How the Xenopus eleutheroembryonic thyroid assay compares to the amphibian metamorphosis assay for detecting thyroid active chemicals.

The Xenopus Eleutheroembryonic Thyroid Assay (XETA) was recently published as an OECD Test Guideline for detecting chemicals acting on the thyroid axis. However, the OECD validation did not cover all mechanisms that can potentially be detected by the XETA. This study was therefore initiated to investigate and consolidate the applicability domain of the XETA regarding the following mechanisms: thyroid hormone receptor (THR) agonism, sodium-iodide symporter (NIS) inhibition, thyroperoxidase (TPO) inhibition, deiodinase (DIO) inhibition, glucocorticoid receptor (GR) agonism, and uridine 5'-diphospho-glucuronosyltransferase (UDPGT) induction. In total, 22 chemicals identified as thyroid-active or -inactive in Amphibian Metamorphosis Assays (AMAs) were tested using the XETA OECD Test Guideline. The comparison showed that both assays are highly concordant in identifying chemicals with mechanisms of action related to THR agonism, DIO inhibition, and GR agonism. They also consistently identified the UDPGT inducers as thyroid inactive. NIS inhibition, investigated using sodium perchlorate, was not detected in the XETA. TPO inhibition requires further mechanistic investigations as the reference chemicals tested resulted in opposing response directions in the XETA and AMA. This study contributes refining the applicability domain of the XETA, thereby helping to clarify the conditions where it can be used as an ethical alternative to the AMA.

Pronamide: Weight of evidence for potential estrogen, androgen or thyroid effects.

Based on the exposure potential to humans and environment, pronamide was one of 52 chemicals on the first list evaluated under US EPA's Endocrine Disruptor Screening Program (EDSP). The purpose of EDSP is to screen chemicals for their potential to interact with estrogen-, androgen-, or thyroid-signaling pathways. A battery of 11 Tier 1 assays was completed for pronamide in accordance with EDSP test guidelines. In addition, Other Scientifically Relevant Information, which included existing data from regulatory guideline studies and published literature, was used in a weight-of-evidence (WoE) evaluation of potential endocrine activity. The WoE conclusion is that pronamide does not interact directly with estrogen, androgen, or thyroid receptors or post-receptor events. Across in vivo studies, the liver is consistently and reproducibly the target organ for pronamide's effects. Pronamide activates hepatocytic nuclear receptors (including constitutive androstane receptor), induces hepatic enzymes, produces hepatocellular hypertrophy and increases liver weights. These changes are coupled with increased metabolic activity and a subsequent increased metabolism and/or clearance of both steroid and thyroid hormones. Thus, while pronamide alters some endocrine-sensitive endpoints in EDSP Tier 1 assays, effects on liver metabolism likely explain altered hormone levels and indirect endocrine changes.

Current testing programs for pesticides adequately capture endocrine activity and adversity for protection of vertebrate wildlife.

The toxicity and ecotoxicity of pesticide active ingredients are evaluated by a number of standardized test methods using vertebrate animals. These standard test methods are required under various regulatory programs for the registration of pesticides. Over the past two decades, additional test methods have been developed with endpoints that are responsive to endocrine activity and subsequent adverse effects. This article examines the available test methods and their endpoints that are relevant to an assessment of endocrine-disrupting properties of pesticides. Furthermore, the article highlights how weight-of-evidence approaches should be applied to determine whether an adverse response in (eco)toxicity tests is caused by an endocrine mechanism of action. The large number of endpoints in the current testing paradigms for pesticides make it unlikely that endocrine activity and adversity is being overlooked. Integr Environ Assess Manag 2023;19:1089-1109. © 2023 Bayer CropScience and The Authors. Integrated Environmental Assessment and Management published by Wiley Periodicals LLC on behalf of Society of Environmental Toxicology & Chemistry (SETAC).

The Extended Amphibian Metamorphosis Assay: A Thyroid-Specific and Less Animal-Intensive Alternative to the Larval Amphibian Growth and Development Assay.

