Transmural Movements of Zinc, Manganese, Cadmium and Mercury by Rat Small Intestine

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Abstract
To elucidate some features of the mechanism of distribution and localization of trace metals in soft mammalian tissues, we studied the simultaneous uptake and transport patterns of 2 essential metals, Zn and Mn, and 2 nonessential metals, Cd and Hg. The radioisotopes of the metals and loops of rat small intestine were used in a new in vitro perfusion method. The characteristic rates of metal uptake and metal transport varied markedly. During the first hour of perfusion, very small amounts of Zn, Hg or Cd were transported across the rat intestinal wall, whereas Mn was transported to an appreciable extent. Except for Mn, when the tissue uptake of a metal was high, its accumulation in the serosal solution was proportionately slow. The relative uptake-to-transport molar ratios for Zn, Hg, Cd and Mn were approximately 20:12:6:1, respectively. The uptake or transport of a metal was strikingly enhanced or depressed by the presence of a second metal particularly when the initial concentration of the second metal exceeded the concentration of the first. The effects of each of the chemical agents, L-ascorbate, dehydroascorbate, phlorhizin, iodoacetate and EDTA on the simultaneous uptake and transport of each of these 4 metals were measured. Zinc, Cd and Mn uptakes were depressed by L-ascorbate, dehydroascorbate or EDTA, whereas their transports were enhanced, except for Mn transport which was unaffected by L-ascorbate. Iodoacetate depressed Zn and Hg transports but markedly enhanced the uptakes of Zn, Hg, and Cd. Phlorhizin enhanced the uptake and depressed the transport of Cd nearly 3-fold. Both Zn uptake and transport were enhanced by phlorhizin. The transmural passages of this tetrad of metals Zn, Mn, Cd, and Hg are not dependent on the expenditure of metabolic energy, are not unidirectional, and are not carried against a concentration difference. Competition among these metals for uptake and transport appears to be primarily a competition for the common available uptake sites. Transport is controlled to a lesser degree by the diffusibility characteristics of the ions of the particular metal.