Na+-dependent transport

Recently, molecular biology studies have been carried out on hepatic uptake transporters. With regard to the Na+-dependent hepatic uptake of bile acids, Na+-taurocholate cotransporting polypeptide (Ntcp/NTCP) has been cloned from both rodents and humans [14-17]. Ntcp/NTCP accepts bile salts, such as taurocholate and glycocholate, as well as some anionic compounds such as dehydroepian-drosterone sulfate and bromosulfophthalein [16, 18]. However, the presence of unidentified Na+-dependent transporters for anionic drugs (e.g., bumetanide) has also been suggested [19, 20]. 

Besides by interfering with the general energy maintenance of the cell, impact of active transport during intracellular accumulation can also be investigated by direct inhibition of certain transport mechanisms. To detect contribution of Na+-dependent transport, the Na+/K+-ATPase, one of the primary active transport systems of the cell, can be inhibited by ouabain at 10 /uM. Higher amounts of the inhibitor may seriously impede with the viability of the cell. To obtain reliable results, the cells are pretreated with ouabain for 15 min prior to addition of the analyte. Usually, the inhibitory effect of ouabain lasts up to 45 min [29],... 

Both specificity studies confirmed that bromosulphthalein (BSP) competitively inhibited taurocholate transport by NTCP and OATP. This is in conflict with reports that BSP transport was not sodium dependent, suggesting that OATP was responsible.The reason for this dilference is not clear but may reflect dilferences in the approaches, using isolated rat hepatocytes or transfection to produce cells that stably express the protein. Choice of cell line may also be important as expression of MEH also showed dilferences, with no demonstrable Na" -dependent transport of taurocholate in Syrian hamster kidney cells or oocytes but Na" -dependent transport was shown in Mardin-Darby canine... 

The transport of amino acids at the BBB differs depending on their chemical class and the dual function of some amino acids as nutrients and neurotransmitters. Essential large neutral amino acids are shuttled into the brain by facilitated transport via the large neutral amino acid transporter (LAT) system [29] and display rapid equilibration between plasma and brain concentrations on a minute time scale. The LAT-system at the BBB shows a much lower Km for its substrates compared to the analogous L-system of peripheral tissues and its mRNA is highly expressed in brain endothelial cells (100-fold abundance compared to other tissues). Cationic amino acids are taken up into the brain by a different facilitative transporter, designated as the y system, which is present on the luminal and abluminal endothelial membrane. In contrast, active Na -dependent transporters for small neutral amino acids (A-system ASC-system) and cationic amino acids (B° system), appear to be confined to the abluminal surface and may be involved in removal of amino acids from brain extracellular fluid [30]. Carrier-mediated BBB transport includes monocarboxylic acids (pyruvate), amines (choline), nucleosides (adenosine), purine bases (adenine), panthotenate, thiamine, and thyroid hormones (T3), with a representative substrate given in parentheses [31]. 

An increase of intracellular adenosine levels can also be achieved by inhibition of nucleoside transport proteins. Mammalian nucleoside transport processes can be classified into two types on the basis of their thermodynamic properties. These classes are the concentrative, Na+-dependent transport processes and the equilibrative, Na+-independent processes. The corresponding transporters are called CNTs (concentrative nucleoside transporters) and ENTs (equilibrative nucleoside transporters) (Pastor-Anglada and Baldwin, 2001). 

Crane, R.K. 1965. Na-dependent transport in the intestine and other animal tissues. Fed Proc 24 1000. 

Na -Dependent Transport A General Mechanism for Energization of Transport 88... 

Two hypotheses have been proposed (Eisenman, 1962 Mullins, 1975) to account for the high Na+/K+ selectivity in Na+ dependent transport systems. Mullins emphasizes a geometrical fit of the cation. Eisenman emphasizes the electrical field strength of the putative binding sites. Many of the predictions made by Eisenman have been borne out experimentally in the ion-coupled systems and are complemented by the construction of ion-specific glass electrodes (Eisenman, 1962). 

Glucagon and exogenous cAMP stimulate the Na+-dependent transport of alanine and certain other amino acids into the perfused liver [176] and isolated hepa-tocytes [177-179], There is a rapid initial stimulation of the transport [177, 178] which is probably related to the stimulation of (Na2+-K+)-ATPase activity and membrane hyperpolarization [177], This is followed after 30-90 min by a larger increase which is blocked by cycloheximide [178]. Kinetic analysis indicates that both the short and long term actions of glucagon result in an increase in the Vmax for transport [177,179,180], and it has been proposed that the slower effect is due to the synthesis of a high affinity amino acid transport component [179,180],... 

A. M. Lynch and J, D, McGivan, Evidence for a single common Na+-dependent transport system for alanine, glutamine, leucine and phenylalanine in brush-border membrane vesicles from bovine kidney, Biochim. Biophys. Acta, 899 176-184 (1987). 

M. R. Hammerman and B. Sacktor, Na +-dependent transport of glycine in renal brush border membrane vesicles Evidence fora single specific transport system, Biochim, Biophys. Acta, 686 189-196 (1982). 

