A study of the effect of the rate of stimulation on the transient outward current in sheep cardiac Purkinje fibres

Polyaromatic hydrocarbons in pollution: a heart-breaking matter

Air pollution is associated with detrimental effects on human health, including decreased cardiovascular function. However, the causative mechanisms behind these effects have yet to be fully elucidated. Here we review the current epidemiological, clinical and experimental evidence linking pollution with cardiovascular dysfunction. Our focus is on particulate matter (PM) and the associated low molecular weight polycyclic aromatic hydrocarbons (PAHs) as key mediators of cardiotoxicity. We begin by reviewing the growing epidemiological evidence linking air pollution to cardiovascular dysfunction in humans. We next address the pollution-based cardiotoxic mechanisms first identified in fish following the release of large quantities of PAHs into the marine environment from point oil spills (e.g. Deepwater Horizon). We finish by discussing the current state of mechanistic knowledge linking PM and PAH exposure to mammalian cardiovascular patho-physiologies such as atherosclerosis, cardiac hypertrophy, arrhythmias, contractile dysfunction and the underlying alterations in gene regulation. Our aim is to show conservation of toxicant pathways and cellular targets across vertebrate hearts to allow a broad framework of the global problem of cardiotoxic pollution to be established. AhR; Aryl hydrocarbon receptor. Dark lines indicate topics discussed in this review. Grey lines indicate topics reviewed elsewhere.

Mathematical models of the electrical action potential of Purkinje fibre cells

Early development of ionic models for cardiac myocytes, from the pioneering modification of the Hodgkin-Huxley giant squid axon model by Noble to the iconic DiFrancesco-Noble model integrating voltage-gated ionic currents, ion pumps and exchangers, Ca(2+) sequestration and Ca(2+)-induced Ca(2+) release, provided a general description for a mammalian Purkinje fibre (PF) and the framework for modern cardiac models. In the past two decades, development has focused on tissue-specific models with an emphasis on the sino-atrial (SA) node, atria and ventricles, while the PFs have largely been neglected. However, achieving the ultimate goal of creating a virtual human heart will require detailed models of all distinctive regions of the cardiac conduction system, including the PFs, which play an important role in conducting cardiac excitation and ensuring the synchronized timing and sequencing of ventricular contraction. In this paper, we present details of our newly developed model for the human PF cell including validation against experimental data. Ionic mechanisms underlying the heterogeneity between the PF and ventricular action potentials in humans and other species are analysed. The newly developed PF cell model adds a new member to the family of human cardiac cell models developed previously for the SA node, atrial and ventricular cells, which can be incorporated into an anatomical model of the human heart with details of its electrophysiological heterogeneity and anatomical complexity.

The Purkinje cell; 2008 style

Cardiac Purkinje fibers, due to their unique anatomical location, cell structure and electrophysiologic characteristics, play an important role in cardiac conduction and arrhythmogenesis. Purkinje cell action potentials are longer than their ventricular counterpart, and display two levels of resting potential. Purkinje cells provide for rapid propagation of the cardiac impulse to ventricular cells and have pacemaker and triggered activity, which differs from ventricular cells. Additionally, a unique intracellular Ca2+ release coordination has been revealed recently for the normal Purkinje cell. However, since the isolation of single Purkinje cells is difficult, particularly in small animals, research using Purkinje cells has been restricted. This review concentrates on comparison of Purkinje and ventricular cells in the morphology of the action potential, ionic channel function and molecular determinants by summarizing our present day knowledge of Purkinje cells.

Electrophysiological effects of ginseng and ginsenoside Re in guinea pig ventricular myocytes

Panax ginseng is a folk medicine with various cardiovascular actions; however, its underlying mechanisms of action are not well known. In the present study, we examined the effects of ginseng and its main component, ginsenoside Re, on action potentials and membrane currents recorded from isolated guinea pig ventricular myocytes with the whole-cell patch clamp technique. Ginseng (1 mg/ml) shortened the action potential duration in a rate-dependent manner. Ginseng depressed the L-type Ca2+ current (I(Ca-L)) in a mode of both tonic block and use-dependent block, and enhanced the slowly activating component of the delayed rectifier K+ current (I(Ks)). Ginsenoside Re 3 microM exhibited similar electrophysiological effects to those of 1 mg/ml ginseng, but of slightly smaller magnitude. Inhibition of I(Ca,L) and enhancement of I(Ks) by ginsenoside Re appear to be one of the main electrophysiological actions of ginseng in the heart, although contributions from other ingredients should be considered.

Effect of acidosis on transient outward potassium current in isolated rat ventricular myocytes

The effect of acidosis on the transient outward K(+) current (I(to)) of rat ventricular myocytes has been investigated using the perforated patch-clamp technique. When the holding potential was -80 mV, depolarizing pulses to potentials positive to -20 mV activated I(to) in subepicardial cells but activated little I(to) in subendocardial cells. Exposure to an acid solution (pH 6.5) had no significant effect on I(to) activated from this holding potential in either subepicardial or subendocardial cells. When the holding potential was -40 mV, acidosis significantly increased I(to) at potentials positive to -20 mV in subepicardial cells but had little effect on I(to) in subendocardial cells. The increase in I(to) in subepicardial cells was inhibited by 10 mM 4-aminopyridine. In subepicardial cells, acidosis caused a +8.57-mV shift in the steady-state inactivation curve. It is concluded that in subepicardial rat ventricular myocytes acidosis increases the amplitude of I(to) as a consequence of a depolarizing shift in the voltage dependence of inactivation.

Mechanisms underlying the shortening of the action potential at high and low stimulus rates in sheep Purkinje fibres

The shortening of the action potential of sheep Purkinje fibres at high and low rates of stimulation has been investigated. The shortening of the action potential at high rates can be entirely accounted for by incomplete recovery of the plateau conductances between beats. When sufficient time is allowed for membrane recovery, a prolongation of the action potential, rather than a shortening, occurs at high frequencies. The effect on electrical activity of increasing the stimulus frequency is similar to decreasing the bathing K concentration. The possibility of a reduction in the cleft K concentration at high frequencies is discussed. The shortening of the action potential at low rates is unaffected by 4-amino pyridine (a blocker of the transient outward current, i to ) is abolished by D600 (a blocker of the second inward current, i st ) and by a rise in the bathing Ca concentration. It is concluded that i si rather than i to is involved in action potential shortening at low rates. Action potential shortening at low rates is closely associated with declines in the maximum diastolic potential and the pacemaker potential; all of these changes are abolished by ouabain (a blocker of the Na-K pump). It is concluded that the shortening of the action potential at low rates may be the result of a decline in i si , which in turn is dependent on a decline in [Na] i . It is suggested that the rate-dependent changes in the maximum diastolic potential, pacemaker potential and tension are also related to [Na] i .

Activity-dependent changes in the electrical behaviour of sheep cardiac Purkinje fibres

Rate-dependent changes in the electrical activity of sheep Purkinje fibres maintained at 37 °C have been investigated. The duration of the action potential is maximal at a frequency of about 60 min -1 . When the rate is increased above 60 min -1 there is a substantial shortening of the action potential; this occurs abruptly in the first beat at the higher rate although subsequently there can be further changes in duration and these can result in a small prolongation, no change, or a small further shortening of the action potential and can take up to 10 min to reach a steady-state. When the rate is reduced from 60 min -1 there is also a shortening of the action potential but it occurs gradually over several hundred seconds. Action potential duration reaches a minimum value at a rate of about 6 min -1 . 70% of preparations studied showed an increase in duration again at rates below 6 min -1 but duration is always constant at frequencies below about 0.1 min -1 . The maximum diastolic potential is more negative and the pacemaker potential is larger at higher rates of stimulation. When the frequency is raised these variables increase over a time course lasting several hundred seconds. At rates below 60 min -1 the slow changes in action potential duration, maximum diastolic potential and pacemaker potential, after a change in the stimulus frequency, all have similar monoexponential time courses (Ƭ ≈ 3 min) and are accompanied by slow changes in tension production over a similar time course. In Purkinje fibres that exhibit spontaneous activity, rapid stimulation results in overdrive excitation: an acceleration of spontaneous activity when stimulation is ceased.

