Song D K, Earm Y E, Ho W K
Department of Physiology and Institute for Medical Science, Keimyung University School of Medicine, Taegu 700-712, Korea.
Pflugers Arch. 1999 Jul;438(2):147-53. doi: 10.1007/s004240050892.
We investigated the actions of various divalent cations on the delayed rectifier K+ currents (IKr) in rabbit sinoatrial node cells using the whole-cell voltage-clamp technique in isotonic K+ solutions. External divalent cations decreased the amplitude of currents, accelerated the time course of deactivation and shifted the activation to positive potentials in a dose-dependent manner. The concentrations for half-maximum inhibition of the steady-state currents (KM) obtained at 0 mV were 0.63, 1.36, 1.65 and 2.16 mM for Ni2+, Co2+, Mn2+ and Ba2+, respectively. The effect was voltage dependent (KM decreased e-fold for 12.2-16.8 mV hyperpolarization), but the dependence did not vary significantly among different cations. Acceleration of the time course of current deactivation by the increase of divalent cation concentration was well fitted by the voltage-dependent block model, and the binding rate constant (k1) was obtained. The binding rates for the ions took the following order: Ni2+ >Co2+ >Mn2+ >Ba2+. The degree of the shift of activation occurred in the same order: Ni2+ >Co2+ >Mn2+ >Ba2+. From these results, we conclude that IKr channels are non-selectively blocked by most divalent cations from the external side and that the binding site is located deep inside the channel, resulting in a steep voltage dependence of the blockade.
我们采用全细胞膜片钳技术，在等渗钾溶液中研究了多种二价阳离子对兔窦房结细胞延迟整流钾电流（IKr）的作用。细胞外二价阳离子降低了电流幅度，加速了失活的时间进程，并使激活电位向更正电位移动，且呈剂量依赖性。在0 mV时，Ni2+、Co2+、Mn2+和Ba2+使稳态电流半数抑制浓度（KM）分别为0.63、1.36、1.65和2.16 mM。这种作用具有电压依赖性（超极化12.2 - 16.8 mV时，KM下降10倍），但不同阳离子之间的这种依赖性差异不显著。二价阳离子浓度增加对电流失活时间进程的加速作用，能很好地用电压依赖性阻断模型拟合，并得到了结合速率常数（k1）。离子的结合速率顺序如下：Ni2+ > Co2+ > Mn2+ > Ba2+。激活电位的偏移程度也按相同顺序排列：Ni2+ > Co2+ > Mn2+ > Ba2+。从这些结果，我们得出结论，IKr通道被大多数来自细胞外侧的二价阳离子非选择性阻断，且结合位点位于通道内部深处，导致阻断具有陡峭的电压依赖性。
