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PubMed1994

The relationship between carbon and water transport in single cells of Chara corallina.

The hydraulic resistance of the plasma membrane was measured on single internodal cells of Chara corallina using the method of transcellular osmosis. The hydraulic resistance of the plasma membrane of high CO2-grown cells was significantly higher than the hydraulic resistance of the plasma membrane in low CO2-grown cells. Therefore we tested the possibility that the "bicarbonate transport system", postulated to be present in low CO2-grown cells, serves as a water channel that lowers the hydraulic resistance of the plasma membrane. We were unable to find any correlation between agents that inhibited the "bicarbonate transport system" and agents that increased the hydraulic resistance of low CO2-grown cells. We did, however, find a correlation between the permeability of the cell to water and CO2. We propose that the reduced hydraulic resistance of the plasma membrane of the low CO2-grown cells is a function of a change in either the structural properties of the lipid bilayer or the activity of a CO2 transport protein so that under conditions of reduced inorganic carbon, the plasma membrane becomes more permeable to CO2, and consequently to other small molecules, including H2O, methanol and ethanol.

Bicarbonates↗
PubMed1984

Calcium effects on electrogenic pump and passive permeability of the plasma membrane of Chara corallina.

Removal of Ca2+ from the medium results in depolarization of the Chara internodal cell and an increase in membrane conductance (Gm). The increase in conductance is associated with an increase in K+ conductance, as judged by Ca2+ effects on the K+ dependence of clamp current. The voltage dependence of Gm is also affected by Ca2+, as is the time course of the response of clamp current to a step change in voltage. Mg2+ restores the low conductance and the fast response to a voltage change, but not hyperpolarization at neutral pH, suggesting that there is an additional, independent effect on the electrogenic pump. The membrane does not show the normal ability to increase proton conductance at high pH in the absence of Ca2+; this is also restored by Mg2+ as well as by Ca2+.

Calcium↗
PubMed1990

Transport of potassium in Chara australis: II. Kinetics of a symport with sodium.

An electrogenic K(+)-Na+ symport with a high affinity for K+ has been found in Chara (Smith & Walker, 1989). Under voltage-clamp conditions, the symport shows up as a change in membrane current upon adding either K+ or Na+ to the bathing medium in the presence of the other. Estimation of kinetic parameters for this transport has been difficult when using intact cells, since K(+)-Na+ current changes show a rapid falling off with time at K+ concentrations above 50 microM. Cytoplasm-enriched cell fragments are used to overcome this difficulty, since they do not show the rapid falling off of current change seen with intact cells. Current-voltage curves for the membrane in the absence or presence of either K+ or Na+ are obtained, yielding difference current-voltage curves which isolate the symport currents from other transport processes. The kinetic parameters describing this transport are found to be voltage dependent, with Km for K+ ranging from 30 down to 2 microM as membrane potential varies from -140 to -400 mV, and Km for Na+ ranging between 470 and 700 microM over a membrane potential range of -140 to -310 mV. Two different models for this transport system have been investigated. One of these involves the simultaneous transport of both the driver and substrate ions across the membrane, while the other allows for the possibility of the two ions being transported consecutively in two distinct reaction steps. The experimental results are shown to be consistent with either of these cotransport models, but they do suggest that binding of K+ occurs before that of Na+, and that movement of charge across the membrane (the voltage-dependent step) occurs when the transport protein has neither K+ nor Na+ bound to it.

Biological Transport↗
PubMed1980

The role of electrogenic pump in Chara corallina.

The conductance, G, and the electromotive force, E, of the Chara membrane were determined accurately by using the current-clamp technique. The measurements at the final steady state of inhibitor poisoning give the conductance, g1, and the electromotive force, E1, of the passive ion conducting pathways. By knowing these values the conductance, g2, and the electromotive force, E2, of the electrogenic pump can be calculated from the measured G and E at each time during the progress of inhibitor poisoning. The local closed circuit current, i, which usually causes a hyperpolarization across the passive conducting pathways, can be calculated by using g1, g2, E1 and E2 thus determined. The values of g2 and i decrease monotonically to zero with the progress of poisoning, while E2 approaches E1 asymptotically after a transient hyperpolarization. During excitation i increases markedly. Such an increased inward current through the passive conducting pathways may help in accelerating the inactivation of the excitatory mechanism.

