Which statement best explains why primary hyperaldosteronism causes hypokalemia?

• A. Aldosterone decreases sodium reabsorption in the distal tubule.
• B. Aldosterone promotes potassium retention in the collecting duct.
• C. Aldosterone increases sodium reabsorption in the distal tubule without affecting potassium.
• D. Aldosterone increases Na reabsorption in the distal tubule in exchange for K and H, causing potassium loss.

Answer: (D)

Elevated aldosterone in primary hyperaldosteronism increases sodium reabsorption in the distal nephron. It does this by acting on mineralocorticoid receptors in the collecting duct and distal tubule, upregulating ENaC channels on the apical membrane and stimulating the Na+/K+ ATPase on the basolateral side. As more Na+ is reabsorbed, the lumen becomes more negative, driving potassium and hydrogen ion secretion into the urine. The result is increased Na+ reabsorption with concurrent loss of K+ (and H+), leading to hypokalemia (and often metabolic alkalosis). The statement that best captures this is that aldosterone raises Na+ reabsorption in the distal tubule in exchange for K+ and H+ secretion, causing potassium loss.