Accumulation and tissue distribution of mercury in the guinea pig during subacute administration of methyl mercury

Female guinea pigs were dosed po daily for 71 days with 0.4, 4, 40, or 400 μg Hg/kg given as radiolabeled methyl mercuric chloride. The accumulation of total mercury was followed in 10 tissues at 6 time intervals. After dosing ceased, the decay profiles of mercury were followed for an additional 35 days. The accumulation pattern for mercury was similar for each dose level, and the tissue mercury concentration on day 71 increased in the following order: blood < cerebellum < hypothalamus < calcarine cortex < frontal lobe < occipital lobe < caudate nucleus < muscle < liver < kidney. Mercury accumulation in all tissues, except kidney at the 4-, 40-, and 400-μg/kg dose levels, approached steady-state values in the 35–71 -day dosing period. The accumulation curves could be fitted by an exponential equation incorporating the mercury half-life obtained from the decay profiles. As the dose level increased, tissue mercury concentrations increased to a greater extent than anticipated. Although doses increased 1000-fold from 0.4 to 400 μg Hg/kg, kidney concentrations increased 3300-fold after 71 days of dosing. At this time, inorganic mercury (Hg²⁺) comprised 42% of the total kidney mercury and 5% of the total liver mercury at the 400 μg/kg dose.Clinical signs of methyl mercury intoxication were induced in guinea pigs after dosing daily for 9 days at 5 mg Hg/kg. The activities of 6 enzymes were monitored and cholinesterase (serum), choline acetylase (caudate nucleus) and carboxylesterase (liver) were significantly lower than control values. The total mercury concentration in whole brain was 28 μg/g (wet weight). Animals dosed at 400 μg Hg/kg for 71 days showed no decrease in the activities of the selected enzymes, there was no change in weight gain when compared to the control and there were no signs of methyl mercury toxicity. The highest brain mercury concentration after 71 days dosing was 11 μg/g (wet weight) in the caudate nucleus.     

Mobilization of Methyl Mercury in Vivo and in Vitro using N–acetyl–DL–penicillamine and other Complexing Agents

Abstract The distribution and excretion of mercury was studied in mice given a single intravenous dose of 5 umol/kg of methyl mercuric chloride. Oral treatment with N–acetyl–DL–penicillamine (3 mmol/kg per day) removed more mercury from the brain and from the whole body than the corresponding treatment with other complexing agents, and it was also effective on delayed treatment. Even more mercury was removed into the faeces and the urine, by higher doses of N–acetyl–DL–penicillamine, and 4 days of treatment with 27 mmol/kg per day of this compound did not give rise to any significant toxic symptoms in the mice. In vitro experiments showed that the chemical affinity of N–acetyl–DL–penicillamine for methyl mercury was higher than that of the other thiols tested, except D–penicillamine. In contrast to the latter, N–acetyl–DL–penicillamine easily penetrated the cellular membranes, and therefore rapidly removed a substantial fraction of methyl mercury from the blood cells. It is assumed that N–acetyl–DL–penicillamine can reduce the mercury concentration in brain cells by converting the intracellularly non–diffusible methyl mercury into a freely diffusible complex.     

Protective Effects of Ca2+ Channel Blockers against Methyl Mercury Toxicity

Abstract: The protective effects of Ca²⁺ channel blockers against the toxicity of methyl mercury were examined by both in vivo and in vitro experiments. In the in vivo study we first examined the effects of the Ca²⁺ channel blockers (20 mg/ kg/day), flunarizine, nifedipine, nicardipine, and verapamil against the toxic level of methyl mercury treatment (5 mg/kg/ day of methyl mercuric chloride for 12 consecutive days). However, there was a difference in potency of the effects among the reagents. All the Ca²⁺ channel blockers prevented a decrease in body weight and/or the appearance of the symptoms of neurological disorders in the rats treated with methyl mercury. In the next experiment, we examined flunarizine at different levels of supplementation (1,25 and 50 mg/kg/day). Flunarizine in a dose-dependent manner prevented a decrease in body weight, appearance of the symptoms of neurological disorder and mortality in the rats treated with methyl mercury. Flunarizine treatment (25 mg/kg/day) for the first 5 days did not affect mercury distribution among the tissues, suggesting that the mechanism of protection against methyl mercury-induced toxicity may be attributed to its own pharmacological effect. In the in vitro study we examined the effect of flunarizine (0, 0.5, 5 and 50 μM) using primary cultures of cerebellar granular cells in 96-well culture plates. Viable cell numbers were estimated 1 and 3 days after treatment with methyl mercury. The estimated 50% lethal concentration (LC50) of methyl mercury was higher in plates treated with 5 and 50 uM of flunarizine both on days 1 and 3, indicating that flunarizine protected the primary cultured cerebellar granular cells against the toxicity of methyl mercury. As such, Ca²⁺ channel blockers protected against the toxicity of methyl mercury both in vivo and in vitro, suggesting that Ca²⁺ plays an important role in the mechanisms of methyl mercury toxicity.     

