Effects of calcium and metabolic inhibitors on trace element uptake in two marine bivalves

The influx of five trace elements (Ag, Cd, Co, Se and Zn) from the dissolved phase into the mussel, Mytilus edulis, and the clam, Macoma balthica, was determined at varying Ca concentrations and in the presence of different metabolic inhibitors. In both bivalves, varying the Ca concentration (but at a constant salinity) did not significantly influence the influx of any of the metals. Influx of Ag, Cd and Zn in mussels was significantly inhibited by verapamil (a Ca channel blocker), whereas trace metal uptake in clams was generally unaffected by this compound. Metabolic inhibitors (ouabain, which inhibits Na⁺/K⁺ ATPase, and 2,4-dinitrophenol which uncouples oxidative phosphorylation) did not influence trace element influx in mussels. N-Ethylmaleimide, which specifically binds to sulfhydryl groups, reduced the influx of Ag, Cd and Zn, implying that the transport of these metals is carrier-mediated by proteins or other SH-containing compounds. There was no evidence of Cd and Zn interaction in their uptake in mussels. The calculated absorption efficiencies of trace elements from the dissolved phases were highest for Ag, followed by Zn > Cd > Co > Se, and were 2.2 to 3.0 times higher in M. edulis than in M. balthica for Ag, Cd and Zn. While metal transport in mussels appears to be a carrier-mediated process for metals that associate with proteins, the Ca channel may also be involved in the transport of Ag and Zn in mussels, but not in clams. Co is primarily taken up in these animals by passive diffusion.