* Reproductive Toxicology Division and
Experimental Toxicology Division, National Health and Environmental Effects Research Laboratory, Office of Research and Development, U.S. Environmental Protection Agency, Research Triangle Park, North Carolina 27711; and
3M, Medical Department, St. Paul, Minnesota 55133
Received February 24, 2003; accepted April 16, 2003
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ABSTRACT |
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Key Words: perfluorooctane sulfonate; maternal; prenatal; toxicity; rodent.
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INTRODUCTION |
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Perfluorooctane sulfonate (PFOS, C8F17SO3-) is a perfluorinated alkane with a sulfonyl group. The intermediate precursor, perfluorooctane sulfonyl fluoride, provides a link to products with other functional groups, such as free acids, metal salts, sulfonyl halides, and sulfonamides. Since the 1950s, with the commercial scale-up of electrochemical fluorination, PFOS and other perfluorinated organic compounds that metabolize into PFOS (as an end-stage metabolite and breakdown product) have been used in a wide variety of industrial and consumer applications that include stain-resistant coatings for fabrics and carpets, oil-resistant coatings for paper products approved for food contact, fire-fighting foams, mining and oil well surfactants, floor polishes, and insecticide formulations (Renner, 2001; Seacat et al., 2002
). In all, PFOS or products degrading to PFOS are used in over 200 products and applications. However, 3M Company, the primary manufacturer of these compounds and products, discontinued production at the end of 2002.
The widespread use of PFOS and its related products, as well as the environmental stability of the perfluorinated organic chemical, have led to documentation of its presence in both human and wildlife populations worldwide (Giesy and Kannan, 2001; Hansen et al., 2001
; Kannan et al., 2001a
,b
; 2002a
,b
,c
;Olsen et al., 2001a
,b
,c
). Olsen et al. (1999)
reported an average serum PFOS level of 2 ppm in 3M production workers, with 5% of them having levels
6 ppm. Current information from a broad survey of individual blood samples from adult Red Cross blood donors, children (ages 212) from a streptococcal A clinical trial, and a group of elderly subjects enrolled in a longitudinal study of cognitive function indicates that the upper bound of the 95th percentile serum concentration is approximately 100 ppb, with a mean of approximately 3040 ppb (Olsen et al., 2001a
,b
,c
). Recent studies by Giesy and coworkers (Giesy and Kannan, 2001
; Kannan et al., 2001a
,b
; a
,b
,c
) reported detection of PFOS in a variety of wildlife species, including fresh-water and marine mammals, fishes, birds, and shellfish. Although distribution of the chemical appears to be global, including remote locations in the Arctic and North Pacific Oceans, concentrations of PFOS in these animals are relatively greater in the more populated and industrial regions. These investigators also suggested that PFOS can be biomagnified in the top levels of the food chain.
Pharmacokinetic studies have shown that PFOS is readily absorbed, distributed, and accumulated in the serum and liver but poorly eliminated (urinary and fecal excretion half-life estimated at >90 days in the rat; Johnson and Ober, 1979; Johnson et al., 1979
, 1984
; Seacat et al., 2003
). In the rat, a serum elimination half-life of 7.5 days was reported after an oral treatment of PFOS (Johnson et al., 1979
); in Cynomolgus monkeys, a half-life of 200 days was described (Seacat et al., 2002
); and in humans, a mean half-life of approximately 8.7 years was recently estimated from retired production workers (Geary Olsen, 3M, 2002, personal communication). The potential mammalian toxicity of PFOS has been investigated. In the rat, reduction of body weight, liver hypertrophy, and decreased serum cholesterol and triglycerides have been reported after subchronic exposure to PFOS (Seacat et al., 2003
). PFOS has been suggested to interfere with mitochondrial bioenergetics, gap junctional intercellular communication, and fatty acid-protein binding in the liver (Berthiaume and Wallace, 2002
; Hu et al., 2002
; Luebker et al., 2002a
; Starkov and Wallace, 2002
). In addition, PFOS-induced hepatic peroxisome proliferation has been indicated in both rat and mouse (Berthiaume and Wallace, 2002
; Haughom and Spydevold, 1992
; Sohlenius et al., 1993
). Seacat and coworkers (2002
) have evaluated PFOS toxicity in Cynomolgus monkeys and reported weight loss, hepatocellular hypertrophy, and lipid vacuolation, as well as reductions in serum cholesterol, triiodothyronine, and estradiol. The potential reproductive and developmental toxicities of PFOS have not been fully elaborated. Case et al. (2001
) examined the effects of PFOS in rabbits and noted reductions of maternal weight gain and fetal weight at 3.75 mg/kg and higher dosage but no significant incidence of malformation. More recently, 3M completed a multigenerational reproduction study with PFOS in rats (Butenhoff et al., 2002
), which did not indicate any adverse effects on mating and fertility; however, significant reductions of body weight and perinatal viability were noted.
