Endothelial Dysfunction in Hypertension: Molecular Mechanisms and Pathophysiology

Overview and Objectives of Vascular Endothelial Function

The vascular system is a complex network designed to deliver oxygen and nutrients to organs throughout the body. The function and regulation of this system depend heavily on the interaction between blood vessels and the heart.

• Path of Blood Flow: Blood is pumped by the heart into the outer muscular arteries and arterioles, eventually reaching the capillaries within organs before returning to the heart through the veins.

• The Endothelium: A critical anatomical feature of all blood vessels is the presence of endothelial cells (EC) in the inner layer (intima). These cells are essential regulators of blood pressure and vascular health.

Primary Learning Objectives

1. Understand the vascular system and the mechanics of smooth muscle contraction.

2. Examine how endothelial cells regulate smooth muscle relaxation and contraction through signaling molecules.

   • Endothelium-Derived Relaxing Factors (EDRFs): Nitric oxide (NO), endothelium-derived hyperpolarization (EDHF), and plastocycline (PGI2).

   • Endothelium-Derived Contracting Factors (EDCFs): Endothelin-1 and trombutism A2.

3. Define hypertension and its classifications.

4. Identify hypertensive animal models.

5. Analyze the molecular mechanisms of endothelial cell dysfunction in hypertension.

Cellular Mechanics of Vascular Smooth Muscle Contraction

Vascular smooth muscle cell (VSMC) contraction is primarily driven by changes in intracellular calcium concentration ($Ca^{2+}$).

• Activation: Contraction begins when an agonist binds to a receptor or when voltage-gated calcium channels are activated.

• Calcium Influx: Influx of calcium increases the concentration within the cytosol.

• Calmodulin Binding: Calcium binds to the protein calmodulin.

• MLCK Activation: The calcium-calmodulin complex activates Myosin Light Chain Kinase (MLCK).

• Phosphorylation: MLCK phosphorylates myosin, which enables the sliding of myosin and actin filaments.

• Result: This process results in the physical contraction of the smooth muscle.

Major Functions of Endothelial Cells

Endothelial cells serve three primary roles in maintaining vascular homeostasis:

• Barrier Function: Located in the intima of all vessels, endothelial cells act as a barrier to prevent the inappropriate migration of blood cells into neighboring tissues.

• Angiogenesis and Vascular Repair:

  • They facilitate the formation of new blood vessels (angiogenesis).

  • They repair injured areas through the migration and progression of neighboring endothelial cells or the migration of endothelial progenitor cells.

• Regulation of Vascular Tone: Endothelial cells actively change vessel tone by releasing EDRFs and EDCFs. They also propagate electrical signals directly to smooth muscle cells via gap junctions.

Mechanisms of Endothelium-Dependent Vascular Relaxation

Relaxation is initiated by triggers such as the neurotransmitter acetylcholine (ACh) binding to muscarinic receptors (G-protein coupled receptors) on the endothelial cell.

Calcium Signaling in Endothelial Cells

1. ACh binds to the GPCR, leading to the production of $IP_3$.

2. $IP_3$ binds to the $IP_3$ receptor on the Endoplasmic Reticulum (ER).

3. As $Ca^{2+}$ concentration is significantly higher in the ER than in the cytosol, the opening of these receptors releases $Ca^{2+}$ into the cytoplasm.

4. Increased cytosolic $Ca^{2+}$ activates three major pathways:

   • PLA2: Produces prostacyclin (PGI2).

   • eNOS (anos): Produces Nitric Oxide (NO).

   • K+ Channels: Activates calcium-activated potassium channels, leading to membrane hyperpolarization.

Nitric Oxide (NO) Pathway

Nitric oxide is produced by the enzyme Nitric Oxide Synthase (NOS). There are three isoforms:

• nNOS: Neuronal NOS.

• iNOS: Inducible NOS.

• ENOS: Endothelial NOS (predominantly expressed in endothelial cells).

Molecular Synthesis of NO:

• Substrate: L-arginine is used as the substrate to produce nitric oxide and L-citrulline.

• Cofactors: Synthesis requires NADPH, tetra hydrolybioptanin (BH4), and calmodulin.

• NOS Coupling: NO production typically requires two NOS molecules to couple. Calmodulin binds and closes the enzyme structure, while BH4 acts as a bridge to couple the two molecules.

• NOS Uncoupling: BH4 is sensitive to reactive oxygen species (ROS). In the presence of ROS, BH4 is oxidized into seventy eight dihydrobioprotein (BH2). High concentrations of BH2 prevent coupling, leading to "uncoupled nurse" (NOS). Uncoupled enzymes produce superoxide anion instead of NO. Superoxide reacts with NO to form nitrate, drastically reducing NO bioavailability.

