Subcellular interactions of dietary cadmium, copper and zinc in rainbow trout (Oncorhynchus mykiss)

Interactions of Cu, Cd and Zn were studied at the subcellular level in juvenile rainbow trout (Oncorhynchus mykiss) fed diets containing (μg/g) 500 Cu, 1000 Zn and 500 Cd singly and as a ternary mixture for 28 days. Livers were harvested and submitted to differential centrifugation to isolate components of metabolically active metal pool (MAP: heat-denaturable proteins (HDP), organelles, nuclei) and metabolically detoxified metal pool (MDP: heat stable proteins (HSP), NaOH-resistant granules). Results indicated that Cd accumulation was enhanced in all the subcellular compartments, albeit at different time points, in fish exposed to the metals mixture relative to those exposed to Cd alone, whereas Cu alone exposure increased Cd partitioning. Exposure to the metals mixture reduced (HDP) and enhanced (HSP, nuclei and granules) Cu accumulation while exposure to Zn alone enhanced Cu concentration in all the fractions analyzed without altering proportional distribution in MAP and MDP. Although subcellular Zn accumulation was less pronounced than that of either Cu or Cd, concentrations of Zn were enhanced in HDP, nuclei and granules from fish exposed to the metals mixture relative to those exposed to Zn alone. Cadmium alone exposure mobilized Zn and Cu from the nuclei and increased Zn accumulation in organelles and Cu in granules, while Cu alone exposure stimulated Zn accumulation in HSP, HDP and organelles. Interestingly, Cd alone exposure increased the partitioning of the three metals in MDP indicative of enhanced detoxification. Generally the accumulated metals were predominantly metabolically active: Cd, 67-83%; Cu, 68-79% and Zn, 60-76%. Taken together these results show both competitive and cooperative interactions dependent on the subcellular fraction, metal, exposure duration and relative metal exposure concentrations. Competitive interactions likely result from ionic mimicry with the metals displacing each other from common binding sites, whereas cooperative interactions suggest increased abundance of metal binding sites and/or existence of metal-specific non-interacting binding sites in some of the fractions. Moreover, the changes in subcellular distribution of the biometals Cu and Zn due to Cd exposure together with the shifts of the metals between MAP and MDP observed may have toxicological consequences.     

Effects of deltamethrin on rainbow trout (Oncorhynchus mykiss)

The aim of this study was to assess the effect of deltamethrin on rainbow trout (Oncorhynchus mykiss). Control and experimental group of fish were exposed to Decis EW 50 pesticide preparation (active substance 50 g/l of deltamethrin). The acute semistatical toxicity test lasting 96 h was performed on rainbow trout juveniles. The 96hLC₅₀ value of Decis EW 50 was 0.02 mg/l. Examination of haematological and biochemical profile and histological tissue examination was performed on 1–2-year-old rainbow trout after 96 h of exposure to Decis EW 50 in a concentration of 0.02 mg/l. The experimental group showed significantly lower values (p < 0.05) of plasma glucose, alanine aminotransferase, cholinesterase and significantly higher (p < 0.05) values of erythrocyte count, haemoglobin content, haematocrit and plasma total protein, albumins, ammonia, aspartate aminotransferase, creatinekinase and calcium compared to the control group. The deltamethrin-based Decis EW 50 pesticide preparation was classified among substances strongly toxic for fish.     

Xanthate effects on cadmium intracellular distribution in rainbow trout (Oncorhynchus mykiss) gills

The uptake of ¹⁰⁹cadmium through perfused rainbow trout gills in the presence of xanthates was studied, and the subcellular distribution of cadmium in perfused gill tissue was determined. Pnenol absorption was also studied because xanthates form hydrophobic Cd complexes with a log P_{octanol/water} similar to that of phenol.

1. Xanthate concentrations higher than 10⁻⁵ M increased the rate of cadmium transfer through the gills and cadmium retention in gill tissue. Cadmium was present as a hydrophobic complex at this and higher xanthate concentrations.

2. A redistribution of cadmium from metallothionein to high molecular weight cadmium binding fractions occurred in the presence of 10⁻⁴ M xanthate.

3. The rate of phenol transfer across the gill epithelium was much higher than the rate of cadmium transfer regardless of whether xanthate was present. The rate of phenol transfer stabilized much faster than the rate of cadmium transfer irrespective of whether xanthate was present, indicating that different uptake mechanisms were involved.

