Measuring Biotic Ligand Model (BLM) Parameters in Vitro: Copper and Silver Binding to Rainbow Trout Gill Cells as Cultured Epithelia or in Suspension.

Biotic ligand models (BLMs) for metals are useful for risk assessment. The modeling of metal complexation by the biotic ligand has received little attention relative to the modeling of organic and inorganic complexation of metals in solution. We used ion selective electrodes (ISEs) to directly characterize copper and silver binding to rainbow trout gill cells, either as cultured reconstructed epithelia, or dispersed in suspension. Preparations were composed of pavement cells (PVCs) alone or mixtures of PVCs (≈85%) and mitochondria-rich cells (MRCs, ≈15%). Mixed cells showed up to an order of magnitude greater binding for both metals, indicating that MRCs were much more important than PVCs. Also, cell orientation had a dramatic effect; cells cultured as epithelia exhibited much greater binding than cells in suspension. Silver and copper demonstrated generally similar binding behavior, with stronger (logK ≈ 10 or greater) and weaker binding sites (logK ≈ 8). Comparisons to existing BLM calibrations show good agreement, but reveal that selection of analytical window can impact which binding sites are titrated. We conclude that cultured gill epithelia in vitro provide a powerful approach to studying metal complexation directly at the biotic ligand.