Demo S D, Yellen G
Howard Hughes Medical Institute, Johns Hopkins University School of Medicine, Baltimore, Maryland 21205.
Biophys J. 1992 Mar;61(3):639-48. doi: 10.1016/S0006-3495(92)81869-6.
We studied the effects of permeant ions on the gating of the large conductance Ca(2+)-activated K+ channel from rat skeletal muscle. Rb+ blockade of inward K+ current caused an increase in the open probability as though Rb+ occupancy of the pore interferes with channel closing. In support of this hypothesis, we directly measured the occupancy of the pore by the impermeant ion Cs+ and found that it strongly correlates with its effect on gating. This is consistent with the "foot-in-the-door" model of gating, which states that channels cannot close with an ion in the pore. However, because Rb+ and Cs+ not only slow the closing rate (as predicted by the model), but also speed the opening rate, our results are more consistent with a modified version of the model in which the channel can indeed close while occupied, but the occupancy destabilizes the closed state. Increasing the occupancy of the pore by the addition of other permeant (K+ and Tl+) and impermeant (tetraethylammonium) ions did not affect the open probability. To account for this disparity, we used a two-site permeation model in which only one of the sites influenced gating. Occupancy of this "gating site" interferes with channel closing and hastens opening. Ions that directly or indirectly increase the occupancy of this site will increase the open probability.
我们研究了通透离子对大鼠骨骼肌大电导钙激活钾通道门控的影响。铷离子对内向钾电流的阻断导致开放概率增加，就好像铷离子占据孔道会干扰通道关闭一样。为支持这一假设，我们直接测量了非通透离子铯离子对孔道的占据情况，发现其与对门控的影响密切相关。这与“脚踏门”门控模型一致，该模型认为通道在孔道中有离子时无法关闭。然而，由于铷离子和铯离子不仅减慢关闭速率（如模型所预测），还加快开放速率，我们的结果更符合该模型的一个修正版本，即通道在被占据时确实可以关闭，但占据会使关闭状态不稳定。通过添加其他通透离子（钾离子和铊离子）和非通透离子（四乙铵）增加孔道的占据情况并未影响开放概率。为解释这种差异，我们使用了一个双位点通透模型，其中只有一个位点影响门控。这个“门控位点”的占据会干扰通道关闭并加速开放。直接或间接增加该位点占据情况的离子会增加开放概率。
