Intracellular microelectrode measurements in small cells evaluated with the patch clamp technique

iPSC-Cardiomyocyte Models of Brugada Syndrome-Achievements, Challenges and Future Perspectives

Brugada syndrome (BrS) is an inherited cardiac arrhythmia that predisposes to ventricular fibrillation and sudden cardiac death. It originates from oligogenic alterations that affect cardiac ion channels or their accessory proteins. The main hurdle for the study of the functional effects of those variants is the need for a specific model that mimics the complex environment of human cardiomyocytes. Traditionally, animal models or transient heterologous expression systems are applied for electrophysiological investigations, each of these models having their limitations. The ability to create induced pluripotent stem cell-derived cardiomyocytes (iPSC-CMs), providing a source of human patient-specific cells, offers new opportunities in the field of cardiac disease modelling. Contemporary iPSC-CMs constitute the best possible in vitro model to study complex cardiac arrhythmia syndromes such as BrS. To date, thirteen reports on iPSC-CM models for BrS have been published and with this review we provide an overview of the current findings, with a focus on the electrophysiological parameters. We also discuss the methods that are used for cell derivation and data acquisition. In the end, we critically evaluate the knowledge gained by the use of these iPSC-CM models and discuss challenges and future perspectives for iPSC-CMs in the study of BrS and other arrhythmias.

Activation of plasma membrane H+-ATPases participates in dormancy alleviation in sunflower seeds

Using various inhibitors and scavengers we took advantage of the size of sunflower (Helianthus annuus) seeds to investigate in vivo the effects of hormones, namely abscisic acid (ABA) and ethylene (ET), and reactive oxygen species (ROS) on the polarization of dormant (D) and non-dormant (ND) embryonic seed cells using microelectrodes. Our data show that D and ND seed cells present different polarization likely due to the regulation of plasma membrane (PM) H⁺-ATPase activity. The data obtained after addition of hormones or ROS scavengers further suggest that ABA dependent inhibition of PM H⁺-ATPases could participate in dormancy maintenance and that ET-and ROS-dependent PM H⁺-ATPase stimulation could participate in dormancy release in sunflower seeds.

The Resting Potential and K+ Currents in Primary Human Articular Chondrocytes

Human transplant programs provide significant opportunities for detailed in vitro assessments of physiological properties of selected tissues and cell types. We present a semi-quantitative study of the fundamental electrophysiological/biophysical characteristics of human chondrocytes, focused on K⁺ transport mechanisms, and their ability to regulate to the resting membrane potential, E_m. Patch clamp studies on these enzymatically isolated human chondrocytes reveal consistent expression of at least three functionally distinct K⁺ currents, as well as transient receptor potential (TRP) currents. The small size of these cells and their exceptionally low current densities present significant technical challenges for electrophysiological recordings. These limitations have been addressed by parallel development of a mathematical model of these K⁺ and TRP channel ion transfer mechanisms in an attempt to reveal their contributions to E_m. In combination, these experimental results and simulations yield new insights into: (i) the ionic basis for E_m and its expected range of values; (ii) modulation of E_m by the unique articular joint extracellular milieu; (iii) some aspects of TRP channel mediated depolarization-secretion coupling; (iv) some of the essential biophysical principles that regulate K⁺ channel function in “chondrons.” The chondron denotes the chondrocyte and its immediate extracellular compartment. The presence of discrete localized surface charges and associated zeta potentials at the chondrocyte surface are regulated by cell metabolism and can modulate interactions of chondrocytes with the extracellular matrix. Semi-quantitative analysis of these factors in chondrocyte/chondron function may yield insights into progressive osteoarthritis.

