The effect of dose, dose rate, route of administration, and species on tissue and blood levels of benzene metabolites

Studies were completed in F344/N rats and B6C3F1 mice to determine the effect of dose, dose rate, route of administration, and rodent species on formation of total and individual benzene metabolites. Oral doses of 50 mg/kg or higher saturated the capacity for benzene metabolism in both rats and mice, resulting in an increased proportion of the administered dose being exhaled as benzene. The saturating air concentration for benzene metabolism during 6-hr exposures was between 130 and 900 ppm. At the highest exposure concentration, rats exhaled approximately half of the internal dose retained at the end of the 6-hr exposure as benzene; mice exhaled only 15% as benzene. Mice were able to convert more of the inhaled benzene to metabolites than were rats. In addition, mice metabolized more of the benzene by pathways leading to the putative toxic metabolites, benzoquinone and muconaldehyde, than did rats. In both rats and mice, the effect of increasing dose, administered orally or by inhalation, was to increase the proportion of the total metabolites that were the products of detoxification pathways relative to the products of pathways leading to putative toxic metabolites. This indicates low-affinity, high-capacity pathways for detoxification and high-affinity, low-capacity pathways leading to putative toxic metabolites. If the results of rodent studies performed at high doses were used to assess the health risk at low-dose exposures to benzene, the toxicity of benzene would be underestimated.

Differences in the pathways for metabolism of benzene in rats and mice simulated by a physiological model

Studies conducted by the National Toxicology Program on the chronic toxicity of benzene indicated that B6C3F1 mice were more sensitive to the carcinogenic effects of benzene than were F344 rats. A physiological model was developed to describe the uptake and metabolism of benzene in rats and mice. Our objective was to determine if differences in toxic effects could be explained by differences in pathways for benzene metabolism or by differences in total uptake of benzene. Compartments incorporated into the model included liver, fat, a poorly perfused tissue group, a richly perfused tissue group, an alveolar or lung compartment and blood. Metabolism of benzene was assumed to take place only in the liver and to proceed by four major competing pathways. These included formation of hydroquinone conjugates (HQC), formation of phenyl conjugates (PHC), ring-breakage and formation of muconic acid (MUC), and conjugation with glutathione with subsequent mercapturic acid (PMA) formation. Values for parameters such as alveolar ventilation, cardiac output, organ volumes, blood flow, partition coefficients, and metabolic rate constants were taken from the literature. Model simulations confirmed that during and after 6-hr inhalation exposures mice metabolized more benzene on a mumole per kilogram body weight basis than did rats. After oral exposure, rats metabolized more benzene than mice at doses above 50 mg/kg because of the more rapid absorption and exhalation of benzene by mice. Model simulations for PHC and PMA, generally considered to be detoxification metabolites, were similar in shape and dose-response to those for total metabolism.(ABSTRACT TRUNCATED AT 250 WORDS)

A physiological model for simulation of benzene metabolism by rats and mice

Studies conducted by the National Toxicology Program on the chronic toxicity of benzene indicated that B6C3F1 mice are more sensitive to the toxic effects of benzene than are F344 rats. A physiological model was developed to describe the uptake and metabolism of benzene in rats and mice and to determine if the observed differences in toxic effects could be explained by differences in the pathways for metabolism of benzene or by differences in uptake of benzene. Major pathways for elimination of benzene included metabolism to hydroquinone glucuronide or hydroquinone sulfate, phenyl glucuronide or phenyl sulfate, muconic acid, and prephenyl mercapturic acid or phenyl mercapturic acid. Model simulations for total benzene metabolized and for profiles of benzene metabolites were conducted for oral or inhalation exposure and compared to data for urinary excretion of benzene metabolites after exposure of rats and mice to [14C]- or [3H]-benzene by inhalation or gavage. Results for total amount of benzene metabolized, expressed per kilogram body weight, indicated that for inhalation exposure concentrations up to 1000 ppm, mice metabolized at least two to three times as much benzene as did rats. Simulations of oral exposure to benzene resulted in more benzene metabolized per kilogram body weight by rats at oral exposures of greater than 50 mg/kg. Patterns of metabolites formed after either route of exposure were very different for F344/N rats and B6C3F1 mice. Rats primarily formed the detoxification metabolite, phenyl sulfate. Mice formed hydroquinone glucuronide and muconic acid in addition to phenyl sulfate. Hydroquinone and muconic acid are associated with pathways leading to the formation of the putative toxic metabolites of benzene. Metabolic rate parameters, Vmax and Km, were very different for hydroquinone conjugate and muconic acid formation compared to formation of phenyl conjugates and phenyl mercapturic acids. Putative toxication pathways could be characterized as high affinity, low capacity whereas detoxification pathways were low affinity, high capacity. Model simulations suggested that for both rats and mice at lower exposure concentrations hydroquinone and muconic acid represented a larger fraction of the total benzene metabolized than at higher exposure concentrations where detoxification metabolites were predominant. Preferential production of a putative toxic metabolite at low exposure concentrations may have important implications in risk assessment for benzene.

Effect of four-week repeated inhalation exposure to unconjugated azodicarbonamide on specific and non-specific airway sensitivity of the guinea pig

Reports of respiratory problems among industrial workers exposed repeatedly by inhalation to azodicarbonamide (ADA) raised concern that ADA might be a pulmonary sensitizer. We used a non-invasive method for measuring specific airway conductance to evaluate the potential for repeated inhalation of unconjugated ADA to cause specific or non-specific pulmonary sensitization in the guinea pig. Two groups of male Hartley guinea pigs were exposed 6 h/day, 5 days/week for 4 weeks to aerosolized ADA at 51 or 200 mg/m3, or to filtered air as controls. One group was tested for specific sensitization to ADA by measuring specific airway conductance during inhalation challenge with ADA before and on the third day after the 4-week ADA exposure. The ADA concentrations for the challenges were identical to the repeated exposure concentrations (51 or 200 mg/m3, 200 mg/m3 for controls). The other group was tested for non-specific airway sensitization by inhalation challenge with aerosolized histamine before and after the 4-week ADA exposure. Histamine was administered in stepwise increasing concentrations to elicit an airway response in each guinea pig. Skin tests for immunological responses to ADA, body weight and histopathology of the respiratory tract and skin test sites were also evaluated. The 4-week exposure to ADA did not result in either specific or non-specific airway sensitization. The ADA exposure did not induce positive skin reactions, influence body weight or cause histopathological responses. These results indicate that ADA, acting alone (i.e. not conjugated to a protein), is not a pulmonary sensitizer in the guinea pig exposed repeatedly for 4 weeks and challenged to simulate a 'Monday morning' exposure.

