Lei Ming, Cooper Patricia J, Camelliti Patrizia, Kohl Peter
Department of Physiology, University of Oxford, Oxford OX1 3PT, UK.
Cardiovasc Res. 2002 Jan;53(1):68-79. doi: 10.1016/s0008-6363(01)00459-x.
(i) to characterize the electrophysiological properties of the slowly activating delayed rectifier potassium current, i(Ks), defined as the 293b-sensitive current, during the action potential (AP) of rabbit sino-atrial node (SAN) pacemaker cells; (ii) to evaluate the contribution of i(Ks) to the pacemaker AP under physiological conditions and during beta-adrenergic stimulation.
Rabbit SAN pacemaker cells were studied using the perforated patch clamp technique in voltage-, AP- and current-clamp modes.
Voltage-clamp findings. Block of i(Ks) by 293b is dose-dependent, with an IC(50) (half block) in rabbit SAN cells of 1.35 microM and an IC(80) (sub-maximal block) of 5 microM. Sub-maximal concentrations of 293b have no significant effects on long-lasting and transient inward calcium currents, i(Ca,L) and i(Ca,T), inward hyperpolarization activated current, i(f), and transient outward current, i(to). AP-clamp experiments. The 293b-sensitive current activates near the peak of the SAN pacemaker action potential, reaches a mean maximal current density of 1.0+/-0.3 pA/pF (n=8, cell capacitances 27 to 62 pF, mean 35+/-4.0 pF) during late repolarization, and inactivates towards the end of repolarization. Additionally, in two smaller cells (cell capacitances 15 and 23 pF), no discernible 293b-sensitive current component was detected. Current-clamp data. In spontaneously beating SAN cells under control conditions, sub-maximal block of i(Ks) by 5 microM 293b has negligible effects on action potential characteristics and does not change average cycle length (n=11). In contrast, after pre-treatment with 10 nM isoprenaline to mimic beta-adrenergic stimulation, cells showed a 293b-induced depolarization of maximum diastolic potential by 2.2+/-1%, a decrease in diastolic depolarization rate by 9.9+/-4%, and a slowing of late action potential repolarization by 28.7+/-10.2%, resulting in a prolongation of spontaneous cycle length by 9.8+/-3.0% (P<0.05, n=10; for all parameters).
Our findings suggest that in rabbit SAN: (i) i(Ks) is activated during the normal pacemaker AP; (ii) the contribution of i(Ks) to beating rate is small under control conditions; and (iii) i(Ks) contributes significantly to spontaneous pacemaker rate during beta-adrenergic stimulation.
（i）表征缓慢激活延迟整流钾电流（i(Ks)，定义为对293b敏感的电流）在兔窦房结（SAN）起搏细胞动作电位（AP）期间的电生理特性；（ii）评估i(Ks)在生理条件下和β-肾上腺素能刺激期间对起搏细胞动作电位的贡献。
使用穿孔膜片钳技术，在电压钳、动作电位钳和电流钳模式下研究兔SAN起搏细胞。
电压钳结果。293b对i(Ks)的阻断呈剂量依赖性，兔SAN细胞中的半数抑制浓度（IC(50)）为1.35微摩尔，80%抑制浓度（IC(80)）为5微摩尔。亚最大浓度的293b对持久和瞬时内向钙电流（i(Ca,L)和i(Ca,T)）、内向超极化激活电流（i(f)）以及瞬时外向电流（i(to)）无显著影响。动作电位钳实验。293b敏感电流在SAN起搏细胞动作电位峰值附近激活，在复极化后期达到平均最大电流密度1.0±0.3皮安/皮法（n = 8，细胞电容27至62皮法，平均35±4.0皮法），并在复极化末期失活。此外，在两个较小的细胞（细胞电容15和23皮法）中，未检测到可辨别的293b敏感电流成分。电流钳数据。在对照条件下自发搏动的SAN细胞中，5微摩尔293b对i(Ks)的亚最大阻断对动作电位特征的影响可忽略不计，且不改变平均周期长度（n = 11）。相反，在用10纳摩尔异丙肾上腺素预处理以模拟β-肾上腺素能刺激后，细胞显示293b诱导最大舒张电位去极化2.2±1%，舒张期去极化速率降低9.9±4%，动作电位晚期复极化减慢28.7±10.2%，导致自发周期长度延长9.8±3.0%（P<0.05，n = 10；所有参数）。
我们的研究结果表明，在兔SAN中：（i）i(Ks)在正常起搏细胞动作电位期间被激活；（ii）在对照条件下，i(Ks)对搏动频率的贡献较小；（iii）在β-肾上腺素能刺激期间，i(Ks)对自发起搏频率有显著贡献。
