[Effects of mercury compounds on growth, cell composition and glucose uptake in Tetrahymena pyriformis]

The effects of the mercuric compounds methylmercuric chloride (MMC) and mercuric chloride (MC) on growth, cell composition, and glucose uptake were studied in the ciliate protozoan Tetrahymena pyriformis GL. MMC was the most effective growth inhibitor of Tetrahymena among mercuric compounds and sulfhydryl inhibitors; it completely inhibited growth of Tetrahymena at concentrations of 1/30-1/3000 of the others. The growth inhibition of Tetrahymena was inversely proportional to concentrations of chemicals in medium. Particularly in the case of MMC and MC the relationship between the concentrations and growth were S-curved in shape. Contents of RNA and DNA in cells were shown to accumulate proportionally to the MMC concentration. Cell division was probably delayed by MMC. The value of the ratio of RNA to DNA content shown by DNA synthesis was inhibited more strongly than RNA at MIC/4 of MMC. MMC inhibited glucose uptake, and the inhibition was likely due to altered permeability of cell membranes. However, no difference was found in lipid compositions between control and MMC cells by thin-layer chromatography.

Mechanisms of methylmercury-induced neurotoxicity

Mercury in both organic and inorganic forms is neurotoxic. Methylmercury (MeHg) is a commonly encountered form of mercury in the environment. Early electrophysiological experiments revealed that MeHg potently affects the release of neurotransmitter from presynaptic nerve terminals. Recently, the hypothesis that these alterations may be mediated by changes in the intracellular concentration of Ca2+ ([Ca2+]i) has been supported. MeHg alters [Ca2+]i by at least two mechanisms. First, it disrupts regulation of Ca2+ from an intracellular Ca2+ pool and second, it increases the permeability of the plasma membrane to Ca2+. MeHg also blocks plasma membrane voltage-dependent Ca2+ and Na+ channels in addition to activating a nonspecific transmembrane cation conductance. Chronic MeHg exposure results in ultrastructural changes and accumulation of MeHg within mitochondria. In vitro, MeHg inhibits several mitochondrial enzymes and depolarizes the mitochondria membrane subsequently reducing ATP production and Ca2+ buffering capacity. Inhibition of protein synthesis is observed after in vivo or in vitro exposures of MeHg and may be an early effect of MeHg. Thus, the early cellular effects of exposure to MeHg are diverse and cell damage likely occurs by more than one mechanism, the effects of which may be additive or synergistic.

Mechanism of methylmercury efflux from cultured astrocytes

To study the mechanism of methylmercury (MeHg) efflux from the central nervous system cells, cultured astroglia obtained from neonatal rats were incubated with 10 microM MeHg-cysteine (CySH) for 30 min. After being washed four times, cells were incubated in Hg-free medium, and the release of MeHg from the cells was monitored. The amount of MeHg released in the medium approached a plateau level (ca. 31% of the loaded amount) at 4 hr. Treatment of the cells with a CySH precursor, 2-oxothiazolidine-4-carboxylic acid (OTC), resulted in a significant increase of cellular levels of CySH and glutathione (GSH). OTC also increased 1.5-fold the MeHg efflux from the loaded cells. Another GSH enhancer, GSH isopropyl ester, also stimulated MeHg export from the cells. Ion-exchange column chromatography using DEAE-Sephadex revealed that the MeHg metabolite thus released was exclusively MeHg-GSH conjugate, both with and without OTC. Since the MeHg efflux was suppressed significantly by the presence of probenecid, the efflux occurred via the probenecid-sensitive organic acid transport system. Even though the cellular GSH levels were depleted drastically by treatment with L-buthionine-(S,R)-sulfoximine (BSO), a considerable level (90% of the control) of Hg efflux was detected. Since neither GSH- nor CySH-MeHg was detected in the culture medium of the BSO-treated cells, GSH depletion may trigger some other secretion system(s) in the cells. These results suggest that conjugation with GSH is the major pathway for MeHg efflux in rat astroglia, and that elevation in the cellular GSH level would possibly be a logical therapy for MeHg poisoning, promoting the accelerated elimination of MeHg from the critical tissues.

