Kinetics of Na inactivation in frog atria

A computational investigation of cardiac caveolae as a source of persistent sodium current

Recent studies of cholesterol-rich membrane microdomains, called caveolae, reveal that caveolae are reservoirs of “recruitable” sodium ion channels. Caveolar channels constitute a substantial and previously unrecognized source of sodium current in cardiac cells. In this paper we model for the first time caveolar sodium currents and their contributions to cardiac action potential morphology. We show that the β-agonist-induced opening of caveolae may have substantial impacts on peak overshoot, maximum upstroke velocity, and ultimately conduction velocity. Additionally, we show that prolonged action potentials and the formation of potentially arrhythmogenic afterdepolarizations, can arise if caveolae open intermittently throughout the action potential. Our simulations suggest that caveolar sodium current may constitute a route, which is independent of channelopathies, to delayed repolarization and the arrhythmias associated with such delays.

Exponential activation of the cardiac Na+ current in single guinea-pig ventricular cells

1. The cardiac Na+ current of guinea-pig was recorded using an improved oil-gap voltage clamp method. When a single ventricular cell was stretched between the internal and external solution compartments across an oil gap of about 40 microns in width, the sealing resistance in the oil gap was higher than 1 G omega and the time constant of the capacitive current was between 10 and 40 microseconds. Effective series resistance (Rs) was less than 50 k omega after Rs compensation. 2. The activation time course (I'Na) was separated from inactivation by dividing the digitized record of Na+ current with the inactivation variable h(t), which was obtained by fitting exponential functions to the decaying phase of current. I'Na started as a single exponential activation at time 0, which was defined by the decay of the capacitive current to 5% of its peak. 3. The Na+ tail current was recorded on repolarization after a short (1.2 ms) depolarizing pulse to -10 mV. Its single exponential decay at potentials negative to -50 mV, or its major exponential component of decay between -50 and -30 mV, was attributed to deactivation. The time constants of deactivation were similar to those of activation which were measured from I'Na on depolarization to comparable potentials. The m1 kinetics gave a better fit for Na+ activation than the m3 kinetics. 4. The time constant of deactivation was a linear function of the membrane potential on a semilogarithmic scale with an e-fold increase per 21.6 +/- 1.3 mV (n = 8) depolarization. The steady-state activation value (m(infinity)) was obtained from the amplitude of I'Na. Fitting a Boltzmann equation indicated a half-activation potential of -21.9 +/- 1.7 mV and a slope factor of 7.9 +/- 0.4 mV (n = 9). 5. m1 kinetics are more pertinent to a description of the cardiac Na+ current. Limitations in analysing the activation kinetics of Na+ current are discussed for the improved oil-gap voltage clamp method.

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.

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.

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.

Effect of tetrodotoxin, lidocaine, and quinidine on the transient inward current of sheep Purkinje fibres

The effect of tetrodotoxin (TTX), lidocaine, and quinidine on the transient inward current (TI) was studied in voltage-clamped sheep cardiac Purkinje fibres. The TI was induced by elevation of extracellular Ca or addition of strophanthidin. Reduction of external Na had a biphasic effect on the steady state TI magnitude; a moderate (less than 50%) reduction of external Na had an enhancing effect on the TI; a further decrease of extracellular Na was accompanied by a decline of TI amplitude. The TI could not be induced in Na-free medium (external Ca less than or equal to 9.0 mM). TTX, lidocaine, and quinidine reduced the magnitude of the TI in a dose-dependent way. The blocking effect of these agents could be compensated for by a moderate (less than 50%) reduction of external Na or an elevation of extracellular Ca. It is suggested that the blocking effect of TTX, lidocaine, and quinidine on the TI is due to a reduction of intracellular Na, which causes a decay of intracellular Ca via the Na-Ca exchange mechanism.

Sodium current in single cells from bullfrog atrium: voltage dependence and ion transfer properties

1. Whole-cell and patch-clamp techniques (Hamill, Marty, Neher, Sakmann & Sigworth, 1981) have been used to make quantitative measurements of the transient inward sodium current (INa) in single cells from bullfrog atrium. This preparation is particularly suitable for the study of INa: (i) the current density is relatively low, (ii) the cells lack a transverse tubule system, (iii) isolated myocytes can be maintained at reduced temperatures (approximately 8-12 degrees C); therefore kinetics can be studied quantitatively. 2. INa was pharmacologically and kinetically isolated from other transmembrane currents by blocking ICa with CdCl2 (0.2-0.5 mM) or LaCl3 (5 x 10(-6) M), and by using only relatively short voltage-clamp depolarizations which did not activate IK (the delayed rectifier). 3. The voltage dependence of INa in bullfrog atrium is similar to that in amphibian node of Ranvier or fast skeletal muscle. The threshold for activation is approximately -50 mV. The peak of the INa vs. membrane potential relation is near -5 to -10 mV. The reversal potential in 'normal' (115 mM-Na+) Ringer solution is +59.0 mV (S.D. +/- 3.4, n = 10). Reduction of external Na+ concentration to one-third of normal resulted in an approximately -27 mV shift of the reversal potential, close to that expected for a highly Na+-selective conductance. 4. Steady-state inactivation of INa (h infinity), measured with a conventional two-pulse voltage-clamp protocol, spanned the membrane potential range from -90 to -50 mV. The potential dependence of h infinity was well described by a single Boltzmann function with half-inactivation at -71 mV and maximum slope of 6.0 mV. 5. Steady-state activation of INa (m infinity) was determined from fits of INa records to a Hodgkin-Huxley model. The potential dependence of m infinity was fitted to a Boltzmann function with half-activation at -33 mV and maximum slope of 9.5 mV. Thus at temperatures around 10 degrees C there was very little overlap of the m infinity and h infinity curves, and only very small steady-state 'window' currents are predicted. 6. The activation time constant, tau m, had a 'bell-shaped' dependence on membrane potential. The peak value of tau m was about 4.2 ms, at a membrane potential of -35 mV (9 degrees C). 7. The time course of inactivation of INa was consistently better described by the sum of two exponentials than by one exponential.(ABSTRACT TRUNCATED AT 400 WORDS)

Revised formulation of the Hodgkin-Huxley representation of the sodium current in cardiac cells

The purpose of this paper is to revise the parameters of the Hodgkin-Huxley formulation for the Na+ current in ventricular myocardial cells. To this end we have assembled much of the recent voltage clamp data on cardiac preparations obtained with modern voltage clamp and patch clamp techniques. The selected activation and inactivation characteristics of the Na+ channel and other membrane parameters represent a good compromise between available experimental measurements and lead to a reasonable average representation of the cardiac Na+ membrane current. The resulting Na+ conductance changes during the action potential upstroke are much larger than in earlier models, so that the upstroke is much faster and the peak depolarization is close to the Na+ equilibrium potential. The firing threshold level is nearly constant for resting potentials in the range of -70 and -90 mV. The maximum rate of rise of the action potential displayed by the new model is quite comparable to experimental observations.

