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(Circulation. 1998;98:2545-2552.)
© 1998 American Heart Association, Inc.

Clinical Investigation and Reports

Novel, Ultraslow Inactivating Sodium Current in Human Ventricular Cardiomyocytes

Victor A. Maltsev, PhD; Hani N. Sabbah, PhD; Robert S. D. Higgins, MD; Norman Silverman, MD; Michael Lesch, MD; Albertas I. Undrovinas, PhD

From the Department of Medicine, Division of Cardiovascular Medicine (V.A.M., H.N.S., M.L., A.I.U.) and Department of Surgery, Division of Cardiac and Thoracic Surgery (R.S.D.H., N.S.), Henry Ford Heart and Vascular Institute, Detroit, Mich.

Correspondence to Albertas I. Undrovinas, PhD, Henry Ford Hospital, Cardiovascular Research, 2799 W Grand Blvd, Detroit, MI 48202-2689.

	Abstract

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Background—Alterations in K⁺ channel expression and gating are thought to be the major cause of action potential remodeling in heart failure (HF). We previously reported the existence of a late Na⁺ current (I_NaL) in cardiomyocytes of dogs with chronic HF, which suggested the importance of the Na⁺ channel in this remodeling process. The present study examined whether this I_NaL exists in cardiomyocytes isolated from normal and failing human hearts.

Methods and Results—A whole-cell patch-clamp technique was used to measure ion currents in cardiomyocytes isolated from the left ventricle of explanted hearts from 10 patients with end-stage HF and from 3 normal hearts. We found I_NaL was activated at a membrane potential of -60 mV with maximum density (0.34±0.05 pA/pF) at -30 mV in cardiomyocytes of both normal and failing hearts. The steady-state availability was sigmoidal, with an averaged midpoint potential of -94±2 mV and a slope factor of 6.9±0.1 mV. The current was reversibly blocked by the Na⁺ channel blockers tetrodotoxin (IC₅₀=1.5 µmol/L) and saxitoxin (IC₅₀=98 nmol/L) in a dose-dependent manner. Both inactivation and reactivation of I_NaL had an ultraslow time course (0.6 seconds) and were independent of voltage. The amplitude of I_NaL was independent of the peak transient Na⁺ current.

Conclusions—Cardiomyocytes isolated from normal and explanted failing human hearts express I_NaL characterized by an ultraslow voltage-independent inactivation and reactivation.

Key Words: heart failure • myocytes • action potentials • saxitoxin • tetrodotoxin

	Introduction

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Mechanisms of ventricular arrhythmias in heart failure (HF) remain poorly understood despite intensive investigation. Studies in isolated ventricular myocardial fibers and cardiomyocytes obtained from failing human hearts demonstrated a prolongation of action potential (AP) duration.^{1 2 3 4} The prolongation was less prominent at rates >150 beats/min¹ and was different in HF of different origins.⁴ AP prolongation was also described in animal HF models.^{1 5 6 7 8} Patch-clamp studies highlighted the importance of alterations in K⁺ channel expression and gating in the prolongation of AP in HF.^{3 4} Given that a delicate balance exists between the inward and outward currents in modulating AP duration, a role for inward currents in the AP prolongation in HF cannot be discounted. A late inward Na⁺ current, which exceeded the duration of a typical AP, has been observed in cardiac ventricular specimens of various mammalian species,^{9 10 11 12 13 14 15} but its existence and potential significance in the human heart has not been reported. We¹⁵ previously showed an increase in the density of a late inward Na⁺ current in ventricular cardiomyocytes of dogs with chronic HF, a finding that may have important implications in the mechanism underlying the impaired repolarization manifested in HF. The present study describes a slowly inactivating and reactivating inward Na⁺ current in ventricular cardiomyocytes of normal and failing human hearts.

