Yu H, Gao J, Wang H, Wymore R, Steinberg S, McKinnon D, Rosen M R, Cohen I S
Department of Physiology and Biophysics, Institute of Molecular Cardiology, State University of New York at Stony Brook 11794-8661, USA.
Circ Res. 2000 May 26;86(10):1062-8. doi: 10.1161/01.res.86.10.1062.
The Ca(2+)-independent portion of transient outward K(+) current (I(to)) exhibits a transmural gradient in ventricle. To investigate control mechanisms for this gradient, we studied canine epicardial and endocardial ventricular myocytes with use of the whole-cell patch-clamp technique. I(to) was larger in amplitude, had a more negative voltage threshold for activation, and had a more negative midpoint of inactivation in epicardium. Recovery from inactivation was >10-fold slower in endocardium. Incubation of epicardial myocytes with angiotensin II for 2 to 52 hours altered I(to) to resemble unincubated endocardium and reduced the amplitude of the phase 1 notch of the action potential. In contrast, incubation of endocardial myocytes with losartan for 2 to 52 hours altered I(to) to resemble unincubated epicardium and induced a phase 1 notch in the action potential. With RNase protection assays, we determined that incubations with angiotensin II or losartan did not alter mRNA levels for either Kv4.3 or Kv1.4; thus, a change in the alpha subunit for I(to) is unlikely to be responsible. To test whether posttranslational modification produced the effects of angiotensin II, we coexpressed Kv4.3 and the angiotensin II type 1a receptor in Xenopus oocytes. Incubation with angiotensin II increased the time constant for recovery from inactivation of the expressed current by 2-fold with an incubation time constant of 3.7 hours. No effect on activation or inactivation voltage dependence was observed. These results demonstrate that the properties of I(to) in endocardium and epicardium are plastic and likely under the tonic-differing influence of the renin-angiotensin system.
瞬时外向钾电流（I(to)）的钙非依赖性部分在心室呈现跨壁梯度。为了研究该梯度的控制机制，我们使用全细胞膜片钳技术对犬心外膜和心内膜心室肌细胞进行了研究。心外膜的I(to)幅度更大，激活的电压阈值更负，失活的中点电压更负。心内膜从失活状态恢复的速度慢>10倍。用血管紧张素II孵育心外膜心肌细胞2至52小时，可使I(to)发生改变，类似于未孵育的心内膜，并降低动作电位1期切迹的幅度。相反，用氯沙坦孵育心内膜心肌细胞2至52小时，可使I(to)发生改变，类似于未孵育的心外膜，并在动作电位中诱导出1期切迹。通过核糖核酸酶保护分析，我们确定用血管紧张素II或氯沙坦孵育不会改变Kv4.3或Kv1.4的mRNA水平；因此，I(to)的α亚基变化不太可能是原因所在。为了测试翻译后修饰是否产生了血管紧张素II的作用，我们在非洲爪蟾卵母细胞中共表达了Kv4.3和血管紧张素II 1a型受体。用血管紧张素II孵育可使表达电流从失活状态恢复的时间常数增加2倍，孵育时间常数为3.7小时。未观察到对激活或失活电压依赖性的影响。这些结果表明心内膜和心外膜中I(to)的特性是可塑的，可能受肾素-血管紧张素系统不同的紧张性影响。
