Resting potential of the mouse neuroblastoma cells: I. The presence of K+ channels activated at high K+ concentration but closed at low K+ concentration including the physiological concentration

The effects of changes in extracellular K⁺ concentration ([K⁺]_o) on the resting membrane potential, the input resistance and ⁸⁶Rb efflux (as a marker of K⁺ efflux) were examined with use of the cultured mouse neuroblastoma cells (N-18 clone). The results obtained are as follows. (1) The membrane potential was depolarized, with an increase in [K⁺]_o at concentrations above 10–20 mM at a rate of 55–58 mV per 10-fold change in [K⁺]_o, but practically unchanged with varying [K⁺]_o below this concentration. (2) Above the critical [K⁺]_o of 10–20 mM, the input membrane resistance decreased sharply by a factor of $\text{1}\text{4}\text{?}\text{1}\text{5}$ with an increase in [K⁺]_o. A similar decrease in the resistance occurred even under the conditions that the membrane potential was held at control level (about ?55 mV) by a steady-state current passage. (3) Elimination of Na⁺ and Cl^? from the external solution brought about practically no change in the membrane potential. (4) A fractional escape rate of ⁸⁶Rb from N-18 cells remained constant at relatively low level (0.125%/min on average) in the low [K⁺]_o range, but increased sharply with increasing [K⁺]_o above 15 mM (e.g., approx. 3.4- and 4.5-fold at 30 and 100 mM [K⁺]_o, respectively). (5) The high K⁺-induced ⁸⁶Rb efflux was not practically inhibited by 1 mM tetraethylammonium or 0.1 mM 4-aminopyridine, indicating that the K⁺ channels activated by an elevation of [K⁺]_o are not the delayed (voltage-dependent) K⁺ channels. The present results favoured the conclusion that N-18 cells carry K⁺ channels which open at high [K⁺]_o but are closed at low [K⁺]_o including the physiological range for the mouse neuroblastoma cells (around 5.4 mM). This conclusion leads to the notion that in the mouse neuroblastoma N-18 cells the K⁺ permeability does not mainly contribute to determining the resting membrane potential under physiological conditions.     

Influence of Ca on the plasma membrane potential and electrogenic uptake of glycine by myeloma cells. Involvement of a Ca-activated K channel

The involvement of Ca²⁺-activated K⁺ channels in the regulation of the plasma membrane potential and electrogenic uptake of glycine in SP 2/0-AG14 lymphocytes was investigated using the potentiometric indicator 3,3′-diethylthiodicarbocyanine iodide. The resting membrane potential was estimated to be −57 ± 6 mV (n = 4), a value similar to that of normal lymphocytes. The magnitude of the membrane potential and the electrogenic uptake of glycine were dependent on the extracellular K⁺ concentration, [K⁺]_o, and were significantly enhanced by exogenous calcium. The apparent V_max of Na⁺-dependent glycine uptake was doubled in the presence of calcium, whereas the K_0.5 was not affected. Ouabain had no influence on the membrane potential under the conditions employed. Additional criteria used to demonstrate the presence of Ca²⁺-activated K⁺ channels included the following: (1) addition of EGTA to calcium supplemented cells elicited a rapid depolarization of the membrane potential that was dependent on [K⁺]_o; (2) the calmodulin antagonist, trifluoperazine, depolarized the membrane potential in a dose-dependent and saturable manner with an IC₅₀ of 9.4 μM; and (3) cells treated with the Ca²⁺-activated K⁺ channel antagonist, quinine, demonstrated an elevated membrane potential and depressed electrogenic glycine uptake. Results from the present study provide evidence for Ca²⁺-activated K⁺ channels in SP 2/0-AG14 lymphocytes, and that their involvement regulates the plasma membrane potential and thereby the electrogenic uptake of Na⁺-dependent amino acids.     

