Studies on the biotransformation of 203Hg-labeled methyl mercury chloride in rats

Organic mercury compounds: human exposure and its relevance to public health

Humans may be exposed to organic forms of mercury by either inhalation, oral, or dermal routes, and the effects of such exposure depend upon both the type of mercury to which exposed and the magnitude of the exposure. In general, the effects of exposure to organic mercury are primarily neurologic, while a host of other organ systems may also be involved, including gastrointestinal, respiratory, hepatic, immune, dermal, and renal. While the primary source of exposure to organic mercury for most populations is the consumption of methylmercury-contaminated fish and shellfish, there are a number of other organomercurials to which humans might be exposed. The antibacterial and antifungal properties of organomercurials have resulted in their long use as topical disinfectants (thimerosal and merbromin) and preservatives in medical preparations (thimerosal) and grain products (both methyl and ethyl mercurials). Phenylmercury has been used in the past in paints, and dialkyl mercurials are still used in some industrial processes and in the calibration of certain analytical laboratory equipment. The effects of exposure to different organic mercurials by different routes of exposure are summarized in this article.

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.

The influence of nutrition on methyl mercury intoxication

This article reviews progress in the research of methyl mercury (MeHg) and nutrient interactions during the past two decades. Special emphasis is placed on the following three major areas: a) effects on kinetics, b) effects on toxicity, and c) possible mechanisms. Dietary information is not usually collected in most epidemiologic studies examining of the effects of MeHg exposure. However, inconsistency of the MeHg toxicity observed in different populations is commonly attributed to possible effects of dietary modulation. Even though the mechanisms of interaction have not been totally elucidated, research in nutritional toxicology has provided insights into the understanding of the effects of nutrients on MeHg toxicity. Some of this information can be readily incorporated into the risk assessment of MeHg in the diets of fish-eating populations. It is also clear that there is a need for more studies designed specifically to address the role of nutrition in the metabolism and detoxification of MeHg. It is also important to collect more detailed dietary information in future epidemiologic studies of MeHg exposure.

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.

Lipid peroxidation in liver of rats administrated with methyl mercuric chloride

Parenteral administration of methyl mercuric chloride (MMC, CH3HgCl) to rats enhanced lipid peroxidation in liver of rats, as measured by the thiobarbituric acid reaction for malondialdehyde (MDA) in fresh tissue homogenates. After sc injection of CH3HgCl (5 mg/kg body wt), MDA concentration in liver became significantly increased at 24 h and further increased at 48 h. Dose-response studies were carried out with male albino rats of the Fisher-344 strain (body wt 170-280 g) injected with 3 or 5 mg Hg/kg as CH3HgCl and sacrificed after 24 h. In time-response studies, animals were administered 5 mg Hg/kg as CH3HgCl and sacrificed after 24 and 48 h. Studies in the authors' laboratory have shown that (1) mercury is accumulated in liver; (2) concentration of MDA is increased in liver of CH3HgCl-treated rats; (3) severity of hepatotoxicity is generally proportional to the elevation of MDA concentration, based upon the dose-effect relationships observed after administration of CH3HgCl to rats. The results of this study implicate that the lipid peroxidation is one of the molecular mechanisms for cell injury in acute CH3HgCl poisoning.

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.

Methylmercury poisoning: long-term clinical, radiological, toxicological, and pathological studies of an affected family

For 3 months in 1969 a family in the United States that included a pregnant mother consumed pork containing methylmercury. Children, aged 20, 13, and 8 years and a neonate, developed severe neurological signs. Twenty-two years later, the 2 oldest had cortical blindness or constricted visual fields, diminished hand proprioception, choreoathetosis, and attentional deficits. Magnetic resonance images showed tissue loss in the calcarine and parietal cortices and cerebellar folia. The youngest had quadriplegia, blindness, and severe mental retardation until their deaths. The brain of the 8-year-old who died at age 30 showed cortical atrophy, neuronal loss, and gliosis, most pronounced in the paracentral and parietooccipital regions. The total mercury level in formalin-fixed, left occipital cortex was 1,974 ng/gm as measured by atomic absorption. Regional brain mercury levels correlated with extent of brain damage. A control patient had 38.5 ng of mercury/gm in the occipital cortex. Systemic organs in the patient and a control subject had comparable mercury levels. In mercury-intoxicated rats, we found that only 5 to 10% of total brain mercury was lost by formalin fixation. Brain inorganic mercury in the patient ranged from 82 to 100%. Since inorganic mercury crosses the blood-brain barrier poorly, biotransformation of methyl to inorganic mercury may have occurred after methylmercury crossed the blood-brain barrier, accounting for its persistence in brain and causing part of the brain damage.

Influence of sodium selenite on 203Hg absorption, distribution, and elimination in male mice exposed to methyl203Hg

To study the effects of long-term selenium supplementation on absorption, distribution, and elimination of methylmercury (MeHg) in mice, three groups of male mice (Balb/c CA) were exposed for 7 wk to 0, 0.6, and 3 ppm sodium selenite in tap water. They were then given a single oral dose of Me203Hg (2 mumol/kg) by gastric intubation, and elimination of 203Hg was followed by whole-body counting for 49 d at the same Se exposure as previously. Twenty-four hours and 49 d after dosage, 6-7 animals/group were sampled for analysis of 203Hg distribution in the body. Glutathione peroxidase (GSH-PX) activity in blood and selenium levels in the liver were used as measures of selenium status. Gastrointestinal absorption of Me203Hg was not influenced by the Se status of the animals. Selenium supplementation of MeHg-exposed mice caused an enhanced whole-body elimination of Hg, but selenium-supplemented animals did not have lower Hg levels in the brain and kidney than nonsupplemented animals. The effect of selenium on the accumulation of Hg in the brain was dose-dependent, a high dose (3 ppm Se) causing a higher initial accumulation of Hg. The intracellular distribution of 203Hg in the liver and kidney was not affected by Se. The results indicate that selenium treatment of MeHg-exposed mice may have a positive effect on the health of the animals by decreasing the total body burden of MeHg.

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.

