Uptake pathways and toxicity of Cd and Zn in the earthworm Eisenia fetida

The uptake of Cd and Zn by the earthworm Eisenia fetida was determined at varying Ca concentrations and with pre-exposure to different metabolic inhibitors in simulated soil solutions over a 48-h period. The presence of Ca in the solution had complex actions on Cd uptake. At a low Cd concentration of 0.1 μM, Ca (0.1–1 mM) slightly but significantly stimulated Cd uptake, whereas it inhibited Cd uptake at a higher Cd level (10 μM). Pre-exposure to a Ca-channel blocker (Lanthanum) inhibited Cd uptake over a relatively wide range of Cd concentrations, but not Zn uptake, suggesting that the uptake of Zn was not exerted at a Ca channel. N-ethylmaleimide, which specifically binds to sulfhydryl groups, inhibited Zn uptake at both 0.1 and 10 μM, implying that the transport of Zn is carrier-mediated by proteins or other SH-containing compounds. The present study provides evidence that the mechanisms of Cd and Zn uptake in earthworms are pharmacologically different, although both metals have similar geochemical and environmental properties. After 24 h pre-exposure to a sublethal concentration (1.0 μM) of Cd, Zn toxicity for E. fetida was significantly reduced with 48-h LC50 values (with 95% confidence interval) increasing from 145 (105–201) to 316 (212–470) μM Zn. Pre-exposure to Zn (1.0 μM) did not, however, affect Cd toxicity. Pre-exposure to Cd significantly changed the subcellular Zn distribution, with a decreasing fraction of Zn associated with Fraction B (associated with granules and cell membranes), which is believed to be most indicative of toxic pressure and an increased fraction associated with Fraction G (associated with cytosol). This most likely explains the observed Zn tolerance of E. fetida after low level Cd pre-exposure. These results help to understand the uptake mechanism and interactions of Zn and Cd in earthworms.