Endothelial function and dysfunction

Normal Endothelial Function - The Vascular Guardian

• Primary Role: Maintains vascular homeostasis as a dynamic, selectively permeable barrier between blood and tissue.
• Key Products & Actions:
  • Vasodilation: Releases Nitric Oxide (NO), synthesized via endothelial Nitric Oxide Synthase (eNOS): $L\text{-arginine} \xrightarrow{\text{eNOS}} NO$. Also produces prostacyclin (PGI₂).
  • Antithrombotic: Surface heparin-like molecules activate antithrombin III. Thrombomodulin binds thrombin, activating Protein C. Releases tissue plasminogen activator (t-PA).
  • Anti-inflammatory: Normally resists leukocyte adhesion by downregulating surface adhesion molecules (e.g., VCAM-1, ICAM-1).

⭐ Endothelial-derived NO is the most critical factor for vasodilation and also possesses potent anti-platelet and anti-proliferative properties.

Endothelial Dysfunction - The Barrier Breaks

• Healthy State (Vasodilatory & Anti-thrombotic):

  • Key mediators: Nitric Oxide (NO), Prostacyclin ($PGI_2$).
  • Function: ↑ cGMP → vasodilation; inhibits platelet aggregation.
  • Maintains an intact, selective barrier.

• Dysfunctional State (Vasoconstrictive & Pro-thrombotic):

  • Triggers: Smoking, HTN, hyperglycemia, dyslipidemia.
  • Core Pathophysiology:
    • ↓ NO bioavailability → impaired vasodilation.
    • ↑ Endothelin release → vasoconstriction.
    • ↑ Adhesion molecules (VCAM-1, ICAM-1) → leukocyte binding.
    • ↑ Permeability → allows LDL & macrophage entry into intima.
    • Prothrombotic surface → ↑ platelet adhesion & thrombosis.

⭐ Oxidized LDL is a key driver, promoting inflammation and reducing nitric oxide synthase (eNOS) activity, initiating atherosclerosis.

Atherosclerosis Pathogenesis - Plaque Attack Pathway

• Initiating Event: Endothelial dysfunction, often due to chronic stressors (e.g., hypertension, smoking, hyperglycemia), ↑ vascular permeability.
• Lipid Invasion: Low-density lipoprotein (LDL) cholesterol enters the tunica intima and becomes oxidized (oxLDL).
• Inflammatory Response:
  • Endothelium expresses adhesion molecules, recruiting monocytes.
  • Monocytes migrate into the intima and differentiate into macrophages.
• Foam Cell Formation: Macrophages engulf oxLDL, becoming lipid-laden foam cells. This is the earliest visible lesion, the fatty streak.

⭐ Location, Location, Location: Atherosclerotic plaques preferentially develop at arterial branch points and curvatures where turbulent, non-laminar blood flow disrupts endothelial integrity.

High‑Yield Points - ⚡ Biggest Takeaways

• Endothelial cells are key regulators of vascular tone and hemostasis.
• Nitric oxide (NO), synthesized from L-arginine by eNOS, is the main vasodilator. It works by ↑ cGMP.
• Endothelial dysfunction is the earliest stage of atherosclerosis, characterized by ↓ NO bioavailability.
• Major causes include hypertension, hyperlipidemia, diabetes, and smoking.
• Dysfunction shifts the balance towards a vasoconstricted, proinflammatory, and prothrombotic state.
• Endothelin-1 is a potent vasoconstrictor released from the endothelium.