Influence of exposure route and oral dosage regimen on 1, 1-dichloroethylene toxicokinetics and target organ toxicity.

The objective of this investigation was to elucidate the effects of route of exposure and oral dosage regimen on the toxicokinetics (TK) of 1,1-dichloroethylene (DCE). Fasted male Sprague-Dawley rats that inhaled 100 or 300 ppm for 2 h absorbed total systemic doses of (10 or 30 mg/kg DCE, respectively. Other groups of rats received 10 or 30 mg/kg DCE by intravenous injection, bolus gavage (by mouth), or gastric infusion (g.i.) over a 2-h period. Serial microblood samples were taken from the cannulated, unanesthetized animals and analyzed for DCE content by gas chromatography to obtain concentration versus time profiles. Inhalation resulted in substantially higher peak blood concentrations and area under blood-concentration time curves (AUC(0)(2)) than did gastric infusion of the same dose over the same time frame at each dosage level, although inhalation (AUC(0)(infinity)) values were only modestly higher. Urinary N-acetyl-beta-D-glucosaminidase (NAG) and gamma-glutamyltranspeptidase (GGT) activities were monitored as indices of kidney injury in the high-dose groups. NAG and GGT excretion were much more pronounced after inhalation than gastric infusion. Administration of DCE by gavage also produced much higher Cmax and AUC(0)(2) values than did 2-h g.i., although AUC(0)(infinity) values were not very different. The 30 mg/kg bolus dose produced marked elevation in serum sorbitol dehydrogenase, an index of hepatocellular injury. Administration of this dose by inhalation and gastric infusion was only marginally hepatotoxic. These findings demonstrate the TK and target organ toxicity of DCE vary substantially between different exposure routes, as well as dosage regimens, making direct extrapolations untenable in health risk assessments.

Evaluation of potassium iodide (KI) and ammonium perchlorate (NH4ClO4) to ameliorate 131I- exposure in the rat.

Nuclear reactor accidents and the threat of nuclear terrorism have heightened the concern for adverse health risks associated with radiation poisoning. Potassium iodide (KI) is the only pharmaceutical intervention that is currently approved by the Food and Drug Administration for treating (131)I(-) exposure, a common radioactive fission product. Though effective, KI administration needs to occur prior to or as soon as possible (within a few hours) after radioactive exposure to maximize the radioprotective benefits of KI. During the Chernobyl nuclear reactor accident, KI was not administered soon enough after radiation poisoning occurred to thousands of people. The delay in administration of KI resulted in an increased incidence of childhood thyroid cancer. Perchlorate (ClO(4)(-)) was suggested as another pharmaceutical radioprotectant for 131I- poisoning because of its ability to block thyroidal uptake of iodide and discharge free iodide from the thyroid gland. The objective of this study was to compare the ability of KI and ammonium perchlorate to reduce thyroid gland exposure to radioactive iodide (131I-). Rats were dosed with 131I- tracer and 0.5 and 3 h later dosed orally with 30 mg/kg of either ammonium perchlorate or KI. Compared to controls, both anion treatments reduced thyroid gland exposure to 131I- equally, with a reduction ranging from 65 to 77%. Ammonium perchlorate was more effective than stable iodide for whole-body radioprotectant effectiveness. KI-treated animals excreted only 30% of the (131)I(-) in urine after 15 h, compared to 47% in ammonium perchlorate-treated rats. Taken together, data suggest that KI and ammonium perchlorate are both able to reduce thyroid gland exposure to 131I- up to 3 h after exposure to 131I-. Ammonium perchlorate may offer an advantage over KI because of its ability to clear 131I- from the body.

Additional thyroid dose factor from transportation sources in Russia after the Chernobyl disaster.

