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)     

Magnesium-resistant excitatory synaptic potentials in the leech retzius cell

Postsynaptic potentials (PSPs) recorded from leech Retzius cells in response to stimulation of interganglionic connective could not be reversed by soma depolarization or abolished by 40 mM Mg ion, nor could input resistance changes be detected during them. Alteration of external Cl and K over a tenfold range provided no clear evidence that the PSPs involved a conductance change to either ion. The method of extrapolation yielded an apparent PSP equilibrium potential of about ?20 mV. The steep portion of the relationship between Retzius cell action potential amplitude and membrane potential extrapolated to an apparent reversal potential of ?13 mV. It is likely that the connective-to-Retzius cell PSPs were principally electrical events. Their apparent reversal potentials could have been in the range associated with chemical synapses because they traversed an electrical synapse with a variable coupling resistance, or because the polarizing currents, passing “backwards” across electrical synapses, changed the amplitude of the presynaptic action potentials.     

Electrophysiological characterization of murine HL-5 atrial cardiomyocytes

HL-5 cells are cultured murine atrial cardiomyocytes and have been used in studies to address important cellular and molecular questions. However, electrophysiological features of HL-5 cells have not been characterized. In this study, we examined such properties using whole cell patch-clamp techniques. Membrane capacitance of the HL-5 cells was from 8 to 62 pF. The resting membrane potential was –57.8 ± 1.4 mV (n = 51). Intracellular injection of depolarizing currents evoked action potentials (APs) with variable morphologies in 71% of the patched cells. Interestingly, the incidence of successful, current-induced APs positively correlated with the hyperpolarizing degrees of resting membrane potentials (r = 0.99, P < 0.001). Only a few of the patched cells (4 of 51, 7.8%) exhibited spontaneous APs. The muscarinic agonist carbachol activated the acetylcholine-activated K⁺ current and significantly shortened the duration of APs. Immunostaining confirmed the presence of the muscarinic receptor type 2 in HL-5 cells. The hyperpolarization-activated cation current (I_f) was detected in 39% of the patched cells. The voltage to activate 50% of I_f channels was –73.4 ± 1.2 mV (n = 12). Voltage-gated Na⁺, Ca²⁺, and K⁺ currents were observed in the HL-5 cells with variable incidences. Compared with the adult mouse cardiomyocytes, the HL-5 cells had prolonged APs and small outward K⁺ currents. Our data indicate that HL-5 cells display significant electrophysiological heterogeneity of morphological appearance of APs and expression of functional ion channels. Compared with adult murine cardiomyocytes, HL-5 cells show an immature phenotype of cardiac AP morphology. action potential; ion channel; muscarinic receptor     

Electrophysiological studies of the isolated posterior moiety of the electroplax of the electric eel

Intracellular microelectrodes were used to measure resting and action potentials of electric eel electroplax cells from which the middle portion of the anterior moiety had been removed. External electrode studies of this preparation have been described by Chagas and Esquibel (C. R. Acad. Sc., Paris, 260: 3172 ('65)). In the present studies it was found that the resting and action potentials were almost as large as normal in the intact portion of the preparation, whereas these potentials were greatly reduced in the areas from which the anterior moiety had been removed. The reduction in amplitude was greater the farther the microelectrode was from the intact portion, indicating the possibility of decremental spread from the intact portions. The action potentials measured with external electrodes appeared to be an average of potentials from the various areas of the preparation. In other experiments the entire anterior moiety of the electroplax was removed, leaving only the innervated plasma membrane plus some adhering cytoplasm and extracellular material. Various high K⁺ solutions were used to bathe the inner surface. No action potentials could be elicited from these preparations, but resting potentials as high as 51 mV were observed using external electrodes. The resting potential could be reduced reversibly by carbamylcholine applied to the outer surface. Carbamylcholine applied to the inner surface had no effect.     

Magnesium-resistant excitatory synaptic potentials in the leech Retzius cell.

Postsynaptic potentials (PSPs) recorded from leech Retzius cells in response to stimulation of interganglionic connective could not be reversed by soma depolarization or abolished by 40 mM Mg ion, nor could input resistance changes be detected during them. Alteration of external Cl and K over a tenfold range provided no clear evidence that the PSPs involved a conductance change to either ion. The method of extrapolation yielded an apparent PSP equilibrium potential of about -20 mV. The steep portion of the relationship between Retzius cell action potential amplitude and membrane potential extrapolated to an apparent reversal potential of -13 mV. It is likely that the connective-to-Retzius cell PSPs were principally electrical events. Their apparent reversal potentials could have been in the range associated with chemical synapses because they traversed an electrical synapse with a variable coupling resistance, or because the polarizing currents, passing "backwards" across electrical synapses, changed the amplitude of the presynaptic action potentials.     

