Effects of dietary calcium and cadmium on cadmium accumulation, calcium and cadmium uptake from the water, and their interactions in juvenile rainbow trout

The objective of this study was to examine the effects of chronically elevated dietary Ca²⁺ (as CaCO₃), alone and in combination with elevated dietary Cd, on survival, growth, and Cd and Ca²⁺ accumulation in several internal compartments in juvenile rainbow trout (Oncorhynchus mykiss). In addition, effects on short-term branchial uptake and internal distribution of newly accumulated waterborne Ca²⁺ and Cd during acute waterborne Cd exposure (50 μg/L as CdNO₃ for 3 h) were monitored using radiotracers (⁴⁵Ca, ⁶⁵Cd). Fish were fed with four diets: 20 mg Ca²⁺/g food (control), 50 mg Ca ²⁺/g food, 300 μg Cd/g food, and 50 mg Ca²⁺/g + 300 μg Cd/g food for 30 days. There were no significant effects on growth, mortality, or total body Ca²⁺ accumulation. The presence of elevated Ca²⁺, Cd, or Ca²⁺ + Cd in the diet all reduced waterborne Ca²⁺ uptake in a short-term experiment (3 h), though the inhibitory mechanisms appeared to differ. The effects were marked after 15 days of feeding, but attenuated by 30 days, except when the diet was elevated in both Ca ²⁺ and Cd. The presence of elevated Ca²⁺ in the diet had only modest influence on Cd uptake from the water during acute Cd challenges but greatly depressed Cd uptake from the diet and accumulation in most internal tissues. None of the treatment diets prevented the decreases in waterborne Ca²⁺ uptake and new Ca²⁺ accumulation in internal tissues caused by acute exposure to waterborne Cd. In conclusion, there are complex interactions between waterborne and dietary effects of Ca²⁺ and Cd. Elevated dietary Ca²⁺ protects against both dietary and waterborne Cd uptake, whereas both waterborne and dietary Cd elevations cause reduced waterborne Ca²⁺ uptake.