Chemosensitisation and its impact on bioaccumulation and toxicity under environmental conditions

In the environment, organisms are exposed to complex xenobiotic pollution. Consequently, they have developed means of reducing internal chemical exposure. In addition to biotransformation, the active efflux of toxicants via membrane-bound, energy-dependent multixenobiotic resistance (MXR) transport proteins contributes to the elimination process. However, MXR transporters may be blocked by so-called chemosensitisers. Among this class of compounds are many common environmental pollutants, such as pesticides, pharmaceuticals, fragrances, and surfactants. The complexity of chemical pollution is hence assumed to overpower MXR transport and result in the increase of bioaccumulation and toxicity. While this has been observed predominantly in cellular and membrane-based in vitro assays with high concentrations of single substances, the biological relevance of chemosensitisation remains unclear on the organism level. Therefore, this thesis aimed at determining the impact of MXR transport inhibition on bioaccumulation and toxicity in complex organisms. For this purpose, a literature research on reported environmental chemosensitiser effect and exposure concentrations was conducted and the values were compared for the respective compounds. Inhibitory concentrations (ICs) remained below the corresponding maximum environmental concentrations for all chemosensitisers. When a concentration additive mixture toxicity model was applied, the threshold value of one was not exceeded. Thus, toxicity was not expected to augment in the presence of known chemosensitisers at their identified ICs. However, this conclusion was based on great uncertainty due to substantial knowledge gaps. In particular, a requirement for the methodological screening of environmental pollutants acting as chemosensitisers and a need for the reporting of freely dissolved exposure concentrations was identified. In addition to single substances tested in vitro, complex environmental mixtures have been reported to increase marker substrate bioaccumulation in aquatic wildlife by chemosensitisation. In this thesis, the change in the internal concentration of hydrophobic organic pollutants was quantified in zebrafish embryos in the presence and absence of the potent model chemosensitiser verapamil. A small volume QuEChERS (quick, easy, cheap, effective, rugged, and safe) method was developed for the extraction of zebrafish embryos and coupled to large volume injection gas chromatography tandem mass spectrometry (LVI-GC-MS/MS) for analysis. Overall, no impact was observed upon verapamil co-exposure. Instead, biological variation by a factor of approximately two was detected. Nevertheless, embryo toxicity could be affected by biological exposure mitigation measures. The excretion of harmful chemicals via MXR transport requires energy, which has been hypothesised to impair other organism functions. Thus, the impact of verapamil on polluted sediment extract toxicity to zebrafish embryos was assessed by passive dosing. While a moderate increase in toxicity was observed for one sediment extract, exposure to another resulted in slightly reduced toxicity. In literature, the impact of chemosensitisation by complex environmental pollution has been reported to be of a comparable magnitude. Commonly, an increase in marker substrate bioaccumulation by a factor of up to three has been reported. In conclusion, chemosensitisation appears to be of limited relevance for the bioaccumulation and toxicity of hydrophobic pollutants in zebrafish embryos. Although many common environmental pollutants possess chemosensitising properties, exposure to high concentrations of single compounds and environmental mixtures does not seem to lead to a relevant increase in the bioaccumulation and toxicity of other pollutants