Ho W K, Brown H F, Noble D
University Laboratory of Physiology, Oxford, UK.
Pflugers Arch. 1994 Jan;426(1-2):68-74. doi: 10.1007/BF00374672.
The ionic selectivity of the hyperpolarization-activated inward current (i(f)) channel to monovalent cations was investigated in single isolated sinoatrial node cells of the rabbit using the whole-cell patch-clamp technique. With a 140 mM K+ pipette, replacement of 90% external Na+ by Li+ caused a -24.5 mV shift of the fully activated current/voltage I/V curve without a significant decrease of the slope conductance. With a 140 mM Cs+ pipette, the i(f) current decreased almost proportionally to the decrease in external [Na+]o as Li+ was substituted. These responses are practically the same as those observed with N-methyl glucamine (NMG+) substitution, suggesting that the relative permeability of Li+ compared with Na+ for the i(f) channel is as low as that of NMG+. When Cs+ or Rb+ was substituted for internal K+, the fully activated I/V relationship for i(f) showed strong inward rectification with a positive reversal potential, indicating low permeability of the i(f) channel for Cs+ and Rb+. These results show that the i(f) channel is highly selective for Na+ and K+ and will not pass the similar ions Li+ and Rb+. Such a high degree of selectivity is unique and may imply that the structure of the i(f) channel differs greatly from that of other Na+ and K+ conducting channels.
利用全细胞膜片钳技术，在单个分离的兔窦房结细胞中研究了超极化激活内向电流（i(f)）通道对单价阳离子的离子选择性。使用含140 mM K+的电极内液，用Li+替代90%的细胞外Na+，导致完全激活的电流/电压I/V曲线负向偏移24.5 mV，而斜率电导无明显降低。使用含140 mM Cs+的电极内液，随着Li+替代细胞外[Na+]o，i(f)电流几乎与细胞外[Na+]o的降低成比例下降。这些反应与用N-甲基葡糖胺（NMG+）替代时观察到的反应基本相同，表明Li+相对于Na+对i(f)通道的相对通透性与NMG+一样低。当用Cs+或Rb+替代细胞内K+时，i(f)的完全激活I/V关系显示出强烈的内向整流，反转电位为正，表明i(f)通道对Cs+和Rb+的通透性低。这些结果表明，i(f)通道对Na+和K+具有高度选择性，不会通透类似的离子Li+和Rb+。这种高度的选择性是独特的，可能意味着i(f)通道的结构与其他Na+和K+传导通道有很大不同。
