Physiologically based toxicokinetic model for maternal transfer of 2,3,7,8-tetrachlorodibenzo-p-dioxin in brook trout (Salvelinus fontinalis)

A physiologically based toxicokinetic (PB-TK) model was developed to describe the uptake, distribution, and elimination of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) in female brook trout during a B-month feeding and depuration study. Dietary uptake was modeled under two assumptions, uptake proceeds to equilibrium between blood exiting the intestinal tract and the contents of the intestinal tract, and uptake is limited by desorption of TCDD from gut contents and/or diffusion from the lumen into tissues. Model outputs were evaluated by comparison with measured TCDD residues. The best model fit to the data was obtained by imposing a moderate diffusion limitation on gut uptake. Of the parameters that comprise the gut submodel, whole body residue predictions were most sensitive to changes in the diffusion rate constant and fecal egestion rate. Chemical residues in fat were indicative of an internal disequilibrium with other tissues during the loading phase of the study. Accurate simulations of this behavior were obtained using a diffusion-limited tissue description. Chemical residues in liver, muscle, and ovaries were adequately described by assuming that uptake and elimination were blood flow limited. Spawning probably resulted in a small increase in whole body TCDD concentration and did not appear to represent an important route of chemical elimination. These results suggest that field-derived whole body TCDD residues could be used to estimate residues in developing ovaries of brook trout to within a factor of two, provided that whole body and ovary lipid content were known.