A study on the biochemical and biological behavior of methylmercury

Biomarkers of mercury toxicity: Past, present, and future trends

Mercury (Hg) toxicity continues to represent a global health concern. Given that human populations are mostly exposed to low chronic levels of mercurial compounds (methylmercury through fish, mercury vapor from dental amalgams, and ethylmercury from vaccines), the need for more sensitive and refined tools to assess the effects and/or susceptibility to adverse metal-mediated health risks remains. Traditional biomarkers, such as hair or blood Hg levels, are practical and provide a reliable measure of exposure, but given intra-population variability, it is difficult to establish accurate cause-effect relationships. It is therefore important to identify and validate biomarkers that are predictive of early adverse effects prior to adverse health outcomes becoming irreversible. This review describes the predominant biomarkers used by toxicologists and epidemiologists to evaluate exposure, effect and susceptibility to Hg compounds, weighing on their advantages and disadvantages. Most importantly, and in light of recent findings on the molecular mechanisms underlying Hg-mediated toxicity, potential novel biomarkers that might be predictive of toxic effect are presented, and the applicability of these parameters in risk assessment is examined.

Effects of diphenyl diselenide on methylmercury toxicity in rats

This study investigates the efficacy of diphenyl diselenide [(PhSe)2] in attenuating methylmercury- (MeHg-)induced toxicity in rats. Adult rats were treated with MeHg [5 mg/kg/day, intragastrically (i.g.)] and/ or (PhSe)2 [1 mg/kg/day, intraperitoneally (i.p.)] for 21 days. Body weight gain and motor deficits were evaluated prior to treatment, on treatment days 11 and 21. In addition, hepatic and cerebral mitochondrial function (reactive oxygen species (ROS) formation, total and nonprotein thiol levels, membrane potential (ΔΨm), metabolic function, and swelling), hepatic, cerebral, and muscular mercury levels, and hepatic, cerebral, and renal thioredoxin reductase (TrxR) activity were evaluated. MeHg caused hepatic and cerebral mitochondrial dysfunction and inhibited TrxR activity in liver (38,9%), brain (64,3%), and kidney (73,8%). Cotreatment with (PhSe)2 protected hepatic and cerebral mitochondrial thiols from depletion by MeHg but failed to completely reverse MeHg's effect on hepatic and cerebral mitochondrial dysfunction or hepatic, cerebral, and renal inhibition of TrxR activity. Additionally, the cotreatment with (PhSe)2 increased Hg accumulation in the liver (50,5%) and brain (49,4%) and increased the MeHg-induced motor deficits and body-weight loss. In conclusion, these results indicate that (PhSe)2 can increase Hg body burden as well as the neurotoxic effects induced by MeHg exposure in rats.

Total blood mercury concentrations in the U.S. population: 1999-2006

We describe the distribution and demographic characteristics of total blood Hg levels in the U.S. general population among persons ages 1 year and older who participated in the 2003-2006 National Health and Nutrition Examination Survey (NHANES). We also describe trends in the total blood Hg of children ages 1-5 (n=3456) and females ages 16-49 during 1999-2006 (n=7245). In the combined 2003-2006 survey periods, the geometric means for non-Hispanic blacks, 0.853microg/L (95% confidence interval [CI], 0.766-0.950microg/L), and non-Hispanic whites, 0.833microg/L (95% CI, 0.752-0.922microg/L), were higher than the geometric mean for Mexican Americans, 0.580microg/L (95% CI, 0.522-0.645microg/L). Also in 2003-2006, regression analysis of log total blood Hg with age, race/ethnicity and gender showed that total blood Hg levels in the population exhibited a quadratic increase with age (p<0.0001), peaking at ages 50-59 in non-Hispanic blacks and whites, at ages 40-49 in Mexican Americans, and then declining at older ages. Over the four survey periods (1999-2006), regression analysis showed that total blood Hg levels increased slightly for non-Hispanic white children and decreased slightly for non-Hispanic black and Mexican American children. Over the same four survey periods, female children had slightly higher total blood Hg levels than males (0.356 vs. 0.313microg/L, p=0.0050) and total blood Hg levels in non-Hispanic black women aged 16-49 years were significantly higher than in non-Hispanic white women (1.081 vs. 0.850microg/L, p<0.0001) and in Mexican American women (1.081 vs. 0.70microg/L, p<0.0001).

