounds are effective indicators of Cd water pollution and explain their potential use in biomonitoring applications.

Distinctive accumulation patterns of Cd(II), Cu(II), and Cr(VI) in tissue of the South American teleost, pejerrey (Odontesthes bonariensis)

The time- and concentration-dependent accumulation patterns of three environmentally relevant metals, with different chemical and biological properties, were comparatively evaluated in tissue of South American fish, pejerrey (Odontesthes bonariensis). Six-month-old juvenile pejerrey were exposed to 1, 5, or 10 microgl(-1) of Cd(2+); 10, 50, or 100 microgl(-1) of Cu(2+); or 100, 500, or 1000 microgl(-1) of Cr(6+) during 16 days. Tested concentrations ranged from those that caused no significant accumulation to those that induced growth reduction or even mortality (Cd and Cu). Concentration-dependent accumulation of Cd and Cr presented similar patterns, characterized by a linear and exponential relationship in the gill and liver, respectively, with a greater slope at longer exposure times. Differently, essential Cu showed a linear relationship in both tissues, and no slope increase was observed in the gill. The gill time-dependent accumulation pattern of Cd exhibited relatively long times to half-saturation (t(1/2(1))=36.6 days) and high-saturation values (S((1))=205.4 nmolg(-1)). The opposite pattern was observed for Cu (t(1/2(100))=0.6 days; S((100))=24.4 nmolg(-1)), while Cr showed an intermediate position (t(1/2(1000))=2.8 days; S((1000))=50.2 nmolg(-1)). In the liver, times to half saturation for the three metals were as follows: Cd=4.7 days; Cu=2.5 days; Cr=1.6 days. Conversely, different saturation concentrations were observed among metals, Cu (S((100))=102) presenting the highest values, Cd (S((10))=4.4) the lowest, and Cr presenting an intermediate position (S((1000))=24.7). Bioconcentration of non-essential Cd was high in the gill (2000-fold) and low in the liver (50-fold). On the other hand, the essential element Cu was poorly retained by the gill (15-fold) and accumulated mainly in the liver (50-fold). Differently, Cr was quickly and evenly accumulated by both organs, but barely bioconcentrating (2.5- and 1.5-fold levels as compared to the original in the gill and liver, respectively). The ratio of gill/liver concentration in exposed fish clearly differed from that of non-exposed fish, and was characteristic for each metal (Cd>1; Cu<1; Cr<9), representing a potential tool to assess exposure. The accumulation patterns of studied metals in the tissue of pejerrey corresponded more with those reported for sensitive than for tolerant fish species. Distinctive tissue accumulation patterns in relation to growth reduction and non-acute lethality suggests different target tissues for sub-chronic effects, and would partially explain differences in the relative toxicity of these metals.

Hepatic metallothionein and total oxyradical scavenging capacity in Atlantic cod Gadus morhua caged in open sea contamination gradients

Biological effects monitoring has seldom been undertaken in offshore pelagic environments. Cages containing hatchery-reared Atlantic cod Gadus morhua were deployed on expected contamination gradients, along a transect from the River Elbe in the German Bight, and in the vicinity of an oil field in the North Sea (Statfjord). Six weeks later, the cod were retrieved and samples taken for a range of biological effects techniques. In this study, metallothionein (MT) and total oxyradical scavenging capacity (TOSC) were measured in liver samples from the caged cod, together with metals (as a measure of bioaccumulation). Both MT and TOSC were highest in cod from the German Bight. In the Statfjord samples MT and TOSC decreased with distance from the oil platform indicating induction in response to anthropogenic sources. The bioavailability of metals appears to be a major factor in MT synthesis, and the measurement of MT and associated metals is shown to be a useful tool for biological exposure and effects monitoring in pelagic systems. There also appears to be a strong linkage between MT and TOSC levels, indicating overlapping capabilities as stress biomarkers. Results suggest that in addition to its role as a specific indicator of metal exposure, MT in cod could act as a more general biomarker of oxidative stress under certain conditions.