Beginning approximately 4 years after the Chernobyl nuclear accident a steady increase in the incidence of thyroid cancer was observed in children and adolescents of the Bryansk Oblast, which received the highest level of radionuclide contaminants in Russia. We examined the spatial relationship between the residence location of patients with identified thyroid cancer (0-18 years old at the time of the accident) and a number of geographic parameters to better account for the etiology of thyroid cancer spatial distribution. Geographic parameters analyzed included spatial distribution of 137Cs and 131I in soil, population demographics, measurements and reconstructions. of absorbed thyroid 131I doses in the population, and maps of major transportation arteries. An interesting finding is the lack of a consistent correlation between the spatial distribution of radionuclides in the soil and thyroid cancer incidence. Instead, most of the thyroid cancer cases were diagnosed in settlements situated on major railways and roads. Correlating population with thyroid cancer cases and transportation arteries reveals a much higher cancer rate on or near major roads and railways than at a distance from them, again independent of radionuclide soil concentration. There are other important factors, of course, that must be considered in future evaluations of this phenomenon. These include the influence of iodine endemic zones, genetic predisposition to thyroid cancer, and duration of residence time in contaminated areas. The feasibility of radionuclide transport on railways and roads is discussed, together with the vectors for transfer of the contaminants to the human population. Developing a model to reconstruct the radiation dose to the thyroid over time in this geographic region is proposed in light of the impact of transportation arteries. Specific studies are outlined to provide the data necessary to develop this model as well as to better characterize the feasibility and scientific validity of the contribution to human health effects of this transport factor. Transport factor refers to the transport of radionuclides on transportation arteries and the transfer of these agents to the human population residing in the vicinity of these arteries. If the impact on thyroid cancer of the transport of radionuclides on major railways and roads is indeed significant, a major reappraisal of the risk of large-scale radioactive release into the environment is necessary.

Biphasic effects of 1,1,1-trichloroethane on the locomotor activity of mice: relationship to blood and brain solvent concentrations.

Despite the central nervous system (CNS) being a target of virtually all solvents, few solvents have been thoroughly studied for their effects on unlearned animal behaviors. Of the solvents that have been studied, little is known about the relationship of exposure concentration to behavioral effect, and quantitative data relating the toxicologically important target organ (i.e., brain) dose to behavioral effect are almost non-existent. To examine the concentration- and time-dependency of effects of 1,1, 1-trichloroethane (TRI) on behavior, male albino Swiss-Webster mice were exposed to TRI (500-14,000 ppm) in static inhalation chambers for 30 min, during which locomotor activity was measured. Separate mice were exposed to the same concentrations under identical conditions for 6, 12, 18, 24, and 30 min, to determine blood and brain concentrations versus time profiles for TRI. This allowed for the relationships between blood and brain concentrations of TRI and locomotor activity to be discerned. The lowest TRI concentrations studied (500-2000 ppm) had no statistically significant effect on activity, intermediate concentrations (4000-8000 ppm) increased activity immediately to levels that remained constant over time, and higher concentrations (10,000-14,000 ppm) produced biphasic effects, i.e., increases in activity followed by decreases. 1,1, 1-Trichloroethane concentrations in blood and brain approached steady-state equilibria very rapidly, demonstrated linear kinetics, and increased in direct proportion to one another. Locomotor activity increased monophasically ( approximately 3.5-fold) as solvent concentrations increased from approximately 50-150 microg/g brain and microg/ml blood. As concentrations exceeded the upper limit of this range, the activity level declined and eventually fell below the control activity level at approximately 250 microg/g brain and microg/ml blood. Regression analyses indicated that blood and brain concentrations during exposure were strongly correlated with locomotor activity, as were measures of internal dose integrated over time. The broad exposure range employed demonstrated that TRI, like some classical CNS depressants, is capable of producing biphasic effects on behavior, supporting the hypothesis that selected solvents are members of the general class of CNS depressant drugs. By relating internal dose measures of TRI to locomotor activity, our understanding of the effects observed and their predictive value may be enhanced.

Comparisons between operant response and 1,1,1-trichloroethane toxicokinetics in mouse blood and brain.

