Simultaneous Measurements of Cytoplasmic K Concentration and the Plasma Membrane Electrical Parameters in Single Membrane Samples of Chara corallina |
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| Authors: | Beilby M J Blatt M R |
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| Affiliation: | Botany School, University of Cambridge, Downing Street, Cambridge CB2 3EA, United Kingdom. |
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| Abstract: | The electrophysiological properties of cytoplasm-rich fragments (single membrane samples) prepared from internodal cells of Chara corallina were explored in conjunction with K+-sensitive microelectrode and current-voltage (I-V) measurements. This system eliminated the problem of the inaccessible cytoplasmic layer, while preserving many of the electrical characteristics of the intact cells. In 0.1 millimolar external K concentration (Ko+), the resting conductance (membrane conductance Gm, 0.85 ± 0.25 Siemens per square meter (±standard error)) of the single membrane samples, was dominated by the proton pump, as suggested by the response of the near-linear I-V characteristic to changes in external pH. Initial cytoplasmic K+ activities (aK+), judged most reliable, gave values of 117 ± 67 millimolar; stable aK+ values were 77 ± 31 millimolar. Equilibrium potentials for K+ (Nernst equilibrium potential) (EK) calculated, using either of these data sets, were near the mean membrane potential (Vm). On a cell-to-cell basis, however, EK was generally negative of the Vm, despite an electrogenic contribution from the Chara proton pump. When Ko+ was increased to 1.0 millimolar or above, Gm rose (by 8- to 10-fold in 10 millimolar Ko+), the steady state I-V characteristics showed a region of negative slope conductance, and Vm followed EK. These results confirm previous studies which implicated a Ko+-induced and voltage-dependent permeability to K+ at the Chara plasma membrane. They provide an explanation for transitions between apparent Ko+-insensitive and Ko+-sensitive (`K+ electrode') behavior displayed by the membrane potential, as recorded in many algae and higher plant cells. |
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