Sodium current kinetics in cat atrial myocytes

1. Na+ current kinetics were studied in isolated atrial myocytes from the adult cat using the single suction-pipette voltage-clamp technique. 2. Current-voltage and conductance-voltage relationships were similar to those described in other cardiac myocyte preparations. 3. Analysis of Na+ current decay using single-pulse, double-pulse and tail current measurements were in agreement and demonstrate a second-order process of current decay. 4. Voltage dependence of steady-state inactivation curves was not symmetrical, having an inflexion at about -90 mV. These results suggest more than a single inactivation process for Na+ channel in the negative potential region. 5. Recovery of Na+ current from inactivation had a sigmoid time course: an initial slow component (delay) followed by a fast and then a second slow component. Increasing the pre-pulse duration slowed the time course of recovery. 6. Taken together, the results were consistent with the presence of multiple inactivated states for the atrial myocyte Na+ channel.

Cardiac cellular electrophysiology: past and present

The time-course of the cardiac action potential can be accounted for in terms of ionic currents crossing the cell membranes. Depolarizing current is carried by Na+ or Ca2+ entering the cells, repolarizing current by K+ leaving the cells. Membrane permeability for the passive movement of these ions is thought to be voltage-dependent as well as time-dependent. Net transfer of charge may also result from active transport, 2 Na+ out against 1 K+ in; or coupled exchange, 3 or 4 Na+ in against 1 Ca2+ out. This review follows the path by which present-day knowledge has been reached. It also gives a few examples to illustrate that electrophysiology has provided concepts useful to clinical cardiology.

Electrophysiological characteristics of cardiac pacemaker cells of the frog Caudiverbera caudiverbera

1. The cardiac pacemaker cells of the frog Caudiverbera caudiverbera are centrally located in the sinus venosus. These cells are rounded, smaller than contractile fibres and have large nuclei. 2. Intracellular recording confirmed the existence of primary and transitional pacemaker cells. 3. Action potentials from primary cells were resistant to blockade by tetrodotoxin (TTX), but were abolished by verapamil suggesting that their bioelectric activity is dependent on a slow inward current. 4. Transitional cells appeared to have two different inward currents contributing to the upstroke: a fast TTX-sensitive and a slow verapamil-sensitive current.

Effects of Goniopora toxin on the membrane currents of bullfrog atrial muscle

The effects of Goniopora toxin (GPT), isolated from the Goniopora species, on the action potential and membrane currents of bullfrog atrial muscle were studied using the double sucrose-gap method. GPT at concentrations above 10 nmol/l prolonged the duration of the action potential and sometimes induced arrhythmias. The prolongation was also induced in the presence of Ca channel blockers (Mn or verapamil). These effects were not reversed by continuous superfusion of GPT-free solution, but were rapidly antagonized by tetrodotoxin (1 mumol/l). The resting potential was not affected by GPT. Voltage clamp experiments revealed that sustained inward current flows following the fast sodium current upon depolarization, in the presence of GPT. The current was elicited by the toxin in Mn-treated fibers and abolished in the presence of TTX. The delayed outward current (Ix) was slightly reduced; the background K current (IK1), inward background current (Ib) and slow inward current (Islow) were not altered by GPT. These results suggest that GPT acts on sodium channels to give rise to a prolonged sodium current which is in turn responsible for the prolongation of the action potential.

Electrophysiological profile of the antiarrhythmic compound asocainol studied on perfused guinea-pig hearts and on isolated cardiac preparations

The studies deal with electrophysiological effects of asocainol [(+/-)-6,7,8,9-tetrahydro-2,12-dimethoxy-7-methyl-6-phenetyl-5 H-dibenz(d,f)azonine-1-ol] on isolated perfused guinea-pig hearts (Langendorff-preparation), on right ventricular papillary muscles, on Purkinje fibres from the guinea pig, and on isolated sinus nodes from the rabbit. In the perfused heart (n = 5) the lowest effective concentration of asocainol is about 0.2 mumol/l. At a concentration of 2 mumol/l the cardiac electrogram shows in spontaneously beating hearts a mean decrease in frequency of 15%, in electrically driven hearts (150/min at 32 degrees C) prolongation of PQ (+31%), of QRS (+24%) and of QT (+5%). In papillary muscles (32 degrees C; K+e 5.9 mmol/l; stimulation rate 0.5 Hz) asocainol (3-30 mumol/l) exerts the following effects: no change of the resting potential, concentration-dependent reduction of the maximum rate of rise (Vmax) of the action potential (AP) (-16 to -67%) as well as of the AP-amplitude (-4 to -16%), and shortening of the AP-duration at 50% repolarisation (-18 to -43%). The steady-state dependence of Vmax on the resting potential (RP) determined by variation of K+e (5.9-15 mmol/l) is shifted by asocainol to more negative potentials. The percentage deviation from controls of the Vmax-RP relationship is more pronounced at lower membrane potentials. The influence of asocainol on the recovery from inactivation of Vmax shows marked time-dependence. Slow response (Ca2+-mediated) APs elicited by strong stimuli in a K+e-rich solution (K+e 20-24 mmol/l) respond to asocainol (3-10 mumol/l) with a marked reduction in amplitude, Vmax and duration.(ABSTRACT TRUNCATED AT 250 WORDS)

A note on the reactivation of the fast sodium current in spherical clusters of embryonic chick heart cells

Reactivation of the fast sodium current in spherical clusters of embryonic chick heart cells was studied using the two-microelectrode voltage-clamp technique. The results show that there are at least two phases of reactivation. The contribution of the two phases to the overall reactivation process is highly dependent on the particular pulse protocol used to measure them.

