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.