In light of the prevalence and persistence of PFOS in both humans and wildlife, this study was undertaken to provide a comprehensive evaluation of the developmental toxicity of this fluorochemical. This article summarizes the observations from the pregnant dams and term fetuses, and a companion article (Lau et al., 2003) will address the postnatal findings in rats and mice.
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MATERIALS AND METHODS |
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Animal Treatment
Timed-pregnant Sprague-Dawley rats and CD-1 mice obtained from Charles River Laboratories (Raleigh, NC) were bred within a 4-h period and overnight, respectively. Those animals with spermatozoa in a vaginal smear and/or with a copulatory plug were considered to be at gestational day (GD) 0. In a separate study, mature female rats weighing 200 g were obtained from the same supplier. Animals were housed individually in polypropylene cages with heat-treated pine shavings for bedding and provided pellet chow (LabDiet 5001, PMI Nutrition International, Brentwood, MO) and tap water ad libitum. Animal facilities were controlled for temperature (2024°C) and relative humidity (4060%), and operated under a 12-h light-dark cycle.
Rats.
PFOS was freshly prepared daily at 1, 2, 3, 5, and 10 mg/ml of 0.5% Tween-20 vehicle and administered to the pregnant dams by gavage at a volume of 1 ml/kg/day from GD 2 through GD 20. Controls received vehicle alone. Throughout gestation and treatment, maternal body weights as well as food and water consumption were recorded. Blood from each dam was collected between 911 A.M. on GDs 7 and 14 from the lateral tail vein and on GD 21 after decapitation. Aliquots of serum from these blood samples were stored at 20°C for subsequent analysis of PFOS concentration, thyroid hormones, corticosterone, prolactin, cholesterol, and lipid content. On GD 21, both maternal and some fetal livers were removed, weighed, and immediately frozen on dry ice and stored at 80°C for PFOS analysis. For other animals, the gravid uterus was removed and examined, and individual live fetuses were weighed and prepared for teratological evaluation.
In a separate study, adult female rats were given either 3 or 5 mg/kg PFOS daily for 20 days; controls received the Tween-20 vehicle. Blood samples were withdrawn from tail vein at 3, 7, and 14 days after the initiation of PFOS exposure, and trunk blood was obtained from decapitation after 20 days of chemical treatment. Serum samples were prepared and stored at 20°C for T3, T4, and thyroid-stimulating hormone (TSH) analyses.
Mice.
PFOS (0.1, 0.5, 1.0, 1.5, and 2.0 mg/ml vehicle) was similarly prepared and administered by gavage at a volume of 10 ml/kg/day from GD 1 through GD 17. Maternal weight as well as food and water consumption were monitored throughout gestation. Some mice were sacrificed on GDs 6 and 12 by CO2 asphyxiation. The remaining dams were sacrificed on GD 18. Blood was collected from the descending aorta, and serum samples were prepared and analyzed for PFOS concentration as well as lipid content. On GD 18, maternal livers from representative animals were dissected, weighed, and immediately frozen on dry ice and stored at -80°C for PFOS analysis. For other animals, the gravid uterus was removed and examined, and individual live fetuses were weighed and prepared for teratological evaluation.
Teratological Evaluation
The gravid uterus of the pregnant rat or mouse was removed and weighed; the numbers and positions of the live or dead fetuses, as well as resorptions, were recorded. Live fetuses were weighed individually, gender-determined, and examined for external abnormalities. Half of the fetuses were prepared for skeletal examination, and the other half were prepared for visceral evaluation.
Skeletal evaluation.
Fetuses were killed with an overdose of pentobarbital, eviscerated, and fixed in 95% ethanol. Specimens were subsequently stained with Alizarin red and Alcian blue to visualize bone and cartilage, respectively. Skeletal morphology was evaluated as described previously (Narotsky and Rogers, 2000).
Visceral evaluation.
