Dose-dependence of methylmercury metabolism. A study of distribution: biotransformation and excretion in the squirrel monkey

Interaction of mercury species with proteins: towards possible mechanism of mercurial toxicology

The nature of the binding of mercurials (organic and inorganic) and their subsequent transformations in biological systems is a matter of great debate as several different hypotheses have been proposed and none of them has been conclusively proven to explain the characteristics of Hg binding with the proteins. Thus, the chemical nature of Hg-protein binding through the possible transportation mechanism in living tissues is critically reviewed herein. Emphasis is given to the process of transportation, and binding of Hg species with selenol-containing biomolecules that are appealing for toxicological studies as well as the advancement of environmental and biological research.

Clinical and Forensic Signs Resulting from Exposure to Heavy Metals and Other Chemical Elements of the Periodic Table

Several heavy metals and other chemical elements are natural components of the Earth's crust and their properties and toxicity have been recognized for thousands of years. Moreover, their use in industries presents a major source of environmental and occupational pollution. Therefore, this ubiquity in daily life may result in several potential exposures coming from natural sources (e.g., through food and water contamination), industrial processes, and commercial products, among others. The toxicity of most chemical elements of the periodic table accrues from their highly reactive nature, resulting in the formation of complexes with intracellular compounds that impair cellular pathways, leading to dysfunction, necrosis, and apoptosis. Nervous, gastrointestinal, hematopoietic, renal, and dermatological systems are the main targets. This manuscript aims to collect the clinical and forensic signs related to poisoning from heavy metals, such as thallium, lead, copper, mercury, iron, cadmium, and bismuth, as well as other chemical elements such as arsenic, selenium, and fluorine. Furthermore, their main sources of occupational and environmental exposure are highlighted in this review. The importance of rapid recognition is related to the fact that, through a high degree of suspicion, the clinician could rapidly initiate treatment even before the toxicological results are available, which can make a huge difference in these patients' outcomes.

The Roles of Oxidative Stress in Regulating Autophagy in Methylmercury-induced Neurotoxicity

Methylmercury (MeHg) is a potential neurotoxin that is highly toxic to the human central nervous system. Although MeHg neurotoxicity has been widely studied, the mechanism of MeHg neurotoxicity has not yet been fully elucidated. Some research evidence suggests that oxidative stress and autophagy are important molecular mechanisms of MeHg-induced neurotoxicity. Researchers have widely accepted that oxidative stress regulates the autophagy pathway. The current study reviews the activation of Nuclear factor-erythroid-2-related factor (Nrf2)-related oxidative stress pathways and autophagy signaling pathways in the case of MeHg neurotoxicity. In addition, autophagy mainly plays a role in the neurotoxicity of MeHg through mTOR-dependent and mTOR-independent autophagy signaling pathways. Finally, the regulation of autophagy by reactive oxygen species (ROS) and Nrf2 in MeHg neurotoxicity was explored in this review, providing a new concept for the study of the neurotoxicity mechanism of MeHg.

Plasma and red blood cells distribution of total mercury, inorganic mercury, and selenium in maternal and cord blood from a group of Japanese women

Fetuses are a high-risk group for methylmercury (MeHg) exposure. The main objective of this study was to compare the characteristic profiles of total mercury (THg), inorganic mercury (IHg), MeHg, and selenium in plasma and red blood cells (RBCs) between maternal and cord blood at parturition collected from a group of Japanese women. Furthermore, correlations of THg in RBCs, which is a biomarker of MeHg, and THg in plasma, which is an IHg exposure biomarker, were examined in maternal and cord blood. Fifty-five pairs of maternal and cord blood samples obtained at parturition were collected from pregnant women in Fukuoka, Japan. THg in RBCs and plasma were significant correlated between maternal and cord blood. THg in RBCs was 13.9 ng/mL for cord and 9.16 ng/mL for maternal blood, with a cord:maternal RBCs ratio for THg of 1.58, suggesting that MeHg is actively transferred from the mother to the fetus via the placenta. THg in plasma showed a positive correlation with THg in RBCs for maternal and cord blood. This result suggests that measuring THg in plasma can overestimate the exposure level to IHg in fish-eating populations. The percentages of IHg in cord plasma and RBCs were 31% and 1.7%, respectively, and those in maternal plasma and RBCs were 46% and 5.9%, respectively. These results suggest that cord blood is rich in MeHg and can easily transfer to the fetal brain. Selenium in cord plasma was 67 ng/mL and that in maternal plasma was 97 ng/mL, with a cord:maternal plasma ratio for Se of 0.69, suggesting that the protective effects of Se against MeHg toxicity in fetuses may be weaker than those expected in adult mothers.

Toxic trace elements at gastrointestinal level

Many trace elements are considered essential [iron (Fe), zinc (Zn), copper (Cu)], whereas others may be harmful [lead (Pb), cadmium (Cd), mercury (Hg), arsenic (As)], depending on their concentration and chemical form. In most cases, the diet is the main pathway by which they enter our organism. The presence of toxic trace elements in food has been known for a long time, and many of the food matrices that carry them have been identified. This has led to the appearance of legislation and recommendations concerning consumption. Given that the main route of exposure is oral, passage through the gastrointestinal tract plays a fundamental role in their entry into the organism, where they exert their toxic effect. Although the digestive system can be considered to be of crucial importance in their toxicity, in most cases we do not know the events that occur during the passage of these elements through the gastrointestinal tract and of ascertaining whether they may have some kind of toxic effect on it. The aim of this review is to summarize available information on this subject, concentrating on the toxic trace elements that are of greatest interest for organizations concerned with food safety and health: Pb, Cd, Hg and As.

Mercury Stable Isotopes in Ornithogenic Deposits As Tracers of Historical Cycling of Mercury in Ross Sea, Antarctica

Production of methylmercury (MeHg) in ocean waters and its bioaccumulation in marine organisms are critical processes controlling the fate and toxicity of mercury (Hg). However, these processes are not well understood in the Antarctic, where high levels of MeHg are observed in the subsurface ocean (100-1000 m). We explored the use of Hg stable isotope compositions in historical and modern biological deposits as a new approach for discerning Hg sources and tracing MeHg cycling in the ocean and bioaccumulation in marine biota. We found similar mass independent isotope fractionation (MIF) of Hg between a sediment profile containing historical penguin and seal feces deposits from coastal Antarctica and modern penguin and seal feces, suggesting that penguin and seal feces were the dominant sources of Hg to the sediments at different time periods. Furthermore, sediments dominated by seal feces displayed a significantly lower MIF slope (Δ(199)Hg/Δ(201)Hg) than those dominated by penguin feces despite similar extents of MIF. Since seals forage at greater depths (>400 m) than penguins (<100 m), the high MIF values and lower Δ(199)Hg/Δ(201)Hg in seal feces suggest that a significant fraction of MeHg accumulated by seals was produced in situ in the subsurface ocean from residual inorganic Hg(II) that sank from the euphotic zone after partial photoreduction. Our results suggest that in situ Hg methylation can be an important source of MeHg for marine biota, and Hg isotope compositions in biological archives can be valuable tracers of MeHg cycling.

