Distinguishing multiple Zn sources in oysters in a complex estuarine system using Zn isotope ratio signatures.

The Pearl River Estuary (PRE), the largest estuary in Southern China, historically has suffered from metal contamination as a result of inputs from different riverine discharges. Determining the sources of metals accumulation in local aquatic flora and fauna remains a great challenge for this estuarine system with complex water circulation. In this study, Zn isotope ratios were measured in local oysters (Crassostrea hongkongensis) collected at 8 locations in the estuary on four occasions from 2014 to 2018, to better understand and assess the contamination sources. The results showed no significant differences (p < 0.05) in δ66Zn values in oysters among the four sampling dates within individual sites. However, approximately a 0.67‰ (range from -0.66‰ to 0.01‰) difference in average δ66Zn values was consistently found in oysters collected from the east side of the estuary compared to the west side, despite their comparable Zn concentrations. A mixing model was subsequently used to estimate the relative contributions from various sources to the δ66Zn values in these oysters. The mixing model predicts that zinc derived from the dissolved fraction (approximately 80 %) was the dominant uptake pathway for oysters collected at the east shore whereas approximately 50 % of the Zn in oysters collected at the west shore was derived from the particulate fraction. The mixing model also was used to estimate the relative impacts of fresh versus saline water on the measured δ66Zn values. Contributions from these two sources also varied between the east and west shores. This study presents the first data for Zn isotope ratios in oysters from the PRE, providing new insight for using Zn isotope ratios in oysters as a powerful tracer of sources in a complex estuarine system.

Rare Earth Element Accumulation and Fractionation in a Lake Ecosystem Impacted by Past Uranium Mining.

Rare earth elements (REEs) are a natural resource of vital economic interest. While REE mining and processing are known for severe environmental issues, REEs are also by-products of other mining processes (e.g. uranium). Here, we provide an in-depth assessment of REE distribution across a lake system impacted by adjacent uranium mining over a long period (Bow Lake, Ontario, Canada). We observed a robust REE-U correlation with a consistent La/U ratio of 2.0 ± 0.2 and La concentrations up to 2200 µg g-1. Selective extraction results demonstrated that 80-94% of REEs were acid extractible, while 3-8% of REEs were extracted by an alkaline solution (i.e. bound to natural organic matter). Analysis of specific REE patterns, together with a strong REE-P correlation, suggest that (co)precipitation with P mineral would be an important mechanism sequestrating REEs into Bow Lake sediments. Moreover, we identified three sources of particles delivering REEs into the lake with unique REE patterns: mine tailings, U ores and Precambrian bedrock. Negative Sm anomalies were detected in three soil samples and associated with the Precambrian bedrock. We also detected positive Gd and La anomalies in the sediments. Lanthanum anomalies were strongly correlated with U authigenic accumulation and thus associated with microbial processes requiring La, such as methanotrophy. This research demonstrates that lake sediments adjacent to U mining could represent ecological risks given that La and other REE concentrations largely exceed the maximum permissible concentrations. Water and sediment quality criteria are therefore required as both primary REE mining and extraction of REEs as by-products from legacy metallurgical tailings are increasing.

A rapid, automated system for the separation, preconcentration and measurement of 90Sr, and U, Am and Pu isotopes.

An online separation and preconcentration method, using an automated flow injection setup and solid phase extraction followed by ICP-MS/MS, was developed for the analysis of 90Sr, and U, Am and Pu isotopes in various liquid sample matrices. The radionuclide analytes were separated from interferences and complex matrices using DGA-branched resin and Sr resin, then specific gases were used in the reaction/collision cell in the ICP-MS/MS to measure the different analytes. The system requires smaller sample volumes (10 mL), less sample preparation and shorter processing time (46 min per sample) compared to traditional radiometric and other MS techniques. Based on a 10 mL sample, the limits of detection were 1.48 pg L-1 (8257 mBq L-1) for 90Sr, 1.75 pg L-1 (0.40 mBq L-1) for 234U, 0.65 pg L-1 (77.65 mBq L-1) for 241Am, and 0.56 pg L-1 (1.25 mBq L-1) for 239Pu when all target analytes were measured in one analysis. The analytical figures of merit were evaluated for a range of sample matrices including lake water, seawater and urine and were comparable to those reported in the literature. This online system thus provides a novel, fully automated analytical tool with faster analysis time, smaller sample requirements, minimum sample preparation, low detection limits and the flexibility to handle single and multiple measurements of various radionuclides.