• Note: BH2 can be converted back to BH4 using dihydrofolate reductase (DHFR).

Relaxation via PKG:

1. NO diffuses from endothelial cells to smooth muscle cells.

2. It activates soluble granules cyclases (sGC) to produce cyclic GMP (cGMP).

3. cGMP activates Protein Kinase G (PKG), which has three functions:

   • Activates MLC Phosphatase: Dephosphorylates myosin to stop actin-myosin sliding.

   • Activates SERCA (sarcoendoplasmic reticulum calcium ATPase): Pumps calcium back into the SR, decreasing cytosolic $Ca^{2+}$.

   • Activates Potassium Channels: Leads to membrane hyperpolarization, closing voltage-dependent calcium channels (VCDD/BDCC), further reducing cytosolic $Ca^{2+}$.

Endothelium-Derived Hyperpolarizing Factor (EDHF)

Relaxation can also occur through electrical signaling and specific hyperpolarizing factors:

• Potassium Channels in EC: Increased calcium in the EC activates small conductance (SK) and intermediate conductance (IK) calcium-activated potassium channels.

• Hyperpolarization: Opening these channels drops the membrane potential.

• Gap Junctions: This electrical hyperpolarization is propagated directly to smooth muscle cells.

• SMC Channels: EDHF activates big conductance (BK) and inwardly rectifying (KIR) potassium channels on the smooth muscle cell membrane, stopping BDCC activity and reducing cytosolic $Ca^{2+}$.

Prostacyclin (PGI2) Pathway

• Production: Arachidonic acid (AA) is released from the plasma membrane by PLA. Cyclooxygenase (COX) converts free AA into PGH2, which is then converted into PGI2.

• Mechanism: PGI2 binds to the IP receptor (a GS-coupled GPCR) on the smooth muscle cell.

• PKA Activation: Activation of adenylyl cyclase (SAC) produces cyclic AMP (cAMP), which activates Protein Kinase A (PKA).

• PKA Functions: Mirrors PKG functions; it activates MLC Phosphatase, activates SERCA, and opens potassium channels to induce relaxation.

Endothelium-Derived Contracting Factors (EDCFs)

Trombutism A2 (TXA2)

• Produced via the COX pathway from PGH2.

• Released TXA2 (TRK) binds to the TP receptor on smooth muscle cells.

• This activates PLC and increases $IP_3$, triggering calcium release from the SR, leading to contraction.

Endothelin-1 (ET-1)

• Produced in endothelial cells as prepro-endothelin, converted to big endothelin-1, and finally to ET-1.

• ET-1 binds to the ETA receptor (predominant) or ETB receptor on smooth muscle cells.

• This binding activates PLC, increases $IP_3$, and raises cytosolic $Ca^{2+}$ to induce contraction.

• Note: Endothelial cells also possess the ETB receptor on their cell membranes.

Definition and Classifications of Hypertension

Hypertension is defined as a persistent increase in blood pressure against the walls of the blood vessels.

Systemic Hypertension

1. Primary (Essential) Hypertension: The definitive cause is unknown. Risk factors include:

   • High salt intake and low potassium intake.

   • Alcohol consumption and obesity.

   • Lack of physical activity and unhealthy diet.

2. Secondary Hypertension: Caused by underlying diseases, such as Ketirin disease or Cushing's syndrome.

Local Hypertension

Pressure is increased only within a specific organ while systemic pressure remains normal (normotension).

• Example: Pulmonary hypertension (elevated pressure only within the lungs). This is a lethal disease that is difficult to treat.

• Other types: Portal, skin, or ocular hypertension.

Clinical Classification (BP in mmHg)

• Normal: Systolic < 120 and Diastolic < 80.

• Stage 1: Systolic 130–139 or Diastolic 80–89.

• Stage 2: Systolic over 140 or Diastolic over 90.

• Hypertensive Crisis: Systolic over 180 and/or Diastolic over 120.

Hemodynamics and Vascular Resistance

Blood pressure is mathematically related to cardiac function and vascular resistance.

Calculations

• Mean Arterial Pressure (MAP):

  • $MAP = \frac{CO \times SVR}{80} + CVP$

  • Where $CO$ is Cardiac Output, $SVR$ is Systemic Vascular Resistance, and $CVP$ is Central Venous Pressure.

• Cardiac Output ($CO$):

  • $CO = \text{Heart Rate} \times \text{Stroke Volume}$

• Stroke Volume ($SV$):

  • $SV = EDV - ESV$ (End Diastolic Volume - End Systolic Volume).

• Vascular Resistance ($R$):

  • $R = \frac{8 \times \text{blood viscosity} \times \text{vessel length}}{\pi \times r^4}$

  • Crucial Insight: The radius ($r$) is raised to the fourth power. A tiny decrease in vessel diameter results in a massive increase in resistance and MAP.