In vivo and in vitro changes in neurochemical parameters related to mercury concentrations from specific brain regions of polar bears (Ursus maritimus)

Mercury (Hg) has been detected in polar bear brain tissue, but its biological effects are not well known. Relationships between Hg concentrations and neurochemical enzyme activities and receptor binding were assessed in the cerebellum, frontal lobes, and occipital lobes of 24 polar bears collected from Nunavik (Northern Quebec), Canada. The concentration-response relationship was further studied with in vitro experiments using pooled brain homogenate of 12 randomly chosen bears. In environmentally exposed brain samples, there was no correlative relationship between Hg concentration and cholinesterase (ChE) activity or muscarinic acetylcholine receptor (mAChR) binding in any of the 3 brain regions. Monoamine oxidase (MAO) activity in the occipital lobe showed a negative correlative relationship with total Hg concentration. In vitro experiments, however, demonstrated that Hg (mercuric chloride and methylmercury chloride) can inhibit ChE and MAO activities and muscarinic mAChR binding. These results show that Hg can alter neurobiochemical parameters but the current environmental Hg exposure level does have an effect on the neurochemistry of polar bears from northern Canada.

Mercury distribution and speciation in different brain regions of beluga whales (Delphinapterus leucas)

The toxicokinetics of mercury (Hg) in key species of Arctic ecosystem are poorly understood. We sampled five brain regions (frontal lobe, temporal lobe, cerebellum, brain stem and spinal cord) from beluga whales (Delphinapterus leucas) harvested in 2006, 2008, and 2010 from the eastern Beaufort Sea, Canada, and measured total Hg (HgT) and total selenium (SeT) by inductively coupled plasma mass spectrometry (ICP-MS), mercury analyzer or cold vapor atomic absorption spectrometry, and the chemical forms using a high performance liquid chromatography ICP-MS. At least 14% of the beluga whales had HgT concentrations higher than the levels of observable adverse effect (6.0 mg kg(-1) wet weight (ww)) in primates. The concentrations of HgT differed between brain regions; median concentrations (mgkg(-1) ww) were 2.34 (0.06 to 22.6, 81) (range, n) in temporal lobe, 1.84 (0.12 to 21.9, 77) in frontal lobe, 1.84 (0.05 to 16.9, 83) in cerebellum, 1.25 (0.02 to 11.1, 77) in spinal cord and 1.32 (0.13 to 15.2, 39) in brain stem. Total Hg concentrations in the cerebellum increased with age (p<0.05). Between 35 and 45% of HgT was water-soluble, of which, 32 to 41% was methyl mercury (MeHg) and 59 to 68% was labile inorganic Hg. The concentration of MeHg (range: 0.03 to 1.05 mg kg(-1) ww) was positively associated with HgT concentration, and the percent MeHg (4 to 109%) decreased exponentially with increasing HgT concentration in the spinal cord, cerebellum, frontal lobe and temporal lobe. There was a positive association between SeT and HgT in all brain regions (p<0.05) suggesting that Se may play a role in the detoxification of Hg in the brain. The concentration of HgT in the cerebellum was significantly associated with HgT in other organs. Therefore, HgT concentrations in organs that are frequently sampled in bio-monitoring studies could be used to estimate HgT concentrations in the cerebellum, which is the target organ of MeHg toxicity.

The Alice in Wonderland syndrome

The Alice in Wonderland syndrome is a term applied to altered bizarre perceptions of size and shapes of a patient's body and illusions of changes in the forms, dimensions, and motions of objects that a patient with this syndrome encounters. These metamorphopsias arise during complex partial seizures, migraine headaches, infections, and intoxications. The illusions and hallucinations resemble the strange phenomena that Alice experienced in Lewis Carroll's Alice's Adventures in Wonderland. Charles Lutwidge Dodgson, whose nom de plume was Lewis Carroll, experienced metamorphopsias. He described them in the story that he wrote for Alice Liddell and her two sisters after he spun a tale about a long and strange dream that the fictional Alice had on a warm summer day. The author of this chapter suggests that Dodgson suffered from migraine headaches and used these experiences to weave an amusing tale for Alice Liddell. The chapter also discusses the neurology of mercury poisoning affecting the behavior of Mad Hatter character. The author suggests that the ever-somnolent Dormouse suffered from excessive daytime sleepiness due to obstructive sleep apnea.

Heavy metal poisoning: management of intoxication and antidotes

Of the known elements, nearly 80% are either metals or metalloids. The highly reactive nature of most metals result in their forming complexes with other compounds such oxygen, sulfide and chloride. Although this reactivity is the primary means by which they are toxic, many metals, in trace amounts, are vital to normal physiological processes; examples include iron in oxygen transport, manganese and selenium in antioxidant defense and zinc in metabolism. With these essential metals toxicity occurs when concentrations are either too low or too high. For some metals there are no physiological concentrations that are beneficial; as such these metals only have the potential to cause toxicity. This chapter focuses on four of these: arsenic, mercury, lead and thallium.

Mercury exposure and public health

Mercury is a metal that is a liquid at room temperature. Mercury has a long and interesting history deriving from its use in medicine and industry, with the resultant toxicity produced. In high enough doses, all forms of mercury can produce toxicity. The most devastating tragedies related to mercury toxicity in recent history include Minamata Bay and Niagata, Japan in the 1950s, and Iraq in the 1970s. More recent mercury toxicity issues include the extreme toxicity of the dimethylmercury compound noted in 1998, the possible toxicity related to dental amalgams, and the disproved relationship between vaccines and autism related to the presence of the mercury-containing preservative, thimerosal.

The toxicology of mercury and its chemical compounds

This review covers the toxicology of mercury and its compounds. Special attention is paid to those forms of mercury of current public health concern. Human exposure to the vapor of metallic mercury dates back to antiquity but continues today in occupational settings and from dental amalgam. Health risks from methylmercury in edible tissues of fish have been the subject of several large epidemiological investigations and continue to be the subject of intense debate. Ethylmercury in the form of a preservative, thimerosal, added to certain vaccines, is the most recent form of mercury that has become a public health concern. The review leads to general discussion of evolutionary aspects of mercury, protective and toxic mechanisms, and ends on a note that mercury is still an "element of mystery."

