[Potassium physiology, hypokalaemia and hyperkalaemia].

Potassium (K+) is a key component of the resting membrane potential of all cells that influences many important biologic events. The clinical importance of K+ is that surpluses or deficits in K+ in the extracellular fluid may predispose the patient to cardiac arrhythmias. The kidneys adjust overall K+ homeostasis by increasing or decreasing the rate of excretion of K+. Urinary excretion of K+ has 2 components: (i) the concentration of K+ in the tubular fluid that depends on the capacity of the cortical collecting duct to secrete K+. The capacity is determined by the lumen-negative transepithelial potential difference generated by the electrogenic reabsorption of Na+. Aldosterone and to a lesser degree HCO3- and Na+ in the tubular fluid are implicated in the generation of the potential difference. This component is evaluated by the transtubular K+ gradient (TTKG). (ii) The volume of fluid delivered to the cortical collecting duct that depends on the osmolar rate of excretion. These 2 components can be calculated if blood osmolality is higher than urine osmolality. Thus, investigating K+ abnormalities is based on the determination of TTKG and osmolar rate of excretion in the cortical collecting duct, on other clinical (extracellular fluid, blood pressure...) and biological data (24-hour K+ excretion, renin, aldosterone...) easily available. First treatment of K+ abnormality is the treatment of its cause. Insulin and glucose supply and dialysis are the best symptomatic treatments of hyperkalaemia. Copyright 2010 Association Société de néphrologie. Published by Elsevier SAS. All rights reserved.