Electrophysiological recordings from spontaneously contracting reaggregates of cultured vascular smooth muscle cells from chick embryos

Single cells were trypsin-dispersed from blood vessels (great vessels near the heart and mesenteric vessels) of 10–20 day chick embryos, and induced to reaggregate into small spheres (0.1–0.5 mm ) either by gyration or by plating on cellophane. Many reaggregates contracted spontaneously or in response to electrical stimulation during culture periods of up to 6 weeks. When the spherical reaggregates were allowed to adhere to a glass substrate, cells emigrated from the spheres to form aprons of monolayered cells which continued to contract. Thick and thin myofilaments (mean diameters of 146 and 65 Å, respectively) were observable in a large fraction of cells studied in electron micrographs. Vascular smooth muscle (VSM) cells were identified in the reaggregates by recording resting potentials of −40 to −60 mV, and by action potential generation. The action potentials were preceded by pacemaker potentials, had slow rates of rise (<20 V/sec), and were insensitive to tetrodotoxin (TTX). Although the action potentials depend on an inward slow current, D-600 did not block the action potentials of the VSM cells. Reaggregates of atrial cells, produced at the same time for comparison, had larger resting potentials (up to −80 mV), less automaticity, fast rates of rise (mean of about 85 V/sec), and complete TTX sensitivity, thus indicating dependence on fast Na⁺ channels. These findings indicate that identifiable VSM cells can be successfully maintained in primary culture for several weeks, and these cells retain electrical and contractile properties similar to those of smooth muscle cells in intact adult blood vessels. This preparation provides a convenient system for electrophysiological and pharmacological studies of VSM cells.     

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.     

Propagation of Action Potential over the Trap-lobes of Aldrovanda vesiculosa

In the trap-lobes of Aldrovanda vesiculosa, an action potentialwas generated in a cell located at the base of a sensory hairstanding on the margin of central portion of the paired lobes,which spread over this portion within about 40 msec. The electricalcoupling ratio for two adjacent cells in the middle layer ofthe lobes was 0.8. This showed that an action potential generatedin a cell of the trap-lobes must spread electrotonically toadjacent cells. An analysis of an equivalent circuit for thecell injected with current and its neighboring cells in themiddle layer of the lobes showed that the resistances of theplasmalemma, tonoplast and junction between two cells were 11.1,4.7 and 0.56 M, respectively. Numerous plasmodesmata in thejunctional walls of the cells were found by electron microscopy.The low resistance of the junction between cells must be dueto the presence of the plasmodesmata which allows an electrotonictransmission of action potential from cell to cell. (Received January 23, 1982; Accepted April 13, 1982)     

Action Potential in Luffa cylindlica and its Effects on Elongation Growth

The action potential of the non-seismonastic plant, Luffa cylindrica,was studied and characterized. Single action potentials of 30to 60 mV were evoked by electrical or cooling stimuli appliedto the stem internode. Action potentials were transmitted withoutdiminution along stimulated internodes shorter than 40 mm inlength. No difference in velocity between acropetal and basipetaltransmission was observed. This behavior is very similar tothat of Mimosa and Biophytum. The velocity of transmission ofaction potentials along internodes of Luffa was dependent upondistance from the point of stimulation; 10 to 20 mm sec^–1upto 40 mm, and 70 mm sec^–1 at around 50 mm from the pointof stimulation. The action potential was not transmitted tothe neighboring internode. Action potentials were also observedin the petiole and leaf. Beyond the threshold stimulus, theamplitude of the transmitted action potential depended on thestrength of the stimulus. The absolute and relative refractoryperiods for the transmission of action potentials were 2 minand 2 to 5 min, respectively. Elongation growth of the stem, recorded using a differentialtransformer, decreased after the generation of a single actionpotential in the upper most internode. This decreased growthrate continued for more than 5 min. (Received March 10, 1986; Accepted June 11, 1986)     

Inotropism and Contracture of Aplysiid Ventricles as Related to the Action of Neurohumors on Resting and Action Potentials of Molluscan Hearts