Ionic currents in single isolated bullfrog atrial cells

Enzymatic dispersion has been used to yield single cells from segments of bullfrog atrium. Previous data (Hume and Giles, 1981) have shown that these individual cells are quiescent and have normal resting potentials and action potentials. The minimum DC space constant is approximately 920 microns. The major goals of the present study were: (a) to develop and refine techniques for making quantitative measurements of the transmembrane ionic currents, and (b) to identify the individual components of ionic current which generate different phases of the action potential. Initial voltage-clamp experiments made using a conventional two-microelectrode technique revealed a small tetrodotoxin (TTX)-insensitive inward current. The small size of this current (2.5-3.0 X 10(-10)A) and the technical difficulty of the two-microelectrode experiments prompted the development of a one-microelectrode voltage-clamp technique which requires impalements using a low-resistance (0.5-2 M omega) micropipette. Voltage-clamp experiments using this new technique in isolated single atrial cells reveal five distinct ionic currents: (a) a conventional transient Na+ current, (b) a TTX-resistant transient inward current, carried mainly by Ca++, (c) a component of persistent inward current, (d) a slowly developing outward K+ current, and (e) an inwardly rectifying time-independent background current. The single suction micropipette technique appears well-suited for use in the quantitative study of ionic currents in these cardiac cells, and in other small cells having similar electrophysiological properties.

Active modulation of electrical coupling between cardiac cells of the dog. A mechanism for transient and steady state variations in conduction velocity

Propagation velocities of action potentials were measured simultaneously along the longitudinal and transverse axes of cardiac fibers in ventricular muscle. The anisotropic distribution of propagation velocities was found to be altered transiently and in the steady state by the rate and pattern of stimulation and by ouabain. The relative amount of velocity change varied with the direction of propagation and was greatest in the direction perpendicular to the long fiber axis. None of the variables usually associated with the membrane ionic mechanism of depolarization--resting potential, Vmax, and taufoot--showed enough variation to account for the observed changes in velocity. A simplified anisotropic propagation model representing the internal current pathway as an alternating sequence of cytoplasmic and junctional resistance is presented, taking into account the larger contribution to the internal resistance made by the cell couplings in the transverse direction than in the longitudinal direction. On the basis of this model, it was concluded that the observed changes in velocity were due to changes in cell coupling. Both transient and steady state velocity changes were found to correspond to changes in the action potential duration, suggesting that there is a common factor, such as the internal calcium and/or sodium concentrations, linking the control of the action potential duration and the coupling resistance between cardiac cells.

Slow channel kinetics in heart muscle

The cardiac slow inward current (Isi) is mediated by a specific conductance system, the slow channel. It is highly selective for Ca and other bivalent cations as for instance Sr, whilst Na permeability is extremely small. The kinetics of activation, inactivation and recovery from inactivation are voltage- and temperature-sensitive. In contrasts to the Hodgkin-Huxley model, development and removal of inactivation operate with different time constants, at least in the ventricular myocardium of cats. Moreover, both processes exhibit a different pharmacological susceptibility. Thus a second inactivation variable having smaller rate constants than the inactivation variable if has to be introduced, which simultaneously suggests the existence of slow inactivation in cardiac slow channels.

Effects of a new antiarrhythmic compound [2-benzal-1-(2' diisopropyl-amino-ethoxy-imino)-cycloheptane hydrogen fumarate] on the electrophysiological properties of mammalian cardiac cells

Intracellular microelectrodes were used to study the effects of Th 494 [2-benzal-1-(2' diisopropyl-amino-ethoxy-imino)-cycloheptane hydrogen fumarate; 1-100 mumol/1) on transmembrane electrical activity of sinus node and Purkinje fibres of the rabbit as well as on atrial trabeculae and papillary muscles of the guinea pig. In the atrial and in the ventricular myocardium (32 degrees C; driving rate 0.3-0.5 Hz) Th 494 exerted the following electrophysiological actions: no change of the resting potential nor of the amplitude of the action potential; concentration- dependent reduction of the maximum rate of rise (dV/dt)max of the action potential; slight increase of the action potential duration at lower concentrations (1-20 mumol/l), loss of the plateau at higher concentrations (above 20 mumol/l). The isometric force of contraction was moderately reduced by Th 494 (about 20% reduction by 2 mumol/l). The h infinity-curve relating (dV/dt)max of the action potential to the membrane potential was depressed by Th 494 without being shifted along the voltage axis. The reduction of (dV/dt)max was considerably more pronounced at higher driving frequencies. After interruption of stimulation for various periods, (dV/dt) max of the first action potential attained a steady-state value in a two-exponential fashion, suggesting use-dependence as well as a change in the recovery kinetics of the fast Na+ channel by Th 494. In Purkinje fibres (37 degrees C) Th 494 reduced (dV/dt) max in a similar manner. The duration of the action potential was considerably decreased at the level of the plateau. In the primary pacemaker region of the sinus node (37 degrees C) Th 494 moderately reduced the rate of diastolic depolarization and diminished at higher concentrations the amplitude of the action potential. All effects of Th 494 were only slowly reversible by drug-free perfusion. In view of its effect on (dV/dt) max, Th 494 resembles quinidine in its potential-dependence, whereas its time-dependence bears greater similarity with lidocaine.

