Permeability characteristics of the human small intestine.

Abstract

Relatively little information is available concern-ing the membrane structure of the mucosal cells of the small intestine. Hdber and Hober (1) and Schanker, Tocco, Brodie, and Hogben (2, 3) studied the absorption of a variety of substances in rats and found that absorption rate increased as lipid solubility increased. From this it has been deduced that mucosal cell membranes are lipoid in nature and that lipid-soluble substances are ab-sorbed by dissolving in the cell membrane. How-ever, it has been known for many years that small molecules, although lipid insoluble, can also be ab-sorbed from the gastrointestinal tract. This has led to the hypothesis that, although essentially lipoidal, cell membranes are interspersed with wa-ter-filled pores, through which small molecules can diffuse. Hbber and Hdber (1) tested this hy-pothesis in the small intestine of the rat by corre-lating the absorption rate of nonlipid-soluble sub-stances with their molecular size. Their results support the thesis that these molecules are ab-sorbed by diffusion through water-filled pores, since small molecules were absorbed more rapidly than larger ones, and beyond a certain size (mo-lecular weight about 180, which corresponds to a molecular radius of about 4 A) no penetration oc-curred. Lindemann and Solomon's studies (4) are in close agreement, since they experimentally determined, by an independent method, the pore radius of the luminal surface of the rat jejunal cells