Effects of coadministered low-molecular-weight thiol compounds on short-term distribution of methyl mercury in the rat

Following iv administration to the rat, methyl mercury was rapidly deposited in the liver, kidneys, and cerebrum. Methyl mercury concentrations in cerebrum 5 min after injection were about equal to the levels attained at 60 min post-treatment. At 5 min after dosing, concentrations of methyl mercury in plasma and tissues were directly proportional to the dose level of methyl mercury. Coadministration of equimolar amounts of l-cysteine increased short-term accumulation of methyl mercury in liver, kidneys, and cerebrum while reducing the levels of methyl mercury in plasma from those found after administration of methyl mercury alone. Equimolar doses of d-cysteine or d-penicillamine lowered plasma methyl mercury levels below those produced by injection of methyl mercury alone, but coadministered N-acetyl-l-cysteine or l-penicillamine did not. Each of these low-molecular-weight thiol compounds given in combination with methyl mercury increased deposition of methyl mercury in liver and kidneys. In addition, l-penicillamine also increased accumulation of methyl mercury in the cerebrum. These results suggest a critical role of plasma methyl mercury levels in the control of short-term methyl mercury distribution. Modification of the distribution pattern by coadministered low-molecular-weight thiol compounds further suggests that methyl mercury-thiol complexes may play a role in the tissue deposition process.     

Comparison of organic and inorganic mercury distribution in suckling rat

Thiomersal is used as a preservative in vaccines given to small children. The metabolic product of thiomersal is ethylmercury and its distribution and kinetics are still not known, especially at this early age. The purpose of this study was to compare the body distribution of two forms of mercury: organic (thiomersal) and inorganic (mercury(2+) chloride) in very young, suckling rats. Mercury was applied subcutaneously three times during the suckling period on days 7, 9 and 11 of pups age, imitating the vaccination of infants. A single dose of mercury was equimolar in both exposed groups, i.e. 0.81 micromol Hg kg(-1). At 14 days of age the animals were killed and the total mercury analysed in blood and organs (kidney, liver and brain). The analytical method applied was total decomposition, amalgamation, atomic absorption spectrometry. The results showed that the level of mercury was higher in the liver and kidney of the inorganic mercury group than in the thiomersal exposed group. However, the brain and blood concentrations of mercury were higher in the thiomersal exposed group. These results need to be clarified by additional data on the kinetic pathways of ethylmercury compared with inorganic mercury.     

无机汞对大鼠脑单胺类神经递质代谢的影响

用大鼠脑组织匀浆、游离脑细胞和脑细胞亚组分作实验材料，较全面地观察了氯化汞对单胺类神经递质含量及其合成、释放、摄取、降解等重要代谢环节的影响。结果表明，汞通过抑制脑线粒体外膜单胺氧化酶使得脑组织中５－羟色胺、多巴胺、去甲肾上腺素含量增高，５－羟吲哚乙酸含量下降，呈良好的剂量－效应关系；汞还明显改变了游离脑细胞对单胺类神经递质的释放和摄取能力。     

The excretion and distribution of inorganic mercury in the rat as influenced by several chelating agents

Among 15 chelating agents tested, sodium-2,3-dimercaptopropane-1-sulfonate (DMPS), 2,3-dimercaptopropanol (BAL), sodium-mercaptoethyliminodiacetate (MEIDA), and D-penicillamine (PA) exerted an influence on the excretion of Hg and its distribution in the organs. The excretion pattern, however, is different for these compounds, and, from the practical point of view, a favourable effect is exhibited only by DMPS which enhances the urinary excretion rate and lowers the Hg-concentration in all organs.     

Intestinal absorption of inorganic mercury in rat.

203Hg-Mercuric chloride was administered intragastrically to female rats. The absorption rate evaluated for a broad range of doses was found constant for low and medium range, and higher for high doses. Mercury was determined in internal organs and intestines. The time-course of intestinal mercury indicated that a deposit formed initially in the mucosa was further absorbed into the circulation. No indication was found of a protection mechanism based on exfoliation of the mucosal deposits of metal.     

Intracellular distribution of inorganic and organic mercury in rat liver after exposure to methylmercury salts

The intracellular distribution of inorganic and organic mercury in rat liver after exposure to methylmercury salts has been reported. The results have been discussed comparing intracellular distribution of mercury after exposure to various mercury compounds. After exposure to methylmercury salts the lysosomes/peroxisomes contain the highest concentration of mercury followed by microsomes and mitochondria. Differences in distribution between various mercury compounds seem to be related to the stability of the carbon-mercury bond, lysosomes accumulating inorganic mercury by preference either injected as such, or released in vivo from the intact organomercurial. Toxicological and pharmacokinetic implications of these conclusions have been discussed.     

