Bioconcentration kinetics of the organophosphorus insecticide chlorpyrifos in guppies (Poecilia reticulata).

The bioconcentration kinetics of chlorpyrifos (O,O-diethyl O-3,5,6-trichloro-2-pyridyl phosphorothioate) in guppies (Poecilia reticulata) were investigated. A static exposure was used to study the uptake of the compound. The amount absorbed was calculated from the difference in disappearance rates from the water phase in aquaria with and without fish. The uptake rate was found to be first-order with respect to the exposure concentration, and amounts to ca. 38 ml g-1 (fish) hr-1. The elimination rate was studied in separate experiments. Guppies were first given the opportunity to accumulate chlorpyrifos and, following transfer to clean water, the clearance of the compound was measured by determining the residual chlorpyrifos in the fish. It was found that the elimination rate was first-order with respect to the concentration in fish, but depended slightly on the length of the preexposure period. The half-life of tissue chlorpyrifos varied from 31 to 38 hr. Release of unchanged chlorpyrifos turned out to be negligible relative to the amount eliminated. This indicates that metabolic breakdown is the only pathway for elimination. From the kinetic rate parameters, a bioconcentration factor was calculated of about 1700. This value is very similar to that measured directly after an 8-day semistatic exposure. A two-compartment model is proposed to describe the kinetics of the bioconcentration process. The conditions limiting the applicability of the indirect method for measuring the toxicokinetics of chlorpyrifos are discussed.