Two modes of gating during late Na+ channel currents in frog sartorius muscle

Developmental regulation of a slowly-inactivating potassium conductance in rat neostriatal neurons

In late embryonic and early post-natal rat neostriatal neurons, the voltage-dependent potassium currents activated by depolarization are largely attributable to a rapidly inactivating A-current and a delayed rectifier current. Over the first 4 weeks of post-natal life, a third potassium current emerges in most cells. This slowly inactivating conductance is distinct from the A-current and delayed rectifier in voltage-dependence, kinetics and pharmacology. The properties of this conductance suggest that it may be of central importance to the integrative behavior of neostriatal neurons by controlling such features as first spike latency and interspike interval.

Inactivation kinetics of the sodium channel in the egg and the isolated, neurally differentiated blastomere of the ascidian

1. Inactivation kinetics of the sodium channel was compared between the egg-type channel in the egg cell and the differentiated-type channel in the cleavage-arrested, neurally differentiated blastomere of the ascidian. The techniques of the two-microelectrode voltage clamp and the cell-attached patch clamp were used. 2. In both types of channel, the time course of inactivation development obtained with a two-pulse protocol at potentials from -40 to -60 mV could be fitted with two exponentials with distinctive parameters. 3. The time course of recovery from inactivation at potentials more negative than -70 mV was compared between the two types of channel. At -80 to -120 mV, a delay of recovery was evident in the egg-type channel, whereas no delay was observed in the differentiated type. 4. In both types of channel, the two time constants of the inactivation of the macroscopic current, derived from the measurements of inward current, inactivation development and recovery from inactivation, had a bell-shaped voltage dependency. The fast time constants had a peak at -55 mV in the differentiated type and -70 mV in the egg type. The slow time constants had a peak around -60 mV in both types. 5. At the single-channel level, the averaged current from the differentiated-type channel showed both fast and slow decays. The frequency of late openings was higher in the differentiated-type channel than in the egg type. 6. The voltage dependence of the decay time constant and the carried charge in the summed current of the single-channel events was found to be shifted in the negative direction by 10-30 mV, compared with that of the macroscopic current. 7. The possibility that the higher frequency of late openings in the differentiated-type channel might be derived from delayed activation was excluded, since first-latency histograms of the single channel were not significantly different between the two types of channel.

Changes in sodium channel gating produced by point mutations in a cytoplasmic linker

Voltage-gated sodium channels are transmembrane proteins of approximately 2000 amino acids and consist of four homologous domains (I through IV). In current topographical models, domains III and IV are linked by a highly conserved cytoplasmic sequence of amino acids. Disruptions of the III-IV linker by cleavage or antibody binding slow inactivation, the depolarization-induced closed state characteristic of sodium channels. This linker might be the positively charged "ball" that is thought to cause inactivation by occluding the open channel. Therefore, groups of two or three contiguous lysines were neutralized or a glutamate was substituted for an arginine in the III-IV linker of type III rat brain sodium channels. In all cases, inactivation occurred more rapidly rather than more slowly, contrary to predictions. Furthermore, activation was delayed in the arginine to glutamate mutation. Hence, the III-IV linker does not simply act as a charged blocker of the channel but instead influences all aspects of sodium channel gating.

Ca channel gating during cardiac action potentials

How do Ca channels conduct Ca ions during the cardiac action potential? We attempt to answer this question by applying a two-microelectrode technique, previously used for Na and K currents, in which we record the patch current and the action potential at the same time (Mazzanti, M., and L. J. DeFelice. 1987. Biophys. J. 12:95-100, and 1988. Biophys. J. 54:1139-1148; Wellis, D., L. J. DeFelice, and M. Mazzanti. 1990. Biophys. J. 57:41-48). In this paper, we also compare the action currents obtained by the technique with the step-protocol currents obtained during standard voltage-clamp experiments. Individual Ca channels were measured in 10 mM Ca/1 Ba and 10 mM Ba. To describe part of our results, we use the nomenclature introduced by Hess, P., J. B. Lansman, and R. W. Tsien (1984. Nature (Lond.). 311:538-544). With Ba as the charge carrier, Ca channel kinetics convert rapidly from long to short open times as the patch voltage changes from 20 to -20 mV. This voltage-dependent conversion occurs during action potentials and in step-protocol experiments. With Ca as the charge carrier, the currents are brief at all voltages, and it is difficult to define either the number of channels in the patch or the conductance of the individual channels. Occasionally, however, Ca-conducting channels spontaneously convert to long-open-time kinetics (in Hess et al., 1984, notation, mode 2). When this happens, which is about once in every 100beats, there usually appears to be only one channel in the patch. In this rare configuration, the channel is open long enough to measure its conductance in 10 Ca/ 1 Ba. The value is 8-10 pS, which is about half the conductance in Ba. Because the long openings occur so infrequently with Ca as the charge carrier, they contribute negligibly to the average Ca current at any particular time during an action potential. However, the total number of Ca ions entering during these long openings may be significant when compared to the number entering by the more usual kinetics.

Voltage activation of purified eel sodium channels reconstituted into artificial liposomes

We report here a characterization of the voltage-activated behavior of sodium channels purified from the electroplax of Electrophorus electricus. Single-channel activity in response to depolarizing pulses was recorded from patches excised from liposomes containing the reconstituted channel. Strong hyperpolarizations were required to elicit channel activity. Channels exhibited two typical gating patterns. They either would open in brief bursts upon depolarization and then inactivate (fast) or would stay opened for prolonged periods that frequently lasted several consecutive depolarizations and showed intense flickering (slow). The single-channel conductance estimated from the slope of the I-V curves ranged between 15 and 30 pS under several experimental conditions. Channels gating in either mode, fast or slow, were indistinguishable in terms of their sizes. No clear difference in their mean open times was observed. In addition to the two gating patterns, we also found a very clear tendency of the channels to stay quiet for long periods.

