Xu H, Guo W, Nerbonne J M
Department of Molecular Biology and Pharmacology, Washington University School of Medicine, St. Louis, Missouri 63110, USA.
J Gen Physiol. 1999 May;113(5):661-78. doi: 10.1085/jgp.113.5.661.
In the experiments here, the time- and voltage-dependent properties of the Ca2+-independent, depolarization-activated K+ currents in adult mouse ventricular myocytes were characterized in detail. In the majority (65 of 72, approximately 90%) of cells dispersed from the ventricles, analysis of the decay phases of the outward currents revealed three distinct K+ current components: a rapidly inactivating, transient outward K+ current, Ito,f (mean +/- SEM taudecay = 85 +/- 2 ms); a slowly (mean +/- SEM taudecay = 1,162 +/- 29 ms) inactivating K+ current, IK,slow; and a non inactivating, steady state current, Iss. In a small subset (7 of 72, approximately 10%) of cells, Ito,f was absent and a slowly inactivating (mean +/- SEM taudecay = 196 +/- 7 ms) transient outward current, referred to as Ito,s, was identified; the densities and properties of IK,slow and Iss in Ito,s-expressing cells are indistinguishable from the corresponding currents in cells with Ito,f. Microdissection techniques were used to remove tissue pieces from the left ventricular apex and from the ventricular septum to allow the hypothesis that there are regional differences in Ito,f and Ito,s expression to be tested directly. Electrophysiological recordings revealed that all cells isolated from the apex express Ito,f (n = 35); Ito,s is not detected in these cells (n = 35). In the septum, by contrast, all of the cells express Ito,s (n = 28) and in the majority (22 of 28, 80%) of cells, Ito,f is also present. The density of Ito,f (mean +/- SEM at +40 mV = 6.8 +/- 0.5 pA/pF, n = 22) in septum cells, however, is significantly (P < 0.001) lower than Ito,f density in cells from the apex (mean +/- SEM at +40 mV = 34.6 +/- 2.6 pA/pF, n = 35). In addition to differences in inactivation kinetics, Ito,f, Ito,s, and IK,slow display distinct rates of recovery (from inactivation), as well as differential sensitivities to 4-aminopyridine (4-AP), tetraethylammonium (TEA), and Heteropoda toxin-3. IK,slow, for example, is blocked selectively by low (10-50 microM) concentrations of 4-AP and by (>/=25 mM) TEA. Although both Ito,f and Ito,s are blocked by high (>100 microM) 4-AP concentrations and are relatively insensitive to TEA, Ito,f is selectively blocked by nanomolar concentrations of Heteropoda toxin-3, and Ito,s (as well as IK,slow and Iss) is unaffected. Iss is partially blocked by high concentrations of 4-AP or TEA. The functional implications of the distinct properties and expression patterns of Ito,f and Ito,s, as well as the likely molecular correlates of these (and the IK,slow and Iss) currents, are discussed.
在本实验中，详细研究了成年小鼠心室肌细胞中与Ca²⁺无关、去极化激活的K⁺电流的时间和电压依赖性特性。在从心室分离的大多数细胞（72个中的65个，约90%）中，对外向电流衰减相的分析揭示了三种不同的K⁺电流成分：一种快速失活的瞬时外向K⁺电流，Ito,f（平均±标准误，衰减时间常数τdecay = 85 ± 2 ms）；一种缓慢（平均±标准误，τdecay = 1,162 ± 29 ms）失活的K⁺电流，IK,slow；以及一种非失活的稳态电流，Iss。在一小部分细胞（72个中的7个，约10%）中，不存在Ito,f，而是鉴定出一种缓慢失活（平均±标准误，τdecay = 196 ± 7 ms）的瞬时外向电流，称为Ito,s；表达Ito,s的细胞中IK,slow和Iss的密度及特性与表达Ito,f的细胞中的相应电流无差异。采用显微解剖技术从左心室心尖和室间隔取下组织块，以直接检验Ito,f和Ito,s表达存在区域差异的假说。电生理记录显示从心尖分离的所有细胞均表达Ito,f（n = 35）；在这些细胞中未检测到Ito,s（n = 35）。相比之下，在室间隔中，所有细胞均表达Ito,s（n = 28），并且在大多数细胞（28个中的22个，80%）中也存在Ito,f。然而，室间隔细胞中Ito,f的密度（在+40 mV时平均±标准误 = 6.8 ± 0.5 pA/pF，n = 22）显著（P < 0.001）低于心尖细胞中Ito,f的密度（在+40 mV时平均±标准误 = 34.6 ± 2.6 pA/pF，n = 35）。除了失活动力学的差异外，Ito,f、Ito,s和IK,slow还表现出不同的恢复（从失活状态）速率，以及对4 - 氨基吡啶（4 - AP）、四乙铵（TEA）和异足蛛毒素 - 3的不同敏感性。例如，IK,slow被低浓度（10 - 50 μM）的4 - AP和（≥25 mM）的TEA选择性阻断。虽然Ito,f和Ito,s都被高浓度（>100 μM）的4 - AP阻断且对TEA相对不敏感，但Ito,f被纳摩尔浓度的异足蛛毒素 - 3选择性阻断，而Ito,s（以及IK,slow和Iss）不受影响。Iss被高浓度的4 - AP或TEA部分阻断。文中讨论了Ito,f和Ito,s不同特性和表达模式的功能意义，以及这些电流（和IK,slow及Iss）可能的分子关联。