Altered intrarenal accumulation of mercury in uninephrectomized rats treated with methylmercury chloride

We tested the hypothesis that the intrarenal accumulation of mercury in rats treated with methylmercury is altered significantly as a result of unilateral nephrectomy and compensatory renal growth. Renal accumulation of mercury was evaluated by radioisotopic techniques in both uninephrectomized (NPX) and sham-operated (SO) rats 1, 2, and 7 days after the animals received a nonnephrotoxic intravenous dose of methylmercury chloride (5 mg/kg Hg). At all times studied after the injection of the dose of methylmercury, the renal accumulation of mercury (on a per gram kidney basis) was significantly greater in the NPX rats than that in the SO rats. The increased accumulation was due to a specific increase in the accumulation of mercury in the outer stripe of the outer medulla. Renal cortical accumulation of mercury was similar in both the NPX and SO rats. The percentage of the administered dose of mercury that was present in the total renal mass of the NPX and SO rats ranged between 5 and 15, depending on the day that the renal accumulation was studied. Approximately 40-50% of the total renal burden of mercury in both the NPX and SO rats was in the inorganic form. However, only less than 1% of the mercury in blood was in the inorganic form at the three times accumulation was studied. Very little mercury was excreted in the urine by either the NPX or SO rats. Only about 2 to 3% of the administered dose of mercury was excreted in the urine in 7 days. By contrast, the cumulative fecal excretion of mercury over 7 days was substantial in the NPX and SO rats, and significantly more mercury was excreted in the feces by the NPX rats (about 19% of the dose) than by that in the SO rats (about 16% of the dose). In conclusion, our findings indicate that unilateral nephrectomy and compensatory renal growth cause a significant increase in the accumulation of mercury in the renal outer stripe of the outer medulla in rats exposed to methylmercury. In addition, the findings indicate that the fecal excretion of mercury is also significantly increased.

Degradation of methyl and ethyl mercury into inorganic mercury by hydroxyl radical produced from rat liver microsomes

Liver microsomes were prepared from Wistar rat by the Ca2+ aggregation method. Under various conditions, ethyl mercury chloride (EtHgCl) or methyl mercury chloride (MeHgCl) was incubated with the microsomal preparations. After the incubation, the amounts of inorganic Hg and hydroxyl radical (.OH) in the preparations were determined. Although the preparations alone produced a small amount of inorganic Hg and .OH, the addition of NADPH to the preparations increased both inorganic Hg and .OH production, which were further accelerated by the addition of KCN. The addition of Fe(III)EDTA, a .OH formation promoter, to the microsome-NADPH-KCN system increased inorganic Hg production, whereas the addition of diethylenetriamine pentaacetic acid, a .OH formation inhibitor, decreased inorganic Hg production. When .OH scavengers such as mannitol and dimethyl sulfoxide were added to this system, the inorganic Hg production decreased. These results suggested that the .OH produced from liver microsomes was responsible for the degradation of MeHg and EtHg. Since both .OH and inorganic Hg production decreased with a concomitant decrease in NADPH-cytochrome P-450 reductase activities, it is suggested that this enzyme may be involved in the microsomal degradation of MeHg and EtHg.

Degradation of methyl and ethyl mercury into inorganic mercury by other reactive oxygen species besides hydroxyl radical

Degradation of methyl mercury (MeHg) and ethyl Hg (EtHg) with reactive oxygens was studied in vitro by using peroxidase-hydrogen peroxide (H2O2)-halide and rose bengal-ultraviolet light A systems. For this purpose, the direct determination method for inorganic Hg was employed. Both systems could effectively degrade EtHg, and MeHg to some extent. Degradation of MeHg and EtHg with the myeloperoxidase (MPO)-H2O2-chloride system was inhibited by MPO inhibitors (cyanide and azide), catalase, hypochlorous acid (HOCI) scavengers (glycine, alanine, serine and taurine), 1,4-diazabicyclo[2,2,2]octane and 2,5-dimethylfuran, but not by hydroxyl radical scavengers (ethanol and mannitol). Iodide was more effective than chloride as the halide component. Lactoperoxidase (LPO) could substitute for MPO in the iodide, but not the chloride system. With MPO-H2O2-chloride, MPO-H2O2-iodide and LPO-H2O2-iodide systems, we observed the increased degradation of EtHg in deuterium oxide (D2O) medium better than that in H2O medium. The D2O effect upon MeHg degradation was extremely weak. These results suggested that HOCl (or HOI) might be also capable of degrading MeHg and EtHg, besides the hydroxyl radical already reported by us. Singlet oxygen could degrade EtHg but not MeHg.

Expression of a high-affinity form of UDP-glucuronosyltransferase in human foetal liver cells in culture on exposure to mercuric chloride

The activity of UDP-glucuronosyltransferase (UDPGT, EC 2.4.1.17) in human foetal liver cells in culture was measured with two acceptor substrates, namely harmol and 1-naphthol. There was a dose-dependent increase of about 10-400% in UDPGT activity when the cells were exposed to 1-30 microM-HgCl2. Above a critical concentration of 30 microM-HgCl2, the heavy metal ion was toxic to the cells. Kinetic studies of the glucuronidation reaction with harmol and 1-naphthol showed that Hg2+ ions seemed to induce the expression of a high-affinity form of UDPGT, which was absent from the normal controls. The dramatic increase in specific activity in UDPGT was accompanied by a parallel increase in Vmax. measured with harmol and UDP-glucuronic acid. The significance of a possible induction of UDPGT in human foetal liver cells by HgCl2 is discussed.

Mercury and selenium interaction: a review

This paper reviews studies on mercury and selenium interaction. It includes the effects of selenium on mercury toxicity on the organism, organ/tissue, and subcellular levels. The paper also touches on possible mechanisms for the "protective action" of selenium against mercury toxicity and deals briefly with the synergism between the two elements.

