The concentration dependency of branchial intracellular cadmium distribution and influx in the zebrafish (Brachydanio rerio)

The role of metallothionein (MT) in the branchial sequestration and influx of cadmium was investigated. This was done by studying the concentration dependency of intracellular Cd distribution and influx in the fish gill. Zebrafish (Brachydanio rerio) were exposed for 24 h to 0.1, 1, 2, 8, 15 and 30 nM Cd in the water (0.011 to 3.4 mu g Cd l(-1)). The apical uptake of Cd into the branchial epithelial cells was proportional to the exposure concentration in the interval 0.1 to 15 nM Cd. Between 15 and 30 nM Cd, however, the increase in uptake appeared to reach a plateau. The intracellular distribution of Cd was altered as the exposure concentration increased, mainly due to a saturation of the Cd-binding capacity of the ligands in the cytosolic pool, among them MT. Concurrently, the Cd influx increased drastically in a non-linear fashion. Thus, both the ability of MT to buffer Cd in the gill and the ability of the gill to restrict influx decreased as the exposure concentration increased. These concentration-dependent alterations of the branchial Cd metabolism may be important determinants for the concentration dependency of toxic effects of Cd in fish.