Site Of Toxic Action Research Articles

Phytotoxicity effect concentrations (ECx) for Ce, Nd and Eu added to soil relative to total and bioaccessible soil REE concentrations, and tissue REE accumulations

Toxicity thresholds (ECx) for radish, tomato, and durum wheat growth endpoints (shoot length, shoot mass, root length) to Ce, Nd or Eu added to a black organic soil were determined from 14-day dose-response growth assays. EC10 expressed as total soil [REE] had a more than twenty-fold range, from 337 mg/kg to more than >8000 mg/kg. Averaged over all REEs and endpoints, durum wheat was more tolerant than radish and tomato; and averaged over all endpoints, Eu appeared to be the most phytotoxic of the three REEs. Bioaccessibility of each REE was determined by extraction with 0.01 M CaCl2, which for all three REEs in this soil was quite low, <0.10% of total. However, bioaccessibility of Eu was five or six times greater than that for Ce and Nd, and thus could explain its apparently greater toxicity, namely that Eu was more likely to be accumulated at the site of toxic action in the plant. To discern inherent toxicity from enhanced bioaccumulation, concentration of each REE in root and shoot tissues was determined, for a tissue-residue approach to toxicity assessment. The EC10 expressed as tissue concentration was lower for Nd than for Ce and Eu, thus the most toxic of the three REEs. As for many cationic inorganic elements, toxicity varies with the chemistry of the exposure medium due to its effects on bioaccessibility. Simple methods to harmonize toxicity thresholds from different media enables greater integration into regulatory standards. When EC25 from this and other studies were normalized to CaCl2-extractable REE in their respective media, the range in Ce EC25 was reduced from 20-fold to 2.5-fold, and the range for Eu EC25 was reduced from 25-fold to 3-fold. This novel and low-input approach to meta-analysis of toxicity thresholds demonstrates the value of considering soil physico-chemical properties as modifiers of soil REE toxicity.

Mercury tissue residue approach in Chironomus riparius: Involvement of toxicokinetics and comparison of subcellular fractionation methods

Along with the growing body of evidence that total internal concentration is not a good indicator of toxicity, the Critical Body Residue (CBR) approach recently evolved into the Tissue Residue Approach (TRA) which considers the biologically active portion of metal that is available to contribute to the toxicity at sites of toxic action. For that purpose, we examined total mercury (Hg) bioaccumulation and subcellular fractionation kinetics in fourth stage larvae of the midge Chironomus riparius during a four-day laboratory exposure to Hg-spiked sediments and water. The debris (including exoskeleton, gut contents and cellular debris), granule and organelle fractions accounted only for about 10% of the Hg taken up, whereas Hg concentrations in the entire cytosolic fraction rapidly increased to approach steady-state. Within this fraction, Hg compartmentalization to metallothionein-like proteins (MTLP) and heat-sensitive proteins (HSP), consisting mostly of enzymes, was assessed in a comparative manner by two methodologies based on heat-treatment and centrifugation (HT&C method) or size exclusion chromatography separation (SECS method). The low Hg recoveries obtained with the HT&C method prevented accurate analysis of the cytosolic Hg fractionation by this approach. According to the SECS methodology, the Hg-bound MTLP fraction increased linearly over the exposure duration and sequestered a third of the Hg flux entering the cytosol. In contrast, the HSP fraction progressively saturated leading to Hg excretion and physiological impairments. This work highlights several methodological and biological aspects to improve our understanding of Hg toxicological bioavailability in aquatic invertebrates.

