Optical Mapping of Sarcoplasmic Reticulum Ca2+ in the Intact Heart

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 V-m (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 (21710 versus 190 +/- 13 ms; 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 (200 mol/L) significantly reduced the pacing threshold for both SR Ca2+ and APD alternans (188 +/- 15 and 173 +/- 12 ms; P<0.05 versus 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.