M11 PM Internal Exposure
Monday, 14 November 2005: 1:50 PM - 5:30 PM in 343-344

(ERI-1117-830729) Uptake and Elimination of Ionizable Organic Chemicals at Fish Gills.

Erickson, R¹, McKim, J¹, Lien, G¹, Hoffman, A¹, Batterman, S¹, ¹ Mid-Continent Ecology Division, U.S. Environmental Protection Agency, Duluth, MN, USA

ABSTRACT- Toxic responses of aquatic organisms to chlorinated phenols have been related to reaching certain critical body burdens. However, water concentrations needed to elicit toxicity increase with exposure water pH in conjunction with increased chemical ionization. This indicates greater bioavailability for the neutral forms of these chemicals, but toxic concentrations and uptake rates do not vary strictly in proportion to these neutral forms. Rather, some bioavailability of the ionized forms is indicated, and this bioavailability appears to vary markedly across chemicals and exposure water pH. Three mechanisms might account for bioavailability of ionized organic molecules. First, alteration of the gill surface pH from metabolic byproducts can change the relative amounts of ionized and neutral molecules compared to bulk exposure water. Second, diffusion of ionized forms to and from epithelial membranes can help maintain high diffusion gradients of neutral compounds across these membranes, and contribute to uptake even if the membranes are impermeable to these ions. Third, epithelial membranes might have some permeability to the ionized forms. These mechanisms were incorporated into a mathematical model for predicting exchange of ionizable organic chemicals at fish gills. Uptake and elimination of several chlorinated phenols at rainbow trout gills were measured as a function of pH and alkalinity to evaluate the importance of these mechanisms and test model performance. Chemical exchange was demonstrated to vary with pH and among chemicals in concordance with the first two mechanisms. The third mechanism was unimportant for the experimental conditions and chemicals examined, but should be important under other circumstances. These efforts also demonstrated that addressing internal exposure of ionizable organic chemicals requires better knowledge of their permeabilities and partitioning within biological tissues, not just for understanding exchange at gills, but also internal distribution of accumulated chemical.

Key words: fish gill chemical flux, ionizable organic chemicals, pH effects, bioavailability