	Methods

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Cell Isolation
Cardiomyocytes were isolated from 10 failing explanted human hearts and from 3 normal donor hearts that for technical reasons were not suitable for transplantation. On harvesting, hearts were immersed in ice-cold cardioplegic solution and delivered to the laboratory within 10 minutes. The composition of the cardioplegic solution was (in mmol/L) NaCl 110, CaCl₂ 1.2, MgCl₂ 16, and KCl 16 (pH 7.8 adjusted with Na₂HCO₃). A transmural tissue block was obtained from the left ventricle apex. Five to 8 midmyocardial longitudinal slices, 10x20 mm and 0.5 to 1 mm thick, were obtained with a blade and rinsed in oxygenated trituration solution (TTS) at room temperature. The composition of TTS was (in mmol/L) NaCl 140, KCl 5.4, MgCl₂ 2, glucose 5, and HEPES 10 (pH 7.4). All subsequent procedures were performed in O₂-saturated and constantly triturated TTS at 37°C. To remove interstitial Ca²⁺, specimens were immersed in 100 mL of TTS for 20 minutes, and the procedure was repeated twice. Slices were transferred into TTS containing 25 µmol/L Ca²⁺ and protease type XXIV (Sigma Chemical Co), 4 U/mL for 3 to 10 minutes, and subsequently treated with a mixture of collagenase (Worthington, type II, 291 U/mg) and hyaluronidase (Sigma, type IV-S) 0.5 mg/mL for 15 to 20 minutes. Finally, slices were incubated for 20 minutes with collagenase only. The cell suspension was centrifuged for 1 minute at 100g, and the cardiomyocyte pellet was resuspended in MEM (Sigma) with 200 µmol/L Ca²⁺. The yield of viable, Ca²⁺-tolerant, rod-shaped myocytes varied from 5% to 50%. The mean capacitance of myocytes was 245±17 pF (n=57). The study was approved by the Henry Ford Health System Human Rights Committee (Institutional Review Board).

Voltage-Clamp and Recording Technique
Ion currents were recorded by whole-cell patch-clamp technique¹⁶ (Axopatch 200A patch-clamp amplifier, Axon Instruments Inc). The resistance of the glass patch pipettes (K150F, WPI Inc) was 600 to 800 k (for solutions, see Table 1). The requirement for stable measurement of the small (in the picoampere range) ion currents was a large total patch-pipette cell resistance (5 to 10 G). The leak current was not subtracted during experiments. However, when characteristics of the late Na⁺ current (I_NaL) were assessed, the leak current was obtained after tetrodotoxin (TTX, 25 µmol/L) application and was subtracted from the current traces. Currents were filtered at 2 or 5 kHz (-3 dB, 4-pole low-pass Bessel filter) and digitized at a sampling rate of 10 kHz (Digidata 1200, Axon Instruments). Ion currents were recorded at room temperature (22°C to 24°C). The quality of the voltage clamp was controlled in each cell as previously described.¹⁷

View this table: [in this window] [in a new window]   Table 1. Extracellular (Bath) and Intracellular (Pipette) Solutions Used in the Study

I_NaL was elicited by 2-second membrane depolarizations from a holding potential of -120 mV applied with a stimulation frequency of 0.25 Hz. Current density was determined from the averaged current measured during a time interval of 200 to 220 ms after the onset of membrane depolarization to -30 mV. This time interval was chosen to avoid contribution of the transient Na⁺ current (I_NaT), which is known to be completely inactivated within 200 ms.¹⁸ We evaluated the steady-state balance of ion currents by measuring the net whole-cell current 500 ms after the onset of membrane depolarization.⁷

AP Recording Technique
APs were recorded in amphotericin-B–perforated patch-clamp configuration⁸ at 37°C in solution B₁ (Table 1). Amphotericin-B (0.32 mmol/L) was added to the pipette solution P₁. Cardiomyocytes were stimulated by use of current pulses of 0.1 ms duration with an amplitude of 2.5 times the excitation threshold. AP duration was measured at the membrane potential level of -65 mV (AP_-65).

Chemicals
Saxitoxin (STX) was purchased from Calbiochem Co. All other chemicals, including TTX, were purchased from Sigma Chemical Co.

