Garber S S
Department of Biochemistry, Brandeis University, Waltham, Massachusetts 02254.
Biophys J. 1988 Nov;54(5):767-76. doi: 10.1016/S0006-3495(88)83014-5.
Single Na+ channels from rat skeletal muscle were inserted into planar lipid bilayers in the presence of either 200 nM batrachotoxin (BTX) or 50 microM veratridine (VT). These toxins, in addition to their ability to shift inactivation of voltage-gated Na+ channels, may be used as probes of ion conduction in these channels. Channels modified by either of the toxins have qualitatively similar selectivity for the alkali cations (Na+ approximately Li+ greater than K+ greater than Rb+ greater than Cs+). Biionic reversal potentials, for example, were concentration independent for all ions studied. Na+/K+ and Na+/Rb+ reversal potentials, however, were dependent on the orientation of the ionic species with respect to the intra- or extracellular face of the channel, whereas Na+/Li+ biionic reversal potentials were not orientation dependent. A simple, four-barrier, three-well, single-ion occupancy model was used to generate current-voltage relationships similar to those observed in symmetrical solutions of Na, K, or Li ions. The barrier profiles for Na and Li ions were symmetric, whereas that for K ions was asymmetric. This suggests the barrier to ion permeation for K ions may be different than that for Na and Li ions. With this model, these hypothetical energy barrier profiles could predict the orientation-dependent reversal potentials observed for Na+/K+ and Na+/Rb+. The energy barrier profiles, however, were not capable of describing biionic Na/Li ion permeation. Together these results support the hypothesis that Na ions have a different rate determining step for ion permeation than that of K and Rb ions.
在存在200 nM 蛙毒素（BTX）或50 μM藜芦定（VT）的情况下，将来自大鼠骨骼肌的单个钠离子通道插入平面脂质双分子层中。这些毒素除了能够改变电压门控钠离子通道的失活外，还可作为这些通道中离子传导的探针。被这两种毒素修饰的通道对碱金属阳离子（Na+≈Li+＞K+＞Rb+＞Cs+）具有定性相似的选择性。例如，双离子反转电位对所有研究的离子而言与浓度无关。然而，Na+/K+和Na+/Rb+反转电位取决于离子种类相对于通道胞内或胞外面的取向，而Na+/Li+双离子反转电位则不依赖于取向。使用一个简单的四屏障、三阱、单离子占据模型来生成与在Na、K或Li离子的对称溶液中观察到的电流-电压关系相似的关系。Na和Li离子的屏障轮廓是对称的，而K离子的屏障轮廓是不对称的。这表明K离子的离子渗透屏障可能与Na和Li离子的不同。用这个模型，这些假设的能量屏障轮廓可以预测Na+/K+和Na+/Rb+观察到的取向依赖性反转电位。然而，能量屏障轮廓无法描述双离子Na/Li离子的渗透。这些结果共同支持了这样一个假设，即Na离子在离子渗透方面具有与K和Rb离子不同的速率决定步骤。
