Plant and Cell Physiology, 1996, Vol. 37, No. 6 770-781
© 1996
Interaction of Extracellular Potassium and Cesium with the Kinetics of Inward Rectifying K+ Channels in the Plasma Membrane of Mesophyll Protoplasts of Avena sativa
Department of Biology, Kline Biology Tower, Yale University New Haven, Connecticut 06511, U.S.A.
1Correspondence to: Dr Joseph I. Kourie, John Curtin School of Medical Research, The Australian National University, PO Box 334, Canberra ACT 2061, Australia. Phone: 06-249-4279. 06- 249-2758. Fax: 06-249-4761.
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 (
a), at 10 mM and 50 mM [K+]0, and of deactivation (d) 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 r, at voltage steps less negative than —125 mV and increased a, for voltage steps between —150 mV and —200 mV. By comparison, the dependence of d 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 (o) increased as a function of hyperpolarized potentials. The mean closed- time (o) 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.
(Received November 28, 1995; Accepted June 14, 1996)
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