Chapter 15 Xenobiotics

Reuse water: Exposure duration, seasonality and treatment affect tissue responses in a model fish

Partially remediated gray (reuse) water will likely find increasing use in a variety of applications owing to the increasing scarcity of freshwater. We aimed to determine if a model fish, the goldfish, could sense reuse water using olfaction (smell), and if 30min or 7d (acute) and 60d (sub-chronic) exposures would affect their olfactory responses to natural odorants. We examined olfaction as previous studies have found that numerous chemicals can impair the olfactory sense, which is critical to carrying out numerous life-sustaining behaviors from feeding to mating. We also examined if fish olfactory and liver tissues would mount a response in terms of biotransformation enzyme gene expression, and whether treatment of reuse water with UV/H₂O₂ ameliorated adverse effects following reuse water exposure. We found that fish olfactory tissue responded to reuse water as it would to a natural odorant and that UV/H₂O₂ treatment had no influence on this. With acute exposures, olfactory impairment was apparent regardless of water type (e.g. responses of 23-55% of control), but in sub-chronic exposures, only the untreated reuse water caused olfactory impairment. The exposure of fish to reuse water increased the expression of one enzyme (CYP1A; >2.5-6.5 fold change) and reuse water treatment with UV/H₂O₂ reversed the effect. There was a seasonal effect that was likely due to changes in water quality (60d summer exposure impaired olfaction whereas spring and fall exposures did not). Overall, the data suggest that reuse water may be detected by olfaction, impair olfactory responses in fish receiving unavoidable exposures, and that exposure duration and season are important factors to consider regarding adverse effects.

Energy allocations to xenobiotic transport and biotransformation reactions in rainbow trout (Oncorhynchus mykiss) during energy intake restriction

Limited energy intake may result in the down-regulation of cellular defense mechanisms, or if maintained, result in trade-offs with other physiological systems. To examine this, juvenile rainbow trout (Oncorhynchus mykiss) were fed full-rations (1.17% body weight [BW]/day), half-rations (0.59% [BW]/day), or fasted for 9 weeks followed by refeeding at full-rations. BW and liver somatic index (LSI), P-glycoprotein (P-gp), ethoxyresorufin-o-deethylase (EROD), and glutathione-S-transferase (GST) activities were measured to determine if they are maintained under limited resources. P-gp and EROD activities were maintained at baseline values in ration-restricted (P-gp: 119+/-29 pg R123/min/mg cells, EROD: 0.58+/-0.17 nmol/min/mg protein) and fasted fish (P-gp: 120+/-14 pg R123/min/mg cells, EROD: 0.47+/-0.14 nmol/min/mg protein), suggesting they may be prioritized systems during fasting. GST activity was attenuated within 6 weeks of fasting (34% decrease from control), but recoverable to baseline values after refeeding. Changes in BW and LSI of calorie-restricted (BW: 16% decrease from control; LSI: 33% decrease from baseline value) and fasted trout (BW: 38% decrease from control; LSI: 44% decrease from baseline value) suggest that resources were mobilized from body stores partly to support these systems. Condition indices and defense activities in groups also varied over time, suggesting that environmental temperature may modulate these parameters.

Energy intake affects the biotransformation rate, scope for induction, and metabolite profile of benzo[a]pyrene in rainbow trout

The metabolic conversion of benzo[a]pyrene (B[a]P) by rainbow trout (Oncorhynchus mykiss) hepatocytes was not significantly different between any group of fed fish (fed one of three isoenergetic diets that varied in protein and lipid content at full satiation levels or half rations), however at 12 weeks, fasted fish exhibited significantly reduced B[a]P biotransformation rates (by 58%). Alterations in metabolite profiles were also seen: fasted fish produced significantly more Phase I metabolites, higher levels of both glucuronide and sulphate conjugates, and lower levels of presumptive glutathione conjugates, compared to fed fish. When fish were fasted, higher proportions of phenols were produced, with lower proportions of quinones, triols and tetrols. Inducing metabolism (using beta-naphthoflavone) increased metabolic scope for B[a]P by 2-fold, regardless of each diet's baseline metabolic rate. However, the balance between Phase I and II reactions was altered with induction and fasting: higher proportions of Phase I metabolites were found, with lower glutathione conjugates and higher proportions of triols/tetrols. Fasting-mediated reductions in glutathione conjugation, and increased induction of oxidation vs. conjugating enzymes, can explain altered metabolite profiles. These results suggest that in contaminated habitats, where pollution-induced reductions in food quantity or quality are combined with the presence of toxic compounds and inducers, detoxification rates can be diminished.