Use of passive samplers for improving oil toxicity and spill effects assessment

Methods that quantify dissolved hydrocarbons are needed to link oil exposures to toxicity. Solid phase microextraction (SPME) fibers can serve this purpose. If fibers are equilibrated with oiled water, dissolved hydrocarbons partition to and are concentrated on the fiber. The absorbed concentration (Cpolymer) can be quantified by thermal desorption using GC/FID. Further, given that the site of toxic action is hypothesized as biota lipid and partitioning of hydrocarbons to lipid and fibers is well correlated, Cpolymer is hypothesized to be a surrogate for toxicity prediction. To test this method, toxicity data for physically and chemically dispersed oils were generated for shrimp, Americamysis bahia, and compared to test exposures characterized by Cpolymer. Results indicated that Cpolymer reliably predicted toxicity across oils and dispersions. To illustrate field application, SPME results are reported for oil spills at the Ohmsett facility. SPME fibers provide a practical tool to improve characterization of oil exposures and predict effects in future lab and field studies.

Advancing environmental toxicology through chemical dosimetry: External exposures versus tissue residues

The tissue residue dose concept has been used, although in a limited manner, in environmental toxicology for more than 100 y. This review outlines the history of this approach and the technical background for organic chemicals and metals. Although the toxicity of both can be explained in tissue residue terms, the relationship between external exposure concentration, body and/or tissues dose surrogates, and the effective internal dose at the sites of toxic action tends to be more complex for metals. Various issues and current limitations related to research and regulatory applications are also examined. It is clear that the tissue residue approach (TRA) should be an integral component in future efforts to enhance the generation, understanding, and utility of toxicity testing data, both in the laboratory and in the field. To accomplish these goals, several key areas need to be addressed: 1) development of a risk-based interpretive framework linking toxicology and ecology at multiple levels of biological organization and incorporating organism-based dose metrics; 2) a broadly applicable, generally accepted classification scheme for modes/mechanisms of toxic action with explicit consideration of residue information to improve both single chemical and mixture toxicity data interpretation and regulatory risk assessment; 3) toxicity testing protocols updated to ensure collection of adequate residue information, along with toxicokinetics and toxicodynamics information, based on explicitly defined toxicological models accompanied by toxicological model validation; 4) continued development of residue-effect databases is needed ensure their ongoing utility; and 5) regulatory guidance incorporating residue-based testing and interpretation approaches, essential in various jurisdictions.

Modelling Bioaccumulation and Toxicity of Metal Mixtures

ABSTRACT Bioaccumulation of metals in mixtures may demonstrate competitive, anticompetitive, or non-competitive inhibition, as well as various combinations of these and/or enhancement of metal uptake. These can be distinguished by plotting (metal in water)/(metal in tissue) against metal in water and comparison to equivalent plots for single-metal exposure. For the special case of pure competitive inhibition where only one site of uptake is involved, inhibition of metal accumulation in any metal mixture can be predicted from bioaccumulation of the metals when present singly. This is consistent with the commonly used Biotic Ligand Model (BLM) but does not explain bioaccumulation of metals in Hyalella azteca. Options for modelling toxicity of metal mixtures include concentration or response addition based on metal concentrations in either water or tissues. If the site of toxic action is on the surface of the organism, if this is the same as the site of metal interaction for bioaccumulation, if there is only one such type of site, and if metal bioaccumulation interactions are purely competitive (as in the BLM), then metal toxicity should be concentration additive and predictable from metal concentrations in either water or tissues. This is the simplest toxicity interaction to model but represents only one of many possibilities. The BLM should, therefore, be used with caution when attempting to model metal interactions, and other possibilities must also be considered.

Time Dependence in Mixture Toxicity with Soft Electrophiles: 1. Combined Effects of Selected SN2- and SNAr-Reactive Agents with a Nonpolar Narcotic

