Slow currents through single sodium channels of the adult rat heart

The sodium channel activator Brevetoxin-3 uncovers a multiplicity of different open states of the cardiac sodium channel

The interaction of Brevetoxin 3 (Pbtx-3), a sodium channel activator, with the cardiac sodium channel was studied at the single channel level. It was found that Pbtx-3 (20 microM) shifted steady-state activation to negative potentials, without major effects on the time course of macroscopic activation or macroscopic currents decay, as calculated from averaged single-channel records. Single-channel open times were found to be prolonged. Under the influence of the toxin, sodium channel openings could be observed frequently even at maintained depolarisation. These openings occurred to at least nine different subconductance levels of the open state with smaller conductivities than the maximal one and differed in their open times. Current amplitudes of these open substates were found to cluster around certain amplitude values. Appearance of substates at maintained depolarisation was dependent on the transmembrane potential (Em): Substates with smaller conductivity appeared more frequently at lower Em values whereas at higher Em values substates with higher conductivity values dominated. Furthermore, it was demonstrated that appearance of substates did not result from incomplete recovery from inactivation. From these observations it was concluded that the open substates observed correspond to different conformational states of the channel's activation gates. Under physiological conditions, when the sodium channel opens directly from its closed state these 'incomplete'-open states of the cardiac sodium channel are obscured by fast gating transitions between the corresponding, electrically silent, preopen states. Thus, Pbtx-3 acts mainly via stabilisation of the channel's preopen and different open states. A classification of sodium channel modifiers, based on their interaction with different conformational states of the channel is suggested.

Direct measurement of L-type Ca2+ window current in heart cells

The activation and inactivation relations of several ion channel currents overlap, suggesting the existence of a steady-state or "window" current. We studied L-type Ca2+ channel window current in single cardiac Purkinje cells using a voltage-clamp protocol by which channels were first inactivated nearly completely during a long-duration depolarizing step, and then the recovery of Ca2+ current was observed during repolarizing steps into the L-type Ca2+ window voltage range. With these conditions, a small-amplitude inward Ca2+ current gradually developed after repolarization to voltages within the window but not after steps to voltages positive or negative to it. Window current was suppressed by Cd2+ (50 microM), nifedipine (1 microM), and nicardipine (1 microM), and it was augmented by isoproterenol (5 microM) and Bay K 8644 (1 microM). At voltages at which window current developed, L-type Ca2+ channels also recovered to a closed state from which they could be reopened by an additional depolarizing step. At voltages positive to the window range, channel recovery to a closed state(s) was absent, whereas at voltages negative to the window range, channel recovery to a closed state(s) increased, as expected from the "steady-state" inactivation relation. Our results provide direct measurement of L-type Ca2+ window current and distinguish it from other processes, such as slow inactivation. Our findings support the postulate that within a window there occur channel transitions from inactivated to closed states, and these channels (re)open, and this process may occur repetitively. Some physiological and pathophysiological roles for L-type Ca2+ window current are discussed.

Synaptically triggered action potentials begin as a depolarizing ramp in rat hippocampal neurones in vitro

1. During just-suprathreshold synaptic activation of CA1 pyramidal cells in rat hippocampal slices in vitro the action potential begins as a slow depolarizing ramp, superimposed on the underlying EPSP and forming an integral part of the action potential. We call this ramp a synaptic prepotential (SyPP). 2. In order to examine the SyPP, a procedure for subtraction of the underlying EPSP was necessary. Because action potentials were only elicited by a subset of EPSPs with larger than average amplitude, a subtraction of the mean subthreshold EPSP would not give valid results. Instead, an EPSP to be subtracted was selected from an assemblage of subthreshold EPSPs, so that its amplitude matched the initial part of the spike-generating EPSP. 3. Virtually all action potentials started with a SyPP. Using an amplitude criterion of 1 S.D. of the mean of the matching subthreshold EPSPs, just-suprathreshold EPSPs gave prepotentials in 72-100% of all action potentials from fifteen randomly selected cells. With a criterion of 2 S.D.S, the frequency of occurrence ranged from 36 to 100%. 4. With a constant stimulus strength, there was a certain variability of the spike latencies. Shorter latency spikes had steeper, but smaller SyPPs than later spikes, suggesting that the slope of SyPP influenced the timing of the cell discharge. 5. The SyPP was best fitted by a single, exponentially rising curve, and was both smaller and slower than the large amplitude action potential. Its amplitude was 1-6 mV and the time constant 1-5 ms, which was 10-50 times slower than that of the upstroke of the action potential. 6. A properly timed hyperpolarizing current pulse could block the large amplitude action potential, thereby unmasking the SyPP as an initial depolarizing ramp. 7. The SyPP was more sensitive than the large amplitude action potential to intracellular injection of QX-314, a lidocaine derivative. At the concentrations used (10 or 30 mM) no detectable changes were seen in the large amplitude action potential. 8. Droplet application of a specific N-methyl-D-aspartate receptor antagonist, DL-2-amino-5-phosphonovaleric acid (1 mM), reduced both the EPSP and the firing probability, but did not change the SyPP. 9. The SyPP amplitude and time course depended upon the membrane potential at which the cell was activated. Depolarization enhanced and prolonged the SyPP, while hyperpolarization gave opposite effects. In part, the depolarization-induced amplitude increase could be attributed to membrane accommodation. 10. Antidromically evoked action potentials never started with a prepotential.(ABSTRACT TRUNCATED AT 400 WORDS)

A persistent sodium current in rat ventricular myocytes

1. The tight seal, whole-cell, voltage-clamp technique was used to record currents from single ventricular myocytes acutely dissociated from adult rat hearts. Subtraction of currents recorded in the presence and absence of tetrodotoxin (TTX, 50 microM) revealed a small, persistent, inward current following a much larger, transient, inward current. 2. Both currents were sodium currents because they reversed close to the sodium equilibrium potential and were depressed when choline was substituted for extracellular sodium. 3. The persistent sodium current could be recorded when the transient current had been inactivated with conditioning depolarization. Only slight inactivation of the persistent current occurred during depolarizing pulses lasting up to 900 ms. 4. A lower concentration of TTX (0.1 microM) blocked the persistent sodium current while having little effect on the transient sodium current. 5. The persistent sodium current was activated at more negative potentials than the transient sodium current. It cannot have been a window current because it was recorded at positive potentials when the transient current was completely inactivated. 6. Because the persistent and transient sodium currents had a different voltage dependence and sensitivity to TTX, it was concluded that different channels are responsible for the two currents.