A toxikinetic model for simulation of benzene metabolism

People exposed to benzene, an important industrial solvent and a common pollutant, can develop aplastic anemia and leukemia. The objectives of this study were to develop a physiological model for the metabolism of benzene, based on studies in laboratory animals, and to use this model to predict benzene metabolism in people to concentrations near the current permissible exposure limits. Model simulations predicted that for 8-h inhalation exposures to below 10 ppm, hydroquinone metabolites would predominate. Hydroquinone is associated with pathways leading to the formation of the putative toxic metabolite, benzoquinone. Lower levels of muconic acid, a marker for the putative toxic metabolite, muconaldehyde, were predicted. At concentrations above 10 ppm, detoxification metabolites such as the phenyl conjugates predominate. Predictions of benzene metabolism in humans based on our physiological model may have important implications for risk assessment. Because there may be preferential production of a putative toxic metabolite at low exposure concentrations, linear extrapolation of toxicity observed at high concentrations may underestimate risk at low exposure concentrations.

The effect of molecular weight/lipophilicity on clearance of organic compounds from lungs

The objective of this study was to test the hypothesis that lipophilicity (as measured by the octanol/water partition coefficient, P) and/or molecular weight are determining factors in the rate of clearance of organic compounds from the lung. Previous work in our laboratory has shown that organic-soluble compounds such as pyrene, benzo[a]pyrene, 1-nitropyrene, 2-aminoanthracene, phenanthridone, dibenzo[c,g]carbazole, 1,3-dichloropropene, and methyl bromide, all of which have a log P less than 6.1, clear the lung rapidly (t 1/2 less than 12 hr). In the present study, organic compounds (mainly anthraquinone dyes) having a wider range of log P's (1.95-8.65) were instilled into rat lungs and the percentage of the compound retained in the lungs at 24 hr was determined. A positive correlation between the log of the theoretical P and the percentage of the compound retained in lungs at 24 hr was found. The lipophilicity of the series of compounds studied was highly dependent on the molecular weight, so that there was also a positive correlation between the molecular weight of the compounds and the percentage of the compound retained in the lung at 24 hr. To help understand the relative importance of lipophilicity and molecular weight in determining lung retention, an additional compound with a high molecular weight but containing a polar functional group [1,5-di(2-sulfo-p-toluidino)anthraquinone] was studied. The results indicated that the lipophilicity was the more important factor in whether the material was retained in the lung. On the basis of the results of this study, organic-soluble compounds with molecular weights less than 300 Da can be expected to clear the lungs rapidly. Nonpolar, organic-soluble compounds with a molecular weight greater than 300 Da can be expected to clear the lungs more slowly.

Disposition of inhaled 1-chloro-2-propanol in F344/N rats

Propylene chlorohydrins, of which 1-chloro-2-propanol (1-CP) is a constituent, used as intermediates in the manufacture of propylene oxide and have been identified as potential air pollutants. The objective of these studies was to determine whether changes in the inhaled exposure concentration would affect the disposition of 1-CP in rats. In addition, experiments were conducted to identify the carbon atom of 1-CP that is metabolized to CO2. Rats were exposed nose-only to [14C]1-CP for 6 hr to 8.3 +/- 1.0 ppm (26.1 +/- 3.2 micrograms/liter air) or 77 +/- 4 ppm (245 +/- 13 micrograms/liter air) (mean +/- SE). There were two major routes of elimination of 14C, urinary and exhalation of CO2, which together accounted for about 80% of the total 14C in excreta and carcass. Half-times for elimination of 14C in urine as 14CO2 were between 3 and 7 hr with no effect of exposure concentration on the elimination half-times for either route. After the end of exposure, kidneys, livers, trachea, and nasal turbinates contained high concentrations of [14C]1-CP equivalents at both exposure concentrations (30-50 nmol 14C/g tissue for the 8 ppm exposure level and 200-350 nmol 14C/g tissue for the 80 ppm exposure level). Elimination of 14C from tissues was biphasic with about 50% of the material in a tissue being rapidly eliminated with a half-time of 1 to 3 hr and the remaining material slowly eliminated with a half-time of 40 to 80 hr. There was no effect of exposure concentration on elimination half-times in tissues. Major metabolites detected in urine and tissues (liver, kidney, and lung) were N-acetyl-S-(hydroxypropyl)cysteine and/or S-(2-hydroxypropyl)-cysteine. Little unmetabolized 1-CP (less than 1%) was detected in analyzed tissues or urine. We propose a metabolic scheme in which the major pathway for metabolism of 1-CP is to CO2 (which is exhaled) and to cysteine conjugates and mercapturic acids that are excreted in the urine. Both carbon-2 and carbon-3 are metabolized in part to CO2.

Uptake of vinylidene fluoride in rats simulated by a physiological model

The purpose of this study was to develop a physiological model to simulate the uptake of vinylidene fluoride (VDF), an important plastics monomer, in laboratory animals. Male Fischer 344/N rats were exposed nose-only for 6 hr to concentrations of VDF ranging from 27 to 16,000 ppm. Tidal volume (mean, 1.51 ml/breath) and respiratory frequency (mean, 132 breaths/min) were not influenced by exposure concentration. Experimentally determined, steady-state blood levels of VDF, obtained by gas chromatography-head space analysis of samples from rats with indwelling jugular cannulas, increased linearly with increasing exposure concentration up to 16,000 ppm. VDF tissue/air partition coefficients were determined experimentally to be 0.07, 0.18, 0.8, 1.0, and 0.29 for water, blood, liver, fat, and muscle, respectively. These values and calculated constants for total body elimination of VDF, Km and Vmax were incorporated into the physiological model. Model predictions agreed with the experimentally determined data. Time to reach steady-state blood levels of VDF was less than 15 min for all concentrations. After cessation of exposure, blood levels of VDF decreased to 10% of steady-state levels by 1 hr. Simulation of the metabolism of VDF indicated that although blood levels of VDF increased linearly with increasing concentration the amount of VDF metabolized per 6-hr exposure period approached a maximum at about 2000 ppm VDF.

Lung, liver, and kidney as potential target organs after exposure to 1-nitropyrene, as determined by the time course of covalently bound material

Previous studies in which rats were exposed to [14C]-1-nitropyrene by inhalation indicated that lung, liver, and kidney consistently accumulated the highest concentrations of 14C after exposure. The purpose of the study described here was to determine the extent to which this 14C was covalently bound to macromolecules. Male F344/N rats were exposed to 360 ng [14C]-1-nitropyrene/l air for 1 h resulting in an average of 2.2 micrograms 1-nitropyrene deposited per rat. An additional group of rats was given 4.2 micrograms [14C]-1-nitropyrene by gavage. Total 14C in 23 tissues was determined for up to 96 h after inhalation exposure and up to 30 d after gavage. Lung, liver, and kidney contained the highest concentrations of 14C. Samples of these tissues were exhaustively extracted to determine the amount of radioactivity covalently bound to macromolecules. Regardless of the route of administration, the kidneys had the highest concentrations of covalently bound 14C. At 96 h after exposure kidneys had overall mean concentrations of 2.7 pmol bound/g tissue.micrograms nitropyrene administered. The overall mean concentration in liver was 0.18 pmol bound/g.microgram and the overall mean concentration in lung was 0.06 pmol/g.microgram at 96 h after exposure. Covalently bound material persisted in kidneys for the duration of the study (30 d postexposure). The calculated half-time for removal of bound 14C from kidneys was 150 d. These data suggest that kidney should be considered as one of the organs at risk after exposure to nitropyrene by inhalation or ingestion.

