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

Botany School, University of Cambridge, Downing Street, Cambridge CB2 3EA, United Kingdom

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 (K_o⁺), the resting conductance (membrane conductance G_m, 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 (a_K+), judged most reliable, gave values of 117 ± 67 millimolar; stable a_K+ values were 77 ± 31 millimolar. Equilibrium potentials for K⁺ (Nernst equilibrium potential) (E_K) calculated, using either of these data sets, were near the mean membrane potential (V_m). On a cell-to-cell basis, however, E_K was generally negative of the V_m, despite an electrogenic contribution from the Chara proton pump. When K_o⁺ was increased to 1.0 millimolar or above, G_m rose (by 8- to 10-fold in 10 millimolar K_o⁺), the steady state I-V characteristics showed a region of negative slope conductance, and V_m followed E_K. These results confirm previous studies which implicated a K_o⁺-induced and voltage-dependent permeability to K⁺ at the Chara plasma membrane. They provide an explanation for transitions between apparent K_o⁺-insensitive and K_o⁺-sensitive (`K⁺ electrode') behavior displayed by the membrane potential, as recorded in many algae and higher plant cells.

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