Doctor of Philosophy (PhD)


Biomedical and Veterinary Medical Sciences - Comparative Biomedical Sciences

Document Type



The ABC transporter P-glycoprotein (Pgp) and membrane permeability are determinant factors for absorption and disposition of xenobiotics. These studies investigated the effects of potential modulators of Pgp and/or membrane function on the disposition, bioavailability and bioaccumulation of environmentally relevant pharmaceuticals, tetracycline and ivermectin, and the carcinogen benzo[a]pyrene in catfish. The pesticide dieldrin and surfactant linear alkylbenzene sulfonate (LAS) were selected as mixture components thought to have potential interactions with these determinants of disposition. Initial in situ experiments demonstrated dieldrin and tetracycline both significantly inhibited biliary excretion of Pgp prototypic substrate Rhodamine-123 in isolated perfused livers. Further, dieldrin (20µM) reduced movement of 3H-tetracycline (8.8nM) into bile (55%) to a greater extent than Pgp prototypic competitive inhibitor verapamil. In contrast to inhibitory effects in situ, a 4-week dietary dieldrin preexposure (0.1mg/day/kg body weight) increased plasma clearance (17%) and reduced tissue concentrations of 3H-tetracycline equivalents (parent and metabolites) for a single intra-aortic administration of 3H-tetracycline (8.31µg/kg body weight) in catfish. A 23% increase in the immunoreactive Pgp level in the hepatic membranes following chronic dietary dieldrin exposure was correlated with in vivo changes in disposition. Additional in situ studies demonstrated LAS treatments (1, 5, and 20µM) reduced movement of Rhodamine-123 (1µM) into bile in isolated perfused livers (18.6, 38.1 and 66.7%, respectively). Fluorescent anisotropy measurements of the corresponding hepatic membranes showed a 29.7% decrease (increase in membrane fluidity) at the 1µM LAS concentration, with little additional change evident at higher concentrations. In sequential in vivo experiments, following six daily diet administrations of 3H-ivermectin (10 µg/day/kg body weight) or 3H-benzo[a]pyrene (40 µg/day/kg body weight) starting on day 7 during a twelve-day waterborne LAS exposure (0, 100 and 300µg/l), 3H-ivermectin and 3H-benzo[a]pyrene equivalent remaining in catfish and their blood and tissue concentrations increased in a dose-dependent fashion with increasing LAS concentrations. The first study indicates dieldrin inhibited Pgp transport at the high concentration with the inductive effect upon Pgp expression predominating at the low concentration exposures in vivo. Findings of the second study suggest that LAS at environmental concentrations altered membrane permeability and/or transporter function so to increase bioaccumulation of other xenobiotics from the diet.



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Committee Chair

Kevin M. Kleinow