The amphibian metamorphosis assay (AMA; US Environmental Protection Agency [USEPA] test guideline 890.1100 and Organisation for Economic Co-Operation and Development test guideline 231) has been used for more than a decade to assess the potential thyroid-mediated endocrine activity of chemicals. In 2013, in the context of the Endocrine Disruptor Screening Program of the USEPA, a Scientific Advisory Panel reviewed the results from 18 studies and recommended changes to the AMA test guideline, including a modification to a fixed-stage design rather than a fixed-time (i.e., 21-d) design. We describe an extended test design for the AMA (or EAMA) that includes thyroid histopathology and time to metamorphosis (Nieuwkoop-Faber [NF] stage 62), to address both the issues with the fixed-time design and the specific question of thyroid-mediated adversity in a shorter assay than the larval amphibian growth and development assay (LAGDA; Organisation for Economic Co-Operation and Development test guideline 241), using fewer animals and resources. A demonstration study was conducted with the EAMA (up to NF stage 58) using sodium perchlorate. Data analyses and interpretation of the fixed-stage design of the EAMA are more straightforward than the fixed-time design because the fixed-stage design avoids confounded morphometric measurements and thyroid histopathology caused by varying developmental stages at test termination. It also results in greater statistical power to detect metamorphic delays than the fixed-time design. By preferentially extending the AMA to NF stage 62, suitable data can be produced to evaluate thyroid-mediated adversity and preclude the need to perform a LAGDA for thyroid mode of action analysis. The LAGDA remains of further interest should investigations of longer term effects related to sexual development modulated though the hypothalamus-pituitary-gonadal axis be necessary. However, reproduction assessment or life cycle testing is currently not addressed in the LAGDA study design. This is better addressed by higher tier studies in fish, which should then include specific thyroid-related endpoints. Environ Toxicol Chem 2021;40:2135-2144. © 2021 The Authors. Environmental Toxicology and Chemistry published by Wiley Periodicals LLC on behalf of SETAC.

Assessment of Transcriptomic and Apical Responses of Daphnia magna Exposed to a Polyethylene Microplastic in a 21-d Chronic Study.

There is global concern regarding the fate and effects of microplastics in the environment, particularly in aquatic systems. In the present study, ethylene acrylic acid copolymer particles were evaluated in a chronic toxicity study with the aquatic invertebrate Daphnia magna. The study design included a natural particle control treatment (silica) to differentiate any potential physical effects of a particle from the intrinsic toxicity of the test material. In addition to the standard endpoints of survival, growth, and reproduction, the transcriptomic profiles of control and ethylene acrylic acid copolymer-exposed D. magna were evaluated at the termination of the 21-d toxicity study. No significant effects on D. magna growth, survival, or reproduction were observed in comparison with both particle and untreated control groups. Significant transcriptomic alterations were induced at the highest treatment level of 2.3 × 1012 particles of the ethylene acrylic acid copolymer/L in key pathways linked to central metabolism and energy reserves, oxidative stress, and ovulation and molting, indicating a global transcriptomic response pattern. To put the results in perspective is challenging at this time, because, to date, microplastic environmental monitoring approaches have not been equipped to detect particles in the nanosize range. However, our results indicate that ethylene acrylic acid copolymer microplastics in the upper nanosize range are not expected to adversely affect D. magna growth, survival, or reproductive outcomes at concentrations of up to 1012 particles/L. Environ Toxicol Chem 2020;39:1578-1589. © 2020 SETAC.

Critical Review of Read-Across Potential in Testing for Endocrine-Related Effects in Vertebrate Ecological Receptors.

