The Na-K pump is an enzyme located in the plasma membrane of virtually every human cell. Its main task is to keep intercellular ion concentrations (those of Sodium and Potassium) at a constant level by translocating three Na ions from there to the extracellular space in exchange for importing two K ions. As the plasma membane is far less permeable for sodium than it is for potassium, an electric potential (negative inside) is the eventual result.
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 "heart glycosides", drugs used to improve heart performance by increasing its force of contraction. Contraction of any muscle is dependant 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 relaxe 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.
