Interaction of extracellular potassium and cesium with the kinetics of inward rectifying K+ channels in the plasma membrane of mesophyll protoplasts of Avena sativa

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 (tau a), at 10 mM and 50 mM [K+]o, and of deactivation (tau 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+]o. The presence of Cs+ in the bathing medium reduced tau a at voltage steps less negative than -125 mV and increased tau a for voltage steps between -150 mV and -200 mV. By comparison, the dependence of tau 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 (tau o) increased as a function of hyperpolarized potentials. The mean closed-time (tau c) 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. saliva.