The determination of a volatile gas, vinylidene fluoride, in blood during a nose-only exposure

A method has been developed for measuring the volatile gas vinylidene fluoride (VDF) in the blood of rats during nose-only exposure. Blood was sampled via a jugular cannula constructed from silastic tubing. The silastic cannula was sutured and glued to the vein and was passed subcutaneously to the back of the rat's neck, where it was externalized and anchored in the same manner. Securing the cannula at these two sites stabilized its position in the vein. A strip of Velcro sutured to the skin of the animal's back served to protect and store the external part of the cannula. VDF in blood was measured by headspace sampling and gas chromatography. The gas chromatography was equipped with a packed column and a flame ionization detector. The method requires 250 microL of blood and has a detection limit of 6 ng/mL of VDF in blood (S/N: 3 X 1). Vinylidene fluoride in the blood reached equilibrium with the headspace within 1.5 h and was stable for up to 4.0 h. The relative standard deviation for blood levels determined under experimental conditions was less than 15%. The interday reproducibility of the standard curve was +/- 3.5%.

Evaluation of a real-time aerosol monitor (RAM-S) for inhalation studies

Measurement of the aerosol concentration in inhalation toxicology studies is generally done by gravimetric and/or chemical analysis of filter samples taken over a known period of time at a fixed sampling flow rate. The value obtained represents the time-averaged concentration in an exposure chamber. However, the filter method does not provide information as to the stability of aerosol concentration in "real-time" nor as to the time required for the aerosol concentration to reach the target value during the start-up of exposures. In order to accomplish evaluation of aerosol stability and chamber rise and fall times, a direct measurement device is required. An available real-time aerosol monitor (RAM-S, GCA Corp., Bedford, MA) is a photometer which collects scattered light from an aerosol cloud at a 70 +/- 25 degrees angle. The output signal is 0 to 10 volt with three ranges corresponding to maximum aerosol concentrations of 200, 20, and 2 mg/m3. The performance of the RAM-S was evaluated in inhalation studies involving nickel sulfate hexahydrate, nickel oxide, nickel subsulfide, and azodicarbonamide. Several RAM-S units were calibrated by obtaining both filter samples and voltage readings of a RAM-S simultaneously. Results indicated that the response of the RAM-S instruments was linear. However, the voltage output per given aerosol concentration was different for each compound used. Furthermore, there was interinstrument variability in the voltage response to aerosol concentration of a given compound. At concentrations higher than 100 mg/m3, modification of the flow system in the RAM-S was made to increase the sheath air around the optical system and also to dilute the aerosol concentration.(ABSTRACT TRUNCATED AT 250 WORDS)

The fate of inhaled azodicarbonamide in rats

Azodicarbonamide (ADA) is widely used as a blowing agent in the manufacture of expanded foam plastics, as an aging and bleaching agent in flour, and as a bread dough conditioner. Human exposures have been reported during manufacture as well as during use. Groups of male F344/N rats were administered ADA by gavage, by intratracheal instillation, and by inhalation exposure to determine the disposition and modes of excretion of ADA and its metabolites. At 72 hr following gavage, 30% of the administered ADA was absorbed whereas following intratracheal instillation, absorption was 90%. Comparison between groups of rats exposed by inhalation to ADA to achieve body burdens of 24 or 1230 micrograms showed no significant differences in modes or rates of excretion of [14C]ADA equivalents. ADA was readily converted to biurea under physiological conditions and biurea was the only 14C-labeled compound present in excreta. [14C]ADA equivalents were present in all examined tissues immediately after inhalation exposure, and clearance half-times on the order of 1 day were evident for all tissues investigated. Storage depots for [14C]ADA equivalents were not observed. The rate of buildup of [14C]ADA equivalents in blood was linearly related to the lung content as measured from rats withdrawn at selected times during a 6-hr inhalation exposure at an aerosol concentration of 25 micrograms ADA/liter. In a study extending 102 days after exposure, retention of [14C]ADA equivalents in tissues was described by a two-component negative exponential function. The results from this study indicate that upon inhalation, ADA is rapidly converted to biurea and that biurea is then eliminated rapidly from all tissues with the majority of the elimination via the urine.

Acute inhalation exposure of azodicarbonamide in the guinea pig

Humans have been exposed to azodicarbonamide (ADA) by inhalation where bulk quantities of ADA are handled in the workplace. Responses of some workers have led to concern for the potential irritant and sensitizing properties of inhaled ADA. This study examined the effects of inhaling ADA on lung structure and function of guinea pigs during and after an acute exposure. Groups of 20 guinea pigs were exposed to each of 3 concentrations of ADA (19, 58, and 97 mg/m3), plus air as a control, for 1 hr. Pulmonary function was measured before exposure (baseline), during exposure, immediately after exposure and 24 hr after exposure. Dynamic compliance (Cdyn), total pulmonary resistance (RL), tidal volume (VT), respiratory frequency and minute volume were measured. In addition, gross necropsies and histological examinations of respiratory tract tissues were done either immediately following the exposure or 24 hr after exposure. There were no effects of ADA exposure on gross necropsy, histology, Cdyn, or RL. Some significant, concentration-related decreases in VT, respiratory frequency and minute volume were seen. The magnitudes of these changes were small: the largest change was seen in minute volume, amounting to a 24% decrease in the high concentration group. Inhalation exposure of guinea pigs to ADA at concentrations of up to 97 mg/m3 resulted in minor changes in pulmonary function without any changes in lung histology.