Automated separation, preconcentration and measurement of 90Sr in liquid samples with complex matrices by online ion exchange chromatography coupled with inductively coupled plasma mass spectrometry (ICP-MS).

An online separation and preconcentration method, employing a lab-on-valve system using solid phase extraction, followed by inductively coupled plasma tandem mass spectrometry (ICP-MS/MS), was developed for the analysis of 90Sr. The 90Sr was separated from 90Zr, an isobaric interference present at high concentrations in many samples, and other matrix components using a dual-column setup (Eichrom DGA-B and Sr resins). Any remaining 90Zr was then chemically resolved from the 90Sr in the ICP-MS/MS using O2 and H2 reaction gases. The system requires small sample volumes (10 mL), minimal sample preparation compared to traditional radiometric and other MS techniques and has a processing time of 22 min per sample. Based on a 10 mL sample size, the system limit of detection, limit of quantification and method detection limit (MDL) were 0.47 Bq L-1 (0.09 pg L-1), 1.57 Bq L-1 (0.32 pg L-1) and 1.79 Bq L-1 (0.34 pg L-1), respectively. The robustness of the system and suitability for use in various sample matrices was demonstrated using spiked lake water, spiked groundwater, spiked seawater and radioactive water samples. Recovery of the IAEA 2018 Proficiency Test Exercise water sample (n = 5) was 99% with an RSD of 11.9%. The method thus provides a powerful tool for the rapid analysis of low levels of 90Sr in various water/wastewater samples.

Using Zn Isotopic Signatures for Source Identification in a Contaminated Estuary of Southern China.

Zinc isotope ratios in water and suspended particles (SP) were measured in the Pearl River Estuary (PRE), China. Site-to-site δ66Zn values in water varied by approximately 1.3‰ (i.e., -0.66‰ to 0.65‰ relative to IRMM-3702 in August 2017). There were larger variations in δ66Zn values in water collected from the east shore (i.e., -0.66‰ to 0.37‰) of the PRE close to industrialized areas, in comparison to those from the western shore (i.e., -0.23‰ to 0.13‰), indicating that the PRE was influenced by different Zn sources. The variations in δ66Zn values in water from estuarine locations were much larger than those collected from river mouths. Similarly, larger variations in δ66Zn values were observed in suspended particles (i.e., -1.45‰ to 0.63‰) relative to the water. Zinc isotopic differences (i.e., Δ66Zn‰) between particles and water were significantly (p < 0.05) and linearly correlated with Zn concentrations in particles between 0.8 and 10 µm in size at most of the estuary stations, suggesting that Zn partitioning between dissolved and particulate phases influences the observed differences in Zn isotope ratios. A significant (p < 0.0001) linear correlation between the predicted δ66Zn values (using variations in water salinities) vs observed δ66Zn values indicates that Zn isotope ratios in water in the PRE can be useful for predicting the mixing processes in the water.

High-resolution mass spectrometry for molybdenum speciation in sulfidic waters.