Data Analysis
Data were analyzed by use of pClamp 6 software (Axon Instruments). Membrane capacitance was measured as previously described.¹⁷ Equilibrium Na⁺ potential (E_Na) at 22°C was calculated in accordance with the Nernst equation: E_Na=25.67 · ln([Na⁺]_o/[Na⁺]_i) (1)

The toxin dose-response curve describing the percentage of the I_NaL block (B%) was determined by a 1-binding-site model:

(2)

Availability (A) for I_NaL was measured by a double-pulse protocol. A prepulse of 2 seconds' duration was followed by a 2-second testing pulse to -30 mV. The relative I_NaL (I_NaLmax/I_NaL) was plotted against the prepulse voltage (V_p) and fitted to the Boltzmann function: A(V_p)=1/[1+e(V_p-V_1/2A)/k_A] (3)

Steady-state activation was evaluated from current-voltage relationships. Maximum Na⁺ conductance (g_max) and reversal potential (V_r) were estimated from a linear fit of the current-voltage relationship in the range from 0 to 60 mV. Na⁺ conductance (g) at a test potential (V_m) was calculated as g=I_peak/(V_m-V_r) (4)

Data points of normalized conductance (G=g/g_max) were fitted to a Boltzmann function: G={1+exp[(V_1/2G-V_m)/k_G]}^-1 (5)

The time course of I_NaL decay was evaluated by a single exponential model: I_NaL(t)=I₀ · e^-t/+I_s (6)

where is the time constant and I_o and I_s are the amplitude and the steady-state component, respectively. I_NaL was fitted within the time interval from 0.2 to 2 seconds after the onset of membrane depolarization.

The recovery time constant (_r) was assessed by a double-pulse protocol. The membrane was depolarized for 2 seconds to -30 mV by a conditioning pulse followed by a recovery period (T). The test pulse to -30 mV was then applied. The amplitude (I_T) of I_NaL elicited by the test pulse was normalized to a maximum I_NaL (I_max) and fitted to a single exponential model: I_T/I_max=1-e^-T/_r (7)

Statistical Analysis
All measurements are reported as mean±SEM. Comparison between mean values was performed with unpaired Student's t test. A value of P<0.05 was considered significant.

	Results

Top Abstract Introduction Methods Results Discussion References

 
Steady-State Current Balance
An inward current limb was found in the steady-state current (I_ss)-membrane potential (V_m) relationship (Figure 1). TTX reversibly shifted the steady-state ion current balance toward outward currents (Figure 1). The difference in I_ss-V_m relationships before and after TTX application (inset in Figure 1) revealed an activation threshold of approximately -60 mV, a maximum of approximately -20 mV, and a reversal potential close to 60 mV. This indicated the presence of an inward current, possibly of Na⁺ origin, that may contribute to the steady-state current balance.

View larger version (22K): [in this window] [in a new window]   Figure 1. Effect of TTX (25 µmol/L) on the net steady-state current density (Iss/C)–membrane potential (Vm) relationship (, before TTX; , after TTX application. Data points are mean±SEM (4 cells each from patients 1, 2, and 3 in Table 2). *P<0.05 at a given Vm. Bath/pipette solutions: B1/P1 (Table 1) at 22°C to 24°C. The leak current was not subtracted. Inset shows difference in current-voltage relationships before and after TTX application.

Current-Voltage Relationship and Na⁺ Selectivity of the Late Current
In the experimental configuration in which K⁺ and Ca²⁺ were blocked, we found a late inward current, I_NaL, that persisted long after I_NaT was completely inactivated. I_NaL was present in cardiomyocytes in 9 of the 10 failing hearts and in 2 of 3 normal donor hearts (Table 2). To rule out the possibility that I_NaL is related to Ca²⁺ current, we also changed [Ca²⁺]_o and measured the I_NaL-voltage relationship. The relationship did not change in response to [Ca²⁺]_o reduction from 1.8 to 0.2 mmol/L (not shown), indicating absence of Ca²⁺ current contribution to I_NaL.

View this table: [in this window] [in a new window]   Table 2. Characteristics of INaL in Human Ventricular Cardiomyocytes

