Effects of sodium on solute transport between compartments in intestinal mucosal epithelium

THE uptake of amino acids and of some monosaccharides into the intestinal mucosal cell across the brush-border membranes requires sodium in the intestinal lumen1–3. An hypothesis of the existence in brush-border membranes of systems for transport that depend on Na—for example, the ‘Na gradient’ hypothesis4—although necessary, is not sufficient to account for all the effects of Na on epithelial transport. Whereas in in vitro conditions5the accumulation of amino acids and monosaccharides within the epithelium depends markedly on Na in the intestinal lumen, movement of these substances from the lumen into the blood in in vivo conditions is less dependent on Na in the lumen6–9.