Intestinal Secretion of Organic Ions

Abstract

Active absorption of nutrients has been known for a long time. Active secretion of foreign compounds (xenobiotics) by the mucosal epithelium of the intestine, however, is a rather recent discovery. It has been demonstrated first for cardiotonic steroids. After i.v. administration a number of these uncharged compounds are concentrated in the intestinal fluid of rats and guinea pigs above the blood level (Lauterbach 1971a,b, 1975, 1977a; for review see Lauterbach 1981). In these as well as in other studies on the phenomenon of intestinal secretion the in vitro method of the isolated mucosa of guinea pig intestine appeared to be especially useful. Permeation by diffusion as well as absorptive and secretory transport processes are easily differentiated by comparing transepithelial fluxes across the tiny mucosal sheet devoid of additional tissue layers and mounted in a flux chamber. Furthermore, determination of tissue content at the end of the experiments allows conclusions on the respective role of the luminal and basolateral membranes of the enterocytes in the transport processes (Fig. 1) (Lauterbach 1977b).