A physiologically based toxicokinetic model for dietary uptake of hydrophobic organic compounds by fish: II. Stimulation of chronic exposure scenarios

A physiologically based toxicokinetic (PBTK) model for dietary uptake of hydrophobic organic compounds by fish was used to simulate dosing scenarios commonly encountered in experimental and field studies. Simulations were initially generated for the model compound [UL-¹⁴C] 2,2′,5,5′-tetrachlorobiphenyl ([¹⁴C] PCB 52). Steady-state exposures were simulated by calculating chemical concentrations in tissues of the predator corresponding to an equilibrium distribution between the fish and water (termed the bioconcentration or BCF residue data set). This residue data set was then varied in a proportional manner until whole-body chemical concentrations exhibited no net change for each set of exposure conditions. For [¹⁴C] PCB 52, the proportional increase in BCF residues (termed the biomagnification factor or BMF) required to achieve steady state in a food-only exposure was 2.24, while in a combined food and water exposure the BMF was 3.11. Additional simulations for the food and water exposure scenario were obtained for a set of hypothetical organic compounds with increasing log K_OW values. Using gut permeability coefficients determined for [¹⁴C] PCB 52, predicted BMFs increased with chemical log K_OW, achieving levels much higher than those reported in field sampling efforts. BMFs comparable to measured values were obtained by reducing permeability coefficients within each gut segment in a log K_OW-dependent manner. This predicted decrease in chemical permeability is consistent with earlier work, suggesting that dietary absorption of hydrophobic compounds by fish is controlled in part by factors that vary with chemical log K_OW. Relatively low rates of metabolism or growth were shown to have a substantial impact on steady-state biomagnification of chemical residues.