Block and modulation of cardiac Na+ channels by antiarrhythmic drugs, neurotransmitters and hormones

The Na+ channel is an important target for the action of antiarrhythmic drugs. Application of contemporary biophysical, biochemical and molecular biological techniques have added considerably to our knowledge of its structure, function, modulation and block by antiarrhythmic drugs. The increased mortality from the use of these drugs for prophylaxis of cardiac arrhythmias has forced a re-evaluation of their use and of the entire pharmacological strategy of arrhythmia management. Gus Grant and David Wendt review recent studies on the block and modulation of cardiac Na+ channels and the place of Na+ channel blockers in future antiarrhythmic drug development.

A source of bias in the analysis of single channel data: assessing the apparent interaction between channel proteins

A recent study of single sodium channel currents in neuroblastoma cells suggested interaction between ion channels in close proximity to one another (T. Kiss and K. Nagy, Eur. Biophys. J. 12, 13, 1985). The opening of one channel appeared to affect the likelihood that neighboring channels might open. Some of the conclusions were based on the analysis of observed channel openings that were segregated depending on whether one channel or more than one channel was open at the same time. We hypothesized that the longer one channel remained open, the more likely another channel operating independently, would open, thereby creating the impression of an apparent coupling of channel behavior. We performed simulations and measurements of single sodium channel currents to determine whether the technique of event segregation could account for apparent channel interactions. The simulations showed that the segregation of overlapping (more than one channel open at the same time) and nonoverlapping events led to a bias in the estimated open time and the derived closing rate. To avoid the bias, we found that random pairing of opening and closing events provided an unbiased estimate of the mean closing rate. Using this random assignment approach, we showed that the mean closing rate of single sodium channels in neonatal rat myocytes decreased with depolarization over a limited range of membrane potential. This suggested that the underlying closure mechanism(s) was voltage dependent. From the analysis of open times, we found no evidence for channel interaction in the time scale of tens of milliseconds. Depolarizing steps without events occurred in runs suggesting the existence of long-lived shut state(s). Double pulse experiments with the prepulse and test pulse above threshold showed significant inactivation of channels that did not open. The rate of inactivation of shut channels was substantially slower than the closure rate of open channels. The rate of inactivation of cardiac sodium channels appeared to be strongly dependent on the initial channel state.

Modulation of single cardiac Na+ channels by cytosolic Mg++ ions

Elementary Na+ currents were recorded in inside-out patches excised from cultured neonatal rat heart myocytes in order to study the influence of cytosolic Mg++ and other bivalent cations present at the cytoplasmic membrane surface on cardiac Na+ channel gating. Exposing the cytoplasmic membrane surface to a Mg(++)-free environment shortened the open state of cardiac Na+ channels significantly. tau open declined to 62 +/- 2% of the value obtained at 5 mmol/l Mgi++. Other channel properties including the tendency to reopen and the elementary current size either changed insignificantly within a 10% range or remained completely unchanged. An almost identical change of tau open can be caused by switching from a Mn++ (5 mmol/l) containing internal solution to a Mn(++)-free internal solution. But tau open failed to significantly respond to a variation in internal Ni++ from 5 mmol/l to 0 mmol/l. The same response to internal Mg++ withdrawal was obtained with (-)-DPI-modified, non-inactivating Na+ channels, indicating that the exit rate from the open state remains as sensitive to cytosolic Mg++ variations as in normal Na+ channels with operating inactivation.

Acute thyroid hormone promotes slow inactivation of sodium current in neonatal cardiac myocytes

Sodium current (INa) inactivation kinetics in neonatal cardiac myocytes were analyzed using whole cell voltage clamp before and after acute treatments with thyroid hormone (3,5,3'-triiodo-L-thyronine, T3). In untreated neonatal myocytes, INa inactivation was predominantly mono-exponential, with 93 +/- 3% (S.D.; n = 9) of the peak amplitude decaying with a time constant, tau h1, of 1.8 +/- 0.5 ms at -30 mV. The remaining 7% of control INa decayed more slowly, with a time constant, tau h2, of 9.3 +/- 3.0 ms at -30 mV. The contribution of slowly-inactivating channels to peak current was increased from 7% to 43 +/- 27% within 5 min of exposure to 5-20 nM T3 (nine cells; P less than 0.005). The time constants for both the fast- and slow-inactivating components of peak current (tau h1 and tau h2) were not significantly changed by acute T3 treatment, nor was steady-state INa inactivation (h infinity) affected. Thyroid hormone action on sodium inactivation was partially reversible by lidocaine. These findings indicate that T3 acts at the neonatal cardiac cell membrane to promote slow inactivation kinetics in sodium channels.

Differential effects on action potential duration of class IA, B and C antiarrhythmic drugs: modulation by stimulation rate and extracellular K+ concentration

1. Standard microelectrode techniques were used to study the effects on the action potential duration (APD) of canine Purkinje fibres of a therapeutic concentration of nine Class I antiarrhythmic drugs. At an extracellular K+ concentration of 5.6 mmol/L all nine agents reduced APD at all drive rates studied (range of interstimulus intervals = 200-1000 ms). At lower levels of K+, quinidine (5 mumol/L) and disopyramide (10 mumol/L) (Class Ia agents) revealed dual effects on APD. At the lowest levels of K+ (2 mmol/L) and the longest interstimulus interval used (2000 ms), both agents significantly prolonged APD. Under all other conditions, APD was either unchanged or reduced. Lignocaine, 15 mumol/L (Class Ib agent) reduced APD at all rates and all K+ concentrations and this effect was greatest at the slowest rates. 2. Flecainide (1 mumol/L) (Class Ic) shortened APD at K+ = 5.6 and 4 mmol/L but had no effect at K+ = 2 mmol/L. 3. We conclude that these data result from opposing drug actions on inward sodium and outward potassium currents flowing during the plateau of the action potential. 4. Class Ia drugs exhibit significant depression of both currents, with the resultant effect on APD being modulated by external K+ concentration and drive rate. 5. Class Ib agents predominantly depress the sodium current and hence shorten APD, and Ic compounds have intermediate actions. 6. These differential effects on APD must be considered when planning antiarrhythmic therapy, and are directly relevant to the proarrhythmic propensities of these agents.