The postsynaptic actions of neurohumors on molluscan musclemay be exerted through control of force as well as by meansof excitation and inhibition. The control of force may appearas potentiation of the response to excitation, as increasedinotropism in spontaneous contractions, or as an increase intonus. We have directed our attention to the alterations offorce induced in aplysiid ventricles by applied postulated neurohumors.The results are interpreted in terms of the known effects ofneurohumors on resting potential and action potentials of molluscanhearts. We recorded compound membrane potentials of aplysiidventricles extracellularly, using a single sucrose gap apparatustogether with a force-displacement transducer to measureforcein contractions or contracture. Aplysia californica ventriclehas a membrane potential of –52.5 ± 9.4 mV. Ventriclesof Aplysia dactylomela or Aplysia californica are depolarizedby increased concentrations of external potassium ion, withan accompanying contracture. After incubation in calcium-freemedium, KCl contracture-force is directly dependent on calciumion concentration. A depolarization of 8.3 ±2.14 mV inpotassium-free medium is blocked by substitution of lithiumfor sodium in the medium, suggesting an electrogenic sodiumpump. There is a sustained depolarization in low chloride medium,which suggests a significant chloride contribution to the restingpotential. Ventricles of Dolabella auriculana or Aplysia dactylomelaare depolarized by acetylcholine (ACh). Threshold for depolarizationis lower than threshold for contracture-force. The ventricleof A. dactylomela is depolarized by 5- hydroxytryptamine (5HT)with threshold at 10^–9 M and a maximum depolarizationof 30 mV at 10^–4M. Depolarization by 5HT may induce beatingbut does not induce contracture. Ventricles of Aplysia californicaare not depolarized by ACh although beating ventricles are inhibited,and a depolarized ventricle in a tonic contracture may be hyperpolarizedand relaxed by low concentrations. The force of contractionof the ventricle of Dolabella auricularia is dependent on theduration of the plateau phase of the cardiac action potential.The plateau is lengthened by 5HT with an accompanying increasein force of beat, and shortened by ACh, with an accompanyingdecrease in force of beat. The action potential in the ventricleof Aplysia californica is not differentiated into spike andplateau phases, and neither ACh nor 5HT has any marked effecton the form of the action potential. Nevertheless, isolatedventricles of Dolabella auricularia, Aplysia dactylomela, andAplysia californica are all excited by 5-hydroxytryptamine,with a threshold at about 10^–M. Both spontaneous beatingand excitation induced in A. californica ventricles by 5HT areblocked by lack of the sodium ion, which may be responsiblefor pacemaker potentials in molluscan hearts.     

Membrane properties of identified embryonic nerve and glial cells of the leech central nervous system

Summary The membrane potential of identified nerve (Retzius) cells and neuropil glial cells from 11 (±1) day-old embryos of the leechHirudo medicinalis was recorded using conventional intracellular microelectrodes. At this stage all ganglia of the segmental nervous system are formed. The membrane potential of Retzius cells was –68±4 mV (±SD,n=8), and showed a slope of 42 mV between 10 mM and 100 mM external K concentration. Retzius cells were able to fire action potentials which had a fast Na-dependent component, and, under appropriate conditions, also generated slow Ca (Ba) action potentials. The mean membrane potential of the neuropil glial cell at physiological K concentration (4 mM) was –83±5 mV (±SD,n=10), and showed a dependence of 56 mV for a tenfold change in the external K concentration (> 4mM). Neuropil glial cells showed no signs of voltage-activated excitability, but they repeatedly depolarized in the presence of 0.1 mM 5-HT.     

Spontaneous Action Potential Initiation and Propagation in Regenerating Cell Segments of Acetabularia mediterranea

Thavarungkul, P., Lertsithichai, S. and Sherlock, R. A. 1987.Spontaneous action potential initiation and propagation in regeneratingcell segments of Acetabularia mediterranea.—J. exp. Bot.38: 1541–1556. The spontaneous action potentials which occur during the regenerationof anucleate isolated stalk segments (ISS's) of the unicellularalga Acetabulana mediterranea have been studied using a novelnon-invasive technique. This involved measurement of spatialsamples of the time dependent potential in the external medium(sea water) and an inverse transformation to give the currentdistribution at the cell surface. The initiation region andsubsequent propagation (if any) of each action potential wasthen analysed from the computed transmembrane currents. Theresults showed that the occurrences of the spontaneous actionpotentials followed a rhythm which had a period of approximately24–30 h. These action potentials initiated more frequentlyat the ends of the ISS's than in the middle region. Our resultsshowed no firm correlation between the regions of action potentialinitiation and the site of the regenerating apex. Generallyboth propagating and non-propagating action potentials wereobserved in the same ISS, the ratio of which seems to dependon the length of the cell. The analysed initiation sites andthe propagating behaviour of the spontaneous action potentialgenerated by the ISS's during the regeneration process do notprovide any clear support to the symmetry breaking role of thepropagating action potentials as proposed by some workers. Key words: Acetabularia, regeneration, action potential     

