Cell specific enzyme markers as indicators of neurotoxicity: effects of acute exposure to methylmercury.

In order to assess the sensitivity of several cell specific enzyme markers (tyrosine hydroxylase (TH), glutamic acid decarboxylase, choline acetyltransferase, glutamine synthetase (GS), neuron specific and non-neuronal enolase and 2',3'-cyclic nucleotide phosphohydrolase (CNP] as indices of neurotoxicity, changes in their activities were monitored after rats were treated with two doses of the neurotoxic agent, methylmercury chloride (MMC). Comparisons were also made of any histopathological changes occurring in the tissues examined. At the low dose rate (3.36 mg Hg/kg, po, for 14 days), the rats exhibited less body weight gain compared to untreated animals. No change in either the neuronal or noneuronal enzyme markers was observed in brain but a significant increase in the myelin marker, CNP, and total enolase activity was seen in the optic nerve. Morphological evaluation by light microscopy indicated no discernible neuronal lesions in MMC-exposed animals. At the higher MMC dose (7.05 mg Hg/kg, po, for 7 days), there was about a 20% loss in the body weight of treated animals and partial hind limb paralysis was observed. Of all the neuronal marker enzymes examined, only TH was found to be decreased in the striatum. The proliferating astroglial marker, GS, was elevated only in the cerebellum. CNP was found to be decreased in both the optic and sciatic nerve. As in the lower dose group no pathological changes were observed at the light microscopic level in the brain of MMC-treated rats. These data suggest that of the cell specific marker enzymes studied, GS in the cerebellum and TH in the striatum may be useful biochemical markers for the neurotoxic action of MMC.

Effect of chronic exposure to hexachlorophene on rat brain cell specific marker enzymes.

The neurotoxicity associated with chronic exposure to hexachlorophene (HCP) was evaluated by measuring the activity of seven cell specific marker enzymes in brain and by comparing these measurements to morphological changes analyzed by light microscopy. Animals were divided into two groups, the experimental group received HCP at a daily dose of 20 mg/kg p.o. for 53 consecutive days whereas the control group received an equivalent amount of the vehicle only. HCP produced no change in the rate of gain in body weight nor did it produce a statistically significant change in brain weight. Furthermore, no overt abnormal neurological symptoms were observed at this level of exposure to HCP. The white matter throughout the brain was extensively vacuolated in the HCP-treated rats, imparting a spongiform structure which was absent in the white matter of the control animal brains. The data obtained reveal that chronic HCP treatment produce little change in any of the neuronal marker enzymes with the exception of a significant decrease in tyrosine hydroxylase activity in the striatum. Of the nonneuronal enzymes assayed, a significant increase in non-neuronal enolase, glutamine synthetase, and 2',3'-cyclic nucleotide phosphohydrolase was observed in the sciatic nerve, hippocampus and optic nerve, respectively.

Placental opioid-enhancing factor (POEF): generalizability of effects.

A substance in amniotic fluid and placenta (POEF for Placental Opioid-Enhancing Factor) has been shown to enhance opiate- or opioid-mediated analgesia in rats. Recent studies have only touched on the generalizability of the phenomenon. The present studies further tested the generalizability of the POEF effect: they examined sex specificity of the mechanism; whether POEF activity exists in afterbirth material of species other than the rat; whether POEF activity exists in tissue other than afterbirth material; whether POEF activity could be demonstrated after injection rather than ingestion of afterbirth material; and whether POEF enhances all opioid-mediated phenomena. We found that (a) POEF is effective in male rats as well as in female rats; (b) POEF activity exists in human and dolphin afterbirth material; (c) ingestion of pregnant-rat liver does not produce enhancement of opioid-mediated analgesia; (d) POEF does not seem to be effective when amniotic fluid is injected either IP or SC; and (e) POEF does not modify morphine-induced hyperthermia.

Effect of vanadium(IV) compounds in the treatment of diabetes: in vivo and in vitro studies with vanadyl sulfate and bis(maltolato)oxovandium(IV).

