A protective effect of dietary calcium against acute waterborne cadmium uptake in rainbow trout

The present study examined the interactions between elevated dietary calcium (as ionic Ca²⁺ in the form of CaCl₂·2H ₂O) and acute waterborne Cd exposure (50μg/l as CdNO₃ for 3h) on whole body uptake and internal distribution of newly accumulated Cd, Ca²⁺, and Na⁺ in juvenile rainbow trout (Oncorhynchus mykiss). Fish were fed with three diets 20 (control), 30 and 60mg Ca ²⁺/g food: for 7 days before fluxes were measured with radiotracers over a 3h period. The two elevated Ca²⁺ diets reduced the whole body uptake of both Ca²⁺ and Cd by >50% and similarly reduced the internalization of both newly accumulated metals in most tissues, effects which reflect the shared branchial uptake route for Ca²⁺ and Cd. As the Ca²⁺ concentrations of the fluid phases of the stomach and intestinal contents were greatly elevated by the experimental diets, increased gastrointestinal Ca²⁺ uptake likely caused the down-regulation of the branchial Ca²⁺ (and Cd) uptake pathway. Waterborne Na ⁺ uptake and internal distribution were not affected. While plasma Ca²⁺ surged after the first two feedings of the 60mg Ca ²⁺/g diet, internal homeostasis was quickly restored. Total Ca ²⁺, Na⁺, and Cl^- levels in tissues were not affected by diets. While dietary Ca²⁺ protected against waterborne Cd uptake, it did not protect against the relative inhibition of waterborne Ca²⁺ uptake caused by waterborne Cd. Acute exposure to 50μg/l Cd reduced the uptake and internalization of newly accumulated Ca²⁺ (but not Na⁺) by 70% or more, regardless of diet. Since elevated dietary Ca²⁺ reduces waterborne Cd uptake, fish eating a Ca ²⁺-rich invertebrate diet may be more protected against waterborne Cd toxicity in a field situation.