High resolution electrospray ionization mass spectrometry (ESI-HRMS) was used to study the speciation of molybdenum in interaction with sulfide and chloride. While reactions between molybdate and sulfide lead to creation of four conventional thiomolybdate species (MoO3S2-, MoO2S22-, MoOS32-, MoS42-), chemical formula assignment of recorded mass spectra confirmed the presence of intermediate thiomolybdate compounds (MoO3S22-, MoO2S32-, MoOS42-, MoS52-, MoS62-) and thio-chloro-molybdate compounds (MoO3Cl-, MoO2SCl-, MoOS2Cl-, MoOS3Cl-). Two of the intermediate thiomolybdate compounds were previously suggested theoretically, and this study provides analytical support for the existence of these compounds. Fast ESI-HRMS analysis has allowed us to conduct a highly-resolved short-term kinetic study of these reactions, and we suggest that the reactivity between molybdate and sulfide is more complex than previously thought, particularly during the first 24 h of interaction. Also, the solution composition will impact reaction pathways, and different outcomes found in the literature may arise from choices of ionic strength and pH adjusting agents in previous studies . The occurrence of the thio-chloro-molybdate species detected in this study should be implemented in future Mo speciation models to better assess Mo reactivity in sulfidic waters and reducing environments.

Zn Isotope Fractionation in the Oyster Crassostrea hongkongensis and Implications for Contaminant Source Tracking.

Variations in stable isotope ratios have been used to trace sources of contaminants as well as their biogeochemical pathways in the environment. In this study, we investigated the influences of internal redistribution among tissues and ambient water conditions on Zn isotope fractionation in oysters. There was no significant difference in Zn isotope ratios during in vivo Zn transportation among various oyster tissues. Estuarine oysters were exposed to additional Zn either at different salinities or at different Zn concentrations, following which the Zn isotope ratios in the oysters were measured. Results showed no significant difference in δ66/64Zn values in the oysters exposed at different salinities. Tissue Zn accumulation increased with increasing Zn levels in water over the 30 day exposure. Within this period, there was a nearly 0.3‰ difference in averaged δ66/64Zn values in the exposed oysters compared to the initial δ66/64Zn values in the oysters prior to exposure. However, there was no evidence of significant difference in δ66/64Zn values in oysters exposed at different Zn levels, with postexposure signatures similar to the δ66/64Zn values of the Zn solution added. Our results suggested that the δ66/64Zn values measured in the oysters were approaching the δ66/64Zn values of the "source" faster with increasing Zn concentrations added in the water. This study highlighted the absence of Zn isotope fractionation during Zn internal distribution and in vivo transport in oysters. The calculation of the contributions of different Zn sources demonstrated that oysters can be a sentinel animal for Zn source tracking in marine environments.

Rare earth elements in the Pearl River Delta of China: Potential impacts of the REE industry on water, suspended particles and oysters.

Rare earth element (REE) concentrations and patterns were measured in surface water, suspended particles (SP) and oysters from the Pearl River Estuary, China. During the rainy season of 2017, higher REE concentrations were found at the stations in the estuary (ΣREE = 0.06-0.42 µg L-1) than those at the river mouths (referred to as 'outlet' stations, ΣREE = 0.001-0.14 µg L-1). However, the reverse occurred in the dry season of 2016 (ΣREE = 0.07-0.16 µg L-1 in the mid-estuary vs. 0.001-0.02 µg L-1in the outlet stations). Elevated concentrations of Pr, Nd, Dy and Ho, relative to the other REEs were found in water in both seasons at most sampling locations. However, in some estuary stations, no anomalies were detected in the SP or in the oysters while some anomalies were seen in SP from the outlet stations. Significant correlations between REE concentrations in SP and oysters as well as between both total REE concentrations and the La/Yb ratio (reflecting enhanced accumulation of light REEs (LREEs)) in oysters indicate that, in the Pearl River Estuary, the dominant REE uptake pathway in oysters is from particles. The elevated concentrations of Pr, Nd, Dy and Ho, which are reported here for the first time suggest that elevated levels of these elements may result from REE recycling and other industrialized activities in this area of southern China. Specific REEs could be used to indicate emerging contamination by the modern REE industry; furthermore, REE anomalies and patterns may be suitable tools to track REE sources.