Recent regulatory testing programs have been designed to evaluate whether a chemical has the potential to interact with the endocrine system and could cause adverse effects. Some endocrine pathways are highly conserved among vertebrates, providing a potential to extrapolate data generated for one vertebrate taxonomic group to others (i.e., biological read-across). To assess the potential for biological read-across, we reviewed tools and approaches that support species extrapolation for fish, amphibians, birds, and reptiles. For each of the estrogen, androgen, thyroid, and steroidogenesis (EATS) pathways, we considered the pathway conservation across species and the responses of endocrine-sensitive endpoints. The available data show a high degree of confidence in the conservation of the hypothalamus-pituitary-gonadal axis between fish and mammals and the hypothalamus-pituitary-thyroid axis between amphibians and mammals. Comparatively, there is less empirical evidence for the conservation of other EATS pathways between other taxonomic groups, but this may be due to limited data. Although more information on sensitive pathways and endpoints would be useful, current developments in the use of molecular target sequencing similarity tools and thoughtful application of the adverse outcome pathway concept show promise for further advancement of read-across approaches for testing EATS pathways in vertebrate ecological receptors. Environ Toxicol Chem 2020;39:739-753. © 2020 The Authors. Environmental Toxicology and Chemistry published by Wiley Periodicals, Inc. on behalf of SETAC.

A critical review of the environmental occurrence and potential effects in aquatic vertebrates of the potent androgen receptor agonist 17ß-trenbolone.

Trenbolone acetate is widely used in some parts of the world for its desirable anabolic effects on livestock. Several metabolites of the acetate, including 17ß-trenbolone, have been detected at low nanograms per liter concentrations in surface waters associated with animal feedlots. The 17ß-trenbolone isomer can affect androgen receptor signaling pathways in various vertebrate species at comparatively low concentrations/doses. The present article provides a comprehensive review and synthesis of the existing literature concerning exposure to and biological effects of 17ß-trenbolone, with an emphasis on potential risks to aquatic animals. In vitro studies indicate that, although 17ß-trenbolone can activate several nuclear hormone receptors, its highest affinity is for the androgen receptor in all vertebrate taxa examined, including fish. Exposure of fish to nanograms per liter water concentrations of 17ß-trenbolone can cause changes in endocrine function in the short term, and adverse apical effects in longer exposures during development and reproduction. Impacts on endocrine function typically are indicative of inappropriate androgen receptor signaling, such as changes in sex steroid metabolism, impacts on gonadal stage, and masculinization of females. Exposure of fish to 17ß-trenbolone during sexual differentiation in early development can greatly skew sex ratios, whereas adult exposures can adversely impact fertility and fecundity. To fully assess ecosystem-level risks, additional research is warranted to address uncertainties as to the degree/breadth of environmental exposures and potential population-level effects of 17ß-trenbolone in sensitive species. Environ Toxicol Chem 2018;37:2064-2078. Published 2018 Wiley Periodicals Inc. on behalf of SETAC. This article is a US government work and, as such, is in the public domain in the United States of America.

Effects of perfluorooctane sulfonate on mallard and northern bobwhite quail exposed chronically via the diet.

Adult mallard ducks and northern bobwhite quail were exposed to 0, 10, 50, or 150mg perfluorooctane sulfonate (PFOS)/kg in the diet for up to 21 weeks. Adult health, body and liver weight, feed consumption, gross morphology and histology of body organs, and reproduction were examined. Due to mortality, birds exposed to 50 or 150mg PFOS/kg feed were terminated by Week 7. In quail, the lowest observable adverse effect level (LOAEL) was 10mg PFOS/kg feed based on decreased survivorship of 14-day-old quail offspring. For adult female quail fed 10mg/kg feed, there was a slight but statistically significantly PFOS-related increase in liver weight when compared to controls. When liver weight was normalized to body weight, the statistically significant differences were still observed indicating that PFOS affected liver size. However, no other pathological effects were observed livers of quail from this treatment group which suggests that this enlargement may have been an adaptive response. For adult mallards, no treatment-related effects on feed consumption, body or liver weight, growth, or reproductive performance were observed. There was a slightly greater incidence of small testes (length) in adult male mallards and quail exposed to 10mg PFOS/kg, feed when compared to controls. However, spermatogenesis was not affected and there was no effect on the rates of egg fertilization. Due to transfer to eggs, concentrations of PFOS measured in the liver and blood at study termination were greater in male birds than female birds.