Vanadyl sulfate (VOSO(4)) was given orally to 16 subjects with type 2 diabetes mellitus for 6 weeks at a dose of 25, 50, or 100 mg vanadium (V) daily [Goldfine et al., Metabolism 49 (2000) 1-12]. Elemental V was determined by graphite furnace atomic absorption spectrometry (GFAAS). There was no correlation of V in serum with clinical response, determined by reduction of mean fasting blood glucose or increased insulin sensitivity during euglycemic clamp. To investigate the effect of administering a coordinated V, plasma glucose levels were determined in streptozotocin (STZ)-induced diabetic rats treated with the salt (VOSO(4)) or the coordinated V compound bis(maltolato)oxovandium(IV) (abbreviated as VO(malto)(2)) administered by intraperitoneal (i.p.) injection. There was no relationship of blood V concentration with plasma glucose levels in the animals treated with VOSO(4), similar to our human diabetic patients. However, with VO(malto)(2) treatment, animals with low plasma glucose tended to have high blood V. To determine if V binding to serum proteins could diminish biologically active serum V, binding of both VOSO(4) and VO(malto)(2) to human serum albumin (HSA), human apoTransferrin (apoHTf) and pig immunoglobulin (IgG) was studied with EPR spectroscopy. Both VOSO(4) and VO(malto)(2) bound to HSA and apoHTf forming different V-protein complexes, while neither V compound bound to the IgG. VOSO(4) and VO(malto)(2) showed differences when levels of plasma glucose and blood V in diabetic rodents were compared, and in the formation of V-protein complexes with abundant serum proteins. These data suggest that binding of V compounds to ligands in blood, such as proteins, may affect the available pool of V for biological effects.

Differences in elimination rates of methylmercury between two genetic variant strains of mice.

This study demonstrates an intraspecies difference in elimination of methylmercury between two genetic variant strains of mice. Following a single non-toxic dose of 203Hg labeled methylmercury (0.5 mg Hg/kg i.p.), the half-time of 203Hg elimination in the CBA/J inbred strain was 7.4 days compared to 3.0 days in the CFW strain. The fecal excretion rate of 203 Hg was the same for both strains, whereas urinary excretion rate was 5-fold higher in the CFW strain than in the CBA/J strain. The higher excretion rate in the CFW strain accounted for significant lowering of mercury concentrations in terminal tissue samples. The mercury burden in the pelt relative to the terminal body burden was similar for the two strains suggesting that elimination via the fur plays no significant role in the difference in overall excretion rate. Intraspecies differences in methylmercury elimination has direct implications for human exposures where a wide variation in kinetics has been observed. This animal model will be useful in studying the mechanism of urinary excretion of methylmercury and provide insight into more effective antidotal procedures.

The role of fluoroacetate-specific dehalogenase and glutathione transferase in the metabolism of fluoroacetamide and 2,4-dinitrofluorobenzene.

2,4-Dinitrofluorobenzene (DNFB) reacts with glutathione to form a stable product similar to that formed with the model glutathione-S-transferase (GST) substrate, 1-chloro-2,4-dinitrobenzene (CDNB). DNFB is approx. 40 times als reactive as CDNB in this chemical reaction. The enzymatic defluorination of DNFB also proceeds at a more rapid rate than that of CDNB in the GST assay. Fluoroacetamide (FAM), like fluoroacetate (FAC), undergoes no discernable chemical defluorination. Its enzymatic defluorination is approx. 10% of that observed for FAC and only 0.2% of the rate for DNFB. An antibody raised to the fluoroacetate specific dehalogenase (FSD) precipitated both FAC and FAM defluorinating activity but had no effect on either CDNB or DNFB activity. The data are consistent with the hypothesis that DNFB is metabolized by the GST while FAM is metabolized by the FSD.

Primary cultures of rabbit renal proximal tubule cells: II. Selected phase I and phase II metabolic capacities.

Specific characteristics of cells vary as a function of time in culture. We have determined the stability of selected Phase I and Phase II biotransformation capacities in rabbit renal proximal tubule cells in primary culture. When grown in hormonally-defined medium, proximal tubule cells lost Phase I metabolic capacity. Cytochrome P-450 content and associated mixed-function oxidase activities present in kidney cortex microsomes were not detectable after 14 days in culture. Phase II glutathione-dependent metabolic functions were well retained in cultured cells compared with freshly isolated proximal tubules (FIPT). Cellular total glutathione content was 2.8 mug/mg protein in FIPT compared with approximately 10 mug/mg protein in stable confluent cultures. A higher total glutathione content of 20.6 mug/mg was noted in preconfluent cultures. The glutathione redox state was initially perturbed in FIPT with 37% of the total glutathione present found in its oxidized form. Tubule cells recovered to a normal ratio (6-13% of total glutathione in the oxidized form) while in culture. The glutathione S-transferase activity in 4-day-old cells in culture was reduced to 50% of the 4 U/mg protein level found in FIPT. No appreciable further decline in glutathione S-transferase activity was detected during 15 days in culture. The level of gamma-glutamyl-transpeptidase (a brush-border enzyme necessary for glutathione uptake into proximal tubule cells) declined from 1499 mU/mg protein in homogenates of FIPT to 636 mU/mg in homogenates of 8-day-old cultured cells. A further decline in activity occurred during the next 7 days in culture. In conclusion, although Phase I metabolic functions were diminished in primary cultured rabbit proximal tubule cells, Phase II metabolic functions were retained at levels comparable with FIPT and well above those found in several established kidney cell lines.