We conclude that in the presence of xanthate concentrations higher than 10⁻⁵ M cadmium is taken up as a hydrophobic Cd(xanthate)₂ complex by the epithelial cells. Within the cell the complex dissociates, and the metal ion is bound to intracellular cadmium-binding ligands. The metal is probably translocated through the basolateral membrane as a free ion.     

In vitro characterization of cadmium transport along the gastro-intestinal tract of freshwater rainbow trout (Oncorhynchus mykiss)

An in vitro gut sac technique was used to examine the mechanism(s) of cadmium (Cd) uptake along the gastro-intestinal tract (GIT) of rainbow trout (Oncorhynchus mykiss). The spatial distribution of Cd between three compartments (mucus-binding, mucosal epithelium, and transport into blood space) was determined using a modified Cortland saline containing 50μM Cd (as CdCl(2)) labeled with (109)Cd radiotracer. Taking into account total surface areas, the order of relative importance for total Cd uptake rate was: posterior intestine>anterior intestine>stomach>mid intestine. Cd transport was not inhibited by experimentally reducing fluid transport rates by manipulation of osmotic gradients using mannitol, but was sensitive to internal luminal pressure changes, suggesting a mechanosensitive pathway. Q(10) values (1, 11, and 19°C) indicated a facilitated transport of Cd in the anterior- and mid-intestine. The effects of 10mM Ca on the kinetics of Cd uptake suggest the presence of a common uptake pathway for Cd and Ca in the stomach, anterior-, and mid-intestine. Further evidence of a shared route of entry was found using three Ca channel blockers, lanthanum, verapamil, and nifedipine: both voltage-insensitive and voltage-sensitive Ca channels appear to be present in either some, or all portions of the GIT. Elevated Fe (500μM), Mg (50mM), and Zn (500μM) showed varying degrees of inhibition of Cd transport depending on the compartment and segment of the GIT. Overall it appears that there are multiple sites, and mechanisms, of Cd uptake along the GIT of rainbow trout.     

Estrogenic effect of dietary 4-tert-octylphenol in rainbow trout (Oncorhynchus mykiss)

The estrogenic effect of dietary 4-tert-octylphenol (octylphenol) in rainbow trout Oncorhynchus mykiss was investigated. Octylphenol was administered orally to sexually immature rainbow trout every second day for 11 days in doses between 0.4 and 50 mgkg(-1)2 d(-1). Plasma vitellogenin was measured at day 0, 6 and 11 and at the end of the experiments, the amounts of octylphenol retained in liver and muscle were determined. Increases in average plasma vitellogenin levels were seen at exposure to 40 mg octylphenol kg(-1) every second day; the most sensitive fish responded to 30 mgkg(-1). Doses below 20 mg octylphenol kg(-1)2 d(-1) had no effect. The ED(50) value for induction of vitellogenin synthesis was 35 mg octylphenol kg(-1)2 d(-1). Only 1 to 2 per thousand of the total amount of octylphenol administered orally over the 11 days experimental period was retained in muscle and liver at the end of the experiment. A clear dose-related increase was observed for concentrations of octylphenol in both liver and muscle of fish exposed to doses between 0.4 and 50 mgkg(-1)2 d(-1). A significant correlation was found between the concentrations of octylphenol in the liver and vitellogenin level in plasma.     

Extrahepatic disposition of 3H-aflatoxin B1 in the rainbow trout (Oncorhynchus mykiss).

Whole-body autoradiography in rainbow trout (Oncorhynchus mykiss) after oral and intravenous administration of 3H-labelled aflatoxin B1 showed labelling of several extrahepatic tissues, such as the uveal melanin and the vitreous humour of the eyes, the trunk and head kidney, the olfactory rosettes and the pyloric caecae. Liquid chromatography of extracts of the vitreous humour showed that unmetabolized 3H-AFB1 was the main labelled material present at this site. Liquid chromatography of extracts of the uveal melanin showed presence of aflatoxicol and aflatoxin B1 in proportions of about 3:1. The binding to the pigment is probably due to a hydrophobic type of interaction with the melanin. Microautoradiography showed that melanin-containing cells in the trunk and head kidney and in the olfactory rosettes also accumulated high amounts of radioactivity. In the trunk kidney there was, in addition, a labelling of the second segment of the proximal tubules and of the distal tubules and the collecting ducts. Studies in vitro with microsomal and 12,000 x g supernatant preparations of the trunk kidney showed formation of DNA- and protein-bound metabolites from the aflatoxin B1. It is probable that the bioactivation of the aflatoxin B1 is confined to the cytoplasm of the cells, may be related to excretion and/or absorption processes. Microautoradiography of the olfactory rosettes, showed labelling of the sensory epithelium, but not the indifferent epithelium. A low formation of protein-bound aflatoxin B1-metabolites was found in incubations with microsomal preparations of this tissue. The same observation was made in incubations with microsomal preparations of the head kidney. In the pyloric caeca bound metabolites were observed in vivo at a level comparable to that found in the trunk kidney. Our results suggest that retention and metabolism in some extrahepatic tissues might be of importance as concerns the toxicologic potential of aflatoxin B1 in the rainbow trout.     

