Branchial cadmium and copper binding and intestinal cadmium uptake in wild yellow perch (Perca flavescens) from clean and metal-contaminated lakes

The Role of Glutathione and Sulfhydryl Groups in Cadmium Uptake by Cultures of the Rainbow Trout RTG-2 Cell Line

The aim of this study is to investigate the role of cellular sulfhydryl and glutathione (GSH) status in cellular cadmium (Cd) accumulation using cultures of the rainbow trout cell line RTG-2. In a first set of experiments, the time course of Cd accumulation in RTG-2 cells exposed to a non-cytotoxic CdCl2 concentration (25 μM) was determined, as were the associated changes in the cellular sulfhydryl status. The cellular levels of total GSH, oxidized glutathione (GSSG), and cysteine were determined with fluorometric high-performance liquid chromatography (HPLC), and the intracellular Cd concentrations were determined with inductively coupled plasma mass spectrometry (ICP-MS). The Cd uptake during the first 24 h of exposure was linear before it approached a plateau at 48 h. The metal accumulation did not cause an alteration in cellular GSH, GSSG, or cysteine levels. In a second set of experiments, we examined whether the cellular sulfhydryl status modulates Cd accumulation. To this end, the following approaches were used: (a) untreated RTG-2 cells as controls, and (b) RTG-2 cells that were either depleted of GSH through pre-exposure to 1 mM L-buthionine-SR-sulfoximine (BSO), an inhibitor of glutathione synthesis, or the cellular sulfhydryl groups were blocked through treatment with 2.5 μM N-ethylmaleimide (NEM). Compared to the control cells, the cells depleted of intracellular GSH showed a 25% reduction in Cd accumulation. Likewise, the Cd accumulation was reduced by 25% in the RTG-2 cells with blocked sulfhydryl groups. However, the 25% decrease in cellular Cd accumulation in the sulfhydryl-manipulated cells was statistically not significantly different from the Cd accumulation in the control cells. The findings of this study suggest that the intracellular sulfhydryl and GSH status, in contrast to their importance for Cd toxicodynamics, is of limited importance for the toxicokinetics of Cd in fish cells.

Cadmium induced bioaccumulation, histopathology, gene regulation in fish and its amelioration - A review

Different anthropogenic activities as well as natural sources contribute enormously towards various heavy metal contaminations in aquatic habitats. Cadmium (Cd) is one of most prevalent and toxic heavy metals with a long half life. Unlike terrestrial animals, exposure of Cd in fishes may happen not only through feeds but also from its habitat water. Bioaccumulation of Cd in fishes occurs in many tissues, but mainly in gill, liver, kidney, skin, and muscle. The concentrations of Cd in fish tissues depend upon the extent and duration of Cd exposure, species and age of fishes, dietary minerals and antioxidant concentrations, and habitat water quality. Specific histopathological observations in liver, kidney, and gill are useful to understand the effects of Cd, which could help to determine the ameliorating methods to be adopted. Exposure of Cd exerts several adverse effects on general growth and development, reproductive processes, osmoregulation, morphological and histological structures, stress tolerance, and endocrine system, mainly due to changes in biological functions induced by differential expressions of several genes related to oxidative stress, apoptosis, inflammation, immunosuppressions, genotoxicity, Cd chelation and carbohydrate metabolism. Chronic biomagnifications of Cd exceeding the permitted level may be harmful not only to the fishes itself but also to humans through food chains. Amelioration of such toxic heavy metal that has been categorized as a potent carcinogenic in humans is of utmost importance. Main modes of amelioration encompas reducing oxidative damages by promoting the antioxidative defenses, decreasing Cd absorption, increasing excretion through excretory system and improving the tolerance of fishes to Cd toxicity. Many amelioration measures such as use of minerals (for example, zinc, calcium, and iron), vitamins (vitamin C, A, and E), different herbs, probiotics and other agents (taurine, bentonite, chitosan, zeolite, and metallothionein) have been explored for their effective roles to reduce Cd bioaccumulation and toxicity symptoms in fishes. The present review discusses bioaccumulation of Cd, histopathological alterations, oxidative stress, synergism-antagonism, and gene regulation in different tissues, and its amelioration measures in fishes.