The role of -SH groups in methylmercuric chloride-induced D-aspartate and rubidium release from rat primary astrocyte cultures

Methylmercuric chloride (MeHgCl) was shown to increase D-aspartate and rubidium (Rb; a marker for potassium) release from preloaded astrocytes in a dose- and time-dependent fashion. Two sulfhydryl (-SH) protecting agents: a cell membrane non-penetrating compound, reduced glutathione (GSH), and the membrane permeable dithiothreitol (DTT), were found to inhibit the stimulatory action of MeHgCl on the efflux of radiolabeled D-aspartate as well as Rb. MeHgCl-induced D-aspartate and Rb release was completely inhibited by the addition of 1 mM DTT or GSH during the actual 5 min perfusion period with MeHgCl (10 microM). However, when added after MeHgCl treatment, this inhibition could not be fully sustained by GSH, while DTT fully inhibited the MeHgCl-induced release of D-aspartate. Neither DTT or GSH alone had any effect on the rate of astrocytic D-aspartate release. Accordingly, it is postulated that the stimulatory effect exerted by MeHgCl on astrocytic D-aspartate release is associated with vulnerable -SH groups located within, but not on the surface of the cell membrane. Omission of Na+ from the perfusion solution did not accelerate MeHgCl-induced D-aspartate release, suggesting that reversal of the D-aspartate carrier cannot be invoked to explain MeHgCl-induced D-aspartate release. Omission of Ca2+ from the perfusion solution increased the time-dependent MeHgCl-induced D-aspartate release.

Effect of methyl mercury exposure on the uptake of radiolabeled inorganic mercury in the brain of rabbits

Exposure to mercuric compounds at high dose levels has previously been shown to alter the integrity and function of the blood-brain barrier in laboratory animals. In the present study, we have investigated the distribution of intravenously administered inorganic 203Hg in rabbits additionally exposed to MeHg. A single dose of 203HgCl2 was administered together with or 5 min. or 24 hr after administration of a single dose (10 or 37.5 mumol/kg b.wt.) of MeHg. In another experiment, 203HgCl2 was administered to rabbits subchronically exposed to MeHg (1 mumol/kg b.wt. daily for three weeks) 24 hr after cessation of treatment. The integrity of the blood-brain barrier was assayed by measuring the uptake of 203Hg in the brain, as the blood-brain barrier usually serves to exclude inorganic Hg from the brain. The concentration of 203Hg within the brain was similar in all MeHg-treated rabbits, corresponding to 0.02% of the administered dose, and not different from that of control animals. Under these conditions, no obvious damage to the blood-brain barrier by MeHg could be observed.

Phospholipase A2 stimulation by methyl mercury in neuron culture

In primary prelabeled cultures of cerebellar granule cells, methyl mercury (MeHg) induced a concentration- and time-dependent release of [3H]arachidonic acid. MeHg-induced [3H]arachidonate release was partially dependent on the extracellular Ca2+ concentration. MeHg at 10-20 microM also stimulated basal 45Ca2+ uptake after 20 min of incubation at 37 degrees C, and at 10 microM inhibited K+ depolarization-stimulated uptake. MeHg stimulated [3H]arachidonate uptake, but had no effect on the rate of phospholipid reacylation. Phospholipase A2 (PLA2) activation preceded cytotoxicity, but at higher concentrations of MeHg such dissociation was not evident. Inhibition of MeHg-induced PLA2 activation by 100 microM mepacrine failed to modify cytotoxicity. MeHg-induced lipoperoxidation, measured as the production of thiobarbituric acid-reacting products, was inhibited by alpha-tocopherol without inhibition of [3H]arachidonate release. The absence of alpha-tocopherol inhibition of MeHg-induced arachidonate release precludes a causal role for lipoperoxide-induced PLA2 activation in this system. Moreover, MeHg induced an increased susceptibility of unilamellar vesicles to exogenous PLA2 in the presence of low Ca2+ concentrations without evidence of lipid peroxidation. [3H]Arachidonate incorporation into granule neuron phospholipids was analyzed by isocratic HPLC analysis. Relatively high proportional incorporation was found in the combined phosphatidylcholine fractions and phosphatidylinositol. With MeHg, an increase in the relative specific activity of incorporation was found in the phosphatidylinositol fraction, indicating a preferential turnover in this phospholipid species in the presence of MeHg.

Effect of CH3HgCl and several transition metals on the dopamine neuronal carrier; peculiar behaviour of Zn2+

CH3Hg+ and metal ions inhibited the specific binding of (1-[2-(diphenylmethoxy)ethyl]-4-(3-phenyl-2-[1-3H]propenyl) piperazine) ([3H]GBR 12783) to the dopamine neuronal carrier present in membranes from rat striatum with a general rank order of potency CH3Hg+ > Cu2+ > Cd2+ > Zn2+ > Ni2+ = Mn2+ = Co2+, suggesting that -SH groups are chiefly involved in this inhibition. Five millimolar dithiothreitol reversed the rather stable block of the specific binding produced by Cd2+ or Zn2+. An increase in the concentration of Na+, or addition of either K+ or Ca2+ reduced the inhibitory effects of metal cations, except Cu2+. Zn2+ (3 microM) reduced the inhibitory potency of Cd2+ on the binding but was ineffective against CH3Hg+ and Cu2+. Zn2+ at 0.3 to 10 microM significantly enhanced the specific binding of [3H]GBR 12783 and [3H]cocaine by 42 to 146%. Zn2+ (3 microM) increased the affinity of all pure uptake inhibitors tested and of the majority of the substrates for the [3H]GBR 12783 binding site. Dissociation experiments revealed that Zn2+ both inhibited and enhanced the [3H]GBR 12783 binding by recognizing amino acids located close to or in the radioligand binding site. Micromolar concentrations of Zn2+ noncompetitively blocked the [3H]dopamine uptake but they did not modify the block of the transport provoked by pure uptake inhibitors. These findings suggest that Na+, K+, Ca2+ and metal ions could recognize some -SH groups located in the [3H]GBR 12783 binding site; low concentrations of Zn2+ could allow a protection of these -SH groups.

