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Action potentials that mimic fibrillation activate sodium current
1University of Georgia College of Pharmacy, Augusta, GA, USA. michael.ujhelyi@medtronic.com
Journal of Molecular and Cellular Cardiology
|September 2, 1999
Summary
Sodium channels remain active during ventricular fibrillation (VF), even with very short rest periods. This sodium current contributes to action potential propagation during VF, challenging previous assumptions.
Area of Science:
- Cardiovascular Physiology
- Ion Channel Electrophysiology
- Cardiac Arrhythmias
Background:
- Ventricular fibrillation (VF) is characterized by extremely brief action potentials and diastolic intervals.
- Under these conditions, the role of sodium channels in action potential generation and propagation is uncertain.
- Previous research suggested sodium channel activation might be compromised during VF due to short diastolic intervals.
Purpose of the Study:
- To investigate whether sodium channels can be activated during simulated ventricular fibrillation conditions.
- To quantify the fast inward sodium current under conditions mimicking VF.
- To determine the impact of short action potential durations and diastolic intervals on sodium channel function.
Main Methods:
- Isolated chick ventricular myocytes were voltage-clamped to measure sodium current.
- A voltage-clamp protocol simulated VF using a 10-pulse train at 10 Hz with varying depolarization intervals (90-20 ms).
- Test pulses were applied after each train to assess sodium current activation, inactivation, and recovery.
Main Results:
- Sodium current was detectable during brief resting intervals as short as 20 ms.
- Short resting intervals (<60 ms) shifted the sodium conductance activation curve to more depolarized potentials.
- Less sodium channel inactivation and faster recovery from inactivation were observed with shorter prepulses, facilitating sodium current during simulated VF.
Conclusions:
- Inward sodium current is evident even with diastolic rest intervals as short as 10-20 ms.
- Sodium channel activation, inactivation, and recovery are influenced by the interplay of rest interval, depolarization length, and membrane potential.
- These findings suggest that sodium channels contribute to wavefront propagation during ventricular fibrillation by maintaining inward sodium current despite brief action potentials and short diastolic intervals.