Targeted intracellular voltage recordings from dendritic spines using quantum-dot-coated nanopipettes

Dendritic spines are the primary site of excitatory synaptic input onto neurons, and are biochemically isolated from the parent dendritic shaft by their thin neck. However, due to the lack of direct electrical recordings from spines, the influence that the neck resistance has on synaptic transmission, and the extent to which spines compartmentalize voltage, specifically excitatory postsynaptic potentials, albeit critical, remains controversial. Here, we use quantum-dot-coated nanopipette electrodes (tip diameters ∼15-30 nm) to establish the first intracellular recordings from targeted spine heads under two-photon visualization. Using simultaneous somato-spine electrical recordings, we find that back propagating action potentials fully invade spines, that excitatory postsynaptic potentials are large in the spine head (mean 26 mV) but are strongly attenuated at the soma (0.5-1 mV) and that the estimated neck resistance (mean 420 MΩ) is large enough to generate significant voltage compartmentalization. Nanopipettes can thus be used to electrically probe biological nanostructures.

Frequency-selective transmission of graded signals in large monopolar neurons of blowfly Calliphora vicina compound eye

The functional roles of voltage-gated K(+)(Kv) channels in visual system interneurons remain poorly studied. We have addressed this problem in the large monopolar cells (LMCs) of the blowfly Calliphora vicina, using intracellular recordings and mathematical modeling methods. Intracellular recordings were performed in two cellular compartments: the synaptic zone, which receives input from photoreceptors, and the axon, which provides graded potential output to the third-order visual neurons. Biophysical properties of Kv conductances in the physiological voltage range were examined in the dark with injections of current in the discontinuous current-clamp mode. Putative LMC types 1/2 and 3 (L1/2 and L3, respectively) had dissimilar Kv channelomes: L1/2 displayed a prominent inactivating Kv conductance in the axon, while L3 cells were characterized by a sustained delayed-rectifier Kv conductance. To study the propagation of voltage signals, the data were incorporated into the previously developed mathematical model. We demonstrate that the complex interaction between the passive membrane properties, Kv conductances, and the neuronal geometry leads to a resonance-like filtering of signals with peak frequencies of transmission near 15 and 40 Hz for L3 and L1/2, respectively. These results point to distinct physiological roles of different types of LMCs.

Nanotechnology and neurophysiology

Neuroscience would be revolutionized by a technique to measure intracellular electrical potentials that would not disrupt cellular physiology and could be massively parallelized. Though such a technology does not yet exist, the technical hurdles for fabricating minimally disruptive, solid-state electrical probes have arguably been overcome in the field of nanotechnology. Nanoscale devices can be patterned with features on the same length scale as biological components, and several groups have demonstrated that nanoscale electrical probes can measure the transmembrane potential of electrogenic cells. Developing these nascent technologies into robust intracellular recording tools will now require a better understanding of device-cell interactions, especially the membrane-inorganic interface. Here we review the state-of-the art in nanobioelectronics, emphasizing the characterization and design of stable interfaces between nanoscale devices and cells.

Neuronal recordings with solid-conductor intracellular nanoelectrodes (SCINEs)

Direct electrical recording of the neuronal transmembrane potential has been crucial to our understanding of the biophysical mechanisms subserving neuronal computation. Existing intracellular recording techniques, however, limit the accuracy and duration of such measurements by changing intracellular biochemistry and/or by damaging the plasma membrane. Here we demonstrate that nanoengineered electrodes can be used to record neuronal transmembrane potentials in brain tissue without causing these physiological perturbations. Using focused ion beam milling, we have fabricated Solid-Conductor Intracellular NanoElectrodes (SCINEs), from conventional tungsten microelectrodes. SCINEs have tips that are <300 nm in diameter for several micrometers, but can be easily handled and can be inserted into brain tissue. Performing simultaneous whole-cell patch recordings, we show that SCINEs can record action potentials (APs) as well as slower, subthreshold neuronal potentials without altering cellular properties. These results show a key role for nanotechnology in the development of new electrical recording techniques in neuroscience.

