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.

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.

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.

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.

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.

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.

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.

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.

Uptake, distribution, and elimination of carbon tetrachloride in rat tissues following inhalation and ingestion exposures.

Carbon tetrachloride (CCl4) has been studied extensively for its hepatotoxic effects. There is a paucity of information, however, about its tissue deposition following administration by different routes and patterns of exposure. The specific objective of this study was to delineate the uptake, distribution, and elimination of CCl4 in tissues of rats subjected to equivalent oral and inhalation exposures. Male Sprague-Dawley rats (325-375 g) were exposed to 1000 ppm CCl4 for 2 hr. The total absorbed dose (179 mg CCl4/kg bw) was administered to other groups of rats as a single oral bolus or by constant gastric infusion over a period of 2 hr. Animals were terminated at selected time intervals during and postexposure and tissues (liver, kidney, lung, brain, fat, skeletal muscle, spleen, heart, and GI tract) removed for measurement of their CCl4 content by headspace gas chromatography. CCl4 levels in all tissues were much lower in the gastric infusion group than in the oral bolus and inhalation groups. Inhalation resulted in relatively high tissue CCl4 concentrations, because inhaled chemicals enter the arterial circulation and are transported directly to organs throughout the body. It seems logical that the liver should accumulate more CCl4 following ingestion than following inhalation. This did not prove to be the case when comparing liver AUC values for the gastric infusion and inhalation groups. Substantially lower CCl4 concentrations in the liver of animals in the gastric infusion group appeared to be due to very rapid metabolic clearance of the relatively small amounts of CCl4 entering the liver over the 2-hr infusion period. It was hypothesized that the capacity of first-pass hepatic and pulmonary elimination could be exceeded, if CCl4 were given as a single, large oral bolus. Indeed, deposition of CCl4 in all tissues was greater in the oral bolus group than in the gastric infusion group. The time courses of uptake and elimination of CCl4 appeared to be governed largely by a tissue's rate of blood perfusion and lipid content. CCl4 was rapidly taken up, for example, by the brain and liver. These organs' CCl4 content then diminished, as CCl4 was metabolized and redistributed to adipose tissue. CCl4 accumulated slowly, but to very high concentrations, in fat and remained elevated for a prolonged period. Thus, concentrations of CCl4 in some tissues may not be reflective of blood levels. The most appropriate measure of internal dose for CCl4 acute hepatotoxicity appears to be the area under tissue concentrations versus time curve from 0 to 30 min. Tissue time-course data sets are essential for the refinement and validation of physiological models for CCl4 and other volatile organic chemicals.

Lack of volatilization and escape of orally administered trichloroethylene from the gastrointestinal tract of rats.

It is possible that a substantial portion of orally administered volatile organic chemicals (VOCs) may volatilize within the warm environment of the gastrointestinal (GI) tract and escape via the esophagus before being absorbed. The objective of this study was to test this hypothesis with a representative VOC, 1,1,2-trichloroethylene (TCE). Upon hepatic portal vein (PV) injection, complete systemic absorption of TCE was assumed. Thus, exhaled TCE after PV injection should originate only from pulmonary exhalation. In contrast, TCE volatilized in the gut may also contribute to the amounts of TCE exhaled by orally (PO) dosed animals. Male Sprague-Dawley rats (320-380 g) were given 8 or 16 mg TCE/kg bw in an aqueous Alkamuls emulsion (RhonePoulenc, Cranbury, New Jersey). For the PO groups both doses were given by gavage, and for the PV groups the lower dose was injected into the PV as a bolus and the higher dose given as a 20 mm infusion. Serial blood samples were taken from an indwelling carotid arterial cannula and analyzed for their TCE content by headspace gas chromatography (GC), so that the areas-under-blood-concentration-versus-time curves (AUCs) could be determined. Serial exhaled air samples were collected from a sampling port in a miniaturized one-way breathing value and TCE measured by GC analysis to delineate exhaled breath concentration-versus-time-curves (EAUCs). Exhaled breath levels of TCE paralleled blood levels of TCE throughout the monitoring periods. Evidence against the aforementioned hypothesis was provided by comparison of ratios of blood AUCs and exhaled breath EAUCs: values for ratios of AUCPO/AUCPV and EAUCPO/EAUCPV were the same, as were EAUC/AUC ratios for the PO and PV groups. The EAUCs in the PO groups should have been higher, had there been substantial extrusion of volatilized TCE fronm the GI tract.