Methylmercury alters intrasynaptosomal concentrations of endogenous polyvalent cations

The effects of the neurotoxic organomercurial methylmercury (MeHg) on intrasynaptosomal polyvalent cation concentrations were examined using fura-2. In the presence of extracellular Ca2+ (Ca2+e), MeHg caused a concentration-dependent, biphasic elevation in the ratio of fluorescence intensity at the emission wavelength of 505 nm following excitation at 340 and 380 nm (340/380 nm ratio). The first phase was independent of Ca2+e and complete within 5 sec. The second phase was dependent upon Ca2+e and was not complete within 6 min. MeHg increased the synaptosomal membrane permeability to Mn2+, suggesting that the second phase was due to influx of Ca2+e. Ruthenium red (20 microM), mitochondrial depolarization (10 mM NaN3 plus 4 micrograms/ml oligomycin), thapsigargin (1 microM), or caffeine (40 mM) did not elevate [Ca2+]i or alter the response of the synaptosomes to MeHg. Upon closer inspection, we noticed that MeHg simultaneously increased the fluorescence intensity at the excitation wavelengths of 340 and 380 nm and at the Ca(2+)-insensitive excitation wavelength of 360 nm. Pretreatment of synaptosomes with the cell-permeant heavy metal chelator TPEN (50 microM) blocked the MeHg-induced elevations in the 360-nm intensity and the 340/380 nm ratio. TPEN given after MeHg reversed the elevations in the 360-nm intensity. The cell-impermeant heavy metal chelator DTPA (150 microM) had no effect. We conclude that MeHg disrupts polyvalent cation homeostasis by at least two mechanisms. The first involves release of endogenous non-Ca2+ polyvalent cations, while the second is due to increased Ca2+ permeability of the plasma membrane.

Methyl mercury, mercuric chloride, and silver lactate decrease superoxide anion formation and chemotaxis in human polymorphonuclear leucocytes

Cytotoxic effects of mercuric chloride, methyl mercury, and silver lactate on polymorphonuclear leucocytes have been examined by assaying superoxide anion formation capability and chemotaxis of metal-exposed cells. Both superoxide anion formation and chemotaxis were negatively affected by all three metal compounds. Both bacteriotoxic functions were affected in a dose-dependent fashion, the functional deficits were seen at doses not affecting cell viability. Dose-response curves were remarkably similar for all three compounds. The bacteriotoxic capacity of polymorphonuclear leucocytes may be hampered by mercury and silver.

Possible involvement of active oxygen species in selenite toxicity in isolated rat hepatocytes

Mechanisms of selenite cytotoxicity were examined using isolated rat hepatocytes. When selenite was added to a suspension of rat hepatocytes, intracellular reduced glutathione (GSH) was decreased and the oxygen consumption rate was increased. Subsequently, thiobarbituric acid-reactive substances (TBA-RS) and lactate dehydrogenase (LDH) leakage were increased. A ferric iron chelator, desferrioxamine (DF), and a synthetic superoxide dismutase (SOD) mimic, desferrioxamine manganese (DFMn), reduced the selenite toxicity. These results suggest that superoxide anion and its reactive metabolites such as the hydroxyl radical may be involved in the cytotoxicity of selenite.

Methyl mercury increases intracellular Ca2+ and inositol phosphate levels in cultured cerebellar granule neurons

In an effort to explain the previously observed methyl mercury (MeHg)-induced stimulation of protein phosphorylation in cerebellar granule neuron cultures, the effect of MeHg on protein kinase activities in cell-free assays and on second messenger systems in cultured neurons has been examined. Using cell-free assays for several protein kinases, no stimulation of enzyme activity was found at any concentration of MeHg tested. After 24 h exposure, 1-5 microM MeHg was found to have no significant effect on neuronal cyclic AMP levels. In contrast, intracellular levels of Ca2+ and rates of 45Ca2+ uptake were elevated 2.2-fold and 3.6-fold, respectively, by 5 microM MeHg. These effects were not observed with mercuric chloride, triethyllead, or lead acetate. Measurement of inositol phosphate production in granule cell cultures revealed a sensitive, pretoxic effect of MeHg with twofold stimulation following 30-min exposure to 5 microM MeHg and 1.6-fold after 24-h exposure to 3 microM MeHg. Detection of inositol phosphate production after 30 min of MeHg was largely neuron-specific. These results suggest that second messenger-mediated activation of select protein kinase enzymes may be the mechanism underlying MeHg-induced stimulation of protein phosphorylation in cerebellar neuronal culture. In addition, these findings indicate a specific interference with neuronal signal transduction and suggest a basis for the selective neurotoxic action of this agent.

