[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.