Potassium

The major intracellular cation in the body is potassium, which is maintained at a concentration of approximately 145mmoll^-1 of intracellular fluid but at much lower concentrations in the plasma and interstitial fluid (3.8-5mmoll^-1 of extracellular fluid). The high intracellular concentration of potassium is maintained via the activity of the Na⁺/K⁺ ATPase pump. Because this enzyme is stimulated by insulin, alterations in the plasma concentration of insulin can affect cellular influx of potassium and thus plasma concentration of potassium. Relatively small changes in the concentration of extracellular potassium greatly affect the extracellular/intracellular potassium ratio and thereby affect nerve transmission, muscle contraction, and vascular tone.