Availability of polychlorinated biphenyls (PCBs) and lindane for uptake byintestinal Caco-2 cells

Children may ingest contaminated soil from hand to mouth. To assess this ex posure route, we need to know the oral bioavailability of the contaminants. Two determining steps in bioavailability of soil-borne contaminants are mo bilization from soil during digestion, which is followed by intestinal abso rption. The first step has been investigated in previous studies that showe d that a substantial fraction of PCBs and lindane is mobilized from soil du ring artificial digestion. Furthermore, almost all contaminants are sorbed to constituents of artificial human small intestinal fluid (i.e., chyme), w hereas only a small fraction is freely dissolved. In this study, we examine the second step using intestinal epithelial Caco-2 cells. The composition of the apical exposure medium was varied by addition of artificial chyme, b ile, or oleic acid at similar or increasing total contaminant concentration s. The uptake curves were described by rate constants. The uptake flux seem ed to be dose-dependent. Furthermore, different exposure media with similar total contaminant concentrations resulted in various uptake rates. This ca n be attributed to different freely dissolved concentrations and carrier ef fects. In addition, the large fractions of contaminants in the cells indica te that PCBs and lindane sorbed to bile, oleic acid, mid digestive proteins contributed to the uptake flux toward the cells. These results can be extr apolated qualitatively to in vivo conditions. Because the sorbed contaminan ts should be considered available for absorption, the first step of mobiliz ation from soil is the most important step for oral bioavailability of the presently investigated soil-borne contaminants.