Current limitations and recommendations to improve testing for the environmental assessment of endocrine active substances.

In the present study, existing regulatory frameworks and test systems for assessing potential endocrine active chemicals are described, and associated challenges are discussed, along with proposed approaches to address these challenges. Regulatory frameworks vary somewhat across geographies, but all basically evaluate whether a chemical possesses endocrine activity and whether this activity can result in adverse outcomes either to humans or to the environment. Current test systems include in silico, in vitro, and in vivo techniques focused on detecting potential endocrine activity, and in vivo tests that collect apical data to detect possible adverse effects. These test systems are currently designed to robustly assess endocrine activity and/or adverse effects in the estrogen, androgen, and thyroid hormone signaling pathways; however, there are some limitations of current test systems for evaluating endocrine hazard and risk. These limitations include a lack of certainty regarding: 1) adequately sensitive species and life stages; 2) mechanistic endpoints that are diagnostic for endocrine pathways of concern; and 3) the linkage between mechanistic responses and apical, adverse outcomes. Furthermore, some existing test methods are resource intensive with regard to time, cost, and use of animals. However, based on recent experiences, there are opportunities to improve approaches to and guidance for existing test methods and to reduce uncertainty. For example, in vitro high-throughput screening could be used to prioritize chemicals for testing and provide insights as to the most appropriate assays for characterizing hazard and risk. Other recommendations include adding endpoints for elucidating connections between mechanistic effects and adverse outcomes, identifying potentially sensitive taxa for which test methods currently do not exist, and addressing key endocrine pathways of possible concern in addition to those associated with estrogen, androgen, and thyroid signaling. Integr Environ Assess Manag 2017;13:302-316. © 2016 The Authors. Integrated Environmental Assessment and Management published by Wiley Periodicals, Inc. on behalf of Society of Environmental Toxicology & Chemistry (SETAC).

Recommended approaches to the scientific evaluation of ecotoxicological hazards and risks of endocrine-active substances.

A SETAC Pellston Workshop® "Environmental Hazard and Risk Assessment Approaches for Endocrine-Active Substances (EHRA)" was held in February 2016 in Pensacola, Florida, USA. The primary objective of the workshop was to provide advice, based on current scientific understanding, to regulators and policy makers; the aim being to make considered, informed decisions on whether to select an ecotoxicological hazard- or a risk-based approach for regulating a given endocrine-disrupting substance (EDS) under review. The workshop additionally considered recent developments in the identification of EDS. Case studies were undertaken on 6 endocrine-active substances (EAS-not necessarily proven EDS, but substances known to interact directly with the endocrine system) that are representative of a range of perturbations of the endocrine system and considered to be data rich in relevant information at multiple biological levels of organization for 1 or more ecologically relevant taxa. The substances selected were 17α-ethinylestradiol, perchlorate, propiconazole, 17ß-trenbolone, tributyltin, and vinclozolin. The 6 case studies were not comprehensive safety evaluations but provided foundations for clarifying key issues and procedures that should be considered when assessing the ecotoxicological hazards and risks of EAS and EDS. The workshop also highlighted areas of scientific uncertainty, and made specific recommendations for research and methods-development to resolve some of the identified issues. The present paper provides broad guidance for scientists in regulatory authorities, industry, and academia on issues likely to arise during the ecotoxicological hazard and risk assessment of EAS and EDS. The primary conclusion of this paper, and of the SETAC Pellston Workshop on which it is based, is that if data on environmental exposure, effects on sensitive species and life-stages, delayed effects, and effects at low concentrations are robust, initiating environmental risk assessment of EDS is scientifically sound and sufficiently reliable and protective of the environment. In the absence of such data, assessment on the basis of hazard is scientifically justified until such time as relevant new information is available. Integr Environ Assess Manag 2017;13:267-279. © 2017 The Authors. Integrated Environmental Assessment and Management published by Wiley Periodicals, Inc. on behalf of Society of Environmental Toxicology & Chemistry (SETAC).