Reversible uncoupling of inactivation in N-type calcium channels

N-type calcium channels are thought to be expressed specifically in neuronal cells and to have a dominant role in the control of neurotransmitter release from sympathetic neurons. But their unitary properties are poorly understood and the separation of neuronal Ca2+ current into components carried by N-type or L-type Ca2+ channels is controversial. Here we show that individual N-type Ca2+ channels in sympathetic neurons can carry two kinetically distinct components of current, one that is rapidly transient and one that is long lasting. The mechanism that gives rise to these two components is unexpected for Ca2+ channels: a test depolarization elicits either a rapidly inactivating, single short burst with an average duration of 40 ms, or sustained, noninactivating channel activity lasting for over 1 s. The switching between inactivating and noninactivating activity is a slow process, the occurrence of each type of unitary kinetic behaviour remaining statistically correlated over several seconds. Variable coupling of inactivation in N-type Ca2+ channels could be an effective mechanism for the modulation of neuronal excitability and synaptic plasticity.

Voltage-sensitive sodium channels: agents that perturb inactivation gating

In summary, the voltage-sensitive sodium channel from eel electroplax provides an optimal preparation for biochemical and biophysical studies of molecular structure and gating. We have demonstrated that the purified and reconstituted protein is capable of functioning normally, exhibiting, among other properties, voltage-dependent activation and inactivation gating mechanisms. We have been able to recreate the classical electrophysiological studies in which inactivation gating can be removed by proteolytic modification of the cytoplasmic surface of the molecule, and have mapped the probable site of modification to the peptide segment lying between subunit domains III and IV. We have demonstrated that the reconstituted protein undergoes interactions with the lidocaine derivative QX-314 which, at low concentrations, results in paradoxical activation of the channel and a facilitation of modification by oxidizing reagents that remove inactivation gating.

A model of the ventricular cardiac action potential. Depolarization, repolarization, and their interaction

A mathematical model of the membrane action potential of the mammalian ventricular cell is introduced. The model is based, whenever possible, on recent single-cell and single-channel data and incorporates the possibility of changing extracellular potassium concentration [K]o. The fast sodium current, INa, is characterized by fast upstroke velocity (Vmax = 400 V/sec) and slow recovery from inactivation. The time-independent potassium current, IK1, includes a negative-slope phase and displays significant crossover phenomenon as [K]o is varied. The time-dependent potassium current, IK, shows only a minimal degree of crossover. A novel potassium current that activates at plateau potentials is included in the model. The simulated action potential duplicates the experimentally observed effects of changes in [K]o on action potential duration and rest potential. Physiological simulations focus on the interaction between depolarization and repolarization (i.e., premature stimulation). Results demonstrate the importance of the slow recovery of INa in determining the response of the cell. Simulated responses to periodic stimulation include monotonic Wenckebach patterns and alternans at normal [K]o, whereas at low [K]o nonmonotonic Wenckebach periodicities, aperiodic patterns, and enhanced supernormal excitability that results in unstable responses ("chaotic activity") are observed. The results are consistent with recent experimental observations, and the model simulations relate these phenomena to the underlying ionic channel kinetics.

Responsiveness of cardiac Na+ channels to antiarrhythmic drugs: the role of inactivation

Elementary Na+ currents were recorded at 9 degrees C in inside-out patches from cultured neonatal rat heart myocytes. In characterizing the sensitivity of cooled, slowly inactivating cardiac Na+ channels to several antiarrhythmic drugs including propafenone, lidocaine and quinidine, the study aimed to define the role of Na+ inactivation for open channel blockade. In concentrations (1-10 mumol/liter) effective to depress NPo significantly, propafenone completely failed to influence the open state of slowly inactivating Na+ channels. With 1 mumol/liter, tau open (at -45 mV) in cooled, (-)-DPI-modified, noninactivating Na+ channels proved to be drug resistant and could not be flicker-blocked by 10 mumol/liter propafenone. The same drug concentration induced in (-)-DPI-modified Na+ channels a discrete block with association and dissociation rate constants of 16.1 +/- 5.3 x 10(6) mol-1 sec-1 and 675 +/- 25 sec-1, respectively. Quinidine, known to have a considerable affinity for activated Na+ channels, in lower concentrations (5 mumol/liter) left tau open unchanged or reduced, in higher concentrations (10 mumol/liter) tau open only slightly to 81% of the predrug value whereas NPo declined to 30%, but repetitive blocking events during the conducting state could never be observed. Basically the same drug resistance of the open state was seen in cardiac Na+ channels whose open-state kinetics had been modulated by the cytoplasmic presence of F- ions. But in this case, propafenone reduced reopening and selectively abolished a long-lasting open state. This drug action is unlikely related to the inhibitory effect on NPo since hyperpolarization and the accompanying block attenuation did not restore the channel kinetics. It is concluded that cardiac Na+ channels cannot be flicker-blocked by antiarrhythmic drugs unless Na+ inactivation is removed.

Na+ channel blockade by cyclic AMP and other 6-aminopurines in neonatal rat heart

Elementary Na+ currents were recorded at 19 degrees C in cell attached and inside-out patches from cultured neonatal rat cardiocytes in order to study the effect of cAMP and other 6-aminopurines. The treatment of the cardiocytes with db-cAMP (1 x 10(-3) mol/liter) led to a decline of reconstructed macroscopic peak INa to 62 +/- 7.6% of the initial control value. This reduction in NPo was mostly accompanied by a decrease in burst activity. Open-state kinetics were preserved even in DPI-modified, noninactivating Na+ channels. Since the stimulator of the adenylate cyclase, forskolin (1 x 10(-6) mol/liter), evoked a similar pattern of response, the NPo decrease can be considered as the functional correlate of Na+ channel phosphorylation brought about by cAMP-dependent protein kinase. As found in inside-out patches, cAMP (1 x 10(-3) mol/liter) remained effective under cell-free conditions and reduced reconstructed macroscopic peak INa to about 50% of the initial control value when the absence of Mg-ATP at the cytoplasmic membrane surface prevents phosphorylation reactions. A very similar response developed in the cytoplasmic presence of other 6-aminopurines including ATP (1 x 10(3) mol/liter), adenosine (1 x 10(-4) mol/liter), adenine (1 x 10(-5) mol/liter) and hypoxanthine (1 x 10(-5) mol/liter). This susceptibility to adenine suggests that cardiac Na+ channels in situ could sense intracellular fluctuations of adenine nucleotides, most likely of ATP.

