Effects of bromodichloromethane on ex vivo and in vitro luteal function and bromodichloromethane tissue dosimetry in the pregnant F344 rat.

Bromodichloromethane (BDCM), a drinking water disinfection by-product, causes pregnancy loss, i.e. full-litter resorption, in F344 rats when treated during the luteinizing hormone (LH)-dependent period. This effect is associated with reduced maternal serum progesterone (P) and LH levels, suggesting that BDCM disrupts secretion of LH. To test the hypothesis that BDCM also affects luteal responsiveness to LH, we used ex vivo and in vitro approaches. For the ex vivo study (i.e., in vivo exposure followed by in vitro assessment), dams were dosed by gavage on gestation days (GD) 6-9 (plug day=GD 0) at 0 or 100 mg/kg/d. One hour after the GD-9 dose, rats were killed, blood was collected, and tissue concentrations of BDCM were assessed. Corpora lutea (CL) were incubated with or without hCG, an LH agonist, to stimulate P secretion. For the in vitro study, CL were pooled from untreated F344 rats on GD 9 and cultured with BDCM at 0, 0.01, 0.10 or 3.0 mM. BDCM was found at highest concentrations in adrenal, ovarian, adipose, and hypothalamic tissues. BDCM treatment decreased serum P and LH levels in vivo. Ex vivo, however, BDCM-exposed CL showed >2-fold increases in P secretion relative to controls. Both control and BDCM-exposed CL displayed a 2.4-fold increase in P secretion in response to hCG challenge. In contrast, in vitro exposures reduced CL responsiveness in a dose-related fashion while baseline levels were unaffected. It is unclear if the ex vivo 'rebound' reflects the removal of the CL from a possible direct inhibitory influence of BDCM, or a response to diminished LH stimulation in vivo. Thus, these data suggest that BDCM disrupts pregnancy in F344 rats via two modes: disruption of LH secretion, and disruption of the CL's ability to respond to LH.

The effects of inhalation exposure to bromo-dichloromethane on specific rat CYP isoenzymes.

Several cytochrome P450 (CYP) isoenzymes may be involved in the metabolism of bromo-dichloromethane (BDCM), a drinking water disinfection byproduct. After 4-h inhalation exposures of male F344 rats to BDCM between 100 and 3200 p.p.m., hepatic microsomal methoxyresorufin demethylase (MROD), ethoxyresorufin de-ethylease (EROD) and pentoxyresorufin dealkylase (PROD) activities showed modest increases at low exposure levels and larger decreases at high exposure levels, compared with controls. Western blots for CYP1A2 and CYP2B1 showed similar trends. In addition, p-nitrophenol hydroxylase (PNP) activity was measured and Western blots for CYP2E1 were performed. CYP2E1 and CYP2B1 isoenzymes are known to metabolize BDCM (Thornton-Manning, J.R., Gao, P., Lilly, P.D., Pegram, R.A., 1993. Acute bromodichloromethane toxicity in rats pretreated with cytochrome P450 inducers and inhibitors. The Toxicologist 13: 361). When compared with a multiple gavage study of BDCM in female F344 rats (Thornton-Manning, J.R., et al., 1994. Toxicology 94, 3-18), the results of the two studies for EROD, PROD, and PNP activities were qualitatively the same; PNP activity did not change, while both PROD and EROD activities decreased at high exposures. In the current work, Western blots for CYP2E1, CYP2B1 and CYP1A2 supported the results from the PNP, PROD and MROD activities, respectively. The decreases in MROD and PROD activities and in Western blots for CYP1A2 and CYP2B1 at high exposures suggest that BDCM may be a suicide substrate for these CYP isoenzymes. Other important conclusions that can be drawn from the comparison between the current and prior work are that the liver response is similar for both sexes, and it is also similar for inhalation and gavage exposures under these conditions. Finally, the decrease in EROD activity at high doses, found in both studies, may be a further reflection of CYP1A2 activity, since little or no CYP1A1 activity is normally found in uninduced rat liver and CYP1A2 is known to metabolize ethoxyresorufin, although much more slowly than CYP1A1.

