Facilitative glucose transporters other

GLUT 5 A facilitative glucose transporter isoform present in the small intestine and other tissues that will transport fructose (and glucose to a lesser extent) across the plasma membrane. 

If insulin acts by facilitating glucose transport through the cell membrane, the mode of action of insulin must be studied at a level of integration other than the one at which a hormone acts directly on an enzyme. 

Amino acid transporters, oligopeptide transporters, glucose transporters, lactic acid transporters, monocarboxylic acid transporters, phosphate transporters, bile acid transporters and other transporters present on the apical membrane of the epithelial cells serve as carriers to facilitate nutrient absorption by the intestine. On the basolateral membrane, amino acid and oligopeptide transporters also exist. Drag moieties possessing similar structures to nutrients that are absorbed by such carriers may also be absorbed in this manner. 

Other cotransporters facilitate the transport of other sugars, osmolytes, and amino acids. In humans, a disorder of intestinal glucose and galactose absorption is due to a defective sodium-glucose transporter. 

In muscle and adipose tissue, insulin promotes transport of glucose and other monosaccharides across cell membranes it al.so facilitates tran.sport of amino icids, potassium ion.s. nucleosides, and ionic phosphate. Insulin also activates certain enzymes—kinases and glycogen. synthetase in muscle und adipose tissue. In adipose tissue, insulin decreases the release of fatty acids induced by epinephrine or glucagon. cAMP promotes fatty acid release from adipose ti.ssue therefore. it is pos.sible that insulin decreases fatty acid release by reducing tissue levels of cAMP. Insulin also facilitates the incorporation of intracellular amino acids into protein. 

Another kind of pore is gramicidin A, which is a simple 15-residue polypeptide that allows potassium and sodium ions to pass through it (Figure 10,22). Still another pore-facilitated system is that of the glucose transport protein of erythrocytes which strongly favors transport of D-glucose over other sugars. 

Glucose, galactose, and fructose formed by the digestive enzymes are transported into the absorptive epithelial cells of the small intestine by protein-mediated Na -dependent active transport and facilitative diffusion. Monosaccharides are transported from these cells into the blood and circulate to the liver and peripheral tissues, where they are taken up by facilitative transporters. Facilitative transport of glucose across epithelial cells and other cell membranes is mediated by a family of tissue-specific glucose transport proteins (GLUT I-V). The type of transporter found in each cell reflects the role of glucose metabolism in that cell. 

As with glucose transport, the Na -dependent carriers of the apical membrane of the intestinal epithelial cells are also present in the renal epithelium. However, different isozymes are present in the cell membranes of other tissues. Conversely, the facilitated tiansport carriers in the serosal membrane of the intestinal epithelia are similar to those found in other cell types in the body. During starvation, the intestinal epithelia, like these other cells, take up amino acids from the blood to use as an energy source. Thus, amino acid transport across the serosal membrane is bidirectional. 

Amino acids that enter the blood are transported across cell membranes of the various tissues principally by Na -dependent cotransporters and, to a lesser extent, by facilitated transporters (Table 37.1). In this respect, amino acid transport differs from glucose transport, which is Na -dependent transport in the intestinal and renal epithelium but facilitated transport in other cell types. The Na dependence of amino acid transport in liver, muscle, and other tissues allows these cells to concentrate amino acids from the blood. These transport proteins have a different genetic basis, amino acid composition, and somewhat different specificity than those in the luminal membrane of intestinal epithelia. They also differ somewhat between tissues. For instance, the N system for glntamine nptake is present in the liver bnt either not present in other tissues or present as an isoform with different properties. There is also some overlap in specificity of the transport proteins, with most amino acids being transported by more than one carrier. 

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