Sodium pump, transport

Scheiner-Bobis G (2002) The sodium pump. Its molecular properties and mechanics of ion transport. Eur J Biochem. 269 2424—2433... 

Figure 2. Sodium and chloride uptake across an idealised freshwater-adapted gill epithelium (chloride cell), which has the typical characteristics of ion-transporting epithelia in eukaryotes. In the example, the abundance of fixed negative charges (muco-proteins) in the unstirred layer may generate a Donnan potential (mucus positive with respect to the water) which is a major part of the net transepithelial potential (serosal positive with respect to water). Mucus also contains carbonic anhydrase (CA) which facilitates dissipation of the [H+] and [HCO(] to CO2, thus maintaining the concentration gradients for these counter ions which partly contribute to Na+ import (secondary transport), whilst the main driving force is derived from the electrogenic sodium pump (see the text for details). Large arrow indicates water flow...

The experiments were continued by Hoffman and Whittam, who concluded that a protein, an ATPase, in the membrane was necessary for active transport and was vectorially organized, with ATP and Na+ being required internally and K+ externally where ouabain was inhibitory. The ATPase was finally identified as the sodium pump by Skou (1957) it vectorially translocated Na+ and K+ across the membrane, and was phosphorylated transiently in the process. 

Amino add reabsorption in the renal tubules Amino acids are small, easily filtered molecules. Efficient reabsorption mechanisms are vital to conserve amino acids which are metabolically valuable resources. Transport of individual amino acids and small peptides is symport carrier mediated mechanisms in which sodium is co-transported. The process is indirectly ATP dependent because Na is returned to the lumen of the nephron by the sodium pump , Na+/K+ dependent ATPase. 

D) Their interaction with the sodium pump results in an inhibition of calcium transport. 

Figure 12.4 Mechanism of action of Na+/K+symport inhibitors (thiazides) on the distal convoluted tubule. As in the other parts of the nephron, Na+movement is powered by the energy-requiring sodium pump (P) in the basolateral membrane which exchanges intracellular Na+for K-i-in the extracellular fluid (ECF). The transport of Na-rand Cl- into the cell from the filtrate against the prevailing electrochemical gradient is facilitated by the symporter (S). The Na-Hons are then transported by the pump mechanism described above and the Cl- ions diffuse passively Into the ECF through ion channels in the basolateral membrane. Thiazide diuretics inhibit the symporter by disabling the Cl- binding site with the loss of Na-rand Cl- in the urine.

The most universal transport systems are those involved in the transport of the ubiquitous inorganic ions, sodium, potassium and calcium1. The sodium pump counteracts passive water movement across the cell membrane by removing sodium ions together with chloride or other anions from the cytoplasm to lower its content of osmotically active substances. In most cells, however, the elimination of sodium ions is connected with an accumulation of potassium ions6. For three sodium ions leaving the cell two potassium ions are taken up9,10). The resulting concentration... 

In addition to the Na+,K+- ATPases there is a very active Ca2+-ATPase which transports two Ca2+ from the inside of cells to the outside while returning two H+ from outside per ATP.510 543a This is the primary transporter by which cells maintain a low internal [Ca2+]. During its action it becomes phosphorylated on Asp 351. However, in neurons, in which the membrane potential is maintained at a high negative value by the sodium pump, an Na+/ Ca2+ ion exchange plays an even more important role.540... 

A full consideration of the mechanism of the sodium pump requires an account of the role of the lipid, the binding sites for Na+, K+, Mg2+ and ATP, the mechanism of hydrolysis of ATP and the way in which this is coupled to the transport of the cation. In addition it should be noted that the enzyme also functions as a K+-dependent phosphatase, a reaction usually studied with p-nitrophenyl phosphate as substrate. Studies with inhibitors have been informative, notably with ouabain and with vanadate. Ouabain binds at one site per pump and so has been of value in quantitatively defining the enzyme in various preparations. 

The best studied example of a Group IIA cation transport system is the calcium pump of the sarcoplasmic reticulum of skeletal muscle. Indeed, the calcium pump and the sodium pump represent the most studied of all transport processes. The calcium pump involves a membrane-bound (Ca2+, Mg2+)-ATPase and uptake of Ca2+ is associated with hydrolysis of ATP. While the... 

This simple experiment was important in that it clearly established the key notion that cellular extrusion of sodium ions by the sodium pump was coupled to metabolism. Because in this and subsequent experiments of the same sort the electrochemical gradient for sodium was known precisely, and since the fluxes of sodium (and later potassium) both into and out of the cell could be measured independently, this study also laid the groundwork for a theoretical definition of active transport, a theory worked out independently by Ussing in the flux ratio equation for transepithelial active transport of ions (see below). 

Mineralocorticoids are believed to increase sodium reabsorption by affecting sodium channels and sodium pumps on the epithelial cells lining the renal tubules.18,58 Mineralocorticoids ability to increase the expression of sodium channels is illustrated in Figure 29-5. These hormones enter the tubular epithelial cell, bind to receptors in the cell, and create an activated hormone-receptor complex.18 This complex then travels to the nucleus to initiate transcription of messenger RNA units, which are translated into specific membrane-related proteins.27,58 These proteins in some way either create or help open sodium pores on the cell membrane, thus allowing sodium to leave the tubule and enter the epithelial cell by passive diffusion.27,83 Sodium is then actively transported out of the cell and reabsorbed into the bloodstream. Water reabsorption is increased as water follows the sodium movement back into the bloodstream. As sodium is reabsorbed, potassium is secreted by a sodium-potassium exchange, thus increasing potassium excretion (see Fig. 29-5). 

Schematic diagram of a cardiac muscle sarcomere, with sites of action of several drugs that alter contractility (numbered structures). Site 1 is Na+/K+ ATPase, the sodium pump. Site 2 is the sodium/calcium exchanger. Site 3 is the voltage-gated calcium channel. Site 4 is a calcium transporter that pumps calcium into the sarcoplasmic reticulum (SR). Site 5 is a calcium channel in the membrane of the SR that is triggered to release stored calcium by activator calcium. Site 6 is the actin-troponin-tropomyosin complex at which activator calcium brings about the contractile interaction of actin and myosin.

Like the 5-HT1A receptor (see Section 2.1), the 5-HT2A receptor can regulate several transport processes. The 5-HT2A receptor activates the type 1 sodium-proton exchanger (NHE-1) in renal mesangial cells (187,227) and vascular smooth muscle cells (222), the Na+K+-AIPase (sodium pump) in airway smooth muscle cells (228), and the Na+/K+/2Cr cotransporter when 5-HT2A receptor transfected... 

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