Voltage dependence and stability of the gating kinetics of the fast chloride channel from rat skeletal muscle

1. The voltage dependence and stability of the gating kinetics of the fast Cl-channel in excised patches of membrane from cultured rat skeletal muscle were studied with the patch clamp technique. Up to 10(6) open and shut intervals were analysed from each of five different patches containing a single fast Cl-channel. 2. To test for kinetic stability, plots of the mean durations of consecutive groups of 5-500 open and shut intervals were examined at each voltage. After excluding infrequent entries into both an apparent subconductance state and a long-lived (inactive) shut state, there were no abrupt and sustained changes in the moving means, indicating the absence of obvious shifts to other kinetic modes. The moving means did, however, fluctuate about the overall mean. 3. A comparison of experimental and simulated data indicated that most, but not all, of the fluctuation in the moving means was due to the stochastic variation inherent in the gating process. The fluctuation not accounted for by stochastic variation was mainly expressed as a slow, low-amplitude, component of drift about the mean. This slow component was unlikely to have arisen from measurement errors. 4. To examine whether the slow drift reflected detectable changes in kinetic modes, the data were divided into consecutive groups of 50,000 intervals. The exponential components describing the distributions were remarkably similar among the different groups, with stochastic variation accounting for most of the observed differences. This finding implies a single kinetic mode throughout the experiment. Thus, any changes in channel activity associated with the slow drift would have little effect on the analysis of gating kinetics presented here. 5. Depolarization increased channel open probability, Popen, for all five channels. This increase had a voltage sensitivity of 17 +/- 4 mV per e-fold change (effective gating charge of 1.6 +/- 0.32 electronic charges at 23 degrees C). Popen was 0.5 at -31 +/- 4 mV. 6. The depolarization-induced increase in Popen typically arose from a decrease in the mean shut time (19 +/- 4 mV per e-fold change; effective gating charge of 1.3 +/- 0.3 at 23 degrees C) and an increase in the mean open time (109 +/- 61 mV per e-fold change; effective gating charge of -0.24 +/- 0.13). 7. Neither plots of Popen versus voltage nor plots of the mean open and mean shut time versus voltage were completely described by a single Boltzmann distribution, suggesting multiple voltage-sensitive steps in channel gating.(ABSTRACT TRUNCATED AT 400 WORDS)

Characteristics of Na+ channels and Cl- conductance in resealed muscle fibre segments from patients with myotonic dystrophy

1. Electrical and contractile properties of resealed fibre segments were investigated by a variety of in vitro techniques. The preparations were removed from skeletal muscles of normal subjects and of eight patients with myotonic dystrophy. 2. Several hours after removal, fibre segments from normal subjects and those patients in whom myotonia was the primary symptom had resting membrane potentials of approximately -80 mV. In contrast, fibre segments obtained from patients in whom muscle dystrophy was more expressed were depolarized (-60 to -70 mV). 3. Contractions induced in fibre segments of myotonic muscle which had normal potentials were characterized by slowed relaxation which was due to electrical after-activity. 4. After single stimuli, long-lasting (3-100) runs of action potentials were recorded intracellularly from the myotonic muscle. In some of these fibre segments complex repetitive discharges were observed: multiple sites of locally gated currents were identified. 5. The three-electrode voltage clamp was used to determine the total membrane conductance, gm, and the ion component conductances. All fibres of a particular patient had similar conductances. However, the Cl- conductance varied from patient to patient from normal (74% of gm) to low values (30% of gm). The K+ conductance was normal in all fibres of all patients. 6. The patch-clamp technique was used to record currents through single Na+ channels of the sarcolemma. After treatment of the fibre segments with collagenase gigaohm seals were routinely obtained. The rate of success was greater when using the cell-attached mode than the inside-out mode. 7. Sodium channel currents were elicited by depolarizing voltage steps which produced an initial burst of Na+ channel openings. Up to ten channels were activated simultaneously when the patch was depolarized to potentials more positive than -30 mV. The Na+ channels re-opened very rarely in controls. The macroscopic sodium current, INa, was reconstructed by averaging depolarizing pulses. The time constant of rapid decay of INa reflecting macroscopic inactivation, the onset of INa and the amplitude of INa were voltage dependent. The mean amplitude of the current produced by re-openings was on average only 0.11 +/- 0.04% of the amplitude of the peak current. 8. Late openings of the Na+ channels were frequent in patches on the myotonic fibre segments. The amplitude of the current produced by re-openings was as high as about 0.75 +/- 0.11% of the amplitude of the peak current.(ABSTRACT TRUNCATED AT 400 WORDS)

