In vitro analysis of intestinal absorption of cadmium and calcium in rainbow trout fed with calcium- and cadmium-supplemented diets

The protective effects of dietary Ca²⁺ supplementation against Cd accumulation in rainbow trout Oncorhynchus mykiss fed with Cd-contaminated food were evaluated in relation to chronic changes in intestinal absorption rates. The changes were measured ' in vitro '. The control diet contained c. 20 mg Ca²⁺ g^?1 food and 0·25 μg Cd g^?1 food; the experimental diets were supplemented with CaCO₃ and Cd(NO₃)₂·4H₂O to levels of 50 mg Ca²⁺ g^?1 food and 300 μg Cd g^?1 food, alone and in combination. The Ca²⁺ and Cd absorption rates were measured using radiotracers (⁴⁵Ca, ¹⁰⁹Cd) at total Ca²⁺ and Cd concentrations of 3·0 and 0·12 mmol l^?1, respectively in the intestinal saline. Chronically elevated dietary Cd caused a significant increase in Cd absorption rate by up to 10-fold at 30 days in the mid-intestine. The high Ca²⁺ diet prevented this up-regulation of Cd transport rate. Conversely, intestinal Ca²⁺ absorption was significantly increased by two- to five-fold by the Ca²⁺-supplemented diet at 30 days in both the mid- and posterior intestine, and this effect was eliminated when Cd was simultaneously elevated in the diet. Ca²⁺ and Cd probably interact at common pathways and transport mechanisms in the intestine, though independent pathways may also exist.