Kuo C C
Department of Physiology, National Taiwan University College of Medicine, and Department of Neurology, National Taiwan University Hospital, Taipei 100, Taiwan, Republic of China.
Biophys J. 1998 Dec;75(6):2845-57. doi: 10.1016/S0006-3495(98)77727-6.
Rapidly inactivating K+ current (KA current) is recorded from rat hippocampal neurons by whole-cell patch-clamp technique and suitable voltage protocols. It is found that imipramine, a commonly prescribed tricyclic antidepressant, is an open KA channel blocker with a binding rate constant of 5.6 x 10(6) M-1 s-1 and an apparent dissociation constant of no more than 6 microM if applied extracellularly in pH 7.4. The inhibitory effect is more pronounced in more alkaline extracellular solution, suggesting that the neutral form of imipramine is much more active than the charged form. In contrast, intracellular imipramine shows no inhibitory effect. Furthermore, the inhibitory effect of imipramine is antagonized by external but not internal K+. These findings suggest an imipramine binding site located close to the external pore mouth. It is also found that the inactivation curve of KA current is not changed by imipramine. Moreover, the recovery of KA current after a step depolarization is accelerated in the presence of imipramine. These findings suggest insignificant binding of imipramine to the fast inactivated KA channel. The selective binding of imipramine to only the activated but not the deactivated or inactivated states seems to suggest continual gating conformational changes in the external pore mouth of these neuronal KA channels during membrane depolarization.
采用全细胞膜片钳技术和合适的电压方案，在大鼠海马神经元中记录到快速失活的钾电流（KA电流）。研究发现，常用的三环类抗抑郁药丙咪嗪是一种开放的KA通道阻滞剂，若在pH 7.4的细胞外应用，其结合速率常数为5.6×10⁶ M⁻¹ s⁻¹，表观解离常数不超过6 μM。在碱性更强的细胞外溶液中，抑制作用更明显，这表明丙咪嗪的中性形式比带电形式活性更强。相反，细胞内的丙咪嗪没有抑制作用。此外，丙咪嗪的抑制作用可被细胞外的钾离子拮抗，但不能被细胞内的钾离子拮抗。这些发现表明丙咪嗪的结合位点靠近外部孔口。还发现丙咪嗪不会改变KA电流的失活曲线。此外，在丙咪嗪存在的情况下，阶跃去极化后KA电流的恢复会加速。这些发现表明丙咪嗪与快速失活的KA通道结合不显著。丙咪嗪仅选择性地结合到激活状态而非失活或去激活状态，这似乎表明在膜去极化过程中，这些神经元KA通道的外部孔口存在持续的门控构象变化。