A Direct Comparison of Whole Cell Patch and Sharp Electrodes by Simultaneous Recording From Single Spinal Neurons in Frog Tadpoles

High-impedance, sharp intracellular electrodes were compared with whole cell patch electrodes by recording from single spinal neurons in immobilized frog tadpoles. A range of neuron properties were examined using sharp or patch test electrodes while making simultaneous recordings with a second control patch electrode. Overall, test patch electrodes did not significantly alter the activity recorded by the control electrode, and recordings from the two electrodes were essentially identical. In contrast, sharp electrode recordings differed from initial control patch recordings. In some cases, differences were due to real changes in neuron properties: the resting membrane potential became less negative and the neuron input resistance ( Ri) fell; this fall was larger for neurons with a higher Ri. In other cases, the control patch electrode revealed that differences were due to the recording properties of the sharp electrode: tip potentials were larger and more variable; resting potentials appeared to be more negative; and spike amplitude was attenuated. However, sharp electrode penetration did not, in most cases, significantly alter the pattern of neuron firing in response to injected current or the normal pattern of activity following sensory stimulation or during fictive swimming. We conclude that sharp electrodes introduce a significant leak to the membrane of tadpole spinal neurons compared with patch electrodes but that this does not change the fundamental firing characteristics or activity of the neurons.

Intracellular microelectrode membrane potential measurements in tobacco cell-suspension protoplasts and barley aleurone protoplasts: interpretation and artifacts

Intracellular microelectrode measurements in plant cell protoplasts have been widely used to study hormone signal transduction processes. However, the interpretation and reliability of such measurements are largely dependent on a detailed evaluation of the measurement conditions, as investigated in the present paper. Upon microelectrode penetration of tobacco cell suspension protoplasts and of barley aleurone protoplasts a fast negative going impalement-induced potential transient of less than a few ms duration could be observed. After reaching a steady-state potential at the ms time scale the measured potential hyperpolarized again and, in most cases, subsequently depolarized to a new steady-state value. Analysis of the electrical equivalent circuit of the measurement configuration showed that the occurrence of the impalement-induced potential transient indicates that these measurements suffer from a microelectrode-induced shunt resistance which loads the measurement. In addition, it is shown that the peak-value of the potential transient is the most reliable indicator of the true membrane potential and of true membrane potential changes of the protoplast, since this value is rather membrane resistance independent. For correct interpretation of steady-state measurements of membrane potential and stimulus-induced membrane potential changes data on membrane and shunt resistance are essential. As an example of the measurement of membrane potential changes the effects of 1-NAA on measured potential values in tobacco protoplasts and the effect of extracellular pH changes on barley aleurone protoplasts are analyzed with regard to the above described conclusions.

Microelectrode measurements of low frequency electric field effects in cells and tissues

The average intensities of electric fields induced into tissue can be calculated if the morphology and conductivities of the tissue are known, and such values provide one estimate of dosage for a given field exposure level. However, the microanatomical structures of living tissue, which include gap junctions, tight junctions, highly charged cell coats, and extracellular matrices, as well as complex cell shapes, precludes a detailed characterization of the field and current distribution near the cells which are actually responding to the electric fields. This suggests that a more useful electric field dose metric may be one based on an induced physical effect on the cells. Electric fields have at least three distinct physical effects on cells: the normal plasma membrane potential will be altered; the ionic currents and ion distributions at the extracellular surface will be modified; and mechanical forces will be imposed at the cell surface. Each of these effects can, in principle, be measured through the application of specific microelectrode techniques. Here, the feasibility of using various intracellular and extracellular recording methods to obtain dosimetric values, as well as the contribution these measurements could make to our understanding of electric field interactions with biological tissue, are discussed.

Voltage-activated K+ conductances in freshly isolated embryonic chicken osteoclasts

Patch-clamp measurements on freshly isolated embryonic chicken osteoclasts revealed three distinct types of voltage-dependent K+ conductance. The first type of conductance, present in 72% of the cells, activated at membrane potentials less negative than -30 to -20 mV and reached full activation at +40 mV. It activated with a delay, reached a peak value, and then inactivated with a time constant of approximately 1.5 s. Inactivation was complete or almost so. Recovery from inactivation, at -70 mV, had a time constant of roughly 1 s. The conductance could be blocked, at least partly, by 4 mM 4-aminopyridine. The second type of conductance (present in all cells) activated at membrane potentials more negative than -40 to -80 mV and reached full activation at -130 mV. Activation potential and maximal conductance were dependent on the extracellular K+ concentration. Inactivation of the conductance first became apparent at membrane potentials more negative than -100 mV and was a two-exponential process. The conductance could be blocked by external 5 mM Cs+ ions. The third type of conductance (present in all cells) activated at membrane potentials more positive than +30 mV. Generally, the conductance did not inactivate.

