Interactions between metals in rat jejunum: implications on the nature of cadmium uptake.

Cadmium uptake from the lumen of the rat jejunum is characterized by apparent saturability and by sensitivity to inhibition by Zn and Ca. A variety of other heavy metals also inhibit intestinal Cd transport. The hypothesis was tested that Cd competes with other metals for their transport systems. The observed discrepancy between the apparent Ki and Km values for interaction between Cd and Zn argues against competition. Further, inhibition of Cd uptake by Zn, Ca and La is independent of Cd concentration over the range in which the system approaches partial apparent saturation (5-100 microM). Relatively high concentrations of other polyvalent cations, including Cd itself and Pb, Ni, Cr3+, Sr, Mg and polylysine, also depress 109Cd uptake. The possibility is suggested that these cations non-specifically neutralize membrane charges, and thus interfere with membrane binding and subsequent internalization of 109Cd. This explanation obviates the need for the unlikely assumption that specific membrane carriers exist to facilitate uptake of a toxic and non-essential metal like Cd.