A voltage-dependent persistent sodium current in mammalian hippocampal neurons

Currents generated by depolarizing voltage pulses were recorded in neurons from the pyramidal cell layer of the CA1 region of rat or guinea pig hippocampus with single electrode voltage-clamp or tight-seal whole-cell voltage-clamp techniques. In neurons in situ in slices, and in dissociated neurons, subtraction of currents generated by identical depolarizing voltage pulses before and after exposure to tetrodotoxin revealed a small, persistent current after the transient current. These currents could also be recorded directly in dissociated neurons in which other ionic currents were effectively suppressed. It was concluded that the persistent current was carried by sodium ions because it was blocked by TTX, decreased in amplitude when extracellular sodium concentration was reduced, and was not blocked by cadmium. The amplitude of the persistent sodium current varied with clamp potential, being detectable at potentials as negative as -70 mV and reaching a maximum at approximately -40 mV. The maximum amplitude at -40 mV in 21 cells in slices was -0.34 +/- 0.05 nA (mean +/- 1 SEM) and -0.21 +/- 0.05 nA in 10 dissociated neurons. Persistent sodium conductance increased sigmoidally with a potential between -70 and -30 mV and could be fitted with the Boltzmann equation, g = gmax/(1 + exp[(V' - V)/k)]). The average gmax was 7.8 +/- 1.1 nS in the 21 neurons in slices and 4.4 +/- 1.6 nS in the 10 dissociated cells that had lost their processes indicating that the channels responsible are probably most densely aggregated on or close to the soma. The half-maximum conductance occurred close to -50 mV, both in neurons in slices and in dissociated neurons, and the slope factor (k) was 5-9 mV. The persistent sodium current was much more resistant to inactivation by depolarization than the transient current and could be recorded at greater than 50% of its normal amplitude when the transient current was completely inactivated. Because the persistent sodium current activates at potentials close to the resting membrane potential and is very resistant to inactivation, it probably plays an important role in the repetitive firing of action potentials caused by prolonged depolarizations such as those that occur during barrages of synaptic inputs into these cells.

Fast and slow gating of sodium channels encoded by a single mRNA

We investigated the kinetics of rat brain type III Na+ currents expressed in Xenopus oocytes. We found distinct patterns of fast and slow gating. Fast gating was characterized by bursts of longer openings. Traces with slow gating occurred in runs with lifetimes of 5 and 30 s and were separated by periods with lifetimes of 5 and 80 s. Cycling of fast and slow gating was present in excised outside-out patches at 10 degrees C, suggesting that metabolic factors are not essential for both forms of gating. It is unlikely that more than one population of channels was expressed, as patches with purely fast or purely slow gating were not observed. We suggest that structural mechanisms for fast and slow gating are encoded in the primary amino acid sequence of the channel protein.

Characteristics of single Na+ channels of adult human skeletal muscle

The patch-clamp technique was used to study Na+ channels of human skeletal muscle. Preparations were from biopsies of quadriceps muscle from adults who were not suffering from neuromuscular diseases. Activity of Na+ channels was recorded from inside-out patches when the membrane potential was stepped from a holding potential of -110 mV to potential above a threshold of about -65 mV. Single channel activity increased within minutes after hyperpolarizing the patch due to recovery from ultra-slow inactivation. Up to ten Na+ channels were active in individual patches. Macroscopic currents were reconstructed by averaging single channel currents. The time-to-peak current declined from 1.6 ms at -60 mV to 0.5 ms at + 10 mV. The currents decayed mono-exponentially with time constants between 12.1 ms at -60 mV and 0.4 ms at + 10 mV (21 C). The conductance of single Na+ channels was 1.65 pS and the mean open time was voltage-dependent. At -50 mV, the mean open time was 0.4 ms, while positive to -10 mV it increased to values above 1 ms. In the threshold potential range, the number of openings per depolarizing pulse was larger than the number of channels under the patch-clamp pipette, indicating reopening of Na+ channels at this potential. Openings could be observed only rarely 10 ms after onset of depolarization and the macroscopic current produced by late openings was less than 0.1% of the peak current. Human skeletal muscle is thus suitable for investigation with the patch-clamp technique and the determination of properties of Na+ channels with this technique could be the basis for an assessment of possible defects of these channels in diseased muscle.

Reconstituted voltage-sensitive sodium channels from eel electroplax: activation of permeability by quaternary lidocaine, N-bromoacetamide, and N-bromosuccinimide

We have investigated the ion permeability properties of sodium channels purified from eel electroplax and reconstituted into liposomes. Under the influence of a depolarizing diffusion potential, these channels appear capable of occasional spontaneous openings. Fluxes which result from these openings are sodium selective and blocked (from opposite sides of the membrane) by tetrodotoxin (TTX) and moderate concentrations of the lidocaine analogue QX-314. Low concentrations of QX-314 paradoxically enhance this channel-mediated flux. N-bromoacetamide (NBA) and N-bromosuccinimide (NBS), reagents which remove inactivation gating in physiological preparations, transiently stimulate the sodium permeability of inside-out facing channels to high levels. The rise and subsequent fall of permeability appear to result from consecutive covalent modifications of the protein. Titration of the protein with the more reactive NBS can be used to produce stable, chronically active forms of the protein. Low concentrations of QX-314 produce a net facilitation of channel activation by NBA, while higher concentrations produce block of conductance. This suggests that rates of modifications by NBA which lead to the activation of permeability are influenced by conformational changes induced by QX-314 binding.

Transient and persistent sodium currents in normal and denervated mammalian skeletal muscle

