Mineralocorticoid actions

Mineralocorticoids - Salty Survival Hormones

• Primary Hormone: Aldosterone, from Zona Glomerulosa.
• Primary Target: Principal cells of the distal convoluted tubule (DCT) & collecting ducts.
• Core Actions:
  • ↑ Na⁺ reabsorption (water follows) → ↑ ECF volume & BP.
  • ↑ K⁺ secretion.
  • ↑ H⁺ secretion (via intercalated cells).

⭐ Aldosterone Escape: In primary hyperaldosteronism, chronic high aldosterone leads to initial Na⁺/water retention, but compensatory mechanisms (like ANP release) cause natriuresis, preventing overt hypernatremia and edema.

Mechanism of Action - The Nuclear Receptor Dance

• Entry & Binding: Lipid-soluble aldosterone freely diffuses into principal cells (distal tubule/collecting duct) and binds to its cytoplasmic Mineralocorticoid Receptor (MR).
• Translocation & Transcription: The hormone-receptor complex translocates to the nucleus, binds to a hormone response element on DNA, and initiates transcription.
• Protein Synthesis: Upregulates synthesis of key transport proteins:
  • Apical membrane: Epithelial Na+ Channels (ENaC)
  • Basolateral membrane: Na+/K+ ATPase pumps

⭐ Cortisol's Cross-Talk: Cortisol can also activate the MR. In aldosterone-sensitive tissues, the enzyme 11β-hydroxysteroid dehydrogenase type 2 (11β-HSD2) locally inactivates cortisol to cortisone, ensuring aldosterone specificity.

Physiological Effects - Salt, Water, Pressure

• Primary Action: Acts on principal cells of the distal nephron (DCT & collecting duct).
• Mechanism: ↑ synthesis and insertion of key transporters.
  • Luminal: ↑ ENaC (Na⁺ reabsorption) & ↑ ROMK (K⁺ secretion).
  • Basolateral: ↑ Na⁺/K⁺-ATPase activity to drive the gradients.
• Net Result:
  • ↑ Salt Retention: Na⁺ reabsorption from urine.
  • ↑ Water Retention: Water follows Na⁺, expanding ECF volume.
  • ↑ Blood Pressure: Due to volume expansion.
  • ↑ K⁺ & H⁺ excretion, leading to potential hypokalemia and metabolic alkalosis.

⭐ Aldosterone Escape: In states of aldosterone excess, high volume/pressure triggers natriuretic peptides (ANP) to limit edema, but urinary K⁺/H⁺ wasting continues, sustaining hypokalemia and metabolic alkalosis.

Regulation - The RAAS & Potassium Show

• Primary Drivers: Angiotensin II & extracellular K⁺ concentration.
• RAAS Cascade: The main pathway for aldosterone release.

• Potassium's Role:
  • Hyperkalemia is a potent, direct stimulator of aldosterone secretion, acting as a key defense against high K⁺.
• Minor Regulators:
  • ACTH: Has a transient, permissive effect.
  • ANP: Inhibits renin and aldosterone release in response to atrial stretch (high volume).

⭐ While the RAAS is the classic pathway, severe hyperkalemia can stimulate aldosterone release independently of Angiotensin II.

Pathophysiology - Too Much or Too Little

• Hyperaldosteronism (Conn's Syndrome): ↑Na⁺/H₂O retention → Hypertension. ↑K⁺/H⁺ excretion → Hypokalemic metabolic alkalosis.
• Hypoaldosteronism: ↓Na⁺/H₂O retention → Hypotension. ↓K⁺/H⁺ excretion → Hyperkalemic metabolic acidosis (Type 4 RTA).

⭐ Licorice (glycyrrhetinic acid) inhibits 11β-HSD2, causing cortisol to activate mineralocorticoid receptors, mimicking hyperaldosteronism.

High‑Yield Points - ⚡ Biggest Takeaways

• Aldosterone primarily acts on the principal cells of the late distal tubule & collecting duct.
• It upregulates ENaC channels to increase Na+ reabsorption and ROMK channels to increase K+ secretion.
• It also stimulates H+ secretion from α-intercalated cells, potentially causing metabolic alkalosis.
• The net effect is increased blood volume and blood pressure.
• Primary regulation is via angiotensin II and serum K+ levels, not ACTH.
• Hyperaldosteronism classically presents with hypertension, hypokalemia, and metabolic alkalosis.