Induction of genetic damage in human lymphocytes and mutations in Salmonella by trihalomethanes: role of red blood cells and GSTT1-1 polymorphism.

The brominated trihalomethanes (THMs) are mutagenic and carcinogenic disinfection by-products frequently found in chlorinated drinking water. They can be activated to mutagens by the product of the glutathione S-transferase-Theta (GSTT1++-1) gene in Salmonella RSJ100, which has been transfected with this gene. To evaluate this phenomenon in humans, we have examined the genotoxicity of a brominated THM, bromoform (BF), using the Comet assay in human whole blood cultures exposed in vitro. No differences were found in the comet tail length between cultures from GSTT1-1(+) versus GSTT1-1(-) individuals (1.67 +/- 0.40 and 0.74 +/- 0.54 microm/mM, respectively, P = 0.28). The high variability was due to the relatively weak induction of comets by BF. Combining the data from both genotypic groups, the genotoxic potency of BF was 1.20 +/- 0.34 microm/mM (P = 0.003). GSTT1-1 is expressed in red blood cells but not in the target cells (lymphocytes), and expression within the target cell (as in Salmonella RSJ100) may be necessary for enhanced mutagenesis in GSTT1-1(+) relative to GSTT1-1(-) cultures. To examine this, we exposed Salmonella RSJ100 and a control strain not expressing the gene (TPT100) to the most mutagenic brominated THM detected in Salmonella, dibromochloromethane (DBCM), either in the presence or absence of S9 or red blood cells from GSTT1-1(+) or GSTT1-1(-) individuals. S9 did not activate DBCM in the non-expressing strain TPT100, and it did not affect the ability of the expressing strain RSJ100 to activate DBCM. As with S9, red cells from either genotypic group were unable to activate DBCM in TPT100. However, red cells (whole or lysed) from both genotypic groups completely repressed the ability of the expressing strain RSJ100 to activate DBCM to a mutagen. Such results suggest a model in which exposure to brominated THMs may pose an excess genotoxic risk in GSTT1-1(+) individuals to those organs and tissues that both express this gene and come into direct contact with the brominated THM, such as the colon. In contrast, those organs to which brominated THMs would be transported via the blood might be protected by erythrocytes. Such a proposal is reasonably consistent with the organ specificity of drinking water-associated cancer in humans, which shows slightly elevated risks for cancer of the colon and bladder but not of the liver.

NOAEL and LOAEL determinations of acute hepatotoxicity for chloroform and bromodichloromethane delivered in an aqueous vehicle to F344 rats.

Chloroform (CHCl3) and bromodichloromethane (BDCM) are generally the two most prevalent disinfection by-products formed during chlorination of drinking water, and both have been shown to be hepatotoxic, nephrotoxic, and carcinogenic in rodents. As the toxicity of these trihalomethanes (THMs) has most often been studied with corn oil as the vehicle of administration, the objectives of this study were to assess hepatotoxicity after exposure to single, low dosages of CHCl3 and BDCM given orally in an aqueous vehicle to estimate a lowest-observed-adverse-effect level (LOAEL) and a no-observed-adverse-effect level (NOAEL) and to compare toxic potency. Ninety-day-old male Fischer 344 rats were gavaged with either 0.125, 0.1875, 0.25, 0.5, 0.75, 1.0, or 1.5 mmol CHCl3 or BDCM/kg body weight in 10% Alkamuls EL-620 (5 ml/kg body weight). At 24 h postgavage, serum was collected for analysis of clinical chemistry indicators of liver damage. Both CHCl3 and BDCM induced dose-dependent hepatotoxicity; serum alanine aminotransferase, aspartate aminotransferase, and sorbitol dehydrogenase were elevated significantly over control at 1.5, 1.0, and 0.5 mmol/kg. At these dose levels after 24 h, the two THMs appeared to be equipotent hepatotoxicants. Additional assessments at later time points demonstrated that BDCM causes more persistent liver damage than CHCl3 (Lilly et al., 1997). At 0.25, 0. 1875, and 0. 125 mmol of either THM/kg, significant increases over control were not detected for any measured endpoint. Therefore, these data indicate that the acute, oral NOAELs and LOAELs for liver toxicity are 0.25 and 0.5 mmol/kg, respectively, for both CHCl3 and BDCM. These determinations should provide a basis to establish new exposure limits for One-Day Health Advisories for these prevalent THMs.