1. Transient and persistent tetrodotoxin-sensitive sodium currents were recorded in response to depolarizing voltage pulses in voltage clamped segments of rat extensor digitorum longus muscle fibres at 20-25 degrees C in a triple Vaseline gap. 2. Appreciable persistent sodium current but little or no transient current was seen in response to depolarizations of up to 15 mV from a holding potential of -100 mV. 3. The maximum amplitude of both transient and persistent sodium currents occurred with depolarizations to -40 mV: the average peak amplitude of the transient current in fibres with a holding potential of -90 mV was -0.22 +/- 0.03 mA/microF (mean +/- 1 S.E.M., seven fibres) and the average amplitude of the persistent current was -0.94 +/- 0.10 microA/microF (mean +/- 1 S.E.M., twelve fibres). With a holding potential of -100 mV, the average amplitudes of the transient and persistent currents were -0.46 +/- 0.10 mA/microF (four fibres) and -1.4 +/- 0.22 microA/microF (five fibres), respectively. 4. The average maximum persistent sodium conductance in seven fibres held at -90 mV was 0.13 +/- 0.0078 microS and the potential for half-maximum conductance was -53 +/- 0.74 mV (mean +/- 1 S.E.M.). 5. When the transient sodium current was completely inactivated with 100 ms conditioning depolarizations to potentials more positive than -50 to -60 mV, there was little inactivation of the persistent current. 6. In six denervated fibres, the average amplitudes of the transient and persistent sodium currents generated by pulses to -40 mV from a holding potential of -90 mV were -0.11 +/- 0.01 mA/microF and -0.88 +/- 0.12 microA/microF, respectively (mean +/- 1 S.E.M.). It was concluded that there was a decrease in transient current but not persistent current amplitude following denervation and that the persistent current in denervated fibres with an increased input resistance could give rise to the spontaneous action potentials responsible for fibrillation.

Pentobarbital suppresses human brain sodium channels

Ion channels, key components in neuronal signal transmission and processing, are likely to be important molecular sites of anesthetic action. Sodium channels from human brain tissue were incorporated into planar lipid bilayers in the presence of batrachotoxin and exposed to the anesthetic pentobarbital. This barbiturate, in a dose-dependent manner and at clinically relevant concentrations, reduced fractional channel open time independent of membrane potential, and interfered with the steady-state activation process.

Binding of tetrodotoxin and saxitoxin to Na+ channels at different holding potentials: fluctuation measurements in frog myelinated nerve

The number of available Na+ channels in nodes of frog nerve fibres was determined from nonstationary Na+ current fluctuations recorded during a train of depolarizing test pulses. Mean numbers in Ringers's solution were 90,000 at a hyperpolarizing holding potential VH = -40 mV, 50,000 at the resting potential (VH = 0 mV) and 30,000 at a depolarizing holding potential VH = 30 mV. Addition of the cationic channel blockers tetrodotoxin (TTX) or saxitoxin (STX) to Ringer reduced the channel number by a factor which was independent of the holding potential. The reduction factor was 4 for 9.3 nM TTX and 3 for 3.5 nM STX. Thus, in the state of repetitive stimulation, TTX or STX blockage of Na+ channels is hardly affected by the membrane potential. Taking into account use-dependent TTX and STX effects [1], it is concluded that binding of both toxins exhibits a weak voltage dependence with toxin affinities decreasing at more negative holding potentials. The results suggest that binding of TTX and STX occurs at an external superficial receptor near the Na+ channel and that the toxin affinity of the receptor may be modulated by fast Na+ activation and slow inactivation gating processes.

Ischemic poison lysophosphatidylcholine modifies heart sodium channels gating inducing long-lasting bursts of openings

The effects of lysophosphatidylcholine (LPC) on Na channels in inside-out patches of adult rat ventricular cells using the patch-clamp technique have been investigated. Application of LPC (9-25 microM) from the inner side of membrane for 4-15 min caused a reduction of averaged Na current (INa) peak and prolonged the time course of inactivation in the potential range of -50 to -10 mV. Analysis of single channel behaviour revealed that after 30-50 min of exposure, in addition to normally functioning Na channels with short openings, LPC induced long-lasting bursts of Na channel openings (up to the 300 ms duration of the test pulses). This resulted in an appearance of noninactivated component of INa. The slope conductance of these modified channels remained the same as in control (11.3 pS - control; 11.6 pS - LPC-treated). The dwell time for modified channels increased significantly.

Sodium and potassium conductances in somatic membranes of rat Purkinje cells from organotypic cerebellar cultures

1. The somatic voltage-gated conductances of Purkinje cells in organotypic cultures (Gähwiler, 1981) were studied using the outside-out patch recording configuration of the patch-clamp technique (Hamill, Marty, Neher, Sakmann & Sigworth, 1981). 2. When activated by step depolarizations, the tetrodotoxin-sensitive voltage-dependent Na+ current presented two distinct phases: an initial surge of inward current fluctuations which activates rapidly upon pulse onset and decays within 20-40 ms, and a later phase in which discrete bursts of single-channel activity are interspersed with silent periods. 3. Ensemble fluctuation analysis of the current fluctuations during the early phase of the Na+ current and measurements of single channels during both early and late phases indicate that a single type of Na+ channel can account for both phases of the Na+ current. This channel has an elementary current amplitude of -2 pA at -40 mV. This amplitude did not vary significantly between -60 and -20 mV. The mean open time depended on membrane potential, increasing by a factor of three between -60 and -20 mV. 4. The early component of the Na+ current activated at a threshold of -60 mV and reached its maximum amplitude at -20, mid-point for the activation curve being -40 mV. Times-to-peak current decreased with membrane potential, from 3.5 ms at -60 mV to 0.3 ms at 0 mV. The decay phase of the current presented two exponential components, with time constants of 1.5 and 10 ms at -40 mV. The steady-state inactivation curve had a mid-point at -75 mV. 5. The late component of the Na+ current was observed in the voltage range from -60 to -20 mV, with a maximum at -40 mV. Its maximum amplitude corresponded to approximately 1.7% of the peak amplitude of the early component. 6. Macroscopic potassium currents were observed upon step depolarizations above a threshold of -30 mV. The currents activated in a voltage-dependent fashion, times-to-peak decreasing with depolarization, and partially inactivated during 40 ms depolarizing steps. Peak current amplitudes at any given membrane potential were decreased by depolarizing the holding potential. The macroscopic properties of the K+ current varied from patch to patch. 7. Two types of single-channel K+ currents were observed during steady-state depolarizations. The unitary current amplitudes were 2.7 and 10.4 pA at 30 mV, corresponding to chord conductances of 28 and 90 pS respectively.(ABSTRACT TRUNCATED AT 400 WORDS)

