Mitcheson J S, Hancox J C
Department of Physiology and Cardiovascular Research Laboratories, School of Medical Sciences, University Walk, University of Bristol, Bristol, BS8 1TD, UK.
Pflugers Arch. 1999 Jun;438(1):68-78. doi: 10.1007/s004240050881.
A transient outward current (Ito) has been observed in the atrioventricular node (AVN), but its characteristics in Ca-tolerant AVN myocytes have not been investigated previously. In this study, Ito was measured from Ca-tolerant rabbit AVN myocytes at 37 degrees C, using the whole-cell patch-clamp technique. With interfering currents inhibited, 500-ms voltage-clamp pulses applied from -80 mV elicited Ito at potentials positive to -30 mV, which increased in magnitude with test potential amplitude. This current was completely blocked by external application of 5 mM 4-aminopyridine (4-AP). During a command pulse, Ito activated rapidly then inactivated with a bi-exponential time-course. Fast and slow time constants of current inactivation (tauf and taus, respectively) showed voltage dependence. At 0 mV, tauf was 14.5+/-2.7 ms and taus was 112.8+/-21. 2 ms, whilst at +60 mV tauf was 6.7+/-1.1 ms and taus was 63.7+/-9.2 ms (n=25). The steady-state inactivation relationship showed half-maximal inactivation at -33.8 mV (n=8). Re-activation of Ito after an inactivating pre-pulse showed a bi-exponential time-course of recovery: tau1 was 196+/-70 ms, and tau2 was 2707+/-1010 ms (n=6, at -80 mV). Repetitive application of voltage-clamp test pulses showed that Ito inactivation accumulated on repetitive stimulation, but reached a steady state rapidly for a given pulse frequency (0. 2-1.0 Hz). AVN Ito was sensitive to the class 1 anti-arrhythmic flecainide (EC50 for peak current of 24 microM), which showed selectivity for the rapidly inactivating current component. Quinidine also inhibited Ito in a dose-dependent fashion, but did not affect the current time-course. Under voltage-clamp conditions, a simulated diastolic depolarisation from -70 to -45 mV did not significantly reduce Ito amplitude, and under current-clamp conditions 4-AP inhibited spontaneous action potentials. Although this is consistent with a significant role for Ito in shaping AVN activity, under the conditions of this study 4-AP also partially blocked the "rapid" delayed rectifier current, IKr, and so the effects of 4-AP on action potentials could not be attributed exclusively to its effects on Ito.
在房室结（AVN）中已观察到瞬时外向电流（Ito），但其在耐钙AVN心肌细胞中的特性此前尚未被研究。在本研究中，采用全细胞膜片钳技术在37℃下从耐钙兔AVN心肌细胞中测量Ito。在抑制干扰电流的情况下，从-80mV施加500ms的电压钳脉冲，在电位高于-30mV时可诱发Ito，其幅度随测试电位幅度增加。该电流可被外部施加5mM 4-氨基吡啶（4-AP）完全阻断。在指令脉冲期间，Ito迅速激活，然后以双指数时间进程失活。电流失活的快速和慢速时间常数（分别为tauf和taus）显示出电压依赖性。在0mV时，tauf为14.5±2.7ms，taus为112.8±21.2ms，而在+60mV时，tauf为6.7±1.1ms，taus为63.7±9.2ms（n = 25）。稳态失活关系显示在-33.8mV时出现半数最大失活（n = 8）。失活预脉冲后Ito的重新激活显示出双指数恢复时间进程：tau1为196±70ms，tau2为2707±1010ms（n = 6，在-80mV时）。重复施加电压钳测试脉冲表明，Ito失活在重复刺激时会累积，但对于给定的脉冲频率（0.2 - 1.0Hz）会迅速达到稳态。AVN Ito对1类抗心律失常药物氟卡尼敏感（峰值电流的EC50为24μM），其对快速失活的电流成分具有选择性。奎尼丁也以剂量依赖性方式抑制Ito，但不影响电流时间进程。在电压钳条件下，从-70mV到-45mV的模拟舒张期去极化并未显著降低Ito幅度，在电流钳条件下4-AP抑制自发动作电位。尽管这与Ito在塑造AVN活动中起重要作用一致，但在本研究条件下，4-AP也部分阻断了“快速”延迟整流电流IKr，因此4-AP对动作电位的影响不能完全归因于其对Ito的作用。