Novel mechanism of voltage-dependent gating in L-type calcium channels

Activation of voltage-dependent calcium channels by membrane depolarization triggers a variety of key cellular responses, such as contraction in heart and smooth muscle and exocytotic secretion in endocrine and nerve cells. Modulation of calcium channel gating is believed to be the mechanism by which several neurotransmitters, hormones and therapeutic agents mediate their effects on cell function. Here we describe a novel type of voltage-dependent equilibrium between different gating patterns of dihydropyridine-sensitive (L-type) cardiac Ca2+ channels. Strong depolarizations drive the channel from its normal gating pattern into a mode of gating characterized by long openings and high open probability. The rate constants for conversions between gating modes, estimated from single channel recordings, are much slower than normal channel opening and closing rates, but the equilibrium between modes is almost as steeply voltage-dependent as channel activation and deactivation at more negative potentials. This new mechanism of voltage-dependent gating can explain previous reports of activity-dependent Ca2+ channel potentiation in cardiac and other cells and forms a potent mechanism by which Ca2+ uptake into cells could be regulated.

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.

Maximum open probability of single Na+ channels during depolarization in guinea-pig cardiac cells

Single Na+ channel currents were recorded from guinea-pig ventricular cells in cell-attached patches. The ensemble average current (I) of multi-channel recordings was used to calculate the variance (sigma 2) of current fluctuations around the mean in individual current recordings. The relationship between sigma 2/I and I was linear and allowed estimation of the number of functional channels in the patch of membrane. The unitary amplitude of channel current obtained from the relation sigma 2/I-I was in agreement with that obtained directly by measuring the original records. The number of channels determined at different depolarizing pulses was almost constant in a given patch. The value was nearly equal to that of the maximum current, measured at high depolarizing potentials when most channels are open, divided by the unitary current. The open probability of the channels at the peak time of mean current was calculated based on the estimated number of channels. It increased with increasing depolarization and saturated at about 0.6 at test potentials above -20 mV. The inactivation time-course of the mean current was fitted by a sum of two exponentials. The current amplitude extrapolated to time zero was much larger than the current which could be generated by all channels. This indicates that the inactivation of the Na+ channel develops with delay after the onset of depolarization. The finding is in agreement with a model in which the inactivation rate is accelerated with activation of the Na+ channel.

Sodium current kinetics in freshly isolated neostriatal neurones of the adult guinea pig

Neurones of the neostriatum were freshly dispersed from the adult guinea pig brain. A fast, transient inward Na+ current (INa) was analysed using the whole-cell patch-clamp technique. Upon depolarizations, INa developed with a sigmoidal time course, which was described by m3 kinetics. INa showed an activation threshold of about -60 mV, a peak current at -30 to -20 mV, and a reversal of polarity at +60 mV. The steady-state activation (m infinity) curve for INa had a slope factor of about 9 mV for a change in the rate constant by a factor of e) in the range between -50 mV and -20 mV. Conversely, the backward rate constant (beta m) decreased as the membrane was depolarized (about 31 mV for an e-fold change). The steady-state inactivation (h infinity) curve was well expressed by the Boltzmann's equation with a half-inactivation potential of -62 mV and a slope parameter of 6 mV. The time course of INa decay followed a second-order process, whereas the recovery from inactivation was described as a first-order process. The tau h curve showed a bell-shaped configuration with a maximum value at -60 mV. The forward rate constants (alpha h) decreased as the membrane was depolarized (about 17 mV for an e-fold change) in the range between -50 mV and -20 mV. The backward rate constants (beta h) increased as the membrane was depolarized (about 10 mV for an e-fold change). There was a significant overlap between m infinity and h infinity curves, suggesting a steady influx of Na+ (window current).

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.

Pyrethroid modifications of the activation and inactivation kinetics of the sodium channels in squid giant axons

The kinetics of sodium channel activation and inactivation were analyzed in the squid giant axons internally treated with various pyrethroids. Pyrethroids increased the steady-state sodium current in squid giant axons by removing the inactivation. The steady-state sodium conductances in control and pyrethroid-treated axons showed the same voltage dependence, indicating that the removal of inactivation by pyrethroids did not lead to an alteration of gating charge transfer. The pyrethroid-modified sodium channels were activated with a biphasic time course involving the movement of at least two gating particles, and both components were voltage-dependent. The slower component was abolished by treatment with either pronase or N-bromoacetamide. The net elementary charges transported in the electric membrane field were reduced in the course of slow activation of the pyrethroid-induced sodium current. It appears that the 'immobilization' of gating charge is related to the slow activation rather than the inactivation of the sodium channel.

Gating currents associated with Na channels in canine cardiac Purkinje cells

Gating currents (Ig) were recorded in single canine cardiac Purkinje cells at 10-12 degrees C. Ig characteristics corresponded closely to macroscopic INa characteristics and appeared to exhibit little contamination from other voltage-gated channels. Charge density predicted by peak INa was 0.14-0.22 fC micron -2 and this compared well with the measured value of 0.19 +/- 0.10 fC micron -2 (SD; n = 28). The charge-voltage relationship rose over a voltage similar to the peak INa conductance curve. The midpoints of the two relationships were not significantly different although the conductance curve was 1.5 +/- 0.3 (SD; n = 9) times steeper. Consistent with this observation, which predicted that a large amount of the gating charge would be associated with transitions close to the open state, an analysis of activation from Hodgkin-Huxley fits to the macroscopic currents showed that tau m corresponded well with a prominent component of Ig. Ig relaxations fitted two exponentials better than one over the range of voltages in which Na channels were activated. When the holding potential was hyperpolarized, relaxation of Ig during step depolarizations to 0 mV was prolonged but there was no substantial increase in charge, further suggesting that early closed-state transitions are less in charge, further suggesting that early closed-state transitions are less voltage dependent. The single cardiac Purkinje cell appears to be a good candidate for combining Ig and single-channel measurements to obtain a kinetic description of the cardiac Na channel.

Kinetic analysis of single sodium channels from canine cardiac Purkinje cells

Single sodium channel events were recorded from cell-attached patches on single canine cardiac Purkinje cells at 10-13 degrees C. Data from four patches containing two to four channels and one patch with one channel were selected for quantitative analysis. The channels showed prominent reopening behavior at voltages near threshold, and the number of reopenings declined steeply with depolarization. Mean channel open time was a biphasic function of voltage with the maximum value (1-1.5 ms) occurring between -50 and -40 mV and lower values at more and at less hyperpolarized levels. Inactivation without opening was also prominent near threshold, and this occurrence also declined with depolarization. The waiting time distributions and the probability of being open showed voltage and time dependence as expected from whole-cell current studies. The results were analyzed in terms of a five-state Markovian kinetic model using both histogram analysis and a maximum likelihood method to estimate kinetic parameters. The kinetic parameters of the model fits were similar to those of GH3 pituitary cells (Horn, R., and C. A. Vandenberg. 1984. Journal of General Physiology. 84:505-534) and N1E115 neuroblastoma cells (Aldrich, R. W., and C. F. Stevens. Journal of Neuroscience. 7:418-431). Both histogram and maximum likelihood analysis implied that much of the voltage dependence of cardiac Na current is in its activation behavior, with inactivation showing modest voltage dependence.