Kinetic diversity of Na+ channel bursts in frog skeletal muscle

Individual Na+ channels of dissociated frog skeletal muscle cells at 10 degrees C fail to inactivate in 0.02% of depolarizing pulses, thus producing bursts of openings lasting hundreds of milliseconds. We present here a kinetic analysis of 87 such bursts that were recorded in multi-channel patches at four pulse potentials. We used standard dwell-time histograms as well as fluctuation analysis to analyze the gating kinetics of the bursting channels. Since each burst contained only 75-150 openings, detailed characterization of the kinetics from single bursts was not possible. Nevertheless, at this low kinetic resolution, the open and closed times could be well fitted by single exponentials (or Lorentzians for the power spectra). The best estimates of both the open and closed time constants produced by either technique were much more broadly dispersed then expected from experimental or analytical variability, with values varying by as much as an order of magnitude. Furthermore, the values of the open and closed time constants were not significantly correlated with one another from burst to burst. The bursts thus expressed diverse kinetic behaviors, all of which appear to be manifestations of a single type of Na+ channel. Although the opening and closing rates were dispersed, their average values were close to those of alpha m and 2 beta m derived from fits to the early transient Na+ currents over the same voltage range. We propose a model in which the channel has both primary states (e.g., open, closed, and inactivated), as well as "modes" that are associated with independent alterations in the rate constants for transition between each of these primary states.

Tetrodotoxin differentially blocks peak and steady-state sodium channel currents in early embryonic chick ventricular myocytes

Single ventricular myocytes were dissociated from 3-day-old embryonic chick hearts and maintained in culture for 9-21 h. The whole-cell patch clamp method was used to record tetrodotoxin (TTX)-sensitive fast Na+ currents. The peak Na+ current recorded at -20 mV ranged from 10 to 70 microA/cm2. At more negative potentials, a component of the current decayed very slowly, resulting in a significant steady-state or "late" Na+ current. The origin of the late Na+ current was revealed through the examination of single Na+ channel currents recorded in outside-out membrane patches. The single Na+ channel conductance was 20 pS. A high percentage of the trials (approximately 16%) displayed multiple reopenings of a single Na+ channel, resulting in bursts of current lasting for greater than or equal to 150 ms. The frequency distributions of the Na+ channel open-times were bi-exponential. The burst-like mode of Na+ channel activity (which underlies the slowly- or non-inactivating currents recorded macroscopically), was blocked to a greater degree by TTX, compared to the peak current. The results suggest that differential blockade may occur as a result of the slow binding and increased affinity of TTX to the open Na channel.

Two molecular transitions influence cardiac sodium channel gating

Sodium channels from diverse excitable membranes are very similar in their structure, yet surprisingly heterogeneous in their behavior. The processes that govern the opening and closing of sodium channels have appeared difficult to describe in terms of a single, unifying molecular scheme. Now cardiac sodium channels have been analyzed by high-resolution single-channel recordings over a broad range of potentials. Channels exhibited both complex and simple gating patterns at different voltages. Such behavioral diversity can be explained by the balance between two molecular transitions whereby channels can exit the open state.

Effects of toxin II from the scorpion Androctonus australis Hector on sodium current in neuroblastoma cells and their modulation by oleic acid

The effects of toxin II (AaH II) isolated from the scorpion Androctonus australis Hector on sodium current in neuroblastoma X glioma NG 108-15 hybrid cells were analysed under patch clamp conditions in the whole cell configuration. AaH II (70 nM) induced a maintained sodium current, as well as increasing both fast and slow inactivation time constants and the amplitude of the peak current. This latter effect occurred via a shift of the activation-voltage curve towards negative voltage values by about 9 mV. Oleic acid (5 microM), which had no effect on INa under control conditions, decreased the AaH II-induced maintained current. It also reversed, or prevented the increase of the peak current induced by AaH II. However, it neither prevented nor modified the AaH II-induced increase in inactivation time constants. The binding of the toxin to its specific site and the number of binding sites for AaH II were not significantly modified by oleic acid. The oleic acid-induced effects could not be related to the activation of protein kinase C since PMA, a potent activator of this enzyme, did not produce oleic acid-like effects. From these results, it is concluded that AaH II has several independent effects on sodium channels, some of which could be modulated by the lipid environment of sodium channels in the membrane.

Late sodium current and its contribution to action potential configuration in guinea pig ventricular myocytes

We used the patch clamp technique to study the nature of the late sodium current in guinea pig ventricular myocytes. In a cell attached mode of single channel recording at room temperature (22-24 degrees C) two kinds of late (100 msec or more after beginning of the depolarizing pulse) sodium channel activities were recognized. One is isolated brief openings appearing once for about 120 depolarizations per channel (background type), while the other type is sustained openings with rapid interruptions (burst type) that occurred only once for 2,700 depolarizations per channel. The time constant obtained from the open time histogram of the burst type (1.05 msec) was about five times longer than that of background type (0.18 msec, measured at the potential 10 mV above the threshold). Magnitude of the late sodium current flowing through the entire surface of a myocyte was estimated with tetrodotoxin (60 microM), a specific inhibitor of sodium channels, in whole-cell clamp experiments. The steady tetrodotoxin-sensitive current of 12 to 50 pA was registered at -40 mV (26 +/- 14 pA, mean +/- SD, n = 5), in good agreement with the late sodium current calculated from the single channel recording. Tetrodotoxin produced small (congruent to 10%) but significant decreases in the action potential duration. These results suggest the presence of a small but significant late sodium current with slow inactivation kinetics and that this current probably plays a significant role in maintaining the action potential plateau and the duration in guinea pig ventricular myocytes.