Contribution of a H+ pump in determining the resting potential of neuroblastoma cells

The aim of this work was to examine the effects of changes in external K⁺ concentration (K _o ) around its physiological value, of various K⁺ channels blockers, including internal Cs⁺, of vacuolar H⁺-ATPase inhibitors and of the protonophore CCCP on the resting potential and the voltage-dependent K⁺ current of differentiated neuroblastoma x glioma hybrid NG108-15 cells using the whole-cell patch-clamp technique. The results are as follows: (i) under standard conditions (K _o =5 mm) the membrane potential was –60±1 mV. It was unchanged when K _o was decreased to 1 mm and was depolarized by 4±1 mV when K_o was increased to 10 mm. (ii) Internal Cs⁺ depolarized the membrane by 21±3 mV. (iii) The internal application of the vacuolar H⁺-ATPase inhibitors N-ethylmaleimide (NEM), NO ₃ ^– and bafilomycin A1 (BFA) depolarized the membrane by 15±2, 18±2 and 16±2 mV, respectively, (iv) When NEM or BFA were added to the internal medium containing Cs⁺, the membrane was depolarized by 45±1 and 42±2 mV, respectively. (v) The external application of CCCP induced a transient depolarization followed by a prolonged hyperpolarization. This hyperpolarization was absent in BFA-treated cells. The voltage-dependent K⁺ current was increased at negative voltages and decreased at positive voltages by NEM, BFA and CCCP. Taken together, these results suggest that under physiological conditions, the resting potential of NG108-15 neuroblastoma cells is maintained at negative values by both voltage-dependent K⁺ channels and an electrogenic vacuolar type H⁺-ATPase.This work was supported by a grant from INSERM (CRE 91 0906).     

Fluid shear stress induces calcium transients in osteoblasts through depolarization of osteoblastic membrane

Intracellular calcium transient ([Ca²⁺]_i transient) induced by fluid shear stress (FSS) plays an important role in osteoblastic mechanotransduction. Changes of membrane potential usually affect [Ca²⁺]_i level. Here, we sought to determine whether there was a relationship between membrane potential and FSS-induced [Ca²⁺]_i transient in osteoblasts. Fluorescent dyes DiBAC₄(3) and fura-2 AM were respectively used to detect membrane potential and [Ca²⁺]_i. Our results showed that FSS firstly induced depolarization of membrane potential and then a transient rising of [Ca²⁺]_i in osteoblasts. There was a same threshold for FSS to induce depolarization of membrane potential and [Ca²⁺]_i transients. Replacing extracellular Na⁺ with tetraethylammonium or blocking stretch-activated channels (SACs) with gadolinium both effectively inhibited FSS-induced membrane depolarization and [Ca²⁺]_i transients. However, voltage-activated K⁺ channel inhibitor, 4-Aminopyridine, did not affect these responses. Removing extracellular Ca²⁺ or blocking of L-type voltage-sensitive Ca²⁺ channels (L-VSCCs) with nifedipine inhibited FSS-induced [Ca²⁺]_i transients in osteoblasts too. Quantifying membrane potential with patch clamp showed that the resting potential of osteoblasts was −43.3 mV and the depolarization induced by FSS was about 44 mV. Voltage clamp indicated that this depolarization was enough to activated L-VSCCs in osteoblasts. These results suggested a time line of Ca²⁺ mobilization wherein FSS activated SACs to promote Na⁺ entry to depolarize membrane that, in turn, activated L-VSCCs and Ca²⁺ influx though L-VSCCs switched on [Ca²⁺]_i response in osteoblasts.     