The reversal potential (68.6 mV) measured in 140 mmol/L NaCl was close to Na⁺ equilibrium potential (E_Na=67 mV; Equation 1; Figure 2). The Na⁺ channel is known to be highly permeable for Li⁺ (ionic permeability ratio P_Li+/P_Na+=0.93¹⁹). Indeed, a current with similar density (92±2%, n=4, measured at -30 mV), current-voltage relationship, and an ultraslow decay (=0.54±0.01 seconds at -30 mV, n=4) was detected when Na⁺ was replaced on an equimolar basis by Li⁺ (Figure 2, A and B). The Na⁺ channel is impermeable for Cs⁺ (P_Cs+/P_Na+ <0.016¹⁹). When Na⁺ was replaced by Cs⁺, the current was almost completely abolished (Figure 2, C and D). The Na⁺-selective current, obtained as the difference in current before and after Na⁺ replacement by Cs⁺, was activated near -60 mV, reached its maximum at -30 mV, and reversed at 64 mV (Figure 2C). These data confirmed the Na⁺ origin of I_NaL and indicated that I_NaL was not related to the electrogenic Na⁺/Ca²⁺ exchange because Li⁺ is not transferred by the exchanger.

View larger version (24K): [in this window] [in a new window]   Figure 2. Monovalent cation selectivity of INaL found in human ventricular cardiomyocytes. A, Late current persists after Na+ replacement by Li+. Shown are non–leak-subtracted current traces recorded before and after Na+ replacement (bath solution B2) by Li+ (bath solution B3; patient 6). B, Current-voltage relationships of late current measured in 140 mmol/L NaCl (B2) and in 140 mmol/L LiCl (B3). The leak current was obtained after TTX (25 µmol/L) application and was subtracted from the data points. C, Substitution of Na+ (B2) by Cs+ (B4) completely abolished the current (patient 5). D, Current-voltage relationship for the Na+-selective current, derived by subtraction of the current before and after Na+ replacement by Cs+. A and C, The voltage-clamp protocol is indicated in the insets, and dashed lines show zero current. Current traces were low-pass filtered (25 Hz) and truncated. B and D, Vr reversal potential evaluated as an intercept with the voltage axis of linear regression (dashed lines) of data points within the voltage range from 0 to 60 mV. Pipette solution P3. Composition of bath and pipette solutions is shown in Table 1. Characteristics of patients are given in Table 2.

Blockade of I_NaL by Specific Toxins
To distinguish between nerve, skeletal, and cardiac Na⁺ channel isoforms, 2 toxins, TTX and STX, were used.²⁰ The cardiac isoform is 10³ times less sensitive to TTX (50% of maximum blockade, IC₅₀=1 to 5 µmol/L) and almost 10² times less sensitive to STX (IC₅₀=100 nmol/L) than nerve and skeletal muscle isoforms.²¹ Both toxins reversibly blocked I_NaL (Figure 3). The IC₅₀ values were 1.53 µmol/L and 98 nmol/L for TTX and STX, respectively, as anticipated for the heart Na⁺ channel.

View larger version (29K): [in this window] [in a new window]   Figure 3. Blockade of INaL by specific toxins. A and B, Non–leak-subtracted whole-cell current recordings showing block of INaL by different concentrations of TTX (patient 5, Table 2) and STX (patient 3), respectively. Voltage-clamp protocols are shown in insets. C and D, Dose-response block of INaL by TTX and STX, respectively. Solid lines represent fit to a single-site binding model (Equation 2). Data points are mean±SEM, n=7 for TTX and n=9 for STX. Current recordings were low-pass filtered (25 Hz) and truncated. Bath/pipette solutions: B2/P2 (Table 1).

Density, Activation, and Inactivation of I_NaL
The density of I_NaL and the midpoint of the availability curve varied widely among patients, whereas the slope (k_A) remained nearly the same (Table 2; Figure 4). Steady-state activation for I_NaL (Figure 4B) was characterized by the midpoint potential (V_1/2G=-34.3±5.6 mV, n=4, patient 5) and the slope (k_G=7.1±0.7 mV).

View larger version (20K): [in this window] [in a new window]   Figure 4. Voltage dependency of steady-state availability and activation of INaL. Example of INaL traces (A) and steady-state availability and activation curves (B) obtained in patient 3. B, Voltage-dependent availability () A(Vp) (Equation 3) for INaL. Voltage-clamp protocol for A(Vp) shown in inset to Figure 4A. Voltage-dependent activation (), A(Vm) (Equation 5) was determined from INaL-voltage relationship (see Methods for details). Bath/pipette solutions: B2/P2 (Table 1).