Terminology of gonadal anomalies in fish and amphibians resulting from chemical exposures.

Given the recent increase in the number of studies describing the ability of chemicals to exert endocrine-disrupting effects, not only in fish but in a variety of other oviparous groups such as amphibians and reptiles, there is an urgent need to harmonize the terminology currently used in describing pathological changes of the gonads. In addition to difficulties in comparing results from different studies, there is also the risk of miscommunication by using terms that imply a certain clinical relevance which may not be true for the species examined. Especially in the case of the recent and controversial issue about potential effects of the triazine herbicide atrazine on amphibians, clinical terminology has been utilized beyond its true meaning by using terms such as "chemical castration" to describe occurrence of TOs or ovarian tissue in the testis of male frogs exposed to environmental chemicals (Hayes 2004). In clinical terminology, castration is defined as the removal of the gonads or their destruction by an external influence, resulting in a nonfertile organism. However, Hayes (2004) did not investigate any possible effects on the fertility of the test animals and thus did not know if these animals were truly castrated. Similarly, terms such as intersex, hermaphrodite, and sex reversal have been used in ways that appear inappropriate with regard to their clinical meaning in a series of different studies with fish or frogs (see previous sections for a detailed discussion). To ensure the appropriate use of certain terminology in a field as controversial and complex as the study of endocrine disruption, we have attempted, in this chapter, to harmonize the terminology used to describe changes in gonadal development of vertebrates such as fish and amphibians, especially frogs (see Table 3). Where appropriate, the terminology suggested was adopted directly from the clinical terminology. However, as outlined here there are substantial differences between the developmental biology of oviparous vertebrates and mammals, and especially humans, that necessitate modification of the definitions of some of the clinical terms. Where appropriate, therefore, the terminology proposed in this manuscript was redefined based on the biological meanings of the terms used in clinical diagnosis. Considering the large increase in research in the area of reproductive endocrine disruption over the past decades, the authors see an increasing need for a harmonization of terms to be used to describe effects observed in the investigated species. Agreement on a common terminology will allow scientists to better communicate and compare their work, and will enable risk assessors to conduct large-scale evaluations of environmental endocrine disruption by fitting the information from individual studies into a synthesis of normal and abnormal conditions of gonadal tissues.

Plasma concentrations of estradiol and testosterone, gonadal aromatase activity and ultrastructure of the testis in Xenopus laevis exposed to estradiol or atrazine.

The ultrastructure of testicular cells of adult male African clawed frogs (Xenopus laevis) exposed to either estradiol (0.1 microg/L) or 2-chloro-4-ethylamino-6-isopropyl-amino-s-triazine (atrazine; 10 or 100 microg/L) was examined by electron microscopy and compared to plasma concentrations of the steroid hormones, testosterone (T) and estradiol (E2), testicular aromatase activity and gonad growth expressed as the gonado-somatic index (GSI). Exposure to E2 caused significant changes both at the sub-cellular and biochemical levels. Exposure to E2 resulted in significantly fewer sperm cells, inhibition of meiotic division of germ cells, more lipid droplets that are storage compartments for the sex steroid hormone precursor cholesterol, and lesser plasma T concentrations. Although not statistically significant, frogs exposed to E2 had slightly smaller GSI values. These results may be indicative of an inhibition of gonad growth and disrupted germ cell development by E2. Concentrations of E2 in plasma were greater in frogs exposed to E2 in water. Exposure to neither concentration of atrazine caused effects on germ cell development, testicular aromatase activity or plasma hormone concentrations. These results suggest that atrazine does not affect testicular function. In contrast, exposure of male X. laevis to E2 led to sub-cellular events that are indicative of disruption of testicular development, and demasculinization processes (decrease of androgen hormone titers). These results indicate that atrazine does not cause responses that are similar to those caused by exposure to E2.