Satellite glial cells In Situ within mammalian prevertebral ganglia express K+ channels active at rest potential

Patch-clamp experiments were performed on satellite glial cells wrapped around sympathetic neurons in the rabbit coeliac ganglion. With the cleaning method used, the glial cells could be kept in place and were directly accessible to the patch-clamp pipettes. Whole-cell recordings showed that glial cells had almost ohmic properties. Their resting potential (–79.1±1.2 mV) was found to be very nearly the same as the K⁺ reversal potential and 20 mV more negative than that of the neurons they encapsulated. Unitary currents from ionic channels present in the glial membrane were recorded in the cell-attached configuration with pipettes filled with various amounts of K⁺, Na⁺ and gluconate. Only K⁺-selective channels with slight inwardly rectifying properties (in the presence of 150 mM [K⁺]₀) were detected. These channels were active (P ₀=0.7–0.8) at the cell resting potential. The channel conductance, but not its opening probability, was dependent on the [K⁺] in the pipette. Cl^–-selective channels (outwardly rectifying and large conductance channels) were detected in excised patches.The properties of the K⁺ channels (increased inward current with [K⁺] and detectable outward current at low [K⁺]) are well suited for siphoning the K⁺ released by active neurons.     

Resting potential of rat cerebellar granule cells during early maturation in vitro

The survival of rat cerebellar granule cells maintained in vitro is enhanced by a KCl-enriched medium. This effect is classically interpreted as resulting from a higher cytosolic calcium concentration. This implies the presence of voltage-dependent Ca²⁺ channels and a membrane potential that can respond to changes in external K⁺. Since previous studies cast a doubt on these two conditions, we reinvestigated the resting membrane potential and Ca²⁺ influxes in rat cerebellar granule neurones during the first week in vitro using a fluorescence imaging approach. Membrane potential was assessed with the fluorescent dye bis-oxonol, and intracellular free calcium with Fura-2. Resting potential was shown to progressively decrease from −40 mV at the first day in vitro to −60 mV at day 7. At all times in culture, as early as day 0, cells were depolarized when external KCl concentration was increased from 5 to 30 mM. This depolarization resulted in an increased cytosolic calcium concentration due to Ca²⁺ influx through L-type and N-type voltage-activated Ca²⁺ channels, functional at day 0. Gross estimations of the permeabilities of Na⁺ and Cl⁻ were obtained at various times in culture by measuring the changes in resting potential brought about by a reduction of their external concentration. A progressive increase of the relative permeability to K⁺ ions seems to underlie the evolution of the resting potential with time. © 1997 John Wiley & Sons, Inc. J Neurobiol 32: 11–21, 1997.     

Contributions of K+, Na+, and Cl− to the membrane potential of intact hamster vascular endothelial cells

The transmembrane potential (V_m) of vascular endothelial cells (EC) is an important property that may be involved in intra- and intercellular signal transduction for various vascular functions. In this study, V_m of intact aortic and vena caval EC from hamsters were measured using conventional microelectrodes. Vascular strips with the luminal surface upwards were suffused in a tissue chamber with krebs solution in physiological conditions. The resting V_m of aortic and vena caval EC was found to be ?40± 1 mV (n = 55) and ?43± 1 mV (n = 15), respectively. The V_m recordings were confirmed to have originated from EC by scanning and transmission electron microscopy combined with the comparison of electrical recordings between normal and endothelium-denuded aortic strips. The input resistance varied from 10–240 MΩ, which implied the presence of electrical coupling between vascular EC. Elevating the K⁺ level in the suffusate from 4.7 mM to 50 and 100 mM depolarized aortic EC by 19% and 29% and vena caval EC by 18% and 29%, respectively. These low percentages indicated a relatively small contribution of [K⁺] to the resting V_m of vascular EC. A positive correlation (r> 0.69) between the resting V_m and the magnitude of depolarization by the high [K⁺]₀ may be related to the involvement of voltage-dependent K⁺ channels. The hyperpolarization caused by lowering both [Na⁺]₀ and [CI^?]₀ suggested the disengagement of some electrogenic transport systems in the membrane, such as a Na⁺ -K⁺ -CI^? cotransporter. The transference number (t_ion), as an index of membrane conductance for specific ions, was calculated for K⁺ (15-20%), Na⁺ (16%), and Cl^? (9-15%), demonstrating that both Na⁺ and Cl^? as well as K⁺ contribute to the overall resting V_m. Our study documented some basic electrophysiology of the vascular EC when both structural and functional properties of the cell were maintained, thus furthering the understanding of the essential role of endothelial cells in mediating vascular functions. © 1993 Wiley-Liss, Inc.     

