Liu D W, Antzelevitch C
Masonic Medical Research Laboratory, Utica, NY 13504.
Circ Res. 1995 Mar;76(3):351-65. doi: 10.1161/01.res.76.3.351.
Recent studies have described regional differences in the electrophysiology and pharmacology of ventricular myocardium in canine, feline, rat, guinea pig, and human hearts. In this study, we use standard microelectrode and whole-cell patch-clamp techniques to examine the characteristics of the action potential and the delayed rectifier K+ current (IK) in epicardial, M region (deep subepicardial to midmyocardial), and endocardial cells isolated from the canine left ventricle. Cells from the M region displayed much longer action potential durations (APDs) at slow rates. At a basic cycle length of 4 s, APD measured at 90% repolarization was 358 +/- 16 (mean +/- SEM), 262 +/- 12, and 287 +/- 11 ms in cells from the M region, epicardium, and endocardium, respectively. Steady state APD-rate relations were steeper in cells from the M region. In complete Tyrode's solution, IK was smaller in myocytes from the M region when compared with those isolated from the epicardium or endocardium. Further characterization of IK was conducted in a Na(+)-, K(+)-, and Ca(2+)-free bath solution to isolate the slowly activating component of the delayed rectifier (IKs) from the rapidly activating component (IKr). IKs was significantly smaller in M cells than in epicardial and endocardial cells. With repolarization to -20 mV, IKs tail current density was 1.99 +/- 0.30 pA/pF (mean +/- SEM) in epicardial cells, 1.83 +/- 0.18 pA/pF in endocardial cells, and 0.92 +/- 0.14 pA/pF in M cells. Voltage dependence and time course of activation and deactivation of IKs were similar in the three cell types. The relative contribution of IKr and IKs among the three cell types was examined by using 6 mmol/L [K+]o Tyrode's solution with and without E-4031, a highly selective blocker of IKr. An E-4031-sensitive current was observed in the presence but not in the absence of extracellular K+. This rapidly activating component showed characteristics similar to those of IKr as described in rabbit and cat ventricular cells. Deactivation of IKr was significantly slower than that of IKs. IKr (E-4031-sensitive component) tail current density was similar in the three cell types, whereas IKs (E-4031-insensitive component) tail current density was significantly smaller in the M cells. Our results suggest that the distinctive phase-3 repolarization features of M cells are due in part to a lesser contribution of IKs and that this distinction may also explain why M cells are the main targets for agents that prolong APD in ventricular myocardium.(ABSTRACT TRUNCATED AT 400 WORDS)
近期研究描述了犬、猫、大鼠、豚鼠和人类心脏心室肌电生理学和药理学的区域差异。在本研究中，我们使用标准微电极和全细胞膜片钳技术，检测从犬左心室分离的心外膜、M区域（心外膜深层至心肌中层）和心内膜细胞的动作电位特征和延迟整流钾电流（IK）。M区域的细胞在慢速刺激时表现出长得多的动作电位时程（APD）。在基础周期长度为4秒时，M区域、心外膜和心内膜细胞在90%复极化时测量的APD分别为358±16（平均值±标准误）、262±12和287±11毫秒。M区域细胞的稳态APD-频率关系更陡峭。在完全的台氏液中，与从心外膜或心内膜分离的细胞相比，M区域的心肌细胞中IK较小。在无钠、钾和钙的浴液中对IK进行进一步特性分析，以从快速激活成分（IKr）中分离出延迟整流器的缓慢激活成分（IKs）。M细胞中的IKs明显小于心外膜和心内膜细胞。复极化至-20 mV时，心外膜细胞中IKs尾电流密度为1.99±0.30 pA/pF（平均值±标准误），心内膜细胞中为1.83±0.18 pA/pF，M细胞中为0.92±0.14 pA/pF。三种细胞类型中IKs的电压依赖性以及激活和失活的时间进程相似。通过使用含和不含E-4031（一种高度选择性的IKr阻滞剂）的6 mmol/L [K+]o台氏液，研究了三种细胞类型中IKr和IKs的相对贡献。在有细胞外钾存在但无细胞外钾时未观察到E-4031敏感电流。这种快速激活成分表现出与兔和猫心室细胞中描述的IKr相似的特征。IKr的失活明显慢于IKs。三种细胞类型中IKr（E-4031敏感成分）尾电流密度相似，而M细胞中IKs（E-4031不敏感成分）尾电流密度明显较小。我们的结果表明，M细胞独特的3期复极化特征部分归因于IKs的贡献较小，这种差异也可能解释了为什么M细胞是延长心室肌APD药物的主要作用靶点。（摘要截短至400字）