Features of cadmium and calcium uptake and toxicity in rainbow trout (Oncorhynchus mykiss) mitochondria

Molecular features of cadmium (Cd) and calcium (Ca) uptake and toxicity in rainbow trout liver mitochondria were studied using modulators of mitochondrial permeability transition pore (MPTP), mitochondrial calcium uniporter (MCU) and rapid uptake mode (RaM). Malate-glutamate energized mitochondria were exposed to 20μM Cd and 50μM Ca, singly and in combination, with and without addition of ruthenium red (RR), cyclosporin A (CsA), bongkrekic acid (BKA) or dithiothreitol (DTT). State 3 mitochondrial respiration was inhibited by 50% by either Cd or Ca, and by 70% when the two cations were added simultaneously. All the modulators tested reduced the inhibition of state 3 respiration with DTT completely reversing the Cd effect. While state 4 respiration was unaffected by Ca and/or Cd, 1.5-3 fold stimulation was observed on addition of the modulators. Uncoupler-stimulated respiration was inhibited by Cd, Ca and Cd+Ca with complete (DTT) and partial (RR, CsA, BKA) protection of the Cd and Cd+Ca effects. All the modulators completely reversed the Ca-induced inhibition. Swelling, the hallmark of MPTP, measured following incubation of mitochondria with 0-100μM of the two cations, singly and in combination, was abolished by all the modulators. Overall these data show the existence of membrane channels in rainbow trout liver mitochondria with some characteristics similar to mammalian MPTP, MCU and RaM. Moreover, entry of Ca and Cd into mitochondria is important in the toxicity of these cations.     

Role of flavin-containing monooxygenase in the in vitro biotransformation of aldicarb in rainbow trout (Oncorhynchus mykiss).

1. The in vitro biotransformation of 14C-aldicarb was examined in liver, kidney, and gill microsomes from the rainbow trout (Oncorhynchus mykiss). 2. In all tissues the major metabolite was aldicarb sulphoxide. Addition of the cytochrome P-450 inhibitor, N-benzylimidazole, failed to alter significantly aldicarb sulphoxide levels, while co-incubation with the flavin-containing monooxygenase substrates, N,N-dimethylaniline or methimazole, caused significant decreases in sulphoxide formation in liver and gill microsomes. 3. Aldicarb sulphoxide formation was optimal at pH 8.0, and had Michaelis-Menten kinetics with an apparent Km of 46.7 microM and a Vmax of 0.216 nmol/min per mg. 4. Aldicarb sulphoxide formation was competitively inhibited by co-incubation with N,N-dimethylaniline in liver microsomes. These data indicate that flavin-containing monooxygenase plays an important role in the in vitro biotransformation of aldicarb in rainbow trout.     

Biotransformation of the xenoestrogen 4-tert-octylphenol in hepatocytes of rainbow trout (Oncorhynchus mykiss)