Degradation of methyl and ethyl mercury into inorganic mercury by oxygen free radical-producing systems: involvement of hydroxyl radical

Degradation of methyl mercury (MeHg) and ethyl Hg (EtHg) with oxygen free radicals was studied in vitro by using three well-known hydroxyl radical (.OH)-producing systems, namely Cu2(+)-ascorbate, xanthine oxidase (XOD)-hypoxanthine (HPX)-Fe(III)EDTA and hydrogen peroxide (H2O2)-ultraviolet light B. For this purpose, the direct determination method for inorganic Hg was employed. MeHg and EtHg were readily degraded by these three systems, though the amounts of inorganic Hg generated from MeHg were one half to one third those from EtHg. Degradation activity of XOD-HPX-Fe(III)EDTA system was inhibited by superoxide dismutase, catalase and the .OH scavengers and stimulated by H2O2. Deletion of the .OH formation promoter Fe(III)EDTA from XOD-HPX-Fe(III)EDTA system resulted in the decreased degradation of MeHg and EtHg, which was enhanced by further addition of the iron chelator diethylenetriamine pentaacetic acid. In all these cases, a good correlation was observed between alkyl Hg degradation and deoxyribose oxidation determining .OH. By contrast, their degradation appeared to be unrelated to either superoxide anion (O2-) production or H2O2 production alone. We further confirmed that H2O2 (below 2 mM) itself did not cause significant degradation of MeHg and EtHg. These results suggested that .OH, but not O2- and H2O2, might be the oxygen free radical mainly responsible for the degradation of MeHg and EtHg.

Action of mercurials on 3H-cAMP binding to the regulatory subunit-II of cAMP-dependent protein kinase

Binding of the regulatory subunit type II (RII) of adenosine 3',5'-cyclic monophosphate (cAMP) dependent protein kinase was inhibited by Hg2+ with an IC50 value of 0.31 microM. Methyl mercury, p-chloromercuribenzoic acid (PCMB), and 5,5'-dithiobis-2-nitrobenzoic acid (DTNB) also inhibited cAMP binding with IC50 values of 70-80 microM for organic mercurials and 130 microM for DTNB. Addition of 1 mM 2-mercaptoethanol and 1 M cysteine to the assay mixture reversed these inhibitions. N-ethylmaleimide (NEM) showed little effect on the binding. On the other hand, Hg2+ and methyl mercury markedly suppressed enzymatic activity of the catalytic subunit. The IC50 value was 0.13 microM for Hg2+ and 0.15 microM for organic mercurials. Scatchard plots of kinetic analysis data for the cAMP binding revealed a noncompetitive type of inhibition by mercurials and DTNB. It is suggested that blockade of sulfhydryl groups resulted in the inhibition of cAMP binding to the RII subunit, which might result in preserving the association of the RII subunit and the catalytic subunits and in preventing further inactivation of the catalytic subunit by Hg2+.

Prenatal exposure to methyl mercury among Greenlandic polar Inuits

During the period 1982 to 1988, 37 paired samples of blood from Inuit women and their newborn children were collected in North Greenland. The samples were analyzed for whole blood content of total mercury (tot-Hg) and for content of methyl mercury (Me-Hg). In maternal blood, 80% of the tot-Hg was found to be methylated in contrast to 98% in cord blood. Concentrations of Me-Hg in maternal and cord blood were significantly correlated, and the mean ratio between fetal and maternal blood Me-Hg was 1.9. Concentrations of Me-Hg in cord blood were closely related to the marine food intake of the mothers. Eighty-four percent of the mothers had blood concentrations of Me-Hg above 0.11 mumol/l (23 micrograms/l), which corresponds to the provisional limit of tolerable intake set by the World Health Organization. This indicates that the majority of the pregnant woman have an unacceptable high intake of methyl mercury.

Interaction of methylmercury compounds with albumin

The nature of interaction between bovine serum albumin (BSA) and methylmercurial compounds has been investigated by ultrafiltration analysis. Four types of BSA samples, mercaptalbumin, its mixed disulfides with glutathione (GSH) and L-cysteine (CySH), and S-carbamidomethylated derivative, were used for binding assays with methylmercury (MM) chloride (MMC) and three kinds of MM mercaptides of low molecular weight thiols, GSH (GS-MM), CySH (CyS-MM) and cysteinylglycine (CG-MM). Among various ligands tested, MMC showed the highest affinity for all BSA species, and the BSA-bound fraction of the ligand did not change with ligand/protein ratio. MMC strongly and stoichiometrically bound to mercaptalbumin even at a molar ratio of 1:1. In contrast, the albumin bound fractions of three other MM ligands increased with concomitant decrease in ligand/protein ratio and with time except for the alkylated albumin, the highest binding being shown by mercaptalbumin. Binding of S-2-nitrophenyl-glutathione, a GSH analog with a hydrophobic S-substituent, to albumin species occurred similarly to that of GS-MM. However, GSH and oxidized glutathione (GSSG) interacted differently with albumin; mercaptalbumin showed the lowest affinity for GSH, and GSSG scarcely interacted with all BSA species. These results suggest that the sulfhydryl group at Cys-34 is not the only site of BSA that interacts with MM compounds and that albumin interacts preferentially with the hydrophobic domains of a mercurial ligand rather than its hydrophilic peptide moiety.

Mercury-induced UDP-glucuronyltransferase (UDPGT) activity in mouse kidney

Administration of mercuric chloride to young adult mice produced a significant increase in the activity of renal UDP-glucuronyltransferase (UDPGT) measured with harmol as the acceptor substrate. This was observed 10 days after a daily oral dose of HgCl2 (6 micrograms Hg2+/g body wt.). The increase in UDPGT activity was correlated with an accumulation of mercury in the renal tissues and was accompanied by an increase in the apparent Vmax of the glucuronidation reaction without a change in the apparent Km values for harmol or UDPGA. Parallel studies with mercuric sulfide however showed negligible retention of mercury in both the liver or kidney nor was there any change in UDPGT activity compared to control values. The difference in solubilities of the two mercuric salts may be responsible for this observation. The possible mode of activation of UDPGT by mercury treatment is discussed.