Application of ESI-HRMS for molybdenum speciation in natural waters: An investigation of molybdate-halide reactions.

High resolution electrospray ionization mass spectrometry (ESI-HRMS) was used to study the speciation of molybdate in interaction with halides (Cl, F, Br). Desolvation during electrospray ionization induced alteration of aqueous species but method optimization successfully suppressed artefact compounds. At low Mo concentrations, chloro(oxo)molybdate and fluoro(oxo)molybdate species were found and in natural samples, MoO3Cl was detected for the first time, to the best of our knowledge. Apparent equilibrium constants for Cl substitution on molybdate were calculated for a range of pH values from 4.5 to 8.5. A minor alteration in speciation during the gas phase (conversion of doubly charged MoO42- to HMoO4-) did not allow investigation of the molybdate acid-base properties; however this could be determined by speciation modeling. This study provides further evidence that ESI-HRMS is a fast and suitable tool to Deceasedassess the speciation of inorganic compounds such as Mo.

Quantitative model of carbon and nitrogen isotope composition to highlight phosphorus cycling and sources in coastal sediments (Toulon Bay, France).

Nutrient loadings from either point or non-point sources to the environment are related to the growing global population. Subsequent negative impacts of nutrient loading to aquatic environments requires a better understanding of the biogeochemical cycling and better tools to track their sources. This study examines the carbon (C), nitrogen (N) and phosphorus (P) discharge and cycling in a Mediterranean coastal area from rivers to marine sediments and assesses the anthropogenic contributions. Carbon and N concentrations and isotope compositions in rivers particles, surface sediments, and sediment cores were investigated to build up a quantitative multiple-end-member mixing model for C and N isotopes. This model predicts the contribution of four natural and one anthropogenic sources to the sediments and highlighted the anthropogenic fraction of P based on the relationship with anthropogenic δ15N. Although P is a monoisotopic element and total P concentration has been the sole index to study P loading, this study suggests an alternative approach to differentiate anthropogenic and non-anthropogenic (diagenetic) P, revealed point and non-point sources of P, and the corresponding P loading. Also, the diagenetic P background has been calculated for the 50-cm sediment layer of the whole Bay.

In vivo fractionation of mercury isotopes in tissues of a mammalian carnivore (Neovison vison).

The use of isotope ratios to trace Hg contamination sources in environmental compartments is now generally accepted. However, for biota and especially for mammals, it is still unknown if and/or how Hg isotopes fractionate in vivo and which tissue is most representative of the source(s) of contamination. We measured fractionation of Hg in mink (Neovison vison) tissues (fur, brain, blood, liver, kidney) collected during a controlled feeding experiment where captive mink were fed differing amounts of methylmercury. There was no significant effect of dietary MeHg concentrations on Hg fractionation in most tissues. Net fractionation of Hg, i.e., fractionation corrected for diet (δ202Hgtissue-δ202Hgdiet) was observed in all tissues with the greatest net fractionation occurring in the mink liver (-1.39‰) and kidney (-0.95‰). Less net fractionation, occurred in the brain (-0.12‰), blood (0.38‰) and fur (0.30‰). In the absence of brain tissue, fur is a suitable proxy which is readily obtainable and can be non-lethally collected. In these mink, it appears that biochemical processes such as demethylation, contribute to significant fractionation of Hg in the liver and kidney, but not as much in the brain and fur, where transport of Hg via thiol-containing complexes may be more important.

Uranium dispersion from U tailings and mechanisms leading to U accumulation in sediments: Insights from biogeochemical and isotopic approaches.