Recording of the potassium efflux during a single action potential in Chara corallina

The potassium efflux from cells of Chara corallina was recordedsimultaneously with an action potential using a flame photometerand a perfusion system. The potassium efflux increased instantaneouslywith the action potential and lasted for 12–76 sec. Thenet potassium efflux during an action potential averaged 308picomoles/cm₂ impulse. (Received December 21, 1974; )     

T-type Ca2+ channel lowers the threshold of spike generation in the newt olfactory receptor cell

Mechanisms underlying action potential generation in the newt olfactory receptor cell were investigated by using the whole-cell version of the patch-clamp technique. Isolated olfactory cells had a resting membrane potential of -70 +/- 9 mV. Injection of a depolarizing current step triggered action potentials under current clamp condition. The amplitude of the action potential was reduced by lowering external Na+ concentration. After a complete removal of Na+, however, cells still showed action potentials which was abolished either by Ca2+ removal or by an application of Ca2+ channel blocker (Co2+ or Ni2+), indicating an involvement of Ca2+ current in spike generation of newt olfactory receptor cells. Under the voltage clamp condition, depolarization of the cell to -40 mV from the holding voltage of -100 mV induced a fast transient inward current, which consisted of Na+ (INa) and T-type Ca2+ (ICa.T) currents. The amplitude of ICa,T was about one fourth of that of INa. Depolarization to more positive voltages also induced L-type Ca2+ current (ICa,L). ICa,L was as small as a few pA in normal Ringer solution. The activating voltage of ICa,T was approximately 10 mV more negative than that of INa. Under current clamp, action potentials generated by a least effective depolarization was almost completely blocked by 0.1 mM Ni2+ (a specific T-type Ca2+ channel blocker) even in the presence of Na+. These results suggest that ICa,T contributes to action potential in the newt olfactory receptor cell and lowers the threshold of spike generation.     

Resting and action potentials of excitabl cells in the main pulvinus of Mimosa pudica

Resting and action potentials of excitable cells in the motortissue of the main pulvinus of Mimosa pudica, recorded withan intracellular microelectrode, were –150 and 110 mV,respectively. The excitable cells were found in both upper andlower halves of the main pulvinus. (Received May 20, 1976; )     

Membrane Potentials in Excitable Cells of Aldrovanda vesiculosa Trap-Lobes

The resting membrane potential in excitable cells of Aldrovandatrap-lobes is composed of diffusion and electrogenic potentials.The diffusion potential, about –100 mV in artificial pondwater, was determined from the external K⁺ and Na⁺ concentrations.The permeability ratio, P_Na/P_K of the membrane was estimatedto be about 0.3. The electrogenic potential hyperpolarized themembrane to about –140 mV. The peak value of the actionpotential increased by +26 mV with a tenfold increase in theexternal Ca²⁺ concentration. The action potential was blockedby an application of the Ca²⁺ chelater or the Ca channel blocker,LaCl₃. Cells showed additional Ca²⁺ influx (7.8 pmole/cm² impulse)during membrane excitation. These facts suggest that the transientincrease in Ca²⁺ influx causes the action potential presentin cells of Aldrovanda trap-lobes. ¹ Present address: Jerry Lewis Neuromuscular Research Center,School of Medicine, University of California Los Angeles, LosAngeles, CA90024, U.S.A. ² Present address: Biological Laboratory, Kyoritsu Women's University,Hachioji 193, Japan. (Received September 21, 1983; Accepted September 7, 1984)     

Voltage-activated currents in guinea pig pancreatic alpha 2 cells. Evidence for Ca2+-dependent action potentials

