The effect of selenium on the biliary excretion and organ distribution of mercury in the rat after exposure to methyl mercuric chloride

Selenomethionine protects against neuronal degeneration by methylmercury in the developing rat cerebrum

Although many experimental studies have shown that selenium protects against methylmercury (MeHg) toxicity at different end points, the direct interactive effects of selenium and MeHg on neurons in the brain remain unknown. Our goal is to confirm the protective effects of selenium against neuronal degeneration induced by MeHg in the developing postnatal rat brain using a postnatal rat model that is suitable for extrapolating the effects of MeHg to the fetal brain of humans. As an exposure source of selenium, we used selenomethionine (SeMet), a food-originated selenium. Wistar rats of postnatal days 14 were orally administered with vehicle (control), MeHg (8 mg Hg/kg/day), SeMet (2 mg Se/kg/day), or MeHg plus SeMet coexposure for 10 consecutive days. Neuronal degeneration and reactive astrocytosis were observed in the cerebral cortex of the MeHg-group but the symptoms were prevented by coexposure to SeMet. These findings serve as a proof that dietary selenium can directly protect neurons against MeHg toxicity in the mammalian brain, especially in the developing cerebrum.

Effects of methylmercury and organic acid mercurials on the disposition of exogenous selenium in rats

Interaction of methylmercury (MM), an environmental and industrial toxicant, with selenium is well known but incompletely understood. Therefore, the effects of MM (10 micromol/kg i.v.) on the disposition of exogenous selenium were compared with those of other organic mercurials (merbromine, mercuribenzene sulfonic acid, and mercuribenzoic acid) in anesthetized bile duct-cannulated rats injected with sodium [(75)Se]selenite (10 micromol/kg i.v.). The mercurial organic acids (10 micromol/kg i.v.) differed strikingly from MM in their influence on selenium disposition. They promoted renal and hepatic accumulation as well as biliary excretion of selenium but decreased distribution to the muscle, testis, and brain as well as the pulmonary excretion of selenium. In contrast, MM altered selenium distribution in an opposite fashion: it diminished the biliary output of selenium and enhanced selenium exhalation. GC-MS analysis verified that this latter paradoxical effect resulted from increased exhalation of dimethyl selenide. Further studies indicated that the MM-induced increase in pulmonary excretion of dimethyl selenide cannot be due to a diminished conversion of this volatile selenium compound to trimethylselenonium ion (TMSe(+)), because MM influenced neither the urinary excretion nor the hepatic and renal concentration of TMSe(+) in selenite-injected rats. Compared to the selenite-exposed rats, the selenite plus MM-injected animals exhibited a significant rise in the hepatic level of S-adenosylmethionine (SAME), the endogenous methyl donor in selenium methylation, and the ratio of SAME to S-adenosylhomocysteine. Based on these and others' observations, it is hypothesized that MM may increase hepatic availability of SAME in selenite-dosed rats by counteracting selenite-induced inactivation of SAME synthetase, thereby facilitating SAME synthesis, and/or by acting as a methyl donor in formation of dimethyl selenide, thereby sparing SAME. In summary, the toxicologically and ecologically relevant interaction of MM and selenite is not mimicked by organic acid mercurials, possibly because it results in formation of lipophilic Hg- and Se-containing common compound(s) and because it also appears to involve methyl transfer from MM to selenium.

Inhibitory effect of selenium on biliary secretion of methyl mercury in rats

The inhibitory effect of sodium selenite on biliary secretion of methyl mercury was examined in rats. The biliary secretion of methyl mercury in rat treated with 1 mumol/kg of methyl mercury was significantly decreased by administration of selenite at doses of 0.05 mumol/kg or higher. In rats given 10 mumol/kg of methyl mercury, marked depression of biliary secretion of mercury was observed when selenite was injected at a dose of 0.2 mumol/kg. On the other hand, secretion of substantial amounts of selenium was observed when biliary secretion of mercury was depressed. When the concentration of selenium in the bile was higher than 5 nmol/ml, biliary secretion of mercury was markedly depressed independently of the dose of methyl mercury administered (1 mumol/kg or 10 mumol/kg). These results suggest that the degree of inhibitory effect of selenite may be determined by the selenium concentration in the liver or the bile after treatment with selenite rather than the molar ratio of the dose of methyl mercury and selenite. We concluded that the decrease in biliary secretion of methyl mercury induced by selenite may result from inhibition of pathway for secretion of methyl mercury from liver to bile rather than the direct formation of a complex between methyl mercury and selenium. Methyl mercury has been considered to be secreted from liver to bile as a complex with glutathione (GSH). However, administration of selenite did not affect biliary secretion of GSH or hepatic glutathione S-transferase activity. Moreover, gel filtration of liver cytosol demonstrated that the distribution pattern of hepatic methyl mercury between macromolecules and GSH was not significantly changed by administration of selenite. These results suggest that selenite does not affect complex formation of methyl mercury with GSH at least in the liver. Selenite might specifically inhibit the activity of the canalicular transporter(s) which transport complexes of methyl mercury and GSH from the liver to bile.