Effects of ethanol on methyl mercury toxicity in rats

This study was designed to investigate the effect of different doses of ethanol on the morbidity, mortality, and distribution of mercury in the tissues of groups of rats treated orally once daily with methyl mercury chloride (MMC: 5 mg/kg . d) for 10 consecutive days. Ethanol potentiated the toxicity of methyl mercury in terms of neurological manifestations (hindleg crossings and abnormal gait) and mortality. The magnitude of effect depended on the concentration of ethanol administered. The concentration of mercury in the kidney and brain also increased with the dose of ethanol given. These findings indicate that epidemiologic studies designed to evaluate methyl mercury toxicity must take into account the multiple environmental burdens that can affect the population cumulatively and simultaneously.     

3H-dopamine uptake and 3H-haloperidol binding in striatum after administration of methyl mercury to rats

Methyl mercury inhibits dopamine (DA) and serotonin (5-HT) uptake by brain synaptosomes and decreases antagonist binding to striatal dopaminergic D₂ receptors in vitro. To assess the effects in vivo, adult rats were given methyl mercury, either as a single dose (10 mg/kg by gavage) or a cumulative total dose of 50 mg/kg in 2 weeks. The repeated dosing decreased body weight and caused neuromuscular dysfunction. In spite of this overt toxicity, neither ³H-DA uptake nor ³H-haloperidol binding changed in striatal synaptosomes. There were no significant alterations in ³H-5-HT uptake by hypothalamic synaptosomes or ³H-flunitrazepam binding in cerebellar synaptosomes. The results suggest that monoaminergic synapses and the benzodiazepine binding sites, associated with cerebellar GABA receptors, remain functionally normal at doses of methyl mercury that are otherwise toxic. The results also emphasize the importance of due care when extrapolating cellular or biochemical data to the level of the whole organism.     

Nitric oxide and sensory afferent neurones modulate the protective effects of low-dose endotoxin on rat gastric mucosal damage

Pretreatment (1 h) with low doses (5–40 μg/kg i.p.) of Escherichia coli endotoxin dose dependently reduced the gastric mucosal damage induced by a 10 min challenge with 1 ml ethanol (50% and 100%) in conscious rats. Treatment with the nitric oxide synthesis inhibitor, oxide synthesis inhibitor, N^G-nitro- -arginine methyl ester (L-NAME, 5 and 10 mg/kg i.p.), significantly inhibited the protective effects of endotoxin (40 μg/kg i.p.). The actions of L-NAME were reversed by the prior administration of -arginine (100 mg/kg i.p.). The protective effects of endotoxin were not influenced by pretreatment with dexamethasone (5 mg/kg s.c. twice) or indomethacin (5 mg/kg s.c.). However, ablation of sensory afferent neurones by capsaicin pretreatment (20, 30 and 50 mg/kg s.c.) abolished the mucosa protective effects of endotoxin (40 μg/kg). These findings suggest that the protection elicited by low doses of endotoxin against ethanol-induced mucosal damage involves synthesis of nitric oxide and activation of sensory neurones.     

Effect of subchronic administration of methyl parathion on in vivo protein synthesis in pregnant rats and their conceptuses

Pregnant rats received daily po doses of the organophosphate methyl parathion (MPTH) from Day 6 through Day 15 or 19 of gestation at doses causing no (1.0 mg/kg) or minimal (1.5 mg/kg) signs of maternal toxicity. Following the dose of MPTH on Day 15 or 19, in vivo protein synthesis was measured 0.5, 1.0, and 2.0 hr after sc injection of L-[1-14C]valine at a dose of 5 microCi/mmol/100 g body wt. The specific activity of [14C]valine in the free amino acid pool and protein bound pool was significantly reduced in various regions of maternal brain and in maternal viscera, placenta, and whole embryos (Day 15), and in fetal brain and viscera (Day 19). The inhibitory effect of MPTH on net protein synthesis was dose dependent, greater on Day 19 than 15 of gestation and more pronounced in fetal than in maternal tissues.     