Deposition, metabolism, and excretion of 1-[14C]nitropyrene and 1-[14C]nitropyrene coated on diesel exhaust particles as influenced by exposure concentration

Nitrated polycyclic aromatic hydrocarbons (nitro-PAH) have been detected in the environment, originating from sources such as diesel exhaust emissions and coal combustion fly ash. 1-Nitropyrene (NP) is a predominant mutagenic and carcinogenic nitro-PAH found in diesel exhaust emissions. Since inhalation of NP is a likely route of exposure in humans, it is important to determine the biological fate of inhaled NP both in its pure form and associated with particles. The purpose of this study was to determine the disposition of NP aerosols inhaled by rats. The studies described in this paper were designed to determine the deposition of [14C]NP over a range of exposure concentrations, identify the pathways and half-times for excretion of absorbed NP, and determine the distribution of inhaled NP and metabolites in tissues. Male F344 rats were exposed nose only to various concentrations of NP and NP coated on diesel exhaust particles (50-1100 ng/liter). The results indicate that, over the range of concentrations tested, pathways for excretion of [14C]NP equivalents in urine and feces were independent of the exposure concentration of NP, whether in its pure form or associated with diesel exhaust particles. In all cases, fecal excretion was the major route of elimination of [14C]NP equivalents, with about 2 times more excreted by this route than by urine. The fractional deposition of [14C]NP in the respiratory tract did not appear to be dependent on exposure concentration. Half-times for elimination of 14C in urine and feces were about 15 to 20 hr. In all exposures, 14C was widely distributed in the tissues examined. Analysis of the tissues for NP and its metabolites indicated that within 1 hr after exposure, greater than 90% of the 14C was NP metabolites. Lungs of rats exposed to [14C]NP coated on diesel exhaust particles contained nearly 5 times more 14C than lungs from rats exposed to pure aerosols of [14C]NP (148 vs 29 pmol/g lung) within 1 hr after exposure. This difference was increased to 80-fold at 94 hr after exposure (80 vs 1 pmol/g lung). Long-term clearance half-times of 14C from various tissues were similar. The results demonstrate that particle association of NP significantly alters the biological fate of inhaled NP.

Disposition and metabolism of 14C-solvent yellow and solvent green aerosols after inhalation

Solvent yellow (2-(2'-quinolinyl)-1,3-indandione) and solvent green (1,4-di-p-toluidinoanthraquinone) are components of colored smoke munitions and may become airborne and be inhaled by workers during the manufacture of the munitions. Little is known about the disposition of either dye after inhalation. To obtain this information, we exposed male F344/N rats to 14C-solvent yellow aerosols (160 nmol solvent yellow/liter air) or a mixture of 14C-solvent yellow and unlabeled solvent green (340 nmol solvent yellow and 370 nmol solvent green/liter air) for 60 min. After either exposure, solvent yellow was rapidly cleared from the respiratory tract, with a t1/2 of 2-3 hr. Solvent green was retained in the lungs with a minimum estimated t1/2 for clearance of 22 days. Solvent green was not detected in other tissues during the 70-hr postexposure period. After either exposure, high-pressure liquid chromatography analysis of tissues extracts indicated that 40 to 75% of the 14C in liver and kidney consisted of solvent yellow metabolites. Greater than 90% of the 14C in the lungs was unmetabolized solvent yellow. The major pathway for excretion of solvent yellow and solvent yellow metabolites was the feces (74% of the initial body burden); the t1/2 for excretion was 14 hr. Urinary 14C accounted for 14% of the initial body burden and the t1/2 for excretion was 10 hr. Over 90% of the 14C excreted in the urine was solvent yellow metabolites. Very little solvent yellow (2%) was metabolized to 14CO2. By 72 hr after exposure, only 10% of the initial 14C deposited remained in the body.

Disposition and metabolism of [14C]dibenzo[c,g]carbazole aerosols in rats after inhalation

Dibenzo[c,g]carbazole (DBC) is a nitrogen-containing polycyclic aromatic hydrocarbon that has been detected in tobacco tars, industrial oils, and diesel engine exhaust fumes. DBC is carcinogenic in respiratory tract tissue of hamsters and in lungs, kidneys, and livers of mice. The purpose of this research was to determine the respiratory tract deposition, distribution in tissues, metabolism, and excretion of DBC in rats after inhalation. Rats were exposed nose-only to 1.1 or 13 micrograms [14C]DBC/liter air for 60 min. Activity median aerodynamic diameters for the two concentrations of DBC ranged from 0.7 to 0.8 micron. Urine, feces, and selected tissues were collected for various times after exposure. The fractional deposition for the 1.1 and 13 micrograms/liter exposure concentrations was similar, 13 and 16%, respectively. The dominant route of excretion of 14C following exposure to either concentration of DBC was the feces, accounting for approximately 95% of the total 14C eliminated. Half-time for fecal excretion was 20 +/- 6 hr (means +/- SE). Gastrointestinal absorption of [14C]DBC was 43%. Radioactivity was widely distributed to all tissues examined, with the respiratory tract (lung, trachea, larynx, and nasal turbinates), upper gastrointestinal tract (stomach and small intestine), the liver, and the adrenals containing the highest concentrations of [14C]DBC equivalents within 1 hr after exposure. At both concentrations of DBC tested, clearance of 14C from tissues was rapid, with approximately 60 to 98% of the initial tissue burden being cleared with half-times ranging from 1 to 16 hr. The remaining 2 to 40% in the tissues was cleared with half-times that ranged from 1.5 to 14 days. Several metabolites were detected in the urine and feces, none of which appeared to be either glucuronide or sulfate conjugates. Small quantities of [14C]DBC were detected in the urine, although quantities were less than 1% of the initial respiratory tract burden of [14C]DBC. The results from this research indicate that DBC was rapidly absorbed from the lungs and translocated to many tissues. Prior to elimination, primarily in the feces, DBC was extensively metabolized. There appeared to be no effect of exposure concentration on the toxicokinetics of inhaled DBC.

Disposition and metabolism of free and particle-associated nitropyrenes after inhalation

The objective of this project was to determine the biological fate of 1-nitropyrene (NP) aerosols in rats. The results from these studies indicate that, over the range of aerosol concentrations tested, pathways for excretion of 14C-NP equivalents in urine and feces were independent of the exposure concentration of NP, either in its pure form or associated with diesel exhaust particles. In all cases, fecal excretion was the major route of elimination of 14C-NP equivalents, with about 2 times more excreted by this route than by urine. Fractional respiratory tract deposition of 14C-NP did not appear to be dependent on exposure concentration. In most cases, half-times for elimination of 14C in urine and feces were about 15 to 20 hours. In all exposures, 14C was widely distributed in the tissues examined. Analysis of the tissues for NP and metabolites indicated that within 1 hour after exposure greater than 90% of the 14C was associated with NP metabolites. Lungs of rats exposed to 14C-NP coated on diesel exhaust particles contained nearly 5 times more 14C than lungs from rats exposed to pure aerosols of 14C-NP (148 vs 29 pmole g lung) within 1 hour after exposure. This difference was increased to 80-fold at 94 hours after exposure (80 vs 1 pmole g lung). Long-term clearance half-times of 14C from various tissues were similar, with values of about 30 to 50 hours measured. Pre-exposure to diesel exhaust prior to exposure to NP may result in increased retention of a small fraction of the NP. Equilibrium organ concentrations predicted for tissues following continuous exposure to NP suggest that both low inhaled concentrations of NP and association of NP with insoluble diesel particles can result in an increased retention of NP in the lungs above what might be predicted using data obtained from animal studies using high concentrations of pure NP. The liver and kidneys are among the other organs predicted to contain the highest amounts of NP.