Modulatory effect of cadmium exposure on deltamethrin-induced oxidative stress in Channa punctata Bloch

The modulatory effect of cadmium pretreatment (0.2 mg/kg b.w. i.p.) on deltamethrin-induced oxidative stress and alterations of antioxidants was studied in freshwater fish Channa punctata Bloch. Lipid peroxidation (LPO) was measured as one of the indicators of oxidative stress. Activities of glutathione peroxidase, glutathione reductase, and glutathione-S-transferase were also studied in liver, kidney, and gills. Levels of reduced glutathione (GSH) were measured in various tissues. Fish pretreated with cadmium and subsequently exposed to deltamethrin showed significantly reduced LPO values when compared with deltamethrin-exposed fish. Conversely, in the kidney, a potentiation response was observed. Deltamethrin exposure in fish resulted in significant alteration in activities of glutathione-dependent antioxidant enzymes. In the cadmium preexposed fish, which were subsequently exposed to deltamethrin, antioxidant enzymes showed a tendency toward normalization over deltamethrin-only exposed fish in liver and gills. GSH also showed a similar pattern in liver and gills but in kidney it remained elevated. Cadmium alone had no significant effect on various parameters at the concentration used in this study. When metallothionein (MT) induction was studied, only liver showed an MT-like protein band in sodium dodecylsulfate-polyacrylamide gel electrophoresis analysis. These results demonstrate a modulatory role of cadmium on the oxidative stress and other related parameters in liver and gills. These findings are important in the context of exposure to a mixture of pollutants in aquatic environment.

Metallothionein induction and heavy metal accumulation in white shrimp Litopenaeus vannamei exposed to cadmium and zinc

Metallothioneins (MTs) have been widely considered for their potential use as specific biomarkers to reflect the existence of heavy metal pollution, because their induction has been observed to be obviously elevated after heavy metal exposure in a large number organism studied. However, relatively fewer efforts have been made in MT-related studies of prawn species, such as the white shrimp Litopenaeus vannamei, a globally important aquaculture species. With the results from gel filtration chromatography, we demonstrate the existence of MTs or MT-like proteins in L.vannamei. We further studied the relationship between MT induction and metals accumulation after long-term exposure to the heavy metals Cd and Zn. From our results, it is very clear that the response of L. vannamei to Cd differs from that to Zn, and this should be considered when using MTs in field applications to monitor metals contamination.

Resistance to water pollution in natural gudgeon (Gobio gobio) populations may be due to genetic adaptation

Anthropogenic disturbances cause the environment to change relatively fast. It is reasonable to assume that it is very unlikely for individuals to develop genetic adaptations to their polluted habitats, since adaptation through natural selection is a relatively slow process. Nevertheless, several studies have shown that such adaptations to changing environmental conditions may develop faster than anticipated. This study investigates the impact of historical metal pollution on a natural population of the gudgeon, Gobio gobio. Specimens from a contaminated site and a reference population were subjected to a series of three exposure experiments to cadmium after an acclimation period to reconstituted fresh water of 36 days. First, we performed an acute toxicity test on a sub-sample of both experimental groups to determine times-to-death (TTD) and lethal body burdens (LBB). The remaining individuals were used in a chronic Cd-exposure experiment, after which total Cd-body concentration, as well as Cd-concentrations and metallothionein-like protein (MTLP) levels in liver and gill tissue were determined. From the specimens that were not sacrificed for these measurements, a random subsample was subjected to a second acute toxicity test to evaluate the effect of chronic Cd-exposure (acclimation) on TTD and LBB. Our results show that, particularly after an extra acclimation period to a sublethal Cd-concentration, specimens originating from the contaminated sample area survived the acute exposure experiments better, despite the fact that neither the average Cd-accumulation rate, nor the lethal body concentrations differed between fishes from both groups. We also find that gudgeons from both populations translocated Cd from the gills (and probably also from other compartments) to the liver, where it can be more efficiently detoxified by MTLPs. Indeed, MTLP levels were found to increase faster in liver and gill tissue of specimens from the contaminated site, resulting in significantly higher MTLP-levels in the organs of these fishes. Although this study does not provide direct evidence for a genetic basis of Cd resistance (i.e. at the gene level), our results indicate that the regulation of MTLP-gene expression may involve a genetic component.

