Sodium channel selectivity. Dependence on internal permeant ion concentration

Area of Science:

• Neuroscience
• Biophysics
• Ion Channel Physiology

Background:

• Sodium channels are crucial for nerve impulse transmission.
• Understanding their selectivity is key to comprehending neuronal excitability.

Purpose of the Study:

• To investigate how varying internal ion concentrations affect sodium channel selectivity in squid axons.
• To determine the primary factors influencing the PNa/PK ratio.

Main Methods:

• Utilized squid axon membrane preparations.
• Measured reversal potentials under different internal ion concentrations (potassium, TMA, Tris, sucrose).
• Calculated selectivity ratios (PNa/PK) based on experimental data.

Main Results:

• The selectivity ratio (PNa/PK) decreased significantly with reduced internal potassium concentration (from 12.8 to 3.5).
• Diluting internal potassium with TMA, Tris, or sucrose similarly reduced PNa/PK, indicating dependence on permeant ion concentration.
• Lowering internal concentrations of cesium, rubidium, guanidinium, or ammonium also decreased PNa/Pion.
• Established a selectivity sequence for the sodium channel: Na > Guanidinium > Ammonium > K > Rb > Cs.

Conclusions:

• Sodium channel selectivity is primarily determined by the concentration of internal permeant ions.
• Factors like ionic strength, membrane potential, and chloride permeability do not significantly influence this selectivity.
• The findings provide insights into the molecular mechanisms governing ion permeation through sodium channels.