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

Abstract

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