A physiologically based pharmacokinetic description of the oral uptake, tissue dosimetry, and rates of metabolism of bromodichloromethane in the male rat.

Bromodichloromethane (BDCM), a trihalomethane (THM) and water chlorination by-product, induces cancer in several tissues in experimental animals, including target tissue sites where increased incidences of human cancer have been linked to consumption of chlorinated water. The purpose of the present study was to examine the effects of vehicle of administration on the pharmacokinetics of orally administered BDCM and to further develop and validate a physiologically based pharmacokinetic (PBPK) model to describe BDCM absorption, tissue dosimetry, and rates of metabolism for both oil and 10% Emulphor vehicles. Estimates of oral absorption rate constants were determined by fitting blood and exhaled breath chamber concentration-time curves obtained following gavage of male F344 rats with 50 or 100 mg BDCM/kg in corn oil or 10% Emulphor using a previously published multicompartmental gastrointestinal tract submodel (Semino et al., Toxicology 117, 25-33, 1997) linked to a PBPK model. Independently estimated oral uptake and metabolic rate constants accurately described kidney BDCM concentrations and plasma bromide ion levels without adjustment. This observation increases our confidence in model structure and values of parameter estimates. Liver BDCM concentrations were simulated, but with less accuracy than kidney dosimetry simulations, following incorporation of BDCM loss to metabolism during sample preparation. This model describes BDCM tissue dosimetry and metabolism following oral gavage and can be utilized in estimating rates of formation of reactive metabolites in target tissues. Estimates of tissue dosimetry and levels of toxic intermediates can be incorporated into a risk assessment model for BDCM-induced toxicity and carcinogenicity.

Effect of dosing vehicle on the developmental toxicity of bromodichloromethane and carbon tetrachloride in rats.

Several halocarbons have been shown to cause full-litter resorption (FLR) in Fischer-344 rats when administered orally in corn oil. Since halocarbons often occur as contaminants of drinking water, we sought to determine the influence of the vehicle, aqueous versus lipid, on the developmental toxicity of two of these agents. In separate assays, bromodichloromethane (BDCM) and carbon tetrachloride (CCl4) were administered by gavage to Fischer-344 rats on gestation days (GD) 6-15 at 0, 25, 50, or 75 mg/kg/day in either corn oil or an aqueous vehicle containing 10% Emulphor EL-620. Dams were allowed to deliver and the litters were examined postnatally. Uteri of females that did not deliver were stained with 10% ammonium sulfide to detect FLR. Effects of both agents on maternal weight gain were slightly more pronounced in the aqueous vehicle at lower doses, but at the highest dose, CCl4 was more maternally toxic in corn oil. Developmentally, both agents caused FLR at 50 and 75 mg/kg in both vehicles. At 75 mg/kg, dams receiving corn oil had significantly higher rates of FLR (83% for BDCM, 67% for CCl4) compared to their aqueous-vehicle counterparts (21% for BDCM, 8% for CCl4). Blood concentrations of BDCM following GD-6 gavage revealed a shorter elimination half-life in the aqueous dosing vehicle (2.7 h) compared to the oil vehicle (3.6 h). Benchmark doses of CCl4 were similar for the oil (18.9 mg/kg) and aqueous (14.0 mg/kg) vehicles. For BDCM, the corn oil vehicle yielded a less conservative (i.e., higher) value (39.3 mg/kg) than the aqueous vehicle (11.3 mg/kg), reflecting different confidence intervals around the estimated 5%-effect dose levels.

Trihalomethane comparative toxicity: acute renal and hepatic toxicity of chloroform and bromodichloromethane following aqueous gavage.