Sodium current in single rat heart muscle cells

1. Rapid inward Na current (INa) was studied in isolated cells from rat ventricular myocardium by a double-suction-pipette voltage clamp technique. All experiments were carried out at 20-22 degrees C. 2. INa elicited by single depolarizing voltage steps from a holding potential, VH, of -80 mV had a threshold between -70 and -60 mV and was maximal at -30 to -20 mV. Peak currents in Krebs-Ringer solution containing 145 mM Na were of the order 0.9-1.8 mA cm-2, assuming an average cell surface area of 8000 square micrometers. 3. The reversal potential for INa was predicted by the Nernst equation for external Na in the range 1.45-145 mM with 16 mM-Na solution perfusing the interior of the cell. 4. Instantaneous I-V plots were linear for potentials of -100 to + 10 mV. Maximum Na conductance (-gNa) was calculated to be 25 mS cm-2 in 145 mM-Na solutions and gNa was constant for potentials positive to -10 mV. 5. INa activated with a time constant of 0.7 msec at -55 mV, decreasing to 100 microsec on depolarizations positive to + 10 mV. 6. Two time constants (tau h1, tau h2) were required to describe INa inactivation during a maintained depolarization, with tau h2 three to four times as long as tau h1. tau h1 was about 2 msec at -50 mV, decreasing to 0.9 msec at -10 mV. 7. The time course for recovery of INa from inactivation also exhibited two time constants (tau r1, tau r2), with the longer tau r2 having a maximum value of the order 100 msec in the potential range -60 to -80 mV. 8. INa in isolated rat cardiac cells has a low sensitivity to tetrodotoxin, requiring a concentration of 30 micrometers for complete blockade.

Active and passive electrical properties of single bullfrog atrial cells

Single cells from the bullfrog (Rana catesbeiana) atrium have been prepared by using a modification of the enzymatic dispersion procedure described by Bagby et al. (1971. Nature [Long.]. 234:351--352) and Fay and Delise (1973. Proc. Natl. Acad. Sci. U.S.A. 70:641--645). Visualization of relaxed cells via phase-contrast or Nomarski optics (magnification, 400--600) indicates that cells range between 150 and 350 micrometers in length and 4 and 7 micrometers in diameter. The mean sarcomere length in relaxed, quiescent atrial cells in 2.05 micrometer. Conventional electrophysiological measurements have been made. In normal Ringer's solution (2.5 mM K+, 2.5 mM Ca++) acceptable cells have stable resting potentials of about -88 mV, and large (125 mV) long-duration (approximately 720 ms) action potentials can be elicited. The Vm vs. log[K+]0 relation obtained from isolated cells is similar to that of the intact atrium. The depolarizing phase of the action potential of isolated atrial myocytes exhibits two pharmacologically separable components: tetrodotoxin (10(-6) g/ml) markedly suppresses the initial regenerative depolarization, whereas verapamil (3 x 10(-6) M) inhibits the secondary depolarization and reduce the plateau height. A bridge circuit was used to estimate the input resistance (220 +/- 7 M omega) and time constant 20 +/- 7 ms) of these cells. Two-microelectrode experiments have revealed small differences in the electrotonic potentials recorded simultaneously at two different sites within a single cell. The equations for a linear, short cable were used to calculate the electrical constants of relaxed, single atrial cells: lambda = 921.3 +/- 29.5 micrometers; Ri = 118.1 +/- 24.5 omega cm; Rm = 7.9 +/- 1.2 x 10(3) omega cm2; Cm = 2.2 +/- 0.3 mu Fcm-2. These results and the atrial cell morphology suggest that this preparation may be particularly suitable for voltage-clamp studies.

Effect of RP 30356 on the fast inward Na current in frog atrial fibres

The effect of a new drug: RP 30356, on action potential and fast inward Na current of frog atrial fibres was studied using the double sucrose gap voltage clamp technique. The drug (10(-4)M) blocked the action potential without noticeably altering the resting membrane potential. RP 30356 inhibited the fast sodium conductance without changing the selectivity of the Na channel. The time to peak of the inward current was not significantly altered by the drug whereas the rate of the Na current inactivation pi h was slowed. The steady state inactivation membrane potential relationship of the Na system was shifted toward negative membrane potentials by the drug. It was also shown that in Ringer solution the reactivation time constant of the Na system (pi re) was faster at more negative membrane potentials than at more positive ones. The drug increased pi re; the increase was more marked when the membrane was depolarized. The block of Na conductance by the drug was partially removed by increasing the membrane potential to values more negative than -70 mV. The drug did not leave the channel or left it very slowly in the absence of clamp stimulation. The inhibition of the Na conductance by RP 30356 was also frequency-dependent. The data suggest that RP 30356 might be effective in the control of cardiac arrhythmias since it mainly decreased the excitability of depolarized fibres.

Voltage clamp measurements of sodium channel properties in rabbit cardiac Purkinje fibres

1. Voltage clamp studies of the excitatory sodium current, INa, were carried out in rabbit cardiac Purkinje fibres using th two-micro-electrode technique. Previous work has shown the rabbit Purkinje fibre to have relatively simple morphology (Sommer & Johnson, 1968) and electrical structure (Colatsky & Tsien, 1979a) compared to other cardiac preparations. 2. Non-uniformities in membrane potential were kept small by reducing the size of INa to less than 50 microA/cm2 of total membrane surface area through prepulse inactivation or removal of external sodium, Nao. Temporal resolution was improved by cooling to 10-26 degrees C. These adjustments did not greatly alter the measured properties of the sodium channel. 3. Under these conditions, sodium currents were recorded satisfying a number of criteria for adequate voltage control. Direct measurement of longitudinal non-uniformity using a second voltage electrode showed only small deviations at the time of peak current. 4. The properties of the sodium channel were examined using conventional protocols. Both peak sodium permeability, PNa, and steady-state sodium inactivation, h infinity, showed a sigmoidal dependence on membrane potential. PNa rose steeply with small depolarizations, increasing roughly e-fold per 3.2 mV, and reaching half-maximal activation at -30 +/- 2 mV. The h infinity -V curve had a midpoint of -74.9 +/- 2 mV and a reciprocal slope of 4.56 +/- 0.13 mV at temperatures of 10-19.5 degrees C, and showed a dependence on temperature, shifting to more negative potentials with cooling (approximately 3 mV/10 degrees C). Recovery of INa from inactivation in double pulse experiments followed a single exponential time course with time constants of 108-200 msec at 19 degrees C for holding potentials near -80 mV. No attempt was made to describe the activation kinetics because of uncertainties about the early time course of the current. 5. These data predict a maximum duration for INa of less than 1-2 msec and a maximum peak current density of about 500 microA/cm2 under physiological conditions, i.e. 37 degrees C and 150 mM-Nao. This current magnitude is sufficient to discharge the membrane capacitance at rates comparable to those measured experimentally (311 +/- 27 V/sec, Colatsky & Tsien, 1979a). 6. The limitations of the method are discussed. The major problem is the longitudinal cable delay which limits the speed of voltage control. This makes it difficult to separate the activation of INa from the decay of the capacity transient for potentials positive to -15 mV. 7. It is concluded that the approach described is valid for measurements of sodium currents in the potential range where action potentials are initiated, making it possible to study cardiac sodium channels in an adult mammalian preparation which is free of enzymatic treatment.

