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

J Membr Biol. 1997 Aug 1;158(3):241-56. doi: 10.1007/s002329900261.

Abstract

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.

Publication types

  • Research Support, Non-U.S. Gov't

MeSH terms

  • Calcium / metabolism
  • Calcium / pharmacology
  • Cations, Monovalent
  • Cell Membrane / physiology
  • Cesium / metabolism
  • Cesium / pharmacology
  • Electrophysiology
  • Fabaceae / physiology*
  • Ion Channel Gating* / drug effects
  • Ion Channel Gating* / physiology
  • Models, Biological
  • Plants, Medicinal*
  • Potassium / pharmacology*
  • Potassium Channels / metabolism*
  • Rubidium / metabolism
  • Rubidium / pharmacology
  • Sodium / metabolism
  • Sodium / pharmacology

Substances

  • Cations, Monovalent
  • Potassium Channels
  • Cesium
  • Sodium
  • Rubidium
  • Potassium
  • Calcium