Vascular Remodeling

Hypertension leads to physical changes in vessels:

• Increased vascular contraction and reduced relaxation.

• Inward media thickness remodeling, which further reduces vessel diameter compared to healthy resistant arteries.

Treatment and Experimental Evidence

Treatment Tiers

• First-line: ACE inhibitors, Angiotensin II receptor blockers (ARBs), Diuretics (to induce urination), and Calcium channel blockers.

• Resistance Hypertension: Diagnosed when a combination of three first-line drugs fails to control pressure, requiring second-line therapies.

Endothelial Dysfunction in Patients (1990 Data)

• In Vivo Study: Brachial artery flow was measured using ultrasound. Control patients showed significant flow increases and decreased resistance upon ACh administration. Hypertensive patients showed significantly lower flow increase and minimal decrease in resistance.

• In Vitro Study: Resistance arteries from patient adipose tissue were cannulated. ACh-induced relaxation was significantly attenuated in hypertensive patients compared to controls. However, relaxation induced by Sodium Nitroprusside (SNP, an NO donor) was unchanged, indicating that smooth muscle function (endothelium-independent relaxation) remains intact while the endothelium itself is dysfunctional.

Hypertensive Animal Models

To study molecular mechanisms, several animal models are utilized:

• Spontaneously Hypertensive Rat (SHR): Found within the Wistakot (Wistar-Kyoto) strain. They reach blood pressures of $180-220\,mmHg$ and exhibit endothelial dysfunction, cardiac hypertrophy, renal dysfunction, insulin resistance, and dyslipidemia.

  • SHR-SP: Stroke-prone SHR; shows more severe cardiovascular disease and frequent cerebral strokes.

  • Other variants: Spontaneous hypertensive obese rats and heart failure rats.

• Dahl Salt-Sensitive Hypertensive Rat: Derived from Sprague-Dawley rats. High salt diets increase MAP to $160\,mmHg$ within 21 days.

• DOCA-salt (Docochinostron acetate): Rats are treated with a subcutaneous DOCA pellet and provided with salt water to induce hypertension.

Endothelial dysfunction plays a important role in the development and progression of hypertension, a condition characterized by persistent high blood pressure. The endothelium, the thin layer of endothelial cells lining the blood vessels, is crucial for maintaining vascular homeostasis. When this endothelial layer becomes dysfunctional, it disrupts the balance of relaxing and contracting factors, ultimately leading to increased vascular resistance and hypertension. Several molecular pathways contribute to this dysfunction, particularly the impairment of nitric oxide (NO) synthesis, altered release of endothelium-derived relaxing factors (EDRFs), and increased production of endothelium-derived contracting factors (EDCFs).

One of the main molecular pathways affected in endothelial dysfunction is the NO pathway. Normally, endothelial cells produce NO through the action of endothelial nitric oxide synthase (eNOS), which converts L-arginine into NO in the presence of cofactors like calmodulin and tetrahydrobiopterin (BH4). NO is a vasodilator that promotes relaxation of smooth muscle cells in blood vessels, leading to reduced arterial pressure. In hypertensive conditions, oxidative stress results in the formation of reactive oxygen species, which can reduce BH4 and disturb the coupling of eNOS. This situation, known as NOS uncoupling, results in reduced NO production and generation of superoxide, a reactive molecule that can further promote vascular inflammation and increase blood pressure. This loss of NO bioavailability is one of the key contributors to endothelial dysfunction and the development of hypertension.

Another significant pathway involved in endothelial dysfunction is the release of EDRFs. In healthy endothelial cells, pathways involving eNOS result in the production of various EDRFs, such as prostacyclin (PGI2) and endothelium-derived hyperpolarizing factors that help to maintain vascular tone and prevent excessive constriction of blood vessels. When endothelial function is compromised, the production of these EDRFs decreases, leading to a lack of vasodilation. Decreased levels of PGI2 can lead to an imbalance favoring vasoconstriction. The smooth muscle cells in the blood vessel walls begin to show increased tension, contributing to elevated blood pressure.

In addition to impaired NO and EDRF pathways, the production of EDCFs is another important aspect of how endothelial dysfunction contributes to hypertension. In a healthy state, endothelial cells release EDCFs like ET-1 and thromboxane A2, which play a role in generating vasoconstriction. However, when endothelial dysfunction occurs, there is often an increased synthesis and release of these contracting factors. For example, ET-1 is produced from its precursor peptide and can bind to its receptors on smooth muscle cells, activating phospholipase C and leading to increased intracellular calcium concentrations, which promotes muscle contraction and further elevates blood pressure. The hyperactivity of EDCFs in the context of endothelial dysfunction makes the situation worse by promoting vasoconstriction, which is detrimental to the regulation of blood pressure.