Effects of calcium on cadmium uptake and binding in the rat intestine

Cadmium uptake in vitro was evaluated in everted duodenal, jejunal, and ileal segments of the rat small intestine. Washing the sacs with EDTA subsequent to cadmium exposure distinguished labile and nonlabile cadmium compartments. These compartments were distinct with respect to regional distribution, kinetics, and effect of calcium. Uptake into the nonlabile cadmium compartment but not into the labile cadmium compartment was time dependent, consistent with transport and adsorption processes, respectively. Initial uptake rates for the nonlabile cadmium compartment showed both saturable and first-order kinetics. Calcium inhibited only the saturable mechanism. Cadmium uptake rate and sensitivity to calcium were greatest in the duodenum due to the predominance of the saturable mechanism in this region. Distal to the duodenum cadmium uptake exhibited primarily first-order kinetics. The Vmax for the saturable process was reduced 90% by calcium. The absence of competitive inhibition indicates that calcium and cadmium do not share a common carrier-mediated uptake mechanism. The amount of cadmium bound in the labile compartment was comparable in all segments and could be described by a nonlinear function which was the sum of the saturable and second-order terms. Addition of calcium had an apparent cooperative effect on cadmium binding, increasing the second-order term of binding in the labile compartment. Thus calcium was found to have a dual effect on cadmium accumulation by intestinal tissue: inhibition of saturable duodenal uptake and stimulation of binding throughout the small intestine.