Species difference in biliary excretion of methylmercury

Toxicokinetics of methylmercury in diabetic KK-Ay mice and C57BL/6 mice

We compared the toxicokinetics of methylmercury (MeHg) in KK-Ay type 2 diabetic mice and C57BL/6J mice to evaluate how metabolic changes associated with diabetes affect MeHg toxicokinetics. A single dose of MeHg (0.2, 1, or 5 mg mercury/kg) was administered orally to 12-week-old KK-Ay and C57BL/6J male mice. Total mercury concentrations in plasma, blood cells, whole blood, and tissues (brain, kidneys, liver, and pancreas) were measured after 4, 7, 11, and 14 days. The volume of distribution/bioavailability and the elimination rate constant per day were higher in KK-Ay mice, while the terminal elimination half-life was lower in almost all samples of KK-Ay mice. The area under the curve was lower in all blood and almost all tissue samples from KK-Ay mice. Total clearance/bioavailability was lower in all blood and tissue samples of KK-Ay mice at all MeHg doses. These results indicate that MeHg is more rapidly absorbed by, and eliminated from, the blood cells, brain, liver, kidney, and pancreas of KK-Ay mice under the experimental conditions. Different patterns of tissue-to-plasma and tissue-to-whole blood partition coefficients suggest that notable differences in MeHg transfer between plasma and blood cells affect its distribution in tissues of the two mouse strains. These findings are useful to understand the selective distribution of MeHg to target organs and the sensitivity to MeHg in pathological states.

High level of methylmercury exposure causes persisted toxicity in Nauphoeta cinerea

Methylmercury (MeHg⁺) is a neurotoxicant abundantly present in the environment. The long-term effects of MeHg⁺ have been investigated in rodents, yet data on the long-term or persisted toxicity of MeHg⁺ in invertebrates is scanty. Here, we examined the acute, intermediate, and chronic effects upon dietary administration of MeHg⁺ in nymphs of Nauphoeta cinerea. Besides, the potential reversibility of the toxic effects of MeHg⁺ after a detoxification period was evaluated. Nymphs were exposed to diets containing 0 (control), 2.5, 25, and 100 μg MeHg⁺/g of diet for 10, 30, and 90 days. Additional groups of nymphs were fed with the same dose of MeHg⁺ for 30 days and then were subjected to a detoxification period for 60 days. The nymphs exposed to 100 μg MeHg⁺/g succumbed to a high mortality rate, along with multiple biochemical (increase of reactive oxygen species production and glutathione S-transferase activity, as well as decrease in the acetylcholinesterase activity) and behavioral alterations. We observed delayed mortality rate and behavioral alterations in nymphs exposed to 100 μg MeHg⁺/g for 30 days and subsequently subjected to 60 days of detoxification. However, the biochemical alterations did not persist throughout the detoxification period. In conclusion, our results established the persistent toxic effect of MeHg⁺ even after a prolonged detoxification period and evidenced the use of N. cinerea as an alternative model to study the toxicity of MeHg⁺.

Mercury exposure and elimination rates in captive bottlenose dolphins

Mercury concentrations in fish, faeces and exhaled air were investigated in order to evaluate total mercury exposure through the gut in captive bottlenose dolphin and excretion via intestine and pulmonary routes. Results showed that faeces account for elimination of 34-48% of dietary mercury; while only 0.9-1.2% of alimentary mercury is eliminated through exhaled air. The remaining 51.2-65.3% of ingested mercury, ranging approximately between 266 and 339 microg per day, is retained within the organism. The complexation of mercury with selenium, forming insoluble tiemannite granules, is discussed as an important mechanism, complementary to excretion, by which odontocetes are able to cope with elevated alimentary exposure to mercury.

Investigation of intracellular factors involved in methylmercury toxicity

Methylmercury is a known pollutant that causes severe central nervous system disorders. It is capable of passing through the blood-brain barrier and accumulates in cerebral cells. However, little is known regarding the mechanism of its toxicity at the molecular level. Using yeast cells, we searched for the genes involved in the expression of methylmercury toxicity, and found that genes encoding L-glutamine.D-fructose-6-phosphate amidotransferase (GFAT) and ubiquitin transferase (Ubc3) confer methylmercury resistance on the cells. It has also been shown that GFAT is the target molecule of methylmercury in yeast cells. These findings provide important clues about the mechanism underlying methylmercury toxicity in mammals.

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.

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.

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.

