Sigg D, Bezanilla F
Department of Physiology, School of Medicine, University of California, Los Angeles 90095, USA.
J Gen Physiol. 1997 Jan;109(1):27-39. doi: 10.1085/jgp.109.1.27.
One measure of the voltage dependence of ion channel conductance is the amount of gating charge that moves during activation and vice versa. The limiting slope method, introduced by Almers (Almers, W. 1978. Rev. Physiol. Biochem. Pharmacol. 82:96-190), exploits the relationship of charge movement and voltage sensitivity, yielding a lower limit to the range of single channel gating charge displacement. In practice, the technique is plagued by low experimental resolution due to the requirement that the logarithmic voltage sensitivity of activation be measured at very low probabilities of opening. In addition, the linear sequential models to which the original theory was restricted needed to be expanded to accommodate the complexity of mechanisms available for the activation of channels. In this communication, we refine the theory by developing a relationship between the mean activation charge displacement (a measure of the voltage sensitivity of activation) and the gating charge displacement (the integral of gating current). We demonstrate that recording the equilibrium gating charge displacement as an adjunct to the limiting slope technique greatly improves accuracy under conditions where the plots of mean activation charge displacement and gross gating charge displacement versus voltage can be superimposed. We explore this relationship for a wide variety of channel models, which include those having a continuous density of states, nonsequential activation pathways, and subconductance states. We introduce new criteria for the appropriate use of the limiting slope procedure and provide a practical example of the theory applied to low resolution simulation data.
离子通道电导对电压的依赖性的一种度量是激活过程中移动的门控电荷量，反之亦然。阿尔默斯（Almers, W. 1978. Rev. Physiol. Biochem. Pharmacol. 82:96 - 190）提出的极限斜率法利用了电荷移动与电压敏感性之间的关系，得出了单通道门控电荷位移范围的下限。实际上，由于需要在非常低的开放概率下测量激活的对数电压敏感性，该技术受到实验分辨率低的困扰。此外，最初理论所限制的线性顺序模型需要扩展，以适应通道激活可用机制的复杂性。在本通讯中，我们通过建立平均激活电荷位移（激活电压敏感性的一种度量）与门控电荷位移（门控电流的积分）之间的关系来完善该理论。我们证明，在平均激活电荷位移和总门控电荷位移与电压的关系图可以叠加的条件下，记录平衡门控电荷位移作为极限斜率技术的辅助手段可大大提高准确性。我们针对多种通道模型探索这种关系，这些模型包括具有连续态密度、非顺序激活途径和亚电导状态的模型。我们引入了极限斜率程序适当使用的新标准，并提供了该理论应用于低分辨率模拟数据的实际示例。
