Hiraoka M, Sunami A, Fan Z, Sawanobori T
Department of Cardiovascular Diseases, Tokyo Medical and Dental University, Japan.
Ann N Y Acad Sci. 1992 Jan 27;644:33-47. doi: 10.1111/j.1749-6632.1992.tb31000.x.
Ionic mechanisms of early afterdepolarization (EAD) induced by the K(+)-free solution or veratridine were studied with guinea-pig ventricular myocytes using the patch-clamp technique of whole-cell and cell-attached patch configurations. In the K(+)-free solution, myocytes exhibited prolonged action potential duration with humps on the final repolarization phase, which eventually turned into EAD starting around -70 mV and induced triggered activity. Application of 0.5 mM Cd2+ inhibited the development of EAD and caused depolarization of maximum diastolic potentials around -30 mV, although Cd2+ did not prevent prolongation of the action potential. Application of 50-100 microM Ni2+ or 30 microM tetrodotoxin had little effects on EAD and diastolic potentials. The background current-voltage relation examined by a ramp voltage clamp showed inhibition of the inward rectifier K+ current, induction of steady inward current between -40 and -10 mV, and increase in the outward tail current upon repolarization in the K(+)-free solution. Cd2+ completely blocked the steady inward current at the plateau level and partially depressed the delayed outward K+ current, while Ni2+ had no effects on the background I-V relation. Tetrodotoxin showed a mild inhibitory effect on the inward component of the background current negative to -50 mV, but left the steady inward current at the plateau level. Therefore, EAD in the K(+)-free condition is mainly formed by decreased inward rectifier K+ current, activation of the L-type Ca2+ current, and time-dependent decay of the delayed outward K+ current upon repolarization. Application of 25-100 microM veratridine caused marked prolongation of action potential with appearance of regenerative EADs. Action potential prolongation and EADs were partially abolished by Cd2+ and completely eliminated by tetrodotoxin. The single channel current recordings showed a decreased current amplitude, and prolonged and delayed openings of the Na+ channel currents by veratridine. Thus, an ensemble average current showed markedly prolonged decay time constant of 609 msec in veratridine from 3.6 msec in the control. These results indicate that veratridine-induced EAD is mainly formed by altered properties of the Na+ channel current and partly by the L-type Ca2+ current due to slowed repolarization. Thus, EAD can be induced by different ionic mechanisms depending on the basal conditions.
采用全细胞和细胞贴附膜片钳技术，研究了无钾溶液或藜芦碱诱导豚鼠心室肌细胞早期后去极化（EAD）的离子机制。在无钾溶液中，心肌细胞动作电位时程延长，终末复极化阶段出现驼峰，最终在约-70 mV左右转变为EAD并诱发触发活动。应用0.5 mM Cd2+可抑制EAD的发生，并使最大舒张电位在-30 mV左右去极化，尽管Cd2+不能阻止动作电位时程的延长。应用50 - 100 microM Ni2+或30 microM河豚毒素对EAD和舒张电位影响不大。通过斜坡电压钳检查的背景电流-电压关系显示，在无钾溶液中，内向整流钾电流受到抑制，在-40至-10 mV之间诱导出稳定的内向电流，复极化时外向尾电流增加。Cd2+完全阻断平台期的稳定内向电流，并部分抑制延迟外向钾电流，而Ni2+对背景电流-电压关系无影响。河豚毒素对背景电流负于-50 mV的内向成分有轻度抑制作用，但对平台期的稳定内向电流无影响。因此，无钾条件下的EAD主要由内向整流钾电流降低、L型钙电流激活以及复极化时延迟外向钾电流的时间依赖性衰减形成。应用25 - 100 microM藜芦碱可使动作电位明显延长，并出现再生性EAD。Cd2+可部分消除动作电位延长和EAD，河豚毒素可完全消除。单通道电流记录显示，藜芦碱使钠通道电流的幅度降低，开放时间延长和延迟。因此，总体平均电流显示，藜芦碱作用下的衰减时间常数从对照时的3.6 msec显著延长至609 msec。这些结果表明，藜芦碱诱导的EAD主要由钠通道电流特性改变形成，部分由复极化减慢导致的L型钙电流形成。因此，根据基础条件的不同，EAD可由不同的离子机制诱导产生。