Evaluation of potential endocrine activity of 2,4-dichlorophenoxyacetic acid using in vitro assays.

The herbicide 2,4-dichlorophenoxyacetic acid (2,4-D) was evaluated in five in vitro screening assays to assess the potential for interaction with the androgen, estrogen and steroidogenesis pathways in the endocrine system. The assays were conducted to meet the requirements of the in vitro component of Tier 1 of the United States Environmental Protection Agency's Endocrine Disruptor Screening Program (EDSP), and included assays for estrogen receptor (ER) binding (rat uterine cytosol ER binding assay), ER-mediated transcriptional activation (HeLa-9903-ERα transactivation assay), androgen receptor (AR) binding (rat prostate cytosol AR binding assay), aromatase enzymatic activity inhibition (recombinant human CYP19 aromatase inhibition assay), and interference with steroidogenesis (H295R steroidogenesis assay). Results from these five assays demonstrated that 2,4-D does not have the potential to interact in vitro with the estrogen, androgen, or steroidogenesis pathways. These in vitro data are consistent with a corresponding lack of endocrine effects observed in apical in vivo animal studies, and thus provide important supporting data valuable in a comprehensive weight of evidence evaluation indicating a low potential of 2,4-D to interact with the endocrine system.

Lessons learned, challenges, and opportunities: the U.S. Endocrine Disruptor Screening Program.

In 1996, the U.S. Congress passed the Food Quality Protection Act and amended the Safe Drinking Water Act (SDWA) requiring the U.S. Environmental Protection Agency (EPA) to implement a screening program to investigate the potential of pesticide chemicals and drinking water contaminants to adversely affect endocrine pathways. Consequently, the EPA launched the Endocrine Disruptor Screening Program (EDSP) to develop and validate estrogen, androgen, and thyroid (EAT) pathway screening assays and to produce standardized and harmonized test guidelines for regulatory application. In 2009, the EPA issued the first set of test orders for EDSP screening and a total of 50 pesticide actives and 2 inert ingredients have been evaluated using the battery of EDSP Tier 1 screening assays (i.e., five in vitro assays and six in vivo assays). To provide a framework for retrospective analysis of the data generated and to collect the insight of multiple stakeholders involved in the testing, more than 240 scientists from government, industry, academia, and non-profit organizations recently participated in a workshop titled "Lessons Learned, Challenges, and Opportunities: The U.S. Endocrine Disruptor Screening Program." The workshop focused on the science and experience to date and was organized into three focal sessions: (a) Performance of the EDSP Tier 1 Screening Assays for Estrogen, Androgen, and Thyroid Pathways; (b) Practical Applications of Tier 1 Data; and (c) Indications and Opportunities for Future Endocrine Testing. A number of key learnings and recommendations related to future EDSP evaluations emanated from the collective sessions.

Differential accumulation of polychlorinated biphenyl congeners in the terrestrial food web of the Kalamazoo River Superfund site, Michigan.