Action Potentials↗
PubMed1990

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.

Calcium↗
PubMed1993

Effects of D2O on permeation and gating in the Ca(2+)-activated potassium channel from Chara.

We studied the effects of H2O/D2O substitution on the permeation and gating of the large conductance Ca(2+)-activated K+ channels in Chara gymnophylla droplet membrane using the patch-clamp technique. The selectivity sequence of the channel was: K+ > Rb+ >> Li+, Na+, Cs+ and Cl-. The conductance of this channel in symmetric 100 mM KCl was found to be 130 pS. The single channel conductance was decreased by 15% in D2O as compared to H2O. The blockade of channel conductance by cytosolic Ca2+ weakened in D2O as a result of a decrease in zero voltage Ca2+ binding affinity by a factor of 1.4. Voltage-dependent channel gating was affected by D2O primarily due to the change in Ca2+ binding to the channel during the activation step. The Hill coefficient for Ca2+ binding was 3 in D2O and around 1 in H2O. The values of the Ca2+ binding constant in the open channel conformation were 0.6 and 6 microM in H2O and D2O, respectively, while the binding in the closed conformation was much less affected by D2O. The H2O/D2O substitution did not produce a significant change in the slope of channel voltage dependence but caused a shift as large as 60 mV with 1 mM internal Ca2+.

Calcium↗
PubMed2001

Tonoplast anion channel activity modulation by pH in Chara corallina.

The patch-clamp technique was used to investigate regulation of anion channel activity in the tonoplast of Chara corallina in response to changing proton and calcium concentrations on both sides of the membrane. These channels are known to be Ca2+-dependent, with conductances in the range of 37 to 48 pS at pH 7.4. By using low pH at the vacuolar side (either pH(vac) 5.3 or 6.0) and a cytosolic pH (pH(cyt)) varying in a range of 4.3 to 9.0, anion channel activity and single-channel conductance could be reversibly modulated. In addition, Ca2+-sensitivity of the channels was markedly influenced by pH changes. At pH(cyt) values of 7.2 and 7.4 the half-maximal concentration (EC50) for calcium activation was 100-200 microm, whereas an EC(50) of about 5 microm was found at a pH(cyt) of 6.0. This suggests an improved binding of Ca2+ ions to the channel protein at more acidic cytoplasm. At low pH(cyt), anion channel activity and mean open times were voltage-dependent. At pipette potentials (V(p)) of +100 mV, channel activity was approximately 15-fold higher than activity at negative pipette potentials and the mean open time of the channel increased. In contrast, at pH(cyt) 7.2, anion channel activity and the opening behavior seemed to be independent of the applied V(p). The kinetics of the channel could be further controlled by the Ca2+ concentration at the cytosolic membrane side: the mean open time significantly increased in the presence of a high cytosolic Ca2+ concentration. These results show that tonoplast anion channels are maintained in a highly active state in a narrow pH range, below the resting pH(cyt). A putative physiological role of the pH-dependent modulation of these anion channels is discussed.

Action Potentials↗
PubMed2002

Acetylcholine-activated Cl- channel in the Chara tonoplast.

Acetylcholine has long been suggested to play a role in controlling physiological processes in plants, but no mechanism has been shown for its action. We show here that a chloride channel in the tonoplast (vacuolar membrane) of Chara corallina responds to acetylcholine. The channel has a conductance of 45 pS. The effect of acetylcholine is enhanced by nicotine, with the open probability increasing from 0.05 in the presence of 4 mM acetylcholine to 0.3 in the presence of 4 mM acetylcholine + 6 mM nicotine. Some effects of acetylcholine were seen at concentrations as low as 20 microM, with a maximum effect between 1 and 10 mM. In the intact cell, acetylcholine prolongs the depolarized phase of the action potential. We propose that this acetylcholine-gated channel has evolved separately from the mammalian acetylcholine-gated channel, and suggest that this represents a third form of acetylcholine signal transduction, after the nicotinic and muscarinic pathways in animal systems.

Acetylcholine↗
PubMed2000

Effects of Na(+) on the predominant K(+) channel in the tonoplast of Chara: decrease of conductance by blocks in 100 nanosecond range and induction of oligo- or poly-subconductance gating modes.