Early changes in membrane potential of Saccharomyces cerevisiae induced by varying extracellular K+, Na+ or H+ concentrations

Recently we introduced a fluorescent probe technique that makes possible to convert changes of equilibrium fluorescence spectra of 3,3’-dipropylthiadicarbocyanine, diS-C₃(3), measured in yeast cell suspensions under defined conditions into underlying membrane potential differences, scaled in millivolts (Plasek et al. in J Bioenerg Biomembr 44: 559–569, 2012). The results presented in this paper disclose measurements of real early changes of plasma membrane potential induced by the increase of extracellular K⁺, Na⁺ and H⁺ concentration in S. cerevisiae with and without added glucose as energy source. Whereas the wild type and the ?tok1 mutant cells exhibited similar depolarization curves, mutant cells lacking the two Trk1,2 potassium transporters revealed a significantly decreased membrane depolarization by K⁺, particularly at lower extracellular potassium concentration [K⁺]_out. In the absence of external energy source plasma membrane depolarization by K⁺ was almost linear. In the presence of glucose the depolarization curves exhibited an exponential character with increasing [K⁺]_out. The plasma membrane depolarization by Na⁺ was independent from the presence of Trk1,2 transporters. Contrary to K⁺, Na⁺ depolarized the plasma membrane stronger in the presence of glucose than in its absence. The pH induced depolarization exhibited a fairly linear relationship between the membrane potential and the pH_o of cell suspensions, both in the wild type and the Δtrk1,2 mutant strains, when cells were energized by glucose. In the absence of glucose the depolarization curves showed a biphasic character with enhanced depolarization at lower pH_o values.     

Effects of K+ Channel Blockers and Nitric Oxide on the Electrical and Contractile Activities of Smooth Muscles of the Rabbit Main Pulmonary Artery

Using a sucrose-bridge technique, we studied electrical and mechanical responses of smooth muscle ring strips of the rabbit main pulmonary artery to applications of blockers of voltage-operated (including Ca²⁺-dependent) K⁺ channels, tetraethylammonium (TEA) and 4-aminopyridine (4-AP), as well to application of nitric oxide (NO); nitroglycerin (NG) was used as a donor of the latter. All experiments were carried out under conditions of blockade of the adreno- and cholinoreceptors in the preparation. Both TEA and 4-AP evoked dose-dependent effects: depolarization of smooth muscle cells (SMC) and their contraction. Simultaneous addition of TEA and 4-AP to the normal superfusate (Krebs solution) resulted in intensification of depolarization and initiated generation of action potentials (AP); contractions became rather intensive and possessed a tetanic pattern. Addition of NG to TEA- and 4-AP-containing Krebs solution effectively suppressed AP generation and contractions, whereas the depolarization level underwent only mild modifications. These findings show that Ca²⁺-dependent high-conductance K⁺ channels (K_Ca channels) and 4-AP-sensitive voltage-operated K⁺ channels (K_V channels) are involved in the formation of the resting membrane potential (RMP) in SMC of the rabbit main pulmonary artery. The impact of the K_Ca channels is greater than that of the K_V channels. We suppose that the effects of NO on SMC are related to inhibition of the activity of high-threshold voltage-operated L-type Ca²⁺ channels and, probably, to lowering of the sensitivity of the contractile SMC apparatus to Ca²⁺.     

