Metallothionein, zinc, and mercury levels in tissues of young rats exposed to zinc and subsequently to mercury.

Several studies have described mercury toxicity and the role of metallothioneins (MT) in the detoxification and regulation of metal homeostasis. However, little data exist on this topic during the specific post-natal developmental phase in young mammals. This developmental phase is particularly important since young animals are more sensitive to toxicants than adults. The objective of this work was to investigate whether MT participates in the mechanism of protection conferred by zinc pre-treatment on the toxic effects induced by mercury in neonate rats. Pups were exposed to ZnCl(2) (5 doses of 27 mg/kg/day, s.c.) and subsequently to HgCl(2) (5 doses of 5 mg/kg/day, s.c.); metal (Zn and Hg) and MT contents were analyzed in the liver, kidney, and blood. MT was induced in the liver and kidney of pups of both Zn-sal and Zn-Hg groups, although the greatest increase was in neonates exposed to Zn only. A direct relationship exists between MT and metals for both hepatic and renal tissues, which indicates that the increase in metal levels occurs in parallel to the increase in MT content. Although the heat-treated cytosolic fraction is rich in MT and metals, higher Zn and Hg contents were detected in the insoluble fraction of all tissues. These results suggest that MT is, at least in part, responsible for preventing Hg accumulation in the liver and blood and decreasing renal toxicity.

Biomarkers in Ruditapes decussatus: a potential bioindicator species.

The clam Ruditapes decussatus is distributed worldwide and due to its ecological and economical interest has been proposed as a bioindicator in areas where mussels are not available. The accumulation of several anthropogenic compounds in their tissues suggests that they possess mechanisms that allow them to cope with the toxic effects of these contaminants. Besides pollutant uptake, the use of biomarkers is pointed out in this paper since it is a promising approach to monitor the effect of these contaminants in the marine environment. Biomarkers complement the information of the direct chemical characterization of different types of contaminants. Therefore, the aim of this paper is to review the role of several biomarkers: (metallothioneins (MT), superoxide dismutase (SOD), catalase (CAT), glutathione peroxidases (GPx) (total and selenium-dependent), lipid peroxidation (measured as MDA, one of the final products of lipid peroxidation), glutathione S-transferase (GST) and acetylcholinesterase (AChE), measured in different tissues of the clam R. decussatus, in laboratory conditions and under various environmental stresses, in two ecosystems (Ria Formosa lagoon- Portugal) and Bizerta lagoon (Tunisia) in a perspective of a multibiomarker approach to assess environmental changes. Experiment and field studies are in good agreement since MT levels, especially in the gills, the first target tissue of these contaminants, can be used as biomarker of exposure to Cd. GPx and MDA may also be determined in this respect. AChE activity is inhibited by pesticide and, to a less extent, by metal exposure in the gills and whole soft body of clams. However, the induction of GST isoforms experimentally demonstrated is not observed in the field because only global GST activity was determined. The whole set of results opens new research perspectives for the use of this species to assess the effect of mixtures of pollutants in the aquatic environment.

Variation of metal and metallothionein concentrations in a natural population of Ruditapes decussatus.

The spatial and seasonal variation of total and subcellular distribution of Cd, Cu, and Zn was followed in different tissues (gills, digestive gland, and remaining tissues) of the clam Ruditapes decussatus collected along a metal contamination gradient in the Ria Formosa lagoon (southern Portugal) and compared with metallothionein (MT) concentrations.Total metal concentrations decreased according to the sequence digestive gland > gills > remaining tissues for Cd, digestive gland approximately gills > remaining tissues for Cu and gills > digestive gland > remaining tissues for Zn. MT concentrations in these tissues decreased according to the same sequence observed for Cd. In all the tissues, the highest subcellular concentration was in the cytosol for Cd and Cu and in the pellet for Zn. Among the three metals, Cd concentrations showed the most evident spatial variation. In all tissues, total and subcellular Cd concentrations decreased from the inner parts of the lagoon toward the ocean. However, no significant spatial or seasonal variation occurred in clam tissues for the other two metals, though marginal elevated Cu concentrations were observed in the inner parts of the lagoon. Therefore, Cu subcellular distribution in clam tissues was not significantly altered by Cu changes in the lagoon and are the baseline levels for normal metabolism of this clam population. The fact that total Zn concentrations remained unchanged both spatial and seasonal suggested that these clams regulate Zn in their tissues. In the three tissues, MT bind most significantly to Cd and Cu, while Zn, although binding to MT, is preferably bound to other ligands. MT concentrations showed the same spatial and seasonal variation of Cd and were significantly related with total and heat-treated cytosolic Cd in all tissues. For Cu a significant relationship between MT and total or cytosolic Cu was only observed in the remaining tissues. No relationship was observed between MT and total or cytosolic Zn concentrations. Metals and MT concentrations increased with the increase in the condition index for the gills and the digestive gland and decreased from the remaining tissues.Cd concentrations in the gills increased only in the heat-treated cytosolic fraction while Zn in this fraction decreased. Thus Cd concentrations in this tissue displaced Zn from the MT-fraction, leading to a modification of the soluble/insoluble Zn ratio once total Zn concentrations remained unchanged. This modification reflects a perturbation in the normal metabolism in this tissue due to the excess of Cd present. With the exception of the gills, Zn subcellular distribution in the other two tissues was similar among sites and season. The model that describes the relationship between MT, metals, and weight in the gills, digestive gland and remaining tissues also indicates that Cd was the only metal that influence MT synthesis significantly in all the tissues. The induced and/or existent MT was sufficient to bind free Cd ions present in the cells, preventing any damage to cellular metabolism in this clam population. Therefore, MT in the gills and digestive gland of R. decussatus can be used as an early warning signal for Cd exposure and are a useful biomarker to assess the toxicological status of this population in the Ria Formosa lagoon.