1. The biotransformation of a 4-tert-alkylphenol in rainbow trout (Oncorhynchus mykiss) liver was studied to determine the possible fate and activity of these xenoestrogens in fish. 2. Primary trout hepatocytes were incubated with 30 μM 4-(1′, 1′,3′, 3′-tetramethylbutyl)[U-¹⁴C]phenol (4-tert-octylphenol; 4-t-OP) for up to 3 h. Radiolabelled metabolites were detected by radio-HPLC and the structures were determined by GC-MS analysis of the conjugated or aglycone products. 3. During the first 15 min, 4-t-OP was metabolized at 1.06 pmol·min^?1·10^?6 cells. The amount of parent compound metabolized was maximum after a 1-h incubation, when 86% of 4-t-OP was transformed to five other products. 4. The major metabolite comprised 61% of the total recovered radioactivity and was identified as 4-t-OP-β-glucuronide. 5. The remaining metabolites were formed from the hydroxylation of 4-t-OP on either the C2 (w-3) or C4 (omega) positions of the alkyl chain, or ortho on the aromatic ring to form a catechol. These oxidized products were also metabolized to glucuronide derivatives conjugated on the phenol ring. 6. The results suggest that oestrogenic alkylphenols could be rapidly transformed in fish liver by both phase I and II metabolic pathways to a number of conjugated products which are unlikely to be active at the oestrogen receptor.     

Biotransformation of the xenoestrogen 4-tert-octylphenol in hepatocytes of rainbow trout (Oncorhynchus mykiss)

1. The biotransformation of a 4-tert-alkylphenol in rainbow trout (Oncorhynchus mykiss) liver was studied to determine the possible fate and activity of these xenoestrogens in fish. 2. Primary trout hepatocytes were incubated with 30 microM 4-(1',1',3',3'-tetramethyl-butyl)[U-14C]phenol (4-tert-octylphenol; 4-t-OP) for up to 3 h. Radiolabelled metabolites were detected by radio-HPLC and the structures were determined by GC-MS analysis of the conjugated or aglycone products. 3. During the first 15 min, 4-t-OP was metabolized at 1.06 pmol x min(-1) x 10(-6) cells. The amount of parent compound metabolized was maximum after a 1-h incubation, when 86% of 4-t-OP was transformed to five other products. 4. The major metabolite comprised 61% of the total recovered radioactivity and was identified as 4-t-OP-beta-glucuronide. 5. The remaining metabolites were formed from the hydroxylation of 4-t-OP on either the C2 (omega-3) or C4 (omega) positions of the alkyl chain, or ortho on the aromatic ring to form a catechol. These oxidized products were also metabolized to glucuronide derivatives conjugated on the phenol ring. 6. The results suggest that oestrogenic alkylphenols could be rapidly transformed in fish liver by both phase I and II metabolic pathways to a number of conjugated products which are unlikely to be active at the oestrogen receptor.     

Effects of nonylphenol on estrogen receptor conformation, transcriptional activity and sexual reversion in rainbow trout (Oncorhynchus mykiss).

Estrogenic potency of 4-n-nonylphenol diethoxylate, 4-n-nonylphenol (NP) and metabolites were tested using two bioassays: rainbow trout hepatocyte culture and recombinant yeast stably expressing rainbow trout estrogen receptor (rtER) and containing estrogen-dependent reporter genes. Since NP was the only compound active in both systems, its interaction with rtER was studied in more detail. Qualitative and quantitative differences were observed in the presence of 17beta-estradiol (E2) or NP when estrogen-dependent promoters containing one to three estrogen-responsive elements were used in yeast. Moreover, limited proteolysis of rtER after E2 or NP binding presented different patterns after SDS-PAGE analysis suggesting that NP induces a differential conformation of rtER compare to E2. This finding may have important implications with respect to the biological activity of NP. Thus, the effects of NP on the activation of an E2-dependent gene and on sexual differentiation were assessed on all-male trout embryos exposed to NP for 1 h per day for 10 days. Although in situ hybridization demonstrated that E2, and to a lesser extend NP, were able to increase rtER mRNA level in the liver of embryos, no indication of total or partial sexual reversion was observed (even in E2 treated fishes) when the gonads were examined 8 months after hatching.     

Effects of Cu on plasma cortisol and cortisol secretion by adrenocortical cells of rainbow trout (Oncorhynchus mykiss)

Fish are exposed to multiple stressors, often acting concurrently, in their environment. To evaluate the potential of Cu to act as a chemical stressor, rainbow trout (Oncorhynchus mykiss) were exposed to Cu (30 or 80 microg/l) for 30 days in the laboratory and they were subjected to a physical stressor (1 min air exposure) before sampling. Physiological stress indicators in the whole fish as well as cortisol secretion by adrenocortical cells in vitro were measured. Fish exposed to Cu had a lower condition factor, hepatosomatic index, plasma glucose, hepatic glycogen and gill Na(+)/K(+)-ATPase activity compared to controls. Exposure to Cu did not have an effect on basal plasma cortisol (fish sampled without air exposure stress) however, the air exposure-induced increase in plasma cortisol was lower in fish exposed to Cu. Cortisol secretion stimulated by ACTH in vitro was greater in adrenocortical cells isolated from fish exposed to Cu in vivo but in vitro exposure to Cu consistently impaired cortisol secretion. Our results indicate that Cu at high concentrations disrupts cortisol secretion through a direct toxic effect on adrenocortical cells while low concentrations resulting from a 30-day exposure to environmentally relevant Cu concentrations enhances cortisol secretion in response to ACTH in vitro.     

