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MECHANISMS OF CADMIUM ABSORPTION IN RATS
Citation:
Foulkes, E., D. Johnson, N. Sugawara, R. Bonewitz, AND C. Voner. MECHANISMS OF CADMIUM ABSORPTION IN RATS. U.S. Environmental Protection Agency, Washington, D.C., EPA/600/1-81/063 (NTIS PB82108184), 1981.
Description:
This study was undertaken in order to help clarify the factors which determine the fractional absorption of an oral load of cadmium (Cd) from the intestine of the rat. The experiments utilized intact segments of intestine, perfused or incubated in situ with their blood supply intact. Absorption of Cd from the jejunal lumen can be ascribed to a saturable membrane system; that is, after short periods of exposure essentially all the metal removed from the lumen was recovered in mucosal tissue (Step I). The second step in Cd absorption, i.e., transfer of the metal from mucosa into blood, proceeded at only 1-2% of the rate of uptake from the lumen (Step I). No evidence was obtained for a role of metallothionein in the mucosal retention of Cd. Step I of Cd absorption was inhibited by a variety of exogenous and endogenous factors. Thus, zinc was found to depress Cd transport in an apparently competitive manner. Addition of milk to the lumen also inhibited Cd uptake, an effect entirely due to the Ca content. Bile salts act as endogenous modulators of Cd absorption; their effect may be related to micelle formation. The research also included studies of duodenal and ileal Cd transport. Ileal Cd absorption differed from that in the jejunum by a relatively much faster Step II. Unlike the low ratio of Steps II/I for the toxic metal in the jejunum, the ratio for the essential metals Cu and Zn was much higher (about 50%). Absorption of Cd by the gut in neonatal rats proceeded much faster than in adults; reasons for this difference have not yet been clarified. Another question remaining under study is the extent to which different metals such as Cd and Zn share common absorptive mechanisms.