Fairley K, Laver D, Walker N A
Biophysics Laboratory, School of Biological Sciences, University of Sydney, New South Wales, Australia.
J Membr Biol. 1991 Apr;121(1):11-22. doi: 10.1007/BF01870647.
Whole-cell sealed-on pipettes have been used to measure electrical properties of the plasmalemma surrounding protoplasts isolated from Black Mexican sweet corn shoot cells from suspension culture. In these protoplasts the membrane resting potential (Vm) was found to be -59 +/- 23 mV (n = 23) in 1 mM Ko+. The mean Vm became more negative as [K+]o decreased, but was more positive than the K+ equilibrium potential. There was no evidence of electrogenic pump activity. We describe four features of the current-voltage characteristic of the plasmalemma of these protoplasts which show voltage-gated channel activity. Depolarization of the whole-cell membrane from the resting potential activates time- and voltage-dependent outward current through K(+)-selective channels. A local minimum in the outward current-voltage curve near Vm = 150 mV suggests that these currents are mediated by two populations of K(+)-selective channels. The absence of this minimum in the presence of verapamil suggests that the activation of one channel population depends on the influx of Ca2+ into the cytoplasm. We identify unitary currents from two K(+)-selective channel populations (40 and 125 pS) which open when the membrane is depolarized; it is possible that these mediate the outward whole-cell current. Hyperpolarization of the membrane from the resting potential produces time- and voltage-dependent inward whole-cell current. Current activation is fast and follows an exponential time course. The current saturates and in some cases decreases at membrane potentials more negative than -175 mV. This current is conducted by poorly selective K+ channels, where PCl/PK = 0.43 +/- 0.15. We describe a low conductance (20 pS) channel population of unknown selectivity which opens when the membrane is hyperpolarized. It is possible that these channels mediate inward whole-cell current. When the membrane is hyperpolarized to potentials more negative than -250 mV large, irregular inward current is activated. A third type of inward whole-cell current is briefly described. This activates slowly and with a U-shaped current-voltage curve over the range of membrane potentials -90 less than Vm less than 0 mV.
全细胞膜片钳已被用于测量从悬浮培养的黑色墨西哥甜玉米茎细胞分离的原生质体周围质膜的电学性质。在这些原生质体中，在1 mM K⁺中膜静息电位（Vm）为-59±23 mV（n = 23）。随着[K⁺]o降低，平均Vm变得更负，但比K⁺平衡电位更正。没有证据表明存在生电泵活性。我们描述了这些原生质体质膜电流-电压特性的四个特征，这些特征显示出电压门控通道活性。从静息电位对全细胞膜进行去极化会激活通过K⁺选择性通道的时间和电压依赖性外向电流。在Vm = 150 mV附近的外向电流-电压曲线中的局部最小值表明这些电流由两种K⁺选择性通道介导。在维拉帕米存在的情况下没有这个最小值表明一种通道群体的激活取决于Ca²⁺流入细胞质。我们识别出两种K⁺选择性通道群体（40和125 pS）的单通道电流，当膜去极化时它们会打开；有可能这些介导外向全细胞电流。从静息电位对膜进行超极化会产生时间和电压依赖性内向全细胞电流。电流激活很快并遵循指数时间进程。电流在膜电位比-175 mV更负时饱和，在某些情况下会降低。这种电流由选择性较差的K⁺通道传导，其中PCl/PK = 0.43±0.15。我们描述了一种低电导（20 pS）的通道群体，其选择性未知，当膜超极化时会打开。有可能这些通道介导内向全细胞电流。当膜超极化到比-250 mV更负的电位时，会激活大的、不规则的内向电流。简要描述了第三种类型的内向全细胞电流。这种电流激活缓慢，在膜电位-90＜Vm＜0 mV范围内具有U形电流-电压曲线。
