Dumaine Robert, Cordeiro Jonathan M
Department of Physiology and Biophysics, University of Sherbrooke, Sherbrooke, Quebec, Canada J1H 5N4.
J Mol Cell Cardiol. 2007 Feb;42(2):378-89. doi: 10.1016/j.yjmcc.2006.10.019. Epub 2006 Dec 19.
The repolarization reserve determines the ability of drugs to prolong the cardiac action potential duration. Differences in K(+) currents between rabbit and dog cardiac Purkinje cells were studied by recording the transient outward K(+) current (I(to)) as well as the delayed rectifier K(+) currents (I(Ks) and I(Kr)) during repolarization. Purkinje fibers were dissected from dog and rabbit hearts and exposed to enzymatic digestion until isolated cells were obtained. Whole cell voltage clamp methods were used to measure K(+) currents in both cell types. Action potential (AP) recordings from Purkinje cells displayed a rapid phase 1 repolarization due to a prominent I(to) with densities of 13.3+/-2.3 and 9.6+/-0.6 pA/pF at +40 mV in dog and rabbit respectively. I(Ks) tail currents were significantly larger in dog Purkinje cells. I(Kr) tail current densities were comparable in Purkinje cell from both species. Rabbit ventricular and Purkinje cell AP waveforms were used for action potential clamp experiments in TSA201 cells expressing human ether a go-go related gene (HERG). HERG currents elicited by the ventricular waveform reached its maximum amplitude during phase 3 repolarization. In contrast, Purkinje cell AP waveform elicited markedly smaller HERG currents even though the action potential duration was longer. The observations suggest that the fast phase 1 and negative plateau of the Purkinje cell AP limits the contribution of I(Kr) to repolarization. These results provide evidence that rabbit Purkinje cells have a smaller repolarization reserve and provide a biophysical explanation for a previously observed higher sensitivity to QT prolonging drugs in rabbit than dog Purkinje fibers.
复极储备决定了药物延长心脏动作电位时程的能力。通过记录复极过程中的瞬时外向钾电流（I(to)）以及延迟整流钾电流（I(Ks)和I(Kr)），研究了兔和犬心脏浦肯野细胞之间钾电流的差异。从犬和兔心脏中分离出浦肯野纤维，进行酶消化直至获得分离的细胞。采用全细胞电压钳方法测量两种细胞类型中的钾电流。浦肯野细胞的动作电位（AP）记录显示，由于突出的I(to)，在复极的快速1期出现复极化，犬和兔在+40 mV时I(to)密度分别为13.3±2.3和9.6±0.6 pA/pF。犬浦肯野细胞中的I(Ks)尾电流明显更大。两种物种的浦肯野细胞中I(Kr)尾电流密度相当。兔心室和浦肯野细胞的AP波形用于在表达人ether a go-go相关基因（HERG）的TSA201细胞中进行动作电位钳实验。由心室波形引发的HERG电流在复极3期达到最大幅度。相比之下，尽管浦肯野细胞动作电位时程更长，但由其AP波形引发的HERG电流明显更小。这些观察结果表明，浦肯野细胞AP的快速1期和负平台期限制了I(Kr)对复极的贡献。这些结果提供了证据，表明兔浦肯野细胞的复极储备较小，并为先前观察到的兔浦肯野纤维对QT延长药物的敏感性高于犬提供了生物物理学解释。