Dose- and sex-dependent alterations in mercury distribution in fetal mice following methylmercury exposure

Methylmercuric chloride was orally given to inbred C57BL/6N mice on d 13 of pregnancy at doses of 2.5, 5, 10, and 20 mg/kg. Animals were sacrificed on each of d 14-18 of pregnancy, and mercury levels in the brain, liver, and kidney of both the fetus and dam were determined. The dose effect on the time course of mercury accumulation in the brain was observed both in the fetus and dam; after the higher doses administered, the brain mercury reached the highest concentration later than it did after the lower doses. In addition, the mercury concentration in the fetal brain was disproportionately higher after a dose of 20 mg/kg, which was toxic in the fetus since the weight of the brain was reduced. The concentration in the fetal brain was 1.6-4.9 times higher than in the maternal brain. The sex difference of fetuses in mercury levels was observed in the brain after a dose of 2.5 mg/kg, in which mercury concentration was higher in females than in males. This corresponded to the previously reported difference in adult mice and rats. However, the sex difference was not seen after doses of 5, 10, or 20 mg/kg.     

The comparative toxicology of ethyl- and methylmercury

Neurotoxicity and renotoxicity were compared in rats given by gastric gavage five daily doses of 8.0 mg Hg/kg methyl- or ethylmercuric chloride or 9.6 mg Hg/kg ethylmercuric chloride. Three or 10 days after the last treatment day rats treated with either 8.0 or 9.6 mg Hg/kg ethylmercury had higher total or organic mercury concentrations in blood and lower concentrations in kidneys and brain than methylmercury-treated rats. In each of these tissues the inorganic mercury concentration was higher after ethyl than after methylmercury.Weight loss relative to the expected body weight and renal damage was higher in ethylmercury-treated rats than in rats given equimolar doses of methylmercury. These effects became more severe when the dose of ethylmercury was increased by 20%. Thus in renotoxicity the renal concentration of inorganic mercury seems to be more important than the concentration of organic or total mercury. In methylmercury-treated rats damage and inorganic mercury deposits were restricted to the P₂ region of the proximal tubules, while in ethylmercury-treated rats the distribution of mercury and damage was more widespread.There was little difference in the neurotoxicities of methylmercury and ethylmercury when effects on the dorsal root ganglia or coordination disorders were compared. Based on both criteria, an equimolar dose of ethylmercury was less neurotoxic than methylmercury, but a 20% increase in the dose of ethylmercury was enough to raise the sum of coordination disorder scores slightly and ganglion damage significantly above those in methylmercury-treated rats.In spite of the higher inorganic mercury concentration in the brain of ethylmercurythan in the brain of methylmercury-treated rats, the granular layer damage in the cerebellum was widespread only in the methylmercury-treated rats. Thus inorganic mercury or dealkylation cannot be responsible for granular layer damage in alkylmercury intoxication. Moreover, histochemistry demonstrated no inorganic mercury deposits in the granular layer.     

Effects of Methyl Mercury on Protein Synthesis in Vitro

Abstract: Methyl mercury has been shown to interact with protein synthesis in vivo and in vitro. In the present paper a brain postmitochondrial supernatant was used for studies in vitro. Inorganic mercury (Hg²⁺) was shown to be a more potent inhibitor of protein synthesis than methyl mercury, puromycin or cycloheximide. The inhibitory effect of methyl mercury was potentiated by puromycin. It is thus possible that methyl mercury causes disintegration of polysomes in brain cells.     

Dissociation of decreases in renal cellular energetics and recovery of renal microsomal translation during chronic cyclosporine A administration

Male Sprague-Dawley rats were given oral cyclosporine A or control vehicle, and renal protein synthesis and renal ATP levels were examined. Acute oral cyclosporine A at 5, 10, 25 and 50 mg/kg/day for 6 days reduced [3H]L-leucine incorporation by isolated renal microsomes to 76.2, 56.8, 44.3 and 29.5% of control incorporations, respectively. No significant changes in renal ATP levels were detected by NMR spectroscopy after acute oral cyclosporine A administration at the doses indicated. However, during chronic exposure to cyclosporine A at doses of 5 and 25 mg/kg/day for 30, 60 and 90 days, there was a recovery of renal microsomal protein synthesis by day 30 at 5 mg/kg/day, and by day 45 at 25 mg/kg/day. NMR spectroscopy of the kidneys of these rats demonstrated decreases in renal ATP level by day 60 in animals given cyclosporine A at 25 mg/kg/day. Cyclosporine A administration produced a renal acidosis and up to a 40% decrease in renal ATP level by day 90 in rats fed cyclosporine A at 25 mg/kg. No apparent histologic abnormalities were observed in the ATP-deficient renal tissue by NMR imaging. Reductions in renal ATP level suggest that the recovery of renal microsomal protein synthesis is aberrant in the continued presence of cyclosporine A, or that mitochondria are direct sites of cyclosporine A toxicity.     