Effects of dietary lipids on whole-body retention and organ distribution of organic and inorganic mercury in mice.

The effect has been investigated of dietary lipids on the whole-body retention and organ distribution of organic and inorganic mercury in mice. A single oral dose of methylmercury chloride or mercuric chloride labelled with 203Hg was given to female NMRI mice fed semi-synthetic diets containing varying amounts (5, 10, 20 or 50%) of energy derived from lipid (coconut oil, soya oil, or cod liver oil). The whole-body retention and relative organ distribution of mercury depended on diet composition. Thus, a significant reduction of the whole-body retention of mercury was seen in mice fed a diet containing 50% cod liver oil compared with mice fed a diet containing 50% coconut oil. After oral administration of mercuric chloride the relative deposition of mercury in the kidneys increased while that in the liver decreased with increasing concentrations of soya oil or coconut oil in the diet. The whole-body retention of mercury after treatment with methylmercury chloride was significantly decreased in mice fed cod liver oil compared with mice fed coconut oil; there was no difference between mice fed cod liver oil and those fed soya oil. The relative disposition of mercury was significantly higher in all organs of mice fed a diet containing 20% energy from cod liver oil compared with mice fed a diet containing 20% energy from soya oil. The present study demonstrates that diet composition is of major importance to the toxicokinetics of methylmercury and mercuric mercury.     

Effects of inorganic mercury on reproductive performance of mice.

Effects of mercuric chloride (MC) on the reproductive performance of mice were evaluated. Both male and female mice were divided into four groups that were subsequently exposed to 0.00, 0.25, 0.50, and 1.00 mg/kg/day of MC, respectively. At the end of pre-mating dosing, males were paired with females receiving the same dose. Dosing continued for males throughout mating, while dosing in females continued throughout mating, gestation, and lactation. The males were necropsied at the conclusion of mating and the females were necropsied at the conclusion of lactation. Fertility indices, parturition, gestation, live birth litter size, survival indices, and implantation efficiency were recorded. Subsequently, these data were statistically analyzed. Fertility and survival indices were significantly reduced in the treated groups. Exposure of mice to MC did not affect their litter size. No evidence of mercury induced target organ toxicity was seen in either the clinical pathology parameters or histomorphologic evaluations. However, in MC treated females, ovary weights were significantly different from the control. There were no histomorphologic or clinical pathology effects induced by MC. These results suggested that oral exposure to 0.25-1.00 mg/kg/day of MC produced adverse effects on the reproductive performance of mice in the absence of overt mercury toxicity.     

Mechanisms of absorption of inorganic mercury from rat small intestine. I. Solvent drag effect on absorption of inorganic mercury

A correlation of mercury absorption with water absorption was investigated by using the perfusion of rat small intestine with buffers containing 10(-4) M HgCl2. With a decrease in the osmolarity of the buffers or with increases in the sodium ion and urea concentrations in the buffers, absorption of water and mercury and accumulation of mercury in the intestinal tissue increased, and the increases in mercury absorption and accumulation were found to correlate with the increase in water absorption. Both the decrease in osmolarity and the increase in sodium ion concentration increased mercury accumulation in the epithelial cell after the perfusion. But these changes did not alter the mercury distribution in subcellular fractions. The results suggest that the increase in water absorption due to the hyptonicity or the increase in concentration of sodium ion or urea increases the mercury absorption and accumulation in the epithelial cell without change in the distributional pattern of mercury in the cell.     

Effects of inorganic mercury on [3H]dopamine release and calcium homeostasis in rat striatal synaptosomes

