ABSTRACT
Reference has been made repeatedly in earlier chapters to the determining role played by cell membranes in the handling of nonlipophilic xenobiotics in the body. The role is critical for such toxicants because they generally do not passively permeate through cell membranes. In consequence, their absorption into the body, their distribution to different organs, and their accumulation at intracellular target sites, as well as frequently their extrusion from cells, in general require mediation by more-or-less specific facilitated diffusion or active transport mechanisms in cell membranes. Fi nally, excretion of xenobiotics from the body, for instance through the liver or the kidneys, also is controlled by cell membranes: A polar solute is not reabsorbed from the glomerular filtrate or secreted into the renal tubule in the absence of special membrane transport mechanisms. In other words, membrane properties and func tion largely determine the toxicokinetics of these toxic agents. In contrast, uptake, distribution, and excretion of lipophilic solutes, to which membranes generally are relatively freely and passively permeable, are little affected by membranes.