Sex difference in acute renal dysfunction induced by methylmercury in mice

To investigate the sex-related difference of susceptibility of renal function to methylmercury (MeHg) toxicity, various doses of MeHg chloride (MMC, 20-200 mumol/kg) were orally administered to C57BL/6N mice of both sexes. On days 1, 3, 5, and 7 after MMC administration, the extent of damage to renal function and the renal Hg levels were examined. After dosing, female mice survived much longer than males. With the increase in the dose level to 200 mumol/kg, the changes of the renal Hg levels 24 h after administration showed biphasic features with a plateau of around 85 micrograms/g. The renal Hg in male mice increased more rapidly to the plateau than in females. The doses by which the renal Hg level reached the plateau were 80 and 120 mumol/kg for males and females, respectively. The time-dependent decrease of the renal Hg became much slower with dose levels exceeding 80 and 160 mumol/kg for males and females, respectively. Inhibition of phenolsulfonphthalein excretion and increase of plasma creatinine after the MMC administration were more marked in males than in females. Inorganic Hg levels in the kidney of MeHg-intoxicated mice were much lower than that of HgCl2-intoxicated mice, indicating that the involvement of inorganic Hg, a product of biotransformation of MeHg, in the renal failure caused by MMC treatment would be negligible. Although pathological changes in the renal proximal tubules of HgCl2-intoxicated mice were marked, those of the MeHg-intoxicated group were slight.(ABSTRACT TRUNCATED AT 250 WORDS)

Mechanism of urinary excretion of methylmercury in mice

To elucidate the mechanisms by which methyl-mercury (MeHg) is eliminated from organisms, male C57BL/6N mice were orally administered with MeHg chloride (5 mg/kg) and the chemical forms of its metabolites in plasma, urine and kidney were determined by column chromatographic analysis. Orally administered MeHg rapidly entered the circulation, accumulated in the kidney and other tissues, and was slowly excreted in the urine. Ultrafiltration and gel filtration analysis revealed that most of plasma MeHg was accounted for by its albumin conjugate. Cell fractionation analysis revealed that about 80% of renal MeHg was recovered from the 15,000 g supernatant fraction of the kidney homogenate. If the kidney was homogenized in the presence of serine-borate complex, a potent inhibitor of gamma-glutamyltranspeptidase (gamma-GTP), about 50% of the MeHg in the supernatant fraction was recovered as its glutathione S-conjugate while the rest was bound to cytosolic protein(s). The major part of urinary MeHg was accounted for by its cysteine conjugate. However, urinary excretion of its glutathione conjugate increased significantly if animals were pretreated with acivicin, an affinity labeling reagent for gamma-GTP. These and other results suggested that MeHg bound to albumin accumulated in the kidney predominantly via some non-filtrating peritubular mechanism, and localized in renal cytosolic compartment as its glutathione- and protein-bound forms. The glutathione S-conjugate of MeHg in the tubule cells might be transferred to the lumenal space, hydrolyzed to the cysteine S-conjugate, and then excreted in urine. These sequential events might constitute an important eliminatory pathway for a hazardous mercurial metabolite in mice.

Combined toxicity of ethanol and methylmercury in rat

The hepatotoxic and nephrotoxic effects of methylmercury (CH3Hg) and ethanol (EtOH) are well known; however, their interaction in vivo is not clearly understood. In order to investigate the combined effects of these 2 substances, 4 groups of male Wistar rats with an initial weight of approximately 190 g were treated for 7 weeks. Each group consisting of 8 rats was gavaged as follows: Group 1 with 5.0 ml/kg body weight of double distilled water, Group 2 with 5.0 ml/kg body weight of 25% EtOH, Group 3 with 2.5 mg/kg of CH3Hg in water, and Group 4 with 2.5 mg/kg of CH3Hg in 25% EtOH. At the termination of the experiment the mean body weights of the rats in Group 3 (372.5 +/- 10.8 S.E.) and Group 4 (383.4 +/- 13.4) were significantly lower than that of Group 1 (433.0 +/- 7.8). Linear regression showed a positive feed conversion efficiency for Groups 1 and 2 (1.07 and 0.83, respectively), and a negative score for this parameter for Groups 3 and 4 (-1.43, -1.53). At necropsy, rat livers from Group 3 exhibited random multifocal tan spots. The relative liver weights were similar to those of controls. Semithin sections of liver revealed an increase in lipid droplets in Groups 2 and 3 compared to those in the other 2 groups while vacuolization was more striking in CH3Hg treated rats (Groups 3 and 4). Severe hepatolysis and portal canal edema were noted in the groups of rats exposed to either EtOH alone or in combination with mercury. The relative weight of left kidney in Group 3 (0.70 +/- 0.03) and Group 4 (0.51 +/- 0.04) rats was significantly greater than that of the control (0.39 +/- 0.03). In gross appearance the kidney was pale and the urine production was significantly higher (P less than 0.05) in Group 3 compared to that of Group 1. Group 4 rats had significantly more (P less than 0.05) Hg in the kidney than Group 3; however, the inorganic percentages in both groups were similar. Morphological examination of the kidney proximal tubules from CH3Hg treated rats (Groups 3 and 4) revealed an increase in lipid droplets, vacuoles, cell sloughing and tubular degeneration compared to Groups 1 and 2. These histological changes in the proximal tubules of Group 4 rats indicate an additive effect of EtOH on the kidney pathology caused by CH3Hg.(ABSTRACT TRUNCATED AT 400 WORDS)

Histological localization of methylmercury in mouse brain and kidney by emulsion autoradiography of 203Hg

Some investigators have abandoned the use of 203Hg emulsion autoradiography in favor of chemical methods of microscopic localization of mercury. However, recent studies indicate that the latter methods identify only inorganic mercury, or some product of inorganic mercury, making them of little or no value for studies of methylmercury toxicity. Doubts about the use of 203Hg for microscopic localization arose because of the high maximum energy of its emissions and the concern that its latent images might be confounded with silver grains produced by chemical reactions between tissue Hg and the silver supplied by photographic emulsions. Examination of the spectrum of emissions from 203Hg demonstrates that its maximum energy emissions are rare. The mean energy of 203Hg emissions is in the 50-ke V range and the modal emissions are close to 0 ke V, indicating sufficient low energy emissions for autoradiography. In preliminary experiments, methylmercury content of mouse brain was shown to be stable through the steps of tissue processing for plastic sections. A direct comparison of autoradiographic grain counts from tissue treated with "cold" or "hot" methylmercury demonstrated that no grains above background were produced in the absence of nuclear emissions--only "hot" samples affected emulsion. In the kidneys of mice killed 24 hr after dosing, grains were most numerous over cortical tubules and significantly less numerous over glomeruli. In the cerebellum, the molecular layer was significantly more heavily labeled than the granular layer. The number of grains was greatly increased in every region by increasing the specific activity of the methylmercury dosing solution while holding the dose of methylmercury constant. Like the differential effect of "hot" vs "cold" tissue, the differential effect of low vs high specific activity confirms that the grain counts reflect nuclear emissions from the sample tissues, rather than a chemical effect dependent only on mercury content. Grain counts provided a measure of methylmercury content that matched the content measured by atomic absorption (AA). For example, the ratio of kidney/brain content was 32 by AA and 31 by grain counts in one experiment. Thus, 203Hg emulsion autoradiography appears to be a useful approach to localization of methylmercury in tissue sections processed for light microscopy.

