Membrane Permeability and Resting Potential

Membrane permeability and resting potential are fundamental concepts in cardiac electrophysiology that describe how ions move across the cardiac cell membrane and how this movement establishes the electrical potential difference essential for normal heart function.

Membrane Permeability

The cell membrane of cardiac myocytes is a selectively permeable barrier, primarily composed of a lipid bilayer embedded with various ion channels, pumps, and exchangers. Membrane permeability refers to the ability of specific ions to pass through the membrane via these proteins. The permeability of the membrane to ions such as potassium (K⁺), sodium (Na⁺), calcium (Ca²⁺), and chloride (Cl⁻) determines the ionic fluxes that influence the electrical state of the cell.

• Ion Channels: These are protein structures that allow ions to cross the membrane down their electrochemical gradients. Channels may be voltage-gated, ligand-gated, or mechanically gated, and their opening and closing regulate membrane permeability dynamically.

• Resting Membrane Permeability: At rest, the cardiac cell membrane is most permeable to K⁺ due to the abundance of open potassium channels (particularly inward rectifier K⁺ channels). This selective permeability is critical in maintaining the resting membrane potential.

• Selective Permeability and Ion Gradients: The differences in ion concentrations inside and outside the cell create electrochemical gradients. For example, high intracellular K⁺ and low extracellular K⁺, combined with low intracellular Na⁺ and high extracellular Na⁺, drive ions to move across the membrane when channels are open.

Resting Potential

The resting membrane potential is the steady-state electrical potential difference across the cardiac cell membrane when the cell is not electrically active (i.e., not depolarizing or generating an action potential). It is typically negative inside the cell relative to the outside, commonly around -85 to -90 millivolts in ventricular myocytes.

• Generation of Resting Potential: The resting potential arises primarily due to the differential permeability of the membrane to K⁺ ions. Because the membrane is highly permeable to K⁺ at rest, K⁺ ions tend to move out of the cell, down their concentration gradient. This outward movement of positive charge leaves behind an excess of negative charge inside the cell, creating the negative resting potential.

• Role of the Nernst Potential: The resting potential approximates the Nernst equilibrium potential for K⁺, which is the electrical potential that exactly balances the chemical gradient for K⁺, preventing net movement across the membrane. The Nernst potential for any ion is given by:

where R is the gas constant, T is temperature in Kelvin, z is the ion charge, F is Faraday's constant, and [ion]_outside and [ion]_inside are the extracellular and intracellular ion concentrations.

• Goldman-Hodgkin-Katz Equation: Because the membrane is permeable to multiple ions, the resting potential is more accurately described by the Goldman-Hodgkin-Katz (GHK) voltage equation, which integrates the permeability and concentration gradients of all permeant ions:

where P_ion indicates membrane permeability for each ion, and the brackets indicate ion concentrations inside and outside the cell.

Ionic Contributions and Dynamics

• Potassium (K⁺): The principal determinant of resting potential, due to the high resting permeability of potassium channels. The outward K⁺ current maintains the inside of the cell negatively charged.

• Sodium (Na⁺): The membrane has low permeability to Na⁺ at rest, but the large electrochemical gradient tends to drive Na⁺ inward. This inward Na⁺ leak is counteracted by active transport mechanisms like the Na⁺/K⁺ ATPase pump.

• Calcium (Ca²⁺): Resting permeability to Ca²⁺ is minimal; voltage-gated Ca²⁺ channels open during excitation, but do not contribute significantly to resting potential.

• Chloride (Cl⁻): Cl⁻ ions are generally passively distributed and contribute to resting potential based on their electrochemical gradient and permeability.

Role of Active Transporters

While membrane permeability to ions and passive diffusion establish the resting potential, active transporters maintain the ionic gradients necessary for this process:

• Na⁺/K⁺ ATPase Pump: Actively transports 3 Na⁺ ions out and 2 K⁺ ions in per ATP hydrolyzed, maintaining the high intracellular K⁺ and low intracellular Na⁺ concentrations.

• Other Pumps and Exchangers: Such as the Ca²⁺ ATPase and Na⁺/Ca²⁺ exchanger, which maintain calcium homeostasis but have less direct impact on resting membrane potential.

Physiological Significance

The resting membrane potential provides the electrical baseline from which cardiac action potentials arise. It ensures that cardiac myocytes are electrically polarized at rest, allowing rapid depolarization when excitatory stimuli open voltage-gated ion channels. This electrical excitability underlies the coordinated contraction of the heart muscle necessary for effective pumping.

Disruptions in membrane permeability or ionic gradients can alter the resting potential, potentially leading to arrhythmias or impaired cardiac function. For example, changes in extracellular K⁺ concentration or mutations affecting ion channel function can depolarize or hyperpolarize the resting potential, modifying excitability and conduction properties.

Summary of Key Points

Understanding membrane permeability and resting potential is essential for comprehending cardiac electrophysiology and the mechanisms underlying normal heartbeat initiation and propagation.