The membrane potential of the cellular slime mold Dictyostelium discoideum is mainly generated by an electrogenic proton pump

Trans membrane potential or ionic current changes may play a role in signal transduction and differentiation in the cellular slime mold dictyostelium discoideum. Therefore, the contribution of electrogenic ion pumps to the membrane potential of D. discoideum cells was investigated. the (negative) peak-value of the rapid potential transient, seen upon microelectrode impalement, was used to detect membrane potential changes upon changes in the external pH in the range of 5.5 to 8.0. The membrane potential was close to the Nernstian potential for protons over the pH range 5.5 to 7.5. The acid-induced changes in membrane potential were consistent with outward-proton pumping. The maximal membrane potential was at pH 7.5. Furthermore, the proton pump inhibitors diethylstilbestrol, miconazole and zearalenone directly depolarize the membrane. Cyanide and temperature decrease cause membrane depolarization as well. During recovery from cyanide poisoning a H+ efflux is present. From these measurements we conclude that the membrane potential of d. discoideum cells is mainly generated by an electrogenic proton pump. Measurements in cells with different extracellular potassium and H+ concentrations suggest a role for potassium in the function of the electrogenic proton pump. These results provide a framework for future research towards a possible role for the proton pump in signal transduction and differentiation.

Voltage-activated ionic channels and conductances in embryonic chick osteoblast cultures

Patch-clamp measurements were made on osteoblast-like cells isolated from embryonic chick calvaria. Cell-attached-patch measurements revealed two types of high conductance (100-250 pS) channels, which rapidly activated upon 50-100 mV depolarization. One type showed sustained and the other transient activation over a 10-sec period of depolarization. The single-channel conductances of these channel types were about 100 or 250 pS, depending on whether the pipettes were filled with a low K+ (3 mM) or high K+ (143 mM) saline, respectively. The different reversal potentials under these conditions were consistent with at least K+ conduction. Whole-cell measurements revealed the existence of two types of outward rectifying conductances. The first type conducts K+ ions and activates within 20-200 msec (depending on the stimulus) upon depolarizing voltage steps from less than -60 mV to greater than -30 mV. It inactivates almost completely with a time constant of 2-3 sec. Recovery from inactivation is biphasic with an initial rapid phase (1-2 sec) followed by a slow phase (greater than 20 sec). The second whole-cell conductance activates at positive membrane potentials of greater than +50 mV. It also rapidly turns on upon depolarizing voltage steps. Activation may partly disappear at the higher voltages. Its single channels of 140 pS conductance were identified in the whole cell and did conduct K+ ions but were not highly Cl- or Na+ selective. The results show that osteoblasts may express various types of voltage controlled ionic channels. We predict a role for such channels in mineral metabolism of bone tissue and its control by osteoblasts.

Electrophysiological properties of Dictyostelium derived from membrane potential measurements with microelectrodes

Electrical membrane properties of the cellular slime mold Dictyostelium discoideum were investigated with the use of intracellular microelectrodes. The rapid potential transients (1 msec) upon microelectrode penetration of normal cells had a negative-going peak-shaped time course. This indicates that penetration of a cell with a microelectrode causes a rapid depolarization, which can just be recorded by the microelectrode itself. Therefore, the initial (negative) peak potential transient value Ep (-19mV) should be used as an indicator of the resting membrane potential Em of D. discoideum before impalement, rather than the subsequent semistationary depolarized value En (-5 mV). Using enlarged cells such as giant mutant cells (Ep = -39 mV) and electrofused normal cells (Ep = -30 mV) improved the reliability of Ep as an indicator of Em. From the data we concluded that Em of D. discoideum cells bathed in (mM) 40 NaCl, 5 KCl and 1 CaCl2 is at least -50 mV. This potential was shown to be dependent on extracellular potassium. The average input resistance Ri of the impaled cells was 56 M omega for normal D. discoideum. However, our analysis indicates that the membrane resistance of these cells before impalement is greater than 1 G omega. Specific membrane capacitance was 1-3 pF/cm2. Long-term recording of the membrane potential showed the existence of a transient hyperpolarization following the rapid impalement transient. This hyperpolarization was associated with an increase in Ri of the impaled cell. It was followed by a depolarization, which was associated with a decrease in Ri. The depolarization time was dependent on the filling of the microelectrode. The present characterization of the electrical membrane properties of Dictyostelium cells is a first step in a membrane electrophysiological analysis of signal transduction in cellular slime molds.