Mercury localization in mouse kidney over time: autoradiography versus silver staining

Several methods of silver staining have been employed to localize mercury in tissue, under the assumption that the techniques represent total Hg, but recent reports have suggested that these stains are specific for a limited fraction of the Hg present in some samples. Magos et al. (1985, Arch. Toxicol. 57, 260-267) hypothesized that the stains actually vary with inorganic mercury content. The purpose of the present study was to compare localization by radiolabeling to localization by one silver stain, the photoemulsion histochemical technique, in tissues prepared to contain a range of levels of total Hg and a range of levels of inorganic Hg. Mice dosed with 8 mg Hg/kg as MeHg were killed 24 hr, 1 week, or 2 weeks after exposure, to allow a decrease in total Hg and an increase in the proportion of demethylated Hg over time. Mice dosed with 4 mg Hg/kg as HgCl2 provided samples in which all the Hg present was in the inorganic form. Atomic absorption of kidneys of mice dosed with MeHg showed that total Hg fell from 55 micrograms/g to 39 to 25 over 2 weeks, while the inorganic fraction climbed from about 2 to 27 to 35%. Grain counts from autoradiographs of 203Hg-labeled sections correlated with total Hg content at +0.88, but silver staining was correlated with inorganic Hg content, appearing only at late termination times in MeHg-exposed animals, but soon after dosing in mice exposed to inorganic Hg. The photoemulsion histochemical technique revealed a substance strictly localized in the proximal tubules, while autoradiographs and grain counts showed total Hg to be present throughout the kidney tissue. These results support the contention that silver stains are selective for inorganic Hg and suggest that the distribution of inorganic Hg, whether introduced experimentally or by gradual demethylation, is different from the distribution of MeHg. If subsequent studies support the association of silver stains with inorganic Hg, it should be possible to localize Hg in histologic sections, distinguishing between organic and inorganic forms, which differ in toxicity.

Effect of sex hormones on the fate of methylmercury and on glutathione metabolism in mice

To investigate the mechanisms for the sex-related difference in the in vivo fate of methylmercury (MeHg), the effects of hormonal manipulation on the distribution and urinary excretion of the mercurial moiety (Hg) of injected MeHg and on hepato-renal metabolism of glutathione were studied in C57BL/6N mice. Twenty-four hours after oral administration of MeHg, urinary Hg levels were significantly higher in males than in females. Tissue Hg levels of males were higher in the kidney, but lower in the brain, liver and plasma than those of females. The fate of injected MeHg in castrated males was similar to that in normal females except for its brain levels. This feminization of the mercurial behavior in the castrated males was restored by treating with testosterone propionate (TP). When control mice were treated with TP, urinary excretion of Hg increased in both sexes, whereas renal Hg level increased only in females. Administration of estradiol benzoate (EB) to males decreased the renal accumulation and urinary excretion of Hg, whereas its hepatic levels increased. However, no significant change in the fate of MeHg was found in females pretreated with EB. Castration of females slightly decreased the urinary excretion of Hg. Thus, tissue distribution and urinary excretion of the administered MeHg seem to be subject to sex hormone control. Since MeHg has a high affinity for GSH, effects of hormonal manipulation on the metabolism of hepato-renal glutathione were also investigated. A significant sex-related difference in glutathione levels was found in plasma but not in the kidney, liver and erythrocytes. The half-lives of glutathione in the liver and kidney were significantly shorter in males than in females as determined by treatment with buthionine sulfoximine, a specific inhibitor of GSH synthesis. This difference was also modulated by the hormonal treatment. Since half-lives of GSH in the liver and kidney predominantly reflect the rate of its efflux from these tissues, the results suggest that GSH metabolism and/or secretory transport may be regulated by sex hormones. These and other observations suggest that the fate of MeHg may be modulated by way of regulating the inter-organ metabolism and transport of glutathione and its derivatives.

The chronic toxicity of aluminium, cadmium, mercury, and lead in birds: a review

The toxicity of chronic dietary metal exposure in birds is reviewed. It is concluded that significant physiological and biochemical responses to such exposure conditions occur at dietary metal concentrations insufficient to cause signs of overt toxicity. Particularly important are reproductive effects which include decreased egg production, decreased hatchability, and increased hatchling mortality. Young, growing birds are typically more sensitive to the toxic effects of chronic metal exposure than adults, and altricial species are often more sensitive than precocial species. Factors which modify the absorption and toxicity of heavy metals, such as Se for the case of Hg, and Ca for the case of Pb and Cd, are discussed. Monitoring strategies for assessing environmental metal exposure in birds are evaluated.

Ultrastructural localization of mercury in adrenals from rats exposed to methyl mercury

Accumulations of mercury have been demonstrated in adrenal glands by light and electron microscopy with a highly sensitive histochemical technique. Rats were exposed to methyl mercury in drinking water (20 mg/l) for 7-180 days, or were given intraperitoneal injections of methyl mercury (daily dose 100 or 200 micrograms). The amount and location of the mercury deposits were dependent upon the exposure time, the method of administration and the amount administered. In rats exposed to methyl mercury in drinking water, accumulations were often observed in both the zona glomerulosa and reticularis. They appeared first in the zona glomerulosa of animals treated for 1 week. In the zona fasciculata, deposits were observed only in the animals treated for 50 to 180 days. In animals treated for 180 days the cytoplasm of the cells in the zona fasciculata was heavily vacuolated and distinct necrotic cells were observed in other cortical zones. In the chromaffin cells, a slight increase in the amount of deposits was observed with increasing exposure time. Both epinephrenic and norepinephrenic cells contained deposits. Only a few deposits were observed in the cortical and chromaffin cells of animals treated with intraperitoneal injections. Ultrastructural deposits were observed in the lysosomes of cortical cells and in both lysosomes and secretory granules of chromaffin cells.