Uranium contamination is a worldwide problem that grows proportionally to human demands for energy and armory. Understanding U cycling in the environment is of eminent interest, mostly concerning ecosystems directly impacted by point sources. In Bow Lake (Ontario, Canada), which is located adjacent to a former U mine, exceptionally high concentrations of U are related to U dispersion from tailings and biogeochemical processes such as biotic reduction and adsorption. This has been shown by a U-Pb isotope composition model. In this study, we use U isotope fractionation (δ238U) to highlight U cycling and the role of bacteria (Geobacteraceae and sulfate-reducing bacteria) in affecting U cycling. Bacteria affected U cycling directly via biotic U reduction and indirectly via reductive dissolution of carrier phases. All the processes are interconnected through diagenetic reactions with the supply of bioavailable organic matter being the primary driving force of the diagenesis. This study is the first to use multiple biogeochemical and isotopic approaches to track U cycling from a contamination point source to U storage in lake sediments.

Uranium Isotope Fractionation during Adsorption, (Co)precipitation, and Biotic Reduction.

Uranium contamination of surface environments is a problem associated with both U-ore extraction/processing and situations in which groundwater comes into contact with geological formations high in uranium. Apart from the environmental concerns about U contamination, its accumulation and isotope composition have been used in marine sediments as a paleoproxy of the Earth's oxygenation history. Understanding U isotope geochemistry is then essential either to develop sustainable remediation procedures as well as for use in paleotracer applications. We report on parameters controlling U immobilization and U isotope fractionation by adsorption onto Mn/Fe oxides, precipitation with phosphate, and biotic reduction. The light U isotope (235U) is preferentially adsorbed on Mn/Fe oxides in an oxic system. When adsorbed onto Mn/Fe oxides, dissolved organic carbon and carbonate are the most efficient ligands limiting U binding resulting in slight differences in U isotope composition (δ238U = 0.22 ± 0.06‰) compared to the DOC/DIC-free configuration (δ238U = 0.39 ± 0.04‰). Uranium precipitation with phosphate does not induce isotope fractionation. In contrast, during U biotic reduction, the heavy U isotope (238U) is accumulated in reduced species (δ238U up to -1‰). The different trends of U isotope fractionation in oxic and anoxic environments makes its isotope composition a useful tracer for both environmental and paleogeochemical applications.

Partitioning and kinetics of methylmercury among organs in captive mink (Neovison vison): A stable isotope tracer study.

Despite the importance of methylmercury (MeHg) as a neurotoxin, we have relatively few good data on partitioning and kinetics of MeHg among organs, particularly across the blood-brain barrier, for mammals that consume large quantities of fish. The objective of this study was to determine the partition coefficients between blood and brain, liver and kidney and fur for MeHg under steady-state conditions and to measure the half-lives for MeHg in these organs. Captive mink (Neovison vison) were fed a diet enriched with two stable isotopes of Hg, Me(199)Hg and Me(201)Hg for a period of 60 days. After a period of 10 days the diet was changed to contain only Me(201)Hg so that, between days 10 and 60, we were able to measure both uptake and elimination rates from blood, brain, liver kidney and fur. Liver and kidney response was very rapid, closely following changes in blood concentrations but there was a small lag time between peak blood concentrations and peak brain concentrations. Half-lives for MeHg were 15.4, 10.2 and 13.4 days for brain, liver and kidney, respectively. There was no measurable conversion of the MeHg to inorganic Hg (IHg) in the brain over the 60 day period, unlike in liver and kidney.

Mechanistic understanding of MeHg-Se antagonism in soil-rice systems: the key role of antagonism in soil.

Methylmercury (MeHg) accumulation in rice has great implications for human health. Here, effects of selenium (Se) on MeHg availability to rice are explored by growing rice under soil or foliar fertilization with Se. Results indicate that soil amendment with Se could reduce MeHg levels in soil and grain (maximally 73%). In contrast, foliar fertilization with Se enhanced plant Se levels (3-12 folds) without affecting grain MeHg concentrations. This evidence, along with the distinct distribution of MeHg and Se within the plant, demonstrate for the first time that Se-induced reduction in soil MeHg levels (i.e., MeHg-Se antagonism in soil) rather than MeHg-Se interactions within the plant might be the key process triggering the decreased grain MeHg levels under Se amendment. The reduction in soil MeHg concentrations could be mainly attributed to the formation of Hg-Se complexes (detected by TEM-EDX and XANES) and thus reduced microbial MeHg production. Moreover, selenite and selenate were equally effective in reducing soil MeHg concentrations, possibly because of rapid changes in Se speciation. The dominant role of Se-induced reduction in soil MeHg levels, which has been largely underestimated previously, together with the possible mechanisms advance our mechanistic understanding about MeHg dynamics in soil-rice systems.

