Potassium channels in the plasmalemma ofChara corallina are multi-ion pores: Voltage-dependent blockade by Cs+ and anomalous permeabilities

Salt tolerance at single cell level in giant-celled Characeae

Characean plants provide an excellent experimental system for electrophysiology and physiology due to: (i) very large cell size, (ii) position on phylogenetic tree near the origin of land plants and (iii) continuous spectrum from very salt sensitive to very salt tolerant species. A range of experimental techniques is described, some unique to characean plants. Application of these methods provided electrical characteristics of membrane transporters, which dominate the membrane conductance under different outside conditions. With this considerable background knowledge the electrophysiology of salt sensitive and salt tolerant genera can be compared under salt and/or osmotic stress. Both salt tolerant and salt sensitive Characeae show a rise in membrane conductance and simultaneous increase in Na(+) influx upon exposure to saline medium. Salt tolerant Chara longifolia and Lamprothamnium sp. exhibit proton pump stimulation upon both turgor decrease and salinity increase, allowing the membrane PD to remain negative. The turgor is regulated through the inward K(+) rectifier and 2H(+)/Cl(-) symporter. Lamprothamnium plants can survive in hypersaline media up to twice seawater strength and withstand large sudden changes in salinity. Salt sensitive C. australis succumbs to 50-100 mM NaCl in few days. Cells exhibit no pump stimulation upon turgor decrease and at best transient pump stimulation upon salinity increase. Turgor is not regulated. The membrane PD exhibits characteristic noise upon exposure to salinity. Depolarization of membrane PD to excitation threshold sets off trains of action potentials, leading to further loses of K(+) and Cl(-). In final stages of salt damage the H(+)/OH(-) channels are thought to become the dominant transporter, dissipating the proton gradient and bringing the cell PD close to 0. The differences in transporter electrophysiology and their synergy under osmotic and/or saline stress in salt sensitive and salt tolerant characean cells are discussed in detail.

The anomalous mole fraction effect in Chara: gating at the edge of temporal resolution

The anomalous mole fraction effect (AMFE) of the K(+) channel in excised patches of the tonoplast of Chara showed a minimum of apparent open-channel current at 20 mM Tl(+) and 230 mM K(+). Time series obtained at a sampling rate of 100 kHz (filter 25 kHz) were analyzed by three methods to find out whether the AMFE results from an effect on gating or on the conductivity of the open state. Fitting the amplitude histograms by a superposition of gaussians showed a broadening in the presence of Tl(+). Dwell-time analysis based on an O-O-C-C-C model failed to evaluate rate constants above the filter frequency. Thus, the absence of any reduction of apparent open-channel current in time series simulated with the evaluated rate constants could not be taken as evidence against the hypothesis of gating. Finally, a direct fit of the measured time series using five different 5-state Hidden Markov models revealed that the presence of Tl(+) changed the rate constants in such a way that the number of transitions into the short-lived open state (30 micros) increased strongly compared to those in the absence of Tl(+). These models explain 25% reduction of apparent single-channel current amplitude through a rapid gating mechanism.

Voltage- and Ca(2+)-dependence of the K+ channel in the vacuolar membrane of Chenopodium rubrum L. suspension cells

Voltage- and Ca(2+)-dependence of the slow-activating SV-K+ channel in the vacuolar membrane of Chenopodium rubrum suspension cells has been analyzed using the patch clamp technique in the vacuole-attached, outside-out and whole-vacuolar configuration. Patch-pipette perfusion was applied to measure Ca2+ dependence of single channels in the attached-configuration. Using the PCLAMP-software (Axon Instruments), an algorithm was developed to extract reliable individual channel data from multi-channel activity records, including open probability, mean open and closed times, as well as time constants for open and closed distributions. The channel conductance of the major open state was about 83 pS (seal resistance > 8 G omega) at 30 mV (transmembrane voltage Vm, vacuole negative), and symmetrical 100 mM KCl. the channel exhibited a strong voltage- and a weak Ca(2+)-activation: increasing Vm from 40 to 100 mV is equivalent to a Ca2+ concentration change from 10(-7) to 10(-4) M. Mean open probabilities at Vm = 30 mV were 0.03 with 1 microM and 0.09 with 100 microM Ca2+. Mean open times were approx. 7 ms, and almost independent of both, voltage and Ca2+. Mean closed times, however, varied in a strongly voltage- and Ca(2+)-dependent manner, e.g., at Vm = 30 mV dropped from 205 to 67 ms, if Ca2+ was raised from 10(-6) to 10(-4) M. Open and closed distributions of events within bursts could be fitted by the sum of two exponentials with time constants between 0.3 and 11 ms.

Probing of pore in the Chara gymnophylla K+ channel by blocking cations and by streaming potential measurements

The current-voltage (I-V) relationship of single K+ channels present in the Chara gymnophylla droplet membrane was studied. The channel presumably contains large mouths at both pore ends which are sufficiently wide to accommodate TEA+ as evidenced by internal and external TEA+ blockade. The voltage dependence of blockade by external Cs+ and Na+ reveals the multi-ion occupancy of the channel. The value of streaming potential (4.0 mV/osmol) measured in the Chara K+ channel indicates that the channel contains up to nine water molecules in the narrow region. It is concluded that the length of this region is around 28 A.