About 85% of the body s phosphate occurs in bones, with 14% in soft tissues and about 1.0% in the extracellular fluids. The normal range of phosphate intake is 20 to 50 mmol/day (0-6-1.5 g phosphorus/day). Phosphate is absorbed throughout the small intestines. In the duodenum, it is absorb by an Na-dependent transport mechanism. Here, transport of the phosphate is coupled with die cotransport of a sodium ion. The rate of Na-dependent transport of phosphate is enhanced by 1,25-(0H)2D3, Phosphate transport in the jejunum and ileum occurs by a passive mechanism. The rate of phosphate transport in this case is dependent mainly on the concentration of phosphate in the lumen and is independent of the levels of other nutrients and independent of energy-using processes. About 200 mg of phosphorus is excreted per day in fluids of the gastnointeslinai tract. About two-thirds of this phosphorus is reabsorbed by the gut. 

Szczepanska-Konkel M, Yusufi ANK, VanScoy M, Webster SK, DousaTP. Phosphonocarboxylic acids as specific inhibitors of Na -dependent transport of phosphate across renal brush border membrane. J Biol Chem 1986 261 6375-6383. 

Osteoblasts secrete osteoid, a matrix rich in type I collagen fibers and vesicles. Precipitation of calcium phosphate is inhibited by a high concentration of pyrophosphate in stromal interstitial fluids, and a high concentration also of albumin and citrate in blood plasma. Pyrophosphate is derived from (1) transport out of the cytosol, and (2) synthesis from nucleoside triphosphates in the stromal interstitial fluid that permeates the osteoid matrix. Precipitation occurs only when calcium and phosphate ions are taken up into vesicles whose inner membrane is composed of phosphatidylserine. The high concentration of calcium and phosphate ions in the vesicle is mediated by annexin and type HI Pi Na-dependent transporters. This overwhelms the pyrophosphate and nucleation occurs. As the precipitate grows and ruptures the membrane, tissue-nonspecific alkaline phosphatase is activated to remove pyrophosphate from the osteoid matrix fluid so that calcium phosphate precipitates around phosphorylated serine residues within the collagen fibers. 

For the absorption of carbohydrates, amino acids, and peptides, a variety of transport systems following facilitated diffusion and active mechanisms have been identified on a molecular and functional level. D-Glucose is mainly absorbed via the Na -dependent transporter SGLTl in the brush-border membrane of enterocytes [18-20]. It is transported across the basolateral membrane by facilitated diffusion via the hexose transporter GLUT-2. Besides SGLTl, the Na +-independent transport protein GLUT-5 is localized in the apical enterocyte membrane, recognizing fructose as a substrate [21]. 

Amino acids are absorbed by Na +-dependent transport proteins, with different proteins for acidic, neutral, and anionic amino acids. Small peptides are absorbed by the peptide transporter PEPTl in the brush-border membrane [11]. The PEPTl mRNA pattern exhibits regional differences with duodenum > jejunum > ileum [22]. The carrier works as an H cotransport system and recognizes 2-3-amino acid-long small peptides as well as drugs with peptide-like structures such as renin inhibitors, ACE inhibitors [23], or P-lactam antibiotics (Figure 9.7). It also serves as a target for... 

Free amino acids are transported into enterocytes by four active, carrier-mediated, Na+-dependent transport systems remarkably similar to the system for glucose. These systems transport, respectively, neutral amino acids basic amino acids (Lys, Arg, His) and cystine aspartic and glutamic acids and glycine and imino acids. Some amino acids (e.g., glycine) have affinities for more... 

Yatsushiro S, Yamada H, Kozaki S, Kumon H, Michibata H, Yamamoto A, Moriyama Y (1997) L-aspartate but not the D form is secreted through microvesicle-mediated exocytosis and is sequestered through Na -dependent transporter in rat pinealocytes. J Neurochem 69 340-347. 

Na+-independent and also Na+-dependent GSH transport systems have been found in lens epithelium, retinal Muller cells, brain endothehal cells and astrocytes [82-84]. The Na+-dependent transport mediates GSH uptake,whereas the Na+-independent carrier appears to be mainly involved in GSH efflux. It is worth noting that these transport systems allow GSH transport across the blood-brain barrier in vivo [85]. 

Transport of amino acids into cells is mediated by specific membrane-bound transport proteins, several of which have been identified in mammalian cells. They differ in their specificity for the types of amino acids transported and in whether the transport process is linked to the movement of Na+ across the plasma membrane. (Recall that the gradient created by the active transport of Na+ can move molecules across membrane. Na+-dependent amino acid transport is similar to that observed in the glucose transport process illustrated in Figure 11.28.) For example, several Na+-dependent transport systems have been identified within the lumenal plasma membrane of enterocytes. Na+-independent transport systems are responsible for transporting amino acids across the portion of enterocyte plasma membrane in contact with blood vessels. The y-glutamyl cycle (Section 14.3) is believed to assist in transporting some amino acids into specific tissues (i.e., brain, intestine, and kidney). 

The effect of CPH-treatment upon the Na -depen-dent transport of the amino acid L-alanine was investigated [75]. The results of these studies showed that Na+-dependent transport of L-alanine was also reduced by the treatment with CPH and the overshoot phenomenon completely eliminated (Figure 7). In con-... 

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