Sodium-potassium ATPase gradients

Pumps move ions and molecules up their electrochemical gradient. Pumps require energy, usually in the form of ATP hydrolysis. Sodium-potassium ATPase is an example of a pump. Cells maintain a higher concentration of potassium inside the cell than they do outside the cell. Sodium is maintained low inside, high outside. Sodium-potassium ATPase pumps three sodium ions from inside the cell to outside. This is the unfavorable direction—Na+ moves from low concentration to a higher one and against the membrane potential. At the same time, it also... 

Nerve stimulation results in a net influx of sodium ions, and normal conditions are restored by the outward transport of sodium ions against an electrochemical gradient. While several earlier workers had identified ATPases in the sheath of giant squid axons, it was Skou who first connected the sodium, potassium ATPase [EC 3.6.1.37] with the ion flux of neurons. This discovery culminated... 

This must obviously be the opposite of passive transport. Active transport does require energy, usually in the form of the consumption of ATP or GTP, because the molecules are moving against the concentration gradient from an area of lower concentration to an area of higher concentration. The most well known active transport system is the Sodium-Potassium-ATPase Pump (Na" "- K+ZATPase) which maintains an imbalance of sodium and potassium ions inside and outside the membrane, respectively. See Figure 3. 

Many substances can be transported into the cell (and vice versa) against a concentration gradient. This is an active transport process, and it requires energy in the form of ATR It is to be distinguished from a passive transport process, which is simple diffusion across membranes. One of the better understood systems of this type is the sodium-potassium ATPase (or Na/K) pump, which maintains high potassium and low sodium levels in the cell. These are up to 160 meq/L for K+ and about 10 meq/L of Na+ inside the cell. Extracellular fluid contains about 145 meq/L of Na+ and 4 meq/L of K+. The simultaneous movement of one substance out of the cell and another into the cell is an antiport. A substantial percentage of the basal metabolic rate (see Chapter 21) is accounted for by the activity of the Na/K pump. ATPase (Na/K pump) is lo-... 

The predominant buffers in the urine are phosphate (HP042 ) and ammonia (NH3). Phosphate is freely filtered by the glomerulus and passes down the tubule where it combines with H+ to form Hd Oy. Hydrogen ions are secreted in exchange for sodium ions the energy for this exchange comes from the sodium-potassium ATPase that maintains the concentration gradient for sodium. 

The plasma membrane of neurons, like all other cells, has an unequal distribution of ions and electrical charges between its two sides. Sodium-potassium ATPase pumps maintain this unequal concentration by actively transporting ions against their concentration gradients sodium in, potassium out. The membrane is positive outside and negative inside. This charge difference is referred to as the resting potential and is measured in millivolts (=—65 mV). 

Figure 8.9. Physiology and molecular biology of intestinal bile acid transport. Bile acids are actively absorbed in enterocytes through a sodium-dependent cotransporter, ASBT. The sodium gradient is maintained by the sodium-potassium ATPase, located at the basolateral membrane. In the ( osol, bile acids are shuttled through the cell by the aid of various proteins, most importantly the ileal hille acid binding protein, iBABP. An anion exchanger transports bile acids across the basolateral membrane into the portal circulation.

Lithium readily enters excitable cells via sodium channels. It is not. however, removed well by the sodium/potassium ATPase exchange mechanism. It therefore accumulates within the cell. Thus, the intracellular concentration of potassium is decreased as the influx of potassium is reduced, both through inhibition of active transport (by Na+/K+ ATPase) and the decrease in the electrical gradient for potassium. Extracellular potassium therefore increases. The reversal in potassium levels results in a decrease in neuronal excitability, producing a therapeutic calming effect. 

Carrier-mediated transport involves cotransport of the absorbable species with a proton. The required proton gradient is hypothesized to be maintained by a Na+-H+ exchanger. The lumen of the intestine is acidic relative to the epithelial cell cytosol. The low cytosolic sodium concentration, required to produce the transporter driving force, is maintained by the Na K ATPase in the basolateral membrane. The sodium/proton exchanger working in concert with the sodium/potassium ATPase, therefore, results in a transport mechanism for the uptake of di- and tripeptides into the intestinal wall (Ganapthy and Leibach, 1985). 