Histopathological alterations of white seabass, Lates calcarifer, in acute and subchronic cadmium exposure

Histopathological alterations to white seabass, Lates calcarifer aged 3 months in acute and subchronic cadmium exposure were studied by light and scanning electron microscopy. The 96-h LC50 values of cadmium to L. calcarifer was found to be 20.12 +/- 0.61 mg/l and the maximum acceptable toxicant concentration (MATC) was 7.79 mg/l. Fish were exposed to 10 and 0.8 mg/l of Cd (as CdCl,H2O) for 96 h and 90 days, respectively. The study showed that gill lamellae and kidney tubules were the primary target organs for the acute toxic effect of cadmium while in the subchronic exposure, the toxic effect to gills was less than that of kidney and liver. Gill alterations included edema of the epithelial cells with the breakdown of pillar cell system, aneurisms with some ruptures, hypertrophy and hyperplasia of epithelial and chloride cells. The liver showed blood congestion in sinusoids and hydropic swelling of hepatocytes, vacuolation and dark granule accumulation. Lipid droplets and glycogen content were observed in hepatocytes at the second and third month of subchronic exposure. The kidney showed hydropic swelling of tubular cell vacuolation and numerous dark granule accumulation in many tubules. Tubular degeneration and necrosis were seen in some areas.

Alterations of tissue glutathione levels and metallothionein mRNA in rainbow trout during single and combined exposure to cadmium and zinc

The objective of this study was to assess the effects of Cd and Zn exposure of rainbow trout (Oncorhynchus mykiss) on (a) hepatic glutathione (GSH) levels; and (b) hepatic and branchial metallothionein (MT) mRNA expression. Juvenile rainbow trout were exposed to waterborne Cd (nominal concentrations: 1.5 or 10 microg Cd l(-1)), Zn (150 or 1000 microg Zn l(-1)) or Cd/Zn mixtures (1.5 microg Cd l(-1) with 200 microg Zn l(-1) or 10 microg Cd l(-1) with 1000 microg Zn l(-1)). After 14 and 28 days of treatment, hepatic concentrations of total glutathione, oxidized glutathione (GSSG) and cysteine were determined by means of fluorometric high performance liquid chromatography (HPLC). Branchial and hepatic expression of MT mRNA was measured by means of semi-quantitative RT-PCR. Exposure of trout to Zn did not result in significantly elevated tissue levels of Zn, whereas Cd accumulation factors changed significantly with time and concentration. Despite of the absence of Zn accumulation, hepatic GSH but not MT mRNA levels were significantly altered in Zn-exposed fish. Cd, on the contrary, affected mainly the MT response but not GSH. Also tissue specific differences in the regulation of the two thiol pools were expressed. The thiol response after exposure to metal mixtures could not be explained by simple addition of the effects of the individual metals. The results indicate that cellular thiol pools show different reaction patterns with respect to specific metals and metal mixtures. Under conditions of long-term, low dose metal exposure, the function of GSH appears to go beyond that of a transitory, first line defense.

Effect of pyrethroids on carbohydrate metabolic pathways in common carp, Cyprinus carpio

The activity levels of succinate dehydrogenase (SDH) and glucose-6-phosphate dehydrogenase (G6PD) were assessed in various tissues of Cyprinus carpio var communis which had been exposed to lethal concentrations of group-II pyrethroids (deltamethrin, cypermethrin, fenvalerate and fluvalinate) for a period of 72 h. The results indicated a steady decrease in SDH activity with a concomitant increase in G6PD activity. The decreased SDH activity indicated inhibition of SDH at mitochondrial level and the increased G6PD activity an enhancement of an alternative pathway of carbohydrate metabolism, viz the hexose monophosphate shunt (HMP) or pentose phosphate pathway as a biochemical adaptation to overcome the toxic stress.

Metal accumulation and metallothionein in brown trout, Salmo trutta, from two Norwegian rivers differently contaminated with Cd, Cu and Zn

In this work we have studied the accumulation of heavy metals in two brown trout (Salmo trutta) populations in their natural environment and the participation of metal binding to metallothionein (MT) in this process. Cd, Cu and Zn concentrations, total MT (including Cu MT) and Cd/Zn MT were measured in the gills, liver and kidney of trout inhabiting two rivers, one Cu-contaminated and the other Cd/Zn-contaminated, located at Røros, Central Norway. In both populations, high levels of Cu were found in the liver, whereas Cd was accumulated in liver and particularly in the kidney. The proportions of Cd/Zn MT and Cu MT in liver and kidney, but not in gills, reflected the accumulated and the environmental concentrations of these metals. The total Cu MT concentrations in the investigated tissues, however, were highest in trout from the river with the lowest ambient Cu concentration. It is suggested that MTs are of less importance in Cu-acclimated trout. The data also suggest that acclimation to a Cu-rich environment involves reduced Cu accumulation or increased Cu elimination. In trout from the Cd-rich environment, this metal was mainly bound to MT, whereas in trout from the Cu-rich environment Cd was also associated with non-MT proteins. These findings emphasize the importance to determine both Cd/Zn MT and Cu MT levels, when the participation of this protein in metal handling in trout tissues is investigated.