A mathematical model of Ca2+ dynamics in rat mesenteric smooth muscle cell: agonist and NO stimulation

A mathematical model of calcium dynamics in vascular smooth muscle cell (SMC) was developed based on data mostly from rat mesenteric arterioles. The model focuses on (a) the plasma membrane electrophysiology; (b) Ca²⁺ uptake and release from the sarcoplasmic reticulum (SR); (c) cytosolic balance of Ca²⁺, Na⁺, K⁺, and Cl⁻ ions; and (d) IP₃ and cGMP formation in response to norepinephrine (NE) and nitric oxide (NO) stimulation. Stimulation with NE induced membrane depolarization and an intracellular Ca²⁺ ([Ca²⁺]_i) transient followed by a plateau. The plateau concentrations were mostly determined by the activation of voltage-operated Ca²⁺ channels. NE causes a greater increase in [Ca²⁺]_i than stimulation with KCl to equivalent depolarization. Model simulations suggest that the effect of [Na⁺]_i accumulation on the Na⁺/Ca²⁺ exchanger (NCX) can potentially account for this difference. Elevation of [Ca²⁺]_i within a concentration window (150-300 nM) by NE or KCl initiated [Ca²⁺]_i oscillations with a concentration-dependent period. The oscillations were generated by the nonlinear dynamics of Ca²⁺ release and refilling in the SR. NO repolarized the NE-stimulated SMC and restored low [Ca²⁺]_i mainly through its effect on Ca²⁺-activated K⁺ channels. Under certain conditions, Na⁺-K⁺-ATPase inhibition can result in the elevation of [Na⁺]_i and the reversal of NCX, increasing resting cytosolic and SR Ca²⁺ content, as well as reactivity to NE. Blockade of the NCX's reverse mode could eliminate these effects. We conclude that the integration of the selected cellular components yields a mathematical model that reproduces, satisfactorily, some of the established features of SMC physiology. Simulations suggest a potential role of intracellular Na⁺ in modulating Ca²⁺ dynamics and provide insights into the mechanisms of SMC constriction, relaxation, and the phenomenon of vasomotion. The model will provide the basis for the development of multi-cellular mathematical models that will investigate microcirculatory function in health and disease.     

Oscillations of membrane potential in L cells

Summary Effects of divalent cations on oscillations of membrane potentials (i.e., spontaneous repetitive hyperpolarizing responses) and on hyperpolarizing responses induced by electrical stimuli as well as on resting potentials were studied in large nondividing L cells. Deprivation of Ca²⁺ from the external medium inhibited these hyperpolarizing responses accompanying slight depolarization of the resting potential. Sr²⁺ or Mn²⁺ applied to the external medium in place of Ca²⁺ was able to substitute for Ca²⁺ in the generation of hyperpolarizing responses, while Mg²⁺, Ba²⁺ or La³⁺ suppressed hyperpolarizing responses. The addition of A23187 to the bathing medium or intracellular injection of Ca²⁺, Sr²⁺, Mn²⁺ or La³⁺ induced membrane hyperpolarization. When the external Ca²⁺, Sr²⁺ or Mn²⁺ concentration was increased, the resting potential also hyperpolarized, in a saturating manner. The amplitude of maximum hyperpolarization produced by high external Ca²⁺ was of the same order of magnitude as those of hyperpolarizing responses and was dependent on the external K⁺ concentration. In the light of these experimental observations, it was deduced that the K⁺ conductance increase associated with the hyperpolarizing excitation is the result of an increase in the intracellular concentration of free Ca²⁺ mainly derived from the external solution.     

The Effects of Various Ions on Resting and Spike Potentials of Barnacle Muscle Fibers

