Effects of Calcium on Cadmium Uptake and Binding in the Rat Intestine

Effects of Calcium on Cadmium Uptake and Binding in the RatIntestine. HOADLEY, J. E., AND JOHNSON, D. R. (1987). Fundam.Appl. Toxicol. 9, 1–9. Cadmium uptake in vitro was evaluatedin everted duodenal, jejunal, and ileal segments of the ratsmall intestine. Washing the sacs with EDTA subsequent to cadmiumexposure distinguished labile and nonlabile cadmium compartments.These compartments were distinct with respect to regional distribution,kinetics, and effect of calcium. Uptake into the nonlabile cadmiumcompartment but not into the labile cadmium compartment wastime dependent, consistent with transport and adsorption processes,respectively. Initial uptake rates for the nonlabile cadmiumcompartment showed both saturable and first-order kinetics.Calcium inhibited only the saturable mechanism. Cadmium uptakerate and sensitivity to calcium were greatest in the duodenumdue to the predominance of the saturable mechanism in this region.Distal to the duodenum cadmium uptake exhibited primarily first-orderkinetics. The V_max for the saturable process was reduced 90%by calcium. The absence of competitive inhibition indicatesthat calcium and cadmium do not share a common carrier-mediateduptake mechanism. The amount of cadmium bound in the labilecompartment was comparable in all segments and could be describedby a nonlinear function which was the sum of the saturable andsecond-order terms. Addition of calcium had an apparent cooperativeeffect on cadmium binding, increasing the second-order termof binding in the labile compartment. Thus calcium was foundto have a dual effect on cadmium accumulation by intestinaltissue: inhibition of saturable duodenal uptake and stimulationof binding throughout the small intestine.