A comparison of ethanolamine and potassium hydroxide as quantitative trapping agents for radiolabeled CO2 in metabolism studies

The efficiency of ethanolamine and potassium hydroxide (KOH) as trapping agents for CO2 was determined using a flow of 500 mL/min through CO2 absorption towers containing trapping solutions. Radiolabeled 14CO2 was produced by acidification of suspensions containing Ba14CO3. Both 100% ethanolamine and 5M ethanolamine in 2-methoxyethanol were evaluated. With 200 mL of either solution, trapping efficiency of 14CO2 decreased when only 5% or less of the amount of ethanolamine available had reacted (22 mmoles of CO2 trapped). In contrast, use of 200 mL of 1M or 5M KOH was effective in retaining 88 mmoles of CO2. This is equivalent to the amount of CO2 produced by a rat over an 8-hr period. In summary, with flow rates commonly used in in vivo metabolism studies, the trapping efficiency of ethanolamine was far less than the theoretical efficiency. In these types of studies KOH would be a more suitable trapping agent or, if used, ethanolamine solutions must be changed frequently.

Effect of vapor concentration on the disposition of inhaled 2,3-dichloropropene in Fischer-344 rats

2,3-Dichloropropene (DCP) is an intermediate used in the manufacture of carbamate herbicides and there is potential for human exposure during the manufacturing process. DCP is a known mutagen in bacteria systems and some structural analogs of DCP are carcinogenic. Since little is known about the disposition of DCP in animals after inhalation, studies were conducted in male Fischer-344 rats to determine the effect of vapor concentration on absorption and excretion. Uptake and elimination of 14C was studied in rats after nose-only inhalation of 17, 240, or 1650 nmol of [14C]DCP vapor/liter of air (0.4, 6, or 40 ppm, respectively, at 760 mm and 25 degrees C) for 6 hr. The percentage of inhaled DCP absorbed averaged 38% and was not statistically different at any vapor concentration, although minute volume was lower during exposure to 1650 nmol/liter. Urine, feces, and expired air were collected from rats for 65 hr after exposure. Rats were sacrificed and tissues, carcass, excreta, and expired air were analyzed for 14C. Routes of 14C excretion were independent of vapor concentration, with 50% of the 14C excreted in urine, 13% in feces, approximately 7% as CO2, and less than 1% as DCP in expired air. Rates of 14C excretion were also independent of vapor concentration, with the half-times averaging 9.9 hr (urine), 13.6 hr (feces), and 0.9 hr (14CO2). Sixty hours after inhalation, 29% of the initial body burden of 14C remained in the carcass. Most was associated with the pelt, but some 14C was found in all tissues. Respiratory tract, GI tract, liver, and kidney were tissues with the highest 14C contents. The results indicate that DCP metabolism and excretion rates are relatively constant throughout the vapor concentration range studied. This suggests that results from more detailed pharmacokinetic studies (and possibly toxicity studies) at one DCP concentration may be extrapolated to other concentrations within this range.

Biliary excretion and enterohepatic circulation of 1-nitropyrene metabolites in Fischer-344 rats

1-Nitropyrene (1-NP), present in diesel engine emissions, is a potent mutagen to bacteria, such as those found in mammalian intestinal tract, which contain nitroreductase enzymes. The purposes of this study were to determine the importance of bile as a route of excretion of 1-NP metabolites and to determine if reabsorption of biliary metabolites required the presence of intestinal bacteria. The bile ducts of male Fischer-344 rats were cannulated, 0.3 or 1.2 mumoles [3H]1-NP was given i.v., and bile, urine, and feces were collected for 24 hr. Biliary excretion accounted for 70 (80%) or 170 (60%) nmoles of [3H]1-NP after the low and high dose, respectively, with half-times for excretion of 1.7 hr +/- 0.3 (+/- S.E.M.) and 3.4 hr +/- 1.6 (+/- S.E.M.). Excretion of [3H]1-NP equivalents in the urine was linearly related to dose, with 6 or 16 nmoles (8%) excreted in 24 hr. At the low dose, more radioactivity appeared in the urine in control rats compared to bile-duct cannulated rats, suggesting that reabsorption of 1-NP metabolites occurred. Pretreatment of rats with orally administered antibiotics prior to i.v. injection of 0.3 mumole [3H]1-NP decreased radioactivity excreted in urine compared to untreated controls, suggesting that intestinal microorganisms may alter the biliary metabolites of 1-NP to facilitate reabsorption. Pretreatment of rats with buthionine sulfoximine, a glutathione depletor, decreased the excretion of certain biliary metabolites, suggesting that they were mercapturic acids of 1-NP metabolites. In summary, the results of these studies indicate that bile was an important route of excretion of nitropyrene metabolites. A portion of the excreted metabolites was reabsorbed from the gut, and this reabsorption required the presence of gut microorganisms.

Pulmonary retention of [14C]benzo[a]pyrene in rats as influenced by the amount instilled

Studies on the pulmonary retention of benzo[a]pyrene after inhalation have shown that clearance is biphasic, with one component clearing with a half-time greater than 1 day and another with a half-time less than 1 day. In the work reported here we demonstrated that the amount of benzo[a]pyrene instilled in the lungs can affect the rate at which the benzo[a]pyrene is cleared into the blood. Fischer-344 rats were given 16, 90 or 6400 ng of [14C]benzo[a]-pyrene/rat by intratracheal instillation. Rats were sacrificed at various times up to 7 days after instillation. Individual lung lobes and trachea were removed, digested, and analyzed by liquid scintillation spectrometry. At 24 h after instillation the amount of 14C covalently bound to lung macromolecules was determined in some rats. Benzo[a]pyrene equivalents remaining in the lungs was expressed as a percentage of the instilled dose as a function of time. A two-component negative exponential function was fit to the data. With increasing dose (16-6400 ng/rat), an increasing percent (89-99.76%) was cleared with a half-time less than 1 day and a decreasing percent (11.3-0.24%) was cleared with a half-time greater than 1 day, suggesting that the mechanism by which the slower clearances occurred had been saturated at higher doses. At 24 h after instillation, from 1 to 2 pmol of [14C]benzo[a]-pyrene equivalents/lung were covalently bound to lung macromolecules. There was no difference in the amount of covalently bound 14C over the range of instillation doses used, suggesting that a small amount of benzo[a]-pyrene equivalents was bound in the lungs regardless of the amount instilled. These results suggested that linear extrapolation from high dose studies to environmental concentrations might underestimate lung burdens of benzo[a]pyrene.