Blood and urine mercury levels in adult amalgam patients of a randomized controlled trial: interaction of Hg species in erythrocytes

Parts of the population are permanently exposed to low levels of Hg degrees and Hg(II) from dental amalgam. It was the aim (1) to investigate the internal exposure to amalgam-related mercury from the kinetics of inorganic Hg in plasma and erythrocytes after amalgam removal, and (2) to estimate the amalgam-related absorbed dose. Dietary coexposure was monitored by determination of blood organic-Hg. Postremoval steady-state Hg concentrations were measured for 18 months. Eighty-two patients had been randomized into three groups: (A) removal of the fillings; (B) removal and non-specific detoxification, and (C) a health promotion program without removal. After amalgam removal, inorganic Hg dropped rapidly in plasma and red cells, stabilizing at 27% of preremoval levels after 60 days. Concentrations of organic Hg in plasma remained unchanged, indicating no change in dietary uptake of organic Hg. The concentration of organic Hg in red cells of group A was in the early postremoval phase lower and in the late postremoval phase higher than the preremoval control (p<0.01 for low-high difference). A protracted increase in organic Hg was also found in red cells of group B after 60 days. Thus, the effect of removal on organic Hg levels in the combined group A+B was compared with the values of group C in a linear mixed effects (LME) model which showed a significant increase with time in group A+B (p=0.028). In all groups, time profiles of urinary concentration and excretion of total-Hg were very similar to those of inorganic-Hg levels in plasma. From extrapolations of blood and urine data it was estimated that the amalgam-related inhalation and ingestion of Hg species were within the limits proposed by WHO, ATSDR and EPA. The integrated daily Hg dose absorbed from amalgam was estimated up to 3 microg for an average number of fillings and at 7.4 for a high amalgam load.

CONCLUSIONS

This is the first study on adult amalgam patients which continuously monitored the postremoval decline of inorganic Hg and the coexposure from dietary organic Hg in a randomized-controlled-trial design. The integrated daily dose of 7.4 microg absorbed from a high amalgam load is well below the tolerable dose of 30 microg (WHO, 1990). The unexpected postremoval increase in erythrocyte organic Hg, which is associated with the depletion of cellular inorganic Hg, might result from binding of organic Hg to cellular sites previously occupied by inorganic Hg.

Mechanisms of mercury disposition in the body

Today the most widespread human exposures to mercury are to mercury vapor emitted from amalgam tooth fillings, to ethylmercury as a preservative in vaccines, and to methylmercury in edible tissues of fish. This review will focus on the mechanisms of transport of these three species of mercury. All three species are freely moveable throughout the body. Inhaled vapor in view of its physical properties as an uncharged atomic gas is believed to be transported by passive diffusion. Methylmercury and ethylmercury also move freely in the body. Methylmercury, and presumably its closely related chemical cousin ethylmercury, cross cell membranes as complexes with small molecular weight thiol compounds, entering the cell in part as a cysteine complex on the large neutral amino acid carriers and exiting the cell in part as a complex with reduced glutathione on endogenous carriers. The implications of these mechanisms with regard to biological monitoring are discussed.

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."

Accumulation of Inorganic Mercury in Hair of Rats Exposed to Methylmercury or Mercuric Chloride

The concentration of methylmercury (MeHg) in human hair is an excellent marker for its exposure, since a portion of MeHg is taken up from the blood circulation to the hair protein in a dose-dependent manner. However, a small portion of the mercury in human hair is found in the inorganic form, though the mechanism of its occurrence is not well established. Here, we examined the hair uptake of inorganic mercury in the rat. Male Wistar rats were exposed to MeHg (1 microg Hg/ml) or HgCl(2) (20 microg Hg/ml) for 84 days through drinking water. The hair, grown from 49 to 84 days, was collected from the MeHg-exposed rats, and the hair samples showed 54.5 microg/g hair of the total mercury concentration, 6.1% of which was in the inorganic form. The inorganic mercury in the plasma (0.022 microg/ml), which would probably be formed from MeHg in rat tissues, accounted for as high as 29% of the total mercury (0.076 microg/ml). The hair uptake rate of inorganic mercury estimated from the hair/plasma ratio was about 1/6 lower than that of MeHg. On the other hand, the total hair mercury level in the HgCl(2)-exposed group at the same time point was 2.86 microg/g, with the inorganic portion being as high as 62%. These findings suggest that the inorganic mercury is also taken up by rat hair from the blood circulation, as is the MeHg, irrespective of the consequences of the biotransformation of MeHg or exposure to inorganic mercury itself. Accordingly, a selective quantification of inorganic mercury in human hair may be useful in detecting inorganic mercury exposure.