The initial inward current in spherical clusters of chick embryonic heart cells

The rapid inward sodium current in spherical clusters of 11-d-old embryonic chick heart cells, ranging in size between 65 and 90 micron diameter, was studied using the two-microelectrode voltage-clamp technique. Using these preparations, it was possible to resolve the activation phase of the rapid inward current for potentials negative to -25 mV at 37 degrees C. The rapid inward current exhibited a voltage and time dependence similar to that observed in other excitable tissues. It was initiated at potential steps more positive than -45 mV. The magnitude of the current reached its maximum value at a potential of approximately -20 mV. The measured reversal potential was that predicted by the Nernst equation for sodium ions. The falling phase of the current followed a single exponential time-course with a time constant of inactivation, tau h, ranging between 2.14 ms at -40 mV and 0.18 ms at -5 mV. The time constant of inactivation, tau h, determined by a single voltage-step protocol was compared to the constant, tau c, determined by a double voltage-step protocol and no significant different between the two constants of inactivation was found. Furthermore, the time constants of inactivation and reactivation at the same potential in the same preparation were similar. The results of this study demonstrate that the sodium current of heart cells recorded at 37 degrees C can be described by Hodgkin-Huxley kinetics with speeds approximately four times faster than the squid giant axon at 15 degrees C.

Effects of ervatamine chlorhydrate on cardiac membrane currents in frog atrial fibres

1 The effects of a new alkaloid, ervatamine, on transmembrane currents of frog atrial fibres were studied by the double sucrose gap voltage clamp technique. 2 Ervatamine (2.8 x 10(-4) M) blocked the action potential without altering the resting membrane potential. 3 The alkaloid depressed the peak INa. The dissociation constant for the blocking effect of ervatamine on gNa fast was 2.35 X 10(-5) M with a one to one relationship between the drug molecule and the Na channel. Ervatamine did not alter the apparent equilibrium potential for Na, as well as the activation and inactivation parameters of gNa fast. This suggests that the alkaloid inhibitory effect on gNa can be attributed to a reduction in gNa. 4 Ervatamine prolonged the rate of reactivation of the Na system. It inhibited gNa in a frequency-dependent manner; this indicates that the alkaloid acts on open Na channels i.e. that the drug has to enter the channel or cross the membrane to produce the block. 5 Ervatamine inhibited Ina slow which occurs in Ca-free, tetrodotoxin-containing solutions and moderately decreased ICa which occurs in Na-free solutions. The drug increased the background K current (IK1) and did not alter the time-dependent K current (Ix1). 6 The present study shows that ervatamine is a good inhibitor of both fast and slow gNa. This drug also shares some common electrophysiological properties with antiarrhythmic drugs namely: the frequency-dependent inhibition of the fast gNa and the ability to slow the reactivation of the Na carrying system.

Ontogenetic increase of the maximal rate of rise of the chick embryonic heart action potential. Relationship to voltage, time, and tetrodotoxin

The maximal rate of rise (Vmax) of the embryonic chick ventricular action potential increased about 2-fold from 112 +/- 6 V/sec and 113 +/- 5 V/sec on the 4th and 6th incubation days, respectively, to 217 +/- 7 V/sec on the 12th incubation day. Neither the steady state inactivation (hinfinity) of Vmax, the temperature-dependent shift of hinfinity, nor the time constant for recovery (tauRec) of Vmax changed significantly when Vmax had doubled. Inhibition of Vmax by tetrodotoxin (TTX) was the same in hearts from the 4th, 6th, and 18th incubation days. The relation between Vmax and TTX concentration could be described by a one-to-one binding curve with an apparent dissociation constant of 2 X 10(-8) M. The increase of Vmax, a valid measure of the early inward Na+ current (iNa) during the rising phase of the cardiac action potential, can be attributed to an elevated maximal Na+ conductance (-gNa) during ontogenesis. These results indicate that the physicochemical properties of the gNa unit responsible for the rapid rising phase of the action potential and of the TTX binding site associated with it may remain constant during a period of embryonic development when -gNa increased significantly.

Effect of phentolamine, alprenolol and prenylamine on maximum rate of rise of action potential in guinea-pig papillary muscles

Effects of phentolamine (13.3, 26.5 and 53.0 micron), alprenolol (3.5, 7.0 and 17.5 micron) and prenylamine (2.4, 4.8 and 11.9 micron) on the transmembrane potential were studied in isolated guinea-pig papillary muscles, superfused with Tyrode's solution. 1. Phentolamine, alprenolol and prenylamine reduced the maximum rate of rise of action potential (.Vmax) dose-dependently. Higher concentrations of phentolamine and prenylamine caused a loss of plateau in a majority of the preparations. Resting potential was not altered by any of the drugs. Readmittance of drug-free Tyrode's solution reversed these changes induced by 13.3 micron of phentolamine and all conconcentrations of alprenolol almost completely but those induced by higher concentrations of phentolamine and all concentrations of prenylamine only slightly. 2. .Vmax at steady state was increased with decreasing driving frequencies (0.5 and 0.25 Hz) and was decreased with increasing ones (2--5 Hz) in comparison with that at 1 Hz. Such changes were all exaggerated by the above drugs, particularly by prenylamine. 3. Prenylamine and, to a lesser degree, phentolamine and alprenolol delayed dose-dependently the recovery process of .Vmax in premature responses. 4. .Vmax in the first response after interruption of stimulation recovered toward the predrug value in the presence of the above three drugs. The time constants of recovery process ranged between 10.5 and 15.0s for phentolamine, between 4.5 and 15.5s for alprenolol. The time constant of the main component was estimated to be approximately 2s for the recovery process with prenylamine. 5. On the basis of the model recently proposed by Hondeghem and Katzung (1977), it is suggested that the drug molecules associate with the open sodium channels and dissociated slowly from the closed channels and that the inactivation parameter in the drug-associated channels is shifted in the hyperpolarizing direction.

