Voltage-Gated Ion Channels in Cardiac Physiology

Overview:

• The heart’s rhythmic contractions depend on precise, timed changes in membrane potential, driven by voltage-gated ion channels (VGICs).

• These channels open or close in response to changes in voltage across the plasma membrane, allowing ion fluxes that underlie cardiac action potentials (APs).

• Understanding VGICs links genotype to phenotype, as mutations in their genes directly lead to cardiac channelopathies such as Long QT and Short QT syndromes.

• The lecture focuses mainly on voltage-gated Na⁺ (NaV) and K⁺ (KV) channels, which determine the initiation and termination of each heartbeat.

Membrane Transport and Ion Gradients:

• Membrane proteins involved in ion movement:

  • Ion channels – allow selective, passive diffusion down an electrochemical gradient.

    • Carriers (facilitated transporters) – bind substrates and change conformation to move them across the membrane.

      • Pumps (active transporters) – move ions against their gradients using ATP hydrolysis (e.g. Na⁺/K⁺-ATPase).
        

• Resting Membrane Potential (RMP):

  • Typical cardiac RMP: –80 to –90 mV.

  • Maintained by:

    • Na⁺/K⁺-ATPase: 3 Na⁺ out / 2 K⁺ in (net + charge out).

    • K⁺ leak channels: allow passive K⁺ efflux → inside becomes negative.
      

  • Intracellular fluid: high [K⁺], low [Na⁺]

  • Extracellular fluid: high [Na⁺], low [K⁺].Nucleic acids and proteins contribute to intracellular fixed negative charge, reinforcing the electrical gradient.

❗Nucleic acids and proteins contribute to intracellular fixed negative charge, reinforcing the electrical gradient.

Action Potential Fundamentals:

• Depolarisation: 

  • Voltage-gated Na⁺ channels open, leading to a rapid Na⁺ influx and so the membrane potential becomes positive.

• Repolarisation:

  • Na⁺ channels inactivate; voltage-gated K⁺ channels open, leading to a K⁺ efflux which restores negativity.

• Hyperpolarisation & Refractory Period:

  • K⁺ permeability remains high briefly which prevents immediate re-excitation, ensuring unidirectional signal conduction.

• Overall:

  • The AP is a transient reversal of electric polarity, converting chemical energy into electrical activity essential for excitation-contraction coupling.

Cardiac Action Potentials:

1. Ventricular Myocytes (Contractile Cells):

• The plateau (Phase 2) is unique to cardiac muscle, preventing tetanus and allowing full relaxation between beats.

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2. Pacemaker Cells (Sinoatrial Node):

No true resting potential — exhibits spontaneous depolarisation.

• Phase 4 (Pacemaker potential):

  • Slow Na⁺ entry via funny current (If, HCN channels) and Ca²⁺ via T-type channels.

• Phase 0:

  • Depolarisation via L-type Ca²⁺ channels (Na⁺ channels are largely absent here).

• Phase 3:

  • Repolarisation via delayed rectifier K⁺ channels.

❗Continuous cycling of depolarisation initiates heartbeat, and the action potential is propagated through the conduction system to the ventricles.

Voltage-Gated Sodium Channels:

Structural Features:

• Main α-subunit: four homologous domains (I–IV), each with six transmembrane segments (S1–S6).

  • S1–S4: Voltage-sensing domain (S4 carries repeated positively charged arginines or lysines).

  • S5–S6: Pore-forming region with selectivity filter.

• β-subunits: Auxiliary proteins modifying gating kinetics and expression.
  

• Selectivity filter (DEKA motif):

  • D (Asp) and E (Glu) are negatively charged → attracts Na⁺.

  • K (Lys) helps repel larger cations (e.g. K⁺).

  • A (Ala) provides neutral spacing.

• Highly specific to Na⁺ ions, discriminating based on hydration shell and ionic radius.

— — — — —

Gating Mechanism:

1. Resting state: activation gate closed, inactivation gate open.

2. Depolarisation: voltage sensor (S4) moves → activation gate opens → Na⁺ influx.

3. Inactivation: intracellular “ball-and-chain” structure occludes pore within milliseconds.

4. Repolarisation: channel resets as potential returns to RMP.

— — — — —

Pharmacology and Toxins:

• Tetrodotoxin (TTX) and saxitoxin: bind extracellular pore, blocking Na⁺ conduction which leads to paralysis, cardiac arrest.