Neurotoxicity of organomercurial compounds

Mercury is a ubiquitous contaminant, and a range of chemical species is generated by human activity and natural environmental change. Elemental mercury and its inorganic and organic compounds have different toxic properties, but all them are considered hazardous in human exposure. In an equimolecular exposure basis, organomercurials with a short aliphatic chain are the most harmful compounds and they may cause irreversible damage to the nervous system. Methylmercury (CH(3)Hg(+)) is the most studied following the neurotoxic outbreaks identified as Minamata disease and the Iraq poisoning. The first description of the CNS pathology dates from 1954. Since then, the clinical neurology, the neuropathology and the mechanisms of neurotoxicity of organomercurials have been widely studied. The high thiol reactivity of CH(3)Hg(+), as well as all mercury compounds, has been suggested to be the basis of their harmful biological effects. However, there is clear selectivity of CH(3)Hg(+) for specific cell types and brain structures, which is not yet fully understood. The main mechanisms involved are inhibition of protein synthesis, microtubule disruption, increase of intracellular Ca(2+) with disturbance of neurotransmitter function, oxidative stress and triggering of excitotoxicity mechanisms. The effects are more damaging during CNS development, leading to alterations of the structure and functionality of the nervous system. The major source of CH(3)Hg(+) exposure is the consumption of fish and, therefore, its intake is practically unavoidable. The present concern is on the study of the effects of low level exposure to CH(3)Hg(+) on human neurodevelopment, with a view to establishing a safe daily intake. Recommendations are 0.4 micro g/kg body weight/day by the WHO and US FDA and, recently, 0.1 micro g/kg body weight/day by the US EPA. Unfortunately, these levels are easily attained with few meals of fish per week, depending on the source of the fish and its position in the food chain.

Nitric oxide-independent effects of nitric oxide donors on energy metabolism in erythrocytes

In order to study the roles of nitric oxide (NO) in various biological events, several types of NO-releasing agents have been extensively used. Although both NO and its donors and/or their decomposed products may have biological activities, most of the cellular responses to these donors have been postulated to reflect NO-dependent events. Among the various NO donors, 1-hydroxy-2-oxo-3-(N-methyl-aminopropyl)-3-methyl-l-triazene (NOC7), 3-morpholinosydnonimine N-ethylcarbamide (SIN-1), S-nitrosoglutathione, S-nitrosocysteine (CysNO), and related nitrosothiols are commonly used agents. To investigate the biological activities of these donors and their decomposed products, we tested their effects on energy metabolism in erythrocytes. When incubated with freshly prepared erythrocytes, NOC7, Cys-NO, and their decomposed products, but not NO and its oxidized metabolites, nitrite and nitrate, decreased cellular ATP levels. Although SIN-1 generates both NO and superoxide radical thereby forming peroxynitrite (ONOO-), this donor had no appreciable effect on cellular ATP levels, even in the presence of superoxide dismutase. These results indicate that NOC7 and CysNO and/or their decomposed product(s), but not NO and its oxidized metabolites, are responsible for the decrease in cellular ATP levels. Thus, the effects of not only NO and its oxidized metabolites (NO2, NO3 ), but also NO donors and their decomposed products, should be taken into account when attempting to understand the mechanism of biological responses induced by NO donors.