Patch-clamp studies on the anomalous mole fraction effect of the K+ channel in cytoplasmic droplets of Nitella: an attempt to distinguish between a multi-ion single-file pore and an enzyme kinetic model with lazy state

Patch-clamp studies have been employed in order to check whether the assumption of a multi-ion single-file pore is necessary for the explanation of the anomalous mole fraction effect or whether this effect can also be explained by a single-barrier enzyme kinetic model. Experiments in the cell-attached configuration were done on the tonoplast membrane of cytoplasmic droplets of Nitella in solutions containing 150 mol m-3 of K+ plus Tl+ with seven different K+/Tl+ ratios. At first sight, the results seem to support the multi-ion single-file pore, because apparent open channel conductivity displays the anomalous mole fraction effect, whereas open-probability has not been found to be dependent on the K+/Tl+ ratio. Changes in open probability would be expected for a single-barrier enzyme kinetic model with a lazy state. On the other hand, the lazy-state model is more successful in explaining the measured I-V curves. The entire slope of the apparent open channel current-voltage curves rotates with changing K+/Tl+ ratios in the whole voltage range between -100 and +80 mV. Numerical calculations on the basis of multi-ion single-file pores could create the anomalous mole fraction effect only in a limited voltage range with intersecting I-V curves. The apparent absence of an effect on open probability which is postulated by the lazy-state model can be explained if switching into and out of the lazy state is faster than can be resolved by the temporal resolution of 1 msec.

Pharmacology of the Ca2(+)-dependent K+ channel in corn protoplasts

We investigated the sensitivity of the Ca2(+)-dependent K+ current, IK(Ca), present in corn protoplasts, to different K+ channel blockers. IK(Ca) was inhibited by external Cs+ (10 mM), Ba2+ (10 mM), and quinine (0.5 mM): reagents which block many types of outward-rectifying K+ channels. In contrast 4-aminopyridine (5 mM), an inhibitor of delayed rectifier or inactivating K+ currents, had no effect. Neither of the peptide toxins, apamin or charybdotoxin, specific for Ca2(+)-dependent K+ channels in animal cells, inhibited currents when used in the nanomolar concentration range. However, higher levels of charybdotoxin (10 microM) caused marked reduction of IK(Ca).

Coupling of K(+)-gating and permeation with Ca2+ block in the Ca2(+)-activated K+ channel in Chara australis

The patch-clamp technique is used here to investigate the kinetics of Ca2+ block in single high-conductance Ca2(+)-activated K+ channels. These channels are detected in the membrane surrounding cytoplasmic drops from Chara australis, a membrane which originates from the tonoplast of the parent cell. The amplitudes and durations of single channel events are measured over a wide range of membrane potential (-300 to 200 mV). Ca2+ on either side of the channel reduces its K+ conductance and alters its ion-gating characteristics in a voltage-dependent manner. This Ca2(+)-induced attenuation of conductance is analyzed using the theory of diffusion-limited ion flow through pores. Interaction of external Ca2+ with the channel's ion-gating mechanism is examined in terms of a kinetic model for ion-gating that includes two voltage-dependent gating mechanisms. The kinetics of channel block by external Ca2+ indicates that (i) external Ca2+ binds at two sites, a superficial site and a deep site, located at 8 and 40% along the trans-pore potential difference, (ii) the external vestibule cannot be occupied by more than one Ca2+ or K+, and (iii) the kinetics of Ca2+ binding at the deep site is coupled with that of a voltage-dependent gate on the external side of the channel. Kinetics of channel block by internal Ca2+ indicates that more than one Ca2+ is involved.

Potassium ion channels in the plasmalemma

The potassium ion is an indispensible cytosolic component of living cells and a key osmolyte of plant cells, crossing the plasmalemma to drive physiological processes like cell growth and motor cell activity. K(+) transport across the plasmalemma may be passive through channels, driven by the electrochemical gradient, K(+) equilibrium potential (E(K) ) - membrane potential (V(m) ), or secondary active by coupling through a carrier to the inward driving force of H(+) or Na(+) . Known K(+) channels are permeable to monovalent cations, a permeability order being K(+) > Rb(+) > NH(4) (+) > Na(+) ≥ Li(+) > Cs(+) . The macroscopic K(+) currents across a cell or protoplast surface commonly show rectification, i.e. a V(m) -dependent conductance which in turn, may be controlled by the cytosolic activity of Ca(2+) , of K(+) , of H(+) , or by the K(+) driving force. Analysis by the patch clamp technique reveals that plant K(+) channels are similar to animal channels in their single channel conductance (4 to 100 pS), but different in that a given channel population slowly activates and may not inactivate at all. Single-channel kinetics reveal a broad range of open times (ms to s) and closed times (up to 100 s). Further progress in elucidating plant K(+) channels will critically depend on molecular cloning, and the availability of channel-specific (phyto)toxins.