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.

Schedule-controlled operant behavior of rats following oral administration of perchloroethylene: time course and relationship to blood and brain solvent levels.

Previous studies have indicated that human exposure to perchloroethylene (PCE) produces subtle behavioral changes and other neurological effects at concentration at or below the current occupational exposure limit. Since comparable effects in animals may be reflected by changes in schedule-controlled operant behavior, the ability of orally administered PCE to alter fixed-ratio (FR) responding for a food reward was investigated in male Sprague-Dawley rats. Furthermore, since behavioral effects of solvents are likely to be more closely related to blood or target tissue (i.e, brain) concentrations than administered dose, the relationship between the pharmacokinetic distribution of PCE and its effects on operant responding was also evaluated. Rats trained to lever-press for evaporated milk on an FR-40 reinforcement schedule were gavaged with 160 or 480 mg/kg PCE and immediately placed in an operant test cage for 90 min. Separate animals gavaged with equivalent doses of PCE were used to determine profiles of blood and brain concentrations versus time. Perchloroethylene produced changes in responding that varied not only with dose but also among animals receiving the same dose. Changes in the response rates of rats receiving 160 mg/kg PCE were either not readily apparent, restricted to the first 5 min of the operant session, or attributable to gavage stress and the dosing vehicle. However, 480 mg/kg produced either an immediate suppression of responding for 15-30 min before a rapid recovery to control rates or a complete elimination of lever-pressing for the majority of the operant session. Although the two doses of PCE produced markedly different effects on operant behavior during the first 30 min of exposure, differences in brain concentrations of PCE were minimal. Furthermore, the majority of animals receiving 480 mg/kg PCE fully recovered from response suppression while blood and brain levels of the solvent continued to rise. Thus, relationships between blood and brain PCE levels and performance impairment were not discernible over the monitored time course. Since the rapid onset of response suppression suggests that the precipitating event occurs within the first few minutes of exposure, it is possible that altered responding is related to the rate of increase in blood or brain concentrations rather than the absolute solvent concentrations themselves. The relationship between the pharmacokinetic distribution of solvents and their effects on the central nervous system is obviously complex and may involve acute neuronal adaptation as well as the dynamics of solvent distribution among the various body compartments.

Effect of route and pattern of exposure on the pharmacokinetics and acute hepatotoxicity of carbon tetrachloride.

The objectives of this study were to evaluate the influence of both route and pattern of exposure on the pharmacokinetics (PK) and target organ toxicity of a common volatile organic chemical, carbon tetrachloride (CCl4). Male Sprague-Dawley rats, 325-375 g, inhaled 100 or 1000 ppm CCl4 for 2 hr through a one-way breathing valve. The total amount of CCl4 retained by each rat (i.e., the systemically absorbed dose) during the 2-hr period was determined to be 17.5 and 179 mg CCl4/kg body wt, respectively. CCl4, in doses of 17.5 and 179 mg/kg body wt, was administered in an aqueous emulsion by bolus gavage or by constant gastric infusion over 2 hr. Serial micro blood samples from the animals were analyzed for CCl4, in order to delineate blood concentration-versus-time profiles. Serum enzyme activities and total liver microsomal cytochrome P450 level and glucose 6-phosphatase activity were measured 24 hr postdosing as indices of CCl4 hepatotoxicity. The pattern of oral exposure, or dosage regimen, had a significant effect on the PK and acute the hepatotoxicity of CCl4. Arterial blood levels in the gastric infusion group were much lower than in the oral bolus group with both doses. Since CCl4 is quickly and extensively absorbed from the GI tract, large amounts of CCl4 in the portal blood following the oral bolus apparently exceeded the capacity of the liver to metabolize the chemical. Thus, substantially higher Cmax and AUC0 integral of infinity values were manifest. Hepatotoxicity was also significantly greater in these animals. The route of exposure also had a significant effect on the PK of CCl4. Levels of CCl4 in the arterial blood were much higher during inhalation than during gastric infusion. However, AUC0 integral of infinity vales for the two groups were not significantly different, due to relatively slow elimination after gastric infusion. There was little difference between the two groups in hepatotoxicity indices 24 hr postdosing.

Physiologically based pharmacokinetic model useful in prediction of the influence of species, dose, and exposure route on perchloroethylene pharmacokinetics.