Heavy Metal Poisoning: Clinical Presentations and Pathophysiology

Humans have had a long and tumultuous relationship with heavy metals. Their ubiquitous nature and our reliance on them for manufacturing have resulted at times in exposures sufficient to cause systemic toxicity. Their easy acquisition and potent toxicity have also made them popular choices for criminal poisonings. This article examines the clinical manifestation and pathophysiology of poisoning from lead, mercury, arsenic, and thallium.

The prediction of methylmercury elimination half-life in humans using animal data: a neural network/rough sets analysis

Artificial neural networks and Rough Sets methodology have been utilized to predict human pharmacokinetic elimination half-life data based on animal data training sets. Methylmercury (Hg) pharmacokinetic data was obtained from 37 literature references, which provided data on species, gender, age, weight, route of administration, dose, dose frequency, and elimination half-life based on either whole-body Hg analysis or blood Hg analysis. Data were categorized into various formats for analysis comparisons. Rough Sets methodology was utilized to identify and remove redundant independent variables. Artificial neural networks were used to produce models based on the animal data, which were in turn used to predict and compare to the human elimination half-life values. These neural network predictions were compared to allometric graphical plots of the same data. The best artificial neural network prediction was based on a "thermometer" categorical representation of the data.

A toxicokinetic model for predicting the tissue distribution and elimination of organic and inorganic mercury following exposure to methyl mercury in animals and humans. I. Development and validation of the model using experimental data in rats

The objective of this study was to develop a biologically based dynamic model for predicting the distribution and elimination of methyl mercury and its metabolite, inorganic mercury, under a variety of exposure scenarios in rats. A model is proposed based on a multicompartment approach; each compartment represents an organ or a group of organs or an excreta. The model translates into a set of coupled differential equations taking into account interorgan rates of exchanges and excretion together with the biotransformation process. The free parameters of the model are determined from statistical fits to the experimental data of the Farris et al. (Toxicol. Appl. Pharmacol. 119, 74-90, 1993) study on the time profiles of blood and tissue concentrations and cumulative excretions. The vast range of time scales that govern tissue absorption, distribution, biotransformation, and excretion served to solve the model step by step. This interplay of time scales in the rates explains the buildups and slow attrition of inorganic mercury in certain key organs such as the brain and the kidney, which are also the sites of the more important toxic effects. The model was validated on additional experimental data provided by Norseth and Clarkson (Arch. Environ. Health 21, 717-727, 1970) and Thomas et al. (Environ. Res. 41, 219-234, 1986; Environ. Res. 43, 203-216, 1987). This approach, when adapted to humans, allows the reconstruction of the time course of blood and tissue concentrations, starting from easily accessible data on hair, urine, and feces.

A toxicokinetic model for predicting the tissue distribution and elimination of organic and inorganic mercury following exposure to methyl mercury in animals and humans. II. Application and validation of the model in humans

The objective of this study was to develop a biologically based dynamical model describing the disposition kinetics of methyl mercury and its inorganic mercury metabolites in humans following different methyl mercury exposure scenarios. The model conceptual and functional representation was similar to that used for rats but relevant data on humans served to determine the critical parameters of the kinetic behavior. It was found that the metabolic rate of methyl mercury was on average 3 to 3.5 times slower in humans than in rats. Also, excretion rates of organic mercury from the whole body into feces and hair were 100 and 40 times smaller in humans, respectively, and urinary excretion of organic mercury in humans was found to be negligible. The human transfer rate of inorganic mercury from blood to hair was found to be 5 times lower than that of rats. On the other hand, retention of inorganic mercury in the kidney appeared more important in humans than in rats: the transfer rate of inorganic mercury from blood to kidney was 19 times higher than in rats and that from kidney to blood 19 times smaller. The excretion rate of inorganic mercury from the kidney to urine in humans was found to be twice that of rats. With these model parameters, simulations accurately predicted human kinetic data available in the published literature for different exposure scenarios. The model relates quantitatively mercury species in biological matrices (blood, hair, and urine) to the absorbed dose and tissue burden at any point in time. Thus, accessible measurements on these matrices allow inferences of past, present, and future burdens. This could prove to be a useful tool in assessing the health risks associated with various circumstances of methyl mercury exposure.

Chronic effects of methylmercury on the cerebral function in rats

We studied the effects of the long-term and small-dose administration of methylmercury chloride (MMC) on the cerebral function in rats. MMC, at a dose of 0.7 mg/kg/day, was subcutaneously injected for 85 consecutive days in nine adult male Sprague-Dawley rats. They were then sacrificed on the final day of exposure (MMC group) after both completing observations on behavioral changes and also determining the local cerebral glucose utilization (LCGU) as an indicator of the cerebral neuronal activities. Histological examinations of the brain and the sciatic nerve were also performed. In addition, seven rats who received physiological saline also served as a control. LCGU significantly decreased in the visual cortex, lateral geniculate nucleus and medial geniculate nucleus without any accompanying histological alterations. Severe axonal degeneration of the sciatic nerve was also observed, which corresponded to the previously described crossed leg phenomenon. The present results suggest that the damage to the peripheral nerve was much more severe than that to the brain, which caused behavioral changes. Although no cerebral morphological changes were observed, brain dysfunction showed a selective involvement of the visual and auditory systems. This finding suggests that LCGU is a sensitive method for detecting the subclinical cerebral dysfunction caused by long-term and small-dose MMC intoxication in the rat brain.

Determination of a site-specific reference dose for methylmercury for fish-eating populations