Effects of activation sequence on monophasic action potential configuration in the dog

The effects of altered activation sequence on the monophasic action potential (MAP) in in situ beating hearts are not known, although its effects on refractory periods are well documented. In nine anesthetized, open-chest dogs, complete atrioventricular block was produced, and the heart was driven by either right ventricular or left ventricular stimulation. The MAPs of the right and left ventricles were recorded by contact electrodes at cycle lengths of 1,000, 800, 600, and 400 ms. The MAP configuration was evaluated with regard to the difference between phase 1 and phase 2 MAP amplitudes and MAP duration at 50 and 90% repolarization. An MAP recorded from the ventricle that was being electrically stimulated was designated an ipsilateral ventricular stimulation, whereas the MAP recorded from the nonstimulated ventricle was termed a contralateral ventricular stimulation. The difference in amplitude and the 50% and 90% MAP durations for ipsilateral ventricular stimulation were consistently larger than for contralateral ventricular stimulation at all cycle lengths tested. Transient outward current did not appear to play a major role in producing such differences in MAP because intravenous treatment with 4-aminopyridine, a blocker of transient outward current, did not affect the configuration of the MAP. These findings provide an insight on the influence of ventricular activation sequence on the shape of the transmembrane action potential.

Activation of protein kinase A partially reverses the effects of 2,3-butanedione monoxime on the transient outward K+ current of rat ventricular myocytes

The transient outward K+ current (Ito) was assessed in single rat ventricular myocytes with the whole-cell patch-clamp technique. Extracellular application of the chemical phosphatase 2,3-butanedione monoxime (BDM) inhibited Ito in a concentration-dependent manner. The IC50 was 14 mM. The on-set of this effect occurred within 20 s after BDM application. Ito recovered almost completely at 2 min after washout of BDM. Application of 20 mM BDM shifted the steady-state inactivation curve of Ito by 9 +/- 2 mV (n = 8) in the negative direction. Addition of 5 microM isoproterenol enhanced Ito amplitude by 16.2 +/- 1.8%. This concentration of isoproterenol partially reversed the BDM-induced inhibition of Ito. Furthermore, application of 10 mM 8-bromoadenosine 3':5'-cyclic monophosphate enhanced the amplitude of Ito and also significantly reversed the BDM-induced suppression of Ito. In contrast, intracellular dialysis with guanosine 3':5'-cyclic monophosphate (cGMP, 1-10 mM) did not affect the BDM-induced inhibition of Ito. The inward rectifier K+ current (Ik1) was relatively insensitive to BDM; i.e., 20 mM BDM inhibited Ito and Ik1 to 35.5 +/- 4.3% (n = 8) and 92.9 +/- 4.0% (n = 4) of the control, respectively. These results indicate that BDM suppressed Ito but not Ik1 of rat ventricular myocytes. We attribute the BDM suppression of Ito to dephosphorylation of the channel protein.

Modulation of action potential duration by inhibition of the transient outward current in sheep cardiac Purkinje fibers

In sheep cardiac Purkinje fibers concentration-dependent inhibition of transient outward current (ito) by 4-aminopyridine (4-AP, 3-1000 mumol/l) was recorded with the two-microelectrode voltage-clamp technique, and correlated effects on action potential duration measured at -70 mV (APD-70) were investigated. Half-maximal inhibition of ito-amplitude occurred at 15 mumol/l 4-AP. The drug exhibited no major effect on voltage-dependent control of inactivation but reduced the maximally available ito-current. At different activation frequencies (0.05 Hz, 0.25 Hz, 1 Hz) an equal amount of ito-current, measured as percentage of the respective control, was inhibited by 4-AP. The APD-70 was on the average increased by 4-AP (3-500 mumol/l) in a concentration-dependent manner up to 151% of control. The drug-induced prolongation, measured as percentage of the respective control, was independent of stimulation frequency (0.05 Hz, 0.25 Hz, 1 Hz). Prolongation of APD-70 was on the average more pronounced for short action potentials (APD-70 < 150 ms: 169% of reference) than for longer ones (APD-70 150-300 ms: prolongation to 117% of reference; 500 mumol/l 4-AP; 0.25 Hz stimulation rate). Few long control signals (APD-70 > 300 ms) were shortened by 4-AP. These results indicate that inhibition of ito-current by appropriate drugs will result in a reduction of inhomogeneity of action potential duration.

Ultra-slow voltage-dependent inactivation of the calcium current in guinea-pig and ferret ventricular myocytes

L-type Ca2+ current, iCa, has been recorded in guinea-pig ventricular myocytes at 36 degrees C using the whole cell patch clamp technique. Intracellular Ca2+ was buffered with ethylenebis(oxonitrilo)tetraacetate (EGTA). An increase in the rate of stimulation from 0.5 to 3 Hz resulted in an abrupt decrease in iCa in the first beat at the high rate, followed by a progressive decrease (tau approx. 7 s) over the next 30 s. The changes were not the result of Ca(2+)-dependent inactivation, because similar changes occurred with either Ba2+ or Na+ as the charge carrier. During 20-s voltage clamp pulses there was an ultra-slow phase of inactivation of Ba2+ or Na+ current through the Ca2+ channel (tau approx. 6 s at 0 mV). This was confirmed by applying test pulses after conditioning pulses of different duration: the Ba2+ current during the test pulse decreased progressively when the duration of the conditioning pulse was increased progressively to 20 s. Ultra-slow inactivation of Ba2+ current was voltage dependent and increased monotonically at more positive potentials. Recovery of Ba2+ current from ultra-slow inactivation occurred with a time constant of 3.7 s at -40 mV and 0.7 s at -80 mV. The gradual decrease in iCa on increasing the rate to 3 Hz may have been the result of the development of ultra-slow voltage-dependent inactivation.

Electrophysiological mechanisms in a canine model of erythromycin-associated long QT syndrome

BACKGROUND

Erythromycin is known to prolong ventricular repolarization and has been associated with the occurrence of torsades de pointes. In this study, we have investigated potential mechanisms in vivo and in vitro for induction of an acquired long QT syndrome by erythromycin.

METHODS AND RESULTS

Ventricular electrograms and endocardial monophasic action potentials were recorded in anesthetized open-chest dogs before and after administration of 40 to 120 mg/kg of erythromycin lactobionate. Conventional microelectrode techniques were used to record transmembrane action potentials in isolated dog Purkinje fibers and papillary muscles. Erythromycin at concentrations > 20 mg/L prolonged action potential duration. At higher concentrations (100 to 200 mg/L), erythromycin induced phase 2 and phase 3 early afterdepolarizations (EADs) both in vivo and in vitro. The effects of erythromycin on repolarization were more marked in Purkinje fibers than in papillary muscle. Pretreatment of Purkinje fibers with erythromycin antagonized the effects of dofetilide, a selective delayed-rectifier potassium channel (IK) blocker. Pretreatment with prazosin or tetrodotoxin had no effect on erythromycin-induced changes in action potential duration.

CONCLUSIONS

These pharmacological studies suggest that erythromycin prolongs repolarization to a large extent by block of IK. In turn, prolongation of action potential duration resulting from erythromycin's actions on IK may promote the development of EADs. The induction of ventricular arrhythmias observed clinically after exposure to erythromycin may be related to the development of EADs. The rarity of occurrence of ventricular arrhythmias suggests that other predisposing factors contribute to the acquired long QT syndrome associated with erythromycin.

Bepridil prolongs the action potential duration of guinea pig ventricular muscle only at rapid rates of stimulation

1. We examined the electromechanical effects of the calcium antagonist, bepridil (1-20 microM), on isolated guinea pig ventricular muscles, driven at various stimulus frequencies (0.1, 0.5, 1, 2 and 5 Hz) in Tyrode's solution containing various K+ concentrations (1.4-43.2 mM). 2. Conventional microelectrode and tension-recording techniques were used. 3. We found that bepridil decreased the maximum upstroke velocity (Vmax) of the action potential with no change in the resting membrane potential (RMP). 4. The former effect depended on both stimulus frequency and the drug concentration used. 5. Bepridil lengthened the duration of the action potential at the level of 25% repolarization (APD25) at the highest frequency (5 Hz), but shortened it at lower frequencies (< or = 2 Hz). 6. The drug also lengthened the APD90 at the highest frequency (5 Hz) but without significant effect at lower frequencies (< or = 2 Hz). 7. Bepridil depolarized the RMP at relatively low extracellular K+ concentrations (< or = 2.7 mM), accompanied by a prolongation of APD90. 8. There were no such effects at much higher K+ concentrations (> or = 5.4 mM), and the drug markedly depressed the Vmax and the action potential amplitude. 9. The drug eliminated the positive staircase phenomenon of twitch contraction, in a concentration-dependent manner. 10. All these findings taken together suggest that bepridil prolongs the action potential duration by inhibiting outward potassium currents (IK and IK1), at rapid rates of stimulation (approximately 300/min), which is comparable to the physiological heart rate of a guinea pig. 11. The prolongation of APD seemed to be secondary to the bepridil-induced reduction of intracellular Ca2+ concentration, [Ca2+]i.