The effect of hypoxia on the refractoriness of the canine ventricular muscle

The effect of hypoxia on the relationship between refractoriness and transmembrane action potential was studied in isolated canine papillary muscle preparations. Test stimuli were applied at variable intervals after every 10 to 15 driving stimuli, and action potentials were recorded through an intracellular microelectrode located near the stimulus site. During hypoxia of 30 to 90 min duration, induced by nitrogenization of the perfusate, action potential duration (APD) was tremendously decreased in association with decline in the amplitude and rising velocity. The resting potential was also reduced. Concurrently the effective refractory period (ERP) was decreased, but its decrement was smaller than that of the APD. The total refractory period (TRP) was, on the other hand, prolonged. In consequence, not only the TRP but the ERP as well fairly outlasted full repolarization as a result of the hypoxia. The duration of the relative refractory period (RRP) was increased during hypoxia as the result of the changes in ERP and TRP, and graded local responses were registered frequently in the early phase of RRP. These facts were thought to be related to the genesis of arrhythmias in the ischemic heart.

Inward current of the rabbit sinoatrial node cell

Inward currents of the rabbit sinoatrial node cell were examined in voltage-clamp experiments using the two-microelectrode technique. A fast and slow inward current could be separated from each other. The slow inward current was blocked by Mn and D 600, but it was insensitive to TTX. On the contrary, the fast inward current was blocked by TTX, but not by Mn and D 600. Both the fast and slow inward currents disappeared on Na removal. The fast inward current system was fully inactivated by holding the membrane potential positive to -40 to -50 mV, while the slow inward current system was recorded with the holding potential up to -20 mV. The voltage dependence of the inactivation of the 2 inward current systems and their dependence on [Na]0 suggests that the rising phase of the spontaneous action potential in the S-A node cell is produced mainly by Na current through the slow inward current system.

Antiarrhythmic and electrophysiological effects of CH-200

A new antiarrhythmic drug CH-200, 5-phenacyl-thieno[3,2-c]yridinium, was compared with procainamide and lidocaine in a two-stage coronary ligation arrhythmia model for its efficacy and electrophysiological properties. CH-200 suppressed arrhythmia in beagle dogs more effectively than did procainamide and lidocaine. The antiarrhythmic effects of CH-200 and procainamide developed slowly and lasted longer than those of lidocaine. Electrophysiological studies with CH-200 showed that it decreased max dV/dt of the action potential. This effect was dependent on the heart rate: the higher the rate, the stronger the effect. CH-200, procainamide and lidocaine prolonged the effective refractory period and this effect seemed to be responsible for suppressing the arrhythmia after coronary ligation. CH-200 and procainamide increased the frequency of ventricular pacemaker activity, while lidocaine decreased it. These effects appear to be unimportant for the antiarrhythmic effects.

Reconstruction of the action potential of ventricular myocardial fibres

1. A mathematical model of membrane action potentials of mammalian ventricular myocardial fibres is described. The reconstruction model is based as closely as possible on ionic currents which have been measured by the voltage-clamp method.2. Four individual components of ionic current were formulated mathematically in terms of Hodgkin-Huxley type equations. The model incorporates two voltage- and time-dependent inward currents, the excitatory inward sodium current, i(Na), and a secondary or slow inward current, i(s), primarily carried by calcium ions. A time-independent outward potassium current, i(K1), exhibiting inward-going rectification, and a voltage- and time-dependent outward current, i(x1), primarily carried by potassium ions, are further elements of the model.3. The i(Na) is primarily responsible for the rapid upstroke of the action potential, while the other current components determine the configuration of the plateau of the action potential and the re-polarization phase. The relative importance of inactivation of i(s) and of activation of i(x1) for termination of the plateau is evaluated by the model.4. Experimental phenomena like slow recovery of the sodium system from inactivation, frequency dependence of the action potential duration, all-or-nothing re-polarization, membrane oscillations are adequately described by the model.5. Possible inadequacies and shortcomings of the model are discussed.

A study of the ion selectivity and the kinetic properties of the calcium dependent slow inward current in mammalian cardiac muscle

1. A voltage-clamp method combining a single surcose gap and two intracellular micro-electrodes was used to measure membrane currents in ventricullar myocardial fibres. 2. The adequacy of the voltage-clamp method is demonstrated by comparing the total current, It, across the gap with the voltage difference, delta V, between the two intracellular micro-electrodes, i.e. another independent way of measuring membrane currents. With both current measurements the slow inward current, Is, shows the same voltage- and time-dependences. 3. The sensitivity of the slow inward current to variation in external Ca and Na concentrations was investigated systematically. The reversal potential of the slow inward current was sensitive to variation of both ion species. 4. From the reversal potential measurements relative permeabilities of the conductance channels of the slow inward current were estimated as PCa/PNa approximately 1/0-01 and PCa/PK approximately 1/0-01 by means of the constant field equation. 5. The activation and inactivation kinetics of the slow inward current were explored in detail and related to the plateau of the action potential.

The effect of heart rate on the membrane responsiveness of rabbit atrial muscle

The maximum rate of rise of action potentials in myocardial fibers of the rabbit atrium decreases with an increase in heart rate. This decrease of the dV/dt max is accompanied by a decrease of the diastolic transmembrane potential prior to the moment of activation (take-off potential). Comparison of the membrane responsiveness curve (relation between dV/dt max and take-off potential) as measured by varying the extracellular potassium concentration at a fixed rate of stimulation, with the effect of changes in the frequency of stimulation on dV/dt max and take-off potential made clear that the fall in dV/dt max after a sudden increase in heart rate was stronger than could be explained by the concomitant decrease of the take-off potential alone. This implicates that the membrane responsiveness itself is heart rate dependent. A possible explanation for this observation is that when heart rate is increased the active Na/K pump is not able to maintain the intracellular concentration of Na and K at the original level. Acceleration of the heart will lead to an intracellular loss of potassium and a gain of sodium. The first causes a diminishment of the diastolic membrane potential which according to the membrane responsiveness curve is attended with a decrease of the dV/dt max. The second results in a decrease of the sodium concentration gradient and therefore in a further reduction of the dV/dt max. This hypothesis was confirmed by experiments with ouabain added to the perfusion fluid. Ouabain, which is known to inhibit the Na/K pump, caused a decrease of both the take-off potential and dV/dt max that was completely comparable with the effects of an increase of the frequency of stimulation. In addition, observation of the time course of the changes in dV/dt max and membrane "resting" potential after a sudden change in the rate of stimulation, gave support to the electrogenic concept of the active Na/K pump in cardiac muscle.