Effects of monovalent cations and some anions on the electrical properties of the barnacle muscle fiber membrane were studied when the intra- or extracellular concentrations of those ions were altered by longitudinal intra-cellular injection. The resting potential of the normal fiber decreases linearly with increase of logarithm of [K⁺]_out and the decrement for a tenfold increase in [K⁺]_out is 58 mv when the product, [K⁺]_out ·[Cl^-]_out, is kept constant. It also decreases with decreasing [K⁺]_in but is always less than expected theoretically. The deviation becomes larger as [K⁺]_in increases and the resting potential finally starts to decrease with increasing [K⁺]_in for [K⁺]_in > 250 mM. When the internal K⁺ concentration is decreased the overshoot of the spike potential increases and the time course of the spike potential becomes more prolonged. In substituting for the internal K⁺, Na⁺ and sucrose affect the resting and spike potentials similarly. Some organic cations (guanidine, choline, tris, and TMA) behave like sucrose while some other organic cations (TEA, TPA, and TBA) have a specific effect and prolong the spike potential if they are applied intracellularly or extracellularly. In all cases the active membrane potential increases linearly with the logarithm of [Ca⁺⁺]_out/[K⁺]_in and the increment is about 29 mv for tenfold increase in this ratio. The fiber membrane is permeable to Cl^- and other smaller anions (Br^- and I^-) but not to acetate^- and larger anions (citrate^-, sulfate^-, and methanesulfonate^-).     

Measurement of membrane potential in polymorphonuclear leukocytes and its changes during surface stimulation

The membrane potential of guinea pig polymorphonuclear leukocytes has been assessed with two indirect probes, tetraphenylphosphonium (TPP⁺) and 3,3′-dipropylthiadicarbocyanine (diS-C₃-(5)). The change in TPP⁺ concentration in the medium was measured with a TPP⁺-selective electrode. By monitoring differences in accumulation of TPP⁺ in media containing low and high potassium concentrations, a resting potential of −58.3 mV was calculated. This potential is composed of a diffusion potential due to the gradient of potassium, established by the Na⁺, K⁺ pump, and an electrogenic potential. The chemotactic peptide fMet-Leu-Phe elicits a rapid efflux of accumulated TPP⁺ (indicative of depolarization) followed by its reaccumulation (indicative of repolarization). In contrast, stimulation with concanavalin A results in a rapid and sustained depolarization without a subsequent repolarization. The results obtained with TPP⁺ and diS-C₃-(5) were comparable. Such changes in membrane potential were observed in the absence of extracellular sodium, indicating that an inward movement of sodium is not responsible for the depolarization. Increasing potassium levels, which lead to membrane depolarization, had no effect on the oxidative metabolism in nonstimulated or in fMet-Leu-Phe-stimulated cells. Therefore, it seems unlikely that membrane depolarization per se is the immediate stimulus for the respiratory burst.     

Persistent Na+ influx drives L-type channel resting Ca2+ entry in rat melanotrophs

Rat melanotrophs express several types of voltage-gated and ligand-gated calcium channels, although mechanisms involved in the maintenance of the resting intracellular Ca²⁺ concentration ([Ca²⁺]_i) remain unknown. We analyzed mechanisms regulating resting [Ca²⁺]_i in dissociated rat melanotrophs by Ca²⁺-imaging and patch-clamp techniques. Treatment with antagonists of L-type, but not N- or P/Q-type voltage-gated Ca²⁺ channels (VGCCs) as well as removal of extracellular Ca²⁺ resulted in a rapid and reversible decrease in [Ca²⁺]_i, indicating constitutive Ca²⁺ influx through L-type VGCCs. Reduction of extracellular Na⁺ concentration (replacement with NMDG⁺) similarly decreased resting [Ca²⁺]_i. When cells were champed at –80 mV, decrease in the extracellular Na⁺ resulted in a positive shift of the holding current. In cell-attached voltage-clamp and whole-cell current-clamp configurations, the reduction of extracellular Na⁺ caused hyperpolarisation. The holding current shifted in negative direction when extracellular K⁺ concentration was increased from 5 mM to 50 mM in the presence of K⁺ channel blockers, Ba²⁺ and TEA, indicating cation nature of persistent conductance. RT-PCR analyses of pars intermedia tissues detected mRNAs of TRPV1, TRPV4, TRPC6, and TRPM3-5. The TRPV channel blocker, ruthenium red, shifted the holding current in positive direction, and significantly decreased the resting [Ca²⁺]_i. These results indicate operation of a constitutive cation conductance sensitive to ruthenium red, which regulates resting membrane potential and [Ca²⁺]_i in rat melanotrophs.     

