Inorganic mercury absorption in mature and immature rat jejunum: transcellular and intercellular pathways in vivo and in everted sacs

The objective of the present work was to test the hypothesis that toxic and nonessential heavy metals in general, and inorganic Hg in particular, are taken up in rat jejunum by a mechanism similar to that previously proposed for Cd. The initiating step in Cd absorption involves electrostatic interaction of the cation with negative membrane charges. In contrast, Hg appears to react with apical membranes as an anion, although overall uptake is not inhibited by the stilbene inhibitor of anion transport DIDS. Further, the ratio of Cd/Hg uptake falls along the jejunum. However, several similarities between Cd and Hg uptakes were observed. Neither process depends on reactive sulfhydryl groups or on oxidative metabolism. Although tissue Hg, unlike Cd, could not be rigorously separated into membrane-bound and intracellular compartments, uptake of both metals includes their relatively temperature-insensitive and rapid influx into a pool readily accessible to suitable extracellular chelators. Another more temperature-sensitive and slower component leads to the filling of a relatively chelation-resistant compartment, identified in the case of Cd as intracellular. Non-specific membrane properties such as surface charge and perhaps membrane fluidity can thus account for mucosal Cd and Hg uptakes; no contribution of specific transport systems need be postulated. Mucosal permeability, calculated in reference to ethanol uptake as measure of absorbing surface area, falls to the same extent for Cd and Hg during postweaning development. Because this fall, however, parallels a diminution in polar diffusion pathways, the mechanism of the faster metal absorption in weanlings may have little bearing on the slower transcellular process in adults. Incidentally, polar diffusion pathways increase in mature jejunum upon preparation of everted sacs, a further limitation on the usefulness of this preparation for the quantitative study of normal intestinal solute transport.