Environmental risk-management decisions in the U.S. involving potential exposures to methylmercury currently use a reference dose (RfD) developed by the U.S. Environmental Protection Agency (USEPA). This RfD is based on retrospective studies of an acute poisoning incident in Iraq in which grain contaminated with a methylmercury fungicide was inadvertently used in the baking of bread. The exposures, which were relatively high but lasted only a few months, were associated with neurological effects in both adults (primarily paresthesia) and infants (late walking, late talking, etc.). It is generally believed that the developing fetus represents a particularly sensitive subpopulation for the neurological effects of methylmercury. The USEPA derived an RfD of 0.1 microg/kg/day based on benchmark dose (BMD) modeling of the combined neurological endpoints reported for children exposed in utero. This RfD included an uncertainty factor of 10 to consider human pharmacokinetic variability and database limitations (lack of data on multigeneration effects or possible long-term sequelae of perinatal exposure). Alcoa signed an Administrative Order of Consent for the conduct of a remedial investigation/feasibility study (RI/FS) at their Point Comfort Operations and the adjacent Lavaca Bay in Texas to address the effects of historical discharges of mercury-containing wastewater. In cooperation with the Texas Natural Resource Conservation Commission and USEPA Region VI, Alcoa conducted a baseline risk assessment to assess potential risk to human health and the environment. As a part of this assessment. Alcoa pursued the development of a site-specific RfD for methylmercury to specifically address the potential human health effects associated with the ingestion of contaminated finfish and shellfish from Lavaca Bay. Application of the published USEPA RfD to this site is problematic; while the study underlying the RfD represented acute exposure to relatively high concentrations of methylmercury, the exposures of concern for the Point Comfort site are from the chronic consumption of relatively low concentrations of methylmercury in fish. Since the publication of the USEPA RfD, several analyses of chronic exposure to methylmercury in fish-eating populations have been reported. The purpose of the analysis reported here was to evaluate the possibility of deriving an RfD for methylmercury, specifically for the case of fish ingestion, on the basis of these new studies. In order to better support the risk-management decisions associated with developing a remediation approach for the site in question, the analysis was designed to provide information on the distribution of acceptable ingestion rates across a population, which could reasonably be expected to be consistent with the results of the epidemiological studies of other fish-eating populations. Based on a review of the available literature on the effects of methylmercury, a study conducted with a population in the Seychelles Islands was selected as the critical study for this analysis. The exposures to methylmercury in this population result from chronic, multigenerational ingestion of contaminated fish. This prospective study was carefully conducted and analyzed, included a large cohort of mother-infant pairs, and was relatively free of confounding factors. The results of this study are essentially negative, and a no-observed-adverse-effect level (NOAEL) derived from the estimated exposures has recently been used by the Agency for Toxic Substances and Disease Registry (ATSDR) as the basis for a chronic oral minimal risk level (MRL) for methylmercury. In spite of the fact that no statistically significant effects were observed in this study, the data as reported are suitable for dose-response analysis using the BMD method. Evaluation of the BMD method used in this analysis, as well as in the current USEPA RfD, has demonstrated that the resulting 95% lower bound on the 10% benchmark dose (BMDL) represents a conservative estimate of the traditional NOAEL, and that it is superior to the use of "average" or "grouped" exposure estimates when dose-response information is available, as is the case for the Seychelles study. A more recent study in the Faroe Islands, which did report statistically significant associations between methylmercury exposure and neurological effects, could not be used for dose-response modeling due to inadequate reporting of the data and confounding from co-exposure to polychlorinated biphenyls (PCBs). BMD modeling over the wide range of neurological endpoints reported in the Seychelles study yielded a lowest BMDL for methylmercury in maternal hair of 21 ppm. This BMDL was then converted to an expected distribution of daily ingestion rates across a population using Monte Carlo analysis with a physiologically based pharmacokinetic (PBPK) model to evaluate the impact of interindividual variability. The resulting distribution of ingestion rates at the BMDL had a geometric mean of 1.60 microg/kg/day with a geometric standard deviation of 1.33; the 1st, 5th, and 10th percentiles of the distribution were 0.86, 1.04, and 1.15 microg/kg/day. In place of the use of an uncertainty factor of 3 for pharmacokinetic variability, as is done in the current RfD, one of these lower percentiles of the daily ingestion rate distribution provides a scientifically based, conservative basis for taking into consideration the impact of pharmacokinetic variability across the population. On the other hand, it was felt that an uncertainty factor of 3 for database limitations should be used in the current analysis. Although there can be high confidence in the benchmark-estimated NOAEL of 21 ppm in the Seychelles study, some results in the New Zealand and Faroe Islands studies could be construed to suggest the possibility of effects at maternal hair concentrations below 10 ppm. In addition, while concerns regarding the possibility of chronic sequelae are not supported by the available data, neither can they be absolutely ruled out. The use of an uncertainty factor of 3 is equivalent to using a NOAEL of 7 ppm in maternal hair, which provides additional protection against the possibility that effects could occur at lower concentrations in some populations. Based on the analysis described above, the distribution of acceptable daily ingestion rates (RfDs) recommended to serve as the basis for site-specific risk-management decisions at Alcoa's Point Comfort Operations ranges from approximately 0.3 to 1.1 microg/kg/day, with a population median (50th percentile) of 0.5 microg/kg/day. By analogy with USEPA guidelines for the use of percentiles in applications of distributions in exposure assessments, the 10th percentile provides a reasonably conservative measure. On this basis, a site-specific RfD of 0.4 microg/kg/day is recommended.

Excretion of methyl mercury in human feces

Fecal excretion of methyl mercury was confirmed in four Japanese male subjects. Perhaps the methyl mercury detected in feces is dependent on (a) the unabsorbed methyl mercury in diet, (b) exfoliation of intestinal cells, (c) hepatic bile, and (d) intestinal methylation of inorganic mercury. The calculated amounts of methyl mercury excreted daily in feces were similar to those found in urine.

Evaluation of the uncertainty in an oral reference dose for methylmercury due to interindividual variability in pharmacokinetics

An analysis of the uncertainty in guidelines for the ingestion of methylmercury (MeHg) due to human pharmacokinetic variability was conducted using a physiologically based pharmacokinetic (PBPK) model that describes MeHg kinetics in the pregnant human and fetus. Two alternative derivations of an ingestion guideline for MeHg were considered: the U.S. Environmental Protection Agency reference dose (RfD) of 0.1 microgram/kg/day derived from studies of an Iraqi grain poisoning episode, and the Agency for Toxic Substances and Disease Registry chronic oral minimal risk level (MRL) of 0.5 microgram/kg/day based on studies of a fish-eating population in the Seychelles Islands. Calculation of an ingestion guideline for MeHg from either of these epidemiological studies requires calculation of a dose conversion factor (DCF) relating a hair mercury concentration to a chronic MeHg ingestion rate. To evaluate the uncertainty in this DCF across the population of U.S. women of child-bearing age, Monte Carlo analyses were performed in which distributions for each of the parameters in the PBPK model were randomly sampled 1000 times. The 1st and 5th percentiles of the resulting distribution of DCFs were a factor of 1.8 and 1.5 below the median, respectively. This estimate of variability is consistent with, but somewhat less than, previous analyses performed with empirical, one-compartment pharmacokinetic models. The use of a consistent factor in both guidelines of 1.5 for pharmacokinetic variability in the DCF, and keeping all other aspects of the derivations unchanged, would result in an RfD of 0.2 microgram/kg/day and an MRL of 0.3 microgram/kg/day.

Speciation of mercury excreted in feces from individuals with amalgam fillings

Investigators established methods for the analysis of total mercury (Hg-total), oxidized mercury and mercury bound to sulfhydryl groups (Hg-S), mercury vapor (Hg0), and mercury from amalgam particles (APs) in fecal samples. Two individuals consumed mercury as a mercury-cysteine complex mercury vapor, and mercury from amalgam particles, and the cumulative excretion of mercury in feces was followed. Investigators found that 80% of the mercury from amalgam particles and mercury bound to sulfhydryl groups was excreted, but only 40% of the mercury vapor was excreted. Speciation of mercury excreted in feces from 6 individuals with a moderate loading of amalgam fillings showed that most of the mercury originating from the fillings consisted of oxidized mercury, which was probably bound to sulfhydryl-containing compounds. The proportion of amalgam particles in fecal samples from these individuals was low, and it did not exceed 26% of the total amount of mercury excreted.