Frequently the toxicity of an organic chemical mixture is close to dose-additive, even when the agents are thought to induce toxicity at different molecular sites of action. These findings appear to conflict with the hypothesis that a strictly dose-additive combined effect will be observed for agents sharing a single molecular site of toxic action within the organism. In this study, several SN2-reactive (alpha-halogen) or S(N)Ar-reactive (halogenated dinitrobenzene) soft electrophiles were tested with a model nonpolar narcotic (NPN) to determine the toxicity of the combinations. A sham combination of the model NPN (3-methyl-2-butanone) was also tested as a positive control. The study design incorporated time-dependent toxicity (TDT) determinations at 15, 30, and 45 minutes using a Microtox (Vibrio fischeri) protocol that included testing seven duplicated concentrations for each single agent and mixture per combination. Additionally, in chemico reactivity was determined for each compound using thiol in glutathione as a model nucleophile. The model NPN alone lacked reactivity and TDT. The SN2-reactive agents individually showed varying levels of both reactivity and TDT alone, while the SNAr-reactive chemicals alone were reactive and had toxicity that was fully time-dependent between 15 and 45 minutes of exposure. Data analyses indicated that the sham combination was dose additive, as expected, whereas three of four SN2:NPN combinations showed effects close to that predicted for dose addition but with some differences. The fourth SN2:NPN combination, which included an alpha-halogen with full TDT, showed a less-than-dose-additive combined effect as did both of the SNAr:NPN pairings. By incorporating TDT values, shapes of the dose-response curves, chemical reactivity data with thiol, reactive mechanisms for the soft electrophiles, and quantitative structure activity relationship information on whether the toxicity of the individual soft electrophiles did or did not exceeded that predicted for baseline narcosis, the results suggested that the alpha-halogens elicited two toxic effects at the concentrations tested (reactivity and narcotizing effects), whereas toxicity induced by the halogenated dinitrobenzenes was essentially limited to reactive effects. Collectively, these results provide experimental evidence consistent with previous explanations as to why binary mixtures of industrial organic chemicals often show combined effects that are close to dose additive, even when the chemicals are thought to induce toxicity at different molecular sites of action.

Biotic Ligand Model, a Flexible Tool for Developing Site-Specific Water Quality Guidelines for Metals

The biotic ligand model (BLM) is a mechanistic approach that greatly improves our ability to generate site-specific ambient water quality criteria (AWQC)for metals in the natural environment relative to conventional relationships based only on hardness. The model is flexible; all aspects of water chemistry that affect toxicity can be included, so the BLM integrates the concept of bioavailability into AWQC--in essence the computational equivalent of water effect ratio (WER) testing. The theory of the BLM evolved from the gill surface interaction model (GSIM) and the free ion activity model (FIAM). Using an equilibrium geochemical modeling framework, the BLM incorporates the competition of the free metal ion with other naturally occurring cations (e.g., Ca2+, Na+, Mg2-, H+), togetherwith complexation by abiotic ligands [e.g., DOM (dissolved organic matter), chloride, carbonates, sulfide] for binding with the biotic ligand, the site of toxic action on the organism. On the basis of fish gill research, the biotic ligands appear to be active ion uptake pathways (e.g., Na+ transporters for copper and silver, Ca2+ transporters for zinc, cadmium, lead, and cobalt), whose geochemical characteristics (affinity = log K, capacity = Bmax) can be quantified in short-term (3-24 h) in vivo gill binding tests. In general, the greater the toxicity of a particular metal, the higher the log K. The BLM quantitatively relates short-term binding to acute toxicity, with the LA50 (lethal accumulation) being predictive of the LC50 (generally 96 h for fish, 48 h for daphnids). We critically evaluate currently available BLMs for copper, silver, zinc, and nickel and gill binding approaches for cadmium, lead, and cobalt on which BLMs could be based. Most BLMs originate from tests with fish and have been recalibrated for more sensitive daphnids by adjustment of LA50 so as to fit the results of toxicity testing. Issues of concern include the arbitrary nature of LA50 adjustments; possible mechanistic differences between daphnids and fish that may alter log K values, particularly for hardness cations (Ca2+, Mg2+); assumption of fixed biotic ligand characteristics in the face of evidence that they may change in response to acclimation and diet; difficulties in dealing with DOM and incorporating its heterogeneity into the modeling framework; and the paucity of validation exercises on natural water data sets. Important needs include characterization of biotic ligand properties at the molecular level; development of in vitro BLMs, extension of the BLM approach to a wider range of organisms, to the estuarine and marine environment, and to deal with metal mixtures; and further development of BLM frameworks to predict chronic toxicity and thereby generate chronic AWQC.

