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 whale-cell and single channel inwardly rectifying K+ currents were measured in enzymatically-isolated protoplasts from A vena sativa mesophyll leaf cells. The hyperpolarization-activated whole-cell current had an initial K+ component (I-Ki) and a time-dependent K+ component which reaches steady state (I-Kss) within 500 ms. After an initial delay, the activation of I-Kss and the deactivation of the tail K+ current (I-KT) 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 (V-m), respectively. The relative number of the activated K+ channel population increased sigmoidally as a function of hyperpolarized V-m. On the other hand, the relative number of the deactivated K+ channel population decreased exponentially as a function of hyperpolarized V-m. The kinetics of the inward rectifying K+ current were dependent on V, but not on extracellular Kf 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(a) on V-m was not altered significantly by changing [Cs+](o). Cs+ reversibly blocked the voltage-dependent sigmoidal rise of Kf 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 (I-K), the probability of the channel being open (P-o) 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. sativa.