Organic mercury: an environmental threat to the health of dietary-exposed societies?

As a natural element, mercury is ubiquitous in the environment. The largest amount of mercury, amounting to approximately 100,000 tons per year, originates from the degassing of the earth's crust. To this amount, such anthropogenic activities as combustion of fossil fuels and releases from industrial activities add approximately 20,000 tons of mercury every year. The emitted mercury, both natural and anthropogenic, is in an inorganic form, predominantly as the metallic vapor (Hgzero). In aquatic environments, however, inorganic mercury is microbiologically transformed into the lipophilic organic compound, methylmercury. The transformation from the hydrophilic to the lipophilic state makes mercury more prone to biomagnification in aquatic food chains. Consequently, populations with a traditionally high dietary intake of food originating from either fresh-water or marine environments have the highest exposure to methylmercury. Because of their traditional pursuit of marine mammals, the Inuits belong to the highest dietary exposure group /1/. This situation is particularly true for the Polar Eskimos in North West Greenland. This population has the most traditional lifestyle among the Inuits and hunts predatory species of whales, such as beluga and narwhal, a combination that results in a high level of exposure to methylmercury. Polar Eskimos in North West Greenland, living in areas with no 'accidental' mercury pollution, but with a high dietary access to methylmercury thus exemplify a population group with a current potential environmental health problem.

Quantitative and qualitative distribution of mercury in organs from arctic sledgedogs: an atomic absorption spectrophotometric and histochemical study of tissue samples from natural long-termed high dietary organic mercury-exposed dogs from Thule, Greenland

Organs from 10 sledgedogs fed methyl mercury-containing organs and meat from predatory marine animals also eaten by humans in the Thule district of Greenland, were examined histochemically for cellular distribution of mercury, and the organ concentrations of mercury were quantified by atomic absorption spectrometry (total Hg). In selected organs the methyl mercuric level was determined by gaschromatography. The highest concentration of total mercury was found in mesenterial lymph nodes followed by liver and kidneys, which indicates that the lymphatic system might play an important role in the regulating transport of mercury to target organs. The concentrations were age-related, and the results suggest that demethylation takes place in all organs except skeletal muscles, but lowest in CNS. The distribution of mercury at cellular and subcellular levels was studied by the autometallographic technique. The atomic absorption spectrometric and autometallographic results were in good agreement. The brain mean concentration in the oldest group was 438 micrograms/kg, a level much lower than what has been reported to cause effects in the human central nervous system. However, if humans over a period of e.g. 50 years eat Arctic marine meat and accumulate mercury in the same way as dogs, the possibility that this may have health implications cannot be entirely excluded.

Effects of dietary alpha-tocopherol and beta-carotene on lipid peroxidation induced by methyl mercuric chloride in mice

Exposure of male CBA mice to methyl mercuric chloride, CH3HgCl, (10-40 mg/l in drinking water) for 2 weeks resulted in dose-related Hg deposition and enhanced lipid peroxidation in liver, kidney and brain. Mice were fed well-defined semisynthetic diets containing different levels of alpha-tocopherol (10, 100 or 1000 mg/kg) or beta-carotene (1000, 10,000 or 100,000 IU/kg) for four weeks, two groups on each diet. The concentrations of alpha-tocopherol and beta-carotene used corresponded to deficient, normal and high levels. During the last two weeks, one group on each diet was given 40 mg CH3HgCl/l of drinking water. High dietary alpha-tocopherol protected against CH3HgCl induced hepatic lipid peroxidation, whereas the alpha-tocopherol deficient diet further enhanced CH3HgCl induced hepatic lipid peroxidation. Similar, though statistically non-significant effects occurred in the kidneys, alpha-Tocopherol did not protect against CH3HgCl induced lipid peroxidation in the brain. Excess dietary beta-carotene further enhanced CH3HgCl induced lipid peroxidation in liver, kidney and brain. CH3HgCl significantly decreased the activity of total glutathione peroxidase (T-GSH-Px) and Se-dependent glutathione peroxidase (Se-GSH-Px) in the kidneys in all dietary groups. High dietary alpha-tocopherol enhanced the activity of Se-GSH-Px in liver and kidney compared to the activity in mice fed the normal level of alpha-tocopherol. This occurred in mice exposed to CH3-HgCl as well as in unexposed mice, and the difference between CH3HgCl exposed and unexposed mice was not diminished. High dietary alpha-tocopherol increased the activity of both Se-GSH-Px and T-GSH-Px in the brain of CH3HgCl-exposed mice. The dietary level of beta-carotene did not affect the activity of the two enzymes in the organs investigated.

Autometallographic detection of mercury in rat spinal cord after treatment with organic mercury

Autometallography was used to localize mercury in rat spinal cord after intraperitoneal administration of methylmercuric chloride (200 micrograms CH3HgCl daily). The technique permits small amounts of mercury sulfides and mercury selenides to be visualized by silver-enhancement. Mercury deposits were observed by light microscopy only in neurons. In all of the spinal cord segments selected (first cervical segment, C1; fifth cervical segment, C5; sixth thoracic segment, T6; and first lumbar segment, L1) the mercury was observed with cumulative dosages of 6000 micrograms CH3HgCl and greater. Laminae VII, VIII, and IX contained the majority of stained neurons, whereas laminae IV, V, VI, and X had a relatively lower density of mercury-containing neurons. Stained neurons were confined to specific cell groups, such as Clarke's column, nucleus intermedio-lateralis, nucleus cervicalis centralis, and nucleus dorsomedialis. At the ultrastructural level, mercury deposits were restricted to lysosomes of neurons and occasional accumulations in the lysosomes of ependymal cells.

Modifications of cell signalling in the cytotoxicity of metals

Many metals act on biological systems at low concentrations and recent epidemiological and experimental research indicates that toxic effects of certain metals occur at levels only marginally higher than those found in healthy subjects. Despite a large number of studies describing metal cytotoxicity, the molecular mechanisms involved are still poorly understood. However, it now seems evident that several metals can interact with enzyme functional groups and that proteins involved in signal transduction, including Ca2+ channels and pumps, may be especially sensitive to this interaction. Impairment of the ability of cells to adequately respond to the stimulation by hormones and growth factors may result in the loss of important cell functions or activation of mechanisms that compromise cell survival. In the following sections we will briefly describe the effects of various metals on cell signalling and present our recent findings on the mechanism by which inorganic mercury affects signal transduction.

