Electrotonic Transmission of an Action Potential between Two Separated Internodal Cells of Chara through a Bridge

Two separated internodal cells of Chara braunii were broughtinto contact with each other longitudinally at their ends andconnected by another pathway composed of a metal bridge beyondthe region of intercellular contact. A conducted action potentialthat arrived at one foot of the bridge electrotonically depolarizedthe other foot of the bridge in the connected cell. The electriccoupling ratio (0.07?0.03), the ratio of the change in the membranepotential of one cell to that of the other cell, was too smallto allow transmission of an action potential. Two cells wereplaced in parallel and connected with two liquid bridges orpools, a' and b'. When the action potential of one cell wasconducted through one connecting pool (pool a)', the other cellwas depolarized electrotonically by the action current via theother connecting pool (pool b'). The coupling ratio was increasedto 0.26?0.07 by the solution bridge, but transmission of theaction potential was rarely observed. Application of 1 mM KC1to pools a' and/or b' slightly improved the frequency of transmissionof the action potential. When pool b' contained 5% urethane,the coupling ratio increased to 0.31?0.08 and transmission ofthe action potential was frequent. (Received August 24, 1989; Accepted March 14, 1990)     

Conduction Velocity and Blockage of Action Potential in Chara Internodal Cells

Conduction velocity of the action potential in Chara brauniiinternodal cells was 0.21 ?0.05 cms in moist air and 1.5?0.9cms in artificial pond water (APW). The action potential waspropagated at an almost constant velocity along the cell inmoist air except within 0.3 cm from an end of the cell, whereasin APW, the velocity increased to 5.7 ? 2.3 cms within 1.8 cmfrom the end of the cell. When part of the cell was put in moistair and the other part was immersed in APW, conduction of theaction potential in moist air decreased in velocity and sometimesstopped in the vicinity of the boundary between moist air andAPW. Some cells from the plants collected in late autumn towinter generated an action potential which could not propagatein moist air. In these cells, an increase in the threshold andpartial cessation of protoplasmic streaming were observed. ¹Present address: Department of Physiology, Tohoku UniversitySchool of Dentistry, Seiryo-machi, Sendai 980 ²Present address: Biology Laboratory, Kyoritsu Women's University,Hachioji, Tokyo 193, Japan. (Received March 10, 1987; Accepted July 6, 1987)     

Jumping Transmission of Action Potential between Separately Placed Internodal Cells of Chora corallina

We report here a new type of cell-to-cell communication. Wenoticed that a characean cell can transmit its action potentialsto one or more cells which lie parallel to it separately inartificial pond water without any special connection betweenthe cells. Two experimental facts proved this transmission notchemical but electrical. No transmission was observed firstwhen an Ag-AgCl wire was placed between two cells, and secondwhen the whole length of the first cell was stimulated so thatnon-propagated action potential was induced. The results showthat the traveling electrical field strength might be the fundamentalfactor in the jumping transmission. (Received November 10, 1989; Accepted December 22, 1989)     

Ephaptic Transmission and Conduction Velocity of an Action Potential in Chara Internodal Cells Placed in Parallel and in Contact with One Another

When two separated Chara internodal cells were kept in contactover a length of 14 mm or more in moist air, an action potentialof one cell could be transmitted to the other cell in about40% of cases (ephaptic transmission). The action potential ofthe former cell was sometimes eliminated transiently when anew action potential was elicited in the latter cell. The newaction potential reactivated the former cell. The conductionvelocity of the action potential was reduced from 0.30?0.11cm/s (mean?SD) to 0.15?0.05 cm/s by ephaptic transmission ofthe action potential. Substitution of an artificial pond waterfor moist air or contact over a shorter length (4 mm insteadof 14 mm or more) between the two cells reduced the couplingratio, the ratio of the change in membrane potential of onecell to that of the other cell, and no transmission was observed.After ephaptic transmission, the action potentials of the twocells were conducted at almost the same velocity along the cells.The velocity was increased to 0.68?0.06 cm/s by simultaneousconduction of the action potentials in the two adjacent cells.When the simultaneously conducted action potentials reachedthe node of one cell, at the point at which another cell madecontact, they were able very frequently to trigger a new actionpotential in the adjoining cell. (Received September 28, 1989; Accepted April 5, 1990)     

Entry of Methylammonium and Ammonium Ions into Chara Internodal Cells

Low concentrations of ammonia and methylamine greatly affectmembrane transport by, and the electrical properties of, cellsof Chara corallina (=C. australis). In the presence of theseamines, influx of Cl^– and efflux of K⁺ increase and alarge depolarizing current flows through the cell membrane. Measurements with [¹⁴C]methylamine show that methylamine isabsorbed rapidly over a wide pH range, and that the absorptionisotherm is complex. Methylamine influx is not affected by presenceor absence of Cl^–, K⁺, or Na⁺, but is decreased by additionof . The depolarizing current is associated with a small increase in membrane conductance, except at highpH, and both these effects are reversible. The current showssaturation with increasing amine concentration; when methylamineis 10–12 times more concentrated than ammonia, it producesa current of the same magnitude. The results show that the amines enter the cells as cations( or CH₃) except where external pH is high, and that a specific, selective electrogenicporter is involved. There is no need to invoke active transport,as there is no evidence that internal amine concentrations exceedthe equilibrium (Nernst) concentrations.     