Hyperthyroidism selectively modified a transient potassium current in rabbit ventricular and atrial myocytes

1. Transient outward potassium currents (I(t)) were compared in single cardiac myocytes obtained from normal and hyperthyroid rabbits. Currents were recorded using the suction electrode whole-cell voltage clamp technique. 2. In ventricular myocytes from hyperthyroid animals (at 22 degrees C and a stimulation rate of 0.2 Hz), I(t) was 4- to 5-fold larger than in normal myocytes, in a potential range of -20 to +60 mV. As in normal myocytes, I(t) in hyperthyroid myocytes was calcium insensitive, and was more than 90% suppressed by 2 mM 4-aminopyridine. 3. The increase in I(t) was observed over a wide range of stimulation rates, even at rates sufficiently slow to enable complete reactivation of the I(t) channels. However, there was a major change in the rate dependence of I(t) in hyperthyroid myocytes, with significant I(t) current still present at rates (e.g. 1-2 Hz) at which it is normally completely suppressed. 4. The augmentation of I(t) in the hyperthyroid myocytes could not be accounted for by changes in the voltage dependence or the kinetics of channel activation or inactivation. There was no change in the reversal potential of I(t), implying no change in the selectivity of the channel. 5. Single-channel activity was recorded using the cell-attached mode of recording. In myocytes from hyperthyroid rabbit we observed the following: (a) active patches (often containing two channels) were obtained more frequently in comparison to control; (b) the unitary conductance of the channel was the same; (c) single-channel openings persisted at high stimulation rates. 6. In contrast to hyperthyroid ventricular cells, I(t) in atrial cells from the same hearts was not substantially changed. 7. The rate dependence of I(t) in atrial cells was also unaffected by hyperthyroidism, in contrast to the large changes observed in ventricular cells. Thus, in atrial cells from hyperthyroid hearts the current was totally suppressed at rates of 1-2 Hz, as in euthyroid conditions. 8. Single-channel recordings in the cell-attached mode showed a unitary conductance similar to that found in normal atrial cells. Channel activity was suppressed at 2 Hz, in contrast to hyperthyroid ventricular cells. 9. In conclusion, I(t) is drastically changed in hyperthyroid rabbit ventricle cells. The changes are in the magnitude of the macroscopic current and its rate dependence. Since the unitary conductance is unchanged (and the peak open probabilities are normally high at positive membrane potential(s) the number of active channels in the membrane must be increased. In atrial cells from the same hyperthyroid hearts no changes are apparent.(ABSTRACT TRUNCATED AT 400 WORDS)

Regional variations in action potentials and transient outward current in myocytes isolated from rabbit left ventricle

1. Regional variations in the shape of early repolarization of the action potential have been correlated to differences in transient outward K+ current, I(t), in myocytes isolated from the epicardial surface, the endocardial trabeculae and the papillary muscles of rabbit left ventricles. Temperature was 35 degrees C during whole-cell, and 22-23 degrees C during cell-attached experiments. 2. Membrane resting potentials were very similar regionally. At 0.1 Hz stimulation the action potential plateau amplitude in papillary muscle cells was significantly higher (104.7 mV) than in epicardial cells (96.47 mV). Exposure to 4-aminopyridine or increases in the rate of stimulation from 0.1 Hz to 3.3 Hz increased plateau height and diminished the initial notch on repolarization. These effects were correlated to the magnitude of I(t) in these cells. At low rates of stimulation I(t) caused a 'spike and dome' morphology of the action potential. 3. Voltage clamp experiments confirmed a higher current density of I(t) in epicardial cells (7.66 pA/pF at +20 mV) than in endocardial (6.45 pA/pF) or papillary muscle cells (3.69 pA/pF). I(t) at 35 degrees C was faster and larger than previously reported and individual currents inactivated almost completely during 100 ms pulses to plateau potentials. No differences in the kinetics or voltage dependence of whole-cell currents were found. Thus, the half-inactivation potential was -37.8 mV in cells from all three regions. 4. Cell-attached recordings from endocardial and epicardial cells showed very similar single-channel amplitudes, burst open probabilities and ensemble averages. The peak channel open probability soon after the start of depolarizing voltage clamp pulses did not change between cell types (P approximately 0.8). The slope conductance of I(t) channels was 13.0 pS with an intercept near the resting potential of the cell. 5. We conclude that regional variations in the shape of initial repolarization in cells from rabbit left ventricle are caused by variations in the magnitude of the transient outward K+ current, I(t). Epicardial cells have the largest, and papillary muscle cells the smallest I(t). The differences are not explained by alterations in the whole-cell kinetics or single-channel kinetics and conductance. The most likely explanation for variations in whole-cell current density is therefore a decrease in channel density in endocardium and papillary muscle compared with epicardial tissue. We estimate the density of I(t) channels per cell to be 1495 (one per 3-4 micron2) in epicardium, 1175 (one per 4-5 micron2) in endocardium, and 875 (one per 6 micron2) in papillary muscle cells.

The cytosolic calcium transient modulates the action potential of rat ventricular myocytes

1. The modulation of the action potential by the cytosolic Ca2+ (Cai2+) transient was studied in single isolated rat ventricular myocytes loaded with the acetoxymethyl ester form of the Ca(2+)-sensitive fluorescent dye Indo-1. Stimulation following rest and exposure to ryanodine were used to change the amount of Ca2+ released from the sarcoplasmic reticulum and thus the size of the Cai2+ transient. The Cai2+ transient was measured as the change, upon stimulation, in the ratio of Indo-1 fluorescence at 410 nm to that at 490 nm (410/490) and action potentials or membrane currents were recorded using patch-type microelectrodes. 2. When stimulation was initiated following rest, the magnitude of the Cai2+ transient decreased in a beat-dependent manner until a steady state was reached. The negative staircase in the Cai2+ transient was accompanied by a similar beat-dependent decrease in the duration of the action potential, manifested primarily as a gradual loss of the action potential plateau (approximately -45 mV). A slow terminal phase of repolarization of a few millivolts in amplitude was found to parallel the terminal decay of the Cai2+ transient. 3. The terminal portion of phase-plane loops of membrane potential (Vm) vs. Indo-1 ratio from all of the beats of a stimulus train followed a common linear trajectory even though the individual beats differed markedly in the duration and amplitude of the action potential and Cai2+ transient. 4. When the stimulation dependence of the Cai2+ transient was titrated away with submaximal exposure to ryanodine, the stimulation-dependent changes in the action potential plateau and terminal phase of repolarization were also eliminated. The same effect was noted in cells which, fortuitously, did not show a staircase in the Cai2+ transient following a period of rest. 5. When action potentials were triggered immediately following spontaneous release of Ca2+ from the sarcoplasmic reticulum, which results in a small depolarization at the resting potential, phase-plane loops (Vm vs. Indo-1 ratio) of the spontaneous events followed the same linear trajectory as the terminal phase of repolarization in the loops of the stimulated beats. 6. Following repolarization from brief voltage clamp pulses (to minimize time and voltage-dependent currents associated with depolarization), an inward current was observed that rose and fell in phase with the Cai2+ transient. This current was present at -70 mV, near the resting potential, and at -40 mV, a potential relevant to the plateau of the action potential.(ABSTRACT TRUNCATED AT 400 WORDS)

Characterization of two distinct depolarization-activated K+ currents in isolated adult rat ventricular myocytes