Toxic Effects of Cadmium on Fish

Large amounts of enriched cadmium (Cd) in the environment seriously threatens the healthy and sustainable development of the aquaculture industry and greatly restricts the development of the food processing industry. Studying the distribution and toxic effects of Cd in fish, as well as the possible toxic effects of Cd on the human body, is very significant. A large number of studies have shown that the accumulation and distribution of Cd in fish are biologically specific, cause tissue differences, and seriously damage the integrity of tissue structure and function, the antioxidant defense system, the reproductive regulation system, and the immune system. The physiological, biochemical, enzyme, molecular, and gene expression levels change with different concentrations and times of Cd exposure, and these changes are closely related to the target sites of Cd action and tissues in fish. Therefore, the toxic effects of Cd on fish occur with multiple tissues, systems, and levels.

Waterborne zinc bioaccumulation influences glucose metabolism in orange-spotted grouper embryos

Fish embryos, as an endogenous system, strictly regulate an energy metabolism that is particularly sensitive to environmental pressure. This study used orange-spotted grouper embryos and stable isotope ⁶⁷Zn to test the hypothesis that waterborne Zn exposure had a significant effect on energy metabolism in embryos. The fish embryos were exposed to a gradient level of waterborne ⁶⁷Zn, and then sampled to quantify ⁶⁷Zn bioaccumulation and mRNA expressions of key genes involved glucose metabolism. The results indicated that the bioaccumulated ⁶⁷Zn generally increased with increasing waterborne ⁶⁷Zn concentrations, while it tended to be saturated at waterborne ⁶⁷Zn > 0.7 mg L^-1. As we hypothesized, the expression of PK and PFK gene involved glycolysis pathway was significantly up-regulated under waterborne ⁶⁷Zn exposure >4 mg L^-1. Waterborne ⁶⁷Zn exposure >2 mg L^-1 significantly suppressed PCK and G6PC gene expression involved gluconeogenesis pathway, and also inhibited the AKT2, GSK-3beta and GLUT4 genes involved Akt signaling pathway. Our findings first characterized developmental stage-dependent Zn uptake and genotoxicity in fish embryos. We suggest fish embryos, as a small-scale modeling biosystem, have a large potential and wide applicability for determining cytotoxicity/genotoxicity of waterborne metal in aquatic ecosystem.

Community recovery dynamics in yellow perch microbiome after gradual and constant metallic perturbations

BACKGROUND

The eco-evolutionary processes ruling post-disturbance microbial assembly remain poorly studied, particularly in host-microbiome systems. The community recovery depends not only on the type, duration, intensity, and gradient of disturbance, but also on the initial community structure, phylogenetic composition, legacy, and habitat (soil, water, host). In this study, yellow perch (Perca flavescens) juveniles were exposed over 90 days to constant and gradual sublethal doses of cadmium chloride. Afterward, the exposure of aquaria tank system to cadmium was ceased for 60 days. The skin, gut and water tank microbiomes in control and treatment groups, were characterized before, during and after the cadmium exposure using 16s rDNA libraries and high throughput sequencing technology (Illumina, Miseq).

RESULTS

Our data exhibited long-term bioaccumulation of cadmium salts in the liver even after two months since ceasing the exposure. The gradient of cadmium disturbance had differential effects on the perch microbiota recovery, including increases in evenness, taxonomic composition shifts, as well as functional and phylogenetic divergence. The perch microbiome reached an alternative stable state in the skin and nearly complete recovery trajectories in the gut communities. The recovery of skin communities showed a significant proliferation of opportunistic fish pathogens (i.e., Flavobacterium). Our findings provide evidence that neutral processes were a much more significant contributor to microbial community turnover in control treatments than in those treated with cadmium, suggesting the role of selective processes in driving community recovery.