• Certain local anaesthetics (e.g., lidocaine) act by binding the intracellular site and stabilising the inactivated state.

Voltage-Gated Potassium Channels (KV):

Structural Features:

• Composed of four separate α-subunits forming a tetrameric pore.

• Each α-subunit: six transmembrane segments (S1–S6) + cytoplasmic N- and C-termini.

  • S1–S4: Voltage-sensing domain.

  • S5–S6: Pore domain containing the selectivity filter (TVGYG sequence) — confers >1000-fold selectivity for K⁺ over Na⁺.

• Auxiliary β-subunits (KCNE family): modulate gating, conductance, and trafficking.

Functional Characteristics:

• Voltage sensing (S4): outward movement upon depolarisation triggers opening.

• Pore helix: stabilises the selectivity filter and ensures precise ion dehydration/re-hydration during transit.

  • Key example:

    • KCNQ1 (α-subunit) + KCNE1 (β-subunit) → forms the slow delayed rectifier current (IKs) crucial for cardiac repolarisation.

Regulation of Cardiac Potassium Channels (KCNQ1):

1. Accessory Subunit KCNE1:

• Slows activation/inactivation → lengthens repolarisation.

  • Provides stability under sustained depolarisation.

    • Mutations (e.g., S74L, R98W) alter kinetics, producing abnormally fast current decay → electrical instability.

2. Phosphatidylinositol 4,5-bisphosphate (PIP₂):

• A phospholipid second messenger anchored in the inner leaflet of the membrane.

  • Binds the intracellular portion of KCNQ1, increasing pore diameter and favouring the open state.

    • Reduced PIP₂ binding (e.g., due to mutation near S4–S5 linker) which leads to impaired activation and a Long QT phenotype.

3. Calmodulin (CaM):

• Calcium-binding regulatory protein interacting with KCNQ1’s C-terminal domain.

  • Modulates channel activity in a Ca²⁺-dependent manner.

    • Competes with PIP₂ for binding which creates fine-tuned feedback control linking cytosolic Ca²⁺ to membrane excitability.

Experimental Techniques:

• Patch-Clamp Electrophysiology:

  • Allows measurement of ionic currents at single-channel or whole-cell level.

    • By applying controlled voltage steps, one can determine activation thresholds, current amplitude, and kinetics.

      • Used to characterise mutant channels (e.g., altered time constants or current-voltage relationships).

• Findings:

  • KCNQ1 alone → fast activation;

  • KCNQ1 + KCNE1 → delayed activation;

  • Mutant subunits → aberrant kinetics consistent with arrhythmic phenotypes.

Long QT and Short QT Syndromes:

Electrocardiogram (ECG) Overview:

• P wave: Atrial depolarisation.

• QRS complex: Ventricular depolarisation.

• T wave: Ventricular repolarisation.

• QT interval: Duration of ventricular depolarisation + repolarisation (normally 360–480 ms).

Long QT Syndrome (LQTS):

• Prolonged QT (>480 ms) leads to delayed repolarisation which causes a predisposition to arrhythmias.

  • Causes:

    • Loss-of-function in K⁺ channels (KCNQ1, KCNH2).

    • Gain-of-function in Na⁺ channels (SCN5A).

  • Types:

    • LQT1: KCNQ1/KCNE1 defect (IKs current).

    • LQT2: KCNH2/HERG defect (IKr current).

    • LQT3: SCN5A mutation (persistent Na⁺ current).

  • Pathophysiology:

    • Delayed repolarisation extends AP duration → early after-depolarisations which leads to ventricular fibrillation.

  • Symptoms: Syncope, seizures, sudden cardiac death, especially under stress or exercise.

— — — — —

Short QT Syndrome (SQTS):

• QT interval <360 ms.

• Mechanism: Accelerated repolarisation.

  • Gain-of-function in K⁺ channels (e.g., KCNH2, KCNQ1).

  • Loss-of-function in Na⁺ or Ca²⁺ channels.

• Clinical features: Palpitations, atrial fibrillation, risk of sudden death due to shortened refractory period.