Effect of temperature and size on metallothionein synthesis in the gill of Mytilus galloprovincialis exposed to cadmium.

Although the degree of metal contamination is expected to be a primary determinant of metallothionein (MT) induction in marine mussels, at least at polluted sites, variability caused by temperature, and biotic factors such as size, may need to be considered when interpreting field data. To test the effects of these variables, mussels, Mytilus galloprovincialis, of different sizes (small: 3.5 +/- 0.5 cm and large: 5.2 +/- 0.7 cm) were exposed to Cd (100 microg l(-1)) at different water temperatures (5, 18 and 25 degrees C) for 34 days. Resultant Cd and MT concentrations in gills were shown to be size dependent and increased significantly with temperature. At the highest temperature tested (25 degrees C) there appears to be a disproportionate effect on Cd accumulation, which raises MT synthesis to exceptionally high levels. The effect of size on MT concentrations was also temperature-dependent: at 18 and 25 degrees C, large mussels exhibited higher MT levels than smaller individuals, whilst at 5 degrees C there were no significant differences between contaminated and control mussels, in either size-group, as a result of the reduced level of MT production at this temperature. When considering the use of MT levels in mussels as a biochemical indicator of metal contamination, the potential influence of factors such as size and temperature on MT-metal relationships needs to be considered. Samples should be of uniform size as far as possible, and collection should be limited to a fixed season (avoiding climatic extremes) to ensure that the effects of these factors on baseline levels of MT is minimised.

Variation of metallothionein and metal concentrations in the digestive gland of the clam Ruditapes decussatus: sex and seasonal effects.

Metallothionein (MT) and metal (Cd, Cu, and Zn) concentrations were determined in the digestive gland of male and female Ruditapes decussatus. Clams were collected monthly during the period of sexual development, from June to September, at two different sites of a coastal lagoon, the Ria Formosa Lagoon, on the south coast of Portugal. The MT concentrations were determined in the heat-treated cytosolic fraction of the digestive gland of both male and female clams. Total metal (Cd, Cu, and Zn) concentrations in the heat-treated cytosol of the digestive gland were also determined in both sexes. The MT and metal concentrations in the digestive gland were not sex dependent in this species. Therefore, these results suggest that random samples can be used to determine MT and metal concentrations in the digestive gland of R. decussatus. Seasonal and site-specific dependency, however, were detected for MT, Cd, and Cu concentrations. The maximum concentration levels of MT, Cd, and Cu were detected in July and August for both sexes in the same area. Zinc concentrations, however, did not show marked seasonal variations, indicating that this species is able to regulate zinc concentrations. The highest MT concentrations were significantly related to the highest metal concentrations.

Comparison of metallothionein induction in response to cadmium in the gills of the bivalve molluscs Mytilus galloprovincialis and Ruditapes decussatus.

Metal binding characteristics and metallothionein induction differ markedly among the gills of the bivalve molluscs Mytilus galloprovincialis and Ruditapes decussatus exposed to sublethal cadmium concentrations (100 micrograms/l). The rates of cadmium uptake as well as the percentage of cadmium bound to the cytosol were significantly higher in the gills of M. galloprovincialis when compared with that of R. decussatus. Similarly metallothionein concentration detected in the gills of M. galloprovincialis increase fourfold while for the clam R. decussatus metallothionein concentrations increased only twofold.