PREDICTING BRANCHIAL AND CUTANEOUS UPTAKE OF 2,2',5,5'-TETRACHLOROBIPHENYL IN FATHEAD MINNOWS (PIMEPHALES-PROMELAS) AND JAPANESE MEDAKA (ORYZIAS-LATIPES) - RATE-LIMITING FACTORS

A physiologically based model was used to predict the bioconcentration of a waterborne, neutral, non-metabolized xenobiotic compound in fath ead minnows (Pimephales promelas) and Japanese medaka (Oryzias latipes ). This study included a quantitative assessment of the primary mechan istic variables regulating uptake across branchial and cutaneous surfa ces in these small fish. Model simulations suggest that branchial and cutaneous surfaces have approx. equal capacity to support exchange of 2,2',5,5'-tetrachlorobiphenyl (TCB). A large cutaneous surface-area-to -volume ratio and a relatively small diffusion distance across the ski n in these fish both contribute to the relatively greater contribution of cutaneous absorption as compared to larger fish. The accuracy of m odel simulations was evaluated by comparison of predicted and observed bioconcentration of TCB in fathead minnows and Japanese medaka. The c omplete model, incorporating simultaneous branchial and cutaneous flux , predicts absorption of TCB that is in basic agreement with bioconcen tration observed in this study. This suggests that the absorption of n eutral waterborne xenobiotics by small fish (< 4 g) can be accurately described in terms of a few fundamental physiological, morphological a nd physico-chemical parameters and that a physiologically based modeli ng approach can be used effectively to predict the bioconcentration of xenobiotics in small fish.