Bromodichloromethane (BDCM) and chloroform (CHCl3) are by-products of drinking water chlorination and are the two most prevalent trihalomethanes (THMs) in finished drinking water. To date, no comprehensive comparison of the acute renal and hepatic effects of BDCM and CHCl3 following oral gavage in an aqueous dosing vehicle has been conducted. To characterize BDCM- and CHCl3-induced nephro- and hepatotoxicity following aqueous gavage and compare directly the responses between these THMs, 95-day-old male F-344 rats were given single oral doses of 0.0, 0.75, 1.0, 1.5, 2.0, or 3.0 mmol BDCM or CHCl3/kg body wt in an aqueous 10% Emulphor solution. Compound-related hepatic and renal damage was evaluated by quantitating clinical toxicity markers in the serum and urine, respectively. Both THMs appear to be equally hepatotoxic after 24 h, but BDCM caused significantly greater elevations in serum hepatotoxicity markers than CHCl3 at 48 h following exposure to 2.0 and 3.0 mmol/kg. In addition to causing more persistent liver toxicity than CHCl3, BDCM also appears to be slightly more toxic to the kidney at lower doses. Potency differences between the two THMs may be due to pharmacokinetic dissimilarities such as greater metabolism of BDCM to reactive metabolites or more extensive partitioning of BDCM into kidneys and fat depots, resulting in prolonged target tissue exposure.

Glutathione S-transferase-mediated mutagenicity of trihalomethanes in Salmonella typhimurium: contrasting results with bromodichloromethane off chloroform.

Trihalomethanes (THMs) are the most prevalent disinfection by-products identified in chlorinated drinking water. Among the THMs, chloroform (CHCl3) generally occurs at the highest concentration in finished water, but the concentrations of each of the brominated THMs (CHBrCl2, CHBr2Cl, and CHBr3) can exceed that of CHCl3. Each of these four THMs was carcinogenic in rodents in chronic oral dosing studies. This study assessed THM mutagenicity in a strain of Salmonella typhimurium TA1535 that was transfected with rat theta-class glutathione S-transferase T1-1 (+GST). The +GST strain and its nontransfected parent strain (-GST) were employed in a plate-incorporation assay and exposed for 24 hr to the vapor of individual THMs at concentrations up to 25,600 ppm in sealed Tedlar bags. Base-substitution revertants were produced in the +GST strain in a dose-dependent fashion by CHBrCl2 but not by CHCl3. At 4800 ppm CHBrCl2, which produced a calculated agar concentration of 0.67 mM, there were 419 +/- 75 revertants per plate compared to a spontaneous level of 23 +/- 5. CHCl3 produced a doubling of revertants only at the two highest concentrations tested (19,200 and 25,600 ppm). These results indicate that bromination of THMs confers the capability for theta-class GST-mediated transformation to mutagenic intermediates at low substrate concentrations, suggesting the possibility of a similar activation route in humans. Further, the very low affinity of the GSH-dependent pathway for CHCl3 demonstrates that different THMs can induce adverse effects via different mechanisms, indicating that risk evaluations of THMs should not treat members of this class as if they shared a common mode of action.

Glutathione S-transferase-mediated induction of GC-->AT transitions by halomethanes in Salmonella.

Halomethanes are among the most common mutagenic and carcinogenic disinfection by-products present in the volatile/semivolatile fraction of chlorinated drinking water. Recent studies have demonstrated that the mutagenicity of dichloromethane (CH2Cl2) and bromodichloromethane (BrCHCl2) can be mediated by a theta-class glutathione S-transferase (GSTT1-1). These studies used strain RSJ100 of Salmonella, which is a derivative of the base-substitution strain TA1535 (hisG46, rfa, delta uvrB), into which has been cloned the GSTT1-1 gene from rat. In the present report, we have extended these studies by demonstrating that the mutagenicity of two additional brominated trihalomethanes, bromoform (CHBr3) and chlorodibromomethane (CICHBr2), are also mediated by GSTT1-1 in RSJ100. Using a Tedlar bag vaporization technique, the mutagenic potencies (revertants/ppm) for these two compounds as well as the compounds tested previously rank as follows: CHBr3 approximately CICHBr2 > BrCHCl2 approximately CH2Cl2. To explore the mutational mechanism, we determined the mutation spectra of all four halomethanes at the hisG46 allele by performing colony probe hybridizations of approximately 100 revertants induced by each compound. The majority (96-100%) of the mutations were GC-->AT transitions, and 87-100% of these were at the second position of the CCC/GGG target. In contrast, only 15% of mutants induced by CH2Cl2 were GC-->AT transitions in the absence of the GSTT1-1 gene in strain TA100 (a homologue of TA1535 containing the plasmid pKM101). The ability of GSTT1-1 to mediate the mutagenicity of these di- and trihalomethanes and the induction of almost exclusively GC-->AT transitions by these compounds suggest that these halomethanes are activated by similar pathways in RSJ100, possibly through similar reactive intermediates. The implications of these findings are discussed in relation to previous experimental work on the GST-mediated bioactivation of dihalomethanes, which includes the possible formation of GSH intermediates and/or GSH-DNA adducts.

