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

Effects of lipophilic complex formation on the disposition of nickel in experimental animals

Dithiocarbamates, thiuram sulphides, xanthates, pyridinethiones and halogenated 8-hydroxyquinolines are groups of compounds which can form lipophilic complexes with Ni2+. These compounds are widely used as drugs and pesticides, and in industry. We have exposed rodents (mice, rats) and fish (brown trout) to substances belonging to these groups of compounds together with 63Ni2+ (as 63NiCl2) and then examined the uptake of the 63Ni2+ in the tissues of the animals. One dithiocarbamate--sodium diethyldithiocarbamate, which is used clinically in nickel carbonyl intoxications--was also examined with regard to effects on the tissue disposition of the metal in mice exposed to 63Ni(CO)4. The studies with 63Ni2+ showed that some of the complexing substances examined caused highly increased tissue levels of the metal in the animals. However, the enhancing effect varied with different complexing compounds. A facilitated penetration of the lipophilic 63Ni2+ complexes across the cellular membranes may underlie the increments in the tissue levels of the metal, but the effects on the disposition of the 63Ni2+ may vary depending on the lipophilicity and the stability of the complexes. In mice exposed to 63Ni(CO)4 by inhalation, sodium diethyldithiocarbamate decreased the levels of the metal in tissues such as the lung, brain and heart. These tissues are targets in nickel carbonyl intoxications and will attain high levels of the metal following inhalation of the compound. The nickel is present in nickel carbonyl as Ni0, but will be oxidized to Ni2+ in the tissues. The experiments presented here indicate that the diethyldithiocarbamate is able to reach and bind the intracellular Ni2+ in the critical target tissues and this property may underlie the ability of the compound to act as an antidote in nickel carbonyl intoxications. However, the ability of diethyldithiocarbamate to act as a nickel antidote may be limited to nickel carbonyl. Generally, the increased uptake of nickel induced by the compounds forming lipophilic complexes with the metal may imply risks of noxious combination effects.

Effects of lipophilic complex formation on the uptake and distribution of some metals in fish

Dithiocarbamates, xanthates, dialkyldithiophosphates and pyridinethiones are groups of compounds which can form lipophilic complexes with heavy metals. These compounds are widely used in industry and in agriculture and forestry and may pollute the aquatic environment. We have exposed fish (brown trouts) to substances belonging to these groups of compounds together with heavy metals (Cd2+, Ni2+, Hg2+, CH3-Hg+ or Pb2+) and then examined the uptake of the metals in the tissues of the fishes. Some of the examined complexing substances were found to cause highly increased tissue levels of the metals in the trouts. However, the enhancing effects varied for different complexing substances and for different metals. A facilitated penetration of the lipophilic complexes over the gill membranes and cellular membranes in other tissues may underlie the increments in the tissue levels of the metals. The lipophilicity and the stability of the complexes may be of importance for the effects of the substances on the disposition of the metals. Studies by other authors, in fishes as well as in other aquatic organisms, have also shown enhanced metal accumulation by compounds forming lipophilic complexes. It is considered that this type of interaction may have important implications for the behaviour of metals in aquatic ecosystems.

Cellular response after mobilization of metals by diethyldithiocarbamate in rat hepatocyte cultures

Cellular effects and mobilization of metals by diethyldithiocarbamate (DTC) were studied in primary cultures of rat hepatocytes incubated with lead acetate (PbAc), lead-diethyldithiocarbamate (Pb(DTC)2), cadmium chloride (CdCl2), cadmium-diethyldithiocarbamate (Cd(DTC)2), mercuric acetate (HgAc) and methylmercuric chloride (MeHgCl). In cells pretreated with inorganic Pb, Cd and Hg, the cellular levels of Cd and Pb were somewhat decreased or unchanged after incubation with DTC, while the cellular concentration of Hg was increased. Cells preincubated with MeHgCl showed a marked decrease in cellular Hg concentration upon DTC treatment. In Pb(DTC)2-treated cells the Pb concentration was increased when incubated with DTC, while DTC caused a decrease in Cd concentrations of cells preincubated with Cd(DTC)2. The activity of alcohol dehydrogenase (ADH) in cells incubated with CdCl2, Cd(DTC)2 and HgAc was significantly decreased. In Hg-treated cells the ADH activity was further decreased after incubation with DTC, whereas the activity in Cd-treated cells recovered gradually after incubation with increasing concentrations of DTC. The activity of the enzyme delta-aminolevulinic acid dehydratase (ALAD) was significantly inhibited in cells treated with PbAc and Pb(DTC)2, but could be restored to 80% and to almost 100%, respectively, of control activity after incubation with DTC. It is suggested that, in the absence of excess DTC, a decomposition of Pb(DTC)2 takes place after penetration into the cell, resulting in inhibition of ALAD by the released Pb. In the presence of excessive amounts of DTC, the equilibrium is shifted towards Pb(DTC)2 and Pb in the complex form is not available for ALAD. These studies suggest that DTC decreases cellular effects of Pb and Cd despite unchanged or even increased cellular concentrations of the metals, while the antidotal efficacy of DTC on inorganic Hg toxicity seems to be of low value.

