1Universitäts-Herzzentrum Freiburg - Bad Krozingen Institut für Experimentelle Kardiovaskuläre Medizin Freiburg im Breisgau, Deutschland; 2Cardiovascular Research Institute Maastricht Department of Physiology Maastricht, Niederlande
Introduction: Stretch modulates atrial electrophysiology and promotes the initiation and maintenance of atrial fibrillation (AF). In ventricles, stretch and arrhythmic behaviors can be linked through the acutely increased spontaneous release of calcium (Ca2+) from the sarcoplasmic reticulum via ryanodine receptors (“Ca2+ sparks”). One possible mechanism is through activation of non-selective cation stretch-activated channels (SACNS), which may increase ryanodine receptor activity via Ca2+ influx. Our aim was to determine whether diastolic stretch enhances SR Ca2+ release in single atrial cardiomyocytes, and if so, to identify specific molecular mechanisms underlying this mechano-sensitive phenomenon.
Methods: Freshly isolated rabbit left atrial cardiomyocytes were glued to glass micro-rods using a bio adhesive (MyoTak) and stretched axially. Confocal imaging was performed to monitor SR Ca2+ release at resting sarcomere length (SL) immediately following a period of pacing (1 min, 2 Hz), and during sustained axial stretch (≈ 12% increase in SL).
Results: The application of diastolic stretch resulted in an increase in Ca2+-spark rate. This stretch-induced increase in Ca2+-spark rate was absent in Na+/Ca2+-free solution suggesting the involvement of cation influx at the sarcolemma. Indeed, streptomycin, a non-selective blocker of SACNS prevented a stretch-induced increase in Ca2+-spark rate. Activation of Piezo1 channels (with Yoda1), resulted in a higher baseline Ca2+-spark rate and the mechanosensitive effect on Ca2+-spark rate upon stretch remained intact. Activation of TRPA1 channels (with AITC) led to an elevated baseline Ca2+-spark rate, without further increase upon stretch. The block of TRPA1 channels (with either A967079 or HC-030031) abolished the stretch-induced change in Ca2+-spark rate. In addition, microtubule integrity, but not presence of reactive oxygen species was required to maintain stretch-induced Ca2+ spark increase.
Conclusion: In atrial cardiomyocytes, diastolic stretch induces an increase in SR Ca2+-release through a mechanism that involves stretch-activated channels, specifically TRPA1, and microtubules, but is independent of redox signaling. Disease-related alterations in the microtubule network, stretch-activated channel expression, or channel gating may alter this mechano-sensitive response and contribute to arrhythmogenesis.