Effect of calcium and copper on zinc absorption in the rat

The effect of cupric and calcium ions on zinc absorption was examined in Sprague-Dawley rats. In in vivo studies test solutions containing 7.5 mM 65ZnCl2 alone or in combination with 500 mM of either CuSO4 or CaCl2 were administered intraduodenally to three groups of rats. 2 h later the animals were sacrificed and their organ 65Zn content was determined. The results of these studies revealed that excess cupric ions had no effect on the 65Zn content of rat internal organs, whereas excess calcium ions decreased organs 65Zn content to 40%. Similar results were obtained in in vitro studies using three sets of rat jejunal sacs each filled with one of the above test solutions and incubated in oxygenated Eagle's medium. To investigate the reason(s) for the observed decreased rate of zinc absorption in the presence of excess luminal calcium, we examined zinc transport in 6 sets of jejunal sacs using six test solutions, each containing 7.5 mM 65ZnCl2 with 0, 12.5, 25, 37.5, 50, or 62.5 mM CaCl2. These experiments showed that 65Zn transport from mucosa to serosa was decreased by 40% in the presence of 25.0 mM CaCl2 but lower calcium concentrations had no effect on zinc transport. Increasing the luminal calcium concentration from 25 to 62.5 mM did not cause a further significant decrease in the rate of zinc transport. These results strongly suggest that zinc absorption in the rat is mediated by a transcellular transport process different from that which mediates copper and calcium absorption. The fact that calcium has been shown to reduce passive cation movements via shunt pathways in the gallbladder mucosa suggests the possibility that calcium in high luminal concentration may depress zinc absorption by reducing the passive component of jejunal zinc absorption.