Extracellular Ba2+ and voltage interact to gate Ca2+ channels at the plasma membrane of stomatal guard cells

FEBS Lett. 2001 Feb 23;491(1-2):99-103. doi: 10.1016/s0014-5793(01)02176-7.


Ca2+ channels at the plasma membrane of stomatal guard cells contribute to increases in cytosolic free [Ca2+] ([Ca2+](i)) that regulate K+ and Cl- channels for stomatal closure in higher-plant leaves. Under voltage clamp, the initial rate of increase in [Ca2+](i) in guard cells is sensitive to the extracellular divalent concentration, suggesting a close interaction between the permeant ion and channel gating. To test this idea, we recorded single-channel currents across the Vicia guard cell plasma membrane using Ba2+ as a charge carrying ion. Unlike other Ca2+ channels characterised to date, these channels activate at hyperpolarising voltages. We found that the open probability (P(o)) increased strongly with external Ba2+ concentration, consistent with a 4-fold cooperative action of Ba2+ in which its binding promoted channel opening in the steady state. Dwell time analyses indicated the presence of a single open state and at least three closed states of the channel, and showed that both hyperpolarising voltage and external Ba2+ concentration prolonged channel residence in the open state. Remarkably, increasing Ba2+ concentration also enhanced the sensitivity of the open channel to membrane voltage. We propose that Ba2+ binds at external sites distinct from the permeation pathway and that divalent binding directly influences the voltage gate.

Publication types

  • Research Support, Non-U.S. Gov't

MeSH terms

  • Barium / metabolism*
  • Calcium Channels / physiology*
  • Cell Membrane / metabolism
  • Cell Membrane / physiology
  • Fabaceae / metabolism
  • Fabaceae / physiology*
  • Fabaceae / ultrastructure
  • Ion Channel Gating*
  • Patch-Clamp Techniques
  • Plant Leaves / metabolism
  • Plant Leaves / physiology
  • Plant Leaves / ultrastructure
  • Plants, Medicinal*
  • Protoplasts / metabolism
  • Protoplasts / physiology
  • Protoplasts / ultrastructure


  • Calcium Channels
  • Barium