Intestinal Permeability of Water-Soluble Nonelectrolytes: Sugars, Amino Acids, Peptides

Abstract

The epithelial barrier is a layer of cells separating the lumen (the external environment) from the internal milieu of the organism. Most of these cells are columnar absorbing cells, also named enterocytes. Their main function is to absorb and/or secrete solutes and water selectively. Many of these solutes are water-soluble nonelectrolyte molecules such as nutrients, e.g., sugars and amino acids. In order to be transported from the lumen to the subepithelial spaces and then to the bloodstream or the lymph, these molecules must cross two in-series plasma membranes: the brush border and the basolateral membrane. However, these substances have dimensions that hinder their movement through water pores of plasma membranes or even through junctional complexes between cells; therefore, specific transport mechanisms have developed to allow the transepithelial movement of these solutes. In order to achieve a net transport, these mechanisms must be asymmetrically distributed in the two opposite plasma membranes: the luminal and the contraluminal (Fig. 1).