Effect of thiol status on nitric oxide metabolism in the circulation

To elucidate the dynamics of nitric oxide (NO) metabolism in the circulation and its relationship with glutathione metabolism, formation of nitrosylhemoglobin (NO-Hb), S-nitrosothiols (RSNO), and nitrite+nitrate (NOx) was determined in blood samples from normal rats and animals that were treated with a loading dose of GSH or L-buthionine-[S,R]-sulfoximine (BSO), a specific inhibitor of GSH synthesis. When incubated in vitro with 0.2 mM NOC7, an NO donor, NO-Hb levels increased rapidly, peaked at 10 min, and decreased thereafter with a half-life of 35 min in blood samples from control, BSO-treated, or GSH-loaded animals. Levels of low-molecular-weight RSNO in plasma samples from the three animal groups also increased transiently, peaked at 10 min, and decreased thereafter. However, the amount of RSNO formed in GSH-loaded rat plasma was significantly greater than in control and BSO-treated animals. Plasma levels of NOx rapidly and similarly increased in all animal groups. Intravenously injected NOC7 also generated NO-Hb in circulating erythrocytes. In control animals, blood levels of NO-Hb increased maximally at 30 min and decreased thereafter with a half-life of 100 min. NO-Hb formed in the GSH-loaded group was significantly lower than in the control group. In contrast, the rate of NO-Hb formation was significantly higher with the BSO-treated group than with the control group. Although NOC7 did not affect the plasma levels of low-molecular-weight RSNO in plasma of both control and BSO-treated groups, it significantly increased RSNO in the GSH-loaded group. Thirty minutes after administration of NOC7, about 20% of the dose was recovered as plasma NOx in all animal groups. These results suggested that GSH status in animals might affect the metabolism of NO.

Prediction of uptake of methyl mercury by rat erythrocytes using a two-compartment model

The uptake of methyl mercury (MeHg) by isolated rat erythrocytes was studied at 37 degrees C using MeHg-cysteine (MeHgCySH), MeHg-glutathione (MeHgGSH), MeHg-mercaptalbumin (MeHgMASH) and the mixture of MeHgCySH with MeHgGSH, MeHgCySH with MeHgMASH, MeHgGSH with MeHgMASH at different MeHg concentrations. The measured MeHg concentrations were analyzed according to the Akaike's information criterion in order to determine the suitable compartment model. After determining a two-compartment model, a model-independent two-compartment model was developed from the kinetics of uptake of MeHg at a concentration of 1 mmol MeHg/l packed erythrocytes using MeHgCySH, MeHgGSH and MeHgMASH, respectively. The developed two-compartment model was validated by predicting the kinetics of uptake of MeHg by rat erythrocytes at different MeHg concentrations and different mixtures of MeHg-complexes. Then, the predicted values were compared with the measured values. The results suggested: 1) MeHg uptake appeared suitable to be described by a two-compartment model, while using MeHgGSH, MeHgMASH, MeHgCySH at lower concentrations and the mixtures of MeHg-complexes; 2) MeHgCySH uptake was slowest among three kinds of MeHg-complexes, although a postulated cysteine-facilitated MeHgCySH transport system might exist in erythrocyte membrane; 3) the mixture of MeHg-complexes might facilitate MeHgCySH uptake; 4) there might be a second MeHg intracellular compartment in rat erythrocytes.

Screening of potential transport systems for methyl mercury uptake in rat erythrocytes at 5 degrees by use of inhibitors and substrates

The current study was designed to screen the potential transport systems for methyl mercury (MeHg) uptake by isolated erythrocytes from rats at 5 degrees. Several inhibitors and substrates were used to test which potential transport system might be involved in MeHg uptake. Probenecid was used to test the organic anion transport system, valinomycin was used to test the effect of the membrane potential, D-glucose and cytochalasin B were used to test the facilitated diffusive D-glucose transport system and colchicine and vinblastine were used to test the microtubule system. The effects of Ca++, Mg++ and Na+ on MeHg uptake have been examined. Ouabain, ATP and glucose were used to test the active transport system, cysteine for the cysteine-facilitated transport system, glycine for system Gly, DL-methionine for system L, and MeHgCl and 4',4-diisothiocyano-2',2-stilbenedisulfonic acid (DIDS) for the Cl- ion transport system. The results showed that MeHg uptake might be involved in the following transport systems at 5 degrees: 1) organic anion transport system; 2) facilitated diffusive D-glucose transport system; 3) cysteine-facilitated transport system; 4) Cl- ion transport system. Moreover, the transport systems for MeHg uptake were sensitive to the membrane potential. Although the mechanisms of interaction of transport systems have not been fully clarified, evidence has been presented which support the existence of several simultaneous transport systems for MeHg uptake.

