The sodium–potassium pump (sodium–potassium adenosine triphosphatase, also known as -ATPase, pump, or sodium–potassium ATPase) is an enzyme (an electrogenic transmembrane ATPase) found in the membrane of all animal cells. It performs several functions in cell physiology.
The -ATPase enzyme is active (i.e. it uses energy from ATP). For every ATP molecule that the pump uses, three sodium ions are exported and two potassium ions are imported. Thus, there is a net export of a single positive charge per pump cycle. The net effect is an extracellular concentration of sodium ions which is 5 times the intracellular concentration, and an intracellular concentration of potassium ions which is 30 times the extracellular concentration.
The sodium–potassium pump was discovered in 1957 by the Danish scientist Jens Christian Skou, who was awarded a Nobel Prize for his work in 1997. Its discovery marked an important step forward in the understanding of how ions get into and out of cells, and it has particular significance for excitable cells such as nerve cells, which depend on this pump to respond to stimuli and transmit impulses.
All mammals have four different sodium pump sub-types, or isoforms. Each has unique properties and tissue expression patterns. This enzyme belongs to the family of P-type ATPases.
Function
The -ATPase helps maintain resting potential, affects transport, and regulates cellular volume. It also functions as a signal transducer/integrator to regulate the MAPK pathway, reactive oxygen species (ROS), as well as intracellular calcium. In fact, all cells expend a large fraction of the ATP they produce (typically 30% and up to 70% in nerve cells) to maintain their required cytosolic Na and K concentrations. For neurons, the -ATPase can be responsible for up to 3/4 of the cell's energy expenditure. In many types of tissue, ATP consumption by the -ATPases have been related to glycolysis. This was first discovered in red blood cells (Schrier, 1966), but has later been evidenced in renal cells, smooth muscles surrounding the blood vessels, and cardiac purkinje cells. Recently, glycolysis has also been shown to be of particular importance for -ATPase in skeletal muscles, where inhibition of glycogen breakdown (a substrate for glycolysis) leads to reduced -ATPase activity and lower force production.
Resting potential
In order to maintain the cell membrane potential, cells keep a low concentration of sodium ions and high levels of potassium ions within the cell (intracellular). The sodium–potassium pump mechanism moves 3 sodium ions out and moves 2 potassium ions in, thus, in total, removing one positive charge carrier from the intracellular space (see for details). In addition, there is a short-circuit channel (i.e. a highly K-permeable ion channel) for potassium in the membrane, thus the voltage across the plasma membrane is close to the Nernst potential of potassium.
Reversal potential
Even if both and ions have the same charge, they can still have very different equilibrium potentials for both outside and/or inside concentrations. The sodium-potassium pump moves toward a nonequilibrium state with the relative concentrations of and for both inside and outside of cell. For instance, the concentration of in cytosol is 100 mM, whereas the concentration of is 10 mM. On the other hand, in extracellular space, the concentration of is 5 mM, whereas the concentration of is 150 mM.
Transport
Export of sodium ions from the cell provides the driving force for several secondary active transporters such as membrane transport proteins, which import glucose, amino acids and other nutrients into the cell by use of the sodium ion gradient.
Another important task of the - pump is to provide a gradient that is used by certain carrier processes. In the gut, for example, sodium is transported out of the reabsorbing cell on the blood (interstitial fluid) side via the - pump, whereas, on the reabsorbing (lumenal) side, the -glucose symporter uses the created gradient as a source of energy to import both and glucose, which is far more efficient than simple diffusion. Similar processes are located in the renal tubular system.