Disruption by methylmercury of membrane excitability and synaptic transmission of CA1 neurons in hippocampal slices of the rat

In order to examine the effects of methylmercury (MeHg) on central synaptic transmission, field potentials were recorded in the CA1 neurons of hippocampal slices by using extracellular microelectrode recording techniques. After stimulation of Schaffer collaterals at low frequency (0.25 Hz), population spikes and excitatory postsynaptic potentials (EPSPs) were recorded at the cell bodies and apical dendrites of CA1 pyramidal cells, respectively. Antidromically activated population spikes were also recorded at the cell bodies of CA1 pyramidal cells by stimulating the alveus. Long-term potentiation (LTP) was induced by application of brief high-frequency stimulation (15 trains of four stimuli per train at 100 Hz) after 25 min of population spike baseline recordings. MeHg was applied to slices acutely by bath perfusion with artificial cerebrospinal fluid (ACSF). At 20-500 microM, MeHg significantly increased and then decreased the amplitudes of, or blocked, the population spikes, EPSPs, and antidromically activated population spikes. Time to increase and time to block of these field potentials were concentration-dependent. Exposure of slices to 4 microM MeHg for 180 min increased but did not reduce the amplitudes of population spikes, EPSPs, or antidromically activated population spikes. The effects of MeHg on population spikes induced by either orthodromic or antidromic stimulation were similar. In the absence of MeHg, application of high-frequency stimulation increased population spike amplitudes by 60-100%. This effect (LTP) could be sustained for more than 2 hr in the absence of MeHg. When 100 microM MeHg was applied concomitantly with high-frequency stimulation, the population spike amplitudes were increased by an additional 20-50% based on the already elevated population spike amplitude by high-frequency stimulation. Subsequently, population spike amplitudes were reduced and finally blocked in a manner similar to the effect of MeHg on population spikes recorded without high-frequency stimulation. Application of MeHg (100 microM) for 20 min prior to high-frequency stimulation did not prevent induction of LTP even though the population spike amplitudes had been decreased by more than 10% of the control level, suggesting that MeHg may not alter induction of LTP. Reversibility of the effects of MeHg was examined by washing slices with MeHg-free ACSF or 1 mM D-penicillamine for 60-120 min after MeHg treatment. Washing slices with MeHg-free ACSF caused at best partial reversal of effects of MeHg. D-Penicillamine, a chelator of MeHg, completely reversed the effects of MeHg on EPSPs but only partially reversed the effects of MeHg on population spikes, antidromically activated population spikes, and LTP.(ABSTRACT TRUNCATED AT 400 WORDS)

Comparison of the interaction of methyl mercury and mercuric chloride with murine macrophages

The toxicity of organic methyl mercury was studied on murine macrophages in cell culture and compared to that of inorganic mercuric chloride. Long-term treatment of macrophage cultures with methyl mercury resulted in decreased cell viability in a concentration-dependent fashion. Experiments showed that 20 microM methyl mercury was highly toxic, causing cell death within a few days, while cultures exposed to lower levels were less severely affected. Comparison of the toxicity of organic and inorganic mercury by cell viability showed no difference between equimolar concentrations of methyl mercury and mercuric chloride. Furthermore, protein synthesis (interferon-alpha/beta) was reduced in a concentration dependent manner and had the same reduced magnitude in cells treated with either methyl mercury or mercuric chloride. However, impairment of random migration and phagocytosis of macrophages appeared at lower concentrations in cells exposed to methyl mercury than in cells exposed to mercuric chloride. Electron microscopy of cells exposed to methyl mercury revealed mercury deposits in lysosomes and dispersed in the cytoplasm and nuclei. The present study shows that methyl mercury and mercuric chloride impair cell viability and protein production in cell cultures at equimolar concentrations, while methyl mercury inhibits macrophage functions such as migration and phagocytosis at lower concentrations than mercuric chloride.

