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T W Clarkson

Publications and source records attributed to T W Clarkson.

At least 19 recordsLinked to original sources
PubMed1995

Environmental contaminants in the food chain.

Both terrestrial and aquatic food chains are capable of accumulating certain environmental contaminants to toxic concentrations. This article focuses on the aquatic food chain because we have less control over contaminant entry into this chain than we have for the terrestrial chain. In general, at least three special properties are required for a contaminant to bioaccumulate in an aquatic food chain: 1) a high octanol-water partition coefficient, 2) chemical and metabolic stability in water and in organisms in the food chain, and 3) a low toxicity to organisms in the chain so that the chain is not broken by loss of an intermediate species. Few of the thousands of chemicals produced by human industry meet these requirements. In terms of organic chemicals, the best known examples of bioaccumulation in aquatic food chains are the polychlorinated biphenyls (PCBs), dioxins, and organochlorine pesticides such as dichlorodiphenyltrichloroethane (DDT). Few examples exist of bioaccumulation of metal compounds. Methylmercury is arguably the most dramatic and best documented example of high bioaccumulation.

Animals↗
PubMed1995

Biological monitoring of iodine, a water disinfectant for long-term space missions.

In order to establish guidelines for exposure of astronauts to iodine, used as a water disinfectant in space, we studied the usefulness of hair, saliva, and urine for biological monitoring in humans and in the human hair/nude mouse model. The monitoring of iodine in patients that received 150 mCi of Na131I (carrier-free) showed similar patterns of elimination for blood, saliva, and urine. The mean correlation coefficient (r) between iodine elimination for blood/saliva was 0.99, for blood/urine, 0.95, and for saliva/urine, 0.97. The absolute value of iodine concentrations in urine revealed marked variability, which was corrected by adjusting for creatinine levels. The autoradiographic studies of human hair demonstrated that iodine is rapidly incorporated into external layers of the hair root and can be removed easily during washing. These data were confirmed after iodine exposure using the human hair/nude mouse model. Hair does not provide satisfactory information about exposure due to unstable incorporation of iodine. The most useful medium for biological monitoring of astronauts exposed to high doses of iodine in drinking water is urine, when adjusted for creatinine, and saliva, if quantitative evaluation of flow rate is provided.

Animals↗
PubMed1995

Methylmercury-thiol uptake into cultured brain capillary endothelial cells on amino acid system L.

Recent in vivo studies suggest that the neurotoxin methylmercury (MeHg) is transported into brain as an L-cysteine complex by amino acid transport system L. To test this hypothesis, the mechanism of MeHg uptake into cultured calf brain capillary endothelial cells, an in vitro model of the blood-brain barrier, was examined. Uptake of Me203Hg-L-cysteine followed Michaelis-Menten kinetics, with a Km of 234 +/- 58 microM (mean +/- S.E.) and a Vmax of 57 +/- 25 pmol.micrograms DNA-1.15 sec-1. Uptake of 10 microM MeHg-L-cysteine was stereoselective and Na+ independent and it was inhibited by the system L substrates L-leucine, 2-amino-2-norbornanecarboxylic acid and L-methionine (5 mM), consistent with transport of MeHg-L-cysteine by the L amino acid carrier. L-Glutamate and methylaminoisobutyric acid, which are transported by the acidic and A amino acid carriers, respectively, had no effect. Moreover, uptake of 3H-L-leucine (5 microM) was inhibited by 1 mM MeHg-L-cysteine is transported into brain capillary endothelial cells by the L carrier. Uptake of other MeHg-thiols was also measured. MeHg-D, L-homocysteine uptake was 82 +/- 11% of MeHg-L-cysteine uptake, whereas uptakes of MeHg complexes of L-penicillamine, dimercaptosuccinic acid, N-acetyl-L-cysteine and glutathione were 57 +/- 16%, 19 +/- 7%, 10 +/- 4% and 8 +/- 5% of MeHg-L-cysteine uptake, respectively. These results illustrate the potential to minimize transport of MeHg across brain capillary endothelium by the careful choice of thiol complexing agent.

Amino Acids↗
PubMed1994

Methylmercury poisoning: long-term clinical, radiological, toxicological, and pathological studies of an affected family.