Cytoplasmic calcium affects the gating of potassium channels in the plasma membrane ofChara corallina: a whole-cell study using calcium-channel effectors

The action of a wide range of drugs effective on Ca(2+) channels in animal tissues has been measured on Ca(2+) channels open during the action potential of the giant-celled green alga,Chara corallina. Of the organic effectors used, only the 1,4-dihydropyridines were found to inhibit reversibly Ca(2+) influx, including, unexpectedly, Bay K 8644 and both isomers of 202-791. Methoxyverapamil (D-600), diltiazem, and the diphenylbutylpiperidines, fluspirilene and pimozide were found not to affect the Ca(2+) influx. Conversely, bepridil greatly and irreversibly stimulated Ca(2+) influx, and with time, stopped cytoplasmic streaming (which is sensitive to increases in cytoplasmic Ca(2+)). By apparently altering the cytoplasmic Ca(2+) levels with various drugs, it was found that (with the exception of the inorganic cation, La(3+)) treatments likely to lead to an increase in cytoplasmic Ca(2+) levels caused an increase in the rate of closure of the K(+) channels. Similarly, treatments likely to lead to a decrease in cytoplasmic Ca(2+) decreased the rate of K(+) channel closure. The main effect of bepridil on the K(+) channels was to increase the rate of voltage-dependent channel closure. The same effect was obtained upon increasing the external concentration of Ca(2+), but it is likely that this was due to effects on the external face of the K(+) channel. Addition of any of the 1,4-dihydropyridines had the opposite effect on the K(+) channels, slowing the rate of channel closure. They sometimes also reduced K(+) conductance, but this could well be a direct effect on the K(+) channel; high concentrations (50 to 100 μM) of bepridil also reduced K(+) conductance. No effect of photon irradiance or of abscisic acid could be consistently shown on the K(+) channels. These results indicate a control of the gating of K(+) channels by cytoplasmic Ca(2+), with increased free Ca(2+) levels leading to an increased rate of K(+)-channel closure. As well as inhibiting Ca(2+) channels, it is suggested that La(3+) acts on a Ca(2+)-binding site of the K(+) channel, mimicking the effect of Ca(2+) and increasing the rate of channel closure.

Tansley Review No. 21 Plant ion channels: whole-cell and single channel studies

Ion channels are proteins which catalyse rapid, passive, electrogenic uniport of ions through pores spanning an otherwise poorly permeable lipid bilayer. Among other processes, fluxes through ion channels are responsible for action potentials - large, transient changes in membrane potential which have been known of in plants for over 100 years. Much disparate information on ion channels in plant cells has accumulated over the past few years. In an attempt to synthesize these data, the properties of at least 18 different ion channels are collated in this review. Channels are initially classified according to ion selectivity (Ca²⁺ , Cl^- , K⁺ and H⁺ ); then gating characteristics (i.e. control of opening and closing), unitary conductance and pharmacology are used to distinguish further different sub-types of channels. To provide a background for this overview, the fundamental properties which define ion channels in animal cells, namely conduction, selectivity and gating, are described. Appropriate techniques for the study of ion channels are also assessed. The review concludes with a discussion on the role of ion channels in plant cells, although any comment on functions beyond turgor regulation and general statements about signalling remains largely speculative. The study of ion channels in plant cells is still at an early stage and it is hoped that this review will provide a framework upon which further work in both algae and vascular plants can be based. CONTENTS Summary 305 I. Introduction: plant electrophysiology 306 II. A general description of ion channels 306 III. Ion channels in plants 310 IV. Ca²⁺ channels 313 V. Cl^- channels 315 VI. K⁺ channels in the plasma membrane 318 VII. K⁺ channels in the tonoplast 322 VIII. Channels in thylakoids 324 IX. H⁺ channels 324 X. Functions of channels 325 XI. Conclusions 328 Acknowledgements 328 References 329.

Electrical tolerance (breakdown) of the Chara corallina plasmalemma: II. Inductive property of membrane and effects of pHo and impermeable monovalent cations on breakdown phenomenon

Changes in the chord conductance G and the membrane electromotive force Em in the so-called breakdown region of large negative potential of the Chara plasmalemma were analyzed in more detail. In addition to the increase in G, the voltage sensitivity of the change in G increased, which was the cause of marked inductive current in the breakdown region. The break-down potential, defined as a critical potential at which both low and high slope conductances of the I-Vm relationship cross, almost coincided with the potential at which an inductive current began to appear. This breakdown potential level changed with pHo in a range between 5 and 9. The Chara plasmalemma was electrically most tolerant around pHo 7. In some cells Em shifted to a positive level as large as +50 approximately +70 mV during the breakdown phenomenon. Such a large positive shift of Em is caused mainly by the increase in conductance of Cl- and partly Ca2+ and K+.