Depolarization-activated outward K+ currents in isolated adult rat ventricular myocytes were characterized using the whole-cell variation of the patch-clamp recording technique. During brief depolarizations to potentials positive to -40 mV, Ca(2+)-independent outward K+ currents in these cells rise to a transient peak, followed by a slower decay to an apparent plateau. The analyses completed here reveal that the observed outward current waveforms result from the activation of two kinetically distinct voltage-dependent K+ currents: one that activates and inactivates rapidly, and one that activates and inactivates slowly, on membrane depolarization. These currents are referred to here as Ito (transient outward) and IK (delayed rectifier), respectively, because their properties are similar (although not identical) to these K+ current types in other cells. Although the voltage dependences of Ito and IK activation are similar, Ito activates approximately 10-fold and inactivates approximately 30-fold more rapidly than IK at all test potentials. In the composite current waveforms measured during brief depolarizations, therefore, the peak current predominantly reflects Ito, whereas IK is the primary determinant of the plateau. There are also marked differences in the voltage dependences of steady-state inactivation of these two K+ currents: IK undergoes steady-state inactivation at all potentials positive to -120 mV, and is 50% inactivated at -69 mV; Ito, in contrast, is insensitive to steady-state inactivation at membrane potentials negative to -50 mV. In addition, Ito recovers from steady-state inactivation faster than IK: at -90 mV, for example, approximately 70% recovery from the inactivation produced at -20 mV is observed within 20 ms for Ito; IK recovers approximately 25-fold more slowly. The pharmacological properties of Ito and IK are also distinct: 4-aminopyridine preferentially attenuates Ito, and tetraethylammonium suppresses predominantly IK. The voltage- and time-dependent properties of these currents are interpreted here in terms of a model in which Ito underlies the initial, rapid repolarization phase of the action potential (AP), and IK is responsible for the slower phase of AP repolarization back to the resting membrane potential, in adult rat ventricular myocytes.

The transient K+ current in rat ventricular myocytes: evaluation of its Ca2+ and Na+ dependence

1. The transient outward K+ current (Ito) was studied in enzymatically isolated rat ventricular myocytes using the whole-cell patch clamp technique. 2. At holding potentials between -100 and -60 mV, depolarizing pulses activated outward current which was composed of transient and maintained components. These components differed from each other in their activation voltage range as well as in their kinetics of inactivation. 3. The transient component, in turn, appeared to be composed of rapidly and slowly inactivating components. Subtraction of ICa from the total current, or nifedipine pre-treatment, eliminated the slowly inactivating component of Ito indicating that the time course of inactivation of Ito may be contaminated by ICa. 4. Reduction of the holding potential from -100 mV to less negative holding potentials reduced all components of Ito, such that at holding potentials of -40 mV, very little or no Ito could be measured. 5. Elevation of [Ca2+]o activated Ito at holding potentials of -40 mV, and substitution of external Ca2+ by Sr2+ suppressed Ito, consistent with findings from other preparations and in support of a Ca(2+)-activated component of Ito. 6. Elevations of [Ca2+]o, however, also shifted the steady-state activation and inactivation parameters of the transient K+ current, such that a greater proportion of Ito channels were activated at the less negative holding potentials. 7. The shifts in the activation and inactivation parameters of the transient outward current were not mimicked by equivalent changes in external Mg2+. 8. Modulators of Ca2+ release from the sarcoplasmic reticulum (SR) such as caffeine and ryanodine suppressed Ito regardless of whether the myocytes were dialysed with low or high concentrations of Ca2+ buffers (EGTA or BAPTA, 0.5-14 mM) or whether nifedipine was used to block ICa. 9. 4-Aminopyridine (4-AP) blocked Ito in a dose-dependent manner, completely suppressing it at 10 mM. Similarly, tedisamil, a new K+ channel blocker, completely and reversibly blocked Ito at 5-20 microM concentrations. 10. TTX (10 microM) or removal of external Na+ decreased Ito, consistent with the idea that a component of Ito was Na+ activated. Both interventions, however, also shifted the voltage dependence of the activation and inactivation of Ito to more negative potentials, such that at -100 mV neither intervention had a significant effect on Ito. Alterations in [Na+]i had no effect on Ito.(ABSTRACT TRUNCATED AT 400 WORDS)

Calcium-sensitive and insensitive transient outward current in rabbit ventricular myocytes

1. A suction pipette whole-cell voltage-clamp technique was used to record membrane currents and potentials of isolated ventricular myocytes from rabbit hearts. 2. Transient outward current (Ito) was activated by voltage steps positive to -20 mV, increasing in amplitude with further depolarization to reach a maximum around +70 mV. The current attained its peak within 10 ms and then it inactivated for 100-200 ms. 3. A large portion of Ito still remained after the calcium current (ICa) was blocked when depolarizing pulses were applied at a frequency of 0.1 Hz or less. Therefore, this current component is referred to as calcium-insensitive Ito or It. 4. It showed voltage- and time-dependent inactivation similar to that observed in Purkinje fibres and other cardiac preparations. 5. The reversal potential of It depended on external K+ concentration, [K+]o, with a slope of 32 mV per 10-fold change in the presence of a normal [Na+]o (143 mM), while the slope was 48 mV per 10-fold change in low [Na+]o (1.0 mM). 6. It was completely inhibited by 2-4 mM-4-aminopyridine. Ito in the presence of ICa was also partially blocked by 4-aminopyridine and the remainder was abolished by 5 mM-caffeine. 7. The calcium-insensitive and caffeine-sensitive Ito differed in their decay rates as well as in their recovery time courses. The former was predominantly available at a slow pulsing rate, while the latter increased its amplitude with high-frequency depolarization. 8. The caffeine-sensitive Ito was inhibited by a blockade of ICa, by replacing Ca2+ with Sr2+, by external application of ryanodine and by internal application of EGTA. This indicates that the current is calcium-sensitive and is dependent on increased myoplasmic Ca2+ through Ca2+ influx via the sarcolemma and Ca2+ release from the sarcoplasmic reticulum. The current is therefore designated as IK, Ca. 9. The physiological functions of IK, Ca and It are indicated by their contribution to ventricular repolarization at fast and slow heart rates, respectively.

Modulation of the cardiac transient outward current by catecholamines

We studied modulation of the transient outward current in single canine Purkinje cells that were voltage clamped under Ca2+-free conditions using the patch pipette. The current showed two exponential time constants of inactivation (48, 352 ms at +58 mV and 53, 325 ms at +78 mV). Norepinephrine or isoproterenol modified the inactivation kinetics of this current without affecting the activation kinetics. The half maximum dose for norepinephrine effect was 1.9 x 10(-8) M and the effect was saturated at 10(-6) M. Norepinephrine or isoproterenol reduced the amplitude of the fast time constant component of inactivation, while increasing the amplitude of the slow component, without changing their time constants. They also increased the amplitude of a time-independent current component. The beta-antagonist, sotalol, blocked the norepinephrine effect on the transient outward current. On the other hand, both activation of adenyl cyclase by forskolin and increase of intracellular cAMP concentration produced the same effect as exposure to norepinephrine. Intracellular perfusion with the catalytic subunit of the cAMP-activated protein kinase reproduced the modulation of the current. These results suggest a role for neurotransmitter regulation of the transient outward current in cardiac cells, perhaps by channel phosphorylation.

Tetrodotoxin-sensitive component in action potential plateau of guinea pig Purkinje fibers: comparison with the papillary muscle

1. The effects of tetrodotoxin on action potentials of isolated guinea pig purkinje fibers were examined and compared the findings with those obtained in the ventricular papillary muscle, by use of conventional microelectrode techniques. 2. Tetrodotoxin (5 x 10(-7)-10(-5) M) decreased the amplitude, overshoot, and maximum upstroke velocity of action potentials of the Purkinje fibers, and shortened the duration of action potential at all levels of repolarization concentration- and stimulus cycle length-dependently. 3. The longer the stimulus cycle length, the greater the shortening by the drug of the action potential duration. 4. In particular, the plateau potential of the Purkinje fibers exposed to tetrodotoxin was remarkably depressed, and which occurred even in case of blockade of K+ conductance, using tetraethylammonium. 5. On the other hand, a high concentration (10(-5) M) of tetrodotoxin did not significantly affect the papillary muscle action potentials. 6. These findings suggest that there is a tetrodotoxin-sensitive component of Na+ current in plateau voltage range of the Purkinje fibers, but little in the papillary muscle, and that the component plays an important role to maintain the plateau of Purkinje fibers action potential.