Metallothionein induction in early larval stages of tilapia (Oreochromis mossambicus)

Amounts of whole-body metallothionein (MT) in tilapia (Oreochromis mossambicus) larvae increased to a peak (1,500 ng mg(-1) protein) 1 d after hatching (H1), decreased rapidly thereafter, and was maintained at a constant level (700 ng mg(-1)) 3 d after hatching (H3). Waterborne Cd(2+) could stimulate MT expression in newly hatched (H0) larvae in dose-dependent and time-dependent patterns. H0 larvae, which were treated with 35 microg L(-1) Cd(2+) for 24 h, showed a 1.7-fold increase in the MT amount (174.0+/-64.7) and a 6. 5-fold increase in accumulated Cd(2+) but no significant change in Ca(2+) content, compared with the H0 control (MT, 102.6+/-48.1). H3 larvae with the same treatment revealed about a 10-fold increase in accumulated Cd(2+), a 10% decrease in Ca(2+) content, but no change in MT (261.2+/-120.0), compared with the H3 control (MT, 330+/-74.0). H0 larvae could synthesize more MT to bind Cd(2+) for detoxification in 35 microg L(-1) Cd(2+), a dose that would not affect normal physiology or survival of H0 larvae. On the other hand, 35 microg L(-1) Cd(2+) caused H3 larvae to experience hypocalcemia, an abnormal physiological condition, in which H3 larvae could not synthesize sufficient MT, thus causing greater than 25% mortality. These results indicate for the first time that the inducibility of MT by waterborne Cd(2+) is development dependent, being correlated with inconsistent sensitivities to Cd(2+) during larval development.

Sublethal effects of repeated intraperitoneal cadmium injections on rainbow trout (Oncorhynchus mykiss)

Acute and chronic effects of cadmium have been widely described for different aquatic organisms and exposure routes. However, there is clearly a lack of information on the potential of cadmium to cause genotoxic effects. This work presents genotoxic and nongenotoxic parameters analyzed in cadmium-exposed rainbow trout. The assessment was performed for sublethal levels after long-term exposure using six intraperitoneal injections of 0.5 mg/kg (Day 1), 1 mg/kg (Days 3, 7 and 11), and 2 mg/kg (Days 15 and 19) to allow precise estimation of the dose. Cadmium accumulation in target tissues, essential metal mobilization by cadmium at the subcellular and tissue levels, and induction of metallothioneins were selected as exposure and effect parameters. Induction of micronuclei and variation in DNA content (expressed as variation coefficient in the G1 phase of the cell cycle) in blood cells, determined by flow cytometry, were selected as biomarkers for genotoxic effects. Cadmium accumulation, induction of metallothioneins, and mobilization of essential metals at the subcellular level were observed in different organs in response to cadmium exposure. The highest metallothionein induction was observed in liver, reaching 270+/-90 nmol/g wet tissue in treated fish versus 2.68+/-1.1 nmol/g wet tissue in controls. The highest cadmium accumulation was also observed in the liver (27.8+/-9.5 microgram Cd/g wet wt in treated animals versus 1.0+/-1.7 in the control group). However, no genotoxic effects were observed in blood cells. The frequency of micronuclei was 0.012+/-0.008 for the control group and 0.013+/-0.021 for treated animals. The variation coefficient of G1-phase nuclei was 3.61+/-0.66 and 3.22+/-0.29 for control and cadmium-exposed groups, respectively. Thus, it is concluded that under the experimental conditions employed here, treatment of rainbow trout with cadmium chloride at doses that produce significant toxicological alterations at the tissue and subcellular levels does not provoke observable alterations in the genotoxic parameters considered in this study.