Kinetics of channel gating in excitable membranes

To a certain degree, the events underlying the action potential are understood. For each ion to which the membrane is permeable, there exists an equilibrium potential, whose physical origin is in the activity gradient for that ion. The membrane potential is then the summation of these individual ionic batteries each weighted according to the membrane permeability to that ion. In nerve fibres at rest the potassium permeability is relatively high and the membrane potential is near to the equilibrium potential for potassium. During excitation there is a transitory increase in the permeability to sodium and a slower transitory increase in the potassium permeability also. The membrane potential, therefore, temporarily moves to a value near to the sodium equilibrium potential and then returns to its resting value. Less well understood are the molecular mechanisms responsible for these selective changes in membrane permeability, i.e. how it is that a channel (pathway bywhich an ion traverses the membrane) changes its availability for ion passage as a response to a change in the membrane potential. This is the gating process.

Action potential changes under varied [Na+]0 and [Ca2+]0 indicating the existence of two inward currents in cells of the rabbit atrioventricular node

In the perfused rabbit heart,the upstroke of the transmembrane action potential of fibers of the atrioventricular (AV) node presents two distinct components. The first depends strongly on extracellular sdoium concentration, but the degree to which it is activated is influenced by extracellular calcium, as indicated by the correlation between its Vmax and [Ca2+]0. The second component depends on calcium and sodium concentrations and is blocked by Mn ions. An analysis comparing action potentials from atrial (A), atrionodal (AN), and nodal (N) fibers shows that the second component of the upstroke of the action potential contributes 12%, 27%, and 34% to the total depolarization. The results suggest that the upstroke of the nodal action potential results from the activation of two inward currents, as in ordinary cardiac fibers. We postulate that (1) the degree of steady state inactivation of gNa is larger in N than in A fibers because of the low resting potential of the former, and (2) the contribution of the second channel to the upstroke depends on the time course of the previous depolarization and the potential level at which this component is activated.

Voltage clamp analysis in isolated cardiac fibres as performed with two different perfusion chambres for double sucrose gap

Voltage clamp experiments were performed on isolated frog atrial trabeculae disposed in 2 different perfusion chambres for double sucrose gap. In one of the perfusion chambres, a "liquid partition system" (L.P.S.) was used; in the other, a vaseline sealing (V.P.S.) method was used to separate the various fluid compartments. From the linear electrical properties, the elements of an apparent equivalent electrical circuit were calculated. The apparent nodal capacity was significantly larger in fibres disposed in the "liquid partition system" than those disposed in the "vaseline partition system". The apparent "error factor" resulting from the presence of the series resistance was larger in the L.P.S. than in the V.P.S. The apparent "error factor" becomes relatively large when the membrane conductance increases, such as during the flow of the inward current. A rapid desactivation of the peak inward current was found on bringing the command potential back to the resting potential in the V.P.S. This was not found in the L.P.S., indicating better voltage control with the V.P.S. than with the L.P.S. Transmembrane microelectrode recordings during voltage clamp experiment in the V.P.S. indicated satisfactory voltage control during the flow of the peak inward current. Adequate voltage control is lost when notches or irregularities appear on the current traces.

[Transmembrane inward currents during excitation of the heart (author's transl)]

During excitation of the myocardial cell 2 transmembrane inward currents occur. The initial fast Na current is responsible for the upstroke of the normal action potential. The slow inward current is triggered at a threshold potential of about -40 mV and causes the plateau phase of action potential. Under physiological conditions Ca ions are the main charge carriers of the slow inward current. Both inward currents are mediated by 2 membrane channels which are independent from each other. The normal excitability of the myocardial cell depends upon the availability of the fast Na channel but the transmembrane Ca supply will be determined by the Ca conductance of the slow channel. After inactivation of the fast Na channel the excitability of the myocardial cell does not disappear completely. In this situation the slow inward current can mediate action potentials (so called Ca action potentials). The slow inward current can be considered as the predominant mediator of the excitation process in the pacemaker cells of the sinoatrial node and the av node. Specific inhibitors of the slow membrane channel (verapamil, D 600, Ni, Co, and Mn ions) block the transmembrane Ca current leading to excitation contraction uncoupling. The excitation process will be impaired only if it is carried by the slow inward current alone. Specific inhibitors of the fast Na channel reduce the Na-dependent excitability of the myocardial cell without significant changes of the Ca current. The existence of 2 separate channels in the ventricular myocardium allows selective alteration of contractility without concomitant changes of the Na-dependent excitation process or, conversely, the reduction of excitability whereas the Ca current remains unchanged.

Effect of lidocaine on the early inward transient current in sheep cardiac Purkinje fibers

We used two experimental techniques to study the effect of lidocaine hydrochloride on the early inward transient (sodium) current as it is reflected by the maximum rate of change of action potential phase 0 (Vmax). We assessed the effect of lidocaine on Vmax as Purkinje fibers were slowly depolarized by increasing the extracellular potassium concentration from 4.0 to 16.0 mM; these voltage-dependent effects were compared with lidocaine's effect on membrane responsiveness (which measures both the time and the voltage dependence of Vmax). We also used a voltage clamp technique to establish the effect of lidocaine on the voltage dependence of Vmax by measuring Vmax 800-1000 msec after transmembrane voltage (Vm) had been changed in small steps. We studied the effect of lidocaine on the time course of early inward transient current reactivation by depolarizing the membrane to -25 +/- 5 mv for 100 msec to inactivate this current, clamping Vm to a repolarized test voltage for various periods, and then measuring phase 0 Vmax of action potentials elicited immediately after termination of the voltage clamp. We showed that lidocaine at 5 mg/liter, but not a 1 mg/liter, shifted the steady-state Vmax- Vm relationship to a more negative position on its voltage axis by about 5 mv and markedly slowed the reactivation of the measure early inward transient current.