For 3 months in 1969 a family in the United States that included a pregnant mother consumed pork containing methylmercury. Children, aged 20, 13, and 8 years and a neonate, developed severe neurological signs. Twenty-two years later, the 2 oldest had cortical blindness or constricted visual fields, diminished hand proprioception, choreoathetosis, and attentional deficits. Magnetic resonance images showed tissue loss in the calcarine and parietal cortices and cerebellar folia. The youngest had quadriplegia, blindness, and severe mental retardation until their deaths. The brain of the 8-year-old who died at age 30 showed cortical atrophy, neuronal loss, and gliosis, most pronounced in the paracentral and parietooccipital regions. The total mercury level in formalin-fixed, left occipital cortex was 1,974 ng/gm as measured by atomic absorption. Regional brain mercury levels correlated with extent of brain damage. A control patient had 38.5 ng of mercury/gm in the occipital cortex. Systemic organs in the patient and a control subject had comparable mercury levels. In mercury-intoxicated rats, we found that only 5 to 10% of total brain mercury was lost by formalin fixation. Brain inorganic mercury in the patient ranged from 82 to 100%. Since inorganic mercury crosses the blood-brain barrier poorly, biotransformation of methyl to inorganic mercury may have occurred after methylmercury crossed the blood-brain barrier, accounting for its persistence in brain and causing part of the brain damage.

Adolescent↗
PubMed1994

Establishment and characterization of methylmercury-resistant PC12 cell line.

Methylmercury (MeHg)-resistant sublines of rat pheochromocytoma (PC12) cells were isolated by repeated exposure to stepwise increased concentrations of MeHg. One of the sublines (PC12/TM) showed an 8- to 10-fold increase in resistance to MeHg compared with parent PC12 cells on the basis of the concentration required for 50% inhibition (IC50) of growth. PC12/TM cells accumulated smaller amounts of MeHg than parent PC12 cells. This reduction in MeHg accumulation in PC12/TM cells resulted from slow uptake and rapid efflux. The intracellular glutathione (GSH) level in PC12/TM cells was four times higher than that of PC12 cells. Pretreatment of PC12/TM cells with buthionine sulfoximine, which decreased the GSH level to that of the parent PC12 cells, increased the sensitivity of PC12/TM cells to MeHg. A close correlation between the MeHg accumulation and MeHg sensitivity was found among seven sublines of PC12 cells and parent PC12 cell line. The GSH level in PC12 sublines was also correlated with their sensitivity to MeHg.

Animals↗
PubMed1993

Reduced methylmercury accumulation in a methylmercury-resistant rat pheochromocytoma PC12 cell line.

Rat pheochromocytoma PC12 cell sublines with increasing resistance to methylmercury (MeHg) were isolated by exposure to MeHg in two sequential steps. The strongest resistance (PC12/TM) among the clones obtained by the first cloning was characterized. PC12/TM cells exhibited about an 8- to 10-fold increase in resistance compared with parent PC12 cells on the bases of the concentration required for 50% inhibition of growth and colony-forming activity. PC12/TM cells accumulated smaller amounts of MeHg (one-half to one-fifth) than parent PC12 cells. This reduced MeHg accumulation in PC12/TM cells resulted from the slow uptake and rapid efflux. A close correlation between reduced MeHg accumulation and MeHg resistance was found among seven sublines of PC12 cells with different sensitivity to MeHg. The reduced retention of MeHg in PC12 sublines was also well correlated with the sensitivity to MeHg. Phenol-3,6-dibromophthalein disulfonate inhibited MeHg efflux from PC12/TM cells and increased its accumulation. These results suggest that efflux of MeHg from PC12/TM cells is associated with glutathione.

Animals↗
PubMed1993

Mercury: major issues in environmental health.

In the past, methylmercury compounds were manufactured as fungicides or appeared as unwanted byproducts of the chemical industry, but today the methylation of inorganic mercury in aquatic sediments and soils is the predominant if not the sole source of methylmercury. This form of mercury is bioaccumulated to a high degree in aquatic food chains to attain its highest concentrations in edible tissues in long-lived predatory fish living in both fresh and ocean waters. It is well absorbed from the diet and distributes within a few days to all tissues in the body. It crosses without hindrance the blood-brain and placental barriers to reach its principal target tissue, the brain. It is eliminated chiefly in the feces after conversion to inorganic mercury. The biological half-time of methylmercury in human tissues is about 50 days, but there is wide individual variation. Adult poisoning is characterized by focal damage to discrete anatomical areas of the brain such as the visual cortex and granule layer of the cerebellum. A latent period of weeks or months may ensue before the appearance of signs and symptoms of poisoning. The latter manifest themselves as paresthesia, ataxia, constriction of the visual fields, and hearing loss. The prenatal period is the most sensitive stage of the life cycle to methylmercury. Prenatally poisoned infants exhibit a range of effects from severe cerebral palsy to subtle developmental delays. Methylmercury is believed to inhibit those processes in the brain specially involved in development and growth such as neuronal cell division and migration.

