Question.

Potassium excretion by the kidney is increased by which one of the following?

  1. Aldosterone
  2. Reduced renal tubular flow
  3. Falling serum pH (acidaemia)
  4. Spironolactone
  5. ACE inhibitors


Answer.

a

Explanation.

Potassium is readily filtered by the glomerulus with around 7-8% of this filtered potassium load ultimately excreted by the kidney. Of the remainder, the majority (70%) is re-absorbed in the proximal tubule (K+-CL- symport and K+-H+ exchanger) and a smaller amount (20%) in the ascending loop of Henle (Na+-K+-2Cl co-transporter) while the distal tubule and collecting ducts can either reabsorb or secrete potassium and are the main point of control of potassium balance.

 

Influences over the absorption or secretion of K+ in the distal tubule and collecting duct include:

  1. Aldosterone increases K+ secretion by increasing both numbers and activity of Na+/K+ ATPase in the distal tubule and collecting ducts. These reabsorb Na+ in exchange for pumping out K+ into tubular fluid. Aldosterone secretion from the adrenal cortex is stimulated by rising plasma [K+].
  2. Secretion of K+ is into tubular fluid also occurs by passive diffusion, and is therefore proportional to the flow rate of tubular fluid; increasing flow causes increased K+ secretion.
  3. A rise in serum [H+] (acidaemia) causes an increase in K+ reabsorption. The H+-K+ antiporter in the proximal tubule and H+/K+ ATPase of the intercalating cells of the collecting duct both reciprocally pump H+ out into the tubular fluid in exchange for K+ into the cell for reabsorption. Upregulation of these transporters to secrete more H+ in acid excess will cause a reciprocal increase in K+ reabsorption. This in one of the mechanisms through which metabolic acidosis and hyperkalaemia are linked. 

     

Of the diuretics, the loop and thiazides will increase K+ excretion through increasing distal tubular flow rates (and both have the complication of hypokalaemia), while spironolactone increases potassium reabsorption through its effect as an aldosterone antagonist (hence known as a potassium sparing diuretic). ACE inhibitors suppress the renin-angiotensin-aldosterone system and so reduce aldosterone secretion and lead to greater potassium reabsorption.