CONCLUSIONS

The short-term metallic disturbance of fish development has important long-term implications for host health. The recovery of microbial communities after metallic exposure depends on the magnitude of exposure (constant, gradual), and the nature of the ecological niche (water, skin, and gut). The skin and gut microbiota of fish exposed to constant concentrations of cadmium (CC) were closer to the control negative than those exposed to the gradual concentrations (CV). Overall, our results show that the microbial assembly during the community recovery were both orchestrated by neutral and deterministic processes. Video Abtract.

The simultaneous uptake of dietary and waterborne Cd in gastrointestinal tracts of marine yellowstripe goby Mugilogobius chulae

Aquatic animals under waterborne metal exposure are also very likely exposed to elevated dietary metals. This study quantified the simultaneous uptake of dietary and waterborne Cd in gastrointestinal tracts (GT) of marine yellowstripe goby using a dual stable isotope tracer method. The Cd spiked diet (10-100 μg g^-1, ¹¹¹Cd as tracers) were fed to the fish as a single meal, and then the fish were exposed to waterborne Cd (0-500 μg L^-1, ¹¹³Cd as tracers) for 48 h, during which the time-course uptake of Cd in the stomach and intestine was determined. The findings revealed that the dietary Cd uptake mainly occurred within 12 h after feeding. The fish exposed to 500 μg L^-1 waterborne Cd showed significantly lower Cd assimilation efficiency (2.07%) than the control group (3.48%) at the dietary Cd of 100 μg g^-1. Moreover, during 4-12 h when there was chyme in the GT, the waterborne Cd uptake in the intestine was lowest but the stomach showed the highest waterborne Cd uptake rate. The uptake of dietary and waterborne Cd, and the relative importance of dietary vs waterborne Cd was positively correlated with the Cd concentration in the chyme. Overall, this research demonstrated that there was interaction between dietary and waterborne Cd uptake in the GT of marine fish. The simultaneous uptake of metal from two routes is far more complex than the situation of a single route of metal uptake, which should be evaluated in determining metal bioaccumulation and toxicity in both laboratory and field metal exposure scenario.

Effects of salinity and copper co-exposure on copper bioaccumulation in marine rabbitfish Siganus oramin

Marine fish living in estuaries and coastal areas commonly encounter the stress of both salinity and metal pollution. In this study, euryhaline rabbitfish Siganus oramin were exposed to 50 μg L^-1 waterborne Cu or 300 μg g^-1 dry wt dietary Cu at salinity 33‰, 25‰, 20‰, 10‰, and 5‰ for 30 days. The Cu accumulation in the liver (>20-fold increase) and intestine (>5-fold increase) significantly increased after either waterborne or dietary Cu acclimation. Moreover, Cu accumulation was further enhanced in the liver, intestine, plasma, and whole body of Cu-exposed fish at lower salinities. Similarly, the waterborne Cu uptake rate constants (k_us) were stable in the control at different salinities but increased significantly (2-4 times higher) after waterborne Cu exposure. Conversely, the dietary Cu assimilation efficiencies (AEs) were significantly lower in the dietary Cu-exposed fish (3-5%) than in the control fish (8-16%) at different salinities, suggesting that dietary Cu acclimation partially alleviated the dietary Cu uptake from the high-Cu diet. The Cu efflux rate constants (k_es) were comparable among all treatments as 0.060-0.071 d^-1. The changes of Cu accumulation by different salinities and Cu exposure were well estimated by the biokinetic modeling. In summary, the present study indicates that rabbitfish can regulate Cu uptake and accumulation when acclimated to different salinities, but the Cu-exposed rabbitfish failed to prevent the elevation of Cu accumulation at low salinities. It therefore suggested that the concurrence of low salinity and high Cu exposure enhances the risks of Cu bioaccumulation and toxicity in rabbitfish.