7V-Acetylpenicillamine Potentiated Excretion of Methyl Mercury in Rat Bile: Influence of S-Methylcysteine

Abstract: N-acetylpenicillamine, 5 mmol/kg body weight increased biliary excretion of methyl mercury more than three fold. Upon simultaneous administration of the same dose of N-acetylpenicillamine and 2,5 mmol/kg body weight of S-methylcysteine biliary excretion of methyl mercury increased only 1.5 fold. In both cases biliary sulfhydryl concentration increased to the same extent, about 5 fold. Decreased biliary excretion of methyl mercury, as a result of liver depletion of reduced glutathione by cyclohexene oxide, could be restored by N-acetylpenicillamine. This restoration could be depressed by S-methylcysteine. The experiments undertaken indicate that N-acetylpenicillamine potentiated methyl mercury excretion occurs by a glutathione S-transferase dependent mechanism. Bile, collected after successive administration of methyl mercuric chloride, cyclohexene oxide, S-methylcysteine and N-acetylpenicillamine contained the methyl mercuric derivatives of N-acetylpenicillamine and glutathione together with other methyl mercury carrying components not present in control bile. Whether these components play any role in the mechanism of N-acetylpenicillamine potentiated methyl mercury excretion cannot be stated from the present investigation.     

Changes in axonally transported proteins in the mature and developing rat nervous system during early stages of methyl mercury exposure

It was established by means of SDS polyacrylamide gel electrophoresis that direct injections of methyl mercury (10 micrograms Hg) into the mature rat vitreous body of the eye decrease protein synthesis in the retina and optic nerve at 4 hours after injection. Although the global spectrum of polypeptides did not change, a specific decrease in the volume of polypeptides of 20-23 K daltons molecular weight was evident. Conversely, systemic exposure to methyl mercury resulted in increased protein synthesis of polypeptides of 20-23 K molecular weight both in adult (8 mgHg/kg/day for 8 days) and neonatal rats (2 mgHg/kg/day for 10 days). In addition, specific changes in the volume of polypeptides 75-90 K molecular weight were noted in sciatic nerves of neonatal rats. These data are consistent with a bimodal response in protein synthesis following MeHg treatment. Local presence of MeHg following direct injection into the eye causes a reduction in protein synthesis, while chronic systemic exposure results in increased synthesis and transport of proteins in both mature and developing optic nerves and neonatal sciatic nerves. Thus, these systems possess the capacity to attempt regenerative processes through induction of a small subset of proteins known as GAPs (Growth-Associated Proteins) during the early stages of systemic methyl mercury exposure. These wide spread and system-specific changes are consistent with growth-specific functions during the early stages of methyl mercury exposure.     

Studies on comparative efficacy of α-linolenic acid and α-eleostearic acid on prevention of organic mercury-induced oxidative stress in kidney and liver of rat

The present study was undertaken to evaluate the effect of α-linolenic acid and α-eleostearic acid, two isomers of linolenic acid, against oxidative stress induced by organic mercury in kidney and liver cells of rat. Male albino rats were divided into six groups. Groups 1, 2 were normal control and methyl mercury chloride (MeHgCl) treated (5 mg/kg BW/day) control, respectively. Groups 3, 4, 5 and 6 were orally treated with different doses of two fatty acids (0.5% and 1.0% of total lipid given for each isomer) along with MeHgCl (5 mg/kg BW). Results showed that activity of antioxidant enzymes viz. catalase (CAT), superoxide dismutase (SOD), glutathione peroxidase (GPx), reduced glutathione (GSH) in liver and kidney decreased significantly due to oxidative stress generated by MeHg. Administration of the linolenic acid isomers almost restored all the altered parameters and also reduced lipid peroxidation and leakage of trans-aminase enzymes from liver to blood due to liver injury when administrated in higher doses. Histopathology of liver and kidney cells showed that administration of α-linolenic acid significantly reduced the damage generated by MeHg. Thus, α-linolenic acid and α-eleostearic acid could serve as cost-effective and natural phytochemical preparation to protect against the adverse effects caused by organic mercury in human.     

