Relative basolateral (a) and apical (b) efflux of fluorescent calcein in immature Caco-2 cells. The MRP1 activity was examined by determining the cumulative level of fluorescent calcein effluxed into the basolateral chamber following two hours of incubation. Data are expressed relative to controls, set at 1; mean ± SD; ; statistical significant difference compared to control; . Legends to the pretreatments: C: control; Cd: cadmium.