Combined effects of temperature changes and metal contamination at different levels of biological organization in yellow perch

In this study, we measured the effects of temperature (9°C, 20°C, and 28°C), metal contamination (cadmium and nickel) and their interaction on yellow perch (Perca flavescens) using liver enzymatic and transcriptomic endpoints and biometric indices. Kidney metal concentrations increased with a rise of temperature. The biometric indices analysed (Fulton condition factor, pyloric cæca, hepatosomatic and gonadosomatic indices) generally decreased with an increase of temperature but not with metal contamination. At the enzymatic level, the activity of superoxide dismutase (SOD), involved in antioxidant response, was affected by both temperature and metal contamination, whereas the activity of glucose-6-phosphate dehydrogenase (G6PDH), involved in energy accumulation but also in antioxidant response, was only affected by metal exposure. The response of perch to the stressors at the transcriptional level differed from the metabolic response. In particular, the transcription level of the cco and g6pdh genes sharply decreased with increasing temperature, while the activities of the corresponding enzymes remained stable. The normal response of the transcription level of the apoptotic gene (diablo) to heat stress was also altered in metal-contaminated fish. The combination of metal and temperature stresses also modified the response of antioxidant metabolism induced by these stressors individually. This study contributes to a better understanding of the influences of natural stressors like temperature on biomarkers commonly used in ecotoxicological studies and will facilitate their interpretation in the context of multiple stressors characteristic of field situations.

Sublethal effects of cadmium on auditory structure and function in fathead minnows (Pimephales promelas)

Aquatic ecosystems are threatened by environmental contaminants, and many heavy metals can influence both the structure and function of sense organs in fishes. The use of these senses is vital to the survival and reproductive success of fish and therefore affects the health of the ecosystem as a whole. The current study examines the effects of cadmium on auditory structure and function in the fathead minnow (Pimephales promelas). In the laboratory, fish were exposed for 96 h to a range of cadmium concentrations and both hearing sensitivity and hair cell morphology were quantified. While hair cell numbers were unaffected, cadmium caused an increase in auditory threshold, with a critical range for toxic effects of cadmium estimated at 2.1-2.9 µg L(-1). Cadmium exposure also caused a decrease in response latency at higher cadmium concentrations. The current study demonstrates the sublethal effects of cadmium on fish sensory function while also pointing to the need for more careful interpretation of cadmium impacts on aquatic populations.

Evidence of common cadmium and copper uptake routes in zebrafish Danio rerio

Cadmium and copper accumulations in gills of zebrafish were measured during a 48 h exposure to 0.025 μM 106Cd and 0.05 or 0.5 μM 65Cu as a single metal or their mixtures. The gill transcript levels of genes involved in the transport of Cu (CTR1 and ATP7A), Na (NHE-2), Ca (ECaC), divalent metals (DMT1), and Zn (ZIP8) were also compared between treatments at 24 and 48 h. Cd uptake was significantly suppressed in the presence of Cu, indicating interaction between Cu and Cd at uptake sites, but Cu uptake was unaffected by Cd. The decrease in Cd accumulation rates in the presence of Cu was associated with an increase in transcript abundance of ECaC at 24 h and DMT1 at 48 h and a decrease in Zip8 transcript levels, all known as routes for Cd uptake. Fish exposed to 0.5 μM 65Cu show an increase in gill ATP7a transcript abundance, suggesting that Cu is removed from the gill and is transferred to other organs for detoxification. A reduction in gill CTR1 transcript abundance was observed during the Cu-Cd exposure; this may be a regulatory mechanism to reduce Cu loading if Cu is entering the gills by other uptake routes, such as ECaC and DMT1.