Development of depolarization‐induced calcium transients in insect glial cells is dependent on the presence of afferent axons

Changes in the intracellular Ca²⁺ concentration ([Ca²⁺]_i) induced by depolarization have been measured in glial cells acutely isolated from antennal lobes of the moth Manduca sexta at different postembryonic developmental stages. Depolarization of the glial cell membrane was elicited by increasing the external K⁺ concentration from 4 to 25 mM. At midstage 5 and earlier stages, less than 20% of the cells responded to 25 mM K⁺ (1 min) with a transient increase in [Ca²⁺]_i of approximately 40 nM. One day later, at late stage 5, 68% of the cells responded to 25 mM K⁺, the amplitude of the [Ca²⁺]_i transients averaging 592 nM. At later stages, all cells responded to 25 mM K⁺ with [Ca²⁺]_i transients with amplitudes not significantly different from those at late stage 5. In stage 6 glial cells isolated from deafferented antennal lobes, i.e., from antennal lobes chronically deprived of olfactory receptor axons, only 30% of the cells responded with [Ca²⁺]_i transients. The amplitudes of these [Ca²⁺]_i transients averaged 93 nM and were significantly smaller than those in normal stage 6 glial cells. [Ca²⁺]_i transients were greatly reduced in Ca²⁺‐free, EGTA‐buffered saline, and in the presence of the Ca²⁺ channel blockers cadmium and verapamil. The results suggest that depolarization of the cell membrane induces Ca²⁺ influx through voltage‐activated Ca²⁺ channels into antennal lobe glial cells. The development of the depolarization‐induced Ca²⁺ transients is rapid between midstage 5 and stage 6, and depends on the presence of afferent axons from the olfactory receptor cells in the antenna. © 2002 Wiley Periodicals, Inc. J Neurobiol 52: 85–98, 2002     

Effect of extracellular Ca2+, K+ and OH− on erythrocyte membrane potential as monitored by the fluorescent probe 3,3′-dipropylthiodicarbocyanine

Changes in fluorescence intensity of thiodicarbocyanine, DiS-C₃(5), were correlated with direct microelectrode potential measurements in red blood cells from Amphiuma means and applied qualitatively to evaluate the effects of extracellular Ca²⁺, K⁺ and pH on the membrane potential of human red cells. Increasing extracellular [Ca²⁺] from 1.8 to 15 mM causes a K⁺-dependent hyperpolarization and decrease in fluorescence intensity in Amphiuma red cells. Both the hyperpolarization and fluorescence change disappear when the temperature is raised from 17 to 37°C. No change in fluorescence intensity is observed in human red cells with comparable increase in extracellular Ca²⁺ in the temperature range 5–37°C. Increasing the extracellular pH, however, causes human red cells to respond to an increase in extracellular Ca²⁺ with a significant but temporary loss in fluorescence intensity. This effect is blocked by EGTA, quinine or by increasing extracellular [K⁺], indicating that at elevated extracellular pH, human erythrocytes respond to an increase in extracellular Ca²⁺ with an opening of K⁺ channels and associated hyperpolarization of the plasma membrane.     