Use-dependent reduction and facilitation of Ca2+ current in guinea-pig myocytes

1. Action potentials, calcium currents (iCa) and cell contraction have been recorded from single guinea-pig myocytes during periods of stimulation from rest. Voltage clamp was carried out using a single microelectrode. Cell contraction was measured optically. All experiments were performed at 18-22 degrees C. 2. An inverse relationship was observed between cell contraction and action potential duration or iCa. Mixed trains of action potentials and voltage clamp pulses preserved this relationship. Long voltage clamp pulses induced negative 'staircases' of iCa and positive 'staircases' of cell contraction. A facilitation of iCa was observed during repetitive stimulation with clamp pulses of 100 ms duration or less and was accompanied by a decrease in cell contraction. 3. The voltage dependence of inward current staircases was found to depend on Ca2+ entry rather than membrane voltage for long voltage clamp pulses and was not affected by 30 mM-TEA or 50 microM-TTX. Current reduction was greatest at 0 mV (P less than 0.05) when iCa was largest. Changes in cell contraction during pulse trains showed a similar voltage dependence. The time constant of current staircases was only mildly voltage dependent. 4. Interference with normal cellular mechanisms for Ca2+ uptake and release by strontium, 1-5 mM-caffeine and 1 microM-ryanodine increased current staircases and could abolish iCa facilitation with short clamp pulses. 5. Variations in the level of Ca2+-dependent inactivation of iCa can explain many features of the changes in iCa during stimulation after rest. Long clamp pulses (or action potentials) may increase cell Ca2+ loading and inhibit iCa. Short clamp pulses reduce available Ca2+ for cell contraction and this may reflect a lowered myoplasmic Ca2+ level which allows facilitation of iCa.

The effect of heart rate on the membrane currents of isolated sheep Purkinje fibres

1. The effect of the rate of stimulation on the membrane currents of sheep Purkinje fibres at 37 degrees C has been examined. 2. In the diastolic range of potentials, the pacemaker current if was unchanged at different stimulus rates. 3. On going from 6 to 60 min-1 the effect on the background current was similar to that of a decrease in the bathing K+ concentration, because there was a decrease in outward current in the plateau range of potentials and an increase in outward current in the diastolic range of potentials. At rates above 60 min-1 there was extra outward background current at all potentials. Partial block of the Na+-K+ pump by ouabain reduced these changes in background current. 4. On going from 6 to 60 min-1 there was an increase in the slow inward current isi, but at rates above 60 min-1 there was usually a decrease in isi. The decrease in isi at rates greater than 60 min-1 was in part the result of insufficient time between action potentials for complete recovery of isi from inactivation. 5. The effect of these rate-dependent changes in membrane current on electrical activity has been considered. 6. The increase in the pacemaker potential at high rates is likely to be the consequence of the decrease in membrane conductance at diastolic potentials as a result of the changes in background current. 7. The increase in the maximum diastolic potential at high rates is likely to be the result of the extra outward background current at diastolic potentials. 8. The prolongation of the action potential on going from 6 to 60 min-1 is likely to be the result of the increase of isi, as well as the decrease in outward background current at plateau potentials. 9. The shortening of the action potential above 60 min-1 is likely to be the result of the decrease in isi although the extra outward background current may also contribute.

4-Aminopyridine sensitive transient outward current in dog ventricular fibres

(1) The depression of slow inward calcium current (ICa) induced by organic or inorganic inhibitors in voltage clamped dog ventricular preparations unmasks an early transient outward current (Ito). (2) Ito is depressed by 4-aminopyridine (1 mM) in a voltage dependent manner. (3) Ito appears in response to voltage steps above 40 mV (from holding voltage = resting voltage) and increases with raising the amplitude of clamp steps. (4) Within physiological range of membrane voltage Ito is smaller and decays several times faster than ICa. Time course of the decline is approximately exponential (tau = 25 +/- 6 ms at 80 mV above resting voltage). (5) Shifts of the holding voltage by 20 mV from the level of resting voltage alters the peak amplitude of Ito. It is increased by hyperpolarization and reduced by depolarization. (6) The recovery of Ito from inactivation at resting voltage was approximated by a single exponential. Time constant (390 ms) is about 15 times longer than the time constant of inactivation at 80 mV positive to the resting voltage.

Adrenergic modulation of the transient outward current in isolated canine Purkinje cells

Single canine Purkinje cells were voltage clamped under Ca2+-free conditions using the patch pipette. Depolarizing pulses from a holding potential of -42 mV induced a time-dependent rapidly activating-slowly inactivating outward current, which was identified as the transient outward current. The current showed two exponential time constants of inactivation (48,352 msec at +58 mV and 53,325 msec at +78 mV). Norepinephrine in concentrations exceeding 10(-9) M modified the inactivation kinetics of this current without affecting the activation kinetics. The half-maximum dose for norepinephrine effect was 1.9 X 10(-8) M, and the effect was saturated at 10(-6) M. Norepinephrine reduced the amplitude of the fast time constant component of inactivation, while increasing the amplitude of the slow component, without changing their time constants. Norepinephrine also increased the amplitude of a time-independent current component. The beta-antagonist sotalol blocked the norepinephrine effect on the transient outward current. On the other hand, both activation of adenyl cyclase by forskolin and increase of intracellular cAMP concentration produced the same effect as exposure to norepinephrine. These results suggest a role for neurotransmitter regulation of the transient outward current in cardiac cells, perhaps by channel phosphorylation.

Transient outward current prominent in canine ventricular epicardium but not endocardium

Previous studies have denied the presence of a transient outward current (Ito) in ventricular myocardium of dog, sheep, and calf. Using conventional microelectrode techniques, we provide evidence for a significant contribution of Ito to epicardial, but not endocardial, activity of canine ventricular myocardium. The epicardial action potential when compared with that of endocardium shows a smaller phase 0 amplitude, a much more prominent phase 1, and a phase 2 amplitude that is greater than that of phase 0. Epicardial action potentials, unlike those of endocardium, display a "spike and dome" morphology that becomes progressively more accentuated at slower stimulation rates. Using the restitution of phase 1 amplitude as a marker for the process responsible for the spike and dome phenomenon, we were able to delineate two exponential components: 1) a slow component that recovers with a time constant of 350-570 msec and 2) a fast component with a time constant of 41-85 msec. The slow component was largely abolished by 1-5 mM 4-aminopyridine, an Ito blocker. The fast component was diminished by 4-aminopyridine, but it was also inhibited by ryanodine and by Sr2+ replacement of Ca2+, which are interventions known to inhibit the Ca2+-activated component of Ito. Following 4-aminopyridine and Sr2+ or ryanodine treatment, the epicardial responses more closely resembled those of endocardium. In summary, the data demonstrate a marked heterogeneity of active membrane properties in canine ventricular muscle. These observations may aid in understanding the basis for rate-dependent changes in the T wave of the ECG, supernormal conduction in ventricular muscle, the greater sensitivity of epicardium to ischemia, and the rate dependence of some cardiac arrhythmias.

Two types of transient outward currents in cardiac ventricular cells of mice

Ventricular cells of adult mice were prepared by an enzyme digestion procedure. Single channel currents were recorded by a conventional patch clamp technique from cell attached patches. Voltage steps from the holding potential of -80 mV to test potentials between -35 and +50 mV caused openings of two types of outward currents through single channels with the conductances of 27 and 12 pS, respectively. The averaged currents reveal transient time courses for both channel types. The current-voltage relations of both single channel currents were linear over the tested voltage range and intersected the voltage axis at -70 mV. This indicates that both single channel currents are mainly carried by potassium ions. All open and closed times were found to be voltage independent. The 27 pS channel had a mean open time of 3.9 +/- 1.0 ms (n = 8). The closed time consisted of two components with tau 1 = 2.1 +/- 0.2 ms and tau 2 = 50 +/- 19 ms (n = 8). The 12 pS channel had a mean open time of 34.0 +/- 5.2 ms (n = 3) and the two components of the mean closed time have been calculated as tau 1 = 8.3 +/- 2.1 ms and tau 2 = 120 +/- 50 ms (n = 3; all mean +/- SD).