Effects of methylmercury and trimethyltin on cardiac, platelet, and aorta eicosanoid biosynthesis and platelet serotonin release

The effects of two organometals on heart, platelet, and aorta prostaglandin biosynthesis were examined in vitro. Methylmercuric chloride (MMC, 39-796 nM) increased the biosynthesis of thromboxane A2 (TxA2) and prostacyclin (PGI2) in the heart and stimulated the biosynthesis of PGI2 in incubates of aorta rings. The aorta biosynthesis of PGI2 was monitored by its inhibition of platelet aggregation and serotonin [5-hydroxytryptamine (5-HT)] secretion, while the metabolites 6-keto-PGF1a and TxB2 were quantified by radioimmunoassay. In platelet experiments, low concentrations of MMC (5 microM) enhanced aggregation to adenosine diphosphate (ADP) and at high concentrations MMC (50-100 microM) directly stimulated aggregation and 5-HT secretion. These effects of MMC were inhibited by nonsteroidal antiinflammatory drug, thromboxane synthetase, and phospholipase A2 inhibitors. Trimethyltin (TMT), another highly toxic organometal, had no effect on prostaglandin biosynthesis in either heart or aorta incubates. TMT did not increase platelet aggregation responses to ADP, nor did it directly trigger aggregation. At moderate concentrations, TMT (20-45 microM) slightly depressed ADP aggregation; however, this was paradoxically associated with increased 5-HT secretion. In platelets pretreated with either NDGA or ASA, TMT in the absence of aggregation enhanced 5-HT secretion in response to ADP. TMT, unlike MMC, did not stimulate platelet TxB2 biosynthesis from exogenous [3H]arachidonic acid, whereas MMC stimulates heart, vascular, and platelet eicosanoid biosynthesis. TMT, unlike MMC, does not directly activate the arachidonic acid cascade.

Effects of inorganic and organic mercury on intracellular calcium levels in rat T lymphocytes

The importance of cytosolic free calcium level ([Ca2+]i) in lymphocyte activation prompted us to investigate changes in [Ca2+]i in T cells caused by mercury compounds, which have been shown to have immunomodulatory and immunotoxic properties. Using fura-2 as fluorescent Ca2+ indicator, we found that both methyl-mercury (MeHg; 0.02-2 microM) and inorganic mercury (HgCl2; 0.01-1 microM) increased [Ca2+]i in lymphocytes from rat spleen in a concentration-dependent manner. The effect of MeHg was rapid and the increase of Ca2+ level was sustained in time, while HgCl2 caused a slow rise in [Ca2+]i. The effects of mercury compounds did not appear to be associated with alterations of membrane integrity, since there was no significant difference in the extent of MnCl2 quench between control and mercury-treated cells. However, HgCl2 (1 microM) and MeHg (2 microM) appeared to cause membrane damage at longer incubation times (15 min). When cells were incubated in Ca(2+)-free medium (in the presence of 1 mM EDTA) MeHg still increased [Ca2+]i, though to a lesser extent, while HgCl2 had no effect. Heparin, an inhibitor of inositol 1,4,5-trisphosphate-induced Ca2+ mobilization partially blocked this rise of [Ca2+]i, while carbonyl cyanide m-chlorophenylhydraxone (CCCP), an inhibitor of mitochondrial function, had a lesser effect. When added together, heparin and CCCP almost completely block the response to MeHg. These results suggest that MeHg and HgCl2 exert their effects of [Ca2+]i in different ways: MeHg-induced increases in [Ca2+]i are due to influx from outside the cells as well as to mobilization from intracellular stores, possibly the endoplasmic reticulum, and, to a minor extent, the mitochondria; on the other hand, HgCl2 causes only Ca2+ influx from the extracellular medium.

Effects of methylmercury on perineurial Na+ and Ca(2+)-dependent potentials at neuromuscular junctions of the mouse

The ability of acute application of the neurotoxicant methylmercury (MeHg) to disrupt the function of presynaptic Ca2+ and Na+ channels at intact neuromuscular junctions was examined using mouse triangularis sterni motor nerves. In Ba(2+)-containing solutions, potential changes arising from Na+ and Ca2+ channel function could be recorded from the perineurial sheath surrounding motor neurons when K+ channels were blocked by tetraethylammonium chloride and 3,4-diaminopyridine. MeHg (100 microM) reduced both Na(+)- and Ba(2+)-dependent components to block within 3-5 min at apparently equivalent rates. Time to block was approximately 7 min after exposure to 50 microM MeHg. In 2 of 5 preparations exposed to 50 microM MeHg, the Ca2+ channel-mediated component was blocked prior to the Na+ channel-mediated component. In the remaining three preparations, Na(+)- and Ba(2+)-dependent potentials were blocked at similar times. Following block by MeHg, neither perfusing the preparation in MeHg-free solutions nor increasing the intensity and/or duration of stimulus to the intercostal nerves resulted in recovery of Na+ or Ca2+ potentials. In the presence of K+ channel blockers, repetitive firing of nerves in response to a single stimulus was observed in 20-30% of the triangularis preparations; in the two preparations treated with MeHg in which repetitive firing was observed, it decreased prior to block of the stimulus-induced Na+/Ba2+ potentials. These results corroborate the results obtained in isolated synaptosomes and pheochromocytoma cells, and suggest that MeHg decreases motor nerve excitability by disrupting Na+ channel function and may block neurotransmitter release by disrupting Na+ and Ca2+ channel function.