Application of the narcosis target lipid model to algal toxicity and deriving predicted‐no‐effect concentrations

The narcosis target lipid model (TLM) was developed to predict the toxicity of chemicals to aquatic organisms that act via narcosis. It is based on the hypothesis that target lipid is the site of toxic action within the organism, that octanol is the appropriate surrogate, and that target lipid has the same physical-chemical properties in all organisms. Here the TLM is extended to available freshwater green algal toxicity data to support a narcosis toxic mode-of-action (TMoA) effect assessment. For each species, significant linear relationships were observed between log(median effective concentration [EC50]) and log(Kow) of the test chemicals. The slope of the log-log relationship statistically was similar to the universal narcosis slope of -0.945 that was derived from an earlier analysis of the TLM to nonalgal species. Critical target lipid body burdens (CTLBB), C(L)* were estimated for each algal species from the intercepts of the regressions and found to be within the range (43-398 micromol/g octanol) reported previously, indicating that algae exhibit a similar sensitivity distribution relative to other aquatic species. The TLM is used to derive the predicted-no-effect concentrations (PNECs) using the hazardous concentration to 5% species (HC5) statistical extrapolation procedure. This calculation requires an analysis of the variability of the universal slope, the C(L)*, and the acute-to-chronic ratio. The PNECs derived using this procedure were consistent with chronic-no-effect concentrations reported for narcotic chemicals. This is in contrast to PNECs derived from limited chemical-specific toxicity data and default application factors. It is concluded that coupling the TLM to the HC5 extrapolation procedure allows optimal use of available toxicity data for deriving environmental quality criteria with a narcotic TMoA.

Gene expression analysis in response to lung toxicants: I. Sequencing and microarray development.

A key challenge in measuring gene expression changes in the lung in response to site-selective toxicants is differentiating between target and nontarget areas. The toxicity for the cytotoxicant 1-nitronaphthalene is highly localized in the airway epithelium. Target cells comprise but a fraction of the total lung cell mass; measurements from whole lung homogenates are not likely to reflect what occurs at the target site. Additionally, the use of generic microarrays to measure expression in airway epithelium may not provide a good representation of transcripts present at the site of toxic action. cDNA libraries from airway and alveolar subcompartments of rat lung were sequenced for the development of a custom microarray representative of these lung regions. We identified 7,460 nonredundant rat lung sequences. Nearly 30% of the sequences on this array are not present on the Affymetrix Rat GeneChip 230. A 20,000-element microarray was developed that delineates differences in gene expression between subcompartments. This is the first in a series of articles employing this microarray for detecting gene expression changes during acute injury produced by 1-nitronaphthalene and subsequent repair.

Bioaccumulation and critical body residue of PAHs in the amphipod, Diporeia spp.: additional evidence to support toxicity additivity for PAH mixtures

Polycyclic aromatic hydrocarbons (PAHs) are considered to act additively when exposed as congener mixtures. Additive internal concentrations at the site of toxic action is the basis for recent efforts to establish a sum PAH guideline for sediment-associated PAH toxicity. This study determined the toxicity of several PAH congeners on a body residue basis in Diporeia spp. These values were compared to the previously established LR 50 value for a PAH mixture based on the molar sum of PAH congeners and demonstrated similar LR 50 values for individual PAH. These results support the contention that the PAH act at the same molar concentration whether present as individual compounds or in mixture. Aqueous exposures were conducted for 28 d, and the water was exchanged daily to maintain the exposure concentration. The concentration in the exposures declined by an average of 22% between water exchanges across all compounds, and ranged from 11% to 32%. The toxicokinetics were determined using both time-weighted-average (TWA) and time-variable water concentrations and were not statistically different between the two source functions. Toxicity was determined for both mortality and immobility (failure to swim on prodding) and on both a TWA water concentration and a body residue basis. The LC 50 values ranged from 1757 μg l −1 for naphthalene after 10 d exposure to 79.1 μg l −1 for pyrene after 28 d exposure, and the EC 50 ranged from 1587 μg l −1 for naphthalene after 10 d exposure to 38.2 μg l −1 for pyrene after 28 d exposure. The LR 50 values for all congeners at all lengths of exposure were essentially constant and averaged 7.5±2.6 μmol g −1, while the ER 50 for immobility averaged 2.6±0.6 μmol g −1. The bioconcentration factor declined with increasing exposure concentration and was driven primarily by a lower uptake rate with increasing dose, while the elimination remained essentially constant for each compound.