Effects of complexing treatment administered with the onset of methyl mercury neurotoxic signs

Complexing therapy was tested in the rat for effectiveness by initiating d-penicillamine (DPA) or 2,3-dimercaptopropanol (BAL) 7 days after the first of three daily exposures to methyl mercury (MM) given po as 14.4 mg MMCl/kg/day. Therapy consisted of five daily doses sc of either 1200 mg/kg DPA or 60 mg/kg BAL. Distribution studies employing ²⁰³Hg-labeled MMCl indicated that, on the 7th day after the first dose, central and peripheral nervous system tissues had reached peak tissue concentrations of mercury. Treatment with BAL proved ineffective either in altering the typical MM-induced pattern of weight change and development of neurotoxic signs. DPA treatment, however, prevented the further development of signs and enhanced weight gain. β-Glucuronidase activity of spinal cord and sciatic nerve indicated degeneration beginning on or before the 14th day after the first MM dose. Treatment initiated just before degeneration or toxic signs have become severe will be effective.     

Acute Toxicity, Uptake and Histopathology of Aqueous Methyl Mercury to Fathead Minnow Embryos

Early life stages of fishes have been shown to be especially susceptible to the toxic effects of heavy metal pollution. In this study, fathead minnow (Pimephales promelas) embryos were exposed in the laboratory to a graded series of aqueous methyl mercury concentrations under continuous-flow conditions. A number of toxicological endpoints were examined including; acute toxicity, bioaccumulation, protein production, impact on mitosis, gross and histopathology. Acute toxicity, reported as LC50 values of methyl mercury, ranged from 221 μg/l (95% C.I. 246–196 μg/l) for 24-h tests to 39 μg/l (95% C.I. 54–24 μg/l) for 96-h exposures. Fathead minnow embryos were shown to rapidly take up mercury from the surrounding water. Mercury levels in embryos reached levels of 2.80 μg/g wet weight after 96 h exposure to 40 μg/l methyl mercury. An initial elevation of total protein in embryo was observed in embryos exposed to 25 μg/l methyl mercury during the first 12 h of development. At later stages, significantly lower levels of protein/μg embryo were observed. Methyl mercury had no effect on mitotic stages (p=0.05) in early, cleaving blastula-stage embryos. Live embryos and serial sections were utilized to characterize changes in embryo morphology and histopathology.     

Effect of activated charcoal on plasma levels of nortriptyline after single doses in man

Summary Six healthy volunteers were given nortriptyline (NT) in doses of 0.86–1.00 mg/kg. There was a significant (P<0.05) decrease in absorption of the drug if 5 g of activated charcoal was administered half an hour afterwards.     

Study of rat serum concentrations of 1-[(4-chlorophenyl)methyl]-1H-indazole-3-car☐ylic acid,new antispermatogenic agent

The correlation between the antispermatogenic activity of 1-[(4-chlorophenyl)methyl]-1H-indazole-3-car?ylic acid (AF 1312/TS) and its serum and testicular concentrations was studied in rats. The doses used were between 100 and 500 mg/kg po or 125, 250 and 500 mg/kg po when the drug was given in a medicated diet. Duration of treatment was between 1 and 30 days. The minimal effective dose following a single administration was 300 mg/kg, corresponding to a peak serum concentration higher than 300 μg/ml, while the minimal effective dose for five daily administrations was 100 mg/kg, corresponding to a peak serum concentration lower than 200 μg/ml. When the drug was incorporated in the diet, the effective dose was 250 mg/kg daily, corresponding to serum concentrations of about 100 μg/ml. The efficacy of the drug was not increased when the duration of treatment was prolonged from 5 to 30 days. The critical testicular concentration for antispermatogenic activity was found to be about 10 μg/g.     

The Effect of Selenium on the Biliary Excretion and Organ Distribution of Mercury in the Rat after Exposure to Methyl Mercuric Chloride

Abstract The influence of selenium compounds on the biliary excretion and the organ distribution of mercury after injection of methyl mercuric chloride (4 μmol/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.     

Inhibition of protein synthesis in mice by ochratoxin A and its prevention by phenylalanine

The sensitivity of protein synthesis to ip doses of ochratoxin A ranging from 1 to 15 mg/kg body weight has been determined in the livers, kidneys and spleens of mice. The incorporation of ¹⁴C-labelled amino acids into protein was measured in the total tissue homogenate, in the 105,000-g supernatant fraction and in the fraction of thermostable soluble proteins. The inhibition of protein synthesis was greatest in spleen and kidney and least in liver. The percentage inhibition of protein synthesis by any one dose of ochratoxin A was similar in all of the protein fractions from any one organ. The degree of inhibition of protein synthesis 5 hr after administration of 1 mg ochratoxin A/kg was 26% in liver, 68% in kidney and 75% in spleen. Phenylalanine (100 mg/kg) injected together with ochratoxin A prevented the inhibition of protein synthesis by a dose of 10 mg OTA/kg in all of these organs.