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.

A simple hanging-drop patch-clamp technique for studying single channel activities in excised membrane patches

A simple hanging-drop technique has been developed for examining the effects of reagents available in limited quantities such as some proteolytic enzymes and site-directed antibodies on single sodium channel activities in patches excised from hippocampal neuronal membranes. The advantages of the technique are as follows: 1. a very small volume (2-4 microliters) of the buffer solution containing the agents to be tested; 2. recording under static conditions without using continuous perfusion systems; 3. reduced background current noise. The use of the technique for studying modifications of single channel parameters under the influence of agents available in limited amounts is illustrated by modification of single sodium channels by treatment with papain.

A reinterpretation of Na channel gating and permeation in terms of a phase transition between a transmembrane S4 alpha-helix and a channel-helix. A theoretical study

A functional model for the S4/IV alpha-helix of the action potential sodium channel is described by means of a thermodynamic approach. The model is based on a phase transition between the alpha-helix and an ion conducting channel-helix which is similar to the well established helix-coil transition in solution. The right hand channel-helix is a peptide chain with an alternating sequence of torsional angles (phi1, psi1) = (87 degrees, 315 degrees) and (phi2, psi2) = (22 degrees, 107 degrees) which yields a helix of 13.5 A per turn. The axial dipole moments of the peptide bonds of this chain of L-amino acids nearly cancel each other out in a similar way to those in the gramicidin A channel, which is formed by alternating D- and L-amino acids. The helix, which does not contain any H-bonds, is stabilized by a helical file of water molecules which includes the permeating ion(s). This file turns around the channel-helix to form a relatively stable "double helix" structure which corresponds to the open channel. Since every third side chain in the S4/IV helix carries a positive charge their environments must be polarized. These polarized regions form a left hand screening-helix around the alpha-helix with 27 A per turn. If all H-bonds of the alpha-helix are broken and the internal alpha-carbon atom is considered as fixed, the outer ten residues leave the membrane while the internal ten residues form the channel-helix. In this configuration every positively charged side chain matches nearly exactly every second polarized region of the screening-helix leaving the three regions in-between exposed to the water file containing the ion(s). This further stabilizes the channel and agrees nicely with the idea of cationic selectivity. An analysis of the energetics of the alpha-helix-channel-helix transition showed that the voltage-independent part of the free energy per helix residue could well be close to 0 kcal/mol and thus be in the range where a transition could occur. Two voltage-dependent contributions were included: the break down of the considerable dipole of the alpha-helix and the outward shift of the positive charges of the side chains upon channel-helix formation. Taking into account the fact that the formation of an alpha-helix is a highly cooperative process the degree of voltage dependence of the probability of formation of a channel-helix proved to be in the same range as experimental values for the open probability of modified Na channels whose inactivation had been removed. With regard to gating currents, the model predicts that 2.74 positive charges are moved in an outward direction. Consequences of the model for other experimental findings are discussed.

Gating in iodate-modified single cardiac Na+ channels

Elementary Na+ currents were recorded at 19 degrees C during 220-msec lasting step depolarizations in cell-attached and inside-out patches from cultured neonatal rat cardiocytes in order to study the modifying influence of iodate, bromate and glutaraldehyde on single cardiac Na+ channels. Iodate (10 mmol/liter) removed Na+ inactivation and caused repetitive, burst-like channel activity after treating the cytoplasmic channel surface. In contrast to normal Na+ channels under control conditions, iodate-modified Na+ channels attain two conducting states, a short-lasting one with a voltage-independent lifetime close to 1 msec and, likewise tested between -50 and +10 mV, a long-lasting one being apparently exponentially dependent on voltage. Channel modification by bromate (10 mmol/liter) and glutaraldehyde (0.5 mmol/liter) also included the occurrence of two open states. Also, burst duration depended apparently exponentially on voltage and increased when shifting the membrane in the positive direction, but there was no evidence for two bursting states. Chemically modified Na+ channels retain an apparently normal unitary conductance (12.8 +/- 0.5 pS). Of the two substates observed, one of them is remarkable in that it is mostly attained from full-state openings and is very short living in nature; the voltage-independent lifetime was close to 2 msec. Despite removal of inactivation, open probability progressively declined during membrane depolarization. The underlying deactivation process is strongly voltage sensitive but, in contrast to slow Na+ inactivation, responds to a voltage shift in the positive direction with a retardation in kinetics. Chemically modified Na+ channels exhibit a characteristic bursting state much shorter than in DPI-modified Na+ channels, a difference not consistent with the hypothesis of common kinetic properties in noninactivating Na+ channels.

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.

Blockade of cardiac sodium channels by lidocaine. Single-channel analysis

The mechanism of interaction of lidocaine with cardiac sodium channels during use-dependent block is not well defined. We examined the blockade of single cardiac sodium channels by lidocaine and its hydrophobic derivative RAD-242 in rabbit ventricular myocytes. Experiments were performed in cell-attached and inside-out patches. Use-dependent block was assessed with trains of ten 200-msec pulses with interpulse intervals of 500 msec and test potentials of -60 to -40 mV. Single-channel kinetics sometimes showed time-dependent change in the absence of drug. During exposure to 80 microM lidocaine, use-dependent block during the trains was associated with a decrease in the average number of openings per step. At -60 mV, mean open time was not significantly changed (control, 1.4 +/- 0.6 msec; lidocaine, 1.2 +/- 0.3 msec, p greater than 0.05). Greater block developed during trains of 200-msec pulses compared with trains of 20-msec pulses at the same interpulse interval at test potentials during which openings were uncommon later than 20 msec (-50 and -40 mV). Prolonged bursts of channels showing slow-gating kinetics were observed both in control and the presence of 80 microM lidocaine. However, lidocaine may decrease the late sodium current by altering the kinetics of slow gating. The hydrophobic lidocaine derivative RAD-242, which has a 10-fold greater lipid solubility than lidocaine, decreased the peak averaged current during pulse train stimulation by 60% without a change in the mean open time. Our results suggest that the major effect of lidocaine during use-dependent block involves the interaction with a nonconducting state of the sodium channel followed by a failure to open during subsequent depolarization.

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.