The effect of selenium on the localization of autometallographic mercury in dorsal root ganglia of rats

The autometallographic technique was used to demonstrate the localization of mercury in dorsal root ganglia of adult Wistar rats. The animals were either exposed to mercury vapour, 100 micrograms Hg m-3, 6 h day-1, 5 days per week, or treated with organic mercury in the drinking water, 20 mg CH3HgCl per litre, for 4 weeks. The effect of orally administered sodium selenite on the pattern of intracellular distribution of mercury in these two situations was investigated. In rats exposed to mercury vapour alone, faint staining was present in ganglion cells. The selenite induced a conspicuous increase in the number of stained cells and in the intracellular staining intensity. In rats treated with organic mercury, mercury deposits were detected within ganglion cells and macrophages. The number of mercury-containing cells was increased by co-administration of selenite. In addition, satellite cells, the capsule and vessel walls were faintly stained. Twenty weeks after cessation of the organic mercury treatment, mercury staining was reduced. Again, selenite treatment enhanced staining intensity. When studied using the electron microscope, mercury was restricted to lysosomes, irrespective of treatments. The present study shows that the deposition of autometallographic mercury in the dorsal root ganglia depends on the chemical type of mercury, the co-administration of selenite and the length of the survival period.

Effect of long-term sodium selenite supplementation on levels and distribution of mercury in blood, brain and kidneys of methyl mercury-exposed female mice

Female Balb/c CA mice were supplemented for seven weeks with 0, 0.6 and 3 p.p.m. Se in tap water and were then exposed to a single oral dose of Me203Hg (2 mumol/kg). Se supplementation continued for 56 days after MeHg dosage. Supplemented animals showed enhanced activity of glutathione peroxidase in the blood. Twenty-four hr after MeHg dosage, the level and distribution of Hg in blood, blood cells, and kidneys were not influenced by Se exposure. However, in the brain the Hg accumulation was increased and Hg distribution was altered by Se supplementation. Fifty-six days after MeHg dosage, 70% to 80% of the dose had been eliminated from the body, and the brain of the 3 p.p.m. group still had a higher Hg level than the control group. Otherwise, there was no consistent effect of Se supplementation on retention of Hg in the animals. It is indicated that Se influences tissue accumulation and intracellular distribution of Hg through tissue-specific mechanisms rather than through a more general effect on Hg sequestration and transport in the blood.

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.

The hepatic glutathione system--influences of xenobiotics

The hepatic glutathione (GSH) system and the influences of xenobiotics have been reviewed. Key steps in the regulation of hepatic GSH are GSH biosynthesis, the GSH-peroxidase/reductase cycle, the cystathionine pathway, and the carrier-mediated export processes. Influences of xenobiotics on these different pathways are discussed. Xenobiotics may lead to liver injury after biotransformation to highly reactive electrophilic metabolites (mainly cytochrome P-450 mediated), which easily conjugate with GSH, thus producing GSH depletion. This GSH depletion and probably an additional loss of protein sulfhydryl groups cause a disturbance of the intracellular calcium homeostasis which leads to an irreversible cell injury. The different acinar distribution of cytochromes P-450 and of GSH and GSH-related detoxication pathways points to a greater susceptibility of perivenous hepatocytes to xenobiotic-induced damage. Also, the intracellular compartmentation of GSH is important for the understanding of hepatocellular injury induced by several xenobiotics.