A series of field studies was conducted to determine the bioaccumulation of polychlorinated biphenyl (PCB) congeners in the terrestrial food web of the Kalamazoo River flood plain. Samples included colocated soils, native plants likely to be consumed by wildlife, several taxa of terrestrial invertebrates, small mammals, passerine bird eggs, nestlings, and adults, and great horned owl plasma and eggs. Mean concentrations of total PCBs in samples from the former Trowbridge impoundment were 6.5 mg/kg dry weight for soils and 0.023, 0.13, 1.3, 1.3, 1.6, and 8.2 mg/kg wet weight for plants, small herbivorous mammals, depurated earthworms, shrews, great horned owl eggs, and house wren eggs, respectively. Historical data from the Kalamazoo River have reported Aroclor-equivalent total PCB concentrations in the terrestrial food web; however, the degree of environmental weathering of the parent PCB mixtures was unknown. In this study, earthworms and composite samples of coleoptera exhibited PCB congener patterns that were similar to patterns in colocated soils. However, in plants, less chlorinated PCBs (e.g., mono-, di-, tri-, and tetrachlorinated biphenyls) were predominant, and in small mammals, there was a notable enrichment of PCBs 153, 180, 138, 118, and 99. In general, concentrations of PCBs were lower in most biota than in soil from the Kalamazoo River Area of Concern (KRAOC) although there was a modest biomagnification of PCBs from lower trophic level biota to highertrophic levels. As a consequence of environmental weathering of PCBs in the terrestrial food web of the KRAOC, the relative potency of the PCBs (expressed as mg TEQs/kg PCBs) decreased from soil to most biota. While there was a general trend, as expected, in which concentrations of 2,3,7,8-tetrachlorodibenzo-p-dioxin equivalents (TEQs) increased with total PCBs, this relationship was rather poor (R2 = 0.13). Taken together, these data suggest that the differential accumulation of PCB congeners in the terrestrial food web can be explained by congener-specific differences in bioavailability from soil, exposure pathways, and metabolic potential of each of the food web components.

Differential accumulation of polychlorinated biphenyl congeners in the aquatic food web at the Kalamazoo River Superfund site, Michigan.

A series of field studies were conducted to gain a better understanding of the bioaccumulation and dynamics of polychlorinated biphenyl (PCB) congeners in the aquatic food web of the Kalamazoo River flood plain. Representative species of passerine birds, mammals, fish, aquatic plants, invertebrates, and colocated sediments were collected from areas located within submerged portions of the former Trowbridge impoundment and also from areas located at an upstream reference location. In most matrixes, total concentrations of PCBs were significantly greater in the downstream study area compared to the upstream reference location. Patterns of PCB congeners varied among trophic levels due to selective bioaccumulation of more chlorinated congeners in upper trophic level organisms. There were no statistically significant differences in total PCB concentrations among sampling grids within either site or temporally among three sampling seasons between May and September. The greatest total PCB concentrations were detected in adult tree swallows (mean = 8.7 mg/kg wet weight (ww)) and fish (mean = 4.4 mg/ kg ww) collected from the former Trowbridge impoundment. Concentrations of total 2,3,7,8-tetrachlorodibenzo-p-dioxin equivalents (TEQs) were greatest in egg, nestling, and adult tree swallows collected from the former Trowbridge impoundment. There was not a significant correlation between concentrations of total PCBs and TEQs at either site in the mammalian or avian food webs. The relative potency of the mixture of PCBs, expressed as the ratio of TEQs to total PCBs, increased with trophic position in the avian and mammalian aquatic food webs located within the former Trowbridge impoundment.

Effects of atrazine on metamorphosis, growth, laryngeal and gonadal development, aromatase activity, and sex steroid concentrations in Xenopus laevis.

African clawed frogs (Xenopus laevis) were exposed to one of eight nominal waterborne concentrations including 0, 0.1, 1.0, 10, or 25 microg/L atrazine, 0.005% ethanol (EtOH), or 0.1mg/L estradiol (E2) or dihydrotestosterone (DHT) containing 0.005% EtOH. Frogs were exposed from 72 h posthatch until 2--3 months postmetamorphosis via a 3-day static renewal exposure regimen. Atrazine at concentrations between 0.1 and 25 microg/L did not significantly affect mortality, growth, gonad development, laryngeal muscle size, or aromatase activity in juvenile X. laevis. Male frogs exposed to 1.0 microg/L atrazine had lower E2 levels compared to controls, but this response was not consistent among other concentrations of atrazine. Male and female frogs exposed to DHT had larger laryngeal dilator muscle areas compared to controls. E2-exposed female frogs had decreased gonadal aromatase activity, and E2-exposed male frogs had statistically greater plasma concentrations of E2 compared to controls.