I_NaL was almost completely inactivated after 2 seconds of membrane depolarization (I_s<0.05xI_o; Equation 6; Figure 5A). The decay time constant of I_NaL was voltage independent within a voltage range from -50 to 40 mV (Figure 5B). The time constant was similar in patients and normal donor hearts (Table 2).

View larger version (16K): [in this window] [in a new window]   Figure 5. Voltage-independent inactivation time course of INaL. A, Representative current traces recorded at -40, -20, and 0 mV are shown along with single exponential fits (Equation 6; patient 5, Table 2). Dashed lines indicate zero current. B, Mean values measured at different membrane potentials (data from 22 cells obtained from 5 patients). Solid line represents linear regression. Slope and correlation coefficient (r2) are indicated. Current recordings were low-pass filtered (25 Hz) and truncated. Bath/pipette solutions: B2/P2 (see Table 1).

The time course of I_NaL recovery from inactivation was assessed at several holding potentials (V_h) by the conventional double-pulse method (Figure 6). The data were well fitted by a single exponential function (Figure 6B). Recovery was slow (_r=0.56±0.04 seconds, V_h=-120 mV, n=7) and, unlike I_NaT,^{21 22} was not voltage dependent (Figure 6C).

View larger version (23K): [in this window] [in a new window]   Figure 6. Recovery of INaL from inactivation. A, Representative INaL traces elicited by a double-pulse voltage-clamp protocol shown in inset (patient 8). B, Normalized current values plotted against conditioning pulse duration. Solid line indicates an exponential fit (Equation 7) to the INaL recovery time course at holding potential Vh=-140 mV. C, Average data on r determined in 7 cells (donor heart 12, Table 2) at various Vh. Dashed line represents linear regression; slope and correlation coefficient (r2) are shown. Bath/pipette solutions: B2/P2 (see Table 1).

I_NaL Is Independent of Peak I_NaT
We investigated the relationship between I_NaL and I_NaT using a protocol shown in the inset of Figure 7A. A portion of I_NaT was inactivated by a short (t=2 to 5 ms) depolarization prepulse to 50 mV preceding a test pulse to -30 mV. The amplitude of the I_NaT elicited by the test pulse was dependent on the prepulse duration. Increase in the prepulse duration gradually reduced the I_NaT peak, but I_NaL remained unchanged (Figure 7), indicating the independence of I_NaL and I_NaT.

View larger version (17K): [in this window] [in a new window]   Figure 7. Independence of INaL and INaT. A, A family of current traces elicited in response to the voltage-clamp protocol shown above. Various INaT peaks were obtained by inactivation of a portion of Na+ channels by a short (t=2 to 5 ms, shown by arrows) depolarization prepulse to 50 mV preceding a test pulse to -30 mV. Zoom shows INaL 200 ms after test pulse start. B, Relationship between INaT peak and INaL obtained from experiment shown in Figure 7A (patient 8, Table 2). Dashed line represents linear regression; slope and correlation coefficient (r2) are indicated. Bath/pipette solutions: B2/P2 (Table 1). Traces shown in A were filtered at 5 kHz and 1 kHz (zoom).

TTX Decreases AP Duration
To test the possible physiological importance of I_NaL, we assayed the effect of TTX on AP duration (Figure 8). TTX decreased AP duration at all 4 stimulation frequencies (0.2, 0.5, 1, and 2 Hz) in cells isolated from a normal donor heart (Table 3). In cardiomyocytes from failing hearts, TTX reduced AP duration and abolished early afterdepolarizations (Figure 8B).

View larger version (18K): [in this window] [in a new window]   Figure 8. Effect of TTX (1.5 µmol/L) on AP in ventricular cardiomyocytes from normal donor (A) and failing human heart (B) (hearts 13 and 1, respectively, Table 2). AP duration reduction by TTX for cardiomyocyte shown in Figure 8B was 75%. Stimulation frequency 0.2 Hz, 37°C, bath/pipette solutions B1/P1 plus amphotericin-B). EADs indicates early afterdepolarizations.