Photosynthetic Fixation of 14Carbon by Internodal Cells of Chara corallina

Maximum fixation rates of 120 and 60 pmol cm^–2 s ^–1wereobtained when exogenous carbon was supplied as ¹CO₂ and H¹⁴CO₃^–respectively. These values are considerably higher than thosepreviously reported for this species. A kinetic analysis wasperformed on this data. Substrate saturation in the concentrationrange 1.0–1.5 mM was observed for both CO₂ and HCO₃^– In the presence of exogenous CO², a linear relationship wasobserved between light intensity and fixation while the HCO₃^–relationship was slightly sigmoidal. Fixation saturated at intensitiesof 15–20 W m^–2 and 13–15 W m^–2 for exogenous¹⁴CO² and H¹⁴CO₃^–respectively. The presence, in this species, of an extremely active HCO₃^–transport system, situated in the plasmalemma, demonstratesthat when alkaline solutions are employed the involvement ofthis ion cannot be ignored during electrical studies on thismembrane. The maximum H¹⁴CO₃^– influxes obtained duringthis study are the largest ionic fluxes measured for any Characeanspecies. It was demonstrated that CO² for fixation can be supplied simultaneouslyby gaseous diffusion and HCO₃^– transport (cf. Raven, 1968).Inhibition of H¹⁴CO₃^– influx was observed in the presenceof Tris, Tricine, and borate buffers, and CO₃^{2 –} alsoappeared to act as a strong inhibitor. The possible mechanism(s)by which this inhibition occurs is discussed.     

Dynamic Properties of Electrotonic Coupling between Cells of Early Xenopus Embryos

Frequency response functions were measured between the cells of Xenopus laevis embryos during the first two cleavage stages. Linear systems theory was then used to produce electronic models which account for the electrical behavior of the systems. Coupling between the cells may be explained by models which have simple resistive elements joining each cell to its neighbors. The vitelline, or fertilization, membrane which surrounds the embryos has no detectable resistance to the passage of electric current. The electrical properties of the four-cell embryo can only be explained by the existence of individual junctions linking each pair of cells. This arrangement suggests that electrotonic coupling is important in the development of the embryos, at least until the four-cell stage.     

Intracellular Distribution of Free Amino Acids between the Vacuolar and Extravacuolar Compartments in Internodal Cells of Chara australis

Distribution of free amino acids between the vacuolar and extra-vacuolar(cytoplasmic) compartments in internodal cells of Chara australiswas studied. Under the control conditions (14-h light : 10-hdark), most (90%) of the cellular free amino acids were foundin the extra-vacuolar compartment. The reverse was true forammonia. The major amino acids were isoasparagine, alanine,glutamic acid, aspartic acid, serine and glycine. The contentsof hydrophobic and basic amino acids were minor and relativelygreater proportions were found in the vacuole except when theircontents were extremely low. When cells were kept for 3 days under continuous light or incontinuous darkness, the total free amino acid content increasedto about 120% (light) and about 150% (dark) that of the control.These increases mainly took place in the vacuole, but the aminoacid species responsible for the increments differed with thelight conditions. In contrast, the cytoplasmic content was relativelyconstant (50–60 mM) even under continuous light or darkness.The results suggest that the vacuole acts in the homeostasisof the cytoplasmic amino acid content. As anion, amino acidsin the cytoplasm compensated for about 10–20% of the reported"anion deficiency" in the cytoplasm. (Received June 7, 1984; Accepted September 11, 1984)     

Metabolic Conversion of Amino Acids Loaded in the Vacuole of Chara australis Internodal Cells

Vacuoles of internodal cells of Chara australis (or Chara corallina) were loaded with a 10 millimolar amount of various amino acids by a perfusion method and incubated under continuous light. After 20 to 24 hours, the cell sap was collected, and free amino acids in it and the rest of the cell (cytoplasm) were analyzed. The only amino acid metabolized completely was alanine. About 40 to 80% of the aspartic acid, glutamine, serine, and glycine were metabolized, whereas less than 30% of the threonine, asparagine, isoasparagine, isoleucine, phenylalanine, γ-aminobutyric acid, lysine, and arginine were metabolized. The figure for glutamic acid fluctuated between 10 and 100%. The main metabolites of alanine were glutamine, glycine and ammonia, which accumulated in the vacuole. Alanine utilization was not affected by l-methionine-d,l-sulfoximine or azaserine, but was strongly inhibited by aminooxyacetate. The cell extract contained enough alanine aminotransferase activity to account for the rate of alanine metabolism.     

