Optical Mapping of Sarcoplasmic Reticulum Ca 2+ in the Intact Heart

Abstract

Rationale: Sarcoplasmic reticulum (SR) Ca 2+ cycling is key to normal excitation–contraction coupling but may also contribute to pathological cardiac alternans and arrhythmia. Objective: To measure intra-SR free [Ca 2+ ] ([Ca 2+ ] SR ) changes in intact hearts during alternans and ventricular fibrillation (VF). Methods and Results: Simultaneous optical mapping of V m (with RH237) and [Ca 2+ ] SR (with Fluo-5N AM) was performed in Langendorff-perfused rabbit hearts. Alternans and VF were induced by rapid pacing. SR Ca 2+ and action potential duration (APD) alternans occurred in-phase, but SR Ca 2+ alternans emerged first as cycle length was progressively reduced (217±10 versus 190±13 ms; P <0.05). Ryanodine receptor (RyR) refractoriness played a key role in the onset of SR Ca 2+ alternans, with SR Ca 2+ release alternans routinely occurring without changes in diastolic [Ca 2+ ] SR . Sensitizing RyR with caffeine (200 μmol/L) significantly reduced the pacing threshold for both SR Ca 2+ and APD alternans (188±15 and 173±12 ms; P <0.05 versus baseline). Caffeine also reduced the magnitude of spatially discordant SR Ca 2+ alternans, but not APD alternans, the pacing threshold for discordance, or threshold for VF. During VF, [Ca 2+ ] SR was high, but RyR remained nearly continuously refractory, resulting in minimal SR Ca 2+ release throughout VF. Conclusions: In intact hearts, RyR refractoriness initiates SR Ca 2+ release alternans that can be amplified by diastolic [Ca 2+ ] SR alternans and lead to APD alternans. Sensitizing RyR suppresses spatially concordant but not discordant SR Ca 2+ and APD alternans. Despite increased [Ca 2+ ] SR during VF, SR Ca 2+ release was nearly continuously refractory. This novel method provides insight into SR Ca 2+ handling during cardiac alternans and arrhythmia.