Chronic nickel bioaccumulation and sub-cellular fractionation in two freshwater teleosts, the round goby and the rainbow trout, exposed simultaneously to waterborne and dietborne nickel

Rainbow trout and round goby were exposed for 30 days to waterborne and dietary Ni in combination at two waterborne concentration ranges (6.2-12 μmol/L, 68-86 μmol/L), the lower of which is typical of contaminated environments. The prey (black worms; Lumbriculus variegatus) were exposed for 48 h in the effluent of the fish exposure tanks before being fed to the fish (ration=2% body weight/day). Ni in gills, gut, and prey was fractionated into biologically inactive metal [BIM=metal-rich granules (MRG) and metallothionein-like proteins (MT)] and biologically active metal [BAM=organelles (ORG) and heat-denaturable proteins (HDP)]. Gobies were more sensitive than trout to chronic Ni exposure. Possibly, this greater sensitivity may have been due to the goby's pre-exposure to pollutants at their collection site, as evidenced by ∼2-fold greater initial Ni concentrations in both gills and gut relative to trout. However, this was followed by ∼2-16× larger bioaccumulation in both the gills and the gut during the experimental exposure. On a subcellular level, ∼3-40× more Ni was associated with the BAM fraction of goby in comparison to trout. Comparison of the fractional distribution of Ni in the prey versus the gut tissue of the predators suggested that round goby were more efficient than rainbow trout in detoxifying Ni taken up from the diet. Assessing sub-cellular distribution of Ni in the gills and gut of two fish of different habitat and lifestyles revealed two different strategies of Ni bioaccumulation and sub-cellular distribution. On the one hand, trout exhibited an ability to regulate gill Ni bioaccumulation and maintain the majority of the Ni in the MT fraction of the BIM. In contrast goby exhibited large Ni spillovers to both the HDP and ORG fractions of the BAM in the gill. However, the same trend was not observed in the gut, where the potential acclimation of goby to pollutants from their collection site may have aided their ability to regulate Ni spillover to the BAM more so than in trout. Overall, chronic mortality observed in goby may be associated more with Ni bioaccumulation in gills than in gut; the former at either 4-d or 30-d was predictive of chronic Ni toxicity. BIM and BAM fractions of the goby gills were equally predictive of chronic (30-d) mortality. However, critical body residue (CBR50) values of the BIM fraction were ∼2-4× greater than CBR50 values of the BAM fraction, suggesting that goby are more sensitive to Ni bioaccumulation in the BAM fraction. There was insufficient mortality in trout to assess whether Ni bioaccumulation was predictive of chronic mortality.

Zn-stimulated mucus secretion in the rainbow trout (Oncorhynchus mykiss) intestine inhibits Cd accumulation and Cd-induced lipid peroxidation

Interest in the interactions between dietary constituents in the gut is increasing, but information remains sparse. In this study rainbow trout were fed non-enriched (186.7±19.0 μg Zn g(-1) (dw)), enriched (20% increase) and hyper-enriched Zn (200% increase) diet for 21 d followed by a single meal of Cd-spiked food (188.6±9.9 μg Cd g(-1) (dw)). Intestinal, hepatic and renal Zn burdens were measured on Days 7, 14 and 21 and Cd concentrations in the same tissue were measured 48 h-post Cd exposure. Oxidative stress was measured as lipid peroxidation in dissected tissues and intestinal mucus was quantified as sialic acid using the thiobarbituric acid assay. Rainbow trout maintained on the hyper-enriched Zn diet experienced significantly increased intestinal mucus secretion (p<0.01), were the only treatment group not to accumulate Cd in the intestine, and there was also no increase in intestinal oxidative damage. Conversely, fish fed the non-enriched and enriched Zn diets did not produce greater than basal levels of intestinal mucus and accumulated significantly greater concentrations of Cd in the intestine (p<0.01) leading to significant localised Cd-induced lipid peroxidation (p<0.01). High levels of mucus production correlated to lower incidences of lipid peroxidation (r(2)=0.54, p<0.05). These results demonstrate that mucus production stimulated by a high Zn diet have an inhibitory effect on Cd accumulation in the intestine and on Cd-induced lipid peroxidation. Mechanistically, it is likely that the elevated mucus production provides a barrier to Cd uptake. This study describes how one dietary constituent directly modifies the gut environment which indirectly influences the fate of another ingested cation.