We present three mechanisms by which Na(+) inhibits the open channel currents of the predominant K(+) channel in the tonoplast of Chara corallina: (i) Fast block, i.e., short (100 ns range) interruptions of the open channel current which are determined by open channel noise analysis, (ii): Oligo-subconductance mode, i.e., a gating mode which occurs preferentially in the presence of Na(+); this mode comprises a discrete number (here 3) of open states with smaller conductances than normal, and (iii): Polysubconductance mode, i.e., a gating mode with a nondiscrete, large number (>30) of states with smaller conductances than the main open channel conductance. This novel mode has also been observed only in the presence of Na(+).

Electric Conductivity↗
PubMed2000

Temperature sensitivity of the tonoplast Ca(2+)-activated K+ channel in Chara: the influence of reversing the sign of membrane potential.

A detailed temperature dependence study of a well-defined plant ion channel, the Ca(2+)-activated K+ channel of Chara corallina, was performed over the temperature range of their habitats, 5-36 degrees C, at 1 degree C resolution. The temperature dependence of the channel unitary conductance at 50 mV shows discontinuities at 15 and 30 degrees C. These temperatures limit the range within which ion diffusion is characterized by the lowest activation energy (Ea = 8.0 +/- 1.6 kJ/mol) as compared to the regions below 15 degrees C and above 30 degrees C. Upon reversing membrane voltage polarity from 50 to -50 mV the pattern of temperature dependence switched from discontinuous to linear with Ea = 13.6 +/- 0.5 kJ/mol. The temperature dependence of the effective number of open channels at 50 mV showed a decrease with increasing temperature, with a local minimum at 28 degrees C. The mean open time exhibited a similar behavior. Changing the sign of membrane potential from 50 to -50 mV abolished the minima in both temperature dependencies. These data are discussed in the light of higher order phase transitions of the Characean membrane lipids and corresponding change in the lipid-protein interaction, and their modulation by transmembrane voltage.

Chlorophyta↗
PubMed1996

Modeling the current-voltage characteristics of Chara membranes: I. The effect of ATP removal and zero turgor.

We have obtained and modeled the electrical characteristics of the plasma membrane of Chara internodal cells: intact, without turgor and perfused with and without ATP. The cells were voltage and space-clamped to obtain the I/V (current-voltage) and G/V (conductance-voltage) profiles of the cell membrane. The intact cells yielded similar I/V characteristics with resting p.d.s. of -221 +/- 12 mV (cytoplasmic clamp, 5 cells) and -217 +/- 12 mV (vacuolar clamp, 5 cells). The cut unperfused cells were depolarized at -169 +/- 12 mV (7 cells) compared to the vacuole-clamped intact cells. The cells perfused with ATP fell into three groups: hyperpolarized group with resting p.d. -175 +/- 12 mV (4 cells) and I/V profile similar to the intact and cut unperfused cells; depolarized group with resting p.d. of -107 +/- 12 mV (6 cells) and I/V profiles close to linear; and excited cells with profiles showing a negative conductance region and resting p.d. at -59 +/- 12 mV (5 cells). The cells perfused with medium containing no ATP showed upwardly concave I/V characteristics and resting p.d. at -81 +/- 12 mV (6 cells). The I/V curves were modeled employing the "Two-state" model for the H+ pump (Hansen et al., 1981). The inward and outward rectifiers were fitted to exponential functions and combined with a linear background current. The excitation state in perfused cells was modeled by including an inward current, iexcit, with p.d.-dependence described by a combination of hyperbolic tangent functions. An inward current, ino-ATP, with a smaller amplitude, but very similar p.d.-dependence was also included in the simulation of the I/V curves from cells without ATP. This approach avoided I/V curve subtraction. The modeling of the total I/V and G/V characteristics provided more information about the parameters of the "Two-state" pump model, as well as more quantitative understanding of the interaction of the major transport systems in the plasmalemma in generation of the resting potential under a range of circumstances. ATP had little effect on nonpump currents except the excitation current; depolarization profoundly affected the pump characteristics.

Adenosine Triphosphate↗
PubMed1999

pH-sensitive gating kinetics of the maxi-K channel in the tonoplast of Chara australis.