Inorganic mercury (Hg2+) in vitro increases spontaneous transmitter release from nerve terminals. The mechanisms of action are not well understood but may involve alterations in intraterminal Ca2+ dynamics. In this study we describe actions of Hg2+ in vitro on isolated mammalian CNS striatal nerve terminals (synaptosomes). Cobalt (2 mM) completely blocked the effect of 2 microM Hg2+ on spontaneous [3H]dopamine release. Cadmium (100 microM) was equipotent to Co2+ in blocking depolarization-dependent [3H]dopamine release, but did not alter the 2 microM Hg2(+)-induced spontaneous [3H]dopamine release. Depolarization-dependent [3H]dopamine release was not altered by 5 microM Hg2+. It appears that the site of action of Hg2+ on spontaneous [3H]dopamine release is not the Ca2+ channel. The effects of Hg2+ on intraterminal ionized Ca2+ [( Ca2+]i) were evaluated using the Ca2(+)-specific fluorescent probe, fura-2. Hg2+ (1-8 microM) had no effect on [Ca2+]i in 1.2 mM Ca2(+)-containing buffers. In nominal Ca2+ media, 4 and 8 microM Hg2+ significantly decreased [Ca2+]i. Following exposure to 4 and 8 microM Hg2+ the quenching of extrasynaptosomal fura-2 by Mn2+ was increased, suggesting that Hg2+ facilitated the leakage of fura-2. This apparent leakage was probably due to a nonspecific increase in membrane permeability since 2 microM Hg2+ produced a Co2(+)-insensitive increase in [3H]deoxyglucose phosphate efflux. Hg2+ did not increase the leakage of either lactate dehydrogenase or soluble protein from synaptosomes. Hg2+ produced a concentration-dependent (1-8 microM) increase in 45Ca2+ efflux from superfused synaptosomes which was insensitive to blockade either by 2 mM Co2+ or by 100 microM Cd2+. These data suggest that the transmitter releasing action of Hg2+ involves interactions with sites that also interact with Co2+ but not with Cd2+. Furthermore, Hg2+ may have direct transmitter releasing actions (i.e., Ca2(+)-mimetic properties), as well as nonspecific actions on plasma membrane permeability which may not necessarily be linked to [3H]dopamine release.     

Identification and distribution of mercury species in rat tissues following administration of thimerosal or methylmercury

Methylmercury (Met-Hg) is one the most toxic forms of Hg, with a considerable range of harmful effects on humans. Sodium ethyl mercury thiosalicylate, thimerosal (TM) is an ethylmercury (Et-Hg)-containing preservative that has been used in manufacturing vaccines in many countries. Whereas the behavior of Met-Hg in humans is relatively well known, that of ethylmercury (Et-Hg) is poorly understood. The present study describes the distribution of mercury as (-methyl, -ethyl and inorganic mercury) in rat tissues (brain, heart, kidney and liver) and blood following administration of TM or Met-Hg. Animals received one dose/day of Met-Hg or TM by gavage (0.5 mg Hg/kg). Blood samples were collected after 6, 12, 24, 48, 96 and 120 h of exposure. After 5 days, the animals were killed, and their tissues were collected. Total blood mercury (THg) levels were determined by ICP-MS, and methylmercury (Met-Hg), ethylmercury (Et-Hg) and inorganic mercury (Ino-Hg) levels were determined by speciation analysis with LC-ICP-MS. Mercury remains longer in the blood of rats treated with Met-Hg compared to that of TM-exposed rats. Moreover, after 48 h of the TM treatment, most of the Hg found in blood was inorganic. Of the total mercury found in the brain after TM exposure, 63% was in the form of Ino-Hg, with 13.5% as Et-Hg and 23.7% as Met-Hg. In general, mercury in tissues and blood following TM treatment was predominantly found as Ino-Hg, but a considerable amount of Et-Hg was also found in the liver and brain. Taken together, our data demonstrated that the toxicokinetics of TM is completely different from that of Met-Hg. Thus, Met-Hg is not an appropriate reference for assessing the risk from exposure to TM-derived Hg. It also adds new data for further studies in the evaluation of TM toxicity.     

Effects of short-term administration of maleic hydrazide or hydrazine on rat hepatic microsomal enzymes

The herbicide maleic hydrazide (MH), in amounts greatly exceeding its residue level in foods, had no apparent effect on the hepatic microsomal enzyme system of Fischer rats. In vivo, up to 1 g/kg of MH did not affect hexobarbital sleep times (HST) and in vitro, 2 g/kg of the sodium salt of MH altered neither the metabolism of benzo(a)pyrene and aniline nor the activities of cytochrome b₅ and P-450. Hydrazine-SO₄, a frequent contaminant of commercial MH preparations, in moderately high doses prolonged HST and diversely altered metabolism in vitro. The effects of hydrazine impurities, if present, could be significant in toxicity tests where excessive doses of MH are administered.     

