Does aldosterone (Anti-Diuretic Hormone) reabsorb sodium and water, while ADH (Antidiuretic Hormone) reabsorbs only free water?

Aldosterone and ADH: Mechanisms of Action in Water and Sodium Regulation

Yes, you are correct - aldosterone primarily reabsorbs sodium and water, while ADH (antidiuretic hormone) primarily reabsorbs free water. The two hormones work through different mechanisms to regulate fluid balance in the body.

Aldosterone Mechanism

• Aldosterone is a mineralocorticoid hormone that acts primarily on the distal tubule and collecting duct of the kidney 1
• It stimulates renal sodium reabsorption by increasing both the permeability of the luminal membrane of principal cells to sodium and the activity of the Na/K ATPase pump in the basolateral membrane 1
• When sodium is reabsorbed under the influence of aldosterone, water follows passively due to osmotic forces, leading to both sodium and water retention 1, 2
• Aldosterone antagonists (like spironolactone) are effective in managing conditions with fluid overload because they block this sodium and water retention mechanism 1

ADH (Vasopressin) Mechanism

• ADH acts primarily on the collecting ducts by increasing water permeability through aquaporin-2 (AQP2) water channels 1
• It allows for free water reabsorption without directly affecting sodium transport 1, 3
• In the absence of ADH, the collecting duct is relatively impermeable to water, resulting in dilute urine 4
• ADH is regulated primarily by plasma osmolality, with increased secretion in response to increased osmolality or decreased effective circulating volume 3

Interaction Between Aldosterone and ADH

• While these hormones have distinct primary functions, they can interact in complex ways 5
• In some circumstances, aldosterone can actually decrease vasopressin-stimulated water reabsorption in the inner medullary collecting duct through a non-genomic mechanism 5
• Both hormones are part of the body's response to decreased effective circulating volume, as seen in conditions like heart failure and cirrhosis 1
• In hypervolemic hyponatremia, both hormones may be inappropriately elevated, contributing to water retention despite low serum sodium 1

Clinical Implications

• Understanding these distinct mechanisms is important for managing fluid disorders:
  • Hyperaldosteronism leads to sodium and water retention with hypertension and hypokalemia 2
  • SIADH (Syndrome of Inappropriate ADH) leads to free water retention without significant sodium retention, causing dilutional hyponatremia 1, 6
• Therapeutic approaches target these specific mechanisms:
  • Aldosterone antagonists (spironolactone, eplerenone) block sodium and water retention 1
  • Vaptans (vasopressin receptor antagonists) block ADH-mediated free water reabsorption, useful in treating hypervolemic hyponatremia 1

Common Misconceptions

• While aldosterone primarily affects sodium reabsorption with secondary water retention, it can also have direct effects on acid-base balance by stimulating proton secretion 2
• ADH can indirectly influence sodium balance through its effects on urine concentration and volume, but its primary direct effect is on water reabsorption 6, 7
• In some pathological states, these hormones may have effects beyond their classical actions, contributing to complex fluid and electrolyte disorders 5