Aldosterone

Aldosterone is a steroid hormone produced by the zona glomerulosa of the adrenal cortex. It belongs to the mineralocorticoids and acts on the renal distal tubules and collecting ducts to promote sodium reabsorption and potassium and hydrogen ion secretion, thereby increasing extracellular fluid volume and blood pressure. The net effect is to conserve sodium and water and to excrete potassium.

The synthesis and release are primarily controlled by the renin–angiotensin–aldosterone system. Low blood pressure or low NaCl delivery to the distal tubule stimulates renin release from the kidney, producing angiotensin II, which stimulates aldosterone synthesis and release. Elevated plasma potassium directly stimulates aldosterone secretion. ACTH provides a transient, permissive effect; atrial natriuretic peptide inhibits aldosterone release.

Mechanism of action in target cells: Aldosterone binds to the intracellular mineralocorticoid receptor, leading to altered gene transcription of channel and pump proteins, including ENaC subunits and the Na+/K+ ATPase, and regulators such as SGK1. These changes increase sodium reabsorption and potassium secretion.

Clinical significance: Disorders include hyperaldosteronism (primary via aldosterone-producing adenoma, or secondary from RAAS activation) presenting with hypertension and hypokalemia; hypoaldosteronism (Addison's disease) with hyperkalemia and hypotension. Treatment includes mineralocorticoid receptor antagonists (spironolactone, eplerenone) for hyperaldosteronism; surgical removal of adenoma; in hypoaldosteronism, mineralocorticoid replacement.

Measurement: Plasma aldosterone concentration and plasma renin activity are used to calculate the aldosterone-to-renin ratio as a screening test for primary hyperaldosteronism; confirm with suppression tests.