TRPA1 mediates stretch-induced increase in atrial Ca2+-spark rate

Introduction: Stretch modulates atrial electrophysiology and promotes the initiation and maintenance of atrial fibrillation (AF). In the ventricles, stretch and arrhythmic behaviors can be linked through an acutely increased spontaneous release of calcium (Ca²⁺) from the sarcoplasmic reticulum via ryanodine receptors (“Ca²⁺ sparks”). One possible mechanism is through activation of cation non-selective stretch-activated channels (SAC_NS), 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: Rabbit freshly isolated left atrial cardiomyocytes were glued to glass micro-rods using a bio-compatible adhesive (MyoTak) and stretched axially. Confocal imaging was performed to monitor SR Ca²⁺ release after a period of pacing (1 min, 2 Hz), at resting sarcomere length (SL; ≈ 1.79 µm) and during sustained axial stretch in the absence of electrical pacing (≈ 12% increase in SL).

Results: Application of sustained stretch resulted in an increase in diastolic Ca²⁺-spark rate. This stretch-induced increase in Ca²⁺-spark rate was absent in Na⁺/Ca²⁺-free solution, even though SR Ca²⁺ content was not significantly altered (caffeine exposure experiments), suggesting an involvement of cation influx via the sarcolemma. Indeed, streptomycin, a non-selective blocker of SAC_NS prevented the stretch-induced increase in Ca²⁺-spark rate. Activation of Piezo1 channels (with Yoda1), resulted in a higher baseline Ca²⁺-spark rate, which was further increased upon stretch. Activation of TRPA1 channels (with AITC) led to an elevated baseline Ca²⁺-spark rate comparing to control, without further increase upon stretch. 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 the presence of reactive oxygen species, was required to maintain the stretch-induced Ca²⁺ spark increase (assessed by exposure to colchicine and N-acetylcysteine, respectively).

Conclusion: In atrial cardiomyocytes, axial stretch induces an increase in diastolic SR Ca²⁺-release through a mechanism that involves transmembrane Na⁺ and/or Ca²⁺ flux, presumably via stretch-activated channels, specifically TRPA1; this requires microtubular integrity, but is independent of redox signalling. Disease-related alterations in the microtubule network, stretch-activated channel expression, and/or channel gating may alter this mechano-sensitive response and contribute to arrhythmogenesis.