The response of single cardiac sodium channels in neonatal rats to the dihydropyridines CGP 28392 and (-)-Bay K 8644

Cell-attached patch clamp recording of elementary Na+ currents were performed at 19 degrees C in neonatal cultured rat heart cells to study Na+ channel properties in the presence of dihydropyridines. Bath application of racemic CGP 28392, at 5 mumol/l, activated Na+ channels. By increasing the open probability, P0, and/or the number of functioning Na+ channels, peak INa in reconstructed macroscopic Na+ currents rose without changes in the decay kinetics. This was accompanied by a prolongation of open time. (-)-Bay K 8644 (1-10 mumol/l) had the same effect. In the presence of either agonist, Na+ channels retained an uniform open state and, as estimated from the mean number of openings per sequence, their initial tendency to reopen. Rarely appearing ultralong opening sequences are unlikely to be drug-induced as Na+ channels can likewise switch into this particular activity mode under drug-free conditions. Racemic CGP 28392, at 50 mumol/l, blocked Na+ channels in an all-or-none fashion suggesting that one enantiomer acts as agonist and the other enantiomer as blocker. A quite different response consisting of the occurrence of a second open state with a several-fold increased life time and a significantly increased reopening was observed with (-)-Bay K 8644 in damaged cardiocytes with hyperpermeable membranes and after patch excision into drug-containing solution. Evidence was obtained from control inside-out patches that this increased reopening is most probably caused by the solvent, ethanol.(ABSTRACT TRUNCATED AT 250 WORDS)

Amiodarone: biochemical evidence for binding to a receptor for class I drugs associated with the rat cardiac sodium channel

Amiodarone has multiple pharmacological effects in heart. Electrophysiological data suggest that among its other effects, amiodarone is a sodium channel blocker. Using a radioligand assay, we determined whether amiodarone interacted with a previously described receptor for type I agents associated with the cardiac sodium channel. The radioligand was [3H]batrachotoxinin A 20 alpha-benzoate ([ 3H]BTXB), a toxin that binds to the activated state of the sodium channel. We have previously shown that class I antiarrhythmic drugs inhibit [3H]BTXB binding. The purpose of this study was to assess whether amiodarone and other class III agents interact with this receptor. Amiodarone inhibited [3H]BTXB binding in a dose-dependent fashion, with an estimated IC50 value of 3.6 microM. This IC50 value is similar to reported clinically effective serum concentrations of amiodarone. In contrast to amiodarone, the IC50 values for other class III drugs (bretylium, sotalol, bethanidine, N-acetylprocainamide) were much higher than their therapeutic concentrations and bore no relation to them. Scatchard analysis of [3H]BTXB binding showed that amiodarone reduced the maximal binding for [3H]BTXB; this finding indicates irreversible inhibition or (more likely) allosteric inhibition by amiodarone. The latter agrees with electrophysiological data suggesting that amiodarone binds to inactivated sodium channels. Sodium channel blockade by amiodarone may contribute to its overall electrophysiological effect.

Single sodium channels from canine ventricular myocytes: voltage dependence and relative rates of activation and inactivation

1. Single sodium channel currents were recorded from canine ventricular myocytes in cell-attached patches. The relative rates of single-channel activation vs. inactivation as well as the voltage dependence of the rate of open-channel inactivation were studied. 2. Ensemble-averaged sodium currents showed relatively normal activation and inactivation kinetics, although the mid-point of the steady-state inactivation (h infinity) curve was shifted by 20-30 mV in the hyperpolarizing direction. This shift was due to the bath solution, which contained isotonic KCl to depolarize the cell to 0 mV. 3. Steady-state activation showed less of a voltage shift. The threshold for eliciting channel opening was around -70 mV and the mid-point of activation occurred near -50 mV. 4. The decline of the ensemble-averaged sodium current during a maintained depolarization was fitted by a single exponential function characterizing the apparent time constant of inactivation (tau h). The apparent rate of inactivation was voltage dependent, with tau h decreasing e-fold for a 15.4 mV depolarization. 5. The relative contributions of the rates of single-channel activation and inactivation in determining the time course of current decay (tau h) were examined using the approach of Aldrich, Corey & Stevens (1983). Mean channel open time (tau o) showed significant voltage dependence, increasing from 0.5 ms at -70 mV to around 0.8 ms at -40 mV. At -70 mV tau h was much greater than tau o, while at -40 mV the two time constants were similar. 6. The degree to which the kinetics of single-channel activation contribute to tau h was studied using the first latency distribution. The first latency function was fitted by two exponentials. The slow component was voltage dependent, decreasing from 19 ms at -70 mV to 0.5 ms at -40 mV. The fast component (0.1-0.5 ms) was not well resolved. 7. Comparing the first latency distribution with the time course of the ensemble-averaged sodium current at -40 mV showed that activation is nearly complete by the time of peak inward sodium current. However, at -70 mV, activation overlaps significantly with the apparent time course of inactivation of the ensemble-averaged current. 8. Using the methods of Aldrich et al. (1983) we also measured the apparent rate of open-channel closing (a) and open-channel inactivation (b). Both rates were voltage dependent, with a showing an e-fold decrease for an 11 mV depolarization and b showing an e-fold increase for a 30 mV depolarization.(ABSTRACT TRUNCATED AT 400 WORDS)

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.

Glial and neuronal forms of the voltage-dependent sodium channel: characteristics and cell-type distribution

Two functionally different forms of the voltage-dependent sodium channel were observed in glia and in neurons of the mammalian nervous system. Both forms had identical conductance and tetrodotoxin sensitivity and displayed steady-state inactivation, a strongly voltage-dependent rate of activation, and a faster but weakly voltage-sensitive rate of inactivation. However, the glial form had significantly slower kinetics and a more negative voltage dependence, suggesting that it was functionally specialized for glia. This form was found in most glial types studied, while the neuronal form was observed in retinal ganglion cells, cortical motor neurons, and O2A glial progenitor cells. Both forms occurred in type-2 astrocytes. The presence of the glial form correlated with the RAN-2 surface antigen.