The most frequently observed K+ channel in the tonoplast of Characean giant internodal cells with a large conductance (ca. 170 pS; Lühring, 1986; Laver & Walker, 1987) behaves, although inwardly rectifying, like animal maxi-K channels. This channel is accessible for patch-clamp techniques by preparation of cytoplasmic droplets, where the tonoplast forms the membrane delineating the droplet. Lowering the pH of the bathing solution, that virtually mimics the vacuolar environment, from an almost neutral level to values below pH 7, induced a significant but reversible decrease in channel activity, whereas channel conductance remained largely unaffected. Acidification (pH 5) on both sides of the membrane decreased open probability from a maximum of 80% to less than 20%. Decreasing pH at the cytosolic side inhibited channel activity cooperatively with a slope of 2.05 and a pKa 6.56. In addition, low pH at the vacuolar face shifted the activating voltage into a positive direction by almost 100 mV. This is the first report about an effect of extraplasmatic pH on gating of a maxi-K channel. It is suggested that the Chara maxi-K channel possesses an S4-like voltage sensor and negatively charged residues in neighboring transmembrane domains whose S4-stabilizing function may be altered by protonation. It was previously shown that gating kinetics of this channel respond to cytosolic Ca2+ (Laver & Walker, 1991). With regard to natural conditions, pH effects are discussed as contributing mainly to channel regulation at the vacuolar membrane face, whereas at the cytosolic side Ca2+ affects the channel. An attempt was made to ascribe structural mechanisms to different states of a presumptive gating reaction scheme.

Chlorophyta↗
PubMed2005

Loss of stability, pH, and the anisotropic extensibility of Chara cell walls.

We investigated the effects of acid conditions on the extensibility of isolated wall segments from growing Chara corallina cells, providing the first detailed multi-azimuthal description of the anisotropic elastic modulus of the walls. The values of anisotropic modulus were obtained by loading a tensile force on wall ribbons excised from the cell walls along twelve different azimuths, and measuring the resulting elongation of the ribbons. Our study differs from previous studies in which mechanical loading of the wall materials was performed under creep conditions. We used ramp-loading conditions which meet the requirements for Loss of Stability. The results show that whereas a linear relationship between wall extension and log time is typical for creep-based experiments, it is not seen under ramp-loading conditions. To clarify the relative values of the wall moduli, the complete all-around anisotropic modulus is presented in polar coordinates, with the value of longitudinal modulus normalized to one unit. Acid pH enhances the extensibility of the wall materials, especially when medium pH<or=4, indicating that it is necessary to eliminate the prolonged plastic extension in order to obtain meaningful modulus values of wall materials immersed in acid environments.

Anisotropy↗
PubMed2000

The pattern of acropetal and basipetal cytoplasmic streaming velocities in Chara rhizoids and protonemata, and gravity effect on the pattern as measured by laser-Doppler-velocimetry.

The spatial pattern of acropetal and basipetal cytoplasmic streaming velocities has been studied by laser-Doppler-velocimetry (LDV) in the positively gravitropic (downward growing) rhizoids of Chara globularis Thuill. and for the first time in the negatively gravitropic (upward growing) protonemata. The LDV method proved to be precise and yielded reproducible results even when tiny differences in velocities were measured. In the apical parts of the streaming regions of both cell types, acropetal streaming was faster than basipetal streaming. Starting at the apical reversal point of streaming, the velocity increased basipetally with the distance from that point and became fairly constant close to the basal reversal point; subsequently, the velocity decreased slightly acropetally as the apical reversal point was again approached. There was no change in velocity at the basal reversal point. However, at the apical reversal point there was an abrupt decrease in velocity. The pattern of the ratio of acropetal to basipetal streaming velocity (VR) was a function of the relative distance of the site of measurement from the apical reversal point rather than a function of the absolute distance. Upon inversion of the rhizoids, the VR decreased on average by 3.8% (+/- 0.4%), indicating that the effect of gravity on the streaming velocity was merely physical and without a physiological amplification. Rhizoids that had developed on the slowly rotating horizontal axis of a clinostat, and had never experienced a constant gravity vector, were similar to normally grown rhizoids with respect to VR pattern. In protonemata, the VR pattern was not significantly different from that in rhizoids although the direction of growth was inverse. In rhizoids, oryzalin caused the polar organization of the cell to disappear and nullified the differences in streaming velocities, and cytochalasin D decreased the velocity of basipetal streaming slightly more than that of acropetal streaming. Cyclopiazonic acid, known as an inhibitor of the Ca2+-ATPase of the endoplasmic reticulum, also reduced the streaming velocities in rhizoids, but had slightly more effect on the acropetal stream. It is possible that the endogenous difference in streaming velocities in both rhizoids and protonemata is caused by differences in the cytoskeletal organization of the opposing streams and/or loading of inhibitors (like Ca2+) from the apical/subapical zone into the basipetally streaming endoplasm.