Extracellular ATP induces a large nonselective conductance in macrophage plasma membranes

Extracellular ATP in its tetra-anionic form (ATP4-) induces ion fluxes and membrane depolarization in the mouse macrophage-like cell line J774.2 and in resident mouse macrophages. We analyzed the effects of extracellular ATP4- by both patch-clamp and intracellular microelectrode techniques. Whole-cell patch-configuration membrane potential measurements on J774.2 cells revealed that ATP4- -induced depolarization occurred within 40 ms of pulsed application of ATP and was completely reversible. The depolarizations were accompanied by a dramatic increase in membrane conductance and showed no sign of adaptation to ATP over a period of 30 min. At 5 mM total ATP (ATPt) the whole-cell conductance was approximately 10 nS, and an upper limit of 20 pS for a single-channel conductance has been established. The reversal potential associated with the ATP-induced depolarization at asymmetric K+, Na+, Ca2+, and Cl- concentrations across the membrane was 0 mV. In patch-clamped cells depolarization was complete at 20 microM ATP4-, and repolarization from full depolarization occurred in approximately 5 s. In contrast, in intact cells measured by microelectrode impalement, complete depolarization occurred at approximately 2 mM ATP4- and repolarization was much slower (approximately 100 min). These findings indicate that the changes in intracellular ionic composition that occur after ATP treatment affect the rate of cell repolarization. At lower concentrations of ATP, potassium conductances modulated the depolarizing effect of ATP. ATP also depolarized mouse peritoneal macrophages, but a variant cell line (ATPR B2), derived from J774.2 cells by prolonged exposure to ATP, was insensitive to ATP. Our results provide a membrane electrophysiological description and analysis of a large nonselective plasma membrane conductance of macrophages induced by extracellular ATP.

Potassium and calcium currents and action potentials in mouse Balb/c 3T3 fibroblasts

The electrical properties of Balb/c 3T3 mouse fibroblasts were studied with the whole-cell patch clamp technique. In current clamp mode a resting potential of -75.5 +/- 2.1 mV was recorded. In voltage clamp mode an inward current was also observed at potentials negative to Vm. This current crossed the 0-current axis at a voltage near Vm, and rectified at more positive potentials; the degree of rectification was dependent on [K+]o. At potentials positive to -30 mV a transient inward current was observed, showing a peak amplitude of -193 +/- 36 pA at +10 mV; the current amplitude was dependent on voltage and [Ca2+]o, it was strongly increased by 20 mM BaCl2 and abolished by 2 microM verapamil and 1 microM nifedipine. These cells, in response to depolarizing stimuli, develop slow action potentials, probably supported by the Ca2+ current.

Factors responsible for oscillations of membrane potential recorded with tight-seal-patch electrodes in mouse fibroblasts

