Toxicokinetics and toxicodynamics of nickel in Enchytraeus crypticus

Metal toxicity is usually determined at a fixed time point, which may bias the assessment of risks associated with varied exposure time. Time-dependent accumulation and toxicity of nickel in the potworm Enchytraeus crypticus were investigated in solutions embedded in an inert quartz sand matrix. Internal Ni concentration and mortality were determined at 7 different time intervals and interpreted from the perspective of toxicokinetics and toxicodynamics. A 1-compartment model was used to describe the uptake and elimination kinetics of Ni. At each exposure concentration, Ni concentration in the organisms increased with increasing exposure time, reaching equilibrium after approximately 14d. Median lethal concentration (LC50) decreased with time and reached an ultimate value of 0.182mg/L. The LC50 values expressed as internal Ni concentrations (LC50inter) were almost constant (16.7mg/kg body dry wt) at each exposure time. The LC50inter was independent of exposure time, suggesting that internal concentration was a better indicator of Ni toxicity than external concentration. The uptake rate constant was 11.9L/kg/d, and elimination rate constants were 0.325/d (based on internal concentration) and 0.070/d (based on survival), indicating that not all internal Ni contributes to toxicity. The present study highlights the importance of taking time into account in future toxicity testing and risk assessment practices.