Mercury compounds in the blood of rats treated with ethylmercuric chloride

Methylmercury-induced neurotoxicity and apoptosis

Methylmercury is a widely distributed environmental toxicant with detrimental effects on the developing and adult nervous system. Due to its accumulation in the food chain, chronic exposure to methylmercury via consumption of fish and sea mammals is still a major concern for human health, especially developmental exposure that may lead to neurological alterations, including cognitive and motor dysfunctions. Mercury-induced neurotoxicity and the identification of the underlying mechanisms has been a main focus of research in the neurotoxicology field. Three major mechanisms have been identified as critical in methylmercury-induced cell damage including (i) disruption of calcium homeostasis, (ii) induction of oxidative stress via overproduction of reactive oxygen species or reduction of antioxidative defenses and (iii) interactions with sulfhydryl groups. In vivo and in vitro studies have provided solid evidence for the occurrence of neural cell death, as well as cytoarchitectural alterations in the nervous system after exposure to methylmercury. Signaling cascades leading to cell death induced by methylmercury involve the release of mitochondrial factors, such as cytochrome c and AIF with subsequent caspase-dependent or -independent apoptosis, respectively; induction of calcium-dependent proteases calpains; interaction with lysosomes leading to release of cathepsins. Interestingly, several pathways can be activated in parallel, depending on the cell type. In this paper, we provide an overview of recent findings on methylmercury-induced neurotoxicity and cell death pathways that have been described in neural and endocrine cell systems.

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.

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.

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.

A study on the biochemical and biological behavior of methylmercury

The biochemical and biological behavior of methylmercury (MeHg) was investigated by measurement of MeHg release rate from erythrocytes (RBC) of selected animal strains and species, by measurement of the intracellular distribution of MeHg in RBC, and by measurement of the binding affinity of hemoglobin (Hb) for MeHg. Methylmercury chloride was used throughout the experiments. Significant strain and species differences were found in the release rate of MeHg from RBC of mice, rats, and man and in the distribution of MeHg in RBC. Significant correlations were found between the above two indexes and the brain/blood ratio of mercury concentration 24 hr after MeHg injection, ip. The affinity of Hb for MeHg was examined by ultrafiltration techniques and Scatchard plots. There were Hbs with only one type of binding site and others with two types of binding sites. Both sites were considered to be cysteinyl residues. Primary sites involved cysteinyl residues oriented externally at the outside of the alpha 1 beta 1 contact junction and cysteinyl residues in the junction, while secondary sites involved only cysteinyl residues in the junction.

Severe acute poisoning from the ingestion of a permanent wave solution of mercuric chloride

1 A woman developed severe mercury intoxication from ingestion of about 2.5 g of mercuric chloride. 2 Antidotal treatment with a dithiol (BAL i.m.) and a monothiol (tiopronin i.v.) was started promptly. 3 Dialysis treatment thereafter markedly increased the elimination of mercury, thus hastening recovery. 4 It is suggested that chelating agents associated with dialysis are an effective treatment for mercury poisoning.

Cell-mediated immunity in the chicken as affected by mercury

Delayed hypersensitivity to phytohemagglutinin and the splenomegaly assay, which is a graft-versus-host response, are accepted indicators of cell-mediated immunity in the chicken. These responses were assessed in chicks that had received either chronic (300 ppm Hg in the drinking water) or acute (five injections of 3 or 12 mg Hg/kg body weight) Hg treatments. Neither of the cell-mediated responses was affected significantly (less than or equal to .05) by the Hg treatments. This contrasts with previous work in our laboratory, which showed that chronic but not acute Hg treatments suppressed humoral immune responses. Cell-mediated immune responses in the chicken are apparently not as susceptible to the toxic effects of Hg as are humoral immune responses.

Mercury induced haematological and biochemical anomalies in Ophiocephalus (Channa) punctatus

The effect of LC50 (1.8 mg/l) and a sublethal (0.3 mg/l) concentration of mercuric chloride on the blood of a teleost fish, Ophiocephalus (Channa) punctatus (the freshwater murrel-soley) was observed at 96 h and 15 and 30 days. Haemoglobin and haematocrit decreased after exposure, but no marked alteration was observed in total plasma protein. Glucose, cholesterol, urea, sodium, chloride, calcium and phosphate increased after acute and chronic exposures. Glutamic-oxalacetic transaminase, glutamic-pyruvic transaminase and amylase activities increased after exposure to LC50 for 96 h and to the sublethal concentration for 15 and 30 days. No marked alteration was observed in acid phosphatase activity. Alkaline phosphatase activity decreased in acute exposure, and increased in chronic exposure. Cholinesterase activity decreased after both acute and chronic exposures.

The mobility of methylmercury in biological systems

Toxicology studies indicate that methylmercury in humans and other species is bonded to sulfhydryl ligands and that the methylmercury in such complexes is labile even though their thermodynamic stability is large. It is shown in this paper that bimolecular nucleophilic displacement of complexed ligand by sulfhydryl-deprotonated ligand is the major pathway for ligand exchange at physiological pH, while at the pH of the stomach the proton-assisted dissociation of the complex is the predominant means by which exchange occurs. The dynamic and equilibrium aspects of the distribution of methylmercury between chloride and sulfhydryl ligands under the solution conditions of the stomach are also considered with respect to a possible role for lipid-soluble CH3HgCl in the absorption of methylmercury from the stomach.