Uptake and excretion of [14C]methyl bromide as influenced by exposure concentration

Methyl bromide is a widely used soil fumigant and poses potential inhalation hazard to workers. Uptake of methyl bromide and pathways for excretion of 14C were investigated in male Fischer-344 rats after nose-only inhalation of 50, 300, 5700, or 10,400 nmol (1.6 to 310 ppm) of [14C]methyl bromide/liter of air for 6 hr. Fractional uptake of methyl bromide decreased at the highest concentrations, 5700 and 10400 nmol/liter, with 37 and 27% of the inhaled methyl bromide absorbed, respectively, compared to 48% at the lower levels. This resulted in the same total amount of methyl bromide being absorbed at the two higher exposure concentrations (650 mumol/kg body wt). Total methyl bromide adsorbed was 9 or 40 mumol/kg body wt after exposure to 50 or 300 nmol/liter, respectively. Elimination of 14C was linearly related to the amount of methyl bromide absorbed as determined from urine, feces, expired CO2, and parent compound collected for 66 hr after the end of exposure. Exhaled 14CO2 was the dominant route of excretion, with from 1.2 to 110 mumol (50% of amount absorbed) exhaled, and was described by a two-component negative exponential function; 85% was exhaled with a t 1/2 of 4 hr, and the remaining 15% was exhaled with a t 1/2 of 17 hr. The rate of exhalation of 14CO2 was not affected by the amount of [14C]methyl bromide absorbed. From 0.4 to 54 mumol was excreted in urine (20% of amount absorbed). The half-time for excretion of 14C in urine was approximately 10 hr, and the rate of excretion was not dependent on the amount of [14C]methyl bromide absorbed. Little 14C was exhaled as methyl bromide (less than 4% of the dose) or excreted in feces (less than 2%). At the end of 66 hr, 25% of the 14C absorbed remained in the rats. Liver, kidneys, adrenals, lungs, thymus, and turbinates (maxilloturbinates, ethmoturbinates, and nasal epithelial membrane) contained the highest concentrations of 14C. Results indicated that uptake of inhaled methyl bromide could be saturated. Any [14C]methyl bromide equivalents absorbed, however, would be excreted by concentration-independent mechanisms.

Disposition of [14C]methyl bromide in rats after inhalation

Methyl bromide is used as a disinfectant to fumigate soil and a wide range of stored food commodities in warehouses and mills. Human exposure occurs during the manufacture and use of the chemical. The purpose of this investigation was to determine the disposition and metabolism of [14C]methyl bromide in rats after inhalation. Male Fischer-344 rats were exposed nose only to a vapor concentration of 337 nmol [14C]methyl bromide/liter air (9.0 ppm, 25 degrees C, 620 torr) for 6 hr. Urine, feces, expired air, and tissues were collected for up to 65 hr after exposure. Elimination of 14C as 14CO2 was the major route of excretion with about 47% (3900 nmol/rat) of the total [14C]methyl bromide absorbed excreted by this route. CO2 excretion exhibited a biphasic elimination pattern with 85% of the 14CO2 being excreted with a half-time of 3.9 +/- 0.1 hr (means +/- SE) and 15% excreted with a half-time of 11.4 +/- 0.2 hr. Half-times for elimination of 14C in urine and feces were 9.6 +/- 0.1 and 16.1 +/- 0.1 hr, respectively. By 65 hr after exposure, about 75% of the initial radioactivity had been excreted with 25% remaining in the body. Radioactivity was widely distributed in tissues immediately following exposure with lung (250 nmol equivalents/g), adrenal (240 nmol equivalents/g), kidney (180 nmol equivalents/g), liver (130 nmol equivalents/g), and nasal turbinates (110 nmol equivalents/g) containing the highest concentrations of 14C. Radioactivity in livers immediately after exposure accounted for about 17% of the absorbed methyl bromide. Radioactivity in all other tissues examined accounted for about 10% of the absorbed methyl bromide. Elimination half-times of 14C from tissues were on the order of 1.5 to 8 hr. In all tissues examined, over 90% of the 14C in the tissues was methyl bromide metabolites. The data from this study indicate that after inhalation methyl bromide is rapidly metabolized in tissues and readily excreted.

Disposition and metabolism of 2,3-[14C]dichloropropene in rats after inhalation

2,3-Dichloropropene (2,3-DCP) is a constituent of some commercially available preplant soil fumigants for the control of plant parasitic nematodes. Human exposure potential exists during manufacture of the chemicals or during bulk handling activities. The purpose of this investigation was to determine the disposition and metabolism of 2,3-[14C]DCP in rats after inhalation. Male Fischer-344 rats were exposed nose-only to a vapor concentration of 250 nmol 2,3-[14C]DCP/liter air (7.5 ppm; 25 degrees C, 620 Torr) for 6 hr. Blood samples were taken during exposure, and urine, feces, expired air, and tissues were collected for up to 65 hr after exposure. Urinary excretion was the major route of elimination of 14C (55% of estimated absorbed 2,3-DCP). Half-time for elimination of 14C in urine was 9.8 +/- 0.05 hr (means +/- SE). Half-time for elimination of 14C feces (17% of absorbed 2,3-DCP) was 12.9 +/- 0.14 hr (means +/- SE). Approximately 1 and 3% of the estimated absorbed 2,3-[14C]DCP were exhaled as either 2,3-[14C]DCP or 14CO2, respectively. Concentrations of 14C in blood increased during 240 min of exposure, after which no further increases in blood concentration of 14C were seen. 14C was widely distributed in tissues analyzed after a 6-hr exposure of rats to 2,3-[14C]DCP. Urinary bladder (150 nmol/g), nasal turbinates (125 nmol/g), kidneys (84 nmol/g), small intestine (61 nmol/g), and liver (35 nmol/g) were tissues with the highest concentrations of 14C immediately after exposure. Over 90% of the 14C in tissues analyzed was 2,3-DCP metabolites. Half-times for elimination of 14C from tissues examined ranged from 3 to 11 hr. The data from this study indicate that after inhalation 2,3-DCP is metabolized in tissues and readily excreted.

Disposition of [14C]2,3-dichloropropene in Fischer-344 rats after oral or intraperitoneal administration

2,3-Dichloropropene (2,3-DCP), a component of commercial fumigants and nematocides, was mixed with [14C]2,3-DCP and given to rats by peroral (p.o.) or intraperitoneal (i.p.) administration. Urine, feces, and expired air were collected over 72 h. Excretion of radioactivity in urine predominated over other routes, with 66% to 75% of the dose excreted in 72 h. Feces contained from 13% to 21% of dose. 8% of the dose was exhaled as 14CO2. At the end of 72 h, only 2% to 3% of the dose remained in the carcass with the highest concentrations of 14C in liver, kidney, testes, and lung. Approx. 91% of the p.o. dose was absorbed from the gastrointestinal (GI) tract.