Fetuses were fixed in Bodians solution (2% formaldehyde, 5% acetic acid, 72% ethanol, 21% water). Examination of the head, thoracic, and abdominal viscera were carried out using a freehand razor dissection.
Serum Chemistry
Serum samples were analyzed for total cholesterol, triglycerides, sorbitol dehydrogenase, glucose, bile acids, and total bilirubin levels using a Cobas Fara II chemistry analyzer (Roche Diagnostics, Basel, Switzerland).
Radioimmunoassays
Concentrations of serum hormones were derived from a standard curve encompassing a range of reference standards specific for each assay. If the value of unknown fell above or below this range, it was arbitrarily assigned the values of the highest or lowest reference standard. Internal standards from rat sera were routinely used to monitor interassay differences.
T4 and T3.
Serum samples were thawed and levels of total thyroxine (T4), free T4, and triiodothyronine (T3) were measured in duplicate using specific radioimmunoassay (RIA) kits (Diagnostics Products Corporation, Los Angeles, CA). Sensitivity of the total T4 assay was 5240 ng/ml; that of the free T4 assay was 1100 pg/ml; and that of the T3 assay was 0.16 ng/ml. Because of the surfactant properties of PFOS, a preliminary experiment was conducted to determine whether the chemical might alter performance of the RIA directly. PFOS (at a final concentration of 5 or 10 mg/ml) was added directly to the assay tubes containing the T4 standards and serum samples from untreated control rats. Under these conditions, PFOS did not interfere with the RIA performance.
TSH and prolactin.
Serum samples were thawed, and the levels of TSH and prolactin were quantified by specific RIA. The TSH assay was performed using the following materials supplied by the National Hormone and Pituitary Program (Torrance, CA): iodination preparation NIDDK-rTSH-I-9, reference preparation NIDDK-rTSH-RP-3, and antiserum NIDDK-antirat TSH-RIA-6. Similarly, the prolactin assay was performed with iodination preparation NIDDK-rPRL-I-6, reference preparation NIDDK-rPRL-RP-3, and antiserum NIDDK-antirat PRL-RIA-9. Iodination materials were radiolabeled with 125I (Perkin Elmer/New England Nuclear, Boston, MA) by a modification of the chloramine-T method of Greenwood et al. (1963). Labeled TSH or prolactin was separated from the unreacted iodide by gel filtration chromatography, as described previously (Goldman et al., 1986
).
Serum was pipetted with the appropriate dilutions to a final assay volume of 500 µl with 100 mM phosphate buffer containing 1% BSA. Reference TSH standards ranging from 0.195 to 200 ng/ml and prolactin standards ranging from 0.39 to 100 ng/ml were prepared by serial dilution. Primary antiserum (200 µl) at a dilution of 1:437,500 prepared in a mixture of 100 mM potassium phosphate, 76.8 mM EDTA, 1% BSA, and 3% normal rabbit serum was then pipetted to each assay tube, vortexed, and incubated at 4°C for 24 h; 100 µl of either 125I-TSH or 125I-prolactin was then added to each tube, vortexed, and incubated for another 24 h. Second antibody (goat antirabbit gamma globulin [Calbiochem, San Diego, CA] at a dilution of 1 U/100 µl) was then added, vortexed, and incubated for a third 24 h. The samples were then centrifuged at 1,260 x g for 30 min; the supernatant was aspirated, and the sample tube with pellet was counted in a gamma counter.
Corticosterone.
Serum samples were thawed and levels of corticosterone were measured in duplicate using an RIA kit (ICN Biomedical Inc., Costa Mesa, CA). Sensitivity of the assay was 251000 ng/ml.
Determination of PFOS Concentrations
Serum samples were diluted and liver samples were homogenized in five volumes of reagent-grade water. An aliquot of each dilution was spiked with the appropriate internal standards. Acetonitrile (5 ml) was added as an extraction solvent, which also served to precipitate the proteins. The samples were shaken at 300 rpm for 20 min and centrifuged at 850 x g for 10 min. The supernatant was transferred to a clean tube, diluted with 40 ml of water, and passed through a preconditioned C18 SPE cartridge. PFOS was eluted from the SPE cartridge with 2 ml methanol and analyzed by high-performance liquid chromatography-electrospray tandem mass spectrometry (HPLC-ES/MS/MS) according to the method described by Hansen et al. (2001).