View this table: [in this window] [in a new window]   Table 3. Effect of TTX (1.5 µmol/L) on AP Duration (AP-65mV)

	Discussion

Top Abstract Introduction Methods Results Discussion References

 
Sodium Channel Underlying I_NaL
A late Na⁺ current, but not similar to the I_NaL reported in the present study, was previously described in the mammalian heart.^{9 10 11 12 13 14} In contrast to our finding, I_NaL has not previously been detected in human cardiomyocytes.^{21 22} The lack of data on I_NaL in human cardiomyocytes may be explained by differences in experimental conditions. In previous studies, to improve voltage control, 5 mmol/L of Na⁺ on both membrane sides was used. In such a voltage-clamp configuration, it is impossible to distinguish I_NaL from the experimental noise (our unpublished observation).

Voltage-dependent Na⁺ channels can be distinguished by their toxin sensitivity. Compared with neuronal and skeletal muscle Na⁺ channels,¹⁹ I_NaL has low sensitivity for both TTX and STX, a property of the cardiac Na⁺ channel clone hH1²³ underlying I_NaT. The I_NaL IC₅₀ for TTX was comparable to that measured for I_NaT in human atrial cardiomyocytes (1.1 µmol/L²¹). The position and shape of the steady-state activation and availability curves for I_NaL are also similar to those documented for the human I_NaT,^{21 22} which suggests that I_NaL is produced by an Na⁺ channel isoform that is similar to hH1. Because the most striking difference between I_NaL and I_NaT was found in their inactivation, the difference, if any, between isoforms would probably be within the intracellular III-IV linker²⁴ but not within the channel vestibule.²⁵

Possible Mechanisms of I_NaL
Bursting Mode of Na⁺ Channel
The mechanism of late currents was believed to be a bursting behavior of the transient Na⁺ channel²⁶ that can function in different gating "modes,"^{27 28} which might have an implication in I_NaL. However, in contrast to the voltage-dependent slow mode, the inactivation and reactivation of I_NaL was found to be voltage independent (Figures 5 and 6).

New Isoform
Given that I_NaL and I_NaT were independent of each other (Figures 6 and 7), it is interesting to speculate that I_NaL may not be simply the result of multiple reopenings of a small fraction of the transient Na⁺ channel but might rather reflect the activity of another channel subtype. A new Na⁺ channel isoform was suggested to produce a late Na⁺ current in rat ventricular myocytes.¹³ Recently, multiple Na⁺ channel subtypes with a slowly inactivating component were found in sensory neurons²⁹ and in human coronary smooth muscle cells.³⁰ Discovery of a second Na⁺ channel gene subfamily, hNa_v2.1, in the human heart³¹ provides an additional evidence for greater evolutionary divergence among voltage-dependent Na⁺ channels and suggests that other Na⁺ channel gene subfamilies may exist that may include the I_NaL reported in the present study.

Na⁺ Channel Modification
Na⁺ channel inactivation can be modulated by channel protein phosphorylation.³² In contrast to the neuronal Na⁺ channel, no modulatory effect by the ß-subunit on the kinetics of the cardiac Na⁺ channel was detected.³³ Transient Na⁺ channel inactivation is dependent on the channel environment, which includes the sarcolemma and underlying cytoskeleton. Sarcolemmal partition of the ischemic phospholipid metabolite lysophosphatidylcholine¹² or modification of the F-actin–based cytoskeleton³⁴ produced transition of some Na⁺ channels into a bursting mode. Although all of above-discussed mechanisms might play a contributory role in the origin of I_NaL, the exact mechanisms remain to be elucidated.

Study Limitations and Implications
One of the limitations of the study is the small number of normal donor hearts. The availability of normal human hearts is always limited for laboratory investigation. I_NaL density as well as the position of the steady-state availability curve varied markedly from patient to patient (Table 2). Reasons for the observed variation may include the cause of the disease, severity of the hemodynamic dysfunction, differences in drug therapy before transplantation, and age and sex differences. Specifically, treatment with mexiletine and amiodarone might attenuate I_NaL.³⁵ Even though we observed a tendency for increased I_NaL density in cardiomyocytes isolated from hearts with dilated cardiomyopathy (Table 2), the small sample size did not allow us to correlate the density of I_NaL with a distinct cause of HF. To study mechanisms of I_NaL in the failing heart, a reliable animal model might be helpful. Indeed, using a dog model of chronic HF, we found that I_NaL underlies AP prolongation (References 8 and 15^{8 15} and our unpublished data).