Physiologically based estimation of in vivo rates of bromodichloromethane metabolism.

Bromodichloromethane (BDCM) is a rodent carcinogen formed by chlorination of drinking water containing bromide and organic precursors. BDCM is a member of the class of disinfection by-products known as trihalomethanes (THMs), compounds that have been shown to be carcinogenic in rodents. A physiologically-based pharmacokinetic (PBPK) model has been developed and applied to provide estimates of the rates of metabolism of BDCM in vivo in rats. The model consists of five compartments (liver, kidney, fat and slowly and rapidly perfused tissues). Tissue partition coefficients were determined using a modified vial equilibration technique and rates of metabolism were estimated by fitting data obtained from stable metabolite (bromide ion, (Br-)) analysis following 4 h constant concentration BDCM inhalation exposure (50-3200 ppm) and closed chamber gas uptake experiments. Metabolism was described using a single saturable pathway representing a high capacity, high affinity process (Vmaxc = 12.8 mg/h/kg; Km = 0.5 mg/l). Rate constants obtained from Br- data adequately described data from gas uptake experiments and literature data on exhalation of 14CO and 14CO2 produced following oral gavage with 14C-BDCM. Pretreatment with trans-dichloroethylene (t-DCE), an inhibitor of CYP2E1, increased the apparent Km from 0.5 to 225 mg/l indicating that CYP2E1 is the major P450 isoform involved in the bioactivation of BDCM to reactive intermediates.

Protective effects of glutathione on bromodichloromethane in vivo toxicity and in vitro macromolecular binding in Fischer 344 rats.

Bromodichloromethane (BDCM), a carcinogenic water disinfection by-product, has been shown to be metabolized to intermediates that covalently bind to lipids and proteins, and this binding has been associated with trihalomethane-induced renal and hepatic toxicity. In this study, the effects of glutathione (GSH) on in vivo BDCM toxicity and in vitro BDCM macromolecular binding were evaluated. The in vivo toxicity of BDCM in animals pretreated with buthionine sulfoximine (BSO, a glutathione synthesis inhibitor) and in untreated male Fischer 344 rats was investigated. In another experiment, covalent binding to protein and lipid was quantified after [14C]BDCM was incubated with hepatic microsomal and S9 fractions and renal microsomes from F344 rats, under aerobic and anaerobic conditions, with and without added GSH. After oral dosing with BDCM, the BSO-pretreated animals had greatly increased levels of serum indicators of hepatotoxicity and serum and urinary indicators of nephrotoxicity compared to those in animals dosed solely with BDCM. Histopathological examination revealed that hepatic necrosis was more severe than renal necrosis in the BSO-treated rats. When GSH was added to an aerobic incubation, protein binding was decreased in hepatic microsomal and S9 fractions by 92 and 83%, respectively. GSH also decreased lipid binding by 55% in hepatic microsomal incubations carried out under anaerobic conditions. Addition of GSH decreased renal microsomal protein (aerobic) and lipid binding (anaerobic) by 20 and 43%, respectively. These data indicate that GSH is an important protective factor in the toxicity associated with BDCM.

Effect of subchronic corn oil gavage on the acute toxicity of orally administered bromodichloromethane.