Metabolism of 1-[14C]nitropyrene in isolated perfused rat livers

1-Nitropyrene (1-NP), a constituent of diesel exhaust, is carcinogenic to rats and is a bacterial and mammalian mutagen. Biliary and fecal excretion of 1-NP metabolites are the major routes of excretion in rats, suggesting that hepatic metabolism plays a dominant role in determining the biological fate of 1-NP. The purpose of this investigation was to quantitate 1-[14C]NP metabolites formed in isolated perfused rat livers and excreted in bile from rats. Perfused rat livers displayed a capacity for oxidation, reduction, acetylation, and conjugation of 1-NP (or its metabolites). Reduction of 1-NP followed by N-acetylation was the major metabolic pathway observed in the perfused livers. Acetylaminopyrene (AAP) was the major metabolite detected, with total quantities (150 nmol) accounting for about 60% of the total 1-[14C]NP dose (258 nmol) added to the perfusate. Considerably smaller quantities of aminopyrene and hydroxynitropyrenes were also detected. Livers perfused with 1-[14C]NP excreted about 36 nmol equivalents of 1-[14C]NP (12% of the total 1-NP dose) in bile after 60 min. Some of the biliary metabolites were tentatively identified as metabolites of the mercapturic acid pathway. The spectrum of biliary metabolites was qualitatively identical to that seen in bile from intact rats. Quantities of 14C covalently bound to hepatic macromolecules from perfused livers were 0.4 nmol 1-NP eq/g liver. The data from this study indicate that the liver may be an important site for metabolism of 1-NP.

Disposition of [14C]methyl bromide in Fischer-344 rats after oral or intraperitoneal administration

Methyl bromide is used as a disinfectant to fumigate soil. The intent of our study was to determine the disposition of methyl bromide following a single acute administration. Male Fischer-344 rats were given 250 mumol of [14C] methyl bromide/kg body wt by either oral or i.p. administration. Urine, feces and expired air were collected and at the end of 72 h the rats were sacrificed and tissues analyzed to determine 14C excretion and tissue distribution. After i.p. administration of methyl bromide, the dominant route of excretion was exhalation of 14CO2, with 46% of the dose exhaled as 14CO2. In contrast, urinary excretion of 14C was the major route of elimination (43% of the dose) when methyl bromide was given orally. Very little of the 14C appeared in the feces (less than 3% of the dose) regardless of route of administration. In rats with bile duct cannulations, 46% of an oral dose appeared in the bile over a 24-h period. Collection of bile significantly decreased the exhalation of 14CO2 and 14C excreted in urine compared to controls. At 72 h after oral or i.p. administration, 14-17% of the 14C remained in the rats, with liver and kidney being the major organs of retention. Results indicate that route of administration can affect the pathways for excretion. In addition, excretion of 14C in bile, coupled with the low levels of radioactivity found in the feces, indicates that reabsorption of biliary metabolites from the gut plays a significant role in the disposition of [14C] methyl bromide.

A double isotope method for characterizing hygroscopic selenious acid aerosols used in inhalation exposures

Methods of generating and characterizing selenious acid aerosols for use in deposition and toxicity studies are described. Selenious acid aerosols were nebulized and heat treated at varied temperatures. Because selenious acid is hygroscopic, a solution containing 3H2O and 75Se was nebulized to determine the water content in selenium aerosols after generation. The stable aerosol was a hydrate of selenious acid, H2SeO3 X 1/2 H2O, at New Mexico ambient relative humidity (ca. 20%). The mass median aerodynamic diameter of the collected droplets was 0.6 micron with a geometric standard deviation of 1.3.

Dog pulmonary macrophage metabolism of free and particle-associated [14C]benzo[a]pyrene

Pulmonary macrophages (PM) are involved in the clearance of inhaled particulate matter from the lung. PM also are capable of metabolizing xenobiotics such as benzo[a]pyrene (BaP). The objective of this investigation was to measure the ability of PM isolated from dogs to metabolize BaP coated onto diesel exhaust particles and to compare this metabolism with that of BaP in solution. PM were isolated from male beagle dogs and incubated with 1 microM [14C]BaP (solution or diesel particle coated) for select times up to 48 h. After incubation of PM with [14C]BaP, both the cells and the media were individually analyzed for [14C]BaP metabolites by high-performance liquid chromatography. Total quantities of [14C]BaP metabolites in both the media (125 pmol/10(6) cells) and cells (45 pmol/10(6) cells) increased with incubation time for up to 48 h. BaP-9,10-diol and BaP-7,8-diol were the major metabolites in organic extracts from the culture media, whereas BaP-7,8-diol and BaP-4,5-diol were the major metabolites in extracts of cells. Small quantities of BaP phenols and BaP quinones were detected in both the cells and media. Total quantities of BaP metabolites (20-30 pmol/10(6) cells) were not significantly different when PM were incubated for 24 h with either [14C]BaP in solution or [14C]BaP coated on diesel particles. The data suggest that particles retained in lungs are capable of being acted upon by PM metabolizing enzymes and that the ensuing metabolism may play an important role in the metabolic fate of organic material inhaled on particulate matter.

Biliary excretion and enterohepatic circulation of 2,4-dinitrotoluene metabolites in Fischer-344 rats

Technical grade dinitrotoluene (DNT) is hepatocarcinogenic when fed to rats. DNT is oxidatively metabolized by hepatic enzymes and reductively metabolized by rat intestinal microflora in vitro. The objectives of the present studies were to determine the importance of bile as a route of excretion for DNT metabolites and to investigate the role of enterohepatic circulation in the metabolism of DNT. The common bile ducts of male and female F-344 rats were cannulated with an uninterrupted cannula at the hepatic and ileal ends. After 24 hr, male rats were given a po dose of 35, 63, or 100 mg 2,4-[14C]DNT/kg; female rats received 35 mg 2,4-[14C]DNT/kg. Immediately prior to dosing, the cannula was snipped and bile was allowed to collect in a glass reservoir, surgically implanted in the peritoneal cavity, which could be sampled externally. In males, excretion of 14C in bile was linearly related to dose. From 9.2 to 29.2 mumol eq of [14C]DNT (approximately 25% of the dose) appeared in bile within 24 hr. Females dosed with 35 mg/kg excreted only 18% of the dose in the bile. Over 90% of the radioactivity in the bile was the glucuronide conjugate of 2,4-dinitrobenzyl alcohol (DNBAlc-G). In comparison to control rats, in which bile flow to the small intestine was uninterrupted, collection of bile decreased the amount of 14C excreted in urine. In both males and females most of the 2,4-DNT dose excreted in the urine was in the form of the oxidized metabolites DNBAlc-G and 2,4-dinitrobenzoic acid. These results indicate that bile is an important route of excretion for 2,4-DNT metabolites and that metabolites excreted in the bile can be reabsorbed from the gut.