Monitoring and assessment of mercury pollution in the vicinity of a chloralkali plant. III. Concentration and genotoxicity of mercury in the industrial effluent and contaminated water of Rushikulya estuary, India

Aquatic mercury pollution of the Rushikulya estuary in the vicinity of the chloralkali plant at Ganjam, India was monitored over a period from October 1987 to May 1989. The concentrations of aquatic mercury in the water samples taken from the effluent channel and from different sites along the course of the estuary covering a distance of 2 km were periodically recorded and ranged from 0 to 0.5 mg/l. The bioconcentration and genotoxicity of aquatic mercury in the samples were assessed by the Allium micronucleus (MNC) assay. The frequency of cells with MNC was highly correlated not only with bioconcentrated mercury (root mercury) but also with the levels of aquatic mercury. The threshold assessment values such as effective concentration fifty (EC50) for root growth, lowest effective concentration tested (LECT), and highest ineffective concentration tested (HICT) for induction of MNC in Allium MNC assay for the present aquatic industrial mercury were determined to be 0.14, 0.06 and 0.02 mg/l, respectively.

Altered intrarenal accumulation of mercury in uninephrectomized rats treated with methylmercury chloride

We tested the hypothesis that the intrarenal accumulation of mercury in rats treated with methylmercury is altered significantly as a result of unilateral nephrectomy and compensatory renal growth. Renal accumulation of mercury was evaluated by radioisotopic techniques in both uninephrectomized (NPX) and sham-operated (SO) rats 1, 2, and 7 days after the animals received a nonnephrotoxic intravenous dose of methylmercury chloride (5 mg/kg Hg). At all times studied after the injection of the dose of methylmercury, the renal accumulation of mercury (on a per gram kidney basis) was significantly greater in the NPX rats than that in the SO rats. The increased accumulation was due to a specific increase in the accumulation of mercury in the outer stripe of the outer medulla. Renal cortical accumulation of mercury was similar in both the NPX and SO rats. The percentage of the administered dose of mercury that was present in the total renal mass of the NPX and SO rats ranged between 5 and 15, depending on the day that the renal accumulation was studied. Approximately 40-50% of the total renal burden of mercury in both the NPX and SO rats was in the inorganic form. However, only less than 1% of the mercury in blood was in the inorganic form at the three times accumulation was studied. Very little mercury was excreted in the urine by either the NPX or SO rats. Only about 2 to 3% of the administered dose of mercury was excreted in the urine in 7 days. By contrast, the cumulative fecal excretion of mercury over 7 days was substantial in the NPX and SO rats, and significantly more mercury was excreted in the feces by the NPX rats (about 19% of the dose) than by that in the SO rats (about 16% of the dose). In conclusion, our findings indicate that unilateral nephrectomy and compensatory renal growth cause a significant increase in the accumulation of mercury in the renal outer stripe of the outer medulla in rats exposed to methylmercury. In addition, the findings indicate that the fecal excretion of mercury is also significantly increased.

Enhancement of gamma-glutamylcysteine synthetase mRNA in rat kidney by methyl mercury

Glutathione (GSH), a major cellular antioxidant, is elevated 2- to 3-fold in kidneys of rats during prolonged treatment with mercury as methyl mercury hydroxide (MMH). Increased renal GSH is accompanied by a dose- and time-related elevation in the relative abundance of mRNA hybridizable to a cDNA probe which encodes renal gamma-glutamylcysteine synthetase (GCS), the rate-limiting enzyme in GSH synthesis. Renal GCS mRNA is maximally elevated 4.4-fold at 3 weeks following initiation of MMH treatment. Enhancement of GSH and GCS mRNA content corresponds to a relative sparing of renal cells from oxidative tissue damage during MMH exposure. These observations suggest that increased synthesis of GSH at the genetic level occurs as an initial adaptive response to mercury-induced oxidative stress in kidney cells.