Data Analysis
Data are presented as means and standard errors. Statistical significance was determined by ANOVA, using individual litter as the statistical unit. Maternal weight gains and food and water consumption were analyzed by ANOVA with repeated measure. When a significant treatment effect or interaction was detected, Duncans multiple-range test or Dunnetts t-test were performed post hoc. Statistically significant differences were determined at p 0.05.
The U.S. Environmental Protection Agency (EPA) now uses the benchmark dose (BMD) approach (Barnes et al., 1995; Crump, 1984
) for noncancer risk assessment (EPA, 1995
). This approach is designed to provide a more quantitative alternative to dose-response assessment than the no-observed-adverse-effect-level (NOAEL) process by constructing mathematical models to fit all data points in the dose-response study and to take data variance into consideration. In this study, BMD5 and BMDL5 values were calculated for maternal and developmental toxicity after PFOS exposure. BMD5 refers to the central estimate of the administered dose predicted to cause a 5% increase in response above background, and BMDL5 is defined as the corresponding lower limit of the 95% confidence interval on the BMD (Allen et al., 1994
). Benchmark Dose Software (EPA, 2000
) was used to calculate the BMD5 values. Selection of a specific curve-fitting model for the BMD determination was based on the Akaikes Information Criterion (AIC) value. The AIC is equal to 2L + 2p, where L is the log-likelihood at the maximum likelihood estimates for the parameters, and p is the number of model parameters estimated. The model that demonstrates "goodness of fit" with the lowest AIC value is presumed to be the most appropriate.
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RESULTS |
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Mouse
PFOS-induced deficits in maternal weight gain were not as pronounced in the mouse as in the rat. Statistically significant differences in body weight gain were observed only in the 20 mg/kg dosage group at late gestation (Fig. 6). Likewise, food and water consumption were less affected by the chemical exposure (Fig. 7
). In contrast, PFOS treatment increased maternal liver weight in a dose-dependent fashion; indeed, in the highest dosage group (20 mg/kg), the livers almost doubled their weight, compared with those in controls (Table 2
). Serum PFOS concentrations in the mouse were comparable with those found in the rat (Fig. 8
); for the 10 mg/kg dosage group, the mean (± SE) maternal rat serum at term was 190 ± 7 µg/ml, and the maternal mouse serum at term was 179 ± 35 µg/ml. Additionally, serum PFOS in the mouse appeared to reach a saturated concentration at 250 µg/ml. A similar pattern of PFOS accumulation and saturation was seen in the maternal mouse liver. Indeed, in the 10 mg/kg dosage group, the rat maternal liver PFOS concentration was 710 ± 44 µg/g, and that for the mouse liver was 560 ± 52 µg/g.
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DISCUSSION |
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Maternal toxicity of PFOS, indicated by deficits in weight gain during pregnancy, was observed in both rat and mouse. In both rodents, the severity of the adverse effects was dose-dependent; at term, a BMD5 for maternal weight reduction is estimated at 0.22 mg/kg and a BMDL5 at 0.15 mg/kg for the rat, and a BMD5 of 15.2 mg/kg and a BMDL5 of 3.1 mg/kg are determined for the mouse (polynomial model). In the rat, the lag in weight gain during pregnancy was particularly pronounced in the two highest dosage groups (5 and 10 mg/kg), which exhibited marked reductions of food and water intake. The PFOS-induced reductions of maternal weight gain in the rat and mouse seen here are comparable to similar alterations produced by the fluorochemical in the rabbit or by N-alkyl perfluorooctanesulfonamido ethyl alcohol in the rat and rabbit (Case et al., 2001), indicating that the adverse effect on maternal weight gain may be a common feature of toxicity for the perfluorochemicals. Liver enlargement with associated histological abnormalities is another feature often seen after exposure to PFOS and related compounds (Case et al., 2001
; Haughom and Spydevold, 1992
; Ikeda et al., 1987
; Seacat et al., 2002
; 2003
; Sohlenius et al., 1993
). A somewhat similar finding was obtained in our studies. An increase of liver weight is generally observed in rodents during pregnancy (by about 24% in rat, Buelke-Sam et al., 1982
; and 56% in mouse, observation in our laboratory, data not shown). Above and beyond this physiological change, significant elevations of hepatic weight were found in the PFOS-exposed mice, and increases in the high-dose groups were as much as twofold over the corresponding controls. Serum triglycerides in these mice were significantly reduced. Interestingly, a comparable PFOS-induced liver enlargement was absent in the pregnant rat; the small increase in the relative liver weight in the 10 mg/kg dosage group largely reflected the reduction of body weight, rather than a net increase of liver weight. Serum cholesterol and triglycerides in the rat were also not altered appreciably by PFOS exposure. An explanation for these disparate findings (compared to the results reported by Seacat et al., 2003
, for instance) is not readily available and may be attributed to the relatively short duration of PFOS exposure (20 days) in our study (compared to 14 weeks in the Seacat study). Nonetheless, the high sensitivity to PFOS-induced liver toxicity in the mouse should be noted, with a BMD5 and a BMDL5 of liver weight increase estimated at 2.61 mg/kg and 1.31 mg/kg (Hill model), respectively.