Mercury-selenium interaction: distribution and excretion of 203Hg2+ in rats after simultaneous administration of selenite or selenate

In female rats intravenously injected with 203HgCl2 (0.6 mg Hg2+ per kg body wt.) the effect of intraperitoneal administration of selenite or selenate (0.525 mg Se per kg body wt.) on distribution and excretion of 203Hg was studied. The content of 203Hg was lower in kidney and higher in liver and blood in the groups treated with selenate or selenite when compared with rats which received only mercury. The brain content of 203Hg was significantly increased in rats injected with selenite. Both selenium compounds injected immediately after mercury significantly decreased urinary as well as biliary excretion of 203Hg. A transient increase in the rate of biliary excretion of 203Hg during the first 2 h after administration was observed in rats treated with selenate. This finding seems to support the idea that the reduction of selenate to selenite in the body is not rapid but takes at least several hours.

A kinetic analysis of the interaction between methylmercury and selenium

Guinea pigs were given radiolabelled methyl(203)mercuric chloride at a dose of 3 mg/kg p.o. alone or with an equimolar dose of sodium selenite, every second day for 3 weeks (10 doses). Whole-body mercury levels were measured during the course of the study and multi-compartment analysis carried out by computer least-squares curve fitting. Results indicated that mercury given alone behaved according to a single compartment model with half-life of 23.6 days, while in the presence of selenium a two-compartment behaviour resulted with one clearing rapidly (half-life 8.7 +/- 5.3 days) and the other clearing more slowly (half-life 40.8 +/- 13.0 days). Selenium decreased excretion of mercury in feces (2-fold) and urine (7-fold). Approximately 70% of the total mercury in the feces and 90% of that in the urine was in organic form. Tissue distribution of total, organic and inorganic mercury was determined 1, 14 and 28 days after the final dose. Selenium decreased the levels of mercury 1 day after the final dose, but produced a slower clearance after that. In all organs examined most of the mercury was in the organic form, except in the kidney which had over 70% inorganic mercury. Selenium increased the relative amounts of inorganic mercury in the liver, spleen, pancreas, large and small bowels, but not in the kidney.

Increased urinary excretion of selenium among workers exposed to elemental mercury vapor

The content of selenium and mercury in urine was measured in 28 male workers exposed to Hg0 and in 21 unexposed male controls. The first group excreted significantly more selenium into urine as compared with the control group. No significant correlation between mercury and selenium excretion was found.

Chronological relationship between neurological signs and electrophysiological changes in rats with methylmercury poisoning - special reference to selenium protection

In order to see chronological relationship between electrophysiological changes and "early" neurological sign (tail rotation) elicited in rats poisoned with methylmercury, we made serial measurements of amplitude of compound action potential and sensory nerve conduction velocity of the tail nerve in rats with five dose schedules [methylmercury vs selenium, (1)20 ppm:0.1 ppm, (2)20: 0.3, (3)20: 0.6, (4)10: 0.1, (5)10: 0.6]. We observed the following sequence in the onset of neuro-electro-physiolo-somatic signs: fall in compound action potential greater than decrease in sensory nerve conduction velocity greater than tail rotation greater than weight loss. Protective potency of dietary selenium against neurotoxicity of methylmercury was observed with regard to both electrophysiological changes and neurological signs.

The many faces of methylmercury poisoning

Methylmercury (MM) is a very potent neurotoxic agent. Its role in polluting the environment is well documented. A vast amount of study over the past several decades has finally provided insight into many aspects of its effect. Exposure to MM may be through ingestion of poisoned fish or inadvertent misuse of grain treated with the poison as a fungicide. Major epidemics have occurred in Japan (Fetal Minamata disease), Iraq, Pakistan, Guatemala, and Ghana. Sporadic incidences have occurred in the United States and Canada. There is no effective antidote to counteract the effect of MM on the central nervous system, although the information documented should provide hope for more effective therapy in acute cases.

Organ distribution and cellular uptake of methyl mercury in the rat as influenced by the intra- and extracellular glutathione concentration