Dose-Response Relationship, Drug↗
PubMed1992

Methylmercury transport across the blood-brain barrier by an amino acid carrier.

The mechanism by which methylmercury (MeHg) crosses the blood-brain barrier was examined in the rat. Previous studies demonstrated that intravenous injection of L-cysteine with MeHg accelerates MeHg uptake into brain. Since the complex of MeHg with L-cysteine is structurally similar to L-methionine, a substrate for the L (leucine-preferring) amino acid transport system, this carrier may be involved in MeHg uptake. To examine this hypothesis, the rapid carotid infusion technique was used in the anesthetized rat. The concentration dependence of 203Hg uptake into brain after injection of Me203Hg-L-cysteine complex was nonlinear, exhibiting characteristics of saturable transport (apparent Michaelis constant 0.39 mM, vmax 33 nmol.min-1.g-1). A slower, nonsaturable uptake was seen after MeHg-L-cysteine uptake was inhibited by methionine and the amino acid analogue 2-aminobicyclo[2.2.1]heptane-2-carboxylic acid (BCH), an L system substrate, but not by alpha-methylaminoisobutyric acid, an alanine-preferring system substrate. Furthermore, L-[14C]methionine transport was inhibited by MeHg-L-cysteine but not by MeHgCl. There was a significant amount of uptake of 203Hg when injected as Me203Hg-glutathione, and this was inhibited by L-methionine and BCH but not D-methionine. S-ethylglutathione also inhibited 203Hg uptake after administration as Me203Hg-glutathione but had no effect on Me203Hg-L-cysteine uptake. These results suggest that MeHg may enter the brain as a cysteine complex via the L system and that plasma MeHg-glutathione serves as a source of MeHg-cysteine.

Amino Acid Transport Systems↗
PubMed1992

Principles of risk assessment.

This review discusses the basic principles of risk assessment as used in general toxicology and in monitoring side-effects of therapeutic treatments. It also outlines how these principles may apply to assessment of biological reactions to dental restorative materials. Mercury exposure from amalgam fillings is used as an example. The calculations performed are intended only as illustrations, and many other factors must be taken into account. Thus, the calculations are not intended as the last word in risk assessment of amalgam fillings.

Body Burden↗
PubMed1991

Biliary-hepatic recycling of a xenobiotic: gallbladder absorption of methyl mercury.

The role of the gallbladder in the disposition of methyl mercury was investigated in guinea pig, hamster, and macaque monkey. 203Hg-labeled methyl mercury or inorganic mercury (5 microM) and [14C]inulin were instilled into the in situ guinea pig or hamster gallbladder. After 2 h, only 27.6 +/- 7.0% of the methyl mercury remained in guinea pig gallbladder fluid as compared with 85.0 +/- 3.2% of the inorganic mercury and 90.7 +/- 4.5% of the [14C]-inulin. In the hamster, 42.5 +/- 4.5% of methyl mercury and 95% +/- 0.9% of inorganic mercury remained after 2 h. When the sulfhydryl-containing compounds L-cysteine, glutathione, and bovine serum albumin (20 microM) were added to the test solution, cysteine increased and albumin decreased absorption of methyl mercury. Ligation of guinea pig cystic artery decreased gallbladder fluid absorption from 72.7 +/- 8.6 to 26.5 +/- 9.8% over 2 h but did not alter methyl mercury absorption. Bile was also sampled from gallbladders of four monkeys exposed chronically to CH3HgCl and from three control monkeys. For one of the exposed and one of the control monkeys, bile was also collected from the common hepatic duct. In both methyl mercury-exposed and control monkeys, the concentration of methyl mercury in gallbladder bile was lower than in hepatic bile. In contrast, the concentration of inorganic mercury in gallbladder bile was four to seven times that of hepatic bile, suggesting that methyl mercury but not inorganic mercury was being reabsorbed. To assess the functional significance of methyl mercury reabsorption by the gallbladder, guinea pig cystic ducts were ligated, the animals were given CH3 203HgCl (10 mumol/kg iv), and body burden of 203Hg was measured over 16 days.(ABSTRACT TRUNCATED AT 250 WORDS)

Absorption↗
PubMed1991

Mercury and monomethylmercury: present and future concerns.