Methylmercury accumulation and elimination in mink (Neovison vison) hair and blood: results of a controlled feeding experiment using stable isotope tracers.

Concentrations of metals in hair are used often to develop pharmacokinetic models for both animals and humans. Although data on uptake are available, elimination kinetics are less well understood; stable isotope tracers provide an excellent tool for measuring uptake and elimination kinetics. In the present study, methylmercury concentrations through time were measured in the hair and blood of mink (Neovison vison) during a controlled 60-d feeding experiment. Thirty-four mink were fed a standard fish-based diet for 14 d, at the end of which (day 0), 4 mink were sacrificed to determine baseline methylmercury (MeHg) concentrations. From day 0 to day 10, the remaining mink were fed a diet consisting of the base diet supplemented with 0.513 ± 0.013 µg Me(199) Hg/g and 0.163 ± 0.003 µg Me(201) Hg/g. From day 10 to day 60, mink were fed the base diet supplemented with 0.175 ± 0.024 µg Me(201) Hg/g. Animals were sacrificed periodically to determine accumulation of Me(201) Hg in blood and hair over the entire 60-d period and the elimination of Me(199) Hg over the last 50 d. Hair samples, collected from each mink and cut into 2.0-mm lengths, indicate that both isotopes of MeHg appeared in the hair closest to the skin at approximately day 10, with concentrations in the hair reaching steady state from day 39 onward. The elimination rate of Me(199) Hg from the blood was 0.05/d, and the ratio of MeHg in the hair to blood was 119. A large fraction of MeHg (22% to >100%) was stored in the hair, suggesting that in fur-bearing mammals the hair is a major route of elimination of MeHg from the body.

In vitro and whole animal evidence that methylmercury disrupts GABAergic systems in discrete brain regions in captive mink.

The effects of mercury (Hg) on key components of the GABAergic system were evaluated in discrete brain regions of captive juvenile male American mink (Neovison vison) using in vitro and in vivo (whole animal) experimental approaches. In vitro studies on cortical brain tissues revealed that inorganic Hg (HgCl(2); IC50=0.5+/-0.2microM) and methyl Hg (MeHgCl; IC50=1.6+/-0.2microM) inhibited glutamic acid decarboxylase (GAD; EC 4.1.1.15) activity. There were no Hg-related effects on [(3)H]-muscimol binding to GABA(A) receptors (IC50s>100microM). HgCl(2) (IC50=0.8+/-0.3microM) but not MeHgCl (IC50>100microM) inhibited GABA-transaminase (GABA-T; EC 2.6.1.19) activity. In a whole animal study, neurochemical indicators of GABAergic function were measured in brain regions (occipital cortex, cerebellum, brain stem, and basal ganglia) of captive mink fed relevant levels of MeHgCl (0 to 2microg/g feed, ppm) daily for 89d. No effects on GAD activity were measured. Concentration-dependent decreases in [(3)H]-muscimol binding to GABA(A) receptors and GABA-T activity were found in several brain regions, with reductions as great as 94% (for GABA(A) receptor levels) and 71% (for GABA-T activity) measured in the brain stem and basal ganglia. These results show that chronic exposure to environmentally relevant levels of MeHg disrupts GABAergic signaling. Given that GABA is the main inhibitory neurotransmitter in the mammalian nervous system, prolonged disruptions of its function may underlie the sub-clinical impacts of MeHg at relevant levels to animal health.