Glucagon-secreting alpha 2 cells were isolated from guinea pig pancreatic islets and used for electrophysiological studies of voltage- activated ionic conductances using the patch-clamp technique. The alpha 2 cells differed from beta cells in producing action potentials in the absence of glucose. The frequency of these potentials increased after addition of 10 mM arginine but remained unaffected in the presence of 5- 20 mM glucose. When studying the conductances underlying the action potentials, we identified a delayed rectifying K+ current, an Na+ current, and a Ca2+ current. The K+ current activated above -20 mV and then increased with the applied voltage. The Na+ current developed at potentials above -50 mV and reached a maximal peak amplitude of 550 pA during depolarizing pulses to -15 mV. The Na+ current inactivated rapidly (tau h approximately 0.7 ms at 0 mV). Half-maximal steady state inactivation was attained at -58 mV, and currents could no longer be elicited after conditioning pulses to potentials above -40 mV. The Ca2+ current first became detectable at -50 mV and reached a maximal amplitude of 90 pA (in extracellular [Ca2+] = 2.6 mM) at about -10 mV. Unlike the Na+ current, it inactivated little or not at all. Membrane potential measurements demonstrated that both the Ca2+ and Na+ currents contribute to the generation of the action potential. Whereas there was an absolute requirement of extracellular Ca2+ for action potentials to be elicited at all, suppression of the much larger Na+ current only reduced the upstroke velocity of the spikes. It is suggested that this behavior reflects the participation of a low-threshold Ca2+ conductance in the pacemaking of alpha 2 cells.     

Inhibition in hippocampal pyramidal neurons during peripheral stimulation

Responses of hippocampal pyramidal neurons were investigated intracellularly in unanesthetized rabbits immobilized with tubocurarine. A single stimulus, applied to the sciatic nerve, evoked prolonged (up to 2.5 sec) hyperpolarization of the cell membrane, accompanied by inhibition of action potentials. The latent period of the evoked hyperpolarization was 48±16.4 msec, and its amplitude 2.5±1.9 mV. In some neurons the development of hyperpolarization potentials was preceded by excitation. The suggestion is made that hyperpolarization of the membrane of pyramidal cells during peripheral stimulation is manifested as an inhibitory postsynaptic potential (IPSP), generated with the participation of hippocampal interneurons. The possibility of prolonged tonic action of interneurons from outside as a cause of prolonged inhibition of the pyramidal neurons is discussed.A. A. Bogomolets Institute of Physiology, Academy of Sciences of the Ukrainian SSR, Kiev. Translated from Neirofiziologiya, Vol. 1, No. 3, pp. 278–284, November–December, 1969.     

Upregulation of gamma-glutamate-cysteine ligase as part of the long-term adaptation process to iron accumulation in neuronal SH-SY5Y cells

Reactive iron is an important prooxidant factor, whereas GSH is a crucial component of a long-term adaptive system that allows cells to function during extended periods of high oxidative stress. In this work, the adaptive response of the GSH system to prolonged iron loads was characterized in human dopaminergic SH-SY5Y neuroblastoma cells. After the initial death of a substantial portion of the cell population, the surviving cells increased their GSH content by up to fivefold. This increase was traced to increased expression of the catalytic and modulatory subunits of -glutamate-cysteine ligase. Under conditions of high iron load, cells maintained a low GSSG content through two mechanisms: 1) GSSG reductase-mediated recycling of GSSG to GSH and 2) multidrug resistant protein 1-mediated extrusion of GSSG. Increased GSH synthesis and low GSSG levels contributed to recover the cell reduction potential from –290 mV at the time of cell death to about –320 mV. These results highlight the fundamental role of GSH homeostasis in the antioxidant response to cellular iron accumulation and provide novel insights into the adaptive mechanisms of neurons subjected to increased iron loads, such as those observed in Parkinson's disease. oxidative stress; glutathione; multidrug resistance protein 1; oxidixed glutathione reductase; neurodegenerative diseases     

Membrane Potentials in the Xylem in Roots of Intact Plants

The membrane potential differences (PDs) of root cells of intact,illuminated Trifolium repens L. and Lolium perenne L. have beenmeasured. In T. repens the PDs were the same for all cell typesexcept for the xylem vessels, which were more positive, andfor some cells immediately adjacent to the xylem vessels whichwere 10 mV more negative. The mean PD for all cells was emdash164.6 ± 0.6 mV and the mean for cells adjacent to thexylem vessels with elevated PDs was 178.4 ± 2.4 mV. Whenthe electrode tip was in a xylem vessel a low but stable PD(mean = emdash 89.9 mV) was recorded. The results for L. perennewere similar except that there were no cells with elevated PDsadjacent to the xylem vessels. An inhibitor of ion transport from the root to the shoot, p-fluorophenylalanine(p-FPA), caused a depolarization of 10 mV in the cell PDs butin the xylem vessels the depolarization was 50 mV. The possibility that the elevated PDs of cells adjacent to thexylem vessels are related to the transport of ions into thevessels is discussed.