Uptake and effects of manufactured silver nanoparticles in rainbow trout (Oncorhynchus mykiss) gill cells

Nanoparticles are already widely used in technology, medicine and consumer products, but there are limited data on their effects on the aquatic environment. In this study the uptake and effect of citrate (AgNP_CIT) and polyvinylpyrrolidone (AgNP_PVP) coated manufactured silver nanoparticles, as well as AgNO₃ (Ag⁺) were tested using primary gill cells of rainbow trout (Oncorhynchus mykiss). Prior to use, the nanoparticles were characterized for size, surface charge and aggregation behavior. Gill cells were cultured either as monolayers on solid support, or as multilayers on a permeable support cell culturing system, enabling transport studies. The uptake of silver nanoparticles and Ag⁺ after exposure to 10 mg L⁻¹ was determined with microscopical methods and inductively coupled plasma mass spectrometry (ICP-MS). Cytotoxicity, in terms of membrane integrity, as well as oxidative stress (depletion of reduced glutathione) was tested at silver concentrations ranging from 0.1 mg L⁻¹ to 10 mg L⁻¹. Results show that AgNP_CIT nanoparticles are readily taken up into gill cell monolayers while uptake was less for AgNP_PVP. In contrast, it appears that the slightly smaller AgNP_PVP were transported through cultured multilayers to a higher extent, with transport rates generally being in the ng cm⁻² range for 48 h exposures. Transport rates for all exposures were dependent on the epithelial tightness. Moderate cytotoxic effects were seen for all silver treatments. Levels of reduced glutathione were elevated in contrast to control groups, pointing on a possible overcompensation reaction. Taken together silver nanoparticles were taken up into cells and did cause silver transport over cultured epithelial layers with uptake and transport rates being different for the two nanoparticle species. All silver treatments had measurable effects on cell viability.     

Effects of atrazine on hepatic metabolism and endocrine homeostasis in rainbow trout (Oncorhynchus mykiss)

The herbicide atrazine (ATZ) is one of the most widely used pesticides in the world and is now under scrutiny for its alleged capacity to disrupt the endocrine system. Exhibiting negligible interaction with the estrogen receptor (ER), ATZ's mode of action remains to be elucidated. ATZ may act as an inducer of the enzyme aromatase, which converts androgens to estrogens, although other mechanisms should also be taken into consideration such as impairment of hepatic metabolism. Therefore we administered juvenile rainbow trout (Oncorhynchus mykiss) a dose of either 2 or 200 μg ATZ/kg, or of carrier control phosphate buffered saline (PBS) and we measured plasma concentrations of testosterone (T), 17beta-estradiol (E2) and vitellogenin (Vtg) 6 days after exposure. Simultaneously we analyzed hepatic gene expression of cytochrome P450 (CYP) 1A and pi-class glutathione S-transferase (GST-P), and catalase (CAT) activity. Although sex steroid levels showed no significant alterations, we found a dose-dependent increase in Vtg and a concomitant decrease in CYP1A. There was no effect of ATZ on GST-P mRNA levels but GST-P was positively correlated with CYP1A. Also, CYP1A was negatively correlated with liver CAT and E2, and varied with T concentrations in a hormetic manner. The results showed that ATZ can alter hepatic metabolism, induce estrogenic effects and oxidative stress in vivo, and that these effects are linked.     

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.     