Mechanisms of pyrethroid neurotoxicity: implications for cumulative risk assessment

The Food Quality Protection Act (FQPA) of 1996 requires the United States Environmental Protection Agency to consider the cumulative effects of exposure to pesticides having a ‘common mechanism of toxicity.’ This paper reviews the information available on the acute neurotoxicity and mechanisms of toxic action of pyrethroid insecticides in mammals from the perspective of the ‘common mechanism’ statute of the FQPA. The principal effects of pyrethroids as a class are various signs of excitatory neurotoxicity. Historically, pyrethroids were grouped into two subclasses (Types I and II) based on chemical structure and the production of either the T (tremor) or CS (choreoathetosis with salivation) intoxication syndrome following intravenous or intracerebral administration to rodents. Although this classification system is widely employed, it has several shortcomings for the identification of common toxic effects. In particular, it does not reflect the diversity of intoxication signs found following oral administration of various pyrethroids. Pyrethroids act in vitro on a variety of putative biochemical and physiological target sites, four of which merit consideration as sites of toxic action. Voltage-sensitive sodium channels, the sites of insecticidal action, are also important target sites in mammals. Unlike insects, mammals have multiple sodium channel isoforms that vary in their biophysical and pharmacological properties, including their differential sensitivity to pyrethroids. Pyrethroids also act on some isoforms of voltage-sensitive calcium and chloride channels, and these effects may contribute to the toxicity of some compounds. Effects on peripheral-type benzodiazepine receptors are unlikely to be a principal cause of pyrethroid intoxication but may contribute to or enhance convulsions caused by actions at other target sites. In contrast, other putative target sites that have been identified in vitro do not appear to play a major role in pyrethroid intoxication. The diverse toxic actions and pharmacological effects of pyrethroids suggest that simple additivity models based on combined actions at a single target are not appropriate to assess the risks of cumulative exposure to multiple pyrethroids.

Relating Environmental Availability to Bioavailability: Soil-Type-Dependent Metal Accumulation in the Oligochaete Eisenia andrei

Body residues are often better estimates of the amount of a chemical at the sites of toxic action in an organism than ambient soil concentrations, because bioavailability differences among soils are explicitly taken into account in considerations of body residues. Often, however, insufficient attention is paid to the rate and extent at which tissue concentrations respond to soil concentrations and soil characteristics. In this contribution the impact of soil characteristics on the environmental bioavailability of heavy metals for the oligochaete worm Eisenia andrei is reported. Uptake of As, Cd, Cr, Cu, Ni, Pb, and Zn in 20 Dutch field soils and in OECD artificial soil was quantified as a function of time. Internal metal concentrations varied less than the corresponding external levels. Metal uptake and elimination were both metal- and species-dependent. Worms typically attained steady-state concentrations rapidly for Cr, Cu, Ni, and Zn. Internal concentrations similar to those in the cultivation medium, linearly increasing body concentrations, or steady-state internal concentrations well above those in the cultivation medium were found for As, Cd, and Pb. Multivariate expressions were derived to describe uptake rate constants, steady-state concentrations, and bioaccumulation factors as a function of soil characteristics. Soil acidity is the most important solid-phase characteristic modulating the availability of As, Cd, and Pb. Although additional semimechanistic calculations yielded evidence of pore-water-related uptake of Cd and Pb modulated by competition between H+ and metal ions at the active sites of the membranes, the findings for Cr, Cu, Ni, and Zn point to additional influences, among which is probably regulation.