Bromodichloromethane (BDCM) is a by-product of water chlorination and is the second most common trihalomethane (THM) in finished drinking water. It has been reported that delivery of THMs in corn oil can influence the site and magnitude of toxic and carcinogenic responses in rodents, perhaps by inducing metabolizing enzymes or altering tissue composition. To determine if corn oil influences the acute toxicity of BDCM, adult male F-344 rats were pretreated 5 days/week for 6 weeks with oral doses of corn oil or water at a volume of 5 ml/kg. Following pretreatment, animals were gavaged with a single dose of 0, 200 or 400 mg BDCM/kg in 10% Emulphor. Urine was collected at timed intervals over a 48-h period following BDCM administration. Rats were sacrificed at this time and organs and blood removed. Urine and serum were analyzed for indicators of toxicity. Corn oil pretreatment did not enhance the acute hepato- or nephrotoxicity of BDCM, suggesting that vehicle effects noted in previous THM toxicity and carcinogenicity studies are more likely due to pharmacokinetic differences between administration in corn oil and aqueous gavage vehicles than to altered tissue composition or physiological changes.

2,3,7,8-Tetrachlorodibenzo-p-dioxin (TCDD) distribution and cytochrome P4501A induction in young adult and senescent male mice.

While the developmental toxicology of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) and its congeners has received considerable attention, the impact of advanced age on the biochemical effects and the pharmacokinetics of dioxins remains largely undetermined. In the present investigation, TCDD tissue distribution and cytochrome P4501A (CYP1A) induction were characterized in male C57BL/6N mice aged 10 weeks and 28 months at 7 days after administration of single oral [3H]TCDD doses ranging from 0.015 to 15 microgram/kg body wt. Determinations of hepatic marker enzyme activities for CYP1A1 (ethoxyresorufin O-deethylation, EROD) and 1A2 (acetanilide-4-hydroxylation, ACOH) indicated that the dose response curves for EROD induction by TCDD were nearly identical for the 2 age groups, but the ACOH induction response was greater in old mice. After receiving the 15 micrograms/kg dose, an increase (approximately 35%) in relative liver weight was observed 7 days after dosing in the 10-week mice, but not in the aged mice, and the hepatic concentration of TCDD was approximately 25% greater in young than old mice. No age difference was found in hepatic nuclear concentrations of TCDD. A dose-dependent increase in liver:fat tissue concentration ratios was noted at both ages, and adipose tissue and blood concentrations of TCDD did not vary significantly with age. In old mice however, TCDD concentrations in skin, kidney and muscle were all approximately twice those of young mice at the 15 micrograms/kg dose. These results suggest that advanced age may have differential effects on Ah receptor-mediated enzyme induction, while increased TCDD concentrations in certain tissues may have toxicological implications for older animals.

Toxicity of bromodichloromethane in female rats and mice after repeated oral dosing.

The carcinogenic water disinfection byproduct, bromodichloromethane (BDCM), produces renal and hepatic toxicity in rodents in acute and subchronic studies. In the present investigation, female rats and mice (n = 6) were dosed daily for 5 consecutive days with BDCM (dissolved in an aqueous, 10% Emulphor solution) by gavage. Rats received 75, 150 and 300 mg BDCM/kg body weight/day and mice received 75 and 150 mg BDCM/kg body weight/day. Two rats in the 300 mg/kg/day treatment group died on day 5. On day 6, the animals were sacrificed and serum samples were taken for analysis of indicators of hepatic and renal toxicity. Livers and kidneys were excised and samples taken for histopathological evaluation. Portions of the livers were also utilized to produce microsomes for analysis of cytochrome P450 enzyme activities and total P450 content. Total hepatic cytochrome P450 was decreased in rats dosed with 150 and 300 mg BDCM/kg body weight/day, but was not significantly affected in BDCM-treated mice. Serum lactate (LDH) and sorbitol (SDH) dehydrogenase, aspartate aminotransferase (AST), creatinine and blood urea nitrogen were increased above those of controls in rats dosed with 300 mg BDCM/kg/day. These data suggested that hepatic and renal damage had occurred in this treatment group. This was confirmed by histopathological analyses which revealed that lesions occurred in both hepatic and renal tissues from rats dosed with 150 and 300 mg BDCM/kg/day. The hepatic lesions were centrilobular and primarily consisted of vacuolar degeneration. The hepatotoxicity indicators alanine aminotransferase (ALT) and SDH were increased in mice dosed with 150 mg BDCM/kg/day. However, no histopathological lesions were observed in these animals. This study shows that BDCM is both hepatotoxic and nephrotoxic to female rats after repeated dosing, but is only weakly hepatotoxic to female mice at the administered doses. Also, reduced activities of hepatic cytochrome P450 were observed in rats, but not mice. These species differences in toxicity and xenobiotic metabolizing enzyme inhibition caused by BDCM suggest that an understanding of the mechanism of toxicity of this compound will be critical when extrapolating rodent toxicity data to humans for this environmental pollutant.