Global atmospheric changes carry the potential to disrupt the normal cycling of mercury and its compounds. Acid rain may increase methylmercury levels in freshwater fish. Global warming and increased ultraviolet radiation may affect the global budget of methylmercury, including its formation and degradation in both biotic and abiotic environments. In this article we review current knowledge on mercury and monomethylmercury with regard to their environmental fate and the potential for human health effects. Recent findings indicate that atmospheric Hg deposition readily accounts for the total mass of Hg in fish, water, and sediment of Little Rock Lake, a representative temperate seepage lake in north-central Wisconsin. It is strikingly evident that modest increases in atmospheric Hg loading could lead directly to elevated levels in the fish stock. It is doubtful, given the experimental limitations in many recent studies, that the temporal pattern for Hg emissions, for background atmospheric Hg concentrations, and for changes in Hg depositional fluxes has been identified. Thus, the present and future questions of whether the environmental impact is of local, regional, or hemispheric significance remain. Contemporary investigations must address these important questions. Human exposure to methylmercury in the United States is probably increasing due to increased consumption of fish and fish products. A recent epidemiological investigation indicates high susceptibility to brain damage during prenatal exposures to Hg. An important objective for future investigation is to establish the lowest effect level for human exposure to methylmercury.

Air Pollutants↗
PubMed1990

Uptake of mercury by the hair of methylmercury-treated newborn mice.

Human hair has unique advantages in monitoring environmental exposures to methylmercury. Using newborn Balb/c mice as a model system, the incorporation of methylmercury into the hair was studied and compared with methylmercury distributions in other tissues. Newborn mice were given intraperitoneal injections of 203Hg-labeled methylmercury at designated times according to hair growth stages of the mouse. Animals were sacrificed 2 days after dosing. Distribution of mercury in pelt and other tissues was measured. The level of mercury in pelt was found to correlate with hair growth. The amount of mercury in pelt peaked when hair growth was most rapid and the total amount of mercury in pelt was significantly higher than that in other tissues, constituting 40% of the whole body burden. However, when the hair ceased growing, the amount of mercury in pelt dramatically dropped to 4% of whole body burden and mercury concentrations in other tissues except brain were elevated. Autoradiographic studies with tritium-labeled methylmercury demonstrated that methylmercury concentrated in hair follicles in the skin. Within hair follicles and hairs, methylmercury accumulated in regions that are rich in high-sulfur proteins. The uptake of inorganic mercury (administered as HgCl2) by pelt was also compared with that of methylmercury. The amount of inorganic mercury found in pelt was less than one-half that of methylmercury in animals with growing hair. Cessation of hair growth did not decrease the inorganic mercury level in pelt to the same extent as in the case of methylmercury.

Animals↗
PubMed1990

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.

Aged↗
PubMed1989

The effects of dose of elemental mercury and first-pass circulation time on exhalation and organ distribution of inorganic mercury in rats.

The lung plays a major role in the removal of dissolved elemental mercury (Hg0) from the bloodstream. During the first passage through the lung after an intravenous dose of Hg0 dissolved in aqueous buffer, from 10 to 17% was exhaled depending on the dose (0.11 or 1.1 micrograms Hg/rat) and the injection site (jugular versus tail vein). Furthermore, evidence is presented that subsequent exhalation over the next 50 s, before the rats were killed and the mercury determined in the lung at that time, was largely Hg0-extracted during the first pass. The total mercury extracted during the 60 s period was in the range of 40-49% of the dose. The oxidation of Hg0 to Hg2+ in red cells is important in limiting the availability of Hg0 to certain tissues. Thus, after a short residence time in blood (0.6 s after jugular vein injection), 12.9-17% is exhaled in the first pass as compared to 10.4-12.2% with a longer residence time (1.8 s after tail vein injection). Furthermore, there was a general tendency, even at 60 s after dosing, for certain tissues - lung, brain, and heart - to have higher values after dosing from the jugular vein. It was estimated that the half-time for oxidation was 3.3 s. Our results confirm previous observations that the form of inorganic mercury greatly influences the short-term deposition in certain tissues. Thus as compared to Hg2+, administration of Hg0 increases lung levels 5-10-fold; brain, 4-fold; and heart, 3-fold. Blood levels are lower after Hg0, particularly after the higher dose. Such findings are consistent with a model wherein Hg0 is in part oxidized by red blood cells, the remainder rapidly diffusing in tissues where it is also oxidized to Hg2+.

Animals↗
PubMed1989

Vasocystostomy: a model for studying male reproductive toxicity in the rat.

A nonsacrificial rat model has been developed which permits the easy collection and measurement of spermatozoa. The ductus deferens is anastomosed to the bladder (Vasocystostomy) and urinary sperm is collected daily. The correlation between testicular histology and sperm counts indicates that this model is reliable. Using this model we demonstrate inhibition of sperm production by fluoroacetate.

Anastomosis, Surgical↗