Inhibiting persistent inward sodium currents prevents myotonia

Area of Science:

• Neuroscience
• Skeletal Muscle Physiology
• Ion Channelopathies

Background:

• Myotonia congenita results from skeletal muscle hyperexcitability caused by loss-of-function mutations in the ClC-1 chloride channel.
• The precise mechanisms triggering the involuntary muscle action potentials (myotonia) in the absence of normal ClC-1 function remain unclear.
• Understanding these triggers is crucial for developing effective treatments for muscle stiffness.

Purpose of the Study:

• To identify the specific ionic currents responsible for initiating spontaneous muscle firing in conditions of reduced ClC-1 chloride channel activity.
• To elucidate the underlying electrophysiological mechanisms of myotonia in a relevant preclinical model.

Main Methods:

• Utilized in vitro intracellular current clamp and voltage clamp electrophysiological recordings.
• Experiments were conducted on muscle tissue derived from a mouse model engineered to mimic myotonia congenita.

Main Results:

• Identified a slow afterdepolarization (AfD) as the direct trigger for myotonic action potentials.
• Demonstrated that the AfD is driven by a tetrodotoxin-sensitive, voltage-dependent persistent sodium current (NaPIC) with slow inactivation kinetics.
• Showed that ranolazine and elevated divalent cations effectively eliminate myotonia by inhibiting both AfD and NaPIC.

Conclusions:

• This study reveals that persistent Na⁺ current (NaPIC) is a key mechanism triggering myotonia, challenging previous therapeutic strategies.
• Inhibiting NaPIC effectively eliminates myotonia, suggesting it as a more promising therapeutic target than blocking transient sodium currents.
• Proposes that ideal myotonia therapies should selectively target NaPIC while preserving normal transient sodium currents essential for muscle contraction.