Ionic selectivity, saturation, and block in sodium channels. A four-barrier model.

Area of Science:

• Neuroscience
• Biophysics
• Physical Chemistry

Background:

• Ionic fluxes through sodium channels in myelinated axons exhibit complex behaviors like competition and block.
• These phenomena deviate from simple flux independence, especially when external sodium ions are substituted.

Purpose of the Study:

• To develop new flux equations and a permeability model for sodium channels that account for observed ionic interactions.
• To explain deviations from simple flux independence using a physically based model.

Main Methods:

• Developed a permeability model for sodium channels based on Eyring rate theory applied to saturable binding sites.
• Utilized voltage clamp measurements on myelinated nerve fibers to study ionic fluxes.
• Fitted experimental data with the model by adjusting parameters related to energy barriers and binding sites.

Main Results:

• The model successfully explains deviations from flux independence due to saturation effects.
• Ionic permeability ratios vary depending on measurement method (zero-current potential vs. current amplitude).
• Block by guanidinium, Tl+, and Na+ ions were accurately fitted by the binding model with specific dissociation constants.

Conclusions:

• The developed model provides a molecular interpretation of ion permeation, involving ion dehydration and interaction with channel sites.
• The model accurately predicts ionic flux behavior and block, offering a more realistic description than simple diffusion models.
• Understanding these ionic dynamics is crucial for comprehending nerve function and potential therapeutic interventions.