The effect of 1,1,1-trichloroethane (TRI) inhalation on operant response was evaluated in relation to the concentration of TRI in blood and brain tissue in mice during exposure. Male CD-1 mice were trained to lever-press for an evaporated milk reinforcer on a variable interval (VI 60) schedule for 2 h. Trained mice were then exposed to either 3500 or 5000 ppm TRI for 100 min, and the changes in the schedule-controlled performance were measured. Additional groups of mice were exposed under the same conditions as those used in the behavioral study and sacrificed at various times during exposure, and the blood and brain samples were collected and subsequently analyzed for TRI content by headspace gas chromatography. Uptake of TRI into blood and brain was rapid, with near steady-state levels reached after approximately 40-60 min of exposure. Inhalation of 5000 ppm, but not 3500 ppm TRI was seen to cause inhibition of operant response, starting approximately 30 min following the initiation of inhalation exposure and beginning to recover after 80 min of exposure. The threshold concentrations for the maximal behavioral inhibition were approximately 110 micrograms/g and 130 micrograms/ml in mouse brain and blood, respectively. It appears that in addition to TRI concentrations in blood and brain tissue, the time it takes to reach the apparent threshold TRI concentration was also a determinant for the onset of TRI neurobehavioral depression.

Genotoxic evaluation of the offgassing products of particle board.

It has been recognized that people are spending more time indoors and that pollutants are being found in elevated concentrations in this environment. Because the constituents of indoor air pollution can vary relative to a large number of factors, the nature of the indoor environment is extremely difficult to study. Of the materials used in construction of buildings which can elute complex mixtures of organic compounds, products such as particle board, plywood and insulation are known to release formaldehyde into the indoor environment. We have employed a modification of the Ames Salmonella/microsome assay with both DNA repair-proficient and -deficient strains and determined that one such material, particle board, emitted mutagenic and genotoxic substances. The materials offgassing from the particle board demonstrated a dose-related response in both mutagenicity and toxicity. It was also observed that incubation at 37 degrees C produced a decrease in both endpoints which was related to time of incubation. In addition, detectable amounts of twelve other organic compounds were identified as offgassing from the incubated particle board.

Schedule-controlled operant behavior of rats during 1,1,1-trichloroethane inhalation: relationship to blood and brain solvent concentrations.

The central nervous system is the principal target of 1,1,1-trichloroethane (TRI), and several studies of this volatile solvent have demonstrated effects on learned animal behaviors. There have been few attempts, however, to quantitatively relate such effects to blood or target organ (brain) solvent concentrations. Therefore, Sprague-Dawley rats trained to lever-press for evaporated milk on a variable interval 30-s reinforcement schedule were placed in an operant test cage and exposed to clean air for 20 min, followed by a single concentration of TRI vapor (500-5000 ppm) for 100 min. Additional rats were exposed to equivalent TRI concentrations for 10, 20, 40, 60, 80, or 100 min to determine blood and brain concentration vs. time profiles. Inhalation of 1000 ppm slightly increased operant response rates, whereas 2000, 3500, and 5000 ppm decreased operant response rates in a concentration- and time-dependent manner. Accumulation of TRI in blood and brain was rapid and concentration dependent, with the brain concentration roughly twice that of blood. Plots of blood and brain TRI concentrations against operant performance showed responding in excess of control rates at low concentrations, and decreasing response rates as concentrations increased. Linear regression analyses indicated that blood and brain concentrations, as well as measures of time integrals of internal dose, were strongly correlated with operant performance. Neurobehavioral toxicity in laboratory animals, as measured by changes in operant performance, can therefore be quantitatively related to internal measures of TRI exposure to enhance its predictive value for human risk assessment.

Respiratory responses in the lower respiratory tract of Sprague-Dawley rats to formaldehyde inhalation.

This study was conducted to determine the characteristic response of Sprague-Dawley rats to formaldehyde (HCHO) challenges to the lower respiratory tract, and whether these response patterns are altered in rats that have received repeated exposures to HCHO. Male Sprague-Dawley rats were exposed to 0, 0.5, or 15 ppm HCHO for 6 hours/day, 5 days/week, for 8 or 16 weeks. Both naive rats and rats repeatedly exposed to HCHO were then administered 30 ppm HCHO test challenges by tracheal exposure, with the minute volume, respiratory rate, and tidal volume responses monitored. The pulmonary response of naive rats to HCHO tracheal challenge involved the correlation of minute volume and tidal volume depression, while respiratory rate was either unaffected or slightly increased. This was also the response pattern for rats that received 8 weeks of repeated exposure to HCHO. The only significant difference in respiratory response patterns between naive and pre-exposed animals existed in a slight increase in the respiratory rate compensatory response in the rats pre-exposed for 16 weeks to 15 ppm. There was substantial recovery of initially depressed respiratory parameters during the tracheal challenge in both naive and pre-exposed rats. The characteristic pulmonary response to HCHO in the lower respiratory tract demonstrated for Sprague-Dawley rats was thus similar to patterns of lower respiratory response to HCHO reported for other rodent species.

