Abstract: Sheep Purkinje fibers were used to investigate early afterdepolarization (EAD) generation and its possible ionic mechanisms with bielectrode voltage clamp technique. Under low K⁺ (2.5mmol/LK⁺) superfusion, two levels (-25 and -55mV) of EAD could be induced on the background of automaticity. The bursts in low membrane level EAD were abolished by nifedipine (2.0μmol/L) whereas those in high membrane level were abolished by lidocaine (10μmol/L) and low temperature (30℃). The quasi-steady-state Ｉ-Ｖ relationship of the background currents obtained with step and ramp voltage clamp showed a negative phase in outward part of the rectification under 5.0mmol/LK⁺ superfusion, while an inward current phase appeared between -55 and -25mV level in the curve under 2.5 K⁺ treatment. Under the condition of EAD generation, the low and high membrane level (-25 and -55mV) of voltage were clamped during the repolarization phase of action potential. On low membrane level of -25mV, a slow inward current similar to Ｉ_Si occurred followed by a slight outward current like I_k, whereas on high level of -55mV, a continued inward current looked like I_f occurred. It was suggested that the two levels of plateau were based on the two zero current point in Ｉ-Ｖ curve under 2.5mmol/LK⁺ superfusion, whereas bursts were based on the Ｉ_Si, mainly I_Ca, for the low membrane level and I_f (and slow inactivated I_Na) for the high one.

Key words: purkinje fibers, action potential, early afterdepolarization, low K⁺, nifedipine, lidocaine, I_Ca, I_f

摘要： 用双电极电压箝制技术研究了羊心蒲氏纤维早后去极化(Early afterdepolarization, EAD)的发生及其离子机制。在低钾生理溶液(2.5mmol/LK⁺)灌流下，于自发活动基础上可诱发出2个膜电位水平(-25与-55mV)的EAD。低电位水平的发放可被硝苯吡啶(2.0μmol/L)消除，而高电位的发放可由利多卡因(10μmol/L)或低温(30℃)所抑制。在正常钾浓度(5.0mmol/L)灌流下，无论用阶梯法或斜坡法进行电压钳制，羊心蒲氏纤维的背景电流I-V曲线，均呈外向整流特性。而在低钾灌流条件下，该曲线在-55及-25mV处分别形成跨越(cross over)，即出现一个内向电流区。在低钾灌流并伴有EAD的情况下，于动作电位复极相的低及高电位处(-25与-55mV)分别进行电压钳制。研究发现：当电压钳制在-25mV时出现一个与I_Ca相似的慢内向偏转，随之为一缓慢的I_K样电流。当电压钳制在-55mV时，出现的内向电流与I_f类似。由以上结果可以推测，产生羊心蒲氏纤维EAD的2个膜电位平台是基于背景电流I-V曲线上跨越区(即-25mV及-55mV)的两个零电流点。低电位水平的触发发放基于I_Ca，而高电位水平的发放基于I_f及慢失活I_Na。

关键词: 羊心蒲氏纤维, 动作电位, 早后去极化, 低钾, 硝苯吡啶, 利多卡因, I_Ca, I_f