Electrical characteristics of frog atrial trabeculae in the double sucrose gap

The electrical behavior of small single frog atrial trabeculae in the double sucrose gap has been investigated. The currents injected during voltage clamp experiments did not behave as predicted from the assumption of spatial uniformity of the voltage across a Hodgkin-Huxley membrane. Much of the difference is due to the geometrical complexities of this tissue. Nonetheless, two transient inward currents have been identified, the faster of which is blocked by tetrodotoxin (TTX). The magnitude of the slower transient varies markedly between preparations but always increases in a given preparation with increase of external calcium. The fast transient current traces, at small to intermediate depolarizations, are often marred by the presence of notches and secondary peaks due most probably to the loss of space clamp conditions. In many preparations these could be removed by reducing the current magnitude through application of a partially-blocking dose of TTX. Conversely, in the preparations whose fast transient was fully blocked by TTX, notches and secondary peaks in the slow transient could by induced through increasing calcium concentration and thereby the slow current magnitude. Previously used techniques for the measurement of the reversal potential of the fast inward transient have been shown to be invalid. In so far as they can be measured, the reversal potentials of the fast and slow inward transient are in the same neighborhood, i.e. around 120 mV from rest. The true values may be quite a bit apart. The total charge flow in the capacitive transient was measured for different sized nodes and preparations. From these data and estimates of plasma membrane area per unit trabecular volume, specific membrane capacitances of around 3 muF/cm2 were calculated for small bundles. The apparent ion current densities on this basis are approximately 1/10 of those measured in axons. The capacitive current occurring in small bundles decayed as the sum of at least three exponential functions of time. On the basis of these data and the anomalously large stable node widths, we suggest a coaxial core model of the preparation with the inner elements in series with an additional large extracellular resistance.

Kinetics of inactivation and recovery of the slow inward current in the mammalian ventricular myocardium

In order to study the kinetics of inactivation and recovery of the slow inward current in the mammalian ventricular myocardium voltage clamp experiments using the double sucrose gap technique were performed on isolated trabeculae and papillary muscles of cats. The separation of the slow inward current from the fast Na current was achieved by use of the conditioning clamp procedure. 1. The decay of the Ca current reflects the inactivation which develops due to depolarization. The rate of inactivation depends upon the membrane potential. Excess Ca (8.8 mM) accelerates the inactivation speed indicating that Ca ions not only act as charge carrier of the slow inward current but might influence in addition the kinetics of the slow membrane channel. In the presence of a lowered temperature a deceleration of inactivation (Q10 2.3) occurs. 2 If the membrane is repolarized a recovery process takes place restoring the availability of the slow membrane channel. As the inactivation the recovery rate depends upon the membrane potential. Excess Ca causes an acceleration whereas a decrease in temperature diminishes the recovery speed (Q10 2.3). Consequently, the Ca supply to the myocardial cell can be modified not only by changes of the transmembrane Ca concentration gradient or by an alteration of the Ca conductance of the slow channel but also by changes in the degree of recovery after a preceding Ca current. 3. Compared with the inactivation the recovery proceeds very slowly. Assuming that this slow recovery represents an inherent kinetic feature of the slow channel the kinetics of inactivation and removal of inactivation are not describable by a single inactivation variable (called as f by Reuter, 1973) which is of the Hodgkin-Huxley type. If a second inactivation variable (called as l) would be introduced additionally a formulation of the inactivation-recovery process of the slow membrane channel on the basis of the Hodgkin-Huxley model becomes feasible.

Quantitative description of the sodium conductance of the giant axon of Myxicola in terms of a generalized second-order variable

A variety of experimental observations in Myxicola and other preparations indicate that the sodium conductance, gNa, has properties quite different from those described by the m and h variables of Hodgkin and Huxley. A new quantitative description of the gNa is presented in which the gNa is assumed to be proportional to the fifth power of a generalized second-order variable, i.e., gNa = g'Na times v to the fifth, v = -Kav + K2U = K3, U = K4U + K5v + K6. This model is shown to be able to quantitatively simulate all of the experimentally observed behavior of the gNa. A view of the sodium gate consistent with these kinetics is to imagine it to be composed of five independent subunits, each of the type A eq. B eq. C eq. A where A represents the resting state, B the conducting state, and C the inactivated state. A model in which the subunit is of the type A eq. B eq. C could not simulate the experimental observations. It was concluded that two processes are sufficient to account for all of the behavior of the gNa.

Slow recovery from inactivation of inward currents in mammalian myocardial fibres

1. Reactivation kinetics of the rapid and slow inward currents in ventricular fibres have been assessed by studying the maximum rate of rise ((dV/dt)max) of the action potential upstroke and the duration of the plateau in progressively earlier premature responses. Reactivation of the slow inward current was also studied by voltage clamp technique in sheep and pig ventricular trabeculae.2. The time constant of recovery of (dV/dt)max was voltage dependent and increased from less than 20 msec when the resting membrane potential was more negative than -80 mV to more than 100 msec when the resting membrane potential was between -65 and -60 mV. Similar results were obtained in Purkinje fibres. These results suggest that the time constant for reactivation is slower than the time constant for inactivation of the rapid inward current system by at least one order of magnitude.3. The time constant of recovery of plateau duration was also voltage dependent and increased from 30 to 70 msec as the membrane potential was changed from -85 to -60 mV.4. The reactivation time constant of the slow inward current determined by voltage clamp experiments were similar to the results obtained by analysis of plateau duration. At potentials less negative than -60 mV the time constant of reactivation became progressively longer. Unlike reactivation time constants of (dV/dt)max, the time constants of reactivation of the slow inward current were similar to the time constants of inactivation.5. Our results indicate that (a) in premature action potentials, time as well as voltage are important determinants of (dV/dt)max in myocardial and Purkinje fibres, (b) the kinetics of reactivation of the rapid inward current in cardiac fibres are different from those in nerve and (c) plateau duration of premature action potentials in ventricular myocardial fibres is largely determined by the kinetics of reactivation of the slow calcium inward current.