Ascorbate is absorbed from the lumen of the human intestine by sodium-ascorbate cotransport in enterocytes, as has been shown by measuring transport activities in luminal (brush-border) membrane vesicles (Malo and Wilson, 2000). This phenomenon couples ascorbate uptake to the concentration gradient of sodinm ion across the plasma membrane that is maintained by sodium/potassium-ATPase. It is likely because of the limited capacity of enterocytes for sodium-ascorbate cotransport that large oral doses of ascorbate are absorbed less completely than are small doses. Absorption sites for ascorbate are found along the entire length of the small intestine (Malo and Wilson, 2000). 

Glucose and galactose are absorbed by a sodium-dependent active process (section 3.2.2.3). The sodium pump and the sodium/potassium ATPase create a sodium gradient across the membrane the sodium ions then re-enter the cell together with glucose or galactose. These two monosaccharides are carried by the same transport protein, and compete with each other for intestinal absorption. 

A well-known example of active transport is the sodium-potassium pump that maintains the imbalance of Na and ions across cytoplasmic membranes. Flere, the movement of ions is coupled to the hydrolysis of ATP to ADP and phosphate by the ATPase enzyme, liberating three Na+ out of the cell and pumping in two K [21-23]. Bacteria, mitochondria, and chloroplasts have a similar ion-driven uptake mechanism, but it works in reverse. Instead of ATP hydrolysis driving ion transport, H gradients across the membranes generate the synthesis of ATP from ADP and phosphate [24-27]. 

With active transport, energy is expended to move a substance against its concentration gradient from an area of low concentration to an area of high concentration. This process is used to accumulate a substance on one side of the plasma membrane or the other. The most common example of active transport is the sodium-potassium pump that involves the activity of Na+-K+ ATPase, an intrinsic membrane protein. For each ATP molecule hydrolyzed by Na+-K+ ATPase, this pump moves three Na+ ions out of the cell and two K+ ions into it. As will be discussed further in the next chapter, the activity of this pump contributes to the difference in composition of the extracellular and intracellular fluids necessary for nerve and muscle cells to function. 

Na+K+ ATPases, so-called sodium-potassium pumps (Figure 4.6), embedded in the basolat-eral membrane. The sodium-potassium pump is a highly conserved integral membrane protein, expressed in virtually all animal cells. The transport of sodium creates both an electrical and a chemical gradient across the plasma membrane. In turn this provides ... 

The structure of the cell membrane and a physiological sodium-potassium pump maintain the neural cell resting membrane potential (RMP). Driven by an AT-Pase enzyme, a physiological sodium potassium pump moves three sodium cations from the inside of the cell for every two potassium cations that it moves back in. The ATPase is necessary because of this movement of cations against their respective resting concentration and electrical gradients. 

Discovery and characterization of the actual molecular pump that establishes the sodium and potassium concentration gradient (Na > -ATPase) earned Jens Skou (Aarhus University, Denmark) half of the 1997 Nobel Prize in Chemistry. The other half went to Paul D. Boyer (UCLA) and John E. Walker (Cambridge) for elucidating the enzymatic mechanism of ATP synthesis. 

An essential requirement for diffusion of Na+ ions is the creation of a concentration gradient for sodium between the filtrate and intracellular fluid of the epithelial cells. This is accomplished by the active transport ofNa+ ions through the basolateral membrane of the epithelial cells (see Figure 19.4). Sodium is moved across this basolateral membrane and into the interstitial fluid surrounding the tubule by the Na+, K+-ATPase pump. As a result, the concentration of Na+ ions within the epithelial cells is reduced, facilitating the diffusion of Na+ ions into the cells across the luminal membrane. Potassium ions transported into the epithelial cells as a result of this pump diffuse back into the interstitial fluid (proximal tubule and Loop of Henle) or into the tubular lumen for excretion in the urine (distal tubule and collecting duct). 

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