NCX-Mediated Subcellular Ca²⁺ Dynamics Underlying Early Afterdepolarizations in LQT2 Cardiomyocytes

Area of Science:

• Cardiovascular Physiology
• Molecular Cardiology
• Computational Biology

Background:

• Long QT syndrome type 2 (LQT2) is a genetic disorder causing life-threatening ventricular arrhythmias.
• LQT2 arises from loss-of-function mutations in hERG potassium channels, leading to delayed repolarization and early afterdepolarizations (EADs).
• Previous studies indicated intracellular calcium handling abnormalities in LQT2 myocytes.

Purpose of the Study:

• To explain how enhanced forward-mode sodium-calcium exchanger (NCX) current occurs in LQT2 despite unchanged whole-cell calcium levels.
• To elucidate the role of aberrant, late, ryanodine receptor (RyR)-mediated calcium releases in LQT2 arrhythmias.
• To investigate the impact of stochastic RyR activity on calcium release and identify NCX as a therapeutic target.

Main Methods:

• Multiscale ventricular myocyte modeling to simulate calcium dynamics and NCX current.
• Analysis of the relationship between local submembrane calcium concentration and NCX current.
• Experimental validation in transgenic LQT2 rabbits.

Main Results:

• A multiscale model demonstrated that discrete, small-amplitude calcium releases can significantly increase whole-cell NCX current without substantially altering overall cytosolic calcium.
• The study provides novel insights into how altered stochastic RyR activity at the single-channel level leads to aberrant late calcium releases.
• Experimental data from LQT2 rabbits confirmed the arrhythmogenic role of NCX.

Conclusions:

• Enhanced forward-mode NCX current in LQT2 is driven by discrete calcium releases, not global calcium increases.
• Aberrant RyR activity and subsequent NCX activation are critical in LQT2-induced arrhythmias.
• NCX represents a promising therapeutic target for antiarrhythmic strategies in LQT2 patients.