Effect of inhibitors and substrates on methyl mercury uptake by rat erythrocytes

Methyl mercury (MeHg) uptake by isolated erythrocytes from rats was studied at 20 degrees C. Inhibitors and substrates were used to test which transport system was involved in MeHg uptake. Ouabain and ATP were used to test the active transport system. Glycine was used to test system Gly. DL-Methionine was used to test system L. Cysteine was used to test the cysteine-facilitated transport system. The effects of Ca2+, Mg2+ and Na+ on MeHg uptake have been examined. MeHgCl and 4,4'-diisothiocyano-2,2'-stilbenedisulfonic acid (DIDS) were used to test C1- ion transport system. D-Glucose and cytochalasin B were used to test the facilitated diffusive D-Glucose transport system. Colchicine and vinblastine were used to test the microtubule system. Probenecid was used to test the organic acid transport system. Valinomycin was used to test the effect of the membrane potential on MeHg uptake. The results showed that MeHg uptake at 20 degrees C might be involved in the following transport systems: 1) an active transport system; 2) a cysteine-facilitated transport system; 3) a C1- ion transport system; 4) a facilitated diffusive D-glucose transport system; 5) an organic acid transport system. The transport systems for MeHg uptake were sensitive to the membrane potential.

Metabolism of mercury in hamster pups administered a single dose of 203Hg-labeled methyl mercury

Golden Syrian hamster pups were administered a single subcutaneous dose of 203Hg-labeled methyl mercury (MeHg), 0.4 nmol/g body weight, seven days after birth, and were sacrificed 2, 7, 14, 21 or 28 days later. The excretion of 203Hg followed a biphasic elimination pattern with an average half-time of 8.7 days for the rapid component. The slow component had a much longer half-time and probably reflects binding of 203Hg to growing hair. The concentration of 203Hg in the liver, kidneys and brain two days after administration was 0.44, 0.38 and 0.19 nmol/g, respectively. The retention of 203Hg was higher in the kidney than in the liver and the brain. The content of inorganic 203Hg in the liver and kidneys increased the first weeks after administration, demonstrating that hamsters are able to demethylate MeHg before two weeks of age.

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.

An explanation for strain and sex differences in renal uptake of methylmercury in mice

The present investigation was designed to elucidate the mechanism for strain and sex differences in renal methylmercury accumulation, in five mouse strains, viz. BALB/cA, C57BL/6N, CBA/JN, C3H/HeN and ICR. Strain and sex comparisons of factors which influence renal mercury accumulation were made. Strain and sex differences were observed in renal mercury accumulation 4 h after methylmercuric chloride (MMC) (1 mumol/kg, s.c.) injection. Glutathione (GSH) content in liver and kidney showed significant strain and sex differences. Pretreatment with 1,2-dichloro-4-nitrobenzene (DCNB), to deplete hepatic GSH without affecting renal non-protein thiol (NPSH) level, led to a dose-dependent decrease in hepatic and plasma GSH concentrations that correlated with decreased mercury levels in the kidney 10 min after MMC (1 mumol/kg, i.v.) injection. This indicates that hepatic and plasma GSH levels are related to mercury accumulation into the kidney. Renal gamma-glutamyltranspeptidase (gamma-GTP) activity significantly varied among the strains, and in BALB/cA and ICR, renal gamma-GTP activity in males was about 2-fold higher than that in females. Renal gamma-GTP activity was also correlated with the renal mercury content. These results suggest that strain and sex differences in renal accumulation of mercury are attributable to differences in tissue GSH content and possibly to differences in renal gamma-GTP activity.