Electrotonic Coupling between Pyramidal Neurons in the Neocortex

Electrotonic couplings (i.e., electrical synapses or gap junctions) are fundamental to neuronal synchronization, and thus essential for many physiological functions and pathological disorders. Interneuron electrical synapses have been studied intensively. Although studies on electrotonic couplings between pyramidal cells (PCs) are emerging, particularly in the hippocampus, evidence is still rare in the neocortex. The electrotonic coupling of PCs in the neocortex is therefore largely unknown in terms of electrophysiological, anatomical and synaptological properties. Using multiple patch-clamp recording with differential interference contrast infrared videomicroscopy (IR-DIC) visualization, histochemical staining, and 3D-computer reconstruction, electrotonic coupling was recorded between close PCs, mainly in the medial prefrontal cortex as well as in the visual cortical regions of ferrets and rats. Compared with interneuron gap junctions, these electrotonic couplings were characterized by several special features. The recording probability of an electrotonic coupling between PCs is extremely low; but the junctional conductance is notably high, permitting the direct transmission of action potentials (APs) and even tonic firing between coupled neurons. AP firing is therefore perfectly synchronized between coupled PCs; Postjunctional APs and spikelets alternate following slight changes of membrane potentials; Postjunctional spikelets, especially at high frequencies, are summated and ultimately reach AP-threshold to fire. These properties of pyramidal electrotonic couplings largely fill the needs, as predicted by simulation studies, for the synchronization of a neuronal assembly. It is therefore suggested that the electrotonic coupling of PCs plays a unique role in the generation of neuronal synchronization in the neocortex.     

N-Ethylmaleimide Blocks the H+ Pump in the Plasma Membrane of Chara corallina Internodal Cells

The sulfhydryl (SH) modifying reagent N-ethylmaleimide (NEM)was applied to the internodal cells of Chara corallina to studythe role of SH residues in the activity of the plasma membraneH⁺ pump. NEM (1 µM) caused a marked depolarizing shiftof the resting potential by 6410mV (n=7) together with depressionof the conductance peak at around —200 mV, indicatinga marked depression of the H⁺ pump activity. This effect ofNEM was partly reversible, the membrane repolarized and theconductance peak was restored after extracellular washing. TheH⁺ pump inhibitor, dicyclohexylcarbodiimide (DCCD), caused noadditive membrane depolarization and/or depression of the H⁺pump conductance, in the presence of NEM. This suggests thatNEM blocks the H⁺ pump and that SH residues play a pivotal rolein maintaining the H⁺ pump activity in Chara corallina. (Received April 10, 1993; Accepted July 29, 1993)     

Studies on Electrogenesis under High K+ Concentrations in Internodal Cells of Chara corallina

+ concentration ([K⁺]_o) on the membrane potential (E_m) of Chara corallina was studied. E_m more negative than -100 mV was maintained even at 100 mM [K⁺]_o. Addition of Ca²⁺ to the external medium further increased this tendency. However, E_m responded sensitively to the increase in [K⁺]_o, when the electrogenic proton pump of the plasma membrane was inhibited by treating cells with dicyclohexylcarbodiimide, an inhibitor of proton pump. Analysis using equivalent circuit model of the plasma membrane suggested that the electrogenic proton pump was activated by the increase in [K⁺]_o. In the presence of 100 mM K⁺, action potentials were generated by electric stimuli. The ionic mechanism of generation of action potentials in the presence of K⁺ at high concentration was discussed. Received 3 October 2000/ Accepted in revised form 6 January 2001     

The Effects of pH on the Ionic and Electrical Properties of the Internodal Cells of Chara australis

The effects of pH on the resting and action potentials and onthe fluxes of potassium, sodium, and chloride across the membranesof internodal cells of Chara australis have been investigated. Experiments were carried out in an artificial pond water (A.P.W.)of standard composition: CaCl₂, 01 mM; KCl, 0.1 mM; NaCl, 1.0mM. The resting potential decreased as the pH was lowered from6.5, being depolarized by about 75 mV at pH 4.5. Measurementsof the ion fluxes as a function of pH suggested that this depolarizationwas caused by an increase in the permeability to sodium anda decrease in permeability to potassium at pH 4.5. Action potentialsof constant peak value can be elicited for some time at pH 4.5,but after 20 min or so the cell becomes refractory. All theseeffects on resting and action potentials are fully reversible.We briefly speculate about the mechanism of these pH effects.     