An electrothermal atomic absorption method for aluminum analysis in plasma: identification of sources of contamination in blood sampling procedures.

A wide range of reported "normal" values for aluminum concentration in serum and plasma prompted the study of an analytical procedure for aluminum determination by atomic absorption and an investigation of possible sources of contamination, which may contribute to the wide variation in results reported in the literature. A method has been developed utilizing the graphite furnace, which gives linear results over an aluminum concentration range of 2.5 to 100 ng Al/mL and a limit of detection of 0.6 ng/mL. Recoveries of added aluminum from plasma ranged from 89 to 100% over this concentration range. The mean plasma concentration of aluminum found in four healthy volunteers was 2.7 ng Al/mL. Blood sampling devices and heparin can have significant amounts of aluminum contamination, which may explain the large discrepancies in quoted "normal" aluminum values in plasma and serum.

Stabilization of glutathione in urine and plasma: relevance to urinary metal excretion studies.

The present studies demonstrate the instability of glutathione (GS) in urine and plasma and illustrate the importance of developing procedures to stabilize GS in biological fluids. The rapid loss of urinary GSH was not prevented by inhibition of gamma-glutamyltranspeptidas (gamma-GTP) with L-(alpha S, 5S)-alpha-amino-3-chloro-4,5-dihydro-5-isoxazoleacetic acid (AT-125), by EDTA, or by sodium citrate. GS in urine could be stabilized for extended periods of time by acidification of samples immediately after collection. In order to accurately measure urine and plasma GS concentrations, samples were analyzed by a known additions technique. The relevance of these analytical procedures to metal excretion studies is discussed.

Standardization of a method for the routine analysis of polychlorinated biphenyl congeners and selected pesticides in human serum and milk.

A methodology is presented for the routine determination of specific polychlorinated biphenyl (PCB) congeners in serum and milk samples. The procedures include standardized extraction, cleanup and quantitation by high-resolution gas chromatography (GC) and comprehensive quality assurance program to minimize systematic and erratic errors. The analyses of 68 PCB congeners and three pesticides, p,p1-dichloro diphenyl dichloro ethylene (DDE), hexachlorobenzene (HCB), and Mirex, at part-per-billion levels include the addition of surrogate congener standards (IUPAC isomers #46 and #142), extraction with hexane after protein precipitation, cleanup with Florisil, and analysis by GC with capillary column and electron capture detection. Quantitation is based on calibration standards and response factors using isomers #30 and #204 as internal standards. The quality control activities consist of analyses of samples in batches of 6 to 10 simultaneously with quality control (QC) samples. The quality assurance program checks that the procedures are under control by the use of control charts and set the criteria for data acceptability. The detection limits for the congeners and pesticides associated with the analyses of 500 serum samples and of 100 milk samples are reported. In addition, typical profiles of congener distribution in both matrices are illustrated.

Extracorporeal regional complexing haemodialysis treatment of acute inorganic mercury intoxication.

A 70-year-old white female presented approximately 24 h after ingesting three 475 mg tablets (1.425 g) of mercuric chloride in a suicide attempt. Acute renal failure necessitated the initiation of haemodialysis approximately 4 d after the ingestion. Treatment with BAL (2,3-dimercaptopropanol) resulted in only small increases in mercury output into dialysate. A new procedure involving the extracorporeal infusion of the chelating agent dimercaptosuccinic acid (DMSA) into the arterial blood line during haemodialysis was initiated. This procedure of Extracorporeal Regional Complexing Haemodialysis (ERCH) had been effective in increasing methylmercury removal in patients poisoned by contaminated grain. The first DMSA-ERCH procedure was performed 6 d after poisoning. There was a dramatic increase in mercury output into the dialysate. During three treatment sessions of 80 min each, 1189 micrograms of mercury were removed from the patient. The dialysed mercury represented the only mercury output since the patient was anuric and not producing faeces. DMSA-ERCH appears to be much more effective than BAL and haemodialysis in the treatment of acute inorganic mercury poisoning. The long interval between poisoning and initiation of treatment probably contributed to the patients ultimate demise, 28 d after poisoning. Efficacy of the DMSA-ERCH procedure for inorganic mercury poisoning is likely to be improved as the interval between exposure and treatment is reduced.

