Journal Article

Interaction of Extracellular Potassium and Cesium with the Kinetics of Inward Rectifying K<sup>+</sup> Channels in the Plasma Membrane of Mesophyll Protoplasts of <i>Avena sativa</i>

Joseph I. Kourie

in Plant and Cell Physiology

Published on behalf of Japanese Society of Plant Physiologists

Volume 37, issue 6, pages 770-781
Published in print September 1996 | ISSN: 0032-0781
e-ISSN: 1471-9053 | DOI: http://dx.doi.org/10.1093/oxfordjournals.pcp.a029012
Interaction of Extracellular Potassium and Cesium with the Kinetics of Inward Rectifying K+ Channels in the Plasma Membrane of Mesophyll Protoplasts of Avena sativa

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Using the patch-clamp technique the kinetics of whole-cell and single channel inwardly rectifying K+ currents were measured in enzymatically-isolated protoplasts from Avena sativa mesophyll leaf cells. The hyperpolarization-activated whole-cell current had an initial K+ component (IKI) and a time-dependent K+ component which reaches steady state (IKSS) within 500 ms. After an initial delay, the activation of IKss and the deactivation of the tail K+ current (IKT) followed an exponential time course. The time-constants of activation (τa), at 10 mM and 50 mM [K+]0, and of deactivation (τd) could be described by exponential and sigmoidal dependence on membrane voltage (Vm), respectively. The relative number of the activated K+ channel population increased sigmoidally as a function of hyperpolarized Vm. On the other hand, the relative number of the deactivated K+ channel population decreased exponentially as a function of hyperpolarized Vm. The kinetics of the inward rectifying K+ current were dependent on Vm but not on extracellular K+ concentration, [K+]0. The presence of Cs+ in the bathing medium reduced τr, at voltage steps less negative than —125 mV and increased τa, for voltage steps between —150 mV and —200 mV. By comparison, the dependence of τd on Vm was not altered significantly by changing [Cs+]o. Cs+ reversibly blocked the voltage-dependent sigmoidal rise of K+ current activation and the voltage-dependent exponential decrease of current deactivation. The Cs+-induced block of K+ current was both voltage and concentration dependent.

Single channel current (IK), the probability of the channel being open (Po) and the mean open-time (τo) increased as a function of hyperpolarized potentials. The mean closed- time (τo) and the mean lag time before the channel opened were exponentially dependent on voltage and they became shorter as the membrane was hyperpolarized. The kinetics of single K+ channels i.e. activation, deactivation and lag time, and the absence of outward single K+ channel currents were consistent with whole-cell current measurements. The inward rectification of single channel currents and simulated cell currents suggest that this current rectification is an intrinsic nature of the inward rectifying K+channel in A. sativa.

Keywords: Activation; Avena sativa protoplast; Cs+-block; Deactivation; External potassium; Inward rectifying K+ current.

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Subjects: Biochemistry ; Molecular and Cell Biology ; Plant Sciences and Forestry

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