Patch clamp of sarcolemmal spheres from stretched skeletal muscle fibers

Stretching frog skeletal muscle fibers to the breaking point results in the rapid formation of numerous large spheres of membrane (5-80 microns diam). The surface of the spheres readily forms gigaohm (G omega) seals against patch pipettes, allowing low-noise single-channel recording. Currents recorded from patches isolated from these spheres indicate that they contain a variety of channels including 1) a small Na+-selective channel seen in the presence of veratridine, 2) a K+-selective channel which is blocked by millimolar Mg-ATP, and 3) a relatively large voltage-dependent Cl- channel which is blocked by Zn2+ and limited in selectivity over other anions [PCl/PMOPS = 3.7; MOPS, 3-(N-morpholino)propanesulfonic acid]. These channels have been described previously and have been identified as markers for sarcolemmal (SL) membrane. Accordingly, this method allows rapid and direct recording of channels in the SL membrane without first having to pretreat fibers with proteolytic enzymes to render the SL accessible to patch pipettes.

Properties of the bursting Na channel in the presence of DPI 201-106 in guinea-pig ventricular myocytes

Single Na channel currents were measured in cell-attached patches of guinea-pig ventricular myocytes in the presence of the S-enantiomer of DPI 201-106. DPI changes the kinetic pattern of channel activity from short living openings at the beginning of a depolarizing pulse (voltage-independent mean open time about 0.4 ms between -60 and -20 mV), into longlasting burst of openings. The single channel current-voltage relation can be approximated by a straight line with a single channel conductance of 15 pS, which is the same as in the absence of DPI, and a reversal potential near the estimated Na equilibrium potential (+74 mV). The ensemble averaged Na current shows a fast peak of inward current, which partially decays within less than 10 ms, but which shows a large component which decays very slowly with a time constant of the order of 1 s (1.31 +/- 0.6 s at -30 mV, 19 measurements in 12 cell-attached patches). The slowly decaying component activates with a half-maximum potential at -55.4 +/- 2.3 mV and a slope parameter s of 4.9 +/- 1.9 mV. The half-maximum potential of the steady-state inactivation is -115.6 +/- 1.8 mV, and the slope parameter is 9.1 +/- 1.5 mV. The open time distribution can be fitted by a single exponential only at potentials negative to -40 mV. The time constant is 1.3 +/- 0.14 ms at -50 mV (7 patches).(ABSTRACT TRUNCATED AT 250 WORDS)

Sodium channel subconductance levels measured with a new variance-mean analysis

The currents through single Na+ channels were recorded from dissociated cells of the flexor digitorum brevis muscle of the mouse. At 15 degrees C the prolonged bursts of Na+ channel openings produced by application of the drug DPI 201-106 had brief sojourns to subconductance levels. The subconductance events were relatively rare and brief, but could be identified using a new technique that sorts amplitude estimates based on their variance. The resulting "levels histogram" had a resolution of the conductance levels during channel activity that was superior to that of standard amplitude histograms. Cooling the preparation to 0 degrees C prolonged the subconductance events, and permitted further quantitative analysis of their amplitudes, as well as clear observations of single-channel subconductance events from untreated Na+ channels. In all cases the results were similar: a subconductance level, with an amplitude of roughly 35% of the fully open conductance and similar reversal potential, was present in both drug-treated and normal Na+ channels. Drug-treated channels spent approximately 3-6% of their total open time in the subconductance state over a range of potentials that caused the open probability to vary between 0.1 and 0.9. The summed levels histograms from many channels had a distinctive form, with broader, asymmetrical open and substate distributions compared with those of the closed state. Individual subconductance events to levels other than the most common 35% were also observed. I conclude that subconductance events are a normal subset of the open state of Na+ channels, whether or not they are drug treated. The subconductance events may represent a conformational alteration of the channel that occurs when it conducts ions.

Conditional open and delay time histograms of sodium channels

Currents through single sodium channels were recorded in neuroblastoma cells. Open time histograms were constructed from openings which appeared between 2.0 and 5.0 ms after the onset of the depolarization. Histograms constructed from openings which were not preceded by other openings showed a maximum at t greater than 0 in contrast to those, which were preceded by other openings. Time constants of delay time histograms fitted by the sum of two exponentials were different for the first, second and third records of runs. The results support the view that sodium channels have multiple open and closed states and the transition probabilities among the states depend on local conditions of the membrane.

Modal gating behavior of cardiac sodium channels in cell-free membrane patches

Single voltage-activated Na+ channel currents were obtained from membrane patches of isolated ventricular cells of guinea pig hearts. The currents were compared when measured from cell-attached patches and from the same patch but at least 20 minutes after manual excision. The averaged currents showed a distinctly delayed decay in the excised patches due to the appearance of long lasting openings or bursts of openings. In contrast to control patches, the open time distribution in excised patches requires at least two exponentials. A short mean open time was voltage independent for cell-attached patches (0.38 ms +/- 0.07 ms between -60 and -20 mV, 6 cell-attached patches; and 0.41 +/- 0.1 ms, 7 excised patches). The long mean open time found in excised patches was clearly voltage dependent and increased from 0.48 +/- 0.14 ms (-80 mV) to 2.87 +/- 0.35 ms (-20 mV, regression coefficient +0.88, 7 patches). Sweeps with long openings appeared in clusters. The clustering of records with long openings, short openings, or without openings (nulls) was quantified by a runs analysis which showed a highly significant nonrandom ordering. The results show that in excised patches inactivation is temporally hibernating.