Distribution of mercury in rabbits subchronically exposed to low levels of radiolabeled methyl mercury

The metabolism of methyl mercury (MeHg) has been studied in rabbits administered 203Hg-labeled methyl mercuric chloride, 0.125 mumol/kg body weight, twice a week for 9 weeks, by intravenous injection. Twelve weeks after cessation of treatment, about 54% of the administered dose had been excreted in faeces and 5% in urine. After one week, the highest concentration of 203Hg was found in fur (8.6 nmol Hg/g). Substantially lower concentrations were found in kidney (2.5 nmol/g), liver (0.9 nmol/g), brain (0.4 nmol/g), muscle (0.3 nmol/g) and blood (0.1 nmol/g). The rate of elimination of 203Hg from brain, muscle and blood was faster (t1/2 about 12 days) than that from kidney and liver (t1/2 about 28 days). The relative amount of inorganic Hg in kidney and liver increased with time after cessation of treatment. The highest fractions were 85 and 70%, respectively. In brain, no significant demethylation of MeHg could be detected.

Effects of acidification on the availability of toxic metals and calcium to wild birds and mammals

The effects of acidification on wildlife inhabiting aquatic or semi-aquatic environments are reviewed, with particular reference to the possibility for increased dietary exposure to Hg, Cd, Pb and/or Al, and decreased availability of essential dietary minerals such as Ca. It is concluded that: (1) piscivores risk increased exposure to dietary methyl-Hg in acidified habitats, and Hg concentrations in prey may reach levels known to cause reproductive impairment in birds and mammals; (2) piscivores do not risk increased exposure to dietary Cd, Pb or Al because these metals are either not increased in fish due to acidification, or increase are trivial from a toxicological perspective; (3) insectivores and omnivores may, under certain conditions, experience increased exposure to toxic metals in some acidified environments. Exposure levels are likely to be sufficiently low, however, that significant risks to health or reproduction are unlikely. More importantly, these wildlife species may experience a drastic decrease in the availability of dietary Ca due to the pH-related extinction of high-Ca aquatic invertebrate taxa (molluscs, crustaceans). Decreased availability of dietary Ca is known to adversely affect egg laying and eggshell integrity in birds, and the growth of hatchling birds and neonatal mammals. Acidification-related changes in the dietary availability of other essential elements, such as Mg, Se and P, have not been established and require further investigation; (4) herbivores may risk increased exposure to Al and Pb, and perhaps Cd, in acidified environments because certain macrophytes can accumulate high concentrations of these metals under acidic conditions. The relative importance of pH in determining the metal concentrations of major browse species, and the toxicological consequences for herbivores wildlife, is not well established and requires further study. A decreased availability of dietary Ca is also likely for herbivores inhabiting acidified environments.

Foetal and maternal distribution of inhaled mercury vapour in pregnant mice: influence of selenite and dithiocarbamates

The distribution of mercury after inhalation of metallic mercury vapour (6-8 mumols 203Hg0/kg b.wt.) was studied in pregnant mice (day 17 of gestation) after pretreatment with selenite (10 mumols Se/kg b.wt. intraperitoneally 1 hr before inhalation), thiram, disulfiram or diethyldithiocarbamate (1 mmol/kg orally 2 hr before inhalation of Hg0). For comparison, the effects of thiram, disulfiram and diethyldithiocarbamate on the distribution of mercury after administration of ionic mercury (7 mumols 203HgCl2/kg b.wt. intravenously) were also studied. Selenite pretreatment caused a longer retention of mercury in maternal tissues but decreased the foetal concentrations after 203Hg0 inhalation, similarly to what has been shown previously after administration of ionic mercury (Hg2+). Pretreatment with the three dithiocarbamates markedly increased the uptake in maternal brain and fat and decreased the foetal concentrations after intravenous injection of 203HgCl2. In contrast, no change in foetal uptake and only slight changes in maternal tissue concentration of mercury were observed after treatment with the dithiocarbamates followed by inhalation of 203Hg0, compared with 203Hg0 inhalation alone. The results are in favour of a firmer binding of mercury after Hg0 inhalation, when oxidation of Hg0 to Hg2+ occurs intracellularly, than after Hg2+ injection. Further studies, using repeatedly low dose administration of selenium, are needed to draw any conclusions concerning the protective effects of selenium after exposure to metallic mercury vapour.

Mercury neurotoxicity: mechanisms of blood-brain barrier transport

Mercury exists in a wide variety of physical and chemical states, each of which has unique characteristics of target organ toxicity. The classic symptoms associated with exposure to elemental mercury vapor (Hg0) and methylmercury (CH3Hg+; MeHg) involve the central nervous system (CNS), while the kidney is the target organ for the mono- and divalent salts of mercury (Hg+ and Hg++, respectively). Physical properties and redox potentials determine the qualitative and quantitative differences in toxicity among inorganic mercury compounds, while the ability of MeHg to cross the blood-brain barrier accounts for its accumulation in the CNS and a clinical picture that is dominated by neurological disturbances. This review gives an up-to-date account of mercury's physical and chemical properties and its interaction with biologically active sites pertinent to transport across the blood-brain barrier, a major regulator of the CNS millieu.

Literature-derived absorption coefficients for 39 chemicals via oral and inhalation routes of exposure

Absorption refers to the amount of a chemical or substance that is able to cross biological membranes and be taken up by the blood for subsequent distribution to target tissues. The term absorption coefficient, as used here, is a numerical descriptor characterizing that fractional uptake by the blood and represents an approximation of the biological "dose" ultimately responsible for toxicity or other effects following exposure or chemical administration. Regulatory agencies utilize absorption coefficients in deriving acceptable daily intake values and health advisory indices, as well as in quantifying radiological risk. However, absorption coefficients do not exist for many chemicals due to a paucity of appropriate toxicological data. As a result, regulatory policy must often provide default options that assume, for example, 100% absorption by all routes to permit evaluation of "data-gap" chemicals. This paper attempts to improve the situation by providing a discrete source of route-specific absorption coefficients that are based on experimental data reported in the open literature. The estimates presented here are the result of an extensive investigation of three data bases (TOXLINE, HSDB, and CIS), many agency documents, and nearly 200 articles from 30 scientific journals. Acknowledging that absorption efficiency varies with dietary status, age, and several other situation-specific factors, the estimates presented here are intended to reflect absorption by the average adult human.

