Fleischhauer R, Mitrovic N, Deymeer F, Lehmann-Horn F, Lerche H
Department of Applied Physiology, University of Ulm, D-89069 Ulm, Germany.
Pflugers Arch. 1998 Oct;436(5):757-65. doi: 10.1007/s004240050699.
The F1473S mutation of the adult human skeletal muscle Na+ channel causes paramyotonia congenita, a disease characterized by muscle stiffness sometimes followed by weakness in a cold environment. The symptoms are relieved by the local anaesthetic mexiletine. This mutation, which resides in the cytoplasmic S4-S5 loop in domain IV of the alpha-subunit, was studied by heterologous expression in HEK293 cells using standard patch-clamp techniques. Compared to wild-type (WT) channels, those with the F1473S mutation exhibit a twofold slowing of fast inactivation, an increased persistent Na+ current, a +18-mV shift in steady-state inactivation and a fivefold acceleration of recovery from fast inactivation; slow inactivation was similar for both clones. Single-channel recordings for the F1473S mutation revealed a prolonged mean open time and an increased number of channel reopenings that increased further upon cooling. The pharmacological effects of mexiletine on cells expressing either WT, F1473S or G1306E channels were studied. G1306E is a myotonia-causing mutation located within the inactivation gate that displays similar but stronger inactivation defects than F1473S. The hyperpolarizing shift in steady-state inactivation induced by mexiletine was almost identical for all three clones. In contrast, this agent had a reduced effectiveness on the phasic (use-dependent) block of Na+ currents recorded from the mutants: the relative order of block was WT>F1473S>G1306E. We suggest that the relative effectiveness of mexiletine is associated with the degree of abnormal channel inactivation and that the relative binding affinity of mexiletine is not substantially different between the mutations or the WT.
成人人类骨骼肌钠通道的F1473S突变会导致先天性副肌强直，该病的特征是在寒冷环境中出现肌肉僵硬，有时随后会出现肌无力。局部麻醉药美西律可缓解这些症状。该突变位于α亚基结构域IV的胞质S4 - S5环中，采用标准膜片钳技术在HEK293细胞中进行异源表达研究。与野生型（WT）通道相比，具有F1473S突变的通道快速失活减慢两倍，持续性钠电流增加，稳态失活向正18 mV偏移，从快速失活恢复的速度加快五倍；两个克隆的慢失活情况相似。F1473S突变的单通道记录显示平均开放时间延长，通道重新开放的次数增加，冷却后进一步增加。研究了美西律对表达WT、F1473S或G1306E通道的细胞的药理作用。G1306E是位于失活门内的一个导致肌强直的突变，其显示出与F1473S相似但更强的失活缺陷。美西律诱导的稳态失活的超极化偏移在所有三个克隆中几乎相同。相比之下，该药物对突变体记录的钠电流的时相性（使用依赖性）阻断效果降低：阻断的相对顺序为WT>F1473S>G1306E。我们认为美西律的相对有效性与通道失活异常的程度有关，并且美西律的相对结合亲和力在突变体或WT之间没有实质性差异。
