Intracellular calcium (Ca²⁺) buffering is essential for cardiac excitation–contraction (EC) coupling and electrophysiological stability. In cardiomyocytes, ~99% of Ca²⁺ is bound to buffers—primarily myofilaments—leaving only a small fraction free to regulate signaling. Even modest changes in buffering affinity can disrupt Ca²⁺ homeostasis and promote arrhythmogenic remodeling.

In a new study published, researchers from the University Medical Center Göttingen and the German Center for Cardiovascular Research (DZHK), examined the effects of blebbistatin (a myosin II ATPase inhibitor commonly used to uncouple contraction) on calcium buffering and ion channel function in human iPSC-derived atrial cardiomyocytes (iPSC-aCM).

Key findings:

• Reduced buffer affinity: Blebbistatin (10 μmol/L) increased the buffer dissociation constant (K_d), indicating lower Ca²⁺ binding affinity. The maximum buffering capacity (B_max) remained unchanged.
• Elevated free Ca²⁺: Treated cells showed significantly higher diastolic and systolic Ca²⁺ levels, increased sarcoplasmic reticulum (SR) Ca²⁺ leak, and more frequent spontaneous Ca²⁺ release events (sparks).
• Electrophysiological changes: Peak I_Na and I_Ca,L densities were unaffected, but I_Ca,L inactivation kinetics were faster, and I_K1 density was reduced.
• Reversibility: All changes were abolished by intracellular Ca²⁺ chelation with EGTA, confirming that effects were secondary to altered buffering.

This study highlights intracellular Ca²⁺ buffering as a modifiable target in arrhythmia research. Blebbistatin-induced desensitization of Ca²⁺ buffers mimics pathological conditions seen in atrial fibrillation and cardiomyopathy. The reversibility of these effects with EGTA suggests that pharmacological modulation of Ca²⁺ buffering (either through sensitizers or chelators) could be a viable strategy for anti-arrhythmic therapy development.

Moreover, blebbistatin incubation may serve as a model system for screening compounds that restore Ca²⁺ homeostasis, offering a translational bridge between cellular mechanisms and therapeutic innovation.

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📖 Read the full article: Blebbistatin reduces calcium buffering in cardiomyocytes: Consequences for cellular electrophysiology (Journal of Physiology, 2025)

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