Abstract
Sarcoplasmic reticulum (SR) Ca2+ cycling is tightly regulated by ryanodine receptor (RyR) Ca2+ release and sarco-endoplasmic reticulum Ca2+-ATPase (SERCA) Ca2+ uptake during each excitation–contraction coupling cycle. We previously showed that RyR refractoriness plays a key role in the onset of SR Ca2+ 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 Ca2+ alternans is not well understood. Simultaneous optical mapping of transmembrane potential (Vm) and SR Ca2+ was performed in isolated rabbit hearts (n = 10) using the voltage-sensitive dye RH237 and the low-affinity Ca2+ 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 Ca2+ reuptake, with the time constant (tau) 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 (p < 0.05 vs. baseline for all doses). At fast pacing frequencies, CPA significantly increased the magnitude of SR Ca2+ and APD alternans, most strongly at 10 μM (pacing cycle length = 220 ms: SR Ca2+ 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; p < 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 Ca2+ release occurred prior to alternation of diastolic SR Ca2+ load as pacing frequency increased. Simultaneous optical mapping of SR Ca2+ and Vm in the intact rabbit heart revealed that SERCA inhibition exacerbates pacing-induced SR Ca2+ and APD alternans magnitude, particularly at fast pacing frequencies. Importantly, SR Ca2+ release alternans always occurred before the onset of SR Ca2+ load alternans. These findings suggest that even in settings of diminished SERCA function, relative refractoriness of RyR Ca2+ release governs the onset of intracellular Ca2+ alternans.
Highlights
In mammalian hearts, Ca2+ release from and reuptake into the sarcoplasmic reticulum (SR) is tightly regulated for proper excitation–contraction coupling (ECC) (Bers, 2002a)
To determine the role of sarco-endoplasmic reticulum Ca2+-ATPase (SERCA) function on transmembrane potential (Vm) and Sarcoplasmic reticulum (SR) Ca2+, increasing doses of cyclopiazonic acid (CPA), a specific SERCA inhibitor (Seidler et al, 1989; Szentesi et al, 2004), were added to the perfusate, while simultaneous imaging of RH237 (Vm) and Fluo5N (SR Ca2+) was performed at 300-ms pacing cycle length (PCL) (Figures 1A,B)
The marked prolongation of action potential duration (APD) in the presence of CPA suggests that alterations in SR Ca2+ reuptake feed back onto Vm to influence action potential (AP) dynamics, potentially due to a reduction in Ca2+-dependent inactivation
Summary
Ca2+ release from and reuptake into the sarcoplasmic reticulum (SR) is tightly regulated for proper excitation–contraction coupling (ECC) (Bers, 2002a). At steady state, during each ECC cycle, Ca2+ is released from the SR through ryanodine receptors (RyRs) during systole, and the same amount of Ca2+ is taken back up by the sarco-endoplasmic reticulum Ca2+-ATPase (SERCA) pump during diastole. Several recent studies have shown that RyR function and expression play a key role in the onset of Ca2+ alternans (Kornyeyev et al, 2012; Wang et al, 2014; Zhong et al, 2016; Sun et al, 2018; Zhong et al, 2018). Using optical mapping of free intra-SR Ca2+ in the intact rabbit heart, we showed that as heart rate increases, SR Ca2+ release begins to alternate without appreciable changes in diastolic SR Ca2+ load, suggesting that refractoriness of RyR Ca2+ release governs the onset of alternans (Wang et al, 2014).
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a callDisclaimer: All third-party content on this website/platform is and will remain the property of their respective owners and is provided on "as is" basis without any warranties, express or implied. Use of third-party content does not indicate any affiliation, sponsorship with or endorsement by them. Any references to third-party content is to identify the corresponding services and shall be considered fair use under The CopyrightLaw.
