Absorption of Chromium Across the Rat Small Intestine

Abstract

Chromium is an essential element in trace quantities, but toxic at high concentrations, particularly in the hexavalent form. In this study the transport of physiological non-toxic amounts of trivalent chromium across different regions of rat small intestine was investigated using the everted sac technique. A very small percentage of chromium was transported to the serosal compartment, and a larger percentage of chromium was rapidly taken up by the intestinal tissue. There was no specific site of absorption for chromium in the rat small intestine. The amount of chromium both transported to the serosal compartment and taken up by the tissue was dependent upon the initial concentration of chromium and no evidence of saturation was observed. There was however a good correlation between water and chromium transport across the jejunum and ileum. Changes in glucose concentration, temperature, or anoxic conditions appeared to have no affect on chromium transport to the serosal compartment. However transport was increased in the presence of decreased concentration of calcium ions and also in the presence of increased concentration of hydrogen ions. Bile salts and E.D.T.A. did not affect the serosal transport of chromium, however, the interaction of chromium with citric acid increased the amount of chromium transported to the serosal compartment. There was strong and tenacious interaction between the intestinal tissue and chromium ions which displayed characteristics of covalent bonding. Variations in glucose concentration and pH, as well as anoxic condition and high concentration of bile salts, markedly influence the interaction of chromium with the intestinal tissue. Lowered luminal volumes resulted in an increase in the tissue-chromium interaction. From the observations a model was developed to describe the transport mechanism for chromium, and factors which influence chromium absorption. Chromium crosses the intestinal epithelium by passive diffusion via the zonulae occludentes. Interaction of chromium with a dietary agent may increase the amount transported to the serosal compartment and result in the complex using an intracellular route. The tenacious binding of chromium to the intestinal tissue, possibly to phosphate groups, suggests that the intestinal barrier acts as a controlling factor for the amount of chromium entering the body pool, and also predicts other conditions that might affect the absorption of chromium.