Localization of mercury in CNS of the rat. IV. The effect of selenium on orally administered organic and inorganic mercury

The distribution and exact cellular localization of mercury in the brain and upper cervical spinal cord of the adult male Wistar rat has been determined using the autometallographic silver-enhancement technique. A detailed atlas of mercury-containing nuclei following oral administration of HgCl2 (20 mg x liter-1 or CH3HgCl (20 mg x liter-1) was prepared. The effect of orally administered Na2SeO3 (2 mg x liter-1) on these patterns was investigated. In animals treated with CH3HgCl, sodium selenite induced a conspicuous increase in mercury staining of nerve cell bodies in specific areas of the central nervous system (CNS) including laminae III-VI in the cerebral cortex, thalamus, hypothalamus, and brain stem nuclei. In the cerebellum, the cortical Purkinje cells and nerve cells in the deep nuclei were targets for appreciable mercury accumulations after CH3HgCl. Again, these deposits were increased by coadministration of selenite. In the spinal cord following administration of CH3HgCl alone, staining was limited to the gray matter. The intensity of this staining was increased by selenite and deposits also appeared in the white matter. Mercury accumulations were present in scattered glia cells in the cuneate and gracile fasciculi. Treatment with HgCl2 alone or in combination with selenite yielded no staining of the Purkinje cells, nor did selenite result in an increase in the density of other stained cell bodies throughout the CNS, as was the case with organic mercury. The most intense neuronal staining was seen in sections taken from rats treated with a combination of CH3HgCl and selenite. Lesser staining was seen in neuroglia, ependymal, and choroidal cells. In the latter two cell types, staining intensity was unaffected by selenite treatment. In HgCl2-treated rats the same cell types were targets for mercury deposits although staining was to a significantly lesser degree. Concurrent treatment with selenite had no visible effect on the staining pattern. Ultrastructurally, the bulk of the mercury was located in lysosomes. Administration of CH3HgCl combined with selenite caused mercury to appear in the nuclei of neurons. Selenium treatment delayed the functional toxic effects of CH3HgCl. Sections prepared from animals treated separately with selenium or demineralized water (used as the solvent for all compounds) were devoid of mercury deposits.     

Mercury distribution in the rat brain after mercury vapor exposure.

Brown Norwegian rats were exposed to mercury vapor at a concentration of approximately 1 mg/m3 for 5 weeks 24 hr/day 7 days a week and 6 hr/day 3 days a week, respectively. The total mercury absorption was calculated to 264 and 35 micrograms per week and 100 g body weight. The mean blood mercury concentration was 0.25 +/- 0.03 and 0.09 +/- 0.01 microgram/g, and the total concentration in the brain was 5.03 +/- 0.73 and 0.71 +/- 0.10 microgram/g tissue, respectively. The mercury distribution in the brains was examined using a method based on chemographic principles. Mercury was found primarily in the neocortex, in the basal nuclei, and in the cerebellar Purkinje cells. This distribution pattern corresponded to the pattern of inorganic mercury described after exposure to methyl mercury. Distribution of mercury after administration of different mercury compounds is discussed.     

Binding of inorganic mercury by subcellular fractions and proteins of rat kidneys

Inorganic mercury, administered to rats in a single dose of 0.5 mg Hg/kg is accumulated in the kidneys mainly in the soluble (54%) and nuclear (30%) fractions, showing decreasing tendency with time. Mitochondrial and microsomal fractions, initially accumulating approx. 11 and 6% of total Hg, show a tendency to increase the absolute level of Hg for the first week after administration. In the soluble fraction low-molecular weight, metallothioneinlike proteins are mainly responsible for the accumulation of mercury, in other fractions proteins of higher molecular weight prevail.This work was supported by the Polish-American agreement No. 05-009-2, with National Institute for Occupational Safety and Health, PHS, USA     

Degradation of methyl and ethyl mercury into inorganic mercury by hydroxyl radical produced from rat liver microsomes

Liver microsomes were prepared from Wistar rat by the Ca²⁺ aggregation method. Under various conditions, ethyl mercury chloride (EtHgCl) or methyl mercury chloride (MeHgCl) was incubated with the microsomal preparations. After the incubation, the amounts of inorganic Hg and hydroxyl radical (·OH) in the preparations were determined. Although the preparations alone produced a small amount of inorganic Hg and ·OH, the addition of NADPH to the preparations increased both inorganic Hg and ·OH production, which were further accelerated by the addition of KCN. The addition of Fe(III)EDTA, a ·OH formation promoter, to the microsome-NADPH-KCN system increased inorganic Hg production, whereas the addition of diethylenetriamine pentaacetic acid, a ·OH formation inhibitor, decreased inorganic Hg production. When ·OH scavengers such as mannitol and dimethyl sulfoxide were added to this system, the inorganic Hg production decreased. These results suggested that the ·OH produced from liver microsomes was responsible for the degradation of MeHg and EtHg. Since both ·OH and inorganic Hg production decreased with a concomitant decrease in NADPH-cytochrome P-450 reductase activities, it is suggested that this enzyme may be involved in the microsomal degradation of MeHg and EtHg.