Effects of Lanthanum on Calcium and Magnesium Contents and Cytoplasmic Streaming of Internodal Cells of Chara corallina

Biological and environmental effects of lanthanide series of elements have received much attention recently due to their wide applications. In this study, effects of La³⁺ treatments on calcium and magnesium concentrations as well as cytoplasmic streaming of internodal cells of Chara corallina were investigated. At all treatment concentrations (10, 100, and 1,000 μM), La³⁺ significantly decreased calcium concentrations in the cell-wall fractions after 5-h treatments. Calcium concentrations in the cell contents and magnesium concentrations in the cell-wall fractions were reduced by 100 and 1,000 μM La³⁺ treatments. However, cytoplasmic streaming as an indicator of [Ca²⁺]_cyt was only inhibited at the highest La³⁺ concentration (1,000 μM). The results suggest that La³⁺ may affect cellular calcium homeostasis by actions other than as a simple Ca²⁺ antagonist. La³⁺ could partially compensate for calcium deficiency at certain concentrations.     

Changes in the Membrane Properties of Nodal Cells of Chara braunii Induced by Exposure to Artificial Pond Water

Nodal cells of Chara braunii were exposed to artificial pondwater (APW) for a month or more, with removal of larger surroundingcells. The membrane resistance, resting potential, amplitudeof the action potential, and threshold voltage measured fromthe resting level were examined. Exposure to APW had markedeffects on membrane properties. (Received April 2, 0990; Accepted August 24, 1990)     

Cytotaxonomical studies of West Bengal charophyta: Karyotype analysis in Chara braunii

The paper deals with cytotaxonomic studies of three forms ofChara braunii-f.coromandelina (n = 14), f.schweinitzii (n = 14) and f.kurzii (n = 28) from West Bengal (India). An improved technique involving the use of a pretreating agent permitted a detailed analysis of the karyotypes of the three taxa studied. It has been concluded that structural alterations of chromosomes and polyploidy are associated with evolution of phenotypic differences in theChara braunii complex. The phylogenetic status ofChara braunii in the light of the present investigation is discussed.     

The Influence of Ca2+ and K+ on H14CO3-Influx in Internodal Cells of Chara corallina

The influences of Ca²⁺-free solutions and increasing K⁺ concentrationson the H¹⁴CO^–₃ influx capacity of Chara corallina wereinvestigated. It was found that contact with Ca^2–freesolutions resulted in a gradual reduction in the H¹⁴CO^–₃influx capacity of these cells. Recovery of this influx capacity,following the return of Ca²⁺ to the experimental solution, followeda ‘mirror-image’ of the time course of decay. Potassium concentrations above a certain critical value (2 mM)induced a rapid reduction in H¹⁴CO^–₃ influx capacity.Normal activity was recovered within 60–90 min followingthe return to 0.2 mMK⁺ solutions. It was also shown that 10mM K⁺ can be used to determine the relative contribution of¹⁴C supplied by diffusion of ¹⁴CO₂ and transport of H¹⁴CO^–₃.The Ca²⁺ and K⁺ results are discussed in relation to the effectsof these treatments on the electrical properties of the plasmalemma.     

Recording of Retinal Action Potentials from Single Cells in the Insect Compound Eye

Electrical responses were recorded intracellularly from the compound eyes of a fly (Lucilia) and of several dragonflies (Copera, Agriocnemis, and Lestes). An ommatidium of the dragonflies is made up of four retinula cells and a rhabdom composed of three rhabdomeres while the Lucilia has an ommatidium of seven independent retinula cells and rhabdomeres. The intracellular responses presumably recorded from the retinula cell had the same wave form in the two groups of insects: The responses were composed of two components or phases, a transient spike-like potential and a slow one maintained during illumination. The membrane potential, in the range of -25 to -70 mv., was influenced by the level of adaptation, and it was transiently depolarized to zero by high levels of illumination.     

Further Experiments on the Low Frequency Excess Noise of the Vacuolar Resting Potential of Chara braunii

The use of the low frequency (0.1–1.0 Hz) power spectraldensity function of the vacuolar resting potential is here appliedto the examination of a number of conditions which might beexpected to influence the active transport of ions. Once again,spectra of the form A/fµif are encountered. µifand p, the total power in the band, are significantly reducedby cyanide and by cyanide + salicylhydroxamic acid, but notby O₂ deprivation, µif and p are powerfully reduced byimidazole and reduced to a lesser extent by 1, 10-phenanthroleneand DCMU. The effects of darkness and deprivation of O₂ and/orCO₂ could not be told from the effects of darkness alone. Theresults of varying the external pH were not unambiguous butdo suggest that µif is maximal near pH 7.35 and that itfalls off for more acidic or more basic values. The implicationsof these spectra arc discussed.