Temperature-dependent subconducting states and kinetics of deltamethrin-modified sodium channels of neuroblastoma cells

The effects of temperature on the properties of sodium channels from mouse neuroblastoma cells modified by the pyrethroid insecticide deltamethrin were investigated using the patch-clamp technique. The study was aimed at determining various states of modified channels which were expected to be revealed by raising the temperature as a result of an increase in channel activity. After exposure to 10 microM deltamethrin, the decay of whole cell sodium current at -30 mV was drastically slowed. It is expressed by two exponential functions at 11 degrees C and by three exponential functions at room temperature (22 +/- 1 degree C). Thus, raising the temperature reveals a new process. Whole cell sodium tail currents associated with step repolarization from -30 mV to -100 mV were best fit by the sum of two exponential functions both at 11 degrees C and at room temperature. The decay of the summed modified single sodium channel currents at -30 mV was expressed by a single exponential function at 11 degrees C, and by two exponential functions at room temperature. In keeping with these results, the open time histograms show the single (11 degrees C) and double (room temperature) exponential distributions. Thus, raising the temperature allows a new single channel process to be revealed. Other modified open states observed previously at 11 degrees C were also found at room temperature including a flickering state and a subconducting state. In addition, several new subconducting states were found at room temperature.(ABSTRACT TRUNCATED AT 250 WORDS)

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.

Kinetic properties of single sodium channels in rat heart and rat brain

Single Na channel currents were compared in ventricular myocytes and cortical neurons of neonatal rats using the gigaseal patch-clamp method to determine whether tissue-specific differences in gating can be detected at the single-channel level. Single-channel currents were recorded in cell-attached and excised membrane patches at test potentials of -70 to -20 mV and at 9-11 degrees C. In both cell-attached and excised patches brain Na channel mean open time progressively increased from less than 1 ms at -70 mV to approximately 2 ms at -20 mV. Near threshold, single openings with dispersed latencies were observed. By contrast, in cell-attached patches, heart Na channel mean open time peaked near -50 mV, was three times brain Na channel mean open time, and declined continuously to approximately 2 ms at -20 mV. Near threshold, openings occurred frequently usually as brief bursts lasting several milliseconds and rarely as prolonged bursts lasting tens of milliseconds. Unlike what occurs in brain tissue where excision did not change gating, in excised heart patches both the frequency of prolonged bursting and the mean open time of single units increased markedly. Brain and cardiac Na channels can therefore be distinguished on the basis of their mean open times and bursting characteristics.

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)

The steady-state distribution of gating charge in crayfish giant axons

Progressive shifts of holding potential (Vh) in crayfish giant axons, from -140 to -70 mV, reduce gating currents seen in depolarizing steps (to 0 mV test potential) while proportionately increasing gating currents in hyperpolarizing steps (to -240 mV). The resulting sigmoid equilibrium charge distribution (Q-Vh curve) shows an effective valence of 1.9e and a midpoint of -100 mV. By contrast, Q-V curves obtained using hyperpolarizing and/or depolarizing steps from a single holding potential, change their "shape" depending on the chosen holding potential. For holding potentials at the negative end of the Q-Vh distribution (e.g., -140 mV), negligible charge moves in hyperpolarizing pulses and the Q-V curve can be characterized entirely from depolarizing voltage steps. The slope of the resulting simple sigmoid Q-V curve also indicates an effective valence of 1.9e. When the axon is held at less negative potentials significant charge moves in hyperpolarizing voltage steps. The component of the Q-V curve collected using hyperpolarizing pulses shows a significantly reduced slope (approximately 0.75e) by comparison with the 1.9e slope found using depolarizing pulses or from the Q-Vh curve. As holding potential is shifted in the depolarizing direction along the Q-Vh curve, an increasing fraction of total charge movement must be assessed in hyperpolarizing voltage steps. Thus charge moving in the low slope component of the Q-V curve increases as holding potential is depolarized, while charge moving with high apparent valence decreases proportionately. Additional results, together with simulations based on a simple kinetic model, suggest that the reduced apparent valence of the low slope component of the Q-V curve results from gating charge immobilization occurring at holding potential. Immobilization selectively retards that fraction of total charge moving in hyperpolarizing pulses. Misleading conclusions, as to the number and valence of the gating particles, may therefore be derived from Q-V curves obtained by other than depolarizing pulses from negative saturated holding potentials.

Kinetic states and modes of single large-conductance calcium-activated potassium channels in cultured rat skeletal muscle

1. Kinetic states and modes of a large-conductance Ca2+-activated K+ 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 seven different excised membrane patches containing a single channel. 2. Plots of the mean durations of consecutive groups of ten to fifty open and shut intervals were made to assess kinetic stability of the channel. Occasional abrupt decreases in the mean open interval duration from normal to different distinct levels, which were maintained for hundreds to thousands of consecutive intervals, indicated entry of the channel into different modes. 3. Four different kinetic modes were identified: normal mode, which included 96% of the intervals; intermediate open mode with 3.2% of the intervals; brief open mode with 0.5% of the intervals; and buzz mode with 0.1% of the intervals. The mean open interval durations were 61% of normal during the intermediate open mode, 12% of normal during the brief open mode, and 2.6% of normal during the buzz mode. 4. Most mode transitions were observed from the normal mode to one of the other modes and then back to normal. Sojourns in the normal mode lasted 5-1000 s. Sojourns in the intermediate open, brief open, and buzz modes lasted 1.5-150, 1-7, and 0.01-1 s, respectively. 5. During normal activity the distributions of interval durations were typically described by the sum of three to four exponential components for the open intervals and six to eight exponential components for the shut intervals, and this was the case for data obtained over a wide range of open channel probability resulting from different Ca2+i. These observations suggest that the channel can enter at least three to four open and six to eight shut states during normal activity. 6. The numbers of detected states for data sets of different sample sizes drawn from normal activity agreed with theoretical predictions, and were essentially independent of the segment of normal activity from which the data sets were drawn. These observations are consistent with relative stability of channel kinetics during normal activity. Detection of each additional open or shut state after the first was found to require a 3- to 10-fold increase in the number of analysed events. 7. The intermediate open mode differed from the normal mode in that the longest open component of the four normal open components was absent.

Predominance of poorly reopening single Na+ channels and lack of slow Na+ inactivation in neonatal cardiocytes

Elementary Na+ currents through single cardiac Na+ channels were recorded at -50 mV in cell-attached patches from neonatal rat cardiocytes kept at holding potentials between -100 and -120 mV. Na+ channel activity may occur as burst-like, closely-timed repetitive openings with shut times close to 0.5-0.6 msec, indicating that an individual Na+ channel may reopen several times during step depolarization. A systematic quantitative analysis in 19 cell-attached patches showed that reopening may be quite differently pronounced. The majority, namely 16 patches, contained Na+ channels with a low tendency to reopen. This was evidenced from the average value for the mean number of openings per sequence, 2.5. Strikingly different results were obtained in a second group of three patches. Here, a mean number of openings per sequence of 3.42, 3.72, and 5.68 was found. Ensemble averages from the latter group of patches revealed macroscopic Na+ currents with a biexponential decay phase. Reconstructed Na+ currents from patches with poorly reopening Na+ channels were devoid of a slow decay component. This strongly suggests that reopening may be causally related to slow Na+ inactivation. Poorly pronounced reopening and, consequently, the lack of slow Na+ inactivation could be characteristic features of neonatal cardiac Na+ channels.

