Carbohydrate absorption. Studies on the glucose transport by isolated brush border membranes. A contribution towards an understanding of the molecular mechanism of sugar absorption.

From investigations, mainly on intact in vitro preparations of small intestine, it has been established that the epithelial cell carries out digestion as well as absorption of carbohydrates, i.e. the luminal (brush border) plasma membrane contains di- and poly-saccharidases for the degradation of carbohydrates to monosaccharides as well as transport systems for monosaccharides. Recently, an intact glucose transport system has been demonstrated in isolated membranes from rat intestinal brush borders. In the isolated membranes, glucose transport has the characteristics expected for 'facilitated diffusion' of glucose coupled to Na+ translocation. The Na+-dependent uptake of glucose can be influenced by electrical potentials across the brush border membrane, which indicates that the positive charge of Na+, associated with sugar uptake, is not compensated by the counter-movement of another cation, e.g. K+, or by the co-movement of an anion via the same glucose 'carrier'. In other words, the Na+-coupled D-glucose transport is linked to a charge translocation - it is electrogenic. The apparent Na+ independence of glucose transport in vivo and the accumulation of glucose against a glucose as well as a Na+ concentration gradient by isolated epithelial cells may be explained by the 'ion gradient' model of Crane and Schultz and Zalusky provided the electrical membrane potential is high under these conditions and the electrochemical potential of Na+ serves as driving force for 'active' glucose transport.