Mercury as a potential hazard for the dental practitioner.

Mercury has been used for centuries for medical, chemical, metallurgical and electrical applications. It is an element of mystery, which in its metallic form is an enticing silvery liquid that can be as fascinating as it is dangerous. Its use in dental amalgam has a potential for continuous occupational exposure of dental practitioners to mercury vapor. It is imperative that the dental practitioner understands the hazards associated with the use of mercury, and controls exposures to prevent the development of any untoward effects. This article provides an overview of the toxicology of the different forms of mercury to which human exposure occurs and addresses safety issues associated with mercury vapor, the primary form of mercury encountered in the practice of dentistry.

Environmental PCB exposure and risk of endometriosis.

BACKGROUND: Hormonally active environmental agents have recently been associated with the development of endometriosis. METHODS: We undertook a study to assess the relationship between endometriosis, an estrogen-dependent gynaecological disease, and 62 individual polychlorinated biphenyl (PCBs) congeners. We enrolled 84 eligible women aged 18-40 years undergoing laparoscopy for study, which included an interview and blood specimen (n=79; 94%). Thirty-two women had visually confirmed endometriosis at laparoscopy while 52 did not. Blood specimens were run in batches of 14 including four quality control samples for toxicological analysis. Each PCB congener was adjusted for recovery; batch-specific reagent blanks were subtracted. All PCB concentrations were log transformed and expressed in ng/g serum first as a sum and then as tertiles by purported estrogenic or anti-estrogenic activity of PCB congeners. RESULTS: Using unconditional logistic regression analysis, a significantly elevated odds ratio (OR) was observed for women in the third tertile of anti-estrogenic PCBs [OR 3.77; 95% confidence interval (CI) 1.12-12.68]. Risk remained elevated after controlling for gravidity, current cigarette smoking and serum lipids (OR 3.30; 95% CI 0.87-12.46). CONCLUSIONS: These data suggest that anti-estrogenic PCBs may be associated with the development of endometriosis.

Methylmercury removal in the dog during infusion of 2,3-dimercaptosuccinic acid (DMSA).

2,3-Dimercaptosuccinic acid (DMSA) has been utilized in chelation therapy and in extracorporal complexing hemodialysis therapy for experimental methylmercury intoxication. In the latter application, substantial excretion of methylmercury occurred by the urinary route. This prompted the current study of the effects of continuous intravenous DMSA infusion therapy on methylmercury kinetics in the dog. Animals previously dosed with 203Hg-labeled methylmercury at 2.5 mg Hg/kg received a priming dose of DMSA, followed by a continuous iv infusion at dose rates that resulted in DMSA concentrations in plasma similar to those observed during DMSA complexing hemodialysis therapy. The kinetics of 203Hg removal in DMSA-infused dogs was compared with both saline-infused controls and DMSA complexing hemodialysis treated dogs. DMSA infusion therapy resulted in a shift in 203Hg binding within systemic blood from the red-cell fraction into plasma. This was consistent with an observed association of DMSA with the plasma fraction. The shift in 203Hg from red cells into plasma was paralleled by an increase in urinary clearance of 203Hg during the DMSA infusion period. In four dogs treated in this fashion, an average of 6.5 micrograms of mercury was removed by the urinary route over the 5-h treatment period, compared to 0.007 micrograms in the saline-infused dogs. Although a similar magnitude of mercury output into urine was observed during DMSA complexing hemodialysis, an additional 5 micrograms was removed by the dialyzer, making that technique 1.5 times as effective as infusion therapy. Comparing 203Hg tissue concentration after DMSA infusion therapy with the saline-treated controls revealed a 6.5-fold decrease in liver, a 3-fold reduction in kidney, and a 27% reduction in cerebrum. No significant differences were observed in medulla or cerebellum. Histopathology revealed no consistent differences between DMSA-treated and saline-treated animals. The DMSA infusion therapy was effective in causing a rapid removal of 203Hg from animals previously dosed with 203Hg-labeled methylmercury. DMSA infusion therapy may provide a useful therapeutic alternative for methylmercury poisoning when rapid removal of the intoxicant is desired and hemodialysis equipment and expertise are not readily available.