Can the biotic ligand model predict Cu toxicity across a range of pHs in softwater-acclimated rainbow trout?

This study examined the effects of pH (5.0-8.5) on the toxicity of waterborne Cu to juvenile rainbow trout (Oncorhynchus mykiss) in soft water under flow-through conditions. Relationships between 96 h or 30 day Cu toxicity and 24 h lethal Cu accumulation on the gills (24 h LA(50-Acute) or 24 h LA(50-Chronic)) were examined in the context of predictions made using the biotic ligand model (BLM). Acute toxicity was relatively constant across pHs except for a 2- to 3-fold higher LC(50) at pH 5.0. In the chronic exposure, the fish had similar tolerance to Cu from pH 5.0 to 8.0, but were 3- to 4-fold more tolerant at pH 8.5. This pattern was not captured by the current BLM which predicts that acute and chronic LC(50) values should increase progressively from pH 5.0 to 8.5, with much greater values than those observed at the higher pH range. BLM-based water quality criteria would not be protective for trout at pH 8.0 or 8.5 in acute exposure to Cu in soft water. The measured 24 h LA(50-Acute) and LA(50-chronic) at pH 8.5 were higher, and 24 h LA(50-Chronic) at pH 5.0 was lower than those at the other pHs. This study indicates that gill Cu bioaccumulation does not explain toxicity at high or low pH, and the BLM needs revision to adequately predict Cu toxicity to trout in soft water.

Cadmium accumulation and in vitro analysis of calcium and cadmium transport functions in the gastro-intestinal tract of trout following chronic dietary cadmium and calcium feeding

Juvenile rainbow trout (Oncorhynchus mykiss) were fed diets made from Lumbriculus variegatus containing environmentally relevant concentrations of Cd (approximately 0.2 and 12 microg g(-1) dry wt) and/or Ca (1, 10, 20 and 60 mg g(-1) dry wt) for 4 weeks. Ten fish per treatment were removed weekly for tissue metal burden analysis. In all portions of the gastro-intestinal tract (GIT) (stomach, anterior, mid, and posterior intestine), chronic exposure to elevated dietary Ca decreased Cd tissue accumulation to varying degrees. At week five, the GITs of the remaining fish were subjected to an in vitro gut sac technique. Pre-exposure to the different treatments affected unidirectional uptake and binding rates of Cd and Ca in different manners, dependent on the specific GIT section. Ca and Cd uptake rates were highly correlated within all sections of the GIT, and the loosely binding rate of Cd to the GIT surfaces predicted the rate of new Cd absorption. Overall, this study indicates that elevated dietary Ca is protective against Cd uptake from an environmentally relevant diet, and that Ca and Cd uptake may occur through both common and separate pathways in the GIT.

Do you smell what I smell? Olfactory impairment in wild yellow perch from metal-contaminated waters

In this study, we sampled yellow perch from three lakes along a metal-contamination gradient and examined their olfactory ability in response to conspecific chemical alarm cues and metal-binding characteristics of their olfactory epithelium (OE). We measured the electrophysiological response at the OE, tested their antipredator behaviour and measured neuronal density at the olfactory rosette and bulb. Yellow perch from contaminated lakes exhibited significantly larger electrophysiological responses to alarm cues than clean lake fish, but showed no antipredator behaviour contrary to clean lake fish. Neuron density did not differ at either the olfactory rosette or bulb between clean and contaminated fish. Unlike fishes raised under laboratory or aquaculture settings, fish from contaminated lakes possessed a functional OE after metal exposure, but similar to laboratory/aquaculture fishes, yellow perch did not exhibit olfactory-mediated behaviours. Thus, wild fish from contaminated lakes can detect chemical stimuli but olfactory signal processing is disrupted which could alter ecological functioning.

