White P J, Ridout M S
Department of Cell Physiology, Horticulture Research International, Wellesbourne, Warwick CV35 9EF, UK.
J Membr Biol. 1999 Mar 1;168(1):63-75. doi: 10.1007/s002329900498.
The depolarization-activated, high-conductance "maxi" cation channel in the plasma membrane of rye (Secale cereale L.) roots is permeable to a wide variety of monovalent and divalent cations. The permeation of K+, Na+, Ca2+ and Ba2+ through the pore could be simulated using a model composed of three energy barriers and two ion binding sites (a 3B2S model), which assumed single-file permeation and the possibility of double cation occupancy. The model had an asymmetrical free energy profile. Differences in permeation between cations were attributed primarily to differences in their free energy profiles in the regions of the pore adjacent to the extracellular solution. In particular, the height of the central free energy peak differed between cations, and cations differed in their affinities for ion binding sites. Significant ion repulsion occurred within the pore, and the mouths of the pore had considerable surface charge. The model adequately described the diverse current vs. voltage (I/V) relationships obtained over a wide variety of experimental conditions. It described the phenomena of non-Michaelian unitary conductance vs. activity relationships for K+, Na+ and Ca2+, differences in selectivity sequences obtained from measurements of conductance and permeability ratios, changes in relative cation permeabilities with solution composition, and the complex effects of Ba2+ and Ca2+ on K+ currents through the channel. The model enabled the prediction of unitary currents and ion fluxes through the maxi cation channel under physiological conditions. It could be used, in combination with data on the kinetics of the channel, as input to electrocoupling models allowing the relationships between membrane voltage, Ca2+ influx and Ca2+ signaling to be studied theoretically.
黑麦（Secale cereale L.）根细胞质膜中去极化激活的高电导“大”阳离子通道对多种单价和二价阳离子具有通透性。钾离子（K⁺）、钠离子（Na⁺）、钙离子（Ca²⁺）和钡离子（Ba²⁺）通过该通道的渗透可用一个由三个能量屏障和两个离子结合位点组成的模型（3B2S模型）来模拟，该模型假定为单通道渗透且存在双阳离子占据的可能性。该模型具有不对称的自由能分布。阳离子之间渗透的差异主要归因于它们在靠近细胞外溶液的通道区域自由能分布的差异。特别是，阳离子之间中央自由能峰的高度不同，且它们对离子结合位点的亲和力也不同。通道内发生显著的离子排斥，且通道口具有相当大的表面电荷。该模型充分描述了在各种实验条件下获得的不同电流-电压（I/V）关系。它描述了钾离子、钠离子和钙离子的非米氏单位电导与活性关系的现象、从电导和渗透率比值测量中获得的选择性序列差异、相对阳离子渗透率随溶液组成的变化以及钡离子和钙离子对通过该通道的钾离子电流的复杂影响。该模型能够预测生理条件下通过大阳离子通道的单位电流和离子通量。它可与通道动力学数据结合使用，作为电耦合模型的输入，从而从理论上研究膜电压、钙离子内流和钙离子信号传导之间的关系。