Inhibition of mitochondrial Ca2+ release diminishes the effectiveness of methyl mercury to release acetylcholine from synaptosomes

The interaction of methyl mercury (MeHg) with nerve-terminal mitochondria as a potential mechanism for its effects on the release of acetylcholine (ACh) was studied using rat brain synaptosomes. The primary goal was to assess the relative contribution of extracellular Ca2+ and Ca2+ released from nerve-terminal mitochondria to the previously described stimulatory effects of MeHg on spontaneous release of ACh. A secondary goal was to address possible mechanisms by which MeHg might interact with nerve-terminal mitochondria to elicit Ca2+ discharge and subsequent release of ACh. MeHg depressed the high-affinity uptake of [3H]choline into synaptosomes by approximately 25 and 45% when synaptosomes were incubated with [3H]choline in the presence of 10 and 100 microM MeHg, respectively. In Ca(2+)-containing solutions, 10 and 100 microM MeHg increased the release of [3H]ACh from [3H]choline-loaded synaptosomes by 10 and 30%, respectively; this effect was maximal at 10 sec. Excluding Ca2+ from the reaction medium diminished the effectiveness of both 10 and 100 microM MeHg for inducing [3H]ACh release by about 30 and 25%, respectively, from that of Ca(2+)-containing solutions; however, significant increases still occurred in nominally Ca(2+)-free solutions. Ruthenium red (RR), an inhibitor of mitochondrial Ca2+ transport, was tested for its ability to disrupt MeHg-induced release. RR alone increased [3H]ACh release by 8-10 and 10-13% at 20 and 60 microM, respectively. RR-induced release was attenuated only slightly in Ca(2+)-free solutions. Preincubation of [3H]choline-loaded synaptosomes with RR reduced the stimulatory effect of MeHg on release of [3H]ACh both in the presence and in the absence of Ca2+. The fluorescent potentiometric carbocyanine dye diS-C2(5) was used to assess the ability of RR to prevent MeHg-induced depolarization of intrasynaptosomal mitochondria. RR (20 microM) itself did not depolarize the mitochondrial membrane potential, nor did it prevent MeHg from depolarizing the mitochondria. The results indicate that extracellular Ca2+ contributes only partially to MeHg-induced spontaneous release of ACh. The results with RR suggest that MeHg interacts with mitochondria to induce release of bound intraterminal Ca2+ stores, resulting ultimately in stimulated release of ACh. The ability of RR to prevent release of mitochondrial Ca2+ and, subsequently, ACh is not due to prevention of access of MeHg to the mitochondria, nor to stabilization of the mitochondrial membrane. Finally, MeHg reduces choline uptake into nerve terminals. Thus, MeHg could interfere with cholinergic neurotransmission by affecting the regulatory step in ACh synthesis and by increasing the spontaneous release of transmitter.(ABSTRACT TRUNCATED AT 400 WORDS)

Inhibition of human lymphocyte coproporphyrinogen oxidase activity by metals, bilirubin and haemin

Coproporphyrinogen III oxidase activity in human lymphocytes was found to be inhibited by cadmium and mercury but not by lead. The organo-metal compounds tributyltin and methylmercury were effective inhibitors of this haem biosynthetic pathway enzyme. Haemin (the ultimate product of the pathway) and bilirubin (a product of haem catabolism) were also shown to be inhibitory. Kinetic studies performed under initial velocity conditions showed that bilirubin was a non-competitive inhibitor and that one bilirubin molecule was bound to both the enzyme and enzyme substrate complex. The analysis also showed haemin to be a non-competitive inhibitor in which two haemin molecules bind to the enzyme whereas the enzyme substrate complex accepts only one haemin molecule. The possible physiological significance of the inhibition of coproporphyrinogen III oxidase activity by haemin and bilirubin is discussed.

Direct and indirect actions of HgCl2 and methyl mercury chloride on permeability and chloride secretion across the rat colonic mucosa

The actions of two mercury compounds, the inorganic HgCl2 and the organic methyl mercury chloride (MeHg), and of CdCl2 on ion transport across the rat colon were studied with the Ussing chamber and the everted sac method. The mercury compounds (5-50 microM), but not CdCl2, administered to the luminal side, induced a large, concentration-dependent increase of tissue conductance (Gt). The transepithelial movement of the extracellular marker, mannitol, was enhanced in the presence of the mercury compounds, indicating that they cause an increase in the permeability of the epithelium. Morphological studies revealed that at least for HgCl2 this increase of the permeability was associated with a loosening of the tight junctions, severe alterations of the enterocytes, and a loss of the continuity of the epithelium. After washing out HgCl2, cells neighboring the altered enterocytes developed lateral processes and in this way restored the continuity of the epithelial layer. In parallel, the mercury compounds induced an increase of short-circuit current (Isc), which is indicative of an induction of Cl- secretion. The increase of Isc, but not that of Gt, was suppressed by indomethacin and, in the case of MeHg, also by tetrodotoxin. These results suggest that MeHg and HgCl2 induce Cl- secretion by an indirect effect on the epithelium, which is mediated by prostaglandins.