Seacat and co-workers (2002) reported reductions of serum T3 and elevations of TSH in monkeys after exposure to PFOS for 182 days. In this study, PFOS produced a much more marked reduction of both T3 and T4 in pregnant rats and at a much earlier onset. For the T4 effects at GD 7, a BMD5 at 0.23 mg/kg and a BMDL5 at 0.05 mg/kg (Hill model) are estimated. Interestingly, the accumulated serum PFOS level where thyroid imbalance was detected in the monkey (171 ppm) is comparable to that in the pregnant rat (53155 ppm). Yet in rats, despite these deficits in circulating hormones, a feedback elevation of TSH through activation of the hypothalamic-pituitary-thyroid (HPT) axis was not apparent. Because the level of serum T4 (and to a lesser extent, T3) falls and that of TSH rises during pregnancy (Versloot et al., 1994
; Figure 4
in this study), these physiological changes might have masked the true effects of PFOS. However, the T3 and T4 results from the study with nonpregnant female rats by and large substantiated the findings in pregnant dams, discounting potential confounding effects of pregnancy. The dose-dependent, paradoxical responses of serum TSH in the nonpregnant rats are intriguing. The near-identical response patterns between total and free T4 rule out the potential involvement of the hormone binding proteins. Although feedback increases of the pituitary hormone in the 3 mg/kg dosage group were relatively small (2750%), compared with the two- to threefold increase induced by propylthiouracil (Cooper et al., 1983
), these changes nonetheless indicate the integrity of the HPT axis. The recovery of TSH after a transient response, despite the persistent reductions of serum T3 and T4, suggests that the homeostatic balance of thyroid hormone economy may have been reset. These findings resemble those previously reported with long-term chemical disruption of the thyroid axis (Biegel et al., 1995
). The absence of serum TSH elevation (through the HPT feedback mechanism) in the 5 mg/kg dosage group is perplexing. In fact, the TSH levels of these rats were slightly depressed in the initial stage of PFOS exposure. These results point toward a more complex, dose-dependent effect of the fluorochemical that is not yet fully understood.
Pregnant mice exhibited a pattern of PFOS-induced T4 reductions similar to that seen in the pregnant rat, with a BMD5 of 0.51 mg/kg and a BMDL5 of 0.35 mg/kg estimated at GD 6 (linear model). This profile of thyroid hormone imbalance (reductions of T3 and T4 without a compensatory elevation of TSH) produced by PFOS, though puzzling, is not unique. Chemically induced decreases of serum T4 and T3 without significant feedback increase of TSH have been reported with polychlorinated biphenyls (PCBs) (Goldey et al., 1995; Liu et al., 1995
). The mechanism(s) underlying the thyroidal effects of PFOS remains to be elaborated. Similar patterns of alterations between total and free T4 rule out involvement of hormone binding proteins. Alternatively, the hepatotoxicity of PFOS presents a prime possibility. Altered thyroid hormone metabolism through induction of hepatic enzymes has been described with chemicals such as phenobarbital, pregnenolone-16
-carbonitrile, 3-methylcholanthrane, PCB, and brominated diphenyl ethers (Byrne et al., 1987
; Liu et al., 1995
; Zhou et al., 2002
). Regardless, the thyroid hormone deficits produced by PFOS during pregnancy are of potential concern, particularly if the feedback mechanism via the HPT axis is compromised. Thyroid hormones play a critical role in the normal development of the lung, inner ears, and nervous system (particularly CNS), and they regulate growth, metabolic rate, cardiac performance, and body temperature (Lucas et al., 1988
; Glinoer, 2001
). During in utero development, the embryo and fetus rely completely on maternal supplies of thyroid hormones through placental transfer until maturation of the fetal thyroid gland toward late gestation. Perinatal hypothyroidism has been shown to cause retardation of neurodevelopment and stunted growth (Porterfield, 1993
), and recent epidemiological findings indicate that even subtle changes of the thyroid economy (subclinical hypothyroidism) during maturation may have long-lasting effects on the development of intellectual and motor skills (Haddow et al., 1999
). Decreased availability of maternal T4 to the developing brain poses an increased risk of poor neuropsychological development, and a direct relationship between the degree of neonatal hypothyroxinemia and subsequent neurodevelopment has been established (Morreale de Escobar et al., 2000
).
