Potassium is the most abundant intracellular cation. Approximately 98 % of the total body potassium content is located within cells, primarily muscle, where its concentration ranges from 100 to 150 mEq/l; the remaining 2 % resides in the extracellular fluid, where the potassium concentration is tightly regulated within a narrow range (3.5–5.0 mEq/l in the adult). The ratio of the intra- to extracellular potassium concentration determines, in large part, the resting membrane potential and is thus critical for normal function of electrically excitable cells, including nerve and muscle. Maintenance of a high intracellular potassium concentration is essential for many cellular processes, including DNA and protein synthesis, cell growth and apoptosis, mitochondrial enzyme function, and conservation of cell volume and pH [1–7]. Because of the many vital processes dependent on potassium homeostasis, multiple complex and efficient mechanisms have developed to regulate the internal distribution of potassium between the intra- and extracellular compartments and the external balance between intake and excretion by the kidney and GI tract.