Effects of diethyldithiocarbamate and tetraethylthiuram disulfide on zinc metabolism in mice

Chronic alcoholics with cirrhosis often develop symptoms of zinc deficiency. Tetraethylthiuram disulfide (TTD) is metabolized to two molecules of diethyldithiocarbamate (DDC). DDC chelates divalent metal ions, including zinc, by forming highly lipophilic neutral bis(dithiocarbamate)-metal complexes. DDC could therefore enhance the intestinal zinc uptake or increase the rate of zinc excretion. Accordingly, treatment of alcoholism with TTD could either aggravate or alleviate zinc deficiency. The present study investigated effects of DDC and TTD on intestinal zinc uptake and on the rate of zinc excretion in mice. When given as very high single oral doses, DDC and TTD increased the intestinal uptake of a single oral dose of zinc. When added to the diet and administered in lower doses, closer to those administered to humans for treatment of alcohol abuse, both compounds were without effect on the rate of excretion of the body's zinc stores. In a long-term experiment, where 65Zn was administered in the drinking water, these doses of TTD and DDC reduced the whole-body retention of 65Zn. No treatment changed the organ distribution of zinc in any of the experiments. In conclusion strong indications emerge from the present study that TTD treatment of alcoholism is more likely to reduce the intestinal zinc absorption than to enhance it as has been suggested by other authors. Thus, the widely used experimental model using single oral exposure to metal and chelator conceivably may give erroneous results, when used to predict effects of prolonged exposures.

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.

Species difference in biliary excretion of methylmercury

Species difference in the biliary excretion of methylmercury was studied in male rats, mice, rabbits and guinea pigs. The rates of mercury excretion (% dose/2 hr) into the bile of the rats, mice, rabbits and guinea pigs during the 2 hr from 2 to 4 hr after the administration of methylmercury were 0.61, 0.091, 0.036 and 0.019, respectively. These results suggest that biliary excretion and enterohepatic circulation of methylmercury in the latter three species may not influence the fate of this compound as significantly as in rats. Most of the methylmercury excreted into the bile of rats was bound to glutathione (GSH). In the mouse bile, 40% of the methylmercury was bound to GSH and the rest was found in a fraction eluted at the void volume of the Sephadex G-15 column. However, in the case of the rabbits and guinea pigs, methylmercury-GSH was scarcely detectable in the bile and almost all of the methylmercury was eluted at the void volume of the column.

The influence of some thiols on biliary excretion of methyl mercury

N-Acetylpenicillamine and thiola increased biliary excretion of methyl mercury and sulfhydryl right after administration. Cysteine increased excretion of methyl mercury in bile after a temporary decrease following administration. During the interval of decreased mercury excretion biliary excretion of cysteine passed through a maximum. This indicates the existence of a common factor of the excretory systems for cysteine and methyl mercury and illustrates that cysteine cannot carry methyl mercury from liver to bile. Relatively large proportions of unchanged thiola and N-acetylpenicillamine were excreted in bile. Bile collected after administration of one of these compounds, in addition to thiola or N-acetylpenicillamine, contained other methyl mercury carrying components not present in control bile. From the experiments undertaken it cannot be stated whether these components play any role in the increased excretion of methyl mercury in bile caused by thiola and N-acetylpenicillamine. The mechanisms of increased biliary excretion of methyl mercury following administration of N-acetylpenicillamine, thiola and cysteine are discussed.

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.

Evaluation of methyl mercury chelating agents using red blood cells and isolated hepatocytes

The relative efficacy of thiol-containing mercurial scavengers was assayed by using cellular suspensions of erythrocytes or isolated hepatocytes. The blood cells incubated in a buffer (pH 7.4) containing 1 mM glucose (10% hematocrit) were exposed to 5 microM methyl mercuric chloride. In the absence of extracellular thiols the red blood cells took up more than 90% of methyl mercury from the surrounding medium during 5--10 min. This uptake was almost completely inhibited by dimercaptosuccinic acid (DMSA) (1 mM) and the same chelant could rapidly remove 80% of the mercury from 'pre-loaded' erythrocytes. Hepatocytes prepared according to the method of Seglen [11] in a suspension of 10(6) cells/ml in a buffer containing 5 mM glucose and 5 mg/ml of bovine serum albumin were also exposed to methyl mercuric chloride (4 microM). Almost 50% of the mercurial was taken up by the cells slowly during the incubation period of 240 min. DMSA (1 mM) almost completely blocked the methyl mercury binding by the hepatocytes. 2-Mercaptopropionylglycin (Thiola) or mercaptosuccinic acid (MSA) was almost as effective mercurial scavengers as DMSA in hepatocytes and in red blood cells. Diethyldithiocarbamate (DDC) and dimercaptopropanol (BAL) were considerably less effective than DMSA to inhibit the mercurial binding to hepatocytes. Experiments in vivo have shown that DMSA is a better mercurial chelator than Thiola or MSA, whereas DDC and BAL may both be considered to be inapplicable in methyl mercury poisonings. Our cellular assay provides preliminary information of the efficiency of chelating thiols and may serve as a useful first approximation when planning further experiments.

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

The influence of selenium compounds on the biliary excretion and the organ distribution of mercury after injection of methyl mercuric chloride (4 mumol/kg) have been tested. Selenite, seleno-di-N-acetylglycine and seleno-methionine strongly inhibited the biliary excretion of mercury. Selenite even in a molar dose of 1/40 of the methyl mercury dose inhibited the biliary excretion of mercury. The less toxic seleno-di-N-acetylglycine was needed in larger molar doses and did not act as rapidly as selenite. Biliary excreted methyl mercury is known to be partly reabsorbed in the gut. Subsequently a part of it is deposited in the kidneys since drainage of the bile lowered the kidney content of mercury. Rats given selenium compounds in combination with bile drainage showed further reduction of the kidney mercury content than bile duct drainage alone. Thus the demonstrated lowering effect of selenium compounds on the kidney mercury content cannot be completely explained by an inhibition of biliary excretion of mercury. The mercury concentration in the brain was increased by the selenium compounds; the effect being dependent of the selenium dose reaching a maximum at an equimolar selenite--to methyl mercury dose ratio. The mechanisms by which selenium influences the methyl mercury kinetics are discussed.