Effects of inhaled 1,1,1-trichloroethane on the regional brain cyclic GMP levels in mice and rats.

As it is known that volatile organic compounds (VOCs) exhibit differential dispositions among anatomically discrete brain regions in rodents as well as in humans, potential toxicological consequences of this pharmacokinetic feature were evaluated using measurements of cyclic GMP (glucose monophosphate). With the knowledge of 1, 1, 1-trichloroethane (TRI) uptake and distribution in the various brain regions, cyclic GMP was evaluated due to (1) known susceptibility to the effects of organic solvents, (2) pivotal physiological role in perpetuating changes in neurochemical pathways, and (3) possible involvement with neurobehavioral functions, whose disruption is one of the primary health effects associated with solvent exposures. Male CD-1 mice and Sprague-Dawley rats inhaled 5000 ppm TRI for 40 and 100 min in dynamic inhalation exposure chambers, and the brain was procured from the animals immediately following termination by microwave irradiation. After 40 min of TRI inhalation, significant decreases in cyclic GMP levels were found in the cerebellum of both species, 55% and 58%, respectively, relative to the controls. There was a further decrease in both species after 100 min of TRI inhalation. Smaller decreases in cyclic GMP were seen in the cortex of both species at both time points of measurement. A decrease in cyclic GMP was observed in the medulla oblongata of mice but not in rats after 40 min of exposure. Due to its signal transduction functions, it might be expected that the effects of TRI on cyclic GMP levels could directly impact neurological function. Comparison of the results from this study with the regional brain distribution of TRI and its effects on behavioral performance seen in previous studies by this laboratory appeared to indicate that alterations in brain cyclic GMP levels are only involved with the neurobehavioral toxicity of TRI in an indirect fashion; consequently, behavioral effects and decreases in cyclic GMP do not appear to be directly related to regionally differential dispositions of TRI in rodent brain.

Regional brain dosimetry of trichloroethane in mice and rats following inhalation exposures.

While certain neuroactive volatile organic compounds (VOCs) have been reported to have an uneven distribution in various anatomically distinctive brain regions, this has not yet been reported for the short-chain aliphatic halogenated hydrocarbons. Therefore, the uptake and regional brain distribution of 1, 1, 1-trichloroethane (TRI) in mice and rats following inhalation exposure were examined. Male Sprague-Dawley rats and CD-1 mice were exposed to TRI at either 3500 or 5000 ppm for 10, 30, 60, or 120 min. Seven brain regions from rats and three from mice were sampled, and TRI concentrations in the blood and brain tissues were determined by headspace gas chromatography. In both species, the medulla oblongata was found to have the highest TRI concentrations, while cortex (in both species) and hippocampus (only sampled in rats) contained the lowest TRI concentrations. Substantial differences were also observed between the two species, as the mice exhibited higher capacity to accumulate TRI in the blood as well as in the brain regions. It appears that lipid content is a main factor influencing the differential disposition of TRI among the brains regions. Physiological differences in the respiratory systems of the two species and the physiochemical properties of the chemical favoring diffusion toward lipid-rich compartments could also have been expected to account for the patterns of regional distribution and species differences.

Effects of acute inhalation exposure to isoamyl nitrite on the hypothalamo-pituitary-adrenal axis in male Sprague-Dawley rats.

