Potassium homoeostasis in the elderly.

Abstract

The mechanisms which are involved in potassium potassium excretion by enhancing potassium homeostasis maintain the serum level within the secretion. relatively narrow normal range of 3.5–5.0 mEq/l Increased renal delivery of sodium5 and water6 to despite widely varying dietary intake and relatively the distal tubule may enhance renal potassium excrehuge intracellular potassium stores (98% of total tion, e.g. following loop diuretics,7 but the physiobody potassium). Abnormalities in renal excretion logical importance of this is uncertain. Conversely, and/or interference with the physiological control of decreased delivery of sodium chloride to the distal intracellular to extracellular potassium gradient may tubule during periods of avid sodium retention upset this fine balance, resulting in abnormalities in hinders potassium secretion, e.g. prerenal azotemia serum potassium.1,2 A complex interaction of renal or severe congestive heart failure.8 Arginine vasopresand extrarenal mechanisms are involved in mainsin secretion also significantly influences potassium taining this balance. Physiological changes associexcretion.9 ated with increasing age would tend to predispose Systemic acidosis and alkalosis exert a powerful the elderly to hyperkalaemia. However, potassium impact on renal potassium balance.10 During homeostasis in the elderly has not been well studied periods of systemic acidosis, hydrogen ion is to date. buffered intracellularly, with a resultant shift of In this commentary, renal and extrarenal potaspotassium from the intracellular to the extracellular sium homeostasis are summarized. Age-related fluid to maintain electrical neutrality. Thus, the alterations in these mechanisms are highlighted, and potassium concentration within the distal tubular the possible clinical implications of such changes cells falls, as does the concentration gradient are discussed. towards the tubular lumen, which culminates in a reduction in potassium secretion. Conversely, a shift of potassium into distal renal tubular cells in systemic alkalosis enhances renal tubular secretion, Renal mechanisms which contributes to an increase in potassium excretion. The above effects are however modified The renal tubules can normally adapt to wide fluctuby the duration of disturbance of acid-base balance, ations in potassium intake. The renal excretory e.g. prolonged acidosis alters fluid delivery, and this capacity for potassium is not exceeded by a dietary complicates the above picture. intake of 6 mEq/kg/day.3 The distal convoluted tubule The late distal convoluted tubule and cortical and collecting ducts constitute the sites which control collecting duct contain intercalated and principal renal potassium excretion (Figure 1). Dietary potascells which serve distinct transport functions.11 The sium loading results in both direct4 and indirect (via a and b subtypes of intercalated cells secrete hydroincreased aldosterone secretion3) stimulation of Na+K+ ATPase. This results in a prolonged increase in gen and bicarbonate ions into the lumen, respect-