Preliminary toxicity studies on bis-[beta-(N,N-dimorpholino) ethyl] selenide (MOSE). A new radioimaging agent.

The present study describes the acute toxicity of MOSE, a proposed radioimaging agent for brain scintigraphy . Acute intraperitoneal administration of MOSE in mice revealed an LD50 between 1.35 and 6.25 g/kg, with convulsions preceding death. Intravenous administration of MOSE in rats resulted in an LD50 between 400 and 800 mg/kg, with death also preceded by convulsions. The rabbit was more sensitive to the acute effects of MOSE than the rat. The LD50 for MOSE given i.v. in the rabbit was 80 mg/kg. The predominant toxic sign was convulsions, which immediately preceded death at high doses. At intermediate doses convulsions were elicited, followed by a period of lethargy which gave way to hyperactivity on the following day. Normal appearances were restored within a week. Hematology and blood chemistries were similar to controls, except for increased serum LDH in animals receiving MOSE when sampled two weeks after dosing. Repeated administration of MOSE by the intravenous route in rabbits at a dose rate of 1 mg/kg/da, five days per week for two weeks, resulted in no signs of toxicity. Hematology, clinical chemistry, and histology revealed no changes in animals receiving MOSE when compared to control. It was concluded that barring any unusual susceptibility in man, the proposed diagnostic dose to man is unlikely to precipitate any acute toxic effects.

Preliminary toxicity studies on N,N,N'-trimetyl-N' (2-hydroxyl-3-methyl-5-iodobenzyl)-1,3-propanediamine (HIPDM), a new brain perfusion imaging agent.

HIPDM is a promising new agent for use in conjunction with single-photon emission tomography (SPECT) for studying local cerebral perfusion. The acute toxicity of HIPDM was studied in the rat and the rabbit. The LD50 for HIPDM in the rat was 36 mg/kg. Animals displayed signs of CNS involvement in toxicity which included convulsions, and locomotor deficits. The LD50 in the rabbit was approximately 20 mg/kg. Both loss of motor coordination and convulsions were the predominant signs of toxicity. Both the rat and rabbit displayed little individual variation in response to HIPDM as indicated by the steepness of the dose response curves. Repeated administration of HIPDM in the rabbit for two weeks at 1.4 mg/kg/day resulted in no effects on weight gain. Similarly there were no adverse effects on clinical chemistry and hematological parameters in serum and blood. Terminal tissue analysis revealed no induction of histopathological lesions in the brain or other tissues. It is unlikely that acute toxic manifestations will result from doses necessary for diagnostic purposes in man.

The enzymatic defluorination of fluoroacetate in mouse liver cytosol: the separation of defluorination activity from several glutathione S-transferases of mouse liver.

The liberation of free fluoride ion from fluoroacetate (FAc) proceeds as an enzyme-catalyzed dehalogenation reaction in the soluble fractions of several organs of the CFW Swiss mouse. Liver contained the highest FAc defluorinating activity. The enzyme activity in other organs decreased in the order kidney greater than lung greater than heart greater than testes. No activity was detected in the brain. Experiments were designed to characterize and identify the enzyme species responsible for FAc metabolism in liver. Enzyme activity was dependent on the concentration of glutathione (GSH) in the assay mixture, with maximal activity occurring above 5 mM. The dehalogenation of FAc had an apparent Km of 7.0 mM when measured in the presence of a saturating concentration of GSH. An increase in the pH of the assay mixture enhanced fluoride release in both phosphate and borate buffer. The defluorination activity was reduced to negligible levels when stored for 24 h at 4 degrees C. The addition of either GSH, dithiothreitol, or 2-mercaptoethanol increased stability, with the latter providing protection for greater than 150 h at a concentration of 15 mM. DEAE anion-exchange chromatography separated the defluorinating activity from 90% of the soluble GSH S-transferase activity measured with 1-chloro-2,4-dinitrobenzene. FAc defluorination activity did not bind to a GSH affinity column which selectively separates it from a group of anionic GSH S-transferases. The GSH-dependent enzyme which dehalogenates FAc has unique properties and can be separated from the liver GSH S-transferases previously described in the literature.