Calcium- and voltage-activated plateau currents of cardiac Purkinje fibers

We have used the two-microelectrode voltage-clamp technique to investigate the components of membrane current that contribute to the formation of the early part of the plateau phase of the action potential of calf cardiac Purkinje fibers. 3,4-Diaminopyridine (50 microM) reduced the net transient outward current elicited by depolarizations to potentials positive to -30 mV but had no consistent effect on contraction. We attribute this effect to the blockade of a voltage-activated transient potassium current component. Ryanodine (1 microM), an inhibitor of sarcoplasmic reticulum calcium release and intracellular calcium oscillations in Purkinje fibers (Sutko, J.L., and J.L. Kenyon. 1983. Journal of General Physiology. 82:385-404), had complex effects on membrane currents as it abolished phasic contractions. At early times during a depolarization (5-30 ms), ryanodine reduced the net outward current. We attribute this effect to the loss of a component of calcium-activated potassium current caused by the inhibition of sarcoplasmic reticulum calcium release and the intracellular calcium transient. At later times during a depolarization (50-200 ms), ryanodine increased the net outward current. This effect was not seen in low-sodium solutions and we could not observe a reversal potential over a voltage range of -100 to +75 mV. These data suggest that the effect of ryanodine on the late membrane current is attributable to the loss of sodium-calcium exchange current caused by the inhibition of sarcoplasmic reticulum calcium release and the intracellular calcium transient. Neither effect of ryanodine was dependent on chloride ions, which suggests that chloride ions do not carry the ryanodine-sensitive current components. Strontium (2.7 mM replacing calcium) and caffeine (10 mM), two other treatments that interfere with sarcoplasmic reticulum function, had effects in common with ryanodine. This supports the hypothesis that the effects of ryanodine may be attributed to the inhibition of sarcoplasmic reticulum calcium release.

Factors affecting intracellular sodium during repetitive activity in isolated sheep Purkinje fibres

1. Intracellular Na+ activity (aiNa) was measured using neutral-carrier Na+-sensitive micro-electrodes in voltage-clamped sheep Purkinje fibres during and after 4 min sequences of depolarizing pulses applied to around 0 mV, at a rate of 2.5 Hz. After trains of pulse duration 50 ms the mean increase in aiNa was 0.65 +/- 0.3 mM (mean +/- S.D., n = 18) whereas with longer pulse durations this rise became progressively smaller. At pulse durations of 300 ms a fall in aiNa was usually found. 2. Recovery of aiNa after a pulse sequence followed a roughly exponential time course. The half-time of decline after a rise in aiNa using 50 ms pulses was 111 +/- 52 s (n = 10), compared with a half-time of 318 +/- 116 s (n = 6) for recovery from a fall in aiNa during a sequence of 300 ms pulses. 3. Application of 2 mM-Cs+ to block the pace-maker current (if) resulted in a decrease in resting aiNa by 0.85 +/- 0.45 mM (n = 6) and an outward current shift. Na+ loading during a depolarizing pulse train was greater in 2 mM-Cs+ than in control solution. The rise in aiNa produced by a train of 50 ms pulses in Cs+ was 1.15 +/- 0.4 mM (n = 10). At short pulse durations in the presence of Cs+, Na+ loading at the end of a pulse train increased as a function of pulse duration, becoming maximal at a duration of approximately 50 ms and then diminishing at longer pulse durations. 4. Application of 2.5 X 10(-5) M-tetrodotoxin (TTX) produced a fall in resting aiNa of 0.55 +/- 0.2 mM (n = 6) and an outward current shift, suggesting that a TTX-sensitive component of steady-state Na+ current exists at potentials in the region -65 to -80 mV. 5. TTX greatly reduced the rise in aiNa during a depolarizing pulse train at all pulse durations tested. A fall in aiNa was now found after trains of shorter pulse duration than in control solution. Similar results were obtained in the absence of TTX if the pulse train was initiated from a holding potential which was positive to the Na+ current (iNa) threshold. When iNa had been blocked, using either TTX or a low holding potential, the mean rise in aiNa after a train of 50 ms pulses was 0.25 +/- 0.2 mM (n = 8).(ABSTRACT TRUNCATED AT 400 WORDS)

Extracellular calcium transients and action potential configuration changes related to post-stimulatory potentiation in rabbit atrium

Extracellular calcium transients were monitored with 2 mM tetramethylmurexide at low calcium (250 microM total, 130 microM free), and action potentials were monitored together with developed tension at normal calcium (1.3 mM) during the production and decay of post-stimulatory potentiation in rabbit left atrial strips. At normal calcium, the contractile potentiation produced by a brief burst of 4 Hz stimulation is lost in three to five post-stimulatory excitations, which correlate with a negative staircase of the late action potential. At low calcium, stimulation at 4 Hz for 3-8 s results in a net extracellular calcium depletion of 5-15 microM. At the subsequent potentiated contraction (1-45 s rest), total extracellular calcium increases by 4-8 microM. The contractile response at a second excitation is greatly suppressed and results in little or no further calcium shift; the sequence can be repeated immediately thereafter. Reducing external sodium to 60 mM (sucrose replacement) enhances post-rest contractions, suppresses the late action potential, nearly eliminates loss of contractility and net calcium efflux at post-rest excitations, and markedly reduces extracellular calcium depletion during rapid stimulation. 4-Aminopyridine (1 mM) markedly suppresses the rapid early repolarization of this preparation at post-rest excitations and the loss of contractility at post-rest stimulation from the rested state; during a post-stimulatory potentiation sequence at low calcium, replenishment of extracellular calcium takes several post-stimulatory excitations. Ryanodine (10 nM to 5 microM) abolishes the post-stimulatory contraction at rest periods of greater than 5 s. If the initial repolarization is rapid, ryanodine suppresses the late action potential, calcium efflux during quiescence is greatly accelerated, and subsequent excitations do not result in an accumulation of extracellular calcium. A positive staircase of the early action potential correlates with the magnitude of net extracellular calcium depletion. These findings demonstrate that negative contractile staircases at post-rest stimulation correspond closely to an accumulation of extracellular calcium at activation and a negative staircase of the late action potential; the correlation of these three events suggests that electrogenic sodium-calcium exchange is the common underlying mechanism.

Transient outward current carried by potassium and sodium in quiescent atrioventricular node cells of rabbits

Single atrioventricular node cells were dispersed by treating the rabbit heart with collagenase. In Tyrode's solution, the cells became rounded, and about 20% of them showed spontaneous activity, whereas the rest remained quiescent. When those quiescent cells were whole-cell clamped, depolarizing clamp pulses from the holding potential of -83 mV induced an outward current which decayed quickly, with a time course similar to that of the transient outward current in the Purkinje fiber. The amplitude of the current became larger when progressively more positive clamp pulses were given from a very negative holding potential. The inactivation time course of this current consisted of two exponential components. Single-channel current recordings from those cells revealed a class of channels that activated more frequently during the initial part of depolarizing pulses. Summation of those unitary currents reproduced activation and inactivation time courses of the macroscopic current well, suggesting that this channel corresponds to the transient outward current. The current-voltage relationship of the channel was linear with the slope conductance of 19.9 +/- 1.8 pS (n = 7), and the reversal potential was near the resting potential of the atrioventricular node cell with 5.4 mM potassium chloride and 134.6 mM sodium chloride in the pipette. The channel was passing mainly potassium ions, but sodium ions also seemed to carry a fraction of the current. The possible role of the transient outward current in the quiescent node cell is discussed.

