Role of anionic medium composition on manganese cytotoxicity and bioavailability in rainbow trout gill cells

Research has shown that the bioaccumulation of manganese (Mn) in animals can lead to neurodegenerative disorders such as Manganism. Despite its potential dangers, limited data exist about this essential metal's uptake mechanism and how the medium composition can affect Mn toxicity and bioavailability. We used a model of the rainbow trout (Oncorhynchus mykiss) gill cells, RTgill-W1 cell line, as an in vitro approach. This is a cell line of vertebrate epithelial cells with conserved metal transporters across vertebrate species, including humans. The role of exposure medium composition was evaluated by manipulating anionic composition to produce these media: (1) L-15/ex, a phosphate-buffered medium; and (2) L-15/HCO₃₋, a bicarbonate-buffered medium. A chemical equilibrium model determined the speciation of Mn in the different media. Cell viability and bioaccumulation assays were performed to determine the cytotoxicity and uptake of manganese at different concentrations in the two media. This approach allowed for the linkage between the role of extracellular metal speciation and intracellular toxicity. The cell viability assay results show that the membrane integrity effective concentration 50% (EC50) value in L-15/ HCO₃₋ is 122.4 µM compared to 70.26 µM in L-15/ex. The results of the bioavailability assay showed that more manganese entered the cells when exposed in L-15/ HCO3- compared to L-15/ex. This finding is interesting considering that L-15/ex produced higher Mn toxicity, according to the cell viability assay. This finding possibly points to an interaction between Mn and anions in the different media, which induces or reduces toxicity. In future research, membrane Mn transporter activity could be quantified to help determine the mechanism of Mn cytotoxicity.