Development of the fast sodium current in early embryonic chick heart cells

Single ventricle cells were dissociated from the hearts of two-, three-, four- or seven-day-old chick embryos, and were maintained in vitro for an additional 6 to 28 hr. Rounded 13 to 18 micron cells with input capacitance of 5 to 10 pF were selected for analysis of fast sodium current (INa). Voltage command protocols designed to investigate the magnitude, voltage dependence, and kinetics of INa were applied with patch electrodes in the whole-cell clamp configuration. INa was present in over half of the 2d, and all 3d, 4d and 7d cells selected. The current showed no systematic differences in activation kinetics, voltage dependence, or tetrodotoxin (TTX) sensitivity with age or culture conditions. Between the 2d and 7d stages, the rate of current inactivation doubled and channel density increased about eightfold. At all stages tested, INa was blocked by TTX at a half-effective concentration of 0.5 to 1.0 nM. We conclude that the lack of Na dependence of the action potential upstroke on the second day of development results from the relatively depolarized level of the diastolic potential, and failure to activate the small available excitatory Na current. The change from Ca to Na dependence of the upstroke during the third to the seventh day of incubation results partly from the negative shift of the diastolic potential during this period, and in part from the increase in available Na conductance.

A high-conductance anion channel in adult amphibian skeletal muscle

Membrane patches were excised from enzymatically dissociated frog toe muscle. High-conductance anion channels could be induced in previously quiet patches by 20-120 s depolarizations beyond +20 mV and then studied in the potential range from -80 to +60 mV for a long time. From reversal potentials the estimated permeability ratios PCl/PNa and PCl/Pglucuronate were near 3.5 and 4, respectively. There were probably 5 or more conductance levels (substates) for a single channel, the most common in symmetrical 110 mM NaCl being 260 and 70 pS at 10 degrees C. Gating was complex, with rapid and slow events and several gating modes, including periods of rapid flickering. Channels closed reversibly at potentials more negative than -50 mV. The channel was blocked by application to the cytoplasmic face of tannic acid, gallic acid, and zinc but not of DIDS or 9-anthracene-carboxylic acid, and it was blocked by extracellular zinc.

Burst kinetics of sodium channels which lack fast inactivation in mouse neuroblastoma cells

1. The kinetics of the slow inactivation process of Na+ channels were examined by recording single-channel currents from cultured neuroblastoma cells. 2. In order to directly examine slow inactivation, fast inactivation was first removed irreversibly by briefly exposing the internal surface of excised membranes to papain. Following treatment, the time constant for the inactivation of averaged membrane Na+ current increased by over two orders of magnitude, while the open time of individual channels increased by a factor of three. The two effects are consistent with the idea that papain can selectively remove fast inactivation of Na+ channels. 3. In the absence of fast inactivation, Na+ channels continued to open during maintained depolarization of the membrane to potentials less negative than -60 mV. Under these conditions, the opening occurred in bursts 50 ms to hundreds of milliseconds long, followed by silent periods lasting many seconds. The average burst length was found to be equal to the time constant of the decline in average evoked current measured at the same potential, indicating that a burst was terminated by entry of the channel into the slow inactivated state. 4. Histograms of open times revealed two populations of open states at any potential. Bursts could also be classified as either short or long bursts. Bursts appeared to be due to the gating of a single channel, and long bursts contained both types of open states, suggesting that a Na+ channel could have more than one open state. 5. The kinetics of bursts of Na+ channels were voltage dependent. As the membrane was depolarized, the burst length, interval between bursts, and open time all increased. Although the probability of an open channel during a burst increased to almost 1.0 with depolarization, any channel was open less than 0.5% of the time when measured throughout the depolarization. The increase in burst duration with depolarization would occur if the rate of slow inactivation is faster from closed states of the channel than from open states. 6. Records of membrane current evoked by a series of step depolarizations were clustered into those with openings of Na+ channels and those without openings. Records in which a channel did not inactivate during the depolarization were less likely to lead to hibernation, suggesting that this phenomenon is caused by the slow inactivation process.

Evidence for multiple open states of sodium channels in neuroblastoma cells

Open times of voltage-gated sodium channels in neuroblastoma cells were measured during repolarization (following a short depolarizing conditioning pulse) and during moderate depolarization. Conditional and unconditional channel open-time histograms were best fitted by the sum of two exponentials. (The conditional open time was measured from the end of the conditioning pulse until an open channel shuts provided it was open at t = 0). Time constants of both histograms depended on the post-pulse and were shifted to more positive potentials with increasing conditioning pulse potential. This shift could be explained by assuming more than two time constants in the histograms, which could not be separated. Channel open-time histograms from single-pulse experiments showed a maximum at t greater than 0. These histograms could be best fitted by an exponential function with three time constants. One term of this function included the difference of two exponentials resulting in a maximum at t greater than 0. Open-time histograms showed a definite time dependence. At 2 to 6.5 msec after the beginning of the depolarization the best fit could be obtained by the difference of two exponentials. To these components another term had to be added at 0 to 2 msec. Between 6.5 and 14.0 msec the sum of two exponentials, and after 14.0 msec a single exponential resulted in a good fit. The results support the hypothesis that sodium channels in neuroblastoma cells may have multiple open states. Two of these states are irreversibly coupled.