Kinetics of methyl mercury in blood and brain during chronic exposure in the monkey Macaca fascicularis

The disposition parameters derived from a compartmental model kinetic analysis of blood Hg levels in nonpregnant, adult female Macaca fascicularis given daily doses of MeHg did not vary with either dosage level (50, 70 or 90 micrograms MeHg/kg b.wt.day) or duration of exposure (up to 507 day). In contrast, blood clearance of Hg in pregnant females was dose-dependent; it being higher at the 90 micrograms MeHg/kg b.wt.day than at the lower dosage levels. Hg levels in the brain of adult fascicularis relative to blood Hg also increased at the highest level of exposure. Blood Hg half-life in neonate fascicularis was similar to half-life in their mothers (adults). Finally, the regional distribution of mercury in the brains of adult and neonate fascicularis exposed to low and intermediate levels of MeHg resembles the reported distribution of mercury in the brains of adult and neonate humans environmentally exposed to MeHg. Consequently, M. fascicularis may be an especially appropriate animal model for studying the neurotoxic mechanisms of chronic methyl mercury exposure.

Metabolism of methylmercury in rabbits and hamsters

The metabolism of 203Hg-labeled methylmercury chloride (MeHg) has been studied in rabbits and hamsters. Rabbits were administered 1.6 mumols MeHgCl/kg bw intravenously, and hamsters 40 mumols/kg bw orally. Urine and feces were collected daily and groups of four animals killed after 1 h, 1 d, or 7 d. The concentration of 203Hg in blood, liver, kidney, spleen, lung, heart, and brain was determined by gamma counting. In both animal species, the clearance of 203Hg in the brain was slower than in other tissues. In the rabbits the brain 203Hg concentration increased during the whole experimental period. Rabbits excreted 203Hg primarily in feces (about 20% of the dose within 1 wk), and much less in urine (less than 2%). In contrast, hamsters very efficiently excreted 203Hg in urine (50% in 1 wk). The fecal excretion was similar to that of the rabbits. Separation of inorganic Hg and MeHg in urine from hamsters by ion exchange chromatography showed that about 90% of the urinary 203Hg was excreted as MeHg. These findings show that rabbits and hamsters are interesting experimental animal systems for studying the metabolism of MeHg.

Accumulation of methylmercury and inorganic mercury in the brain

Differences in metabolism between different mercury species are well recognized. Conclusions that only a minor demethylation of methylmercury takes place in the brain are based primarily on results from short term studies. Results from a number of studies on humans exposed for many years to methylmercury have shown high concentrations of inorganic mercury in the brain in relation to total mercury. Similar evidence is available from studies on monkeys exposed for several years to methylmercury. The results indicate that a significant accumulation of inorganic mercury takes place with time despite the fact that the demethylation rate is slow. Differences in biological halftimes between different mercury species will explain the results. Some data do still need confirmation using different analytical methods. There is reason to believe that the one-compartment model for methyl mercury cannot be used without reservations. Inorganic mercury has a complicated metabolism. After exposure to metallic mercury vapor, inorganic mercury, probably bound to selenium, accumulates in the brain. A fraction of the mercury is excreted, with a long biological halftime. Studies on rats and monkeys indicate that inorganic mercury penetrates the blood-brain barrier only to a very limited extent.

Brain and tissue levels of mercury after chronic methylmercury exposure in the monkey

Estimated half-lives of mercury following methylmercury exposure in humans are 52-93 d for whole body and 49-164 d for blood. In its most recent 1980 review, the World Health Organization concluded that there was no evidence to suggest that brain half-life differed from whole-body half-life. In the present study, female monkeys (Macaca fascicularis) were dosed for at least 1.7 yr with 10, 25, or 50 micrograms/kg.d of mercury as methylmercuric chloride. Dosing was discontinued, and blood half-life was determined to be about 14 d. Approximately 230 d after cessation of dosing, monkeys were sacrificed and organ and regional brain total mercury levels determined. One monkey that died while still being dosed had brain mercury levels three times higher than levels in blood. Theoretical calculations were performed assuming steady-state brain:blood ratios of 3, 5, or 10. Brain mercury levels were at least three orders of magnitude higher than those predicted by assuming the half-life in brain to be the same as that in blood. Estimated half-lives in brain were between 56 (brain:blood ratio of 3) and 38 (brain:blood ratio of 10) d. In addition, there was a dose-dependent difference in half-lives for some brain regions. These data clearly indicate that brain half-life is considerably longer than blood half-life in the monkey under conditions of chronic dosing.

New aspects on the distribution and metabolism of essential trace elements after dietary exposure to toxic metals

Under present environmental conditions, an increase in pollution owing to metals such as cadmium (Cd), lead (Pb), and methylmercury (MeHg) must be expected. The resulting effects would be seen particularly in the food chain. The daily intake of toxic metals in various parts of the world is different and depends on both the dietary habits and the concentration in foodstuffs. Oral ingestion of these toxic metals perturbs the metabolism of essential elements, especially zinc (Zn), copper (Cu), iron (Fe), and selenium (Se). The elemental composition of body tissues and fluids is an indicator of the nutritional and pathological status of humans. This review will describe the dietary intake and gut absorption of essential and toxic elements. Furthermore, it will discuss threshold values, toxic effects in relation to body burden of toxic metals, the biological indices of exposure, and the interaction between toxic and essential elements. The overall ratio of Cu, Zn, Fe, and Se concentration to Cd in the human kidney is the lowest in comparison to Hg and Pb. Increased kidney copper and urinary losses may be common denominators in the manifestation of renal toxicity induced by heavy metals. Factors affecting availability and loss of copper should be identified and measured. The critical kidney concentration for Cd, Pb, and MeHg should be revised in relation to essential elements.

Changes in axonally transported proteins in the mature and developing rat nervous system during early stages of methyl mercury exposure

It was established by means of SDS polyacrylamide gel electrophoresis that direct injections of methyl mercury (10 micrograms Hg) into the mature rat vitreous body of the eye decrease protein synthesis in the retina and optic nerve at 4 hours after injection. Although the global spectrum of polypeptides did not change, a specific decrease in the volume of polypeptides of 20-23 K daltons molecular weight was evident. Conversely, systemic exposure to methyl mercury resulted in increased protein synthesis of polypeptides of 20-23 K molecular weight both in adult (8 mgHg/kg/day for 8 days) and neonatal rats (2 mgHg/kg/day for 10 days). In addition, specific changes in the volume of polypeptides 75-90 K molecular weight were noted in sciatic nerves of neonatal rats. These data are consistent with a bimodal response in protein synthesis following MeHg treatment. Local presence of MeHg following direct injection into the eye causes a reduction in protein synthesis, while chronic systemic exposure results in increased synthesis and transport of proteins in both mature and developing optic nerves and neonatal sciatic nerves. Thus, these systems possess the capacity to attempt regenerative processes through induction of a small subset of proteins known as GAPs (Growth-Associated Proteins) during the early stages of systemic methyl mercury exposure. These wide spread and system-specific changes are consistent with growth-specific functions during the early stages of methyl mercury exposure.