Simultaneous ultrastructural demonstration of heavy metals (silver, mercury) and acid phosphatase

A histochemical technique which permits the simultaneous visualization of heavy metals and acid phosphatase at the ultrastructural level is described. The technique was applied to the anterior pituitary gland, the spinal cord and the liver. In all of the tested organs, both mercury and silver were found to accumulate primarily in the lysosomes, although small amounts of both metals could be observed in other organelles including endocytotic vesicles. In the anterior pituitary, few mercury deposits were found in the secretory granules.

Toxicity, distribution, and elimination of thiol complexes of methylmercury after intracerebral injection

Intracerebral injection of CH3Hg and CH3Hg complexed with glutathione (GSH), cysteine (cys), cysteinylglycine (cys-gly), and homocysteine (homocys) resulted in differences in toxicity. Criteria based on neurological indices, mortality, and weight loss indicated that the cys-gly complex of CH3Hg was significantly less toxic than CH3Hg or the other complexes. The other complexes of CH3Hg (GSH, homocys, and cys) were also found to be less toxic than CH3Hg. The selenium status of the animal did not seem to significantly influence the toxicity of CH3Hg and the complexes. While CH3Hg complexed to cys-gly was significantly less toxic than CH3Hg alone, there were no differences observed in the CH3Hg half-life values or in the distribution of these compounds in the kidneys, brain, liver, and blood. It was observed, however, that the CH3Hg--cys-gly complex had higher fecal excretion on d 3 and 4 following intracerebral injection.

Effects of methylmercury and mercuric chloride on preimplantation mouse embryos in vivo

This report compares the effects of methylmercuric chloride (MMC) and mercuric chloride (MC) on the development of mouse preimplantation embryos in vivo. Female mice were injected with a single intravenous dose of 0.5-20.0 mg Hg/kg MMC or 0.5-2.5 mg Hg/kg MC on day 0 of gestation. The embryos were recovered by flushing excised oviduct and uterus on day 3.5 of pregnancy, and were examined for abnormalities. In the groups treated with doses of 0.5 and 1.0 mg Hg/kg of both compounds, the rates of abnormal embryos were not significantly different from that in the control group. The 50% effective dose of MMC was twice as great as that of MC. With increasing dose, the difference became more obvious; the 80% effective doses differed by a factor of ten. The body weight of dams decreased in terms of the dose of mercury in MC-treated groups, but did not vary in MMC-treated groups. The sensitive developmental stage for mercury toxicities could not be determined clearly, although the high sensitivity was reported in the blastocyst stage in vitro. The embryos treated in vivo were less sensitive than those reported in vitro.

Enhanced and inhibited biotransformation of methyl mercury in the rat spleen

Biotransformation of methyl mercury in rats was studied by enhancing or inhibiting its biotransformation with various procedures. A new sensitive method developed to determine specifically inorganic mercury in the presence of organic mercury was used. Biotransformation was enhanced by treating the rat with phenylhydrazine. The increase of inorganic mercury was highest (four to five times) and rapid in the spleen. Inhibited biotransformation of methyl mercury was observed in splenectomized rats. The inorganic portion of total mercury in the macrophage-rich fraction of spleen cells was clearly higher than that in unfractionated spleen cells. The biotransformation of methyl mercury was inhibited by treating the rat with carrageenan, a well-known substance blocking macrophage function. These results suggest that the spleen is an important site for the formation of inorganic mercury, and that the macrophage participates in this biotransformation.

Methyl mercury and selenium interaction in relation to mouse kidney gamma-glutamyltranspeptidase, ultrastructure, and function

The effects of methyl mercury (CH3Hg) and selenium (Se) on renal ultrastructure were investigated and correlated to changes in renal gamma-glutamyl transpeptidase (gamma-GTPase) activity, mercury (Hg) accumulation, and renal function (serum creatinine and urea nitrogen). Three experimental protocols were used to investigate CH3Hg and Se interactions of both Se-sufficient and Se-deficient mice involving ip injection of the following administered alone or in combination: CH3Hg (4.0 mg/kg) and Se (0.16 mg/kg) daily for 7 days, CH3Hg (1.0 mg/kg) and Se (0.08 mg/kg) daily for 20 days, and a single acute dose of CH3Hg (8.0 mg/kg). Acivicin (12 to 50 mg/kg), an antitumor glutamine antagonist, was also used as a highly effective specific inhibitor of the gamma-GTPase. Our results show that CH3Hg administered to Se-deficient mice for 7 or 20 days resulted in significant (p less than or equal to 0.05) but only moderate inhibition (20%) of gamma-GTPase activity and extensive renal ultrastructural damage. Acivicin-treated mice had significant inhibition of gamma-GTPase activity (80%) following a single injection while ultrastructural damage was substantial only after several days of administration. These results may indicate different modes of action of acivicin and CH3Hg. Acivicin inhibited gamma-GTPase prior to renal damage while CH3Hg produced greater pathological effects with only moderate gamma-GTPase inhibition. Renal damage from acute and chronic CH3Hg toxicity occurred after distinct neurological signs were present. Selenium administered to Se-deficient mice ameliorated both the neurotoxic effects and nephrotoxic action of CH3Hg. While Se and CH3Hg treatments caused some of the same ultrastructural pathology as the treatment with CH3Hg alone (cytoplasmic vacuolation, increased lysosomal profile, mitochondrial swelling, and extrusion of cellular masses into the tubular lumen), degeneration was not as extensive. Although the total doses administered during both the 7- and the 20-day studies were similar, mice from the chronic 20-day study showed greater ultrastructural pathological effects from CH3Hg. The primary effects of CH3Hg appeared to be on the lysosomal system, while acivicin exerted its effects on the mitochondrial and endoplasmic reticulum systems. The accumulation studies on Hg suggest that dietary Se may have only an initial protective effect against Hg accumulation in the kidney while injected Se offers longer protection.(ABSTRACT TRUNCATED AT 400 WORDS)

Involvement of glutathione in the enhanced renal excretion of methyl mercury in CFW Swiss mice

