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.

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The mechanisms of cellular lead uptake were characterized using a fluorescence method in cells loaded with indo-1. Pb 2؉ bound to intracellular indo-1 with much higher affinity than Ca 2؉ Lead is a ubiquitous environmental contaminant that can damage various organs, including those of the neurological, hematological, renal, and reproductive systems. Although it is readily taken up by many tissues, very little is known about the mechanisms of lead transport into cells. Lead is taken up in human red blood cells via anion exchange, probably as PbCO 3 (1). Based on in vivo studies, it has been suggested that lead transport across the rat blood-brain barrier is passive and pH-dependent and that the transported species is PbOH ϩ (2). Pb 2ϩ has been reported to enter electrically excitable bovine chromaffin cells via L-type voltage-sensitive Ca 2ϩ channels (VSCCs) 1 (3, 4). Pb 2ϩ is known to substitute for Ca 2ϩ in many cellular processes and to interfere with reactions that require Ca 2ϩ . The present investigation was carried out to determine whether Pb 2ϩ can enter cells through Ca 2ϩ channels. We studied the involvement of both VSCCs and voltage-insensitive channels responsible for the Ca 2ϩ uptake that occurs in response to depletion of intracellular Ca 2ϩ stores, which is referred to as capacitative or store-operated Ca 2ϩ uptake (5-8). Three different cell lines were used. Electrically excitable GH 3 cells, a line derived from rat pituitary, were chosen because they have very well characterized L-type VSCCs (9). C 6 , a nonexcitable rat glioma line, was used because it is derived from brain, one of the principal target organs for Pb 2ϩ toxicity. Nonexcitable 301 cells, a subline of HEK293 that is stably transfected with the G protein-coupled, Ca 2ϩ -mobilizing receptor for thyrotropin-releasing hormone (TRH) (10), were used because they display a large capacitative Ca 2ϩ uptake in response to TRH. A new method for monitoring the uptake of Pb 2ϩ into cells, using the fluorescent dye indo-1, is also described. This commonly used Ca 2ϩ indicator has a very high affinity for Pb 2ϩ . Binding to Pb 2ϩ quenches indo-1 fluorescence at all wavelengths, and fluorescence can be monitored using an emission wavelength at which fluorescence is insensitive to changes in Ca 2ϩ concentration. This method offers the advantages of convenience, sensitivity, rapidity, and the opportunity to monitor Pb 2ϩ uptake in real time by fluorescence spectroscopy or microscopy. Using this fluorimetric method, we have found a novel pathway for Pb 2ϩ influx into cells. We report here that Pb 2ϩ enters cells via voltage-insensitive cation channels that are activated by the depletion of intracellular Ca 2ϩ stores. EXPERIMENTAL PROCEDURESMaterials-Cell culture reagents and physiological salt solutions were obtained from Life Technologies, Inc. Tissue culture plasticware was from Corning. Indo-1 acetoxymethyl ester, indo-1 pentapotassium salt, fura-2 pentapotassium salt, diethylenetriamine-pentaacetic anhydride (DTPA), and tetrakis-(2-pyridylmethy...

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ContextUS EPA proposed a Reference Concentration for Libby amphibole asbestos based on the premise that pleural plaques are adverse and cause lung function deficits.ObjectiveWe conducted a systematic review to evaluate whether there is an association between pleural plaques and lung function and ascertain whether results were dependent on the method used to identify plaques.MethodsUsing the PubMed database, we identified studies that evaluated pleural plaques and lung function. We assessed each study for quality, then integrated evidence and assessed associations based on the Bradford Hill guidelines. We also compared the results of HRCT studies to those of X-ray studies.ResultsWe identified 16 HRCT and 36 X-ray studies. We rated six HRCT and 16 X-ray studies as higher quality based on a risk-of-bias analysis. Half of the higher quality studies reported small but statistically significant mean lung function decrements associated with plaques. None of the differences were clinically significant. Many studies had limitations, such as inappropriate controls and/or insufficient adjustment for confounders. There was little consistency in the direction of effect for the most commonly reported measurements. X-ray results were more variable than HRCT results. Pleural plaques were not associated with changes in lung function over time in longitudinal studies.ConclusionThe weight of evidence indicates that pleural plaques do not impact lung function. Observed associations are most likely due to unidentified abnormalities or other factors.

Inhalation of naphthalene causes olfactory epithelial nasal tumors in rats (but not in mice) and benign lung adenomas in mice (but not in rats). The limited available human data have not identified an association between naphthalene exposure and increased respiratory cancer risk. Assessing naphthalene's carcinogenicity in humans, therefore, depends entirely on experimental evidence from rodents. We evaluated the respiratory carcinogenicity of naphthalene in rodents, and its potential relevance to humans, using our Hypothesis-Based Weight-of-Evidence (HBWoE) approach. We systematically and comparatively reviewed data relevant to key elements in the hypothesized modes of action (MoA) to determine which is best supported by the available data, allowing all of the data from each realm of investigation to inform interpretation of one another. Our analysis supports a mechanism that involves initial metabolism of naphthalene to the epoxide, followed by GSH depletion, cytotoxicity, chronic inflammation, regenerative hyperplasia, and tumor formation, with possible weak genotoxicity from downstream metabolites occurring only at high cytotoxic doses, strongly supporting a non-mutagenic threshold MoA in the rat nose. We also conducted a dose–response analysis, based on the likely MoA, which suggests that the rat nasal MoA is not relevant in human respiratory tissues at typical environmental exposures. Our analysis illustrates how a thorough WoE evaluation can be used to support a MoA, even when a mechanism of action cannot be fully elucidated. A non-mutagenic threshold MoA for naphthalene-induced rat nasal tumors should be considered as a basis to determine human relevance and to guide regulatory and risk-management decisions.

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