Optical Mapping of Sarcoplasmic Reticulum Ca2+ in the Intact Heart: Ryanodine Receptor Refractoriness During Alternans and Fibrillation
Rationale: Sarcoplasmic reticulum (SR) Ca2+ cycling is key to normal excitation-contraction coupling but may also contribute to pathological cardiac alternans and arrhythmia.
Objective: To measure intra-SR free [Ca2+] ([Ca2+]SR) changes in intact hearts during alternans and ventricular fibrillation (VF).
Methods and Results: Simultaneous optical mapping of Vm (with RH237) and [Ca2+]SR (with Fluo-5N AM) was performed in Langendorff-perfused rabbit hearts. Alternans and VF were induced by rapid pacing. SR Ca2+ and action potential duration (APD) alternans occurred in-phase, but SR Ca2+ alternans emerged first as cycle length was progressively reduced (217±10ms vs. 190±13ms, p<0.05). Ryanodine receptor (RyR) refractoriness played a key role in the onset of SR Ca2+ alternans, with SR Ca2+ release alternans routinely occurring without changes in diastolic [Ca2+]SR. Sensitizing RyR with caffeine (200μM) significantly reduced the pacing threshold for both SR Ca2+ and APD alternans (188±15ms and 173±12ms, p<0.05 vs. baseline). Caffeine also reduced the magnitude of spatially discordant SR Ca2+ alternans, but not APD alternans, the pacing threshold for discordance, or threshold for VF. During VF, [Ca2+]SR was high, but RyR remained nearly continuously refractory, resulting in minimal SR Ca2+ release throughout VF.
Conclusions: In intact hearts RyR refractoriness initiates SR Ca2+ release alternans, that can be amplified by diastolic [Ca2+]SR alternans and lead to APD alternans. Sensitizing RyR suppresses spatially concordant, but not discordant SR Ca2+ and APD alternans. Despite increased [Ca2+]SR during VF, SR Ca2+ release was nearly continuously refractory. This novel method provides insight into SR Ca2+ handling during cardiac alternans and arrhythmia.
- SR Calcium
- optical mapping
- sarcoplasmic reticulum
- cardiac arrhythmia
- ventricular fibrillation
- Received August 31, 2013.
- Revision received February 18, 2014.
- Accepted February 25, 2014.