Intravenous administration of CH3HgCl (4 mumol/Kg) premixed with glutathione or cysteine (8 mumole/kg) to female rats caused a rapid uptake of mercury in the kidney and a depressed content in the liver and blood as compared to CH3HgCl given alone. GSH depletion in the tissues, produced by injection of diethylmaleate, DEM (3.9 mmole/kg) did not influence the kidney uptake of mercury from administered (CH3Hg+-GSH, whereas the uptake of injected CH3HgCl was depressed. Both GSH and cysteine (8 mumole/kg) promoted the biliary excretion of methyl mercury. In suspensions of rat erythrocytes and isolated hepatocytes, additions of GSH reduced the cellular uptake of CH3Hg+ from the medium, whereas this was increased in the hepatocytes by adding cysteine or methionine. Cysteine addition slightly reduced the uptake of CH3Hg+ in the erythrocytes. GSH-depletion as obtained by DEM pretreatment of the cells, reduced the Ch3Hg+ uptake into hepatocytes by 40%, in contrast to only a negligible effect on the erythrocytes. Our results support previous reports that a physiological CH3Hg+-GSH-complexation takes place intracellularly, at least in liver cells. Our results are furthermore consistent with the assumption that biliary excreted CH3Hg+-GSH, which can be reabsorbed, only to a limited extent is taken up by the liver, whereas this GSH-complexation and reabsorption is of importance for the Ch3Hg+-uptake in the kidneys.

Formation and possible role of bis(methylmercuric) selenide in rats treated with methylmercury and selenite

The metabolic fate of methylmercury after administration of [203Hg]-methylmercuric chloride in combination with sodium selenite was investigated in rats. Whole body autoradiography and radioassay showed that administration of selenite decreased the mercury concentration in the liver and kidney, and increased that in the brain. The rapid changes of methylmercury concentration in the tissues after selenite injection were accompanied by increase in mercury extractable with benzene at neutral pH. The maximum levels of benzene-extractable mercury in the blood, kidney and liver were attained 30 min after selenite injection and were 30, 23 and 8 percent, respectively, of the total mercury. Thin-layer chromatography showed that the benzene-extractable mercury was a complex of methylmercury with selenium, bis(methylmercuric) selenide. These findings indicate that selenite alters the distribution of methylmercury in the tissues by formation of a diffusible complex with methylmercury, bis(methylmercuric) selenide.

Hepatobiliary transport and organ distribution of silver in the rat as influenced by selenite

Bile from rats injected with 110mAgNO3 (1 micromol/kg) were fractionated on Sephadex G-15 revealing binding of silver to one high molecular weight substance and one low molecular weight substance eluting corresponding to the void volume and glutathione (GSH) respectively. Fractionation of AgNO3 and GSH mixed in vitro gave rise to a polynuclear complex and a 1 : 1 complex of Ag+-GSH which both eluted corresponding to silver in bile. Depletion of GSH in the liver by diethylmaleate (3.9 mmol/kg) caused a parallel decrease in the biliary excretion of both silver and reduced GSH. These findings support the hypothesis that silver is excreted into bile by a GSH-dependent mechanism most likely using GSH as a carrier molecule. Selenite (1 micromol/kg) inhibited the biliary excretion of silver while AgNO3 (1 mumol/kg) did not influence the excretion of selenium into bile. Pretreatment with selenite (1 micromol/kg) also caused a retention of silver (AgNO3, 1 micromol/kg) in the blood, kidney and brain. The liver content of silver was decreased and the organ to plasma ratio of silver was unchanged for erythrocytes, but decreased for the brain, kidney and liver, respectively. The effects caused by selenite are attributed to the formation of Ag2Se complexes which are nearly water insoluble and probably unavailable for biliary excretion. Selenium metabolites (GSSeSG, GSSeH) which are excreted into bile are probably not available for complexing with Ag+.

The influence of selinium on methyl mercury toxicity in rat hepatoma cells, human embryonic fibroblasts and human lymphocytes in culture

The effect of methyl mercury and two selenium compounds have been studied in cell cultures. Methyl mercury in concentrations above 1 microM had a pronounced inhibiting effect on the growth of rat Morris hepatoma cells. Glucose and lactate uptake in relation to cell protein was appreciably stimulated by the organic mercury compound. Selenite in low concentration (0.5 microM) and seleno-di-N-acetyl glycine in thousandfold higher concentrations offered considerable protection against these effects of methyl mercury. The same selenite concentration (0.5 microM), which did not affect cell growth, caused an appreciable protection against methyl mercury (6 microM), even if it was added 3 days after methyl mercury. The methyl mercury inhibited the growth of human embryonic fibroblasts and the DNA-synthesis in the human lymphocytes. However, no protective effect of selenite were observed in these cell types. These results suggest that selenium compounds exert their protective effect through cell specific processes rather than by a direct chemical reaction between selenite and methyl mercury.