The ability of a physiologically based pharmacokinetic (PBPK) model to predict the uptake and elimination of perchloroethylene (PCE) in venous blood was evaluated by comparison of model simulations with experimental data for two species, two routes of exposure, and three dosage levels. Unanesthetized male Sprague-Dawley rats and beagle dogs were administered 1, 3, or 10 mg PCE/kg body weight in polyethylene glycol 400 as a single bolus, either by gavage or by intraarterial (ia) injection. Serial blood samples were obtained from a jugular vein cannula for up to 96 h following dosing. The PCE concentrations were analyzed by headspace gas chromatography. For each dose and route of administration, terminal elimination half-lives in rats were shorter than in dogs, and areas under the blood concentration-time curve were smaller in rats than in dogs. Over a 10-fold range of doses, PCE blood levels in the rat were well predicted by the PBPK model following ia administration, and slightly underpredicted following oral administration. The PCE concentrations in dog blood were generally overpredicted, except for fairly precise predictions for the 3 mg/kg oral dose. These studies provide experimental evidence of the utility of the PBPK model for PCE in interspecies, route-to-route, and dose extrapolations.

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.

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.

Use of tissue disposition data from rats and dogs to determine species differences in input parameters for a physiological model for perchloroethylene.

Tissue disposition of perchloroethylene (PCE) was determined experimentally in two mammalian species of markedly different size in order to derive input parameters for the development of a physiologically based pharmacokinetic (PBPK) model, which could forecast the disposition of PCE in each species. Male Sprague-Dawley rats and male beagle dogs received a single bolus of 10 mg PCE/kg body wt in polyethylene glycol 400 by gavage. Serial samples of brain, liver, kidney, lung, heart, skeletal muscle, perirenal fat, and blood were taken for up to 72 hr following PCE administration. Blood and tissue PCE concentrations were analyzed using a gas chromatography headspace technique. Dogs exhibited considerably longer tissue and blood half-lives than did rats. The dogs also exhibited larger area under tissue concentration versus time curves for all tissues except the liver. Whole body clearance of PCE in the rat was greater than that in the dog. Model simulations indicated this could be attributed to more rapid and extensive PCE exhalation and metabolism by the rat. The in vivo blood:air partition coefficient determined for rats was similar to an in vitro value previously reported. In vivo tissue: blood partition coefficients, however, were 1.4 to 2.8 times greater than published in vitro values. The PCE in vivo blood:air partition coefficient for the dog was twice that of the rat, but tissue:blood partition coefficients were 1.5 to 3.0 times greater in the rat than in the dog. These results demonstrated the existence of significant differences in partition coefficients in two species commonly used in toxicity testing. The PBPK model was shown to have utility in predicting the impact of metabolism and exhalation on pharmacokinetics of PCE in different species of widely differing size.

Development of a physiologically based pharmacokinetic model for perchloroethylene using tissue concentration-time data.

The tissue disposition of perchloroethylene (PCE) was characterized experimentally in rats in order to (1) obtain input parameters from in vivo data for the development of a physiologically based pharmacokinetic (PBPK) model, and (2) use the PBPK model to predict the deposition of PCE in a variety of tissues following inhalation exposure. For the derivation of model input parameters, male Sprague-Dawley rats received a single bolus of 10 mg PCE/kg body wt in polyethylene glycol 400 by ia injection through an indwelling carotid arterial cannula. Other male Sprague-Dawley rats inhaled 500 ppm PCE for 2 hr in dynamic exposure inhalation chambers. Serial samples of brain, liver, kidney, lung, heart, skeletal muscle, perirenal fat, and blood were taken for up to 72 hr following ia injection, during the 2-hr inhalation exposure, and for up to 72 hr postexposure. Blood and tissue PCE concentrations were analyzed using a gas chromatography headspace technique. Following ia administration, the tissues exhibited similar terminal elimination half-lives (t1/2). As comparable tissue t1/2 are consistent with a blood-flow-limited model, tissue:blood partition coefficients were calculated for noneliminating compartments by division of the area under the tissue concentration-time curve (AUC) by the blood AUC. Liver PCE concentration versus time data were employed in the calculation of in vivo metabolic rate constants. A PBPK model was developed using these parameters derived from the ia data set and used to predict tissue PCE concentrations during and following PCE inhalation. Predicted tissue levels were in close agreement with the levels measured over time in the seven tissues and in blood. Tissue concentration-time data can thus provide valuable input for parameter estimation and for validation of PBPK model simulations, as long as independent in vivo data sets are used for each step.