Cytochalasin D↗
PubMed2001

Fracture mechanics of the cell wall of Chara corallina.

Previous mechanical studies using algae have concentrated on cell extension and growth using creep-type experiments, but there appears to be no published study of their failure properties. The mechanical strength of single large internode cell walls (up to 2 mm diameter and 100 mm in length) of the charophyte (giant alga) Chara corallina was determined by dissecting cells to give sheets of cell wall, which were then notched and fractured under tension. Tensile tests, using a range of notch sizes, were conducted on cell walls of varying age and maturity to establish their notch sensitivity and to investigate the propagation of cracks in plant cell walls. The thickness and stiffness of the walls increased with age whereas their strength was little affected. The strength of unnotched walls was estimated as 47+/-13 MPa, comparable to that of some grasses but an order of magnitude higher than that published for model bacterial cellulose composite walls. The strength was notch-sensitive and the critical stress intensity factor K1c was estimated to be 0.63+/-0.19 MNm(-3/2), comparable to published values for grasses.

Cell Wall↗
PubMed2002

The effect of physical and chemical treatment on the mechanical properties of the cell wall of the alga Chara corallina.

Single large internode cells of the charophyte (giant alga) Chara corallina were dissected to give sheets of cell wall, which were then notched and their mechanical properties in tension determined. The cells were subjected to a thermal treatment in excess water (cf. cooking), which had little effect on strength but increased the stiffness, contrasting with the behaviour of higher-plant tissues. Extraction in CDTA (cyclohexane-trans-1,2-diamine-N,N,N',N'-tetraacetate) or 4 M KOH reduced the strength from 17 MPa to 10 MPa, although sequential extraction in CDTA and 4 M KOH reduced the strength further to 4 MPa. The stiffness decreased from 500 MPa to 300 MPa on extraction in CDTA or 4 M KOH, while falling to 70 MPa after extraction in CDTA followed by 4 M KOH. Conventional sequential extraction in CDTA, Na2CO3 at 1 degrees C and 20 degrees C, and KOH at 0.5 M, 1 M, 2 M and 4 M caused a gradual decrease in stiffness and strength after the CDTA treatment to the same lower values. This result is in keeping with mechanical properties for plant tissues, but in contrast to the removal of pectic polysaccharides from model cell wall systems, which does not reduce the stiffness.

Algorithms↗
PubMed1992

Effects of local anesthetics on the Chara plasmalemma.

The effects of lidocaine, tetracaine, procaine and bupivacaine (less than 1000 microM) on the Chara corallina internodal cell were studied. These local anesthetics depolarized the membrane at rest, while they affected the rising phase and the peak level of action potential not appreciably. Instead, they prolonged the time course of the falling phase of action potential as slowly as the repolarization was imperfect, even after enough lapse beyond the refractory period. Consequently, an action potential appeared to enhance the degree of depolarization at rest. Such a depolarization with stimulus/excitation was named use-dependent depolarization, while the depolarization without excitation, the resting one. The order of the potency of the use-dependent depolarization almost coincided with that of the nerve-blocking potency. During depolarization the change in membrane conductance was not simple. However, the conductance-voltage (Gm-Vm) relationship curve in the presence of local anesthetic suggested that depolarization was due to, not only the decrease in the electrogenic H(+)-pump, but also the increase in the diffusion conductance.

Action Potentials↗
PubMed1992

Enflurane is a potent inhibitor of high conductance Ca(2+)-activated K+ channels of Chara australis.

The volatile anaesthetic, enflurane, is commonly used in surgery for inducing the state of general anaesthesia. It is assumed, that general anaesthetics act on ion channels, but little is known of how they do so and what kinds of channels are sensitive. We found, that enflurane inhibits a large conductance Ca(2+)-activated K+ channel of the green alga, Chara australis. Effects occur at clinically relevant concentrations are fully reversible. The actions of enflurane are distinct from those of charybdotoxin and tetraethylammonium, which are well known blockers of this channel type. Kinetic analysis of single-channel data demonstrates multiple effects of enflurane on the channel protein.

Calcium↗