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

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.