Isoamyl nitrite (IAN) is a member of the family of volatile organic nitrites that exert vasodilatory effects and have recently exhibited a considerable potential for inhalation abuse. In an effort to provide mechanistic insight into the neurotoxic effects and abuse potential of these agents, the present study was designed to evaluate the acute effects of IAN on the hypothalamo-pituitary-adrenal (HPA) axis. Attempts were also made to correlate the neuroendocrine effects of IAN with its pharmacokinetic profile. Male Sprague-Dawley rats were exposed to 600 or 1200 ppm IAN by inhalation for 10 or 30 min. Following exposure, adrenocorticotropic hormone (ACTH) and corticosterone in plasma and corticotropin-releasing factor (CRF) in three brain regions (hypothalamus, hippocampus, and frontal cortex) were determined by radioimmunoassay. Levels of IAN in the three brain regions as well as in blood were measured by gas chromatography to determine the target tissue concentrations responsible for neuroendocrine changes. Uptake of IAN into blood and all brain regions was very rapid, as stable concentrations were achieved within 10 min of exposure and maintained for 30 min of continuous inhalation. Plasma corticosterone decreased significantly after 10 min inhalation of both IAN doses, and returned to control levels after 30 min. Moreover, plasma ACTH was significantly increased by 10 and 30 min of exposure to 600 and 1200 ppm IAN, while hypothalamic CRF increased significantly after 30 min of exposure to the 600 ppm dose. These latter findings suggest activation of the hypothalamus and pituitary due to a reduction in negative feedback resulting from the initial decrease in corticosterone. Although plasma ACTH was greatly increased after 30 min, plasma corticosterone levels were unchanged, indicating that IAN primarily acts to inhibit the synthesis or secretion of adrenal steroids and that activation of the HPA axis is not involved in the behavioral manifestations of IAN inhalation. These compensatory effects of HPA axis regulation, and possibly the vasodilatory properties of IAN, also likely precluded the establishment of definitive relationships between observed changes in hormone levels and blood or regional brain concentrations of the inhalant.

Effects of acute inhalation exposure to 1,1,1-trichloroethane on the hypothalamo-pituitary-adrenal axis in male Sprague-Dawley rats.

1,1,1-Trichloroethane (TRI) is a commonly used industrial solvent with a considerable potential for inhalation abuse. Previous studies in our laboratory and elsewhere have shown that this agent exerts a suppressant effect on operant responding, as well as a number of additional neurobehavioral effects that are similar to those of central nervous system (CNS) depressant drugs. In an effort to provide information relevant to potential mechanisms involved in the behavioral effects and abuse potential of TRI, the present study evaluated the acute effects of this agent on the activity of the hypothalamo-pituitary-adrenal (HPA) axis . Male Sprague-Dawley rats were exposed to 3500 or 5000 ppm TRI by inhalation for 10 or 30 min. Following exposure, plasma levels of adrenocorticotropic hormone (ACTH) and corticosterone and levels of ACTH and corticotropin-releasing factor (CRF) in three brain regions--hypothalamus, hippocampus, and frontal cortex--were determined by selective radioimmunoassays. Levels of TRI in the three brain regions as well as blood were measured by headspace gas chromatography to determine the target tissue concentrations responsible for neuroendocrine changes. Uptake of TRI in blood and all brain regions was very rapid, with stable concentrations apparently achieved within 10 min and maintained for 30 min. During this time course, a significant decrease in plasma corticosterone was produced at 30 min but no significant change in plasma ACTH was observed with 3500 ppm TRI. However, after exposure to 5000 ppm, both plasma ACTH and plasma corticosterone were significantly reduced at 10 and 30 min. ACTH levels in the three brain regions were not significantly changed by TRI, while hypothalamic CRF was significantly increased during exposure to 3500 ppm. However, hypothalamic concentrations of CRF declined following 30 min at 3500 ppm and were not significantly changed by 5000 ppm. This complexity of effects on the regulation of HPA axis activity likely precluded the establishment of consistent relationships between changes in hormonal levels and blood or regional brain concentrations of the inhalant. However, these actions of TRI were strikingly similar to those previously reported for the benzodiazepines.

Investigation of the radioadaptive response in brain and liver of pUR288 lacZ transgenic mice.