Dose-dependent vehicle differences in the acute toxicity of bromodichloromethane.

Bromodichloromethane (BDCM) is a disinfection by-product of drinking water chlorination and is the second most common trihalomethane (THM) in finished drinking water. THMs have generally been administered to experimental animals in corn oil, rather than drinking water, which can influence the site and magnitude of toxicity. To examine the effects of gavage vehicle on the acute renal and hepatic toxicity of orally administered BDCM, 95-day-old male F344 rats were given single doses of 0, 200, or 400 mg BDCM/kg in corn oil or an aqueous 10% Emulphor solution. Activities of serum hepatoxicity indicators were significantly greater 48 hr after administration of 400 mg BDCM/kg in corn oil compared to the aqueous vehicle, but delivery of the low dose in either dosing vehicle had little effect on serum enzymes. In contrast, significant elevations in urinary renal toxicity indicators were noted at 200 and 400 mg BDCM/kg in both vehicles after 24 hr, indicating that the kidney is more sensitive to low doses of BDCM than the liver. Significantly greater increases were observed in urinary indicators after delivery of 200 mg BDCM/kg in 10% Emulphor compared to corn oil. However, administration of the high dose in corn oil resulted in greater nephrotoxicity than in the aqueous vehicle. Significant interactions between vehicle of administration and BDCM dose observed for both urinary and serum parameters further indicate that vehicle differences noted in BDCM acute toxicity are dose dependent. This observation may be due to pharmacokinetic differences in gastrointestinal rates of absorption of BDCM from corn oil as compared to an aqueous solution.

Effect of caloric restriction on aflatoxin B1-induced DNA synthesis, AFB1-DNA binding and cell proliferation in Fischer 344 rats.

Young adult male Fischer rats maintained on a reduced calorie diet (60% of ad libitum food consumption) for 6 weeks showed a decrease in the binding of aflatoxin B1 (AFB1) to hepatic or renal nuclear DNA and a reduction of AFB1-induced hepatocellular damage. Repeated dosing of rats with AFB1 resulted in the inhibition of hepatic and renal DNA synthesis measured by [3H]thymidine incorporation. However, the rate of DNA synthesis was greater in ad libitum (AL) rats than in calorically restricted (CR) animals. Three days after AFB1 dosing, the rate of DNA synthesis had recovered to the control level. Cell cycle analyses measured by a flow cytometric method on kidney cells of both AL and CR rats showed that there were no significant changes in cell populations in the S phase between these two groups of rats. AFB1 inhibited the cell proliferation on an average of 33%. The restoration of the cell proliferation in kidney cells was found on the third day after AFB1 dosing. The rate of the regenerative cell proliferation was found to be slightly greater in AL rats than in CR animals. The AFB1-induced regenerative DNA synthesis in both liver and kidney was retarded by CR.

Determining health risks associated with disinfectants and disinfection by-products: research needs.

In order to minimize the levels of potentially toxic disinfectants and disinfection by-products in treated water while maintaining adequate protection against microbiological contamination, the total risks associated with disinfection have to be measured and compared with the risks from microbial agents. Because much work has already been carried out on chlorination and its by-products, it is recommended that research focus on major disinfection alternatives, i.e., ozonation, chloramination, carbon dioxidation, and the most practical combinations of these options. The primary research needs are (1) assessment of the relative toxicological hazards of the disinfectants and their by-products and (2) development of biologically based models for the dose-response relationships of these chemicals.

