Cadmium transport in isolated enterocytes of freshwater rainbow trout: interactions with zinc and iron, effects of complexation with cysteine, and an ATPase-coupled efflux

The present study investigated the mechanisms of intestinal cadmium (Cd) uptake and efflux, using isolated enterocytes of freshwater rainbow trout (Oncorhynchus mykiss) as the experimental model. The apical uptake of free Cd(2+) in the enterocytes was a saturable and high-affinity transport process. Both zinc (Zn(2+)) and iron (Fe(2+)) inhibited cellular Cd(2+) uptake through a competitive interaction, suggesting that Cd(2+) enters enterocytes via both Zn(2+) (e.g., ZIP8) and Fe(2+) (e.g., DMT1) transport pathways. Cellular Cd(2+) uptake increased in the presence of HCO(3)(-), which resembled the function of mammalian ZIP8. Cellular Cd(2+) uptake was unaffected by Ca(2+), indicating that Cd(2+) does not compete with Ca(2+) for apical uptake. Interestingly, Cd uptake was influenced by the presence of l-cysteine, and under the exposure condition where Cd(Cys)(+) was the predominant Cd species, cellular Cd uptake rate increased with the increased concentration of Cd(Cys)(+). The kinetic analysis indicated that the uptake of Cd(Cys)(+) occurs through a low capacity transport mechanism relative to that of free Cd(2+). In addition, Cd efflux from the enterocytes decreased in the presence of an ATPase inhibitor (orthovanadate), suggesting the existence of an ATPase-coupled extrusion process. Overall, our findings provide new mechanistic insights into the intestinal Cd transport in freshwater fish.