Does dietary Ca protect against toxicity of a low dietborne Cd exposure to the rainbow trout?

We examined the toxicity of Cd, provided in a natural diet and at an environmentally relevant concentration ( approximately 12microgg(-1) dry wt.), to the juvenile rainbow trout (Oncorhynchus mykiss). In addition, we tested the protection by elevated dietary Ca against both the accumulation and toxicity of dietary Cd from this natural diet (background Ca approximately 1mgg(-1) dry wt.). Food pellets were made from blackworms (Lumbriculus variegatus), and spiked with Cd and either no additional Ca or elevated ( approximately 60mgg(-1) dry wt.) concentrations for each of the treatment diets. Survival was unaffected for trout fed diet with 12microgg(-1) dry wt. Cd for a month, but growth was potentially reduced. Tissue burden analysis revealed that the stomach, liver and kidney accumulated the most Cd, with concentrations progressively increasing in the liver and kidney over the whole exposure period. Cd concentrations in the plasma and red blood cells were unaffected by the different treatments, but subcellular fractionation analysis indicated that a higher concentration of Cd was associated with the metal-sensitive fractions of red blood cells of the fish that were exposed to the dietborne Cd. Dietary Cd exposure also caused potential toxicity to cells of the stomach in that they bound more Cd to heat-denaturable proteins. However, detoxification appeared to take place in the Cd-exposed fish because more Cd was bound to metallothionein-like proteins by week 4 of exposure. Elevated Ca in the Cd diet generally protected against accumulation and toxicity of dietborne Cd. The protection against Cd accumulation was almost complete at the gills, robust in the stomach and whole body (> or =50% reductions), but not significant in the liver, kidney, carcass, plasma, or red blood cells. Elevated dietary Ca also reduced Cd accumulation in the organelles of the fish stomach and red blood cells. In addition, dietborne Ca not only reduced the uptake of Cd by the cells, but also altered how the cells handled Cd intracellularly. In general, our results have demonstrated the need to use diets with natural compositions for dietary toxicity studies.

In vitro characterization of cadmium and zinc uptake via the gastro-intestinal tract of the rainbow trout (Oncorhynchus mykiss): Interactive effects and the influence of calcium

An in vitro gut sac technique was employed to study whether Cd and Zn uptake mechanisms in the gastro-intestinal tract of the rainbow trout are similar to those at the gills, where both metals are taken up via the Ca transport pathway. Metal accumulation in surface mucus, in the mucosal epithelium, and transport into the blood space were assayed using radiolabelled Cd or Zn concentrations of 50micromolL(-1) in the luminal (internal) saline. Elevated luminal Ca (10 or 100mmolL(-1)versus 1mmolL(-1)) reduced Cd uptake into all three phases by approximately 60% in the stomach, but had no effect in the anterior, mid, or posterior intestine. This finding is in accordance with recent in vivo evidence that Ca is taken up mainly via the stomach, and that high [Ca] diets inhibit Cd accumulation from the food specifically in this section of the tract. In contrast, 10mmolL(-1) luminal Ca had no effect on Zn transport in any section, whereas 100mmolL(-1) Ca stimulated Zn uptake, by approximately threefold, into all three phases in the stomach only. There was no influence of elevated luminal Zn (10mmolL(-1)) on Cd uptake in the stomach or anterior intestine, or of high Cd (10mmolL(-1)) on Zn uptake in these sections. However, high [Zn] stimulated Cd transport into the blood space but inhibited accumulation in the mucosal epithelium and/or mucus-binding in the mid and posterior intestine, whereas high [Cd] exerted a reciprocal effect in the mid-intestine only. We conclude that Cd uptake occurs via an important Ca-sensitive mechanism in the stomach which is different from that at the gills, while Cd transport mechanisms in the intestine are not directly Ca-sensitive. Zn uptake does not appear to involve Ca uptake pathways, in contrast to the gills. These results are discussed in the context of other possible Cd and Zn transport pathways, and the emerging role of the stomach as an organ of divalent metal uptake.

