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Effects of cytosolic calcium and limited, possible dual, effects of G protein modulators on guard cell inward potassium channels
Author(s) -
Kelly Walter B.,
Esser John E.,
Schroeder Julian I.
Publication year - 1995
Publication title -
the plant journal
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 3.058
H-Index - 269
eISSN - 1365-313X
pISSN - 0960-7412
DOI - 10.1046/j.1365-313x.1995.8040479.x
Subject(s) - bapta , egta , biophysics , chemistry , cytosol , guard cell , calcium , inward rectifier potassium ion channel , bk channel , potassium channel , sk channel , gtp' , potassium , patch clamp , g protein , biochemistry , ion channel , intracellular , biology , signal transduction , receptor , enzyme , organic chemistry
Summary The cellular mechanisms that regulate potassium (K + ) channels in guard cells have been the subject of recent research, as K + channel modulation has been suggested to contribute to stomatal movements. Patch clamp studies have been pursued on guard cell protoplasts of Vicia faba to analyze the effects of physiological cytosolic free Ca 2+ concentrations, Ca 2+ buffers and GTP‐binding protein modulators on inward‐rectifying K + channels. Ca 2+ inhibition of inward‐rectifying K + currents depended strongly on the concentration and effectiveness of the Ca 2+ buffer used, indicating a large Ca 2+ buffering capacity and pH increases in guard calls. When the cytosolic Ca 2+ concentration was buffered to micromolar levels using BAPTA, inward‐rectifying K + channels were strongly inhibited. However, when EGTA was used as the Ca 2+ buffer, much less inhibition was observed, even when pipette solutions contained 1 µM free Ca 2+ . Under the imposed conditions, GTPγS did not significantly inhibit inward‐rectifying K + channel currents when cytosolic Ca 2+ was buffered to low levels or when using EGTA as the Ca 2+ buffer. Furthermore, GDPβS reduced inward K + currents at low cytosolic Ca 2+ , indicating a novel mode of inward K + channel regulation by G‐protein modulators, which is opposite in effect to that from previous reports. On the other hand, when Ca 2+ was effectively elevated in the cytosol to 1 µM using BAPTA, GTPγS produced an additional inhibition of the inward‐rectifying K + channel currents in a population of cells, indicating possible Ca 2+ ‐dependent action of GTP‐binding protein modulators in K + channel inhibition. Assays of stomatal opening show that 90% inhibition of inward K + currents does not prohibit, but slows, stomatal opening and reduces stomatal apertures by only 34% after 2 h light exposure. These data suggest that limited K + channel down‐regulation alone may not be rate‐limiting, and it is proposed that the concerted action of proton‐pump inhibition and additional anion channel activation is likely required for inhibition of stomatal opening. Furthermore, G‐protein modulators regulate inward K + channels in a more complex and limited, possibly Ca 2+ ‐dependent, manner than previously proposed.