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

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

Toxicokinetics of mercuric chloride and methylmercuric chloride in mice

Future human exposure to inorganic mercury will probably lead to a few individuals occupationally exposed to high levels and much larger populations exposed to low or very low levels from dental fillings or from food items containing inorganic mercury; human exposure to methylmercury will be relatively low and depending on intake of marine food. Ideally, risk assessment is based on detailed knowledge of relations between external and internal dose, organ levels, and their relation to toxic symptoms. However, human data on these toxicokinetic parameters originate mainly from individuals or smaller populations accidentally exposed for shorter periods to relatively high mercury levels, but with unknown total body burden. Thus, assessment of risk associated with exposure to low levels of mercury will largely depend on data from animal experiments. Previous investigations of the toxicokinetics of mercuric compounds almost exclusively employed parenteral administration of relatively high doses of soluble mercuric salts. However, human exposure is primarily pulmonary or oral and at low doses. The present study validates an experimental model for investigating the toxicokinetics of orally administered mercuric chloride and methylmercuric chloride in mice. Major findings using this model are discussed in relation to previous knowledge. The toxicokinetics of inorganic mercury in mice depend on dose size, administration route, and sex, whereas the mouse strain used is less important. The "true absorption" of a single oral dose of HgCl2 was calculated to be about 20% at two different dose levels. Earlier studies that did not take into account the possible excretion of absorbed mercury and intestinal reabsorption during the experimental period report 7-10% intestinal uptake. The higher excretion rates observed after oral than after intraperitoneal administration of HgCl2 are most likely due to differences in disposition of systemically delivered and retained mercury. After methylmercury administration, mercury excretion followed first-order kinetics for 2 wk, independently of administration route, strain, or sex. However, during longer experimental periods, the increasing relative carcass retention (slower rate of excretion) caused the elimination to deviate from first-order kinetics. Extensive differences in the toxicokinetics of methylmercury with respect to excretion rates, organ deposition, and blood levels were observed between males and females.(ABSTRACT TRUNCATED AT 400 WORDS)

Methyl mercuric chloride toxicokinetics in mice. I: Effects of strain, sex, route of administration and dose

The toxicokinetics of methyl mercury is studied most intensively in the rat. However, the toxicokinetics of methyl mercury in man is closer to the toxicokinetics in the mouse. This study describes the effects of dose, route of administration, and strain and sex on the toxicokinetics of methyl mercuric chloride in mice. Half-time values, fractional whole-body retentions and relative organ distributions of mercury were compared after a single oral or intraperitoneal administration of methyl mercuric chloride. The intestinal absorption was almost complete in accordance with earlier published results. The route of methyl mercury administration did not affect the whole-body retention of mercury significantly, but male mice retained lower amounts of mercury than did female mice. The elimination of mercury was demonstrated to follow first order kinetics during the two week study period independently of administration route, strain or sex. An inverse relationship between administered dose and whole-body retention was observed and by indirect evidence demonstrated not to be caused by an effect on the intestinal uptake mechanism. Absorbed and retained mercury at day 14 was primarily deposited in the carcass, but major deposits were also found in liver, kidneys and intestinal tract. Dose and route of administration did not affect the relative organ distribution of mercury significantly. However, the relative kidney deposition in male mice was about twice that in females. A significant difference in whole-body retention of mercury was observed between different strains of inbred mice at day 14 after administration. The relative organ distribution of mercury also varied significantly between different strains of mice.

Species variations in biliary excretion of glutathione-related thiols and methylmercury

The biliary excretion of methylmercury is thought to be related to the biliary excretion of nonprotein thiols in rats. Species differences in biliary excretion of glutathione (GSH) and related thiols are unknown; therefore, the relationship between the biliary excretion of GSH-related thiols and methylmercury in five species was studied. The biliary excretion rate of GSH-related thiols and disulfides was 369, 192, 94, 50, and 19 nmol/min/kg for mice, rats, hamsters, guinea pigs, and rabbits, respectively. The main thiol in mouse, hamster, and rat bile was GSH, whereas guinea pig and rabbit bile contained mainly cysteinylglycine (Cys-Gly). The larger percentage of Cys-Gly in guinea pig and rabbit bile was correlated with their greater hepatic gamma-glutamyltranspeptidase (GGT) activity than that observed in the other species. The biliary excretion rate (nmol/min/kg) of methylmercury was approximately 0.8 in mice, rats, and hamsters compared to significantly lower rates in guinea pigs and rabbits (0.15 and 0.03, respectively). It is concluded that the species-specific composition of GSH-related thiols and disulfides in bile is related to species variations in hepatic GGT activity, and that the species variation in biliary excretion of GSH-related thiols does not entirely account for the species variation in methylmercury excretion, indicating other factors are also apparently involved in determining the rate of biliary excretion of methylmercury.

Possible role of hepatic glutathione in transport of methylmercury into mouse kidney

The mechanism of the renal uptake of methylmercury was studied in mice. Preadministration of 1,2-dichloro-4-nitrobenzene (DCNB), which is a reagent that depletes hepatic glutathione (GSH) without affecting the renal GSH level, 30 min before injection of methylmercury significantly decreased the renal accumulation of mercury. The renal accumulation of mercury in mice receiving methylmercury-GSH intravenously was significantly higher than that in mice receiving methylmercuric chloride. These results suggest the possibility that hepatic GSH, as a source of extracellular GSH, plays an important role in the renal accumulation of methylmercury. No significant difference in renal mercury accumulation between bile duct-cannulated mice and normal mice was observed, indicating that the enterohepatic circulation of methylmercury is not an important factor in the renal accumulation of methylmercury in mice. Pretreatment of mice with acivicin, a potent inhibitor of gamma-glutamyl transpeptidase (gamma-GTP), significantly depressed the renal uptake of methylmercury and increased the urinary excretion of GSH and methylmercury. In in vitro reactions, methylmercury-GSH was degraded into methylmercury-cysteinylglycine by gamma-GTP, and this product was then converted to methylmercury-cysteine by dipeptidase. These results suggest that methylmercury is transported into the kidney as a complex with GSH, and then incorporated into the renal cells after degradation of the GSH moiety by gamma-GTP and dipeptidase, although the methylmercury bound to extracellular GSH can be reversibly transferred to plasma proteins in the bloodstream.