Slow changes in currents through sodium channels in frog muscle membrane

Area of Science:

• Muscle Physiology
• Ion Channel Biophysics
• Skeletal Muscle Electrophysiology

Background:

• Sodium channels are crucial for muscle excitation.
• Understanding their regulation is key to muscle function.
• Previous studies focused on fast inactivation mechanisms.

Purpose of the Study:

• To investigate the electrophoretic mobility of sodium channels in skeletal muscle.
• To identify novel inactivation mechanisms of sodium channels.
• To characterize the kinetics and voltage dependence of slow inactivation.

Main Methods:

• Utilized patch clamp techniques on frog and rat skeletal muscle membrane patches.
• Applied steady lateral electric fields to assess channel movement.
• Measured sodium currents in response to controlled voltage changes.

Main Results:

• Observed a slow, reversible decrease in functional sodium channels upon negative pipette potential application.
• Demonstrated that this decrease was due to sarcolemmal depolarization, not channel electrophoretic mobility.
• Quantified rate constants for slow inactivation and recovery in frog muscle (approx. 0.1 min-1 at 17°C).
• Identified a similar slow inactivation in rat skeletal muscle.
• Found an additional, intermediate inactivation mechanism in both species.

Conclusions:

• Skeletal muscle exhibits a significant, slow sodium channel inactivation process.
• This slow inactivation is distinct from fast inactivation and is voltage-dependent.
• Membrane lipids may not influence fast inactivation, or their mobility is restricted around sodium channels.