Protection and spontaneous recovery from cisplatin-induced hearing loss.

Cisplatin [cis-diamminechloroplatinum(II)] has proved itself as a potent antineoplastic agent. However, nephrotoxicity, neurotoxicity, gastrointestinal toxicity, myelosuppression, and ototoxicity interfere with its therapeutical efficacy. Forced diuresis reduces nephrotoxicity, effectively leaving neurotoxicity and ototoxicity as the major side effects of concern, and gastrointestinal toxicity and myelosuppression as the secondary side effects. So far, attempts to reduce these side effects by developing equally potent platinum analogs have been unsuccessful. Some success has been achieved, however, by co-treatment with protective agents. Nearly all these agents are sulfur- or sulfhydryl-containing compounds (thio compounds), known as antioxidants and potent heavy metal chelators. These thio compounds may provide protection from cisplatin toxicity either (1) by direct interaction between the cisplatin and the thio moiety, (2) by displacing platinum from its site of toxic action, (3) by preventing platinum from interfering with superoxide dismutase, or (4) by scavenging of cisplatin-induced free radicals. In particular the first two protective mechanisms bear the risk of reducing the antineoplastic activity of cisplatin. Since nephrotoxicity can be controlled effectively by forced diuresis, a more specific approach of coping with ototoxicity by focusing on protection at the sensorineural level was chosen. Being familiar with the neuro-protective and neurotrophic properties of ACTH-related neuropeptides, specifically against cisplatin-induced peripheral neuropathies, it was judged expedient to test for a possible otoprotective action of these neuropeptides. The results were positive, although tainted with high interanimal variability. When testing for the possibility that the neuropeptides would merely delay cisplatin-induced ototoxicity rather than reduce it, it was discovered in control series without neuropeptide co-treatment that the ear can recover spontaneously from cisplatin-induced hearing loss. This was found both electrophysiologically and in outer hair cell (OHC) counts. Although these preliminary findings require further investigation, they strongly suggest that spontaneous recovery of cochlear injury can occur in the mature mammalian cochlea. Moreover, the otoprotective action of the ACTH-related neuropeptides suggests that it may be possible to stimulate recovery from acute hearing loss using neuropeptides.

The Role of Soil Microbial Tests in Ecological Risk Assessment: Differentiating between Exposure and Effects

The use of soil microorganisms in ecological risk assessment is hampered by an unclear dose-response relationship for most contaminants. Establishing dose-response curves for soil microbial communities requires that one have a clear estimate of exposure at the site of toxic action and a response free of confounding environmental factors. It is not clear what methods can estimate toxicant dose at the site of toxic action or determine microbial response to a toxicant. Pollution-induced community tolerance (PICT) is one possible estimate of microbial toxicant exposure. The PICT hypothesis is that the tolerance of a microbial community is proportional to the in situ dose. This method automatically corrects for differences due to differences in soil physical-chemical variables between samples. Various components of the soil nitrogen cycle can act as microbial bioindicators of toxicant impacts. Estimating denitrifica-tion activity presents a number of advantages over other components of the nitrogen cycle. Denitrifying bacteria come from a diversity of habitats, can be autotrophic or heterotrophic, and denitrification is a well-defined enzymatic system, which allows the use of molecular tools. Determining denitrification may be a good estimate of effects of toxicants on microbial communities. However, given the state of our ignorance regarding soil microbial community structure and function, redundant estimates of exposure and effect are necessary to adequately characterize the response of microbial communities to toxicants.