In giant fibroblastic L cells, penetration of a conventional microelectrode brought about marked decreases in the membrane potential and input resistance measured with a patch electrode under tight-seal whole-cell configuration, and repeated hyperpolarizations were often observed upon penetration. Therefore, the question arose whether such leakage artifact is a causal factor for generation of the membrane potential oscillation even in giant L cells. During whole-cell recordings, however, regular potential oscillations were observed in the cells that had not been impaled with a conventional microelectrode, as far as the Ca2+ buffer was not strong in the pipette solution. Oscillatory changes in the intracellular potential were detected by extracellular recordings with a tight-seal patch electrode in the cell-attached configuration. Thus, the potential oscillation occurs even in the absence of penetration-induced leakage or without rupture of the patch membrane. Withdrawal of a micropipette from one cell was often found to induce marked cell damage and elicit oscillatory hyperpolarizations in a neighboring cell with a certain time lag. The longer the distance between the injured and recorded cells, the greater was the time lag. Application of the cell lysate on the cell surface also gave rise to oscillatory hyperpolarizations. After repeated applications of the lysate, the membrane became unresponsive (desensitized), suggesting the involvement of receptors for the lysate factor. The lysates of different cell species (mouse lymphoma L5178Y cells or human epithelial Intestine 407 cells) produced similar effects. The effective component was heat stable and distinct from ATP. Lysate-induced hyperpolarizations were inhibited by deprivation of extracellular Ca2+ and by application of a Ca2+ channel blocker (nifedipine) or a K+ channel blocker (quinine) in the same manner as spontaneous oscillatory hyperpolarizations. It is concluded that the mouse fibroblast exhibits membrane potential oscillations, when the cell was activated, presumably via receptor systems, by some diffusible factors released from damaged cells.

Phagocytosis by human macrophages is accompanied by changes in ionic channel currents

The present study has shown that changes in ionic channel currents accompany the phagocytosis of particles by mononuclear phagocytes. The patch-clamp technique in the cell-attached configuration was applied to human monocyte-derived macrophages to measure the activity of single transmembrane ionic channels in intact cells. During such measurements, IgG-opsonized and non-opsonized latex particles were offered for phagocytosis under continuous video-microscopical observation. Single particles were presented to the phagocytes at a membrane location some distance from that of the patch electrode. After a lag period following particle attachment, enhanced inward and outward time-variant single channel currents coinciding with particle engulfment were observed. On the basis of current-voltage characteristics and membrane potential measurements, the outward-directed channels were identified as K+ channels. Phagocytosis was also accompanied by slow transient changes in background membrane currents, probably due to changes in the membrane potential of the phagocytosing cell. Phagocytosis of IgG-coated latex particles differed from phagocytosis of uncoated or albumin-coated particles by a shorter lag time between particle attachment and the onset of enhanced ionic channel activity.

Intracellular K+, Na+ and Cl- concentrations and membrane potential in human monocytes

The relationship between the resting membrane potential and the intracellular ionic concentrations in human monocytes was investigated. Cell volume, cell water content, and amount of intracellular K+, Na+, and Cl- were measured to determine the intracellular concentrations of K+ (Ki), Na+ (Nai) and Cl- (Cli) of monocytes, and of lymphocytes and neutrophils. Values found for monocytes were similar to those for neutrophils, i.e., cell volumes were 346 and 345 micron3, respectively, cell water content 78%, and Ki, 128 and 125, Nai, 24 and 26, and Cli, 102 and 103 mmol/l cell water, respectively. Lymphocytes, however, had different values: 181 micron3 cell volume, 77% cell water content, and for Ki, Nai, and Cli, 165, 37, and 91 mmol/l cell water, respectively. The resting membrane potential of cultured human monocytes (range -30 to -40 mV), determined by measurement of the peak potential occurring within the first milliseconds after microelectrode entry, was most dependent on extracellular K+, followed by Cl-, and Na+. The membrane permeability ratio of Cl- to K+ was estimated by use of the constant field equation to be 0.23 (range 0.22 to 0.30).

The effect of noradrenaline on the ion permeability of isolated mammalian hepatocytes, studied by intracellular recording