Trophic transfer and dietary toxicity of Cd from the oligochaete to the rainbow trout

Dietary toxicity of metals on fish is often studied using commercial pellet food, and there is a lack of investigation on the toxicity of metals that are biologically incorporated into the natural food from the aquatic environment. In this study, we investigated the toxicity of dietborne Cd from the oligochaete Lumbriculus variegatus to the rainbow trout Oncorhynchus mykiss. The oligochaete worms were exposed to waterborne Cd (0.1, 5, 20, and 200 microgL(-1)) for 1 week and the fish were fed this food exclusively (daily ration=3.5% body wet weight) for 1 month. Cd concentrations in the worms averaged 0.1, 0.6, 2.2, and 30.3 microgg(-1) wet weight respectively, whereas the whole fish accumulated 0.002, 0.005, 0.019, and 0.387 microg Cd g(-1) wet weight respectively, after feeding upon control or Cd-contaminated worms for 4 weeks. Highest concentrations of Cd were retained in the gut, followed by the kidney and liver of the fish, with the latter two increasing over time; however, gut tissue accounted for >80% of whole body Cd burdens at all times. The trophic transfer efficiency of Cd was low (0.9-6.4%) although higher than in previous studies using Cd-spiked commercial diets, and was only weakly correlated to the internal Cd storage in the worms. The level of Cd in the contaminated worms did not affect Cd trophic transfer efficiency, but was reduced over the dietary exposure period. Dietborne Cd did not interfere with whole body Ca uptake from the water or alter plasma [Ca], but reduced growth by 50% in the trout exposed to the highest Cd dose. Cd stored in the metallothionein-like proteins of the fish gut tissue increased while that in the heat-denaturable proteins was reduced, suggesting detoxification over time. This study suggests a higher bioavailability and toxicity of Cd from the natural diets than from the commercial diets used in previous studies.

Pre-exposure to waterborne nickel downregulates gastrointestinal nickel uptake in rainbow trout: indirect evidence for nickel essentiality

Nickel (Ni) may be both a toxicant and a micronutrient, but its essentiality to aquatic animals is not established. Interactions between branchial and gastrointestinal routes of metal uptake are important for understanding metal regulation and essentiality in aquatic animals. Adult rainbowtrout (Oncorhynchus mykiss) were pre-exposed to a sublethal concentration of waterborne Ni (7.43 micromol L(-1)) or a control water (0.12 micromol L(-1)) for 45 days, and subsequently, a gastrointestinal dose of radiolabeled Ni (1.08 micromol kg(-1) wet wt) was infused into the stomach of both non-pre-exposed and Ni pre-exposed trout to test whether pre-exposure to waterborne Ni would affect gastrointestinal uptake. The fish pre-exposed to waterborne Ni exhibited a markedly greater level of total Ni in the blood plasma (approximately 10-fold) but not in red blood cells (RBC). Pre-exposure downregulated the gastrointestinal uptake of radiolabeled Ni (new Ni) in the plasma and RBCs, providing evidence for the first time of homeostatic interaction between the two routes of Ni uptake. The plasma and RBC concentrations of new Ni in the non-pre-exposed and Ni pre-exposed groups were linear in the first 2 h and then approached a plateau. Only a small fraction of the infused dose (1.6-3.7%) was found in the internal organs of both groups at 24 h. Waterborne Ni, but not the infused Ni, greatly increased total Ni levels in the gills (6.1 fold), kidney (5.6 fold), scales (4.2 fold), and gut tissues (1.5-4.2 fold). It appears that gut, kidney and scales play important roles for Ni homeostasis by providing uptake, clearance and storage sites. Overall, our results suggest that Ni is subject to homeostatic regulation in the rainbow trout, a property that is characteristic of essential metals.