Cell Physiology Laboratory, Cardiovascular Research Centre CSIC-ICCC, and Cardiology Department, Hospital de Santa Creu i Sant Pau, 08025 Barcelona, Spain.
J Physiol. 2011 Jul 1;589(Pt 13):3247-62. doi: 10.1113/jphysiol.2010.197715. Epub 2011 Apr 26.
Irregularities in intracellular calcium on a beat-to-beat basis can precede cardiac arrhythmia, but the mechanisms inducing such irregularities remain elusive. This study tested the hypothesis that sarcoplasmic reticulum (SR) and L-type calcium channel activity determine the beat-to-beat response and its rate dependency. For this purpose, patch-clamp technique and confocal calcium imaging was used to record L-type calcium current (ICa) and visualize calcium in human atrial myocytes subjected to increasing stimulation frequencies (from 0.2 to 2 Hz). The beat-to-beat response was heterogeneous among a population of 133 myocytes, with 30 myocytes responding uniformly at all frequencies, while alternating and irregular responses were induced in 78 and 25 myocytes, respectively. Myocytes with uniform responses had the lowest frequency of calcium wave-induced transient inward currents (ITI; 0.4 ± 0.2 min⁻¹), ICa density (1.8 ± 0.3 pA pF⁻¹) and caffeine-releasable calcium load (6.2 ± 0.5 amol pF⁻¹), while those with alternating responses had the highest ITI frequency (1.8 ± 0.3 min⁻¹,P =0.003) and ICa density (2.4 ± 0.2 pA pF⁻¹, P =0.04). In contrast, the calcium load was highest in myocytes with irregular responses (8.5 ± 0.7 amol pF⁻¹, P =0.01). Accordingly, partial ICa inhibition reduced the incidence (from 78 to 44%, P <0.05) and increased the threshold frequency for beat-to-beat alternation (from 1.3 ± 0.2 to 1.9 ± 0.2 Hz, P <0.05). Partial inhibition of SR calcium release reduced the ITI frequency, increased calcium loading and favoured induction of irregular responses, while complete inhibition abolished beat-to-beat alternation at all frequencies. In conclusion, the beat-to-beat response was heterogeneous among human atrial myocytes subjected to increasing stimulation frequencies, and the nature and stability of the response were determined by the SR and L-type calcium channel activities, suggesting that these mechanisms are key to controlling cardiac beat-to-beat stability.
在逐搏基础上细胞内钙的不规则可能先于心律失常，但诱导这种不规则的机制仍不清楚。本研究测试了这样一个假设，即肌浆网 (SR) 和 L 型钙通道活性决定了逐搏反应及其频率依赖性。为此，使用膜片钳技术和共聚焦钙成像技术记录 L 型钙电流 (ICa) 并可视化在递增刺激频率 (从 0.2 到 2 Hz) 下的人心房肌细胞中的钙。在 133 个心肌细胞的群体中，逐搏反应具有异质性，其中 30 个心肌细胞在所有频率下均呈均匀反应，而交替和不规则反应分别在 78 个和 25 个心肌细胞中诱导。具有均匀反应的心肌细胞具有最低频率的钙波诱导瞬时内向电流 (ITI；0.4±0.2 min⁻¹)、ICa 密度 (1.8±0.3 pA pF⁻¹) 和咖啡因可释放钙负荷 (6.2±0.5 amol pF⁻¹)，而具有交替反应的心肌细胞具有最高的 ITI 频率 (1.8±0.3 min⁻¹，P=0.003) 和 ICa 密度 (2.4±0.2 pA pF⁻¹，P=0.04)。相比之下，具有不规则反应的心肌细胞的钙负荷最高 (8.5±0.7 amol pF⁻¹，P=0.01)。相应地，部分 ICa 抑制降低了逐搏交替的发生率 (从 78%降至 44%，P<0.05)，并增加了逐搏交替的阈值频率 (从 1.3±0.2 至 1.9±0.2 Hz，P<0.05)。部分抑制 SR 钙释放降低了 ITI 频率，增加了钙负荷，并有利于诱导不规则反应，而完全抑制则在所有频率下消除了逐搏交替。总之，在递增刺激频率下，人心房肌细胞的逐搏反应具有异质性，反应的性质和稳定性由 SR 和 L 型钙通道活性决定，表明这些机制是控制心脏逐搏稳定性的关键。