Accumulation of mercury and selenium in the brain of river otters (Lontra canadensis) and wild mink (Mustela vison) from Nova Scotia, Canada.

Total Hg, methyl-Hg (MeHg) and Se levels were measured in the brain of river otters (Lontra canadensis) and wild mink (Mustela vison) carcasses collected from Nova Scotia, Canada. Total Hg concentrations in the otters' brain were highly variable, ranging from 0.3 to 18.0 microg/g dw and were significantly higher in animals caught from inland areas of the province versus coastal animals. Similarly, inland otters contained significantly more MeHg in the brain than did coastal otters. MeHg was highly correlated with total Hg in both inland and coastal otters and represented on average approximately 82% of the total Hg. Selenium concentrations in the otter brain ranged from 1.0 to 7.8 microg/g dw but unlike Hg, there was no significant difference in Se levels between inland and coastal otters. There was a significant positive relationship between Se and total Hg concentrations in the otters with the molar ratio of Se:Hg approximately 1:1 for animals having an Hg concentration>18 nmol/g dw. The non-random sampling protocol for the mink precluded extensive statistical analysis of the data. However, unlike otters, virtually all (i.e. 98%) of the total Hg in mink brain was present as MeHg in both inland and coastal mink. Also unlike the otter results, Se concentrations showed no relationship with either total or MeHg in both inland and coastal mink. The data suggest that mink and otters may have different mechanisms for managing high levels of Hg in the brain.

Mercury concentrations in wild mink (Mustela vison) and river otters (Lontra canadensis) collected from eastern and Atlantic Canada: relationship to age and parasitism.

Total mercury (Hg) concentrations were measured in the fur, brain and liver of wild mink (Mustela vison) and river otters (Lontra canadensis) collected from eastern and Atlantic Canada. Total Hg concentrations in fur were strongly correlated with levels in the brain and liver. There was no difference in tissue concentrations between male and female mink; however, female otters had significantly higher fur, brain and liver Hg levels than males. Similarly, there was not a significant relationship between Hg concentration and age of mink, whereas in otters, Hg concentrations in all three tissues decreased significantly with age. In both species, only a very small percentage of the variability in Hg concentration was explained by age. After adjusting the data for site-to-site differences in Hg levels, Hg concentrations in the fur of mink infected by the parasite, Dioctophyma renale, were found to be significantly higher than Hg levels in uninfected mink.

The effects of mercury on muscarinic cholinergic receptor subtypes (M1 and M2) in captive mink.

A combination of in vitro (competitive binding assays) and in vivo (tissues from animals exposed to dietary methyl mercury, MeHg) experimental procedures was employed to assess the effects of mercury (MeHg, HgCl(2)) on the two-key muscarinic cholinergic (mACh) receptor subtypes (M1, M2) in two brain regions (occipital cortex, brain stem) of captive mink (Mustela vison). In vitro, HgCl(2) and MeHg were equipotent in inhibiting [(3)H]-pirenzipine binding to the M1 receptor in the occipital cortex, but in the brain stem, MeHg was about 65x more potent than HgCl(2). For the M2 receptor, both HgCl(2) and MeHg were more potent at inhibiting [(3)H]-AFDX-384 binding in the occipital cortex than in the brain stem. Within each brain region, HgCl(2) was more potent at inhibiting [(3)H]-AFDX-384 binding than MeHg. In vivo exposure of captive mink to MeHg (0.5, 1, and 2ppm MeHg in the diet for 89 days) resulted in greater binding of radioligands to the M1 and M2 receptor in the occipital cortex, but not in the brain stem, when compared to control animals. Based on the in vitro results, we could not conclude which mACh receptor subtype or brain region was most sensitive to Hg, but the in vivo findings suggest that Hg preferentially affects mACh receptor subtype (M1 and M2) levels in the occipital cortex. By studying distinct mACh receptors, these results extend upon previous studies in laboratory rodents and wildlife that showed Hg to affect the global population of mACh receptors.