Lecture Notes on Aldosterone

Introduction

• Aldosterone is crucial for regulating blood pressure and blood volume.
• It increases sodium reabsorption by the kidneys.

Secretion and Transport

• Secreted from the zona glomerulosa of the adrenal cortex.
• Uses cholesterol to produce aldosterone and other mineral corticoids.
• Being hydrophobic, it requires transport in the blood bound to albumin.

Regulation of Aldosterone Secretion

• Released under low blood pressure conditions.
• Increase in secretion:
  • High levels of angiotensin II.
  • High potassium levels in blood and extracellular fluid.
• Decrease in secretion:
  • High sodium ion concentration in extracellular fluid.

Mechanism of Action

• Diffuses into principal epithelial cells of the late distal tubules and cortical collecting tubules.
• Forms aldosterone-receptor complex in the cytoplasm.
• Complex enters the nucleus, upregulates DNA transcription into mRNA.
• mRNA is translated into proteins that enhance:
  • Sodium reabsorption.
  • Potassium and hydrogen secretion.

Effects on Cellular Activities

• Increases formation of sodium-potassium pumps on basolateral membrane.
• Increases epithelial sodium channels (ENaC) on the apical membrane.
• Sodium attracts water, leading to increased blood volume and pressure.

Potassium and Hydrogen Secretion

• Principal cells secrete potassium due to high intracellular concentration.
• Hydrogen secretion is increased by the hydrogen-potassium pump activity in type A intercalated cells, leading to potential alkalosis.

Primary Aldosteronism (Kahn Syndrome)

• Caused by a benign tumor in zona glomerulosa cells secreting excess aldosterone.
• Symptoms include:
  • Hypokalemia.
  • Mild metabolic alkalosis.
  • Increased extracellular fluid (ECF) and blood volume.
  • High blood pressure, without edema.
  • Decrease in plasma renin levels.
• Treatment:
  • Surgical removal of the tumor.
  • Management of hypertension and hypokalemia.
  • Use of aldosterone antagonists like spironolactone.

Secondary Aldosteronism

• Caused by extra-adrenal stimuli leading to high aldosterone levels.
• Often due to increased renin production, raising angiotensin II levels.
• Causes include:
  • Shock, dehydration, liver failure (decreased blood colloid osmotic pressure).
  • Conditions leading to less blood delivery to kidneys (e.g., renal artery stenosis, heart failure).
  • Renin-secreting tumors.