Metal transport in cells: cadmium uptake by rat hepatocytes and renal cortical epithelial cells.

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Abstract
The toxic metals appear to use the transport pathways that exist for biologically essential metals. In this regard interactions between the toxic and essential metals are possible. This report summarizes recent findings on the transport of cadmium in rat hepatocytes and renal cortical epithelial cells in the presence or absence of certain essential metals. The transport of cadmium in hepatocytes does not require energy and, therefore, is not an active process. It occurs primarily (80%) by temperature-sensitive processes, i.e., ion channels and carriers, that involve interaction with sulfhydryl groups. These processes apparently exist for the transport of essential metals like copper, zinc and calcium. The remaining 20% of the cadmium in hepatocytes is transported via a temperature-insensitive process, possibly by diffusion. In comparison with the hepatocytes, a smaller fraction (30%) of the cadmium transport through the basolateral membrane and none from the apical membrane of the renal cortical epithelial cells is temperature-sensitive. Total accumulation through the basolateral membrane is about twice that through the apical membrane. A majority of the cadmium transport in the renal cells is by diffusion. As in hepatocytes, copper, zinc and mercury antagonize cadmium transport through the apical membranes of the renal cells. The relative antagonism by copper is the same (25%); however, the antagonism by zinc (16%) and mercury (10%) is 4- to 6-fold lower than in hepatocytes. It appears that the relative contribution of various transport pathways available for cadmium uptake is different in each cell type and apparently depends on the morphological and functional differences between the cell membranes.