1. The influence of noradrenaline on the membrane potential and conductance of isolated guinea-pig and rabbit hepatocytes in short-term (2-8 h) tissue culture has been studied by intracellular recording. 2. Resting hepatocytes had linear current-voltage relationships, with input resistances of 166 and 216 M omega in guinea-pig and rabbit cells respectively. The recorded membrane potential was -18 mV in each species, though the true resting potential is likely to have been up to 10 mV greater. 3. The hepatocytes sometimes slowly hyperpolarized during intracellular recording, and this was associated with a fall in membrane resistance, and an increase followed by a decrease in membrane potential noise. These effects were abolished by quinine (200 microM) but not by apamin (50 nM), and are attributable to a K+ conductance activated by cell swelling. 4. Noradrenaline (2 microM, in the presence of propranolol at 1 microM) was applied to individual hepatocytes by pressure ejection (puffer pipette technique). After a short latency, the cells hyperpolarized by a mean of 18 mV in both guinea-pig and rabbit preparations. This was associated with a large rise in membrane conductance (50 nS in guinea-pig, 54 nS in rabbit cells). The reversal potential for this action was -38 mV. 5. The experiments were repeated in the presence of apamin (50 nM) to block the Ca2+-dependent K+ permeability which noradrenaline activates in these cells. Noradrenaline still caused some hyperpolarization and a substantial increase (approximately 40 nS) in conductance, with a reversal potential (Er) of -31 mV. This can be attributed to an increase in Cl- conductance. 6. In keeping with this interpretation, noradrenaline applied in the absence of Cl- (replaced by isethionate or gluconate) caused a much greater hyperpolarization (58 mV in guinea-pig, 40 mV in rabbit cells) associated with a smaller rise in conductance (approximately 12 nS). Er for this action was -95 mV (guinea-pig) and -68 mV rabbit), suggesting that the conductance increase was now mainly to K+. 7. The magnitudes of the conductance changes produced by noradrenaline under the various experimental conditions suggest that the increase in the conductance to Cl- (delta GCl) is 3-fold greater than that to K+ (delta GK). 8. The activation of delta GCl occurs either at the same time as delta GK, or (in ca. one cell in ten) a few seconds later.(ABSTRACT TRUNCATED AT 400 WORDS)

Microelectrode Measurements on Red Beet Vacuole : Biological Effect of Na OR NO(3) Ions, Diffusing from the Microelectrode

Glass microelectrodes filled with 3 molar KCl are widely used to measure intracellular potentials and it is usual to try to minimize their electrolyte loss. In these experiments we have used the ionic leak of our microelectrodes, filled with various salt solutions, to introduce a given ion into the red beet vacuole. This allowed us to show that NO(3) (-) ions reduce the magnitude of the current spectral density while they do not change the resistance of the tonoplast. This is true when NO(3) (-) is either added to the external medium or used as the microelectrode filling solution. This can be interpreted by a NO(3) (-) effect on the vacuolar side of the tonoplast, resulting in an inhibition of the ion transporting ATPase. Replacing K(+) by Na(+) ions in the medium has no effect on tonoplast resistance (R(s)). On the contrary, when ions leaking from the microelectrode are H(+), Li(+) or K(+), R(s) is close to 4 kilohm square centimeter, whereas R(s) is of the order of 30KOmega square centimeter when Na(+) are the leaking ions. We also found a possible correlation between the presence of a Lorentzian in the current spectral density (cut-off frequency = 2 hertz) and a Cl(-) efflux from the vacuole. This could be explained by the existence of Cl(-) channels on the tonoplast.

Ionic channels and membrane hyperpolarization in human macrophages

Microelectrode impalement of human macrophages evokes a transient hyperpolarizing response (HR) of the membrane potential. This HR was found to be dependent on the extracellular concentration of K+ but not on that of Na+ or Cl-. It was not influenced by low temperature (12 degrees C) or by 0.2 mM ouabain, but was blocked by 0.2 mM quinine or 0.2 mM Mg2+-EGTA. These findings indicate that the HR in human macrophages is caused by the activation of a K+ (Ca2+) conductance. Two types of ionic channels were identified in intact cells by use of the patch-clamp technique in the cell-attached-patch configuration, low and high-conductance voltage-dependent K+ channels. The low-conductance channels had a mean conductance of 38 pS with Na+-saline and 32 pS with K+-saline in the pipette. The high-conductance channels had a conductance of 101 and 114 pS with Na+- and K+-saline in the pipette, respectively. Cell-attached patch measurements made during evocation of an HR by microelectrode penetration showed enhanced channel activity associated with the development of the HR. These channels were also high-conductance channels (171 pS with Na+- and 165 pS K+-saline in the pipette) and were voltage dependent. They were, however, active at less positive potentials than the high-conductance K+ channels seen prior to the microelectrode-evoked HR. It is concluded that the high-conductance voltage-dependent ionic channels active during the HR in human macrophages contribute to the development of the HR.