Effects of anesthesia (tricaine methanesulfonate,MS222) on liver biotransformation in rainbow trout (Oncorhynchus mykiss)

The effect of tricaine methanesulfonate (MS222) on rainbow trout liver biotransformation rates was investigated with a microsomal model; an in vitro preparation that can be employed with or without the use of an anaesthetic. Two experimental sets of rainbow trout microsomes were tested; one representing in vivo or surgical tricaine exposures and the other representing in vitro tissue/organ collection tricaine exposures. Microsomal incubations were performed on these two experimental groups with phenol as substrate to assess the effects of tricaine on Phase I (ring-hydroxylation) and II (glucuronidation) liver biotransformation by monitoring production of hydroquinone (HQ), catechol (CAT), and phenylglucuronide (PG). The use of a 2-h 100 mg/l exposure of tricaine for surgical anesthesia with or without 24-h recovery did not significantly (P< or =0.05) affect rates of phenol (Phase I and II) biotransformation rates; nor, did the 5-min 300 mg/l tricaine exposure for isolated organ/tissue collection significantly (P< or =0.05) affect phenol (Phase I and II) biotransformation rates. There were also no significant statistical differences (P< or =0.05) in P450 protein levels, or 7-ethoxyresorufin-O-deethylase (EROD) activity in these microsomal assays between any of the tricaine treated rainbow trout and controls.     

Reciprocal inhibition of Cd and Ca uptake in isolated head kidney cells of rainbow trout (Oncorhynchus mykiss).

Some environmental pollutants, including cadmium (Cd) and zinc (Zn), can act as endocrine disruptors in fish, either in vivo or through a direct action on steroidogenic cells, as has been demonstrated in vitro. We have previously characterized Cd uptake in head kidney (homologue of mammalian adrenal) cells of rainbow trout (Oncorhynchus mykiss) and have provided evidence for a Cd/Ca interaction. Here, we pursued our investigation of metal competition for uptake. Our results show that inorganic speciation conditions favour Cd uptake with optimal level of accumulation for Cd2+ compared to chlorocomplexes (CdCl(n)(2-n)). Calcium uptake was studied for the first time in the fish head kidney cells and Ca was found to be less efficiently accumulated compared to Cd. A specific saturable mechanism of transport was characterized for Ca uptake but voltage-gated or La-sensitive cationic channels are unlikely to contribute appreciably. A concentration-dependent reciprocal inhibition was observed between Ca and Cd, whereas, Zn proved to inhibit Cd uptake exclusively. Additive inhibitory effect on Cd uptake was obtained with co-exposure to Ca and Zn. We conclude that Cd, but not Zn, may decrease Ca availability to the head kidney tissue. Also, Zn may partially protect against Cd toxicity but Zn would not protect against Cd-induced perturbation of Ca homeostasis.     

Effects of salinity on aldicarb toxicity in juvenile rainbow trout (Oncorhynchus mykiss) and striped bass (Morone saxatilis x chrysops).

Fluctuations in several environmental variables, such as salinity, can influence the interactions between organisms and pollutants in aquatic organisms, and, therefore, affect the toxicity of xenobiotics. In this study, after 2 species of fish, rainbow trout (Oncorhynchus mykiss) and hybrid striped bass (Morone saxatilis x chrysops) were acclimated to 4 salinity regimens of 1.5, 7, 14, and 21 ppt for 1 week and then exposed to 0.5 mg/l aldicarb. Mortality, brain, and muscle cholinesterase levels were measured after 96 h. Rates of (14)C-aldicarb sulfoxide formation were determined in kidney (trout only), liver, and gill microsomes from each species acclimated to the 4 salinity regimens. Salinity significantly enhanced aldicarb toxicity, cholinesterase inhibition, and (14)C-aldicarb sulfoxide formation in rainbow trout but not in striped bass. In vitro incubations with (14)C-aldicarb and the cytochrome P450 (CYP) inhibitor, N-benzylimidazole, did not significantly alter aldicarb sulfoxide formation in tissue microsomes from either species of fish, indicating CYP did not contribute to aldicarb sulfoxidation. Salinity increased flavin-containing monooxygenase (FMO) mRNA expression and catalytic activities in microsomes of liver, gill, and kidney of rainbow trout, which was consistent with the salinity-induced enhancement of aldicarb toxicity. Salinity did not alter FMO mRNA expression and catalytic activities in striped bass, which was also consistent with the lack of an effect of salinity on aldicarb toxicity in this species. These results suggest that salinity-mediated enhancement of aldicarb toxicity is species-dependent, and at least partially due to the salinity-related upregulation of FMOs, which, in turn, increases the bioactivation of aldicarb to aldicarb sulfoxide, which is a more potent inhibitor of cholinesterase than aldicarb.