Bucchi A, Tognati A, Milanesi R, Baruscotti M, DiFrancesco D
Laboratory of Molecular Physiology and Neurobiology, Department of Biomolecular Sciences and Biotechnology, University of Milano, Italy.
J Physiol. 2006 Apr 15;572(Pt 2):335-46. doi: 10.1113/jphysiol.2005.100776. Epub 2006 Feb 16.
Ivabradine is a 'heart rate-reducing' agent able to slow heart rate, without complicating side-effects. Its action results from a selective and specific block of pacemaker f-channels of the cardiac sinoatrial node (SAN). Investigation has shown that block by ivabradine requires open f-channels, is use dependent, and is affected by the direction of current flow. The constitutive elements of native pacemaker channels are the hyperpolarization-activated cyclic nucleotide-gated (HCN) channels, of which four isoforms (HCN1-4) are known; in rabbit SAN tissue HCN4 is expressed strongly, and HCN1 weakly. In this study we have investigated the blocking action of ivabradine on mouse (m) HCN1 and human (h) HCN4 channels heterologously expressed in HEK 293 cells. Ivabradine blocked both channels in a dose-dependent way with half-block concentrations of 0.94 microm for mHCN1 and 2.0 microm for hHCN4. Properties of block changed substantially for the two channels. Block of hHCN4 required open channels, was strengthened by depolarization and was relieved by hyperpolarization. Block of mHCN1 did not occur, nor was it relieved, when channels were in the open state during hyperpolarization; block required channels to be either closed, or in a transitional state between open and closed configurations. The dependence of block upon current flow was limited for hHCN4, and not significant for mHCN1 channels. In summary our results indicate that ivabradine is an 'open-channel' blocker of hHCN4, and a 'closed-channel' blocker of mHCN1 channels. The mode of action of ivabradine on the two channels is discussed by implementing a simplified version of a previously developed model of f-channel kinetics.
伊伐布雷定是一种“降低心率”的药物，能够减慢心率且无复杂的副作用。其作用源于对心脏窦房结（SAN）起搏f通道的选择性和特异性阻断。研究表明，伊伐布雷定的阻断作用需要开放的f通道，具有使用依赖性，并受电流方向影响。天然起搏通道的组成元件是超极化激活的环核苷酸门控（HCN）通道，已知有四种亚型（HCN1 - 4）；在兔SAN组织中，HCN4表达强烈，而HCN1表达较弱。在本研究中，我们研究了伊伐布雷定对在HEK 293细胞中异源表达的小鼠（m）HCN1和人（h）HCN4通道的阻断作用。伊伐布雷定以剂量依赖性方式阻断这两种通道，mHCN1的半阻断浓度为0.94微摩尔，hHCN4的半阻断浓度为2.0微摩尔。两种通道的阻断特性有很大变化。hHCN4的阻断需要开放通道，去极化可增强阻断作用，超极化可解除阻断。当通道在超极化期间处于开放状态时，mHCN1不会发生阻断，也不会解除阻断；阻断需要通道处于关闭状态，或处于开放与关闭构型之间的过渡状态。hHCN4的阻断对电流的依赖性有限，而mHCN1通道则不显著。总之，我们的结果表明，伊伐布雷定是hHCN4的“开放通道”阻断剂，是mHCN1通道的“关闭通道”阻断剂。通过实施先前开发的f通道动力学模型的简化版本，讨论了伊伐布雷定对这两种通道的作用模式。