ATP-Dependent Silver Transport across the Basolateral Membrane of Rainbow Trout Gills

Silver has been shown to be extremely toxic to freshwater teleosts, acting to inhibit Na+ uptake at the gills, due to the inactivation of branchial Na+/K+-ATPase activity. However, the gills are also a route by which silver may enter the fish. Therefore, this study focuses on the mechanism of transport of this nonessential metal across the basolateral membrane of the gill cell, using basolateral membrane vesicles (BLMV) prepared from the gills of freshwater rainbow trout. Uptake of silver by BLMV was via a carrier-mediated process, which was ATP-dependent, reached equilibium over time, and followed Michaelis–Menten kinetics, with maximal transport capacity (Vmax) of 14.3 ± 5.5 (SE) nmol mg membrane protein-1 min-1 and an affinity (Km) of 62.6 ± 43.7 μM, and was inhibited by 100 μM sodium orthovanadate (Na3VO4). The ionophore monensin (10 μM) released transported silver from the BLMV. Acylphosphate intermediates, of a 104 kDa size, were formed from the BLMV preparations in the presence of ATP plus Ag. These results demonstrate that there is a P-type ATPase present in the basolateral membrane of the gills of rainbow trout that can actively transport silver, a process which will remove this heavy metal from its site of toxic action, the gill.

Halogenated Aromatic Hydrocarbons Suppress CA1 Field Excitatory Postsynaptic Potentials in Rat Hippocampal Slices

Halogenated aromatic hydrocarbons (HAHs), such as polychlorinated biphenyls (PCBs) and dibenzo-p-dioxins (PCDDs), alter cognitive function and learning. The cellular basis of HAH-induced alteration of brain function is not well-understood. The hippocampus is a likely site of toxic action because of its well-known roles in learning and memory, as well as its propensity to accumulate environmental neurotoxicants. A hippocampal function that can be measured readily is evoked excitatory postsynaptic potentials (EPSPs), which are an index of excitatory synaptic function. In this study, effects of HAHs on EPSPs were characterized in hippocampal slices from adolescent to adult maleSprague—Dawley rats. 2,3,7,8-Tetrachlorodibenzo-p-dioxin (TCDD) and 1,2,3,4-TCDD were used because these HAHs are prototypical potent and weak aryl hydrocarbon (Ah) receptor agonists, respectively. 2,2′,5,5′-Tetrachlorobiphenyl (TCB) was used as a prototypical ortho-substituted PCB, which acts through Ah receptor-independent pathways. For each hippocampal slice, peak amplitudes of EPSPs during a 15-min recording period (1 recording/min) were averaged and used as baseline (100%). Subsequent EPSPs were expressed as percentage of baseline. TCDD and 1,2,3,4-TCDD did not alter EPSPs in slices from the middle third of the hippocampus. However, in ventral slices, TCDD significantly decreased EPSPs, whereas 1,2,3,4-TCDD was inactive. TCB decreased EPSPs in both middle and ventral slices at half-maximal stimulation. An unexpected reversal of inhibition was observed within 30 min of continuous application of TCDD or TCB. In ventral slices,l-type calcium channel blocker nifedipine blocked inhibition of EPSPs induced by TCDD but not EPSPs inhibited by TCB. These results suggest that, while TCB-induced inhibition of EPSPs occurs through an unknown mechanism, TCDD-induced inhibition of EPSPs was mediated byl-type calcium channel activity in a congener-specific manner.

Main Features of Basal Cytoxicity: Sites of Toxic Action and Interaction in the Pollen Tube Cell

Pollen tubes have frequently been used to determine the cytotoxic potentials of various chemical compounds and to study the effects of toxic action in the tube cell. In this paper, the main results of these studies are used to develop a model for understanding basal cytotoxicity. The following eight intracellular sites or functions, which are known to play a significant role as targets for toxic action, are considered: mitochondria, intracellular transport, membrane flow, the endoplasmic reticulum and Golgi apparatus, lipid and protein synthesis, carbohydrate synthesis, the cytoskeleton, and the plasma membrane. The reactions of these targets to certain representative cytotoxic compounds frequently applied to pollen tubes are reviewed. These reactions, most of which were observed by using electron microscopy and immunofluorescence microscopy, are discussed in relation to cell growth inhibition. In addition, interactions between the target sites are described and schematically presented. The set of targets mentioned above is representative of what is found in the majority of eukaryotic cell types. Therefore, it is not surprising that many of the cell types which are used in cytotoxicology produce values within the same logarithmic range.