The effects of mercury ingestion on hepatic mitochondrial membranes of chicks

Cytochrome c oxidase activity was measured in livers of chicks receiving 0, 200,250 or 300 p.p.m. mercury (as HgC12) in the drinking water beginning at one week of age. In the first experiment treatment with 200 p.p.m. mercury for up to 4 weeks tended to increase the activity of cytochrome c oxidase and decrease the proportion of free (measured in untreated liver preparations) to total (measured in Triton X-100 treated liver preparations) enzyme activity as compared with controls. However, the differences, which appeared after 1 week of treatment were not always statistically significant. The fatty acid composition of mitochondrial lipids was not affected by treatment with 200 p.p.m. mercury for 4 weeks. In the second experiment, treatment with 300 p.p.m. mercury for 4 weeks resulted in significantly higher total cytochrome c oxidase specific activity and a significantly lower proportion of free to total enzyme activity as compared with controls. However, mitochondrial susceptibility to rupture by freezing and thawing was not affected. In the third experiment chicks were given 250 p.p.m. mercury in the drinking water from 1 through 9 weeks of age. The mercury-treated chicks exhibited severe tremors at 9 weeks while the controls were unaffected. Cytochome c oxidase total specific activity was significantly reduced in trembling chicks, and the proportion of free to total enzyme activity was significantly increased as was the susceptibility of isolated mitochondria to rupture by freezing and thawing. The data suggest that moderate levels of mercury administered over an 8 week period can affect adversely the integrity of mitochondrial membranes.

Mercury inhibition of avian fatty acid synthetase complex

(1) Subcutaneous or intra-abdominal injections of 8 mg of HgCl2/100 g body weight markedly depressed hepatic fatty acid synthetase activity of chicks at 1 h post-injection. The depression occurred despite the fact that the chicks continued to eat up until the time they were killed. Under these same conditions, the hepatic activity of acetyl-CoA carboxylase (EC 6.4.1.2) was not affected by HgCl2, while the activity of the mitochondrial system of fatty acid elongation was stimulated. (2) When 2-mercaptoethanol was included in the incubation medium for a highly purified preparation of fatty acid synthetase, 500 muM HgCl2 was required to show definite inhibition of the enzyme. When 2-mercaptoethanol was omitted, 50 muM HgCl2 was inhibitory and 100 muM HgCl2 abolished enzyme activity. (3) 2 mM dithiothreitol completely protected the purified fatty acid synthetase preparation from inhibition by 100 muM HgCl2. When dithiothreitol was added after the addition of enzyme to the mercury-containing medium, protection of the enzyme was not complete. (4) Dialysis of cytosol fractions from chicks injected with HgCl2 against 500 vol. of 0.2 M potassium phosphate buffer (pH 7.0) containing 1 mM EDTA and 10 mM dithiothreitol for 4 h at 4 degrees stimulated the fatty acid synthetase activity of the fractions. Dialysis of cytosol fractions from noninjected chicks under the same conditions was without effect on fatty acid synthetase activity. (5) These data support the hypothesis that the inhibitory effect of HgCl2 administered in vivo on hepatic fatty acid synthetase activity in chicks is mediated through the interaction of mercury with the sulfhydryl groups of the enzyme.

The erythrocyte transport and transfer of methylmercury to the tissues of the rainbow trout (Salmo gairdneri)

Methylmercury (MeHg) was found to be taken up rapidly and almost completely by trout red blood cells (RBC) both in vitro and in vivo. The binding of MeHg within the RBC was freely reversible both in vitro, as shown by the efflux of MeHg from RBCs suspended in protein solutions, and in vivo following intracardial (i.c.) injection of RBC-bound MeHg. Hemoglobin (Hb) appeared to be the main MeHg transport protein in trout blood since it bound 90% of whole blood Hg following an intragastric dose of Me203HgCl. MeHg, injected i.c. as MeHgS-cysteine, was found to be present in blood bound almost completely to hemoglobin 10 days post-injection. This suggests an ability of hemoglobin to compete for and bind MeHg bound to other sulfhydryl (-SH) compounds. The number of reactive -SH groups per molecule of trout Hb was determined to be 4 by amperometric titration with MeHgCl. The concentration of Hb reactive -SH groups in the trout RBC was calculated to be at least 20 mM. This accounts for the high affinity of the RBC for MeHg.

Mercury inhibition of fatty acid synthesis in chicks

Male chicks were fed a commercial ration and were given drinking water which contained 0, 50, 100, 150, 200 or 300 mug of mercury/ml as mercuric chloride from hatching to 3 weeks of age. In one experiment, the mercuric chloride was administered by injection into the abdominal cavity rather than in the drinking water. At 3 weeks the chicks were killed, and the livers were removed and weighed. The activity of fatty acid synthetase in the 800 X gav supernatant fractions of the liver homogenates and in vivo incorporation of [14C]acetate into liver and carcass fatty acids and respiratory 14CO2 was determined as indicated. Administration of mercury at a treatment level of 300 mug/ml of drinking water depressed growth, feed and water consumption, liver weight, hepatic fatty acid synthetase activity, and in vivo incorporation of [14C]acetate into liver and carcass fatty acids, and increased the production of respiratory 14CO2 as compared with controls. In experiments in which graded doses of mercury were administered, body weights, liver weights, and feed and water intakes of the chicks receiving 0, 50 and 100 mug of mercury/ml of drinking water were similar to each other, but these parameters were severely depressed by 200 mug of mercury/ml of drinking water. Mercury caused a dose-related decrease of fatty acid synthetase activity. Incorporation of [14C]acetate into carcass fatty acid was depressed by 50 and 200 mug of mercury/ml of drinking water; incorporation into liver fatty acids and production of respiratory 14CO2 was not affected by mercury. Intra-abdominal injection of 6 mg of mercury/100 g body weight (as mercuric chloride) into well alimented chicks depressed hepatic fatty acid synthetase activity at 1 h post-injection. The data are consistent with the hypothesis that a portion of the effects of mercury on fatty acid synthesis are direct rather than a secondary effect of inanition.