Characterization of the effects of binary metal mixtures on short-term uptake of Cd, Pb, and Zn by rainbow trout (Oncorhynchus mykiss)

Biotic Ligand Models (BLMs) for individual metals improve our ability to regulate metals in the aquatic environment by considering the effects of water quality parameters (ionic composition, pH, DOC) on metal bioavailability. However, in natural aquatic systems, organisms are often simultaneously exposed to multiple metals and these interactions are not currently considered in BLMs or most environmental regulations. Recently, several different mixture BLMs (mBLMs) have been developed to begin assessing this issue. Some of these models assume competitive interactions between all metals, while others assume only metals with similar modes of action (e.g., Na+ or Ca2+ antagonists) will competitively interact. In this study, we used standard in vivo 3-h gill metal binding assays to characterize the uptake of Cd, Pb, and Zn individually and in binary mixtures with Ag, Cd, Cu, Pb, Ni, and Zn across a range of concentrations that encompassed the 96-h LC50 for each metal. Inhibition of Cd, Pb, and Zn uptake at the gill by introduction of a second metal was consistent with mode of action in some cases, but not others. Further, contrary to expectations, inhibition was always either non-competitive or could not be defined statistically. We also observed one example of stimulated metal uptake (Ni stimulated Zn uptake). Consistent with our previous experiments on Ag, Cu, and Ni, these studies suggest that current mBLM frameworks will need revision to better reflect the mechanisms underlying metal mixture interactions.