Proposed pathways for movement of magnesium (Mg) across the intestinal epithelium. Two possible routes exist for the absorption of Mg across intestinal epithelial cells: the transcel-lular route and the intercellular pathway. Although a transcellular route has not yet been demonstrated, its existence is inferred from several observations. No large chemical gradient exists for Mg movement across the cell membrane; however, a significant uphill electrical gradient exists for the exit of Mg from cells. This finding suggests the existence and participation of an energy-dependent mechanism for extrusion of Mg from intestinal cells. If such a system exists, it is believed it would consist of two stages. 1) Mg would enter the apical membrane of intestinal cells by way of a passive carrier or facilitated diffusion. 2) An active Mg pump in the basolateral section of the cell would extrude Mg. The intercellular movement of Mg has been demonstrated to occur by both gradient-driven and solvent-drag mechanisms. This intercellular path may be the only means by which Mg moves across the intestinal epithelium. The change in transport rates at low Mg concentrations would reflect changes in the "openness" of this pathway. High concentrations of luminal Mg (eg, after a meal) are capable of altering the morphology of the tight junction complex. High local Mg concentrations near the intercellular junction also can affect the activities of local membrane-associated proteins (eg, sodium-potassium adenosine triphosphate [Na-K ATPase]) near the tight junction and affect its permeability (see Fig. 4-6) [13-15].