Ivermectin Toxicokinetics in Rainbow Trout (Oncorhynchus mykiss) following P-glycoprotein Induction

Alterations in ivermectin (IVM, 22,23-dihydro avermectin B1a+22,23-dihydro avermectin B1b) toxicokinetics following P-glycoprotein (P-gp) induction by clotrimazole (CTZ) were examined in rainbow trout (Oncorhynchus mykiss) to assess the potential importance of P-gp activity levels in xenobiotic distribution and kinetics in fish. Control and fish pretreated with CTZ (30 µmol/kg) were administered 175 µg/kg 3H-IVM into the caudal vasculature. At various time points (0.25, 0.5, 1, 3, 24, 48, 96, and 168 h) following injection, tissues (blood, liver, kidney, gill, intestines, brain [5 regions], eye, gonad and fat) were removed analyzed for IVM-derived radioactivity. IVM concentration declined in blood, liver, kidney and gill, and concentrations in other tissues remained constant over the sampling period. The highest measured concentrations were found in kidney, followed by liver, with the lowest values found in brain, eye and gonad. The highest % of the administered dose was found in the liver and kidney in the immediate hours post-administration, and in the intestines and fat at 24 h post-administration. P-gp induction by CTZ did not alter IVM distribution or any calculated toxicokinetic parameter (AUC, mean residence time, T1/2, clearance rate, volume of distribution), suggesting that P-gp induction may be limited or that P-gp plays a lesser role in xenobiotic kinetics in fish compared to mammals.

Effects of the chemosensitizer verapamil on P-glycoprotein substrate efflux in rainbow trout hepatocytes

The ATP-dependent membrane transporter P-glycoprotein (P-gp) is associated with resistance to a wide variety of chemical substrates, as well as the multi-drug resistance (MDR) phenotype in mammals. Less is known regarding P-gp's function and relevance in teleosts; this study expanded the range of known substrates and the inhibitory effects of a model chemosensitizer verapamil. The P-gp-mediated uptake and efflux dynamics of 5 known mammalian substrates (berberine, cortisol, doxorubicin, rhodamine 123 [R123], and vinorelbine) were examined in isolated rainbow trout (Oncorhynchus mykiss) hepatocytes with and without co-exposure to varying doses of verapamil. Initial substrate uptake rates (pmol/106 cells/min) varied widely and were in order: berberine (482 ± 94) > R123 (364 ± 67) > doxorubicin (158 ± 41) > cortisol (20.3 ± 5.9) > vinorelbine (15.3 ± 3.5). Initial efflux rates (pmol/106 cells/min) were highest in berberine (464 ± 110) > doxorubicin (341 ± 57) > R123 (106 ± 33) > cortisol (26.6 ± 6.1) > vinorelbine (9.0 ± 2.4). Transport of vinorelbine and R123 is verapamil sensitive, but verapamil had no effect on transport of berberine, cortisol, or doxorubicin. Cortisol and doxorubicin showed evidence of high P-gp affinity, thus displacing verapamil from their shared P-gp binding site. Cortisol, doxorubicin, R123, and vinorelbine transport by rainbow trout P-gp was confirmed, while berberine could not be confirmed or excluded as a substrate. Binding sites and affinities were similar between mammalian and trout P-gp for doxorubicin, R123, and vinorelbine, while fish P-gp had a higher affinity for cortisol than mammalian P-gp. This study demonstrated that the range of substrates, as well as binding sites and affinities, of fish P-gp are well-aligned with those in mammals.

Embryonic exposure to model naphthenic acids delays growth and hatching in the pond snail Lymnaea stagnalis

Naphthenic acids (NAs), a class of structurally diverse carboxylic acids with often complex ring structures and large aliphatic tail groups, are important by-products of many petrochemical processes including the oil sands mining activity of Northern Alberta. While it is evident that NAs have both acute and chronic harmful effects on many organisms, many aspects of their toxicity remain to be clarified. Particularly, while substantive data sets have been collected on NA toxicity in aquatic prokaryote and vertebrate model systems, to date, nothing is known about the toxic effects of these compounds on the embryonic development of aquatic invertebrate taxa, including freshwater mollusks. This study examines under laboratory conditions the toxicity of NAs extracted from oil sands process water (OSPW) and the low-molecular weight model NAs cyclohexylsuccinic acid (CHSA), cyclohexanebutyric acid (CHBA), and 4-tert-butylcyclohexane carboxylic acid (4-TBCA) on embryonic development of the snail Lymnaea stagnalis, a common freshwater gastropod with a broad Palearctic distribution. Evidence is provided for concentration-dependent teratogenic effects of both OSPW-derived and model NAs with remarkably similar nominal threshold concentrations between 15 and 20 mg/L and 28d EC₅₀ of 31 mg/L. In addition, the data provide evidence for substantial toxicokinetic differences between CHSA, CHBA and 4-TBCA. Together, our study introduces Lymnaea stagnalis embryonic development as an effective model to assay NA-toxicity and identifies molecular architecture as a potentially important toxicokinetic parameter in the toxicity of low-molecular weight NA in embryonic development of aquatic gastropods.