Lecture Notes on Aldosterone

Introduction to Aldosterone

• Aldosterone: A hormone crucial for regulating blood pressure and blood volume.
• Functions by increasing sodium reabsorption in the kidneys.
• Secreted by the zona glomerulosa of the adrenal cortex.

Synthesis and Transport

• Source: Zona glomerulosa cells use cholesterol to synthesize aldosterone and other mineralocorticoids.
• Transport: Being hydrophobic, aldosterone binds to albumin for blood transport.

Regulation of Secretion

• Increases with:
  • Low blood pressure.
  • High levels of angiotensin II or potassium in blood and extracellular fluid.
• Decreases with:
  • High sodium ion concentration in extracellular fluid.

Mechanism of Action

• Cellular Action:
  • Diffuses into principal epithelial cells of late distal tubules and cortical collecting tubules.
  • Binds to mineralocorticoid receptor forming aldosterone-receptor complex.
  • Translocates into the nucleus to upregulate transcription of genes responsible for sodium reabsorption and potassium/hydrogen secretion.
• Protein Synthesis: Promotes formation of sodium-potassium pumps and epithelial sodium channels (ENaC) on cell membranes.

Effects on Body

• Sodium and Water Reabsorption:
  • Increased sodium reabsorption leads to increased fluid volume and blood pressure.
  • Sodium attracts water, enhancing blood volume.
• Potassium Secretion:
  • Via sodium-potassium pump, high intracellular potassium diffuses out through apical potassium channels.
• Hydrogen Secretion:
  • Increases hydrogen-potassium pump activity in type A intercalated cells, resulting in potential alkalosis.

Disorders and Conditions

• Primary Aldosteronism (Kahn Syndrome):
  • Caused by a benign tumor in zona glomerulosa cells, leading to excessive aldosterone.
  • Symptoms include hypokalemia, mild metabolic alkalosis, increased ECF/blood volume, high blood pressure.
  • Edema rarely present due to atrial natriuretic hormone compensating fluid balance.
  • Diagnostic features: Decreased plasma renin levels.
  • Treatment: Surgical removal of the tumor or use of aldosterone antagonists like spironolactone.

Secondary Aldosteronism

• Causes: Triggered by extra-adrenal stimulus.
• Common Causes:
  • Increased renin production due to decreased circulating blood volume (shock, dehydration, liver failure).
  • Conditions like renal artery stenosis and heart failure.
  • Renin-secreting tumors.
• Connection to Liver Failure: Decreased albumin production lowers blood colloid osmotic pressure, affecting circulating volume.

Summary

• Aldosterone plays a versatile role in maintaining electrolyte balance and blood pressure.
• Understanding its regulation and effects is crucial for managing related disorders.