Rate-related suppression and facilitation of conduction in isolated canine cardiac Purkinje fibers

Previous studies have shown that antegrade conduction through damaged His Purkinje tissue may be suppressed following rapid ventricular pacing (overdrive suppression of conduction). We studied this phenomenon using isolated Purkinje fibers placed in a three-chamber bath. Superfusates for the left, middle, and right segments of the fiber were altered to produce action potentials that resembled those of normal bundle branch, damaged His bundle, and normal His bundle, respectively. To produce anisotropic conduction, the left segment of the fiber was adjusted to be three to four times longer than the right segment. Pacing the right segment at intermediate rates produced maximal action potential amplitude in the middle segment and 1:1 right-to-left conduction, whereas pacing at faster or slower rates reduced action potential amplitude and produced block. Pacing the left segment at fast or slow rates also reduced action potential amplitude in the middle segment, but conduction was maintained (anisotropy). After rapid or slow left segment pacing, action potential amplitude in the middle segment remained low during subsequent right segment pacing at intermediate rates, and transient block occurred (overdrive or underdrive suppression of conduction). With time, action potential amplitude normalized and conduction resumed. In other more severely depressed preparations, conduction block occurred even at intermediate right segment pacing rates prior to left segment pacing. Under these conditions, pacing the left segment at intermediate rates increased action potential amplitude in the middle segment and temporarily permitted 1:1 conduction at intermediate right segment pacing rates (overdrive facilitation of conduction).(ABSTRACT TRUNCATED AT 250 WORDS)

A model of cardiac electrical activity incorporating ionic pumps and concentration changes

Equations have been developed to describe cardiac action potentials and pacemaker activity. The model takes account of extensive developments in experimental work since the formulation of the M.N.T. (R. E. McAllister, D. Noble and R. W. Tsien, J. Physiol., Lond. 251, 1-59 (1975)) and B.R. (G. W. Beeler and H. Reuter, J. Physiol., Lond . 268, 177-210 (1977)) equations. The current mechanism i K2 has been replaced by the hyperpolarizing-activated current, i f . Depletion and accumulation of potassium ions in the extracellular space are represented either by partial differential equations for diffusion in cylindrical or spherical preparations or, when such accuracy is not essential, by a three-compartment model in which the extracellular concentration in the intercellular space is uniform. The description of the delayed K current, i K , remains based on the work of D. Noble and R. W. Tsien ( J. Physiol., Lond . 200, 205-231 (1969 a )). The instantaneous inward-rectifier, i K1 , is based on S. Hagiwara and K. Takahashi’s equation ( J. Membrane Biol . 18, 61-80 (1974)) and on the patch clamp studies ofB. Sakmann and G. Trube ( J. Physiol., Lond . 347, 641-658 (1984)) and of Y. Momose, G. Szabo and W. R. Giles ( Biophys. J . 41, 311a (1983)). The equations successfully account for all the properties formerly attributed to i K2 , as well as giving more complete descriptions of i K1 and i K . The sodium current equations are based on experimental data of T. J. Colatsky ( J.Physiol., Lond. 305, 215-234 (1980)) and A. M. Brown, K. S. Lee and T. Powell ( J.Physiol., Lond. , Lond. 318, 479-500 (1981)). The equations correctly reproduce the range and magnitude of the sodium ‘window’ current. The second inward current is based in part on the data of H. Reuter and H. Scholz ( J. Physiol., Lond . 264, 17-47 (1977)) and K. S. Lee and R. W. Tsien ( Nature, Lond . 297,498-501 (1982)) so far as the ion selectivity is concerned. However, the activation and inactivation gating kinetics have been greatly speeded up to reproduce the very much faster currents recorded in recent work. A major consequence of this change is that Ca current inactivation mostly occurs very early in the action potential plateau. The sodium-potassium exchange pump equations are based on data reported by D. C. Gadsby ( Proc. natn. Acad. Sci. U. S. A. 77, 4035-4039 (1980)) and by D. A. Eisner and W. J. Lederer ( J. Physiol., Lond . 303, 441-474 (1980)). The sodium-calcium' exchange current is based on L. J. Mullins’ equations ( J. gen.. Physiol. 70, 681-695 (1977)). Intracellular calcium sequestration is represented by simple equations for uptake into a reticulum store which then reprimes a release store. The repriming equations use the data of W. R. Gibbons & H. A. Fozzard ( J. gen. Physiol . 65, 367-384 (1975 b )). Following Fabiato & Fabiato’s work ( J. Physiol., Lond. 249, 469-495 (I975)), Ca release is assumed to be triggered by intracellular free calcium. The equations reproduce the essential features of intracellular free calcium transients as measured with aequorin. The explanatory range of the model entirely includes and greatly extends that of the M.N.T. equations. Despite the major changes made, the overall time-course of the conductance changes to potassium ions strongly resembles that of the M.N.T. model. There are however important differences in the time courses of Na and Ca conductance changes. The Na conductance now includes a component due to the hyperpolarizing-activated current, i r , which slowly increases during the pacemaker depolarization. The Ca conductance changes are very much faster than in the M.N.T. model so that in action potentials longer than about 50 ms the primary contribution of the fast gated calcium channel to the plateau is due to a steady-state ‘window’ current or non-inactivated component. Slower calcium or Ca-activated currents, such as the Na-Ca exchange current, or Ca-gated currents, or a much slower Ca channel must then play the dynamic role previously attributed to the kinetics of a single type of calcium channel. This feature of the model in turn means that the repolarization process should be related to the inotropic state, as indicated by experimental work. The model successfully reproduces intracellular sodium concentration changes produced by variations in [Na]0, or Na-K pump block. The sodium dependence of the overshoot potential is well reproduced despite the fact that steady state intracellular Na is proportional to extracellular Na, as in the experimental results of D. Ellis J. Physiol., Lond . 274, 211-240 (1977)). The model reproduces the responses to current pulses applied during the plateau and pacemaker phases. In particular, a substantial net decrease in conductance is predicted during the pacemaker depolarization despite the fact that the controlling process is an increase in conductance for the hyperpolarizing-activated current. The immediate effects of changing extracellular [K] are reproduced, including: (i) the shortening of action potential duration and suppression of pacemaker activity at high [K ]; (ii) the increased automaticity at moderately low [K ]; and (iii) the depolarization to the plateau range with premature depolarizations and low voltage oscillations at very low [K]. The ionic currents attributed to changes in Na-K pump activity are well reproduced. It is shown that the apparent K m for K activation of the pump depends strongly on the size of the restricted extracellular space. With a 30% space (as in canine Purkinje fibres) the apparent K m is close to the assumed real value of 1 mM . When the extracellular space is reduced to below 5% , the apparent K m increases by up to an order of magnitude. A substantial part of the pump is then not available for inhibition by low [K] b . These results can explain the apparent discrepancies in the literature concerning the K m for pump activation.

A transient outward current related to calcium release and development of tension in elephant seal atrial fibres

Membrane currents and development of tension in atrial trabeculae from elephant seal hearts were studied using a single sucrose-gap voltage-clamp technique. A transient outward current (Ito) was observed with kinetics, voltage and beat dependence, similar to those of tension. Ito had a bell-shaped voltage dependence similar to that of tension and the slow inward current (Isi). Ito, unlike Isi, showed beat dependence quite similar to developed tension. Increases in [Ca]o, frequency of stimulation, and addition of adrenaline enhanced Ito and developed tension. Ito was suppressed by addition of Mn2+, tetracaine, or by depolarizing pre-pulses (to -40 mV for 250 ms). Caffeine at low concentrations (1 mM) blocked beat dependence of Ito. At higher concentrations (greater than 5 mM) caffeine suppressed the activation of Ito, phasic tension, and the second component of the birefringence signal (related to Ca2+-releasing activity of the sarcoplasmic reticulum (s.r.]. Similar to Isi phasic tension and Ito, the voltage dependence of the second component of the birefringence signal was bell-shaped. Our studies suggest that activation of Ito is related to triggered release of Ca2+ from the s.r. which generates the phasic tension. An excitation-contraction coupling scheme is presented which incorporates these findings and suggests that Ito may be responsible for shorter action potentials found in atrial fibres.

The surprising heart: a review of recent progress in cardiac electrophysiology

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Cat ventricular muscle treated with D600: effects on calcium and potassium currents

In single sucrose-gap experiments on cat ventricular muscle strands stimulated with 300 ms pulses at 0.33 Hz, 2 microM-D600 reduced the Ca-dependent slow inward current (ICa) by 50% within 5 min and more than 90% in 90-120 min. The late outward current was reduced by up to 30%. During the exposure to D600, Ca channels could be unblocked by hyperpolarizing pulses and blocked again by stimulation with depolarizing pulses. Since the degree of unblocking depended on voltage, and the degree of blocking depended on stimulation pattern, ICa amplitude could be rapidly manipulated to probe the dependence of K conductance on ICa. Under control conditions, an increase in stimulation rate from 0.02 to 1 Hz reduced ICa by 15% and increased the late outward current by a smaller amount. During exposure to D600, a similar intervention provoked a 60% reduction in ICa, but a control-like increase in the late outward current. Two other series of experiments failed to disclose a link between ICa and K conductance: when a block of Ca channels was reimposed following their unblocking, the outward currents were independent of ICa amplitude. Unblock-block experiments also provided information on the extent of steady-state ICa at 0 mV. The fraction of Ca channels not undergoing inactivation appears to be very small. During full D600 block, the inward peak of the current wave form is broad and very much delayed in comparison with pre-drug currents or currents on the first pulse following unblocking. A similar wave form was recorded in D600-treated ventricular myocytes from cat but not guinea-pig. The likely explanation is that D600 unmasks a small transient outward current in cat ventricle.