A methylmercury toxicity model to test for possible adverse effects resulting from chelating agent therapy.

A daily dosing model for methylmercury (MM) intoxication was developed for the purpose of testing for possible adverse effects resulting from the administration of complexing agents used in the treatment of MM poisoning. The dithiol complexing agents 2,3-dimercaptopropanol (BAL) and meso-2,3-dimercaptosuccinic acid (DMSA) were chosen to test the discriminative ability of this model, since the former is contraindicated for MM poisoning and causes an increase in target organ MM burden, while the latter compound is known to be efficacious in reducing both toxicity and brain MM content. The basic design of the model called for daily observation of treated animals with identification of the following signs of MM intoxication: loss of body weight, onset of signs of toxicity, and mortality. The degree of toxicity was evaluated, and a toxicity score (0-5) was provided for each animal. A dose-dependent decrease in body weight was found in MM-treated mice. The latent period for development of signs of intoxication varied inversely with the dose rate. The rate of progression of severity of signs of intoxication was also dependent upon the dose. A dose rate of 14 mg Hg per kg per day was utilized to test the effects of BAL and DMSA on the onset and progression of signs of MM intoxication. Onset and progression of signs of methylmercury intoxication were similar for animals receiving methylmercury either alone or with administration of BAL at 2 mg per kg per day. Animals which received BAL at a dose rate of 20 mg per kg per day developed signs of intoxication significantly earlier.(ABSTRACT TRUNCATED AT 250 WORDS)

Involvement of glutathione in the enhanced renal excretion of methyl mercury in CFW Swiss mice.

The present studies attempted to identify the mechanism for the elevated urinary excretion rate for methyl mercury (MM) previously reported in CFW Swiss mice. Strain comparisons of factors which could conceivably influence renal excretion of MM were made. The biotransformation of MM to the inorganic form did not appear to play a significant role. No significant strain differences were observed in the distribution of MM between plasma and red cells under in vivo or in vitro conditions. The percentage of total plasma MM present in the low-molecular-weight fraction did not differ statistically between the CFW and CBA/J strains. Strain comparisons of total reduced nonprotein thiol concentrations in liver, kidneys, whole blood, and plasma revealed no significant strain differences. A significant strain difference in plasma oxidized glutathione (GSSG) concentrations was observed. However, plasma concentrations of reduced glutathione (GSH), the form of glutathione (GS) which interacts with MM, did not significantly vary between the strains. The rate of total glutathione (TGS) excretion in urine was approximately 2-fold higher in CFW mice than in CBA/J mice. The significantly higher urinary GS excretion in CFW mice was accompanied by a 1.6-fold lower urinary gamma-glutamyltranspeptidase (gamma-GTP) activity in this strain.

Effect of L-(alpha S,5S)-alpha-amino-3-chloro-4,5-dihydro-5-isoxazoleacetic acid on urinary excretion of methylmercury in the mouse.

The gamma-GTP inhibitor L-(alpha S,5S)-alpha-amino-3-chloror-4,5-dihydro-5-isoxazoleacetic acid (AT-125) was administered to CBA/J mice pretreated with 203Hg-methylmercury (MM) (0.5 mg/kg) to determine whether increasing urinary GS in this strain would result in a simultaneously increased urinary elimination of MM. Doses at AT-125 ranging from 3.0 to 30 mg/kg increased urinary GS in a dose-related fashion. The peak effect was attained at 2 hr after injection. AT-125 (7.5 mg/kg) increased urinary GS to peak value of 450 microM. When this dose was administered to mice 24 hr after MM, neither the rate of decline in body burden of 203Hg nor the rate of 203Hg output into urine or feces varied significantly from control values. A dose of 15 mg of AT-125 per kg increased urinary GS to 1.0 mM and caused a significant increase in urinary excretion of 203Hg when compared to the 7.5-mg dose group. The greatest effect was seen at the 30-mg AT-125/kg dose which produced a 1.9 mM concentration of total GS in urine and a 2-fold increase in the urinary 203Hg excretion rate. The dose-dependent changes in urinary excretion elicited by AT-125 were paralleled by increased rates of decline of 203Hg body burden and decreased rates of excretion in the feces. The results suggest that urinary GS must approach the millimolar range before affecting a redistribution of MM across the luminal membrane.