Functional comparison of the metal-regulated transcriptional control regions of metallothionein genes from cadmium-sensitive and tolerant fish species

The promoter region of teleost metallothioneins (MTs) contains multiple metal-responsive elements (MREs) organized in proximal and distal clusters which together mediate gene induction by heavy metals. This arrangement of MREs is found both in cadmium-sensitive species, such as the rainbow trout, and in cadmium-tolerant species such as the pike and the stone loach. On comparison of the putative regulatory elements identified within the 5'-flanking region of these genes the major differences are that the number of MREs differ between the different species and that, while both the stone loach and rainbow trout MT genes contain TATA boxes, the pike MT gene has a TTTA box. In order to investigate if the metal sensitivity of a species is correlated to the regulatory potential of the putative MT detoxification system the promoter regions of MT genes from all three species were assessed for their ability to enhance transcription in response to the heavy metals Zn, Cd and Cu. The polymerase chain reaction was used to produce nested deletion sets of each promoter region and these were cloned into the mammalian expression vector pGL-2 upstream of the firefly luciferase gene. The inducibility of the different constructs in response to heavy metal challenge was tested in two cell lines, one fish cell line (homologous to rainbow trout and heterologous to the two other species), the rainbow trout hepatoma, RTH-149, cell line and one cell line that was heterologous to all studied species, the human hepatoblastoma; HepG2, cell line. Maximum inducibility of each gene was achieved with constructs containing both the proximal and the distal MRE clusters. Both the rainbow trout and the stone loach MT genes showed inducibility of comparable amplitude whilst the pike MT gene on the other hand was less inducible, partly due to fewer MREs and partly due to the TTTA box. These data indicate that more than one mechanism is responsible for the differences in cadmium sensitivity of these three teleost species. Although MT is the main heavy-metal detoxifying system in most vertebrates and appears to be contributing to the differences seen between rainbow trout and pike, the present study shows that the relative sensitivity of these species is not primarily due to MT.

Evaluation of differential pulse polarography for the quantification of metallothionein--a comparison with RIA

Two methods to quantify metallothionein (MT), differential pulse polarography (DPP) and radioimmunoassay (RIA), were compared for MT analysis of liver from Zn- and Cd-injected perch (Perca fluviatilis). Nine perch were intraperitoneally injected, twice a week during 2 weeks with ZnSO4 and CdCl2 to yield a total dose of 30 mg Zn and 3 mg Cd per kilogram body weight. Two samples, 100 and 200 mg from each liver, were homogenized separately and further prepared for DPP, RIA, and atomic absorption spectroscopy. MT values obtained by DPP were in good agreement with the MT values determined by RIA (r = 0.92). The relationship between the MT values analyzed with the two methods is described by the formula MTRIA = MTDPP x 0.99-0.048. Analysis of MT was not affected by sample size. MT values from individual liver samples plotted against the Cd and Zn content of the corresponding samples provided a high correlation. The correlation coefficient was 0.86 for MT values obtained by DPP and 0.92 for MT measured by RIA. It is concluded that DPP is a reliable method for analyzing MT in liver.

The importance of metallothionein for the accumulation of copper, zinc and cadmium in environmentally exposed perch, Perca fluviatilis

A field study of the role of metallothionein (MT) in the binding of heavy metals in perch (Perca fluviatilis), exposed to moderate levels of copper, zinc and cadmium, was performed. Perch were sampled at four sites in Sweden in September during two consecutive years. Two sites were located in the vicinity of a brassworks and two outside the emission range. The first year, fish from the two brassworks sites and from one of the uncontaminated sites were collected. The second year, fish from the most contaminated site and from the two uncontaminated sites were caught. The levels of hepatic copper, zinc and cadmium reflected the concentrations of these metals in water and were increased in fish from the two contaminated sites. The level of cadmium in liver was relatively low. MT was induced in liver of perch caught at the most contaminated site. The hepatic MT content in individual livers correlated well to the accumulation of copper (r = 0.85, P less than 0.001) and zinc (r = 0.75, P less than 0.001). There was a low but significant correlation between the levels of MT and cadmium in the liver (r = 0.48, P less than 0.001). The relationship between MT and metals was very similar both years. Subcellular fractionation of the metals in the liver revealed that an induction of MT was followed by an increased amount of copper, zinc and cadmium bound to the protein. The relative fraction of the total hepatic copper and cadmium bound to MT was increased at the most contaminated site, whereas there was no difference in subcellular distribution of zinc between the sites. In perch from the most contaminated site, the portions of hepatic copper, zinc and cadmium found in the cytosolic fraction were 70, 57 and 81%, respectively. Seventy-one % of the copper, 29% of the zinc and 84% of the cadmium found in hepatic cytosol of fish from the same site, eluted together with MT after gel filtration chromatography. The analysis of the subcellular distribution of copper, zinc and cadmium demonstrates that MT is responsible for the binding of a large amount of the total hepatic cellular content of copper and cadmium in perch.