The present studies attempted to identify the mechanism for the elevated urinary excretion rate for methyl mercury (MM) previously reported in CFW Swiss mice. Strain comparisons of factors which could conceivably influence renal excretion of MM were made. The biotransformation of MM to the inorganic form did not appear to play a significant role. No significant strain differences were observed in the distribution of MM between plasma and red cells under in vivo or in vitro conditions. The percentage of total plasma MM present in the low-molecular-weight fraction did not differ statistically between the CFW and CBA/J strains. Strain comparisons of total reduced nonprotein thiol concentrations in liver, kidneys, whole blood, and plasma revealed no significant strain differences. A significant strain difference in plasma oxidized glutathione (GSSG) concentrations was observed. However, plasma concentrations of reduced glutathione (GSH), the form of glutathione (GS) which interacts with MM, did not significantly vary between the strains. The rate of total glutathione (TGS) excretion in urine was approximately 2-fold higher in CFW mice than in CBA/J mice. The significantly higher urinary GS excretion in CFW mice was accompanied by a 1.6-fold lower urinary gamma-glutamyltranspeptidase (gamma-GTP) activity in this strain.

The dependence of biliary methylmercury secretion on liver GSH and ligandin

The biliary secretion of methylmercury was investigated in male rats which were given i.p. 400 mumoles/kg azathioprine or 96 mumoles/kg benziodarone 2 hr after the i.v. injection of 5 mumoles/kg MeHgCl. A group of rats were given 400 mg/kg trans-stilbene oxide (TSO) for 4 days before treatment with 10 mumoles/kg MeHgCl. A common link between these three compounds is their interference with ligandin. Azathioprine is a competitive inhibitor of glutathione S-transferase, benziodarone is covalently bound to ligandin and TSO is an inducer of liver ligandin. Although only azathioprine depletes liver GSH stores, both azathioprine and benziodarone inhibited the biliary secretion of methylmercury. As there is published proof that the reaction of MeHg+ with GSH does not require enzymatic help, the inhibitory effect of azathioprine and benziodarone confirms the role of ligandin in the transport of methylmercury or its GSH complex. However, the biliary secretion of methylmercury was increased only slightly by TSO pretreatment, but when 2 hr after the injection of MeHgCl animals received 2 mmoles/kg GSH, secretion increased twice as much in TWO pretreated than in control rats. This indicates the dual dependance of biliary methylmercury secretion on liver GSH and ligandin.

Methylmercury stimulates the exhalation of volatile selenium and potentiates the toxicity of selenite

The aim of the present experiments was to investigate whether a single dose of 24 mumol/kg methylmercuric chloride (MeHgCl) in rats can influence the effect of an equimolar dose of sodium selenite (Na2SeO3) on body weight or the exhalation of dimethylselenide, a volatile metabolic product of selenium. Due to the difference in their single-dose toxicities, only selenite depressed body weight gain, when given alone. The experiments indicated that methylmercury, irrespective of whether it was given 1-2 h before, or at the same time as sodium selenite, potentiated the effect of the latter on body weight. Methylmercury also increased the exhalation of volatile selenium, but this effect decreased when the administration of selenite was delayed.

Differences in the distribution of methyl mercury in erythrocytes, plasma, and brain of Japanese quails and rats after a single oral dose

Distribution of a single oral dose of methyl mercury (10 mg Hg/kg body weight) was followed from 90 min up to 120 h in plasma, erythrocytes, and brain of Japanese quails and rats. Significantly higher Hg concentrations were observed in plasma and brain of quails and red blood cells of rats. Blood/brain ratio decreased in quails from 6 to 2 at 24 h and 120 h respectively, whereas it increased in rats. Erythrocyte/plasma ratio in quails was about three times lower and averaged 54. The differences in Hg distribution were accompanied by a more than 3-fold higher acute toxicity in quails under adequate experimental conditions.

The comparative effects of methylmercuric chloride and mercuric chloride upon DNA synthesis in mouse fetal astrocytes in vitro

The relative effects of direct exposure to methylmercuric chloride (MMC) and mercuric chloride (MC) upon [3H]thymidine incorporation were determined using cultured mouse fetal astrocytes. The labeling indices of cells exposed to MMC were significantly lower than those of cells exposed to equimolar concentrations of MC. These results indicate that on a mole-for-mole basis, MMC is much more deleterious upon DNA synthesis of mouse fetal astrocytes than those of MC.

Effects of diet on mercury metabolism and excretion in mice given methylmercury: role of gut flora

Mice fed either (1) a pelleted rodent diet, (2) evaporated milk, or (3) a synthetic diet (high protein, low fat) exhibited different rates of whole body mercury elimination and fecal mercury excretion after exposure (per os) to methylmercuric chloride. The percentage of the total mercury body burden present as mercuric mercury was highest (35.3%) in mice fed the synthetic diet (which had the highest rate of mercury elimination) and lowest (6.6%) in the animals having the lowest mercury elimination rate (milk-fed mice). Mice fed the synthetic diet had lower mercury concentrations and had a higher proportion of mercuric mercury in their tissues than the mice from the other dietary groups. Treatment of the mice with antibiotics throughout the experimental period to suppress the gut flora reduced fecal mercury excretion and the dietary differences in whole body retention of mercury. Tissue mercury concentrations and proportion of organic mercury in feces, cecal contents, liver, and kidneys were increased by antibiotic treatment of mice fed the pelleted or synthetic diets. These results are consistent with the theory that demethylation of methylmercury by intestinal microflora is a major factor determining the excretion rate of mercury.

[Transformation reactions of special metals in organisms and in the environment. 2. Abiotic methylation reactions of mercury, especially by methyltin compounds and humic and fulvic acids]

Following a literature survey concerning abiological methylation reactions including transmethylation of mercury we present the results of transmethylation experiments. Model experiments in aqueous solution show an instantaneous methylation of mercuric chloride by mono-, di-, and trimethyltin chloride, the dimethyl compound having the highest rate of formation. The same is valid for similar experiments with soils, in which influences of the kind of soil also play a certain role. The methylation experiments of HgCl2 with humic and fulvic acids isolated from soil yielded positive results only in some few of the extracts and only with humic acid fractions.

