Simultaneous Measurements of Cytoplasmic K⁺ Concentration and the Plasma Membrane Electrical Parameters in Single Membrane Samples of Chara corallina

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 G., 0.85 ± 0.25 Siemens per square meter (tstandard 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 (V.). On a cell-to-cell basis, however, EK was generally negative of the V.,, despite an electrogenic contribution from the Chara proton pump. When K+ was increased to 1.0 millimolar or above, G. rose (by 8-to 10-fold in 10 millimolar K.+), the steady state I-V characteristics showed a region of negative slope conductance, and V. followed EK. These results confirm previous studies which implicated a K.+-induced and voltage-dependent permeability to K+ at the Chara plasma membrane. They provide an explanation for transitions between apparent K&+-insensitive and K.+-sensitive ('K+ electrode') behavior displayed by the membrane potential as recorded in many algae and higher plant cells.In the Characeae, as in higher plants, potassium has been thought to be the major permeant ion (see [12,14] for review). From radioactive tracer flux measurements, the contribution by K+ to the total membrane ionic permeability was found to be about an order of magnitude greater than the contributions of Na+ and Cl-. Adding the expected partial conductances together, however, gave values which were still well below those obtained through direct electrical measurements.The "missing" conductance has been ascribed to protons, for which no suitable tracer exists. At low K.. (about 0.1 mM)2 ' Present address:

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