Changes in the electrical activity of dog cardiac Purkinje fibres at high heart rates

Rate-dependent changes in the electrical activity of dog Purkinje fibres have been studied. At high rates of stimulation the rate of repolarization is greater, the action potential is shorter, the maximum diastolic potential is increased, the pace-maker potential is reduced in amplitude, and on cessation of rapid stimulation there can be a suppression of spontaneous activity. After an increase of the stimulus frequency there is an abrupt shortening of the action potential, which can be attributed to incomplete recovery of the plateau currents; this is followed by a progressive decline in action potential duration over the next several hundred seconds. The factor responsible for the slow changes in duration could also be responsible for the accompanying increase in maximum diastolic potential because this develops along a similar time course. These slow changes in electrical activity have been investigated with the phase-plane technique. They are the result of an increase in the net outward current over a wide range of potentials (approximately -10 to approximately -90 mV) during the repolarization phase of the action potential. In voltage-clamp experiments background current has been observed to be strongly rate dependent: the background current during a test voltage-clamp pulse after a train of action potentials is more outward at higher stimulus frequencies. When the frequency is increased, background current slowly becomes more outward over several hundred seconds, and this change therefore occurs along the appropriate time course to explain the slow alteration in electrical activity under these conditions. The extra outward background current at high rates is relatively independent of membrane potential in the range from -110 to -40 mV (more circumstantial evidence indicates that this range may extend to at least +10 mV); this potential dependence is similar to that of the Na-K-pump current (Eisner & Lederer, 1980). Strophanthidin and ouabain, agents known to block the Na-K pump, alter both the changes in background current and the slow rate-dependent changes in electrical activity. Although after an increase in rate there is a gradual change in background current that can be explained by an increase in electrogenic Na-K-pump activity, the initial effect of switching rate is to produce a change in current that is consistent with an increase of the extracellular K concentration. A transient increase in the K concentration of restricted extracellular clefts has been recorded under these conditions in dog Purkinje strands by Kline & Kupersmith (1982) using K-sensitive microelectrodes. The effect on electrical activity of these changes is discussed.(ABSTRACT TRUNCATED AT 400 WORDS)

Transient outward current and rate dependence of action potential duration in rabbit cardiac ventricular muscle

A conventional (single sucrose gap) voltage clamp technique was employed to investigate the rate dependence of ionic currents activated in the plateau range of potential in the rabbit ventricular muscle. A transient outward current of increasing amplitude was observed when the period of rest preceding the test voltage clamp pulse was increased from 0.7-60 s. The action potential duration was short when the transient outward current peak (100-150 ms after the voltage clamp pulse beginning) was high under the studied conditions of stimulation (interbeat intervals 0.7-60 s). The rate dependent transient outward current was small at low levels of depolarization above the resting potential (40 mV), had a maximum at some 90-100 mV and decreased at more positive potentials. This current was sensitive to the simultaneous application of 4-aminopyridine and calcium substitution with strontium in the Tyrode solution. It is suggested that the transient outward current is probably responsible for the changes of the action potential duration in rabbit papillary muscles when the interbeat interval varies from some 0.7-60 s.

Voltage- and frequency-dependent block of diltiazem on the slow inward current and generation of tension in frog ventricular muscle

The effects of a new "Ca2+ -antagonist", diltiazem, were studied in frog ventricular myocardium. The single sucrose gap voltage clamp technique was used to control membrane potential and measure membrane current and tension. Diltiazem had no effect on the resting potential, but reduced the slow inward current (Isi) and increased the net outward current. Membrane conductance measurements suggest that the increase in the total membrane current may be due to the suppression of a maintained inward current. The blocking action of diltiazem is frequency-dependent such that at higher frequencies of stimulation, the steady-state amplitude of twitch is reduced. The diltiazem-induced suppression of tension and Isi could be partially reversed by hyperpolarizing the surface membrane for brief intervals. These results suggest that diltiazem blocks Isi in a voltage- and time-dependent manner. These effects of diltiazem on Isi seem to be, in part, responsible for the tension-suppressant effect of the drug.

Heterogeneity of the action potential in isolated rat ventricular myocytes and tissue

The objectives of this study were to measure action potential parameters in enzyme-dissociated, adult rat ventricular myocytes stimulated at 1 Hz, to compare these measurements with those obtained from intact ventricular tissue, and to determine myocyte and tissue responses at stimulus frequencies between 0.1 and 5 Hz. Action potentials were characterized in terms of amplitude, overshoot, resting potential, duration at 25% and 75% repolarization (APD25, APD75), and Vmax. Based on statistical differences in APD25 and APD75, myocyte action potentials were classified as type I (3.1 +/- 1.0 and 21.5 +/- 3.6 msec), type II (7.4 +/- 1.1 and 38.2 +/- 6.7 msec), or type III (14.5 +/- 1.9 and 46.0 +/- 4.1 msec). Action potentials corresponding to type I were found in right ventricular endocardium and right papillary muscles, and those corresponding to types II and III in the left ventricular endocardium [apex, middle (II); base (III)] and left papillary muscles (II). Myocytes and papillary muscles responded to increases in driving rate with nearly identical lengthening of APD25 and shortening of APD75. The one exception was at 5 Hz where a lengthening of the APD75 occurred in some myocytes. We conclude that action potential configuration in rat ventricle is heterogeneous, and that this is reflected by the different types of action potentials in isolated myocytes. It is likely that the magnitude of a transient outward current is a determinant of action potential configuration, and that slow reactivation of this current is a significant factor underlying the stimulus frequency response.

Block of outward current in cardiac Purkinje fibers by injection of quaternary ammonium ions

We have studied the effects of iontophoretic injection of the quaternary ammonium compounds tetraethylammonium (TEA) and tetrabutylammonium (TBA) in cardiac purkinje fibers. We find that TBA(+) is a more effective blocker than TEA(+), but injection of either compound reduces the time-dependent outward plateau currents, transient outward current (I(to)), and the delayed rectifier (I(x)). Our findings provide evidence that these outward cardiac currents are carried by channels that in some respects are pharmacologically similar to squid axon potassium channels. We demonstrate that this procedure is a new tool that can be useful in the analysis of membrane currents in the heart.

Effect of rate-dependent changes in the transient outward current on the action potential in sheep Purkinje fibres

1. The rate of membrane potential change during the initial phase of rapid repolarization of the action potential in sheep Purkinje fibres has been measured by electronic differentiation. 2. Phase-plane analysis has revealed that the potential dependence of rapid repolarization corresponds to the potential range over which the transient outward current , Ito, is recorded in voltage clamp experiments. 3. The initial rate of repolarization is strongly rate-dependent and it is markedly reduced at high rates. 4. The effect of high rates of stimulation on the phase-plane diagram is consistent with a reduction in I. 5. After an increase or decrease in rate there is an abrupt change in the initial rate of repolarization in the first response followed by slower changes over several hundred responses. 6. Recovery of the initial rate of repolarization occurs in two distinct phases after repetitive to activity: there is a rapid, approximately exponential phase of recovery in the first 10 s which is followed by a slower phase of recovery lasting several hundred seconds. 7. The rate-dependent changes in the initial rate of repolarization are abolished by 4-aminopyridine, 0.5-1.0 mmol/l. 8. It is concluded that the rate-dependent changes in the initial phase of repolarization are due to the similar changes in Ito described in a companion paper (Boyett, 1981). 9. Rate-dependent changes in peak tension have been measured and they bear no relationship to the changes in the initial rate of repolarization. It is concluded that the major component of the transient outward current in sheep Purkinje fibers is unlikely to be a Ca-activated current