Sex and strain differences of susceptibility to methylmercury toxicity in mice

Excretion and organ distribution of mercury and susceptibility to methylmercury (MeHg) toxicity were compared between strains and sexes after successive oral administration of MeHg chloride (5 mg/kg per day) using BALB/cA (C) and C57BL/6N (B6) mice. Every mouse died several days after initiation of toxic symptoms, and significant strain and sex differences were found with regard to length of survival. C mice of both sexes died earlier than B6 mice. B6 males survived much longer (greater than 6 weeks) than B6 females (3 weeks), whereas C males died earlier than C females. B6 male mice showed remarkably higher urinary Hg excretion and lower Hg levels in the brain, liver, kidney and blood than the other 3 groups. With daily MeHg administration, the Hg levels in all tissues except the kidney showed linear increase until the manifestation of toxic symptoms. Mercury accumulation in the kidney, the tissue with the greatest uptake of Hg in the mice examined herein, was biphasic: accumulation was rapid for 7-10 days after which the rate of increase was greatly reduced until death. It is suggested that conditions resulting in saturation of the rate of kidney Hg uptake might cause inhibition of urinary Hg excretion via some disturbance of renal function. Subsequently, Hg accumulation would be accelerated in various tissues, including the brain, leading to manifestation of toxic symptoms.

Accumulation of mercury in the anterior pituitary of rats following oral or intraperitoneal administration of methyl mercury

A sensitive histochemical technique has been used to visualize the ultrastructural localization of mercury in the anterior pituitary of rats which have been exposed to methyl mercury. After administration of methyl mercury in the drinking water (20 mg X l-1 methyl mercury in distilled water) or intraperitoneally (daily dose 100 ug or 200 ug methyl mercury) intracellular accumulations of mercury were found in the lysosomes and granules of secretory cells (somatotrophs, thyrotrophs and corticotrophs). In non-secretory cells (follicular cell and marginal layer cells) mercury deposits were found in lysosomes. In orally treated rats, the number of mercury deposits increased significantly with time up to day 21. In rats exposed intraperitoneally, a continuous increase was seen in intracellular mercury accumulation. Apart from vacuolation of lysosomes, no structural damage was observed in the cells containing mercury.

Alteration of putative amino acid levels and morphological findings in neural tissues of methylmercury-intoxicated mice

Methylmercury chloride was administered PO to male Kud: ddY mice at a dose of 5 mg/kg/day for 20 days. The contents of taurine, aspartate, glutamate, glycine, and gamma-aminobutyric acid were determined in tissue and crude synaptosomal (P2) fraction of cerebellum, cerebral cortex, and spinal cord of methylmercury-treated mice with or without ataxia. In the cerebellum of ataxic mice, increased levels of taurine and glycine were found in the tissue and P2 fraction, and increased levels of glutamate were found in the P2 fraction. In the cerebral cortex, the levels of gamma-aminobutylic acid decreased in the tissue and in the P2 fraction of ataxic mice, but increased levels were found in the tissue of non-ataxic mice. A decreased aspartate level in the cerebral cortex of ataxic mice and an increased taurine level in the cerebral cortex of non-ataxic mice were also found. In the spinal cord of ataxic mice, taurine increased in the tissue and in the P2 fraction. Glutamate level decreased in the spinal cord of ataxic mice, but increased in the P2 fraction of non-ataxic mice. Increased glycine levels in the P2 fraction of the spinal cord were also found in non-ataxic mice. Histologically, some degenerative changes were demonstrated in the cerebral and cerebellar cortices of ataxic mice. Such changes were also present to a mild degree in non-ataxic mice. In conclusion, methylmercury treatment altered the levels of putative neurotransmitter amino acids in neural tissue of mice. These alterations might be caused by specific neural cell dysfunction and could be related to the appearance of ataxia.

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 effect of methylmercuric chloride on arachidonic acid metabolism by platelet lipoxygenase

It has been reported that the addition of nonaggregating concentrations of collagen, epinephrine, thrombin or arachidonic acid to platelets potentiate their responses to subsequent subthreshold concentrations of aggregating agents. We have previously reported that low concentrations of methylmercury will potentiate human platelet ATP secretion and that this effect is blocked by aspirin. Using rat platelets we have examined the effect of low concentrations of methylmercury on the metabolism of arachidonic acid. Low concentrations of methylmercury (20-100 microM) inhibit washed platelet lipoxygenase activity. In experiments using platelet rich plasma a higher concentration of methylmercury was required to produce a comparable inhibition of arachidonic acid metabolism. At these concentrations no inhibition of platelet thromboxane synthetase was seen. These findings suggest that methylmercury potentiates platelet aggregation and secretion by reducing the inhibitory effect of 12 HPETE on thromboxane biosynthesis.

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.