Effect of caloric restriction on the induction of hepatic cytochrome P-450 and Ah receptor binding in C57BL/6N and DBA/2J mice.

While it is known that caloric restriction alters activities of certain xenobiotic metabolizing enzymes, the mechanism(s) by which this occurs have not been determined. In this study, caloric restriction (CR) increases activities of liver cytochrome P450IA1 dependent ethoxyresorufin-O-deethylase (EROD) and aryl hydrocarbon hydroxylase (AHH) and cytochrome P450IIB1 dependent pentoxyresorufin-O- dealkylase (PROD) in DBA/2J or C57BL/6N mice. However, the cytosolic Ah receptor binding in both strains of mice was not increased. The hepatic cytochrome P450IA1 activity was increased by CR in DBA/2J mice (a strain lacking normal Ah receptor binding), indicating that this induction need not be mediated by the Ah receptor. The effects of CR, sex and strain on P450IA1 induction by 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) were also determined. Specific induction of cytochrome P450IA1 by TCDD was greater in females than in males of both strains, whereas the P450 isozymes induced in male DBA/2J mice had less specificity toward 7-ethoxyresorufin than those induced in C57BL/6N mice. Moreover, P450IA1 induction by TCDD was significantly potentiated by CR in the DBA/2 strain, indicating the interactive involvement of different regulatory mechanisms.

Effects of aging and caloric restriction on the genotoxicity of four carcinogens in the in vitro rat hepatocyte/DNA repair assay.

The effects of aging and chronic caloric restriction (CR) on the genotoxicity of four carcinogens, representing four different classes of chemicals, in the in vitro rat hepatocyte/DNA repair assay were investigated. Hepatocyte cultures were isolated from young, middle-aged, and old male Fischer (F344) rats which were maintained on either an ad libitum (AL) or a CR diet (60% of AL). Hepatocyte cultures from old AL rats, treated with 2-acetylaminofluorene (2-AAF), aflatoxin B1 (AFB1), 7,12-dimethylbenz[a]anthracene (DMBA) and dimethylnitrosamine (DMN), exhibited age-related decreases in DNA repair as compared to young AL rats. By contrast, cultures from young CR rats exhibited significant diet-related decreases in DNA repair with 2-AAF, AFB1, DMBA and DMN, when compared to results from young AL diet-fed rats. Old CR F344 rat derived cultures exhibited no significant age-related dose-dependent decrease in the DNA repair response with any of the chemicals tested. However, in cultures from old CR rats 10.0 microM AFB1 produced an age-related decrease in DNA repair from the response observed in young CR rats. When hepatocytes were isolated from Aroclor 1254-induced rats, increases in DNA repair were observed. These data indicate an age- and diet-related decrease in DNA repair and/or DNA damage and suggest that this decrease is due to a decrease in metabolic activation of these carcinogens to genotoxic species.

The effects of dietary restriction and aging on in vivo and in vitro binding of aflatoxin B1 to cellular DNA.

Laboratory animals maintained on a reduced calorie but nutritionally adequate diet have extended life spans and lowered incidences of spontaneous and chemically induced cancers compared to ad libitum- fed counterparts. Many of the effects of dietary restriction on laboratory animals have been suggested to be related to a deceleration of the aging process. The inhibition of age-related changes in xenobiotic metabolizing enzymes activities by dietary restriction has previously been reported. Alterations of these enzyme activities may cause changes in metabolic activation of carcinogens and, therefore, carcinogen-DNA binding. DNA-repair capability has also been reported to be enhanced in diet-restricted rats. Using AFB1 as a model carcinogen, we have studied in vivo and in vitro hepatic AFB1-DNA binding, demonstrating that dietary restriction (60% of ad libitum consumption) may decrease the metabolic activation of AFB1, and subsequently reduce AFB1-DNA binding. Our preliminary results obtained from the AFB1-DNA binding experiments in isolated hepatocytes suggest that the observed age-dependent reduction in AFB1-DNA binding which may be attributed to a loss of metabolic activating capability was delayed in the diet-restricted rats.