Send to

Choose Destination
J Membr Biol. 1997 Aug 1;158(3):241-56.

K(+)-sensitive gating of the K+ outward rectifier in Vicia guard cells.

Author information

Laboratory of Plant Physiology and Biophysics, University of London, Kent, UK.


The effect of extracellular cation concentration and membrane voltage on the current carried by outward-rectifying K+ channels was examined in stomatal guard cells of Vicia faba L. Intact guard cells were impaled with double-barrelled microelectrodes and the K+ current was monitored under voltage clamp in 0.1-30 mM K+ and in equivalent concentrations of Rb+, Cs+ and Na+. From a conditioning voltage of -200 mV, clamp steps to voltages between -150 and +50 mV in 0.1 mM K+ activated current through outward-rectifying K+ channels (IK,out) at the plasma membrane in a voltage-dependent fashion. Increasing [K+]o shifted the voltage-sensitivity of IK,out in parallel with the equilibrium potential for K+ across the membrane. A similar effect of [K+]o was evident in the kinetics of IK,out activation and deactivation, as well as the steady-state conductance-(g kappa-) voltage relations. Linear conductances, determined as a function of the conditioning voltage from instantaneous I-V curves, yielded voltages for half-maximal conductance near -130 mV in 0.1 mM K+, -80 mV in 1.0 mM K+, and -20 mV in 10 mM K+. Similar data were obtained with Rb+ and Cs+, but not with Na+, consistent with the relative efficacy of cation binding under equilibrium conditions (K+ > or = Rb+ > Cs+ > > Na+). Changing Ca2+ or Mg2+ concentrations outside between 0.1 and 10 mM was without effect on the voltage-dependence of g kappa or on IK,out activation kinetics, although 10 mM [Ca2+]o accelerated current deactivation at voltages negative of -75 mV. At any one voltage, increasing [K+]o suppressed g kappa completely, an action that showed significant cooperativity with a Hill coefficient of 2. The apparent affinity for K+ was sensitive to voltage, varying from 0.5 to 20 mM with clamp voltages near -100 to 0 mV, respectively. These, and additional data indicate that extracellular K+ acts as a ligand and alters the voltage-dependence of IK,out gating; the results implicate K(+)-binding sites accessible from the external surface of the membrane, deep within the electrical field, but distinct from the channel pore; and they are consistent with a serial 4-state reaction-kinetic model for channel gating in which binding of two K+ ions outside affects the distribution between closed states of the channel.

[Indexed for MEDLINE]

Supplemental Content

Full text links

Icon for Springer
Loading ...
Support Center