In vitro studies on methyl mercury distribution in human blood

Studies comparing the methyl mercury (mHg) content of maternal and newborn blood have shown increased levels in the newborn. This has been attributed to facilitated transplacental diffusion because of high fetal hematocrit (Hct). This study shows the converse, that the diffusion of mHg diminishes progressively with increasing Hct. The diffusion of m203Hg across a Millipore membrane (0.45 microns) separating compartments A and B of a diffusion cell was studied. When both compartments contained saline or plasma alone, equilibration from A to B occurred in 5 h. Introduction of human red blood cells (RBC) in saline (Hct 20%) into B resulted in a twofold increase in diffusion of mHg when compared to saline alone. Increasing Hct in saline in compartment B resulted in a progressive decrease in diffusion (r = -0.95, P less than 0.001). The presence of RBC in plasma (Hct 20%) in B resulted in a 70% decrease in diffusion; with increasing Hct, diffusion was further reduced (r = -0.95, P less than 0.001). Direct addition of mHg to RBC in saline resulted in 98% RBC uptake. Increasing concentrations of plasma (at a constant Hct) resulted in a progressive decrease in RBC uptake. In undiluted plasma at Hct 14%, RBC uptake of mHg was 35%. Plasma electrophoresis showed that much of the mHg was associated with a high-molecular-weight lipoprotein fraction. Plasma components appear to be important in the distribution of mHg in blood, and may be a factor in the relatively higher blood levels in the fetus.

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.

Methyl mercury toxicity in the chick embryo

The toxicity of methyl mercury (mHg) in the developing chick embryo was investigated. The relationship of dose, time of administration (i.e., days 4-9 of development), and body levels of mercury was examined. The LD50 for mHg injected into the yolk sac on day 5 of incubation was 40-50 micrograms. Embryos dying within 24 hours showed increased total body mHg levels when compared to survivors (219 +/- 67 vs. 105 +/- 41 micrograms/gm, mean +/- SD). Absorption was dose-related, with a good correlation between mortality and body, blood, and brain levels. Daily analysis of body mHg levels after injection on day 5 showed continued mHg accumulation (0.88 +/- 0.35 micrograms/embryo/day). However, the rate of embryo growth exceeded the rate of mHg absorption, resulting in a progressive decrease in mHg in concentration in tissues (from 94.5 +/- 34.2 micrograms/gm on day 6 to 45.3 +/- 13.4 on day 9). Administration after day 5 resulted in a significant reduction in levels of mHg in the brain on day 18 (from 11.4 +/- 2.1 micrograms/gm when given on day 5 to 8.4 +/- 2.3 when given on day 9) and in mortality (from 64% to 33%). Because blood mHg levels remained unchanged, the increased brain levels and higher mortality early in embryogenesis may reflect facilitated transfer of mHg across a poorly developed blood-brain barrier. Later in development, the reduced mortality and lower brain mHg levels correspond to the formation of specialized interendothelial junctions and a more effective blood-brain barrier.

Hereditary analysis of the strain difference of methylmercury distribution in mice

Hereditary analyses of strain differences in the distribution of methylmercury (MeHg) were carried out with various strains of mice. First, dose-response relationships were examined with 6-week-old male mice of four strains at seven dose levels from 0.25 to 6.0 mg CH3HgCl/kg. Significant strain differences in dose-response were found for both blood and brain. Second, the frequency distribution of blood mercury concentration was examined with two inbred strains, C3H and C57BL, their hybrid (F1), F2 generation, and back-cross mice. The F1 generation showed an intermediate value between their parents, and characteristic hereditary segregations were found in the frequency of blood mercury concentration in F2 and back-cross mice. Third, the relationship between blood mercury concentration and the molecular structure of mouse hemoglobins (Hb) was examined with 14 strains of inbred mice and a single wild mouse strain. Five strains with Hb-beta d and one strain with Hb-beta p showed blood mercury concentrations twice as high as the other Hb-beta strains. Through these experiments, Hb structure, especially the number and position of cysteinyl residue in the molecule, was found to play a primary role in binding with MeHg and in determining blood mercury concentration.

Methylmercury removal in the dog during infusion of 2,3-dimercaptosuccinic acid (DMSA)

2,3-Dimercaptosuccinic acid (DMSA) has been utilized in chelation therapy and in extracorporal complexing hemodialysis therapy for experimental methylmercury intoxication. In the latter application, substantial excretion of methylmercury occurred by the urinary route. This prompted the current study of the effects of continuous intravenous DMSA infusion therapy on methylmercury kinetics in the dog. Animals previously dosed with 203Hg-labeled methylmercury at 2.5 mg Hg/kg received a priming dose of DMSA, followed by a continuous iv infusion at dose rates that resulted in DMSA concentrations in plasma similar to those observed during DMSA complexing hemodialysis therapy. The kinetics of 203Hg removal in DMSA-infused dogs was compared with both saline-infused controls and DMSA complexing hemodialysis treated dogs. DMSA infusion therapy resulted in a shift in 203Hg binding within systemic blood from the red-cell fraction into plasma. This was consistent with an observed association of DMSA with the plasma fraction. The shift in 203Hg from red cells into plasma was paralleled by an increase in urinary clearance of 203Hg during the DMSA infusion period. In four dogs treated in this fashion, an average of 6.5 micrograms of mercury was removed by the urinary route over the 5-h treatment period, compared to 0.007 micrograms in the saline-infused dogs. Although a similar magnitude of mercury output into urine was observed during DMSA complexing hemodialysis, an additional 5 micrograms was removed by the dialyzer, making that technique 1.5 times as effective as infusion therapy. Comparing 203Hg tissue concentration after DMSA infusion therapy with the saline-treated controls revealed a 6.5-fold decrease in liver, a 3-fold reduction in kidney, and a 27% reduction in cerebrum. No significant differences were observed in medulla or cerebellum. Histopathology revealed no consistent differences between DMSA-treated and saline-treated animals. The DMSA infusion therapy was effective in causing a rapid removal of 203Hg from animals previously dosed with 203Hg-labeled methylmercury. DMSA infusion therapy may provide a useful therapeutic alternative for methylmercury poisoning when rapid removal of the intoxicant is desired and hemodialysis equipment and expertise are not readily available.