Cadmium exposure reduces the density of a specific ionocyte subtype in developing zebrafish.

The present study examined the effects of waterborne cadmium (Cd) exposure on ionic balance and ionocyte density in developing zebrafish (Danio rerio) (0–4 days post-fertilization). Fish exposed to 1 or 10 μg Cd/L exhibited an increase in whole body Cd level. Exposure to 10 μg Cd/L also significantly reduced whole body content of Ca2+, but not other major ions (e.g., Na+, K+ and Mg2+). Such reduction was accompanied by a decrease in the density of Ca2+-transporting ionocytes, the Na+/K+-ATPase-rich cells (NaRCs). However, the densities of other ionocyte subtypes (e.g., Na+-transporting ionocytes) remained unchanged after exposure to 10 μg Cd/L. The potential interactive effects between water chemistry and Cd exposure on ionocyte density were examined further in Cd-exposed larvae acclimated to different water NaCl or Ca2+ levels. The results demonstrated that NaRC density increased in fish acclimated to low Ca2+ water, presumably increasing Ca2+ uptake for maintaining Ca2+ homeostasis. However, Cd exposure completely abolished the increased NaRC density in low water Ca2+ environments. The increased NaRCs over development was also reduced in Cd-exposed larvae. In conclusion, our study suggested that Cd exposure reduces the density of NaRCs and suppresses the compensatory regulation of NaRCs during acclimation to low water Ca2+ level. These inhibitory effects by Cd exposure ultimately disrupt Ca2+ balance in the early life stages of zebrafish.