Dependence of membrane potential on Ca2+ transport in cultured cytotrophoblasts of human immature placentas

The electrical membrane properties of cultured human cytotrophoblast were examined by means of a standard electrophysiological technique. The mean values of the membrane potential (E_m) and the membrane resistance in a physiological medium were around ?49 mV and 12 MΩ, respectively. The membrane potential was dependent, to a large extent, on the external Ca²⁺ concentration ([Ca²⁺]₀). Deprivation of external Ca²⁺ reduced membrane potential to about ?20 mV, and an increase in [Ca²⁺]₀ caused a hyperpolarization in a saturable manner. The Ca²⁺-dependency of membrane potential was affected remarkably by [K⁺]₀, but not by [Na⁺]₀ or [Cl^?]₀. The intracellular Ca²⁺ injection hyperpolarized the membrane in a Ca²⁺-free medium. A Ca²⁺ channel blocker, verapamil, completely abolished the Ca²⁺-dependent E_m. The Ca²⁺-dependent E_m was also suppressed by cooling or by the application of metabolic inhibitors. It is suggested that the Ca²⁺-dependent E_m in cultured human cytotrophoblast is caused by a Ca²⁺ influx which, in turn, increases the K⁺ conductance of the cell membrane, presumably due to stimulation of Ca²⁺-activated K⁺ channel.     

Cyanine dye fluorescence used to measure membrane potential changes due to the assembly of complement proteins C5b-9

Summary The fluorescent potentiometric indicator diS–C₃-(5) has been used to investigate changes in membrane potential due to assembly of the C5b-9 membrane attack complex of the complement system. EAC1-7 human red blood cells and resealed erythrocyte ghosts—bearing membrane-assembled C5b67 complexes—were generated by immune activation in C8-deficient human serum. Studies performed with these cellular intermediates revealed that the membrane potential of EAC1-7 red cells and ghosts is unchanged from control red cells (–7 mV) and ghosts (0 mV), respectively. Addition of complement proteins C8 and C9 to EAC1-7 red cells results in a dose-dependent depolarization of membrane potential which precedes hemolysis. This prelytic depolarization of membrane potential—and the consequent onset of hemolysis—is accelerated by raising external [K⁺], suggesting that the diffusional equilibration of transmembrane cation gradients is rate limiting to the cytolytic event. In the case of EAC1-7 resealed ghosts suspended at either high external [K⁺] or [Na⁺], no change in membrane potential (from 0 mV) could be detected after C8/C9 additions. When the membrane potential of the EAC1-7 ghost was displaced from 0 mV by selectively increasing the K⁺ conductance with valinomycin, a dose-dependent depolarization of the membrane was observed upon addition of C8 and C9. In these experiments, lytic breakdown of the ghost membranes was <5%. Conclusions derived from this study include: (i) measured prelytic depolarization of the red cell Donnan potential directly confirms the colloid-osmotic theory of immune cytolysis. (ii) The diffusional transmenbrane equilibration of Na⁺ and K⁺ through the C5b-9 pore results in a dose-dependent depolarization of the membrane potential (E _m ) which appears to be rate-limiting to cytolytic rupture of the target erythrocyte. (iii) Enhanced immune hemolysis observed in high K⁺ media cannot be attributed to cation-selective conductance across the C5b-9 pore, and is probably related to the nearequilibrium condition of potassium-containing red cells when suspended at high external K⁺. These experiments demonstrate that carbocyanine dye fluorescent indicators can be used to monitor electrochemical changes arising from immune damage to the plasma membrane under both cytolytic and noncytolytic conditions. Potential application of this method to the detection of sublytic pathophysiological changes in the plasma membrane of complement-damaged cells are discussed.     

Potassium channels in growth cones of leech retzius cells

• 1.1. Potassium-selective channels were analysed in growth cones of cultured leech Retzius cells.
• 2.2. In the cell-attached mode and at physiological bath and pipette solution little channel activity was observed at resting membrane potential. The channel open probability (p_o) increased with cell depolarization, and the slope conductance of the single K⁺ channel current was about 60 pS.
• 3.3. With symmetrical high KCl solution on both sides of the excised membrane patch three K⁺ -selective channels could be discriminated. Two channels exhibited a linear current-voltage relation of about 18 pS and 106 pS, respectively.
• 4.4. The most frequently observed K⁺ channel showed a non-linear current-voltage relation and p_o increased with increasing free cytoplasmic Ca²⁺ and during cell hyperpolarization.