A study of the electrical characteristics of sodium currents in single ventricular cells of the frog

1. The generation of action potentials elicited from enzymatically dispersed ventricular cells from the frog, Rana catesbeiana, has been shown to be due to the influx of both Na+ and Ca2+. The maximum rate of rise, the amplitude and the duration at 50% repolarization of the action potential were estimated to be 26.4 +/- 5.1 V/s (n = 8), 110 +/- 2.7 mV (n = 8) and 601 +/- 180 ms (n = 8) at 15 degrees C, respectively. 2. Inward Na+ current (INa) was studied in these ventricular cells by the whole-cell patch clamp technique in a medium where Ca2+ current was eliminated by substituting extracellular Mg2+ for Ca2+ and K+ current was suppressed by applying Cs+ intracellularly. All the voltage clamp experiments were carried out at 4 degrees C. 3. INa elicited by single depolarizing steps from a holding potential (VH) of -80 mV had a threshold of -50 mV and was maximal at -20 mV. Peak currents in normal Ringer solution containing 113.5 mM-Na+ were of the order of 0.01-0.02 mA/cm2. Maximum Na+ conductance (gNa) was calculated to be 5.9 mS/cm2. 4. Under normal conditions the reversal potential for INa was determined to be 50 mV, which is close to the value predicted from the Nernst equation. The reversal potential changed by 59 mV per tenfold change in the activity of extracellular Na+ (aNa). 5. The instantaneous relation between INa tail currents and membrane potential is linear, crossing the abscissa at the reversal potential for INa. 6. Reconstructions of INa were made in terms of the parameters of the Hodgkin-Huxley model for the squid axon, using constants obtained from the frog ventricular cells. 7. The falling phase of INa and the development of inactivation measured by the double-pulse method could be well fitted by a single-exponential function. 8. The time course for recovery of INa from inactivation exhibited a single time constant.

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.

Block of single cardiac Na+ channels by antiarrhythmic drugs: the effect of amiodarone, propafenone and diprafenone

Cell-attached patch-clamp experiments were performed in cultured cardiocytes of neonatal rats at 19 degrees C to analyze elementary currents through single Na+ channels under control conditions and in the presence of the class 1 antiarrhythmic drugs amiodarone, propafenone, and diprafenone. As observed in a cell-attached patch with only one functioning Na+ channel, repetitive stepping of the membrane at 0.4 Hz triggered periodically channel openings except during a silent period of about 1.5 min. The latter began and ceased abruptly and did not fit the monoexponential distribution of the run length of sweeps without activity (blank sweeps). Treating the cardiocytes with amiodarone, propafenone or diprafenone (10 to 20 mumol/liter) led rapidly to a blockage and reduced the likelihood that membrane depolarization triggers the opening of Na+ channels. The number of blank sweeps increased at the expense of the number of sweeps with activity. The fraction of activity sweeps with superpositions, indicating the simultaneous activation of two or more Na+ channels, also declined. As tested with amiodarone, the run length of blank sweeps is voltage- and time-dependent, analogous to the intensity of the block of macroscopic Na+ currents. Open time, open-time distribution, unitary current size and the tendency to reopen did not differ in unblocked cardiac Na+ channels (i.e. that channel fraction capable of opening in the presence of amiodarone or propafenone) from the respective control values obtained before superfusing the cardiocytes with these drugs. Apart from its blocking action, the propafenone derivative diprafenone exerted additionally a modifying effect and reduced mean open time by up to 45%. In contrast to the block, this reduction in conducting state proved insensitive to changes in holding potential, at least between -130 and -150 mV, the range tested. This means that block was attenuated on hyperpolarization whereas the reduction in open time persisted. It is concluded that, in the presence of these drugs, unblocked cardiac Na+ channels share a number of properties with normal Na+ channels in the absence of these drugs. Shortening of channel lifetime by diprafenone may be indicative of a channel modification brought about possibly by a receptor-mediated facilitation of the transition from the open to the inactivated state.

Calcium block of guinea-pig heart sodium channels with and without modification by the piperazinylindole DPI 201-106

1. External Ca2+ block of Na+ channels was studied by a gigaohm-seal patch clamp technique in single cardiac ventricular cells from guinea-pig. Single-channel currents were recorded from cell-attached patches. 2. Increasing external Ca2+ concentrations in the patch pipette from 0.1 to 20 mM reduced the single-channel conductance of normal Na+ channels from 27 to 14 pS without causing flickering (obtained from linear regression, eight patches). 3. Exposed to external Ca2+ concentrations of 20 mM, the single-channel currents decreased at potentials negative to -60 mV in spite of an increased driving force for inward Na+ currents. 4. An external concentration of 35 mM-Mg2+, which is supposed to exert a screening of surface charges nearly equal to that of 20 mM-Ca2+ (Hille, Woodhull & Shapiro, 1975), reduced the single-Na+-channel conductance only from 26 (1 mM-Mg2+) to 20 pS (linear regression, eight patches). A weaker voltage-dependent block at potentials negative to -50 mV was observed in 35 mM-Mg2+ than in 20 mM-Ca2+. Therefore, surface charge effects cannot explain the obvious reduction of the conductance of single Na+ channels found when the external Ca2+ concentration was increased. 5. Single Na+-channel currents increased with an increase in the external Na+ concentration [( Na+]o) but showed saturation. The Na+o-single-channel current relationship could be described by i = imax/(1 + kd/[Na+]o) with imax = 5.4 pA and kd = 359 mM (seventeen patches). 6. The mean open time of Na+ channels varied between 0.18 and 0.59 ms (potentials between -80 and -20 mV). No significant changes in the mean open time could be obtained when Ca2+ was varied between 0.1 and 20 mM. 7. The piperazinylindole compound DPI 201-106 was used as a tool to prolong the open time of single Na+ channels. If the external Ca2+ concentration was increased from 0.1 to 20 mM the currents through the modified channels were reduced. The reduction of single-channel currents was accentuated at potentials negative to -60 mV (20 mM-Ca2+) similar to the control channels. 8. In contrast to non-modified Na+ channels, the mean open time of DPI 201-106-modified channels proved extremely voltage and Ca2+ dependent.(ABSTRACT TRUNCATED AT 400 WORDS)