MEC-induced excitation (‘mechano-arrhythmogenesis’ [MA]) occurs in diastole, when cardiomyocytes (CM) are in their resting phase. In systole (during the action potential [AP]), the heart is protected from MA due to electrical refractoriness and the tight coordination of cardiac electro-mechanical activity. Pathological states that alter this coordination (e.g., disturbances in the relative recovery dynamics of membrane potential and cytosolic Ca²⁺, termed ‘repolarisation-relaxation coupling’ [RRC]) may increase vulnerability to systolic MA. Specifically, a disturbance in RRC may result in the appearance of a vulnerable period (VP_RRC) for Ca²⁺-mediated MA during late repolarisation, as Ca²⁺ remains elevated in progressively re-excitable tissue.

Experiments were performed using rabbit isolated left ventricular (LV) CM or LV/atrial tissue slices. CM were exposed to pinacidil (to activate K_ATP channels), simulated ischemic (SI) solution, or SI after pre-treatment with glibenclamide (to block K_ATP channels), and were transiently stretched during diastole or the VP_RRC using carbon fibres. Pharmacological agents were used to buffer Ca²⁺ (BAPTA), stabilise ryanodine receptors (dantrolene), non-selectively block stretch-activated channels (streptomycin), block (HC-030031) or activate (AITC) TRPA1 channels, or to chelate (NAC) or block (DPI) reactive oxygen species (ROS). Tissue slices were mounted in MyoDish chambers for measurement of force development. Voltage-Ca²⁺ dynamics were simultaneously monitored in CM with fluorescent dyes (di-4-ANBDQPQ, Fluo-5F AM) and a single camera-optical splitter system, and in tissue slices with fluorescent dyes (di-4-ANBDQPQ, Rhod-2 AM) and alternating pulsed excitation.

In CM, pinacidil and SI cause a greater shortening of the AP than of the Ca²⁺ transient. With stretch during the resulting VP_RRC, there is an increase in MA, which is driven by Ca²⁺-induced membrane depolarisation. For pinacidil, MA is decreased by BAPTA, streptomycin, or HC-030031. For SI, glibenclamide reduces VP_RRC duration, with an associated decrease in MA, which is also decreased by BAPTA, HC-030031, NAC, or DPI. With stretch during diastole, pinacidil or AITC increase MA, which is blocked by BAPTA or HC-030031. In tissue slices, simultaneous force development and voltage-Ca²⁺ dynamics can be measured, and are currently being used to investigate the impact of disturbed RRC on systolic MA in native myocardium.

These results indicate that disturbed RRC creates a VP_RRC for systolic MA, which is dependent on TRPA1, Ca²⁺, and ROS, and may represent a novel anti-arrhythmic target.