Comparative action of methylmercury and divalent inorganic mercury on nerve terminal and intraterminal mitochondrial membrane potentials

Both methylmercury (MeHg) and inorganic divalent mercury (Hg++) alter the flux of ions and small molecules across nerve terminal membranes by mechanisms that may involve membrane depolarization. We compared the effects of MeHg and Hg++ on plasma (psi p) and mitochondrial membrane potentials (psi m) in synaptosomes using the potentiometric carbocyanine dye 3,3'-diethylthiadicarbocyanine iodide [diS-C2(5)]. Both mercurials (1-20 microM) produced concentration-dependent increases in dye fluorescence after 5 min of exposure which were not altered by removal of Ca++ from the medium. To determine directly effects of mercurials on psi p, predepolarization of psi m using NaN3 and oligomycin was necessary. Under this condition, MeHg- and Hg(++)-induced increases in fluorescence were associated with depolarization of psi p. A second approach was used to assess changes in psi p. In synaptosomes, the magnitude of the increase in fluorescence resulting from depolarization of psi p with a stimulus of constant intensity is a function of the resting psi p. The fluorescence response to depolarization of synaptosomes previously exposed to either MeHg or Hg++ (1-20 microM each) was reduced in a concentration-dependent manner relative to mercury-free controls. The concentration-dependent depolarization of psi p calculated in this manner correlated (r = 0.958) with calculations of psi p using direct measurements of increases in fluorescence intensity. MeHg- and Hg(++)-induced depolarizations were not altered by lowering Na+e or by the addition of the Na+ and Ca++ channel blockers tetrodotoxin and Co++, respectively. Thus, the effects of these two neurotoxic mercurials on synaptosomal membrane potentials were similar with respect to their loci but differed in magnitude.(ABSTRACT TRUNCATED AT 250 WORDS)

Deferoxamine inhibits methyl mercury-induced increases in reactive oxygen species formation in rat brain

It has been suggested that methyl mercury may express its neurotoxicity by way of iron-mediated oxidative damage. Therefore, the effect of deferoxamine, a potent iron-chelator, on methyl mercury-induced increases in reactive oxygen species formation was studied in rat brain. The generation rate of reactive oxygen species was estimated in crude synaptosomal fractions using the probes 2',7'-dichlorofluorescin diacetate and dihydrorhodamine 123. The formation rate of the fluorescent oxidation products was used as the measure of reactive oxygen species generation. Seven days after a single injection of methyl mercury (5 mg/kg, ip), the formation rate of reactive oxygen species was significantly increased in the cerebellum. Pretreatment with deferoxamine (500 mg/kg, ip) completely prevented the methyl mercury-induced increase in cerebellar reactive oxygen species generation rates. The oxidative consequences of in vitro exposure to methyl mercury (20 microM) were also inhibited by deferoxamine (100 microM). The formation of the iron-saturated complex ferrioxamine was not affected by a 10-fold excess of methylmercuric chloride or mercuric chloride, suggesting that a deferoxamine-mercurial complex does not form. The findings in this study: (1) provide evidence that iron-catalyzed oxygen radical-producing reactions play a role in methyl mercury neurotoxicity, (2) demonstrate the potential of fluorescent probes as a measure of reactive oxygen species formation, and (3) provide support for iron-chelator therapy in protection against xenobiotic-induced oxidative damage.

Methylmercury induced alterations in the nerve growth factor level in the developing brain

Pre- and early postnatal stages in the development of the central nervous system (CNS) are very sensitive to the toxic effects of methylmercury. The influence of methylmercury on the level of nerve growth factor (NGF) during the development of CNS was studied. Sprague-Dawley rats were exposed indirectly throughout the fetal and suckling periods until weaning on postnatal day 25 (P 25) via their dams given methylmercury in the diet (3.9 mg/kg diet). In addition, after weaning offsprings were exposed directly to methylmercury via the diet until postnatal day 50 (P 50). The level of NGF was analyzed in cortical areas and in the septum with a sensitive enzyme immunoassay. The pups exposed to MeHg exhibited a 50% elevation in the level of NGF in the hippocampus on P 25 and P 50 compared to control animals. Concomitantly, the level of NGF decreased by 30% in the septum on P 25 and P 50, suggesting that the retrograde transport of NGF from hippocampus to septum could be affected by the exposure of methylmercury. The exact mechanism by which the low level of mercury is affecting the NGF concentration in the developing brain is yet unknown. The increase of NGF in the hippocampus and the decrease of NGF measured in the septum could reflect altered conditions for neurotrophic support in these areas of the brain as a result of the exposure to heavy metal. Thus, this finding might indicate a connection between exposure of heavy metals and neurodegeneration, such as that found in the basal forebrain in Alzheimer's disease.