Because potassium (K+) is the major intracellular cation, discussion of disorders of K+ balance must take into consideration changes in the exchange of intra- and extracellular K+ stores. (Extracellular K+ constitutes <2% of total-body K+ content.) Insulin, β2-adrenergic agonists, and alkalosis tend to promote K+ uptake by cells; acidosis, insulinopenia, or acute hyperosmolality (e.g., after treatment with mannitol or D50W) promotes the efflux or reduced uptake of K+. A corollary is that tissue necrosis and the attendant release of K+ can cause severe hyperkalemia, particularly in the setting of acute kidney injury. Hyperkalemia due to rhabdomyolysis is thus particularly common, due to the enormous store of K+ in muscle; hyperkalemia may also be prominent in tumor lysis syndrome.
The kidney plays a dominant role in K+ excretion. Although K+ is transported along the entire nephron, it is the principal cells of the connecting segment and cortical collecting duct that play a dominant role in K+ excretion. Apical Na+ entry into principal cells via the amiloride-sensitive epithelial Na+ channel (ENaC) generates a lumen-negative potential difference, which drives passive K+ exit through apical K+ channels. This relationship is key to the bedside understanding of potassium disorders. For example, decreased distal delivery of Na+ tends to blunt the ability to excrete K+, leading to hyperkalemia. Abnormalities in the renin-angiotensin-aldosterone system (RAAS) can cause both hypo- and hyperkalemia; aldosterone has a major influence on potassium excretion, increasing the activity of ENaC channels and the basolateral Na+/K+-ATPase, thus amplifying the driving force for K+ secretion across the luminal membrane of principal cells.
Section 1. Care of the Hospitalized Patient