The radioadaptive response, where a small priming dose of ionizing radiation can lessen the effects of subsequent exposure to a higher radiation challenge dose, was investigated in brain and liver within transgenic mice. Although it is well characterized in models in vitro, current radioadaptive response research has focused on particular cell types (i.e., lymphocytes) and does not provide comparative data for responses of multiple tissues within an organism. Transgenic animals are useful for such comparisons, because the transgene is integrated into all cells in the body. The pUR288 lacZ plasmid-based transgenic mouse model utilizes a plasmid vector allowing highly efficient recovery of mutational targets, including large size-change mutations that result from radiation exposure. Female C57BI/6 pUR288 lacZ mice were exposed to priming doses of 0.075- to 0.375-Gy x-rays over a 3-d period. After 3 wk, they received an acute challenge dose of 2.5-Gy x-rays. Spontaneous mutant frequencies in lacZ were significantly higher in liver than in brain (6.62 x 10(-5) vs. 3.51 x 10(-5)). In the absence of a priming dose, the 2.5-Gy challenge doubled the mutant frequency of both liver and brain (13.38 x 10(-5), and 7.63 x 10(-5) respectively). Priming doses of 0.15, 0.225, and 0.375 Gy significantly reduced (by 40%) the mutagenic effects of the 2.5-Gy challenge in brain. Restriction enzyme analysis of rescued mutant plasmids revealed a decrease in large size-change mutations at the three priming doses in brain. This study demonstrates the utility of this model for the investigation of radiological processes of large size-change mutations, as well as showing a radioadaptive response in brain, but not liver, of mice in vivo.

Barium: rationale for a new oral reference dose.

The U.S. Environmental Protection Agency (EPA) has an established oral reference dose (RfD) value for Ba of 0.07 mg Ba/kg/d based on a 1984 investigation that reported hypertension. In this study, the toxicological data for Ba has been reevaluated and a revised oral RfD is proposed. The toxicokinetic, acute, and chronic toxicity, carcinogenicity, and reproductive animal studies as well as epidemiological and occupational health human studies for Ba exposure were reviewed for applicability to an oral RfD. The available human studies have some utility but suffer from either a small population size, a short exposure regimen, or difficulties in identifying definitive Ba exposure in the study population. As a result, the available long-term animal studies were found to be more appropriate for the RfD derivation. A dose-response assessment of no-observed-adverse-effect level (NOAEL) and lowest-observed-adverse-effect level (LOAEL) values determined that kidney effects are the most sensitive endpoint for adverse health effects related to chronic soluble Ba ingestion in mammals. The most complete animal studies were conducted by the National Toxicology Program (NTP, 1994) and the lowest species NOAELs were 75 mg Ba/kg/d in male mice and 60 mg Ba/kg/d for male rats. The male rats were identified to be the most sensitive population tested and their NOAEL value was selected for extrapolation to an oral RfD. Application of overall uncertainty factors to the lowest NOAEL value from a chronic animal study of either 90 (based on an approach proposed by Dourson, 1994) or the generally accepted 100 results in an oral RfD of 0.66 mg Ba/kg/d or 0.6 mg Ba/kg/d, respectively. It is proposed to use the more conservative value of 0.6 mg Ba/kg/d. This reassessment results in nearly an order of magnitude increase in the U.S. EPA oral RfD for Ba.

The pharmacokinetics of inhaled 1,1,1-trichloroethane following high milk intake in mice.

In the evaluation of lipophilic halocarbons for neurobehavioral toxicity in operant testing, animals often receive large amounts of milk as a behavioral reinforcer over time. If this increase of fat in the diet sufficiently impacted the lipid depots of the animal, the pharmacokinetics of lipophilic test compounds might be significantly affected and thus obscure the accompanying neurobehavioral effects. The effects of milk intake, comparable to what was consumed as behavioral reinforcer during operant behavioral sessions, on the pharmacokinetics of inhaled 1,1,1-trichloroethane (TRI) were therefore examined in the blood and nine organ tissues of mice. Male CD-1 mice were food restricted so that their body weights would be reduced to and maintained at 80% of their original, and received a single gavage dose of 1.0 ml evaporated milk daily for three weeks. A control group with similar food restrictions was dosed with the same volume of water. Inhalation exposures to 3500 ppm TRI for 100 minutes were conducted at the end of the treatment period. Blood and nine organ tissues were sampled at a series of time points, and their TRI contents were analyzed by headspace gas chromatography. The uptake of TRI was rapid, with near steady state approached in blood and most tissues after 40-60 minutes of exposure. All of the tissues except fat had similar TRI time-concentration profiles, while TRI concentrations in fat tissue were about 20-30 times higher than in other tissues. There was no statistically significant difference in the tissue concentrations between the milk-dosed group and water-dosed group at all of the time points for all tissues measured. Therefore, it appears unlikely that this level of milk intake as a reinforcer in behavioral studies will affect the results of operant testing evaluations by altering the pharmacokinetics of lipophilic halocarbons such as TRI.