Binding and detoxification of heavy metals in lower vertebrates with reference to metallothionein

1. The metabolism of Cu, Zn, Cd and Hg in lower vertebrates is described, using fish as a model. 2. The main part of this review deals with metallothionein and the role of this protein for the storage and detoxification of these metals. 3. Factors influencing the bioavailability and probable uptake routes are identified. 4. The distribution of the metals within the organism is outlined. The distribution between tissues is described and the subcellular distribution discussed with reference to metallothionein.

Ascorbate-2-sulfate as a dietary vitamin C source for Atlantic salmon (Salmo salar): 2. Effects of dietary levels and immunization on the metabolism of trace elements

Atlantic salmon fingerlings were fed a vitamin C deficient diet for four months. The fish were then provided a dry, practical fishmeal based diet supplemented with 0, 500 or 5000 mg vitamin C/kg as L-ascorbic acid or equivalent amounts of ascorbate-2-sulfate. After six weeks on these diets ten fish in each group were injected with a soluble antigen (NIP11-LPH). Six weeks thereafter blood, liver, kidney, spleen and vertebrae were examined for trace elements. The livers were also analysed for metallothionein.The vitamin C deficient fish were anemic despite the significantly elevated iron concentrations in the liver. Vitamin C had no positive effect in lowering tissue levels of cadmium. The highest level of dietary vitamin C given as ascorbic acid reduced the liver selenium concentrations. In response to antigen injection, the fish in all groups showed increased levels of hepatic metallothionein, copper, zinc and cadmium, while hepatic selenium and iron levels were less affected. The elemental composition in other organs was affected by the antigen injection to a minor extent.

A radioimmunoassay for perch (Perca fluviatilis) metallothionein

A sensitive radioimmunoassay (RIA) for the measurement of metallothionein (MT) from perch (Perca fluviatilis) has been developed. The method is a double-antibody RIA with rabbit anti-perch MT serum as first antibody, goat anti-rabbit immunoglobulin G as second antibody, and perch MT conjugated to 125I-labeled Bolton-Hunter reagent as tracer. The rabbit antiserum raised against perch MT recognizes rainbow trout (Oncorhynchus mykiss) MT, but shows little cross-reactivity with horse MT. At a dilution of 1:2000, the MT antibodies bind 36% of the tracer when no cold ligand is present. The sensitivity of the assay is 15 pg perch MT per tube and the practical working range is 0.15-250 ng perch MT per tube. The RIA allows determination of MT in plasma or lysed blood cells at concentrations as low as 3.0 ng/ml and in tissues at levels above 9.0 ng/g (wet weight). Intra- and interassay coefficients of variation were 6 and 10%, respectively.

Endogenous and heavy-metal-ion-induced metallothionein gene expression in salmonid tissues and cell lines

Endogenous levels of metallothionein (MT) mRNA were detected by RNA probes in several somatic and germ-line tissues of rainbow trout, such as eggs, ovaries and immature testis. These levels may be related to metal-ion homeostasis in the observed tissues. The induction kinetics of trout MT isoform B (MT-B) mRNA were studied after single intraperitoneal injections of CdCl2, CuCl2 and ZnCl2. MT-B mRNA was induced within 12 h in liver, kidney, spleen and gills. However, over the 48-h experimental period, the kinetics of MT-B mRNA accumulation differed in response to the three metal salts, possibly due to differential handling of the salts by these tissues. Multiple metal-salt injections induced high levels of MT-B mRNA in the four tissues studied. In the rainbow trout hepatoma cell line, ZnCl2 was a better inducer of the MT-B gene, as compared to CdCl2 and CuCl2. The expression of the exogenous trout MT-B promoter in Chinook salmon embryonic cell line indicates the presence of MT regulatory factors. In contrast, the endogenous MT genes in these cells are quiescent, possibly due to the methylation of their promoter region.