Radiation-induced remodeling of excitation–contraction coupling in human iPSC-derived ventricular cardiomyocytes

Background Stereotactic cardiac radioablation (cRA) has emerged as a non-invasive treatment for refractory ventricular tachycardia (VT). Although clinical efficacy has been demonstrated, the cellular mechanisms remain incompletely understood. As disturbed intracellular Ca²⁺ handling is central to ventricular arrhythmogenesis, we investigated the effects of cRA on calcium dynamics in human induced pluripotent stem cell–derived ventricular cardiomyocytes (hiPSC-vCM). Methods hiPSC-vCM were exposed to 0, 15, or 30 Gy irradiation. Confocal line-scan imaging of Fluo-4–loaded cells was performed during 1 Hz stimulation. Calcium transients were analyzed under baseline conditions (1.8 mM Ca²⁺) and pro-arrhythmic stress (3 mM Ca²⁺ or isoproterenol 1 µM). Time to peak (TTP), decay constant (TAU), and peak amplitude (F/F₀) were quantified. Results Under baseline conditions, irradiation significantly shortened TTP at 15 Gy and 30 Gy compared with control (both p < 0.001), indicating accelerated Ca²⁺ release kinetics. This effect was preserved under high-calcium conditions (3 mM), with further TTP reduction at 30 Gy (p < 0.0001 vs. control), suggesting enhanced responsiveness of the Ca²⁺ release machinery. Calcium reuptake dynamics were differentially affected. In high-calcium buffer, 30 Gy significantly reduced TAU compared with control (p = 0.0168), consistent with accelerated Ca²⁺ decay. In contrast, under β-adrenergic stimulation (isoproterenol 1 µM), TAU was significantly prolonged at 30 Gy compared with 15 Gy (p = 0.0273), indicating dose-dependent modulation of Ca²⁺ handling under stress conditions. Peak Ca²⁺ amplitude (F/F₀) was largely preserved across groups under baseline and high-calcium conditions, with only a modest reduction at 30 Gy under isoproterenol, suggesting that irradiation primarily affects kinetics rather than systolic Ca²⁺ load. Conclusion Stereotactic irradiation induces dose-dependent remodeling of cardiomyocyte Ca²⁺ handling. This may represent a direct antiarrhythmic mechanism of cRA or, alternatively, reflect selective persistence of functionally robust cardiomyocytes following irradiation.