Effect of subchronic formaldehyde inhalation on minute volume and nasal deposition in Sprague-Dawley rats.

Since respiratory depression during formaldehyde (HCHO) inhalation is an important mechanism in reducing the dose received and potentially the toxicity in the nasal passages of exposed animals, this study was conducted to determine if changes in the pattern of minute volume response and nasal deposition occurred in nosepiece challenges to rats after long-term repeated exposures to HCHO. Male Sprague-Dawley rats were exposed to 0, 0.5, 3, or 15 ppm HCHO for 6 h/d, 5 d/wk, for 8 or 16 wk. The preexposed animals and age-specific controls were then submitted to a HCHO nosepiece challenge at the same concentration that was received in the subchronic exposure. Very high nasal deposition was demonstrated in all measurements. There was a diminished maximum minute volume depression in the 16-wk group relative to the 8-wk group. The difference in response was not statistically associated with the subchronic preexposure concentration. The substantial recovery of all initially depressed responses that occurred during the challenges probably diminished the impact of the decreased maximum responses on the resulting nasal deposition over the course of the long-term exposures.

Effect of dosing vehicles on the pharmacokinetics of orally administered carbon tetrachloride in rats.

The primary objectives of this investigation were to determine whether oil and aqueous dosage vehicles alter the pharmacokinetics of orally administered carbon tetrachloride (CCl4) in rats, and to relate vehicle effects on CCl4 absorption and bioavailability to alterations of the acute hepatotoxicity of CCl4 seen in a companion study (H.J. Kim, S. Odend'hal, and J. V. Bruckner, 1990, Toxicol. Appl. Pharmacol. 102, 34-49). Fasted 200- to 230-g male Sprague-Dawley rats with indwelling arterial cannulas received 25 mg CCl4/kg body wt by gavage: in corn oil; as an Emulphor aqueous emulsion; in water; and as pure undiluted chemical. The 25 mg/kg dose was also given iv in PEG 400 through an indwelling jugular cannula. Serial blood samples were taken from the iv and gavage animals and analyzed for CCl4 content to obtain blood concentration-versus-time profiles. CCl4 was absorbed very rapidly from the GI tract, as peak concentrations of CCl4 in the blood were reached within 3-6 min of dosing in the aqueous emulsion and water groups. These peak levels were higher than those in the undiluted CCl4 group and substantially higher than those in the corn oil group. Corn oil markedly delayed the absorption of CCl4 from the GI tract and produced secondary peaks in the blood concentration-versus-time profiles. Elimination of CCl4 from the bloodstream of the iv group followed a triexponential pattern. CCl4 was eliminated from the blood at approximately the same rate in the iv and po groups, as reflected by similar elimination rate constant and half-life values. There was a high degree of correlation of both Cmax and AUC0(120) with hepatotoxicity. CCl4 was apparently less acutely hepatotoxic in corn oil due to delay and prolongation of CCl4 absorption, resulting in a marked decrease in the concentration of the chemical in the arterial blood. These findings suggest that corn oil has sufficient effect on the pharmacokinetics of orally administered CCl4 to require an appraisal of its use in studies of the acute oral toxicity of CCl4 and other volatile organic chemicals (VOCs). The use of aqueous Emulphor emulsions appears appropriate in studies of VOC contaminants of drinking water, in that the emulsion did not substantially alter the pharmacokinetics or hepatotoxicity of CCl4 from that ingested in water.

The influence of inhaled formaldehyde on rat lung cytochrome P450.

