Department of Bioengineering, University of Pittsburgh, Pittsburgh, PA 15261, USA.
Am J Physiol Heart Circ Physiol. 2013 Mar 15;304(6):H848-60. doi: 10.1152/ajpheart.00787.2012. Epub 2013 Jan 11.
Bradycardia prolongs action potential (AP) durations (APD adaptation), enhances dispersion of repolarization (DOR), and promotes tachyarrhythmias. Yet, the mechanisms responsible for enhanced DOR and tachyarrhythmias remain largely unexplored. Ca(2+) transients and APs were measured optically from Langendorff rabbit hearts at high (150 × 150 μm(2)) or low (1.5 × 1.5 cm(2)) magnification while pacing at a physiological (120 beats/min) or a slow heart rate (SHR = 50 beats/min). Western blots and pharmacological interventions were used to elucidate the regional effects of bradycardia. As a result, bradycardia (SHR 50 beats/min) increased APDs gradually (time constant τf→s = 48 ± 9.2 s) and caused a secondary Ca(2+) release (SCR) from the sarcoplasmic reticulum during AP plateaus, occurring at the base on average of 184.4 ± 9.7 ms after the Ca(2+) transient upstroke. In subcellular imaging, SCRs were temporally synchronous and spatially homogeneous within myocytes. In diastole, SHR elicited variable asynchronous sarcoplasmic reticulum Ca(2+) release events leading to subcellular Ca(2+) waves, detectable only at high magnification. SCR was regionally heterogeneous, correlated with APD prolongation (P < 0.01, n = 5), enhanced DOR (r = 0.9277 ± 0.03, n = 7), and was gradually reversed by pacing at 120 beats/min along with APD shortening (P < 0.05, n = 5). A stabilizer of leaky ryanodine receptors (RyR2), 3-(4-benzylcyclohexyl)-1-(7-methoxy-2,3-dihydrobenzo[f][1,4]thiazepin-4(5H)-yl)propan-1-one (K201; 1 μM), suppressed SCR and reduced APD at the base, thereby reducing DOR (P < 0.02, n = 5). Ventricular ectopy induced by bradycardia (n = 5/15) was suppressed by K201. Western blot analysis revealed spatial differences of voltage-gated L-type Ca(2+) channel protein (Cav1.2α), Na(+)-Ca(2+) exchange (NCX1), voltage-gated Na(+) channel (Nav1.5), and rabbit ether-a-go-go-related (rERG) protein [but not RyR2 or sarcoplasmic reticulum Ca(2+) ATPase 2a] that correlate with the SCR distribution and explain the molecular basis for SCR heterogeneities. In conclusion, acute bradycardia elicits synchronized subcellular SCRs of sufficient magnitude to overcome the source-sink mismatch and to promote afterdepolarizations.
心动过缓延长动作电位（AP）持续时间（APD 适应），增强复极化离散度（DOR），并促进心动过速。然而，导致 DOR 和心动过速增强的机制在很大程度上仍未得到探索。当以生理（120 次/分钟）或缓慢心率（SHR = 50 次/分钟）起搏时，通过高（150×150μm2）或低（1.5×1.5cm2）放大倍数从 Langendorff 兔心脏光学测量钙瞬变和 AP。使用 Western blot 和药理学干预来阐明心动过缓的区域效应。结果表明，心动过缓（SHR 50 次/分钟）逐渐延长 APD（时间常数 τf→s = 48 ± 9.2s），并在 AP 平台期间引起肌浆网的二次钙释放（SCR），平均在钙瞬变上升后 184.4 ± 9.7ms 发生在基底。在亚细胞成像中，SCR 在心肌细胞内具有时间同步性和空间均匀性。在舒张期，SHR 引起可变的异步肌浆网 Ca2+释放事件，导致亚细胞 Ca2+波，仅在高放大倍数下才可检测到。SCR 呈区域性异质性，与 APD 延长相关（P<0.01，n=5），增强 DOR（r=0.9277±0.03，n=7），并随着起搏而逐渐逆转120 次/分钟的 APD 缩短（P<0.05，n=5）。肌浆网渗漏型ryanodine 受体（RyR2）稳定剂 3-（4-苄基环己基）-1-（7-甲氧基-2,3-二氢苯并[f][1,4]噻嗪-4（5H）-基）丙-1-酮（K201；1μM）抑制 SCR 并降低基底的 APD，从而降低 DOR（P<0.02，n=5）。心动过缓引起的心室异位（n=5/15）被 K201 抑制。Western blot 分析显示电压门控 L 型 Ca2+通道蛋白（Cav1.2α）、Na+-Ca2+交换（NCX1）、电压门控 Na+通道（Nav1.5）和兔醚-a-go-go 相关（rERG）蛋白的空间差异[但不是 RyR2 或肌浆网 Ca2+-ATP 酶 2a]，与 SCR 分布相关，并解释了 SCR 异质性的分子基础。总之，急性心动过缓引发足够幅度的同步亚细胞 SCR，以克服源-汇不匹配并促进后除极。