The Use of Basal Cytotoxicity and Target Organ Toxicity Tests in Hazard Identification and Risk Assessment

Non-animal procedures, including in vitro test systems and test strategies, can already make a significant contribution to the background to risk assessment — in predicting both the toxic potential and toxic potency of chemicals, as well as, in some circumstances, the toxic hazard they may represent under specified conditions of exposure. They can be particularly useful for investigating molecular and cellular mechanisms of chemical-induced toxicity, and for identifying species-specific effects, which greatly limit the value of data from laboratory animal studies in the human risk assessment process. Attention is focused on the need for greater effort to be invested in the development of non-animal procedures for evaluating the biokinetic factors which will determine the ultimate form and concentration of a particular chemical at possible sites of toxic action. The relative merits of correlative and mechanistic approaches to test development and test validation are discussed. The need for realism is emphasised, not only in relation to our expectations of the validation process, but also in terms of the current and future status of regulatory toxicology, in vitro or in vivo, as a scientific discipline. Finally, it is concluded that the intelligent and strategic use of in vitro test systems, in conjunction with predictive computer modelling, could markedly improve the scientific basis of human risk assessment.

Flux measurements as indices of H + and metal effects on freshwater fish

The gills are the principal site for uptake and losses of major body electrolytes, acid-base equivalents, ammonia, respiratory gases, and many waterborne contaminants in freshwater fish. Measurements of these fluxes offer a powerful tool for assessing the toxic effects of extreme water pH's and dissolved metals. Advantages of the flux approach over conventional methods based on blood or tissue sampling include: (i) 10–400-fold greater sensitivity; (ii) non-invasiveness; (iii) simplicity and applicability to field work; and (iv) measurements made at the site of toxic action (the branchial epithelium). Limitations of the approach include: (i) changes in water quality during the flux determination; (ii) extra-branchial routes of flux: and (iii) adsorption and precipitation phenomena leading to artifactual flux values. Current ideas on the mechanisms of O 2, CO 2, Na +, Cl −, Ca 2, K +, acid-base equivalent, and ammonia fluxes across the gills are reviewed, together with the actions of low pH, high pH, and metals (AI, Cu. Cd, La, Zn, Hg, Mn) on these fluxes. Recent kinetic analyses of flux relationships arc described which provide new information on the mechanism(s) of toxicant action.

Fetal toxicity of cadmium chloride: The pharmacokinetics in the pregnant Wistar rat

The pharmacokinetics of a single subcutaneous injection of 40 mumol/kg CdCl2 in pregnant rats on day 18 were studied during an 18-hour time period. Previous studies demonstrated that this dose of CdCl2 induced fetal death, placental necrosis, and a reduction of uteroplacental blood flow without maternal lethality; direct fetal injections of CdCl2 indicated that the site of toxic action was not in the fetus. Cadmium was rapidly absorbed from the intrascapular subcutaneous depot with the highest blood levels occurring within 5 minutes of injection. The release from the depot appeared to be multiphasic with both rapid and slow absorption components observed in the blood. Uptake of cadmium was greatest in the liver, followed by the placenta, kidney, and pancreas. The best fit to the data was obtained by assuming the existence of two pools of cadmium in the organs: one freely exchangeable with the blood and the other nonexchangeable. Deviations from model predictions were observed for the placenta and adrenals; these deviations are consistent with the concomitant diminished blood flow to these organs. Cadmium uptake by the fetus was also investigated, and the results support the hypothesis that the placenta is relatively impermeable to the toxicant. It is concluded that the rapid and extensive accumulation of cadmium by the placenta may play a role in the development of early placental cellular damage and the eventual induction of fetal death through uteroplacental dysfunction.