The effect of formaldehyde (HCHO) inhalation on total cytochrome P450 in the lungs of Sprague-Dawley rats was assessed after single and repeated exposures to 0, 0.5, 3, and 15 ppm HCHO. Whole-body exposures were conducted in dynamic, monitored exposure systems for 6 hr/day, 5 days/week, for periods of exposure of 1 day, 4 days, 12 weeks, or 24 weeks. Lung microsomal fractions were prepared and total protein and cytochrome P450 were measured 18 hr after the end of exposure at each time point. Two separate sets of exposure studies were conducted, thus duplicating all measurements for each dose group and at each time point. There were no detectable levels of total lung P450 in any of the rats that received a single 6-hr exposure to all three HCHO doses, while control lung P450 levels were similar to that found for 4-day and 12-week control rats. After 4 days of repeated exposures, however, there was a highly significant, reproducible, and dose-dependent increase in lung P450 levels relative to controls, with the 0.5, 3, and 15 ppm groups demonstrating 387, 1026, and 1123% of control values, respectively. Lung P450 levels remained elevated at all HCHO concentrations through 12 and 24 weeks of exposure, although the percentage difference between exposed and control rats continually dropped throughout the course of long-term repeated exposures. While HCHO-exposed rats did have decreased total body weight relative to controls, lung microsomal protein and lung weight of nearly all of the HCHO-exposed rats was not significantly different from the controls. The initial inactivation of lung P450 after a single HCHO exposure is apparently a transient phenomenon, with dose-dependent induction of the total P450 levels in the lung as the pattern of response to repeated exposures to inhaled HCHO.

The uptake and elimination of 1,1,1-trichloroethane during and following inhalation exposures in rats.

The pharmacokinetics of 1,1,1-trichloroethane (TRI) was studied in male Sprague-Dawley rats in order to characterize and quantify TRI uptake and elimination oby direct measurements of the inhaled and exhaled compound. Fifty or 500 ppm TRI was inhaled for 2 hr through a one-way breathing valve by unanesthetized rats of 325-375 g. Repetitive samples of the separate inhaled and exhaled breath streams, as well as arterial blood, were collected concurrently both during and following TRI inhalation and analyzed for TRI by gas chromatography. Respiratory rates and volumes were continuously monitored during and following exposure and were used in conjunction with the pharmacokinetic data to characterize profiles of uptake and elimination. TRI was very rapidly absorbed from the lung, in that substantial levels were present in arterial blood at the first sampling time (i.e., 2 min). Blood and exhaled breath concentrations of TRI increased rapidly after the initiation of exposure, approaching but not reaching steady state during the 2-hr exposures. The blood and exhaled breath concentrations were directly proportional to the exposure concentration during the exposures. Percentage uptake of TRI decreased 30-35% during the first hour of inhalation, diminishing to approximately 45-50% by the end of the exposure. Total cumulative uptake in the 50 and 500 ppm groups over the 2-hr inhalation exposures was determined to be 6 and 48 mg/kg body wt, respectively. By the end of the exposure period, 2.1 and 20.8 mg, respectively, of inhaled TRI was eliminated from rats inhaling 50 and 500 ppm TRI. A physiological pharmacokinetic model for TRI inhalation was utilized to predict blood and exhaled breath concentrations for comparison with observed experimental values. Overall, values predicted by the physiological pharmacokinetic model for TRI levels in the blood and exhaled breath were in close agreement with measured values both during and following TRI inhalation.

A method for direct measurement of systemic uptake and elimination of volatile organics in small animals.

A method was developed which was suitable for toxicokinetic studies requiring direct, repetitive sampling of blood and breath during exposures of small animals to volatile chemicals. Anesthetized male Sprague-Dawley rats were tracheostomized, and a specially fabricated one-way breathing valve was inserted directly into the trachea, in order that the inhaled and exhaled breath streams could be separated and independently sampled. The desired concentration of 1,1,1-trichloroethane was generated in a gas sampling bag, which was connected to the inlet side of the valve for inhalation exposures. Rats dosed with 1,1,1-trichloroethane by gavage inhaled ambient air through the valve. A pneumotachograph was positioned in the influent air stream to measure respiration. Respiratory rate, minute volume, and core temperature were continuously recorded on a physiograph. Samples of inspired and expired air were periodically collected from sampling ports immediately adjacent to the breathing valve and were analyzed for their 1,1,1-trichloroethane content by gas chromatography. Using this system, it was possible to determine percentage systemic uptake at any given time and to monitor cumulative uptake (i.e., total dose received) over the course of inhalation exposures. Alveolar concentrations were calculated by taking into account the respiratory indexes and dead space volume. A femoral vein was cannulated, so that concentrations of the chemical in the blood and breath could be measured concurrently. Data from rats dosed orally and by inhalation with 1,1,1-trichloroethane are presented to illustrate the applicability of the method in kinetics studies of volatile chemicals.