Isolated muscle cells as a physiological model

Summary of a symposium presented by the American Physiological Society (Cell and General Physiology Section and Muscle Group) at the 70th Annual Meeting of the Federation of American Societies for Experimental Biology, St. Louis, Missouri, April 15, 1986, chaired by M. Lieberman and F. Fay. This symposium reflects a growing interest in seeking new technologies to study the basic physiological and biophysical properties of cardiac, smooth, and skeletal muscle cells. Recognizing that technical and analytical problems associated with multicellular preparations limit the physiological significance of many experiments, investigators have increasingly focused on efforts to isolate single, functional embryonic, and adult muscle cells. Progress in obtaining physiologically relevant preparations has been both rapid and significant even though problems regarding cell purification and viability are not fully resolved. The symposium draws attention to a broad, though incomplete, range of studies using isolated or cultured muscle cells. Based on the following reports, investigators should be convinced that a variety of experiments can be designed with preparations of isolated cells and those in tissue culture to resolve questions about fundamental physiological properties of muscle cells.

Zonisamide enhances slow sodium inactivation in Myxicola

In voltage-clamped Myxicola giant axons Zonisamide (1,2-benzisoxazole-3-methanesulfonamide) caused a hyperpolarizing shift in the steady-state fast inactivation curve and retarded recovery from fast and slow Na+ inactivation. The effects of Zonisamide on steady-state fast inactivation could be described assuming a single binding site with a dissociation constant of 12 microM. Slow inactivation was significantly more sensitive, with a Kd of 1 microM from both steady-state and kinetic data. While these results account for anticonvulsant activity, the differential sensitivity suggests Zonisamide may also be useful in studies of the slow inactive state of the Na+ channel.

Mechanisms of closure of cardiac sodium channels in rabbit ventricular myocytes: single-channel analysis

We have examined the kinetics of closure of sodium channels using single-channel recordings in cell-attached and excised membrane patches of rabbit ventricular myocytes. Sodium-channel closure was dependent on membrane potential. The closing rate initially decreased with depolarization. The rate then passed through a minimum and increased at strongly depolarized potentials. We attempted to determine the separate voltage dependence of the deactivation and inactivation rate constants using the method of Aldrich, Corey, and Stevens. In a majority of experiments, the method did not give internally consistent results. As an alternative approach, batrachotoxin was used to remove inactivation and determine the voltage dependence of deactivation rate. The deactivation rate decreased with depolarization. To account for the increase in the closing rate at strongly depolarized test potentials, one must postulate voltage dependence of inactivation. The ensemble average current relaxed with a time course that was usually best described by the sum of two exponentials. The larger of the two rate constants that described the relaxation was strongly voltage-dependent, increasing with depolarization. The larger rate constant may reflect voltage-dependent inactivation. We found evidence of two possible mechanisms for the slow component of relaxation: 1) cardiac sodium channels may open repetitively during a given depolarizing epoch, and 2) channels may return from the inactivated state with low probability and burst for as much as 200 msec with open times that are longer than those during usual gating. The slow component appears to be more prominent in cardiac muscle than in nerve and may play an important role in the control of the action potential duration and the inotropic state of the heart.

Spontaneous opening at zero membrane potential of sodium channels from eel electroplax reconstituted into lipid vesicles

The voltage-dependent sodium channel from the eel electroplax was purified and reconstituted into vesicles of varying lipid composition. Isotopic sodium uptake experiments were conducted with vesicles at zero membrane potential, using veratridine to activate channels and tetrodotoxin to block them. Under these conditions, channel-dependent uptake of isotopic sodium by the vesicles was observed, demonstrating that a certain fraction of the reconstituted protein was capable of mediating ion fluxes. In addition, vesicles untreated with veratridine showed significant background uptake of sodium; a considerable proportion of this flux was blocked by tetrodotoxin. Thus these measurements showed that a significant subpopulation of channels was present that could mediate ionic fluxes in the absence of activating toxins. The proportion of channels exhibiting this behavior was dependent on the lipid composition of the vesicles and the temperature at which the uptake was measured; furthermore, the effect of temperature was reversible. However, the phenomenon was not affected by the degree of purification of the protein used for reconstitution, and channels in resealed electroplax membrane fragments or reconstituted solely into native eel lipids did not show this behavior. The kinetics of vesicular uptake through these spontaneously-opening channels was slow, and we attribute this behavior to a modification of sodium channel inactivation.

Gating of Na channels. Inactivation modifiers discriminate among models

Macroscopic Na currents were recorded from N18 neuroblastoma cells by the whole-cell voltage-clamp technique. Inactivation of the Na currents was removed by intracellular application of proteolytic enzymes, trypsin, alpha-chymotrypsin, papain, or ficin, or bath application of N-bromoacetamide. Unlike what has been reported in squid giant axons and frog skeletal muscle fibers, these treatments often increased Na currents at all test pulse potentials. In addition, removal of inactivation gating shifted the midpoint of the peak Na conductance-voltage curve in the negative direction by 26 mV on average and greatly prolonged the rising phase of Na currents for small depolarizations. Polypeptide toxins from Leiurus quinquestriatus scorpion and Goniopora coral, which slow inactivation in adult nerve and muscle cells, also increase the peak Na conductance and shift the peak conductance curve in the negative direction by 7-10 mV in neuroblastoma cells. Control experiments argue against ascribing the shifts to series resistance artifacts or to spontaneous changes of the voltage dependence of Na channel kinetics. The negative shift of the peak conductance curve, the increase of peak Na currents, and the prolongation of the rise at small depolarization after removal of inactivation are consistent with gating kinetic models for neuroblastoma cell Na channels, where inactivation follows nearly irreversible activation with a relatively high, voltage-independent rate constant and Na channels open only once in a depolarization. As the same kind of experiment does not give apparent shifting of activation and prolongation of the rising phase of Na currents in adult axon and muscle membranes, the Na channels of these other membranes probably open more than once in a depolarization.