Dose- and sex-dependent alterations in mercury distribution in fetal mice following methylmercury exposure

Methylmercuric chloride was orally given to inbred C57BL/6N mice on d 13 of pregnancy at doses of 2.5, 5, 10, and 20 mg/kg. Animals were sacrificed on each of d 14-18 of pregnancy, and mercury levels in the brain, liver, and kidney of both the fetus and dam were determined. The dose effect on the time course of mercury accumulation in the brain was observed both in the fetus and dam; after the higher doses administered, the brain mercury reached the highest concentration later than it did after the lower doses. In addition, the mercury concentration in the fetal brain was disproportionately higher after a dose of 20 mg/kg, which was toxic in the fetus since the weight of the brain was reduced. The concentration in the fetal brain was 1.6-4.9 times higher than in the maternal brain. The sex difference of fetuses in mercury levels was observed in the brain after a dose of 2.5 mg/kg, in which mercury concentration was higher in females than in males. This corresponded to the previously reported difference in adult mice and rats. However, the sex difference was not seen after doses of 5, 10, or 20 mg/kg.

Changes in the distribution of histochemically localized mercury in the CNS and in tissue levels of organic and inorganic mercury during the development of intoxication in methylmercury treated rats

The development of neurotoxicity in rats after exposure to methylmercuric chloride was monitored using behavioural indices. At selected time points the cellular localization of mercury and the relative amounts of organic and inorganic mercury were determined in several regions of the CNS, and in some non-neural tissues. The CNS showed an affinity for organic mercury, the levels of inorganic mercury remaining low throughout symptomatic intoxication. Histopathological changes were not closely related to the regional tissue content of the organic or inorganic forms, nor to mercury localized histochemically at the cellular level. The stained deposits, which had focal cytoplasmic distribution, appeared in glial cells initially then in larger neurones as the intoxication progressed. These observations may represent changes in the mercury content of different cell types or reflect differences in the way that they handle a similar burden of mercury. A transitory accumulation of mercury in glial cells may be a factor contributing to the occurrence of a latent period and sequestration of mercury in cytoplasmic organelles may serve to protect some cell types from injury.

Review of the health effects of methylmercury

The study critically reviews recent data relating to the health effects of methylmercury in man and the attendant dose-response relationships. New data obtained from animal studies, including pre-and postnatal exposure, are also examined. The consumption of fish and fish produce represents the major source of methylmercury exposure in the general population. Reported mercury concentrations in fish throughout the world are examined, particularly in the Mediterranean Sea. Here there is limited knowledge of methylmercury intake in critically exposed populations such as fishermen, employees of the fish industries and their families. The measurement of mercury in hair is now regarded as the most useful indicator of exposure but more experimental data are still required to increase the value of this index. The threshold levels of methylmercury in blood, hair and for dietary intake, as estimated by the World Health Organization, have been largely endorsed. However, new information from Japan and Canada suggests the existence of a latency period for some effects, so that the frequency or probability of their occurrence is inversely related to the duration of exposure. Incorporation of such findings would therefore lead to the designation of lower threshold values than are presently recognized. Pregnant women and the fetus have been identified as groups that are at special risk. The fetal blood mercury level is up to twice that of the mother and the sensitivity of both mother and fetus may be higher than in non-pregnant adults. This should be taken into account when assigning protective threshold concentrations.

Demethylation and excretion of methyl mercury by the guinea pig

Female guinea pigs were dosed po with 1.0 mg CH3 203Hg/kg as methylmercuric chloride, 10 times over a 3-week period. Tissue distribution, excretion, and accumulation of inorganic and organic mercury were studied. The highest concentration of mercury was found in the kidney. The greatest decreases of mercury levels were observed in the small bowel, red blood cells, liver, and cerebrum. The half-life of whole body clearance, based on a single compartment model, was 31.6 days. Mercury in the kidney, liver, and cerebrum was bound mainly by nuclear and soluble fractions. The highest ratio of inorganic to total mercury was seen in the kidney, 60% of this being as inorganic mercury. Excretion of mercury in the feces was measured throughout the experiment. The relationship of organic to inorganic mercury was relatively constant at about 1:3. Data on the effects of methyl mercury on tissue concentrations of zinc and copper show that the only change in the copper content was a marked increase in the kidney.

Biochemical and morphological studies of monkeys chronically exposed to methylmercury

This study was designed to correlated autopsy findings with the effects on cage behavior, laboratory values, and mercury clearance of long-term, low-dose exposure of primates to methylmercury. Six rhesus monkeys were given daily methylmercury hydroxide (MeHg) orally in apple juice on a preplanned dosage schedule. Three were sacrificed while receiving MeHg (group I) and the other 3 were sacrificed 2-5 mo after cessation of MeHg administration (group II). Whole-blood Hg levels (organic and inorganic) were assayed weekly, and major organ levels were assayed at autopsy. Whole-blood Hg levels were maintained between 1 and 2 micrograms/ml when the monkeys were given a MeHg dose of 80-125 micrograms/kg . d for up to 1 yr. The Hg burden of the major organs appeared to be dose- and duration-related. After periods of clearance (2.5-5 mo), intestinal wall Hg burden decreased to less than 1 microgram/g, and the hepatic Hg burden was still between 1.12 and 2.37 micrograms/g. However, the kidneys had a higher concentration of Hg, ranging from 10.34 to 29.54 micrograms/g. Whenever there was a high concentration of Hg, significant ultrastructural changes were observed. In the kidneys there were intracytoplasmic vacuoles and electron-dense inclusion bodies. In the small intestine of the animals cleared of mercury (group II), there were normal Paneth cells, as well as some degenerative cells characterized by dilation of endoplasmic reticulum and the presence of intracellular inclusion bodies. These findings suggest the long turnover time of Hg in these cell populations. During the period of study, weekly routine laboratory data including hematology, blood chemistry, and liver and kidney function tests did not reveal any significant changes.

The effects of CCl4 on the accumulation of mercury in rat tissues after methylmercury injection

Rats were administered a dose of 10 mg of CH3HgCl/kg by single s.c. injection. They were subsequently treated with 0.5 ml of CCl4/kg by three i.p. injections at 48-h intervals. The treatment with CCl4 resulted in the rise of total mercury level in brain, although the increase of mercury level was transitory and the difference between with and without CCl4 treatment gradually diminished. In addition, the mercury levels in other tissues involving liver, kidney, and muscle were increased by the treatment with CCl4. These results suggest that a hepatic damage by CCl4 is responsible for the duration of mercury accumulation in brain and other tissues after the single injection of methylmercury.