Absorption, distribution, and retention of inhaled selenious acid and selenium metal aerosols in beagle dogs

We studied the distribution and retention of inhaled selenious acid and selenium metal aerosols which were similar in size and chemical form to selenium aerosols that may be produced during fossil fuel combustion. Beagle dogs were given 10 to 61 micrograms Se/kg of body weight by inhalation. Aerosols generated for the inhalation exposures were also collected and instilled into the upper respiratory tracts or stomachs of additional dogs to measure systemic absorption at these sites. Selenium-75, incorporated into the aerosols, was used to determine the Se content in the whole animal, excreta, and individual tissues as a function of time. Virtually all of the inhaled selenious acid aerosol was rapidly absorbed into the blood from the lungs, gastrointestinal tract, and the nasal membranes. Selenium metal aerosols were less rapidly absorbed. Selenium that was absorbed into the blood was translocated to the liver, kidney, spleen, and heart. Selenium-75 in these organs had a biological half-life of 30 to 40 days. Approximately 50% of the deposited Se was eliminated with a biological T1/2 of 1.2 days. Urine was the major route of excretion, accounting for 70 to 80% of the excreted Se. The long-term component of the whole-body retention function for both inhaled aerosols had a half-life of about 34 days and accounted for about 20% of the initial Se dose. The data suggested that although absorption of selenious acid into blood following inhalation was more rapid than absorption of selenium metal, once absorbed the disposition of both compounds was similar.

Development of hepatic lesions in male Fischer-344 rats fed AIN-76A purified diet

The suitability of the AIN-76A diet for Fischer-344 rats was investigated. This diet, proposed by the American Institute of Nutrition for use when a purified diet composed of refined ingredients and added vitamins and minerals is required, was tested in Sprague-Dawley rats. Male weanling Fischer-344 rats were fed three different lots of the AIN-76A diet from two suppliers. Increase in body weights and food consumption were compared to animals fed a cereal-based control diet. Animals were sacrificed at various intervals and tissues were taken for histopathological observation. By 8 weeks moderate to marked periportal lipidosis developed in livers of all rats fed the AIN-76A diet. Liver-body weight ratios over the 8-week period were significantly higher in rats fed AIN-76A diets compared to rats fed the control diet. However, growth rates of rats fed the AIN-76A diet were similar to growth rates of controls. Some rats fed the AIN-76A diet developed severe hemorrhagic lesions. The AIN-76A diet in its present form is not suitable for use with male Fischer-344 rats.

Sex-dependent metabolism and biliary excretion of [2,4-14C] dinitrotoluene in isolated perfused rat livers

The incidence of hepatocellular carcinoma is higher in male rats fed 35 mg of dinitrotoluene (DNT) per kg than in female rats fed the same dose. Sex differences in DNT disposition and/or metabolism may account for this difference. This study characterized the metabolism and biliary excretion of 2,4-DNT in male and female isolated perfused rat livers. Livers from both sexes displayed a capacity for oxidation, reduction, acetylation and conjugation of 2,4-DNT (or its metabolites). Oxidation of 2,4-DNT to 2,4-dinitrobenzyl alcohol followed by glucuronidation to 2,4-dinitrobenzyl alcohol glucuronide (DNBalcG) was the major route of 2,4-DNT metabolism in both sexes. Formation of DNBalcG accounted for 10 to 30% of the total initial 2,4-DNT concentration in perfusates. After perfusion with 20 microM 2,4-DNT, male livers excreted larger quantities of DNBalcG in the bile (392 nmol) than female livers (172 nmol); at the same 2,4-DNT concentration, perfusates from female livers contained over 3 times as much DNBalcG as male perfusates. These data suggested that female livers transported DNBalcG into the bile at slower rates than male livers. The transport of DNBalcG into the bile of male, but not female, livers appeared to be saturated after perfusion with 20 micro M 2,4-DNT. No sex differences in the hepatic macromolecular covalent binding were observed after perfusion of livers with either 20 or 70 micro M 2,4-DNT. These data suggest that the major difference in the in vitro metabolism of 2,4-DNT between male and female rats is the larger quantities of DNBalcG excreted in the bile of male rats than female rats.

Systemic absorption of selenious acid and elemental selenium aerosols in rats

Absorption of Se from the nasal passages, lungs, gastrointestinal tract, and skin was studied in Fischer-344 rats. Radiolabeled selenious acid and elemental Se particles were administered by inhalation, gavage, nasal instillation, and iv injection. Selenious acid was always absorbed into the general circulation more rapidly and to a greater extent than elemental Se. By 4 h after inhalation of selenious acid and elemental Se aerosols, 94% of the selenious acid and 57% of the elemental Se deposited in lungs was absorbed into blood. Of the selenious acid instilled into nasal passages, 18% was absorbed into blood; 16% of the elemental Se was absorbed. Gastrointestinal absorption was 87% for selenious acid and 50% for elemental Se. Selenious acid solutions were also painted onto the pelts of rats. From 10 to 30% of the selenious acid was absorbed through the skin. Following inhalation or injection of either Se compound, most of the Se was excreted in the urine. Significantly more Se appeared in feces of animals receiving elemental Se by gavage than animals receiving selenious acid. Results indicate that if people were to absorb inhaled Se from the upper respiratory tract in a manner similar to that of rats, one-third more selenious acid would be absorbed into the general circulation than elemental Se. All Se deposited in the lungs would be absorbed into blood. However, selenious acid would be absorbed more rapidly than elemental Se.

Toxicity of selenium compounds to alveolar macrophages

Selenium compounds released into urban atmospheres as a result of fossil fuel combustion may pose an inhalation hazard to people. Two chemical forms of selenium produced during coal combustion and present in combustion effluent are selenious acid. H2SeO3, and elemental selenium, Se. In an attempt to determine the toxicity of selenium compounds relative to other trace elements, the cytotoxicity of H2SeO3 and Se to rabbit alveolar macrophages in vitro was investigated. Macrophages were obtained by lung lavage and exposed in tissue culture after 20 h. Neither selenious acid nor elemental selenium caused cell lysis at concentrations which decreased total cell viability. Selenious acid was an order of magnitude more toxic then elemental selenium. Elemental selenium was similar in toxicity to environmental contaminants such as CdCl2 and V2O5. These in vitro cytotoxicity data can be used to predict the risk posed to people inhaling selenium compounds at levels found in urban atmospheres.