In view of the profound deficits in maternal weight gain in PFOS-exposed rats, it was surprising to find little adverse effect on the viability of the fetuses at term. In fact, only small decrements of fetal weight were noted. Similar results were obtained with the mouse, even at higher exposures. On the other hand, anasarca, craniofacial malformation (cleft palate), cardiac defects (ventricular septal defects, enlargement of the right atrium), and delayed ossification (sternebrae, phalanges) were detected in the PFOS-exposed fetuses. A BMD5 for the sternal defects is estimated at 0.31 mg/kg, with a BMDL5 at 0.12 mg/kg (logistic model); and a BMD5 for cleft palate at 8.85 mg/kg, with a BMDL5 at 3.33 mg/kg (logistic model). The enlarged right atrium may be associated with complications of pulmonary function. The mouse essentially produced an identical teratological profile. For comparison, BMD5 and BMDL5 for the sternal defects in the mouse are estimated at 0.06 mg/kg and 0.02 mg/kg, respectively (logistic model); those for cleft palate are 7.03 mg/kg and 3.53 mg/kg, respectively (NCTR model). These results are in agreement with previous teratological findings with lithium perfluorooctane sulfonate and N-ethylperfluorooctanesulfonamido ethyl alcohol in the rat (Case et al., 2001; Henwood et al., 1994
) and PFOS in the rabbit (Case et al., 2001
). Nonetheless, it should be noted that a preponderance of these structural abnormalities was found in the highest PFOS dosage group (10 mg/kg for the rat and 20 mg/kg in the mouse). Although a significant reduction of weight gain and food consumption was noted in this group of pregnant rats, malnutrition is not likely the sole factor accounting for the induction of birth defects. Indeed, equivalent or higher incidence of malformations was seen in the mouse fetuses, yet the deficits of weight gain and food consumption in the mouse dams were much less extensive than those of the rat.
Previous studies have shown that PFOS can interfere with cholesterol synthesis through inhibition of HMG CoA reductase activity (Haughom and Spydevold, 1992). Because cholesterol is known to play a role in development through the molecular signaling of sonic hedgehog (Brewer et al., 1993
), alterations of this metabolic precursor may be involved in the mechanism of dysmorphogenesis (Fitzky et al., 2001
). Yet in this study, maternal serum cholesterol was not significantly lowered by PFOS treatment in either rodent species. Indeed, results from a preliminary study (Luebker et al., 2002b
) indicated that cholesterol or mevalonic acid supplement failed to ameliorate PFOS-induced developmental toxicity in the rat. Alternatively, altered thyroid status in the dam may raise concerns regarding developmental toxicity. Thyroid hormone effects on cell proliferation and differentiation, as well as on organ growth and maturation, have been well documented. On the other hand, changes of these parameters are often subtle (for instance, at a functional rather than morphological level) and not easily discernible by standard teratological assessment. Hence, evaluations for potential developmental toxicity of PFOS have been extended to postnatal examination, and the results are described in a companion article (Lau et al., 2003
).
In summary, exposure to PFOS during pregnancy led to significant physiological alterations in the rat and mouse that are indicative of maternal toxicity, as well as to anatomical defects observed in the fetuses at term at high dosages. These adverse outcomes are dose-dependent and can be correlated with body burden of the fluorochemical. Generally, the mouse appeared to be a less sensitive species than the rat in regard to the PFOS-induced toxicity. A species comparison of the benchmark doses for various parameters is provided in Table 3.
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ACKNOWLEDGMENTS |
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NOTES |
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1 To whom correspondence should be addressed at: Mail Drop 67, U.S. Environmental Protection Agency, Research Triangle Park, NC 27711. Fax: (919) 541-4017. E-mail: lau.christopher{at}epa.gov.
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