cells and sodium pumps

Na+/K+-ATPase ATPase pump, like H+/K+-ATPase and Ca+/K+-ATPase, is an enzyme which plays the role of pump. It ensures the transmembrane transfer of the cations Na+ and K+. Na+/K+-ATPase is located in the cytoplasmic membranes. It consists of two alpha catalytic subunits and of two beta subunits. Na+/K+-ATPase uses the energy released by the hydrolysis of the ATP in the presence of magnesium to ensure the transport of three Na+ ions outside the cell and of two K+ ions inside.

Na+/K+-ATPase has three main functions:

-To maintain inside the cell a low concentration of sodium and a high concentration of potassium. It is an electrogenic pump which creates a potential difference between both sides of the cytoplasmic membrane.

- To ensure the polarization of excitable and contractile tissues: depolarization and repolarization correspond respectively to a sodium influx and a potassium exit. Na+/K+-ATPase restores the equilibrium.

-To create a potential energy, related to the ion gradient on both sides of the plasma membrane. This energy is used in particular for the secondary active transport, generally coupled to that of sodium.

Principal inhibitors of Na+/K+-ATPase typically bind to the extracellular part of enzyme i.e. that binds potassium, when it is in a phosphorylated state, to transfer potassium inside the cell.

Consequences of the inhibition of Na+/K+-ATPase: increase of intracellular sodium which is exchanged for calcium

The inhibition of Na+/K+-ATPase induces a rise in sodium concentration inside cells. This sodium increase induces in its turn an increase in the intracellular calcium concentration, via the sodium-calcium exchanger. The sodium-calcium exchanger is particularly active in myocardium and in smooth vascular muscles.

The rise in intracellular calcium increases the force of contraction of the heart/skeletal muscle and the contracture of smooth vascular muscles. The inhibition of Na+/K+-ATPase reduces cellular polarization (depolarizing effect).