Background

Despite the many studies carried out over the past 40 years, the contribution of the HCN4 encoded hyperpolarization-activated ‘funny’ current (I_f) to pacemaker activity in the mammalian sinoatrial node (SAN), and the human SAN in particular, is still controversial and not fully established.

Objective

To study the contribution of I_f to diastolic depolarization of human SAN cells and its dependence on heart rate, cAMP levels, and atrial load.

Methods

HCN4 channels were expressed in human cardiac myocyte progenitor cells (CMPCs) and HCN4 currents assessed using perforated patch-clamp in traditional voltage clamp mode and during action potential clamp with human SAN-like action potential waveforms with 500–1500 ms cycle length, in absence or presence of forskolin to mimic β-adrenergic stimulation and a −15 mV command potential offset to mimic atrial load.

Results

Forskolin significantly increased the fully-activated HCN4 current density at −140 mV by 14% and shifted the steady-state activation curve by +7.4 mV without affecting its slope. In addition, forskolin significantly accelerated current activation but slowed deactivation. The HCN4 current did not completely deactivate before the subsequent diastolic depolarization during action potential clamp. The amplitude of HCN4 current increased with increasing cycle length, was significantly larger in the presence of forskolin at all cycle lengths, and was significantly increased upon the negative offset to the command potential.

Conclusions

I_f is active during a human SAN action potential waveform and its amplitude is modulated by heart rate, β-adrenergic stimulation, and diastolic voltage range, such that I_f is under delicate control.

中文翻译：

---

人类窦房结样动作电位期间的 HCN4 电流

背景

尽管在过去 40 年中进行了许多研究，但HCN4编码的超极化激活“有趣”电流 (I _f ) 对哺乳动物窦房结 (SAN) 尤其是人类 SAN 的起搏器活动的贡献仍然是有争议且未完全成立。

客观的

研究 I _f对人类 SAN 细胞舒张期去极化的贡献及其对心率、cAMP 水平和心房负荷的依赖性。

方法

HCN4 通道在人心肌祖细胞 (CMPC) 中表达，在传统电压钳模式下使用穿孔膜片钳评估 HCN4 电流，并在动作电位钳期间使用具有 500-1500 ms 周期长度的人 SAN 样动作电位波形进行评估，在不存在的情况下或存在毛喉素以模拟 β-肾上腺素能刺激和 -15 mV 命令电位偏移以模拟心房负荷。

结果

Forskolin 在-140 mV 时将完全激活的 HCN4 电流密度显着增加了 14%，并将稳态激活曲线移动了 +7.4 mV，而不影响其斜率。此外，毛喉素显着加速了电流激活，但减慢了失活。在动作电位钳夹期间，HCN4 电流在随后的舒张去极化之前没有完全失活。HCN4 电流的幅度随着循环长度的增加而增加，在存在毛喉素的情况下在所有循环长度下都显着增大，并且在对命令电位负偏移时显着增加。

结论

I _f在人类 SAN 动作电位波形期间是活跃的，其幅度受心率、β-肾上腺素能刺激和舒张电压范围的调节，因此 I _f处于精细控制之下。