Sodium-potassium pump

NaKATPase (also known as the Na⁺/K⁺ pump or Na⁺/K⁺ exchanger) is an enzyme located in the plasma membrane (specifically an electrogenic transmembrane ATPase). It is evolutionarily conserved, found in the plasma membrane of virtually every human cell and is common to all cellular life. It helps maintain cell potential and regulate cellular volume.

To maintain the cell potential, cells need to keep a low concentration of sodium ions, and high levels of potassium ions within the cell (intracellular). Outside cells (extracellular), there are high concentrations of sodium and low concentrations of potassium, so diffusion occurs through ion channels in the plasma membrane. To keep the appropriate concentrations the sodium-potassium pump pumps sodium out and potassium in.

The mechanism is:

• The pump, with bound ATP, binds 3 intracellular Na+ ions.
• ATP is hydrolyzed, leading to phosphorylation of a pump and release of ADP.
• A conformational change in the pump exposes the Na+ ions to the outside. The phosphorylated form of the pump has a low affinity for sodium ions and they are released.
• The pump binds 2 extracellular K+ ions, leading to the dephosphorylation of the pump.
• ATP binds and the pump reorients to release potassium ions inside the cell and the pump is ready to go again.

(Cardiac glycosides can inhibit pump operation.)

As the plasma membane is far less permeable for sodium than it is for potassium, an electric potential (negative intacellularly) is the eventual result. The electrical and concentration gradient established by the sodium-potassium ATPase supports not only the cell resting potential but the action potentials of nerves and muscles. Export of sodium from the cell provides the driving force for several facilitated transporters, which import glucose, amino acids and other nutrients into the cell. Translocation of sodium from one side of an epithelium to the other side creates an osmostic gradient that drives the absorption of water.

Another important task of the Na-K pump is to provide a Na gradient which is used by certain carrier processes. In the gut, for example, sodium is transported out of the resorbing cell on the blood side via the Na-K pump, while on the resorbing side, the Na-Glucose co-transporter uses the created Na gradient as a source of energy to import both Na and Glucose, which is far more efficient than simple diffusion. Similar processes are located in the renal tubular system.

The Na-K pump found in the membrane of heart cells is an important target of cardiac glycosides (for example Oubain and strophantidin), drugs used to improve heart performance by increasing its force of contraction. (Cardiac glycosides can also be used as toxins/poisons). Contraction of any muscle is dependent on a 100-10,000 times higher-than-normal intercellular Ca concentration, which, as soon as it is put back again on its normal level by a carrier enzyme in the plasma membrane, will relax this muscle. Since this carrier enzyme (Na-Ca translocator) uses the Na gradient generated by the Na-K pump to remove Ca from the intercellular space, slowing down the Na-K pump results in a permanently higher Ca level in the muscle, which will eventually lead to stronger contractions.

See also: active transport, Jens C. Skou