Role of Reduced Sarco-Endoplasmic Reticulum Ca-ATPase Function on Sarcoplasmic Reticulum Ca Alternans in the Intact Rabbit Heart.

Sarcoplasmic reticulum (SR) Ca cycling is tightly regulated by ryanodine receptor (RyR) Ca release and sarco-endoplasmic reticulum Ca-ATPase (SERCA) Ca uptake during each excitation-contraction coupling cycle. We previously showed that RyR refractoriness plays a key role in the onset of SR Ca alternans in the intact rabbit heart, which contributes to arrhythmogenic action potential duration (APD) alternans. Recent studies have also implicated impaired SERCA function, a key feature of heart failure, in cardiac alternans and arrhythmias. However, the relationship between reduced SERCA function and SR Ca alternans is not well understood. Simultaneous optical mapping of transmembrane potential (V) and SR Ca was performed in isolated rabbit hearts ( = 10) using the voltage-sensitive dye RH237 and the low-affinity Ca indicator Fluo-5N-AM. Alternans was induced by rapid ventricular pacing. SERCA was inhibited with cyclopiazonic acid (CPA; 1-10 μM). SERCA inhibition (1, 5, and 10 μM of CPA) resulted in dose-dependent slowing of SR Ca reuptake, with the time constant () increasing from 70.8 ± 3.5 ms at baseline to 85.5 ± 6.6, 129.9 ± 20.7, and 271.3 ± 37.6 ms, respectively ( < 0.05 vs. baseline for all doses). At fast pacing frequencies, CPA significantly increased the magnitude of SR Ca and APD alternans, most strongly at 10 μM (pacing cycle length = 220 ms: SR Ca alternans magnitude: 57.1 ± 4.7 vs. 13.4 ± 8.9 AU; APD alternans magnitude 3.8 ± 1.9 vs. 0.2 ± 0.19 AU; < 0.05 10 μM of CPA vs. baseline for both). SERCA inhibition also promoted the emergence of spatially discordant alternans. Notably, at all CPA doses, alternation of SR Ca release occurred prior to alternation of diastolic SR Ca load as pacing frequency increased. Simultaneous optical mapping of SR Ca and V in the intact rabbit heart revealed that SERCA inhibition exacerbates pacing-induced SR Ca and APD alternans magnitude, particularly at fast pacing frequencies. Importantly, SR Ca release alternans always occurred before the onset of SR Ca load alternans. These findings suggest that even in settings of diminished SERCA function, relative refractoriness of RyR Ca release governs the onset of intracellular Ca alternans.