Koumi S, Sato R, Nagasawa K, Hayakawa H
Division of Cardiology, Department of Medicine and the Feinberg Cardiovascular Research Institute, Northwestern University School of Medicine, Chicago, Illinois, 60611, USA.
J Membr Biol. 1997 May 1;157(1):71-81. doi: 10.1007/s002329900217.
Muscarinic receptor-linked G protein, Gi, can directly activate the specific K+ channel (IK(ACh)) in the atrium and in pacemaker tissues in the heart. Coupling of Gi to the K+ channel in the ventricle has not been well defined. G protein regulation of K+ channels in isolated human ventricular myocytes was examined using the patch-clamp technique. Bath application of 1 microM acetylcholine (ACh) reversibly shortened the action potential duration to 74.4 +/- 12.1% of control (at 90% repolarization, mean +/- SD, n = 8) and increased the whole-cell membrane current conductance without prior beta-adrenergic stimulation in human ventricular myocytes. The ACh effect was reversed by atropine (1 microM). In excised inside-out patch configurations, application of GTPgammaS (100 microM) to the bath solution (internal surface) caused activation of IK(ACh) and/or the background inwardly-rectifying K+ channel (IK1) in ventricular cell membranes. IK(ACh) exhibited rapid gating behavior with a slope conductance of 44 +/- 2 pS (n = 25) and a mean open lifetime of 1.8 +/- 0.3 msec (n = 21). Single channel activity of GTPgammaS-activated IK1 demonstrated long-lasting bursts with a slope conductance of 30 +/- 2 pS (n = 16) and a mean open lifetime of 36.4 +/- 4.1 msec (n = 12). Unlike IK(ACh), G protein-activated IK1 did not require GTP to maintain channel activity, suggesting that these two channels may be controlled by G proteins with different underlying mechanisms. The concentration of GTP at half-maximal channel activation was 0.22 microM in IK(ACh) and 1.2 microM in IK1. Myocytes pretreated with pertussis toxin (PTX) prevented GTP from activating these channels, indicating that muscarinic receptor-linked PTX-sensitive G protein, Gi, is essential for activation of both channels. G protein-activated channel characteristics from patients with terminal heart failure did not differ from those without heart failure or guinea pig. These results suggest that ACh can shorten the action potential by activating IK(ACh) and IK1 via muscarinic receptor-linked Gi proteins in human ventricular myocytes.
毒蕈碱受体偶联的G蛋白Gi可直接激活心房和心脏起搏组织中的特异性钾通道（IK(ACh)）。Gi与心室中钾通道的偶联尚未明确。采用膜片钳技术研究了分离的人心室肌细胞中G蛋白对钾通道的调节作用。在人心室肌细胞中，浴槽中加入1μM乙酰胆碱（ACh）可使动作电位时程可逆性缩短至对照值的74.4±12.1%（复极化90%时，平均值±标准差，n = 8），并且在未预先进行β-肾上腺素能刺激的情况下增加全细胞膜电流电导。ACh的作用可被阿托品（1μM）逆转。在膜片外翻的膜片钳记录模式下，向浴槽溶液（内表面）中加入GTPγS（100μM）可激活心室细胞膜中的IK(ACh)和/或背景内向整流钾通道（IK1）。IK(ACh)表现出快速门控行为，斜率电导为44±2 pS（n = 25），平均开放寿命为1.8±0.3毫秒（n = 21）。GTPγS激活的IK1的单通道活性表现为持续的爆发，斜率电导为30±2 pS（n = 16），平均开放寿命为36.4±4.1毫秒（n = 12）。与IK(ACh)不同，G蛋白激活的IK1不需要GTP来维持通道活性，这表明这两种通道可能由具有不同潜在机制的G蛋白控制。IK(ACh)通道激活达半数最大效应时的GTP浓度为0.22μM，IK1为1.2μM。用百日咳毒素（PTX）预处理的心肌细胞可阻止GTP激活这些通道，表明毒蕈碱受体偶联的对PTX敏感的G蛋白Gi对于激活这两种通道至关重要。终末期心力衰竭患者的G蛋白激活通道特性与无心力衰竭患者或豚鼠的无差异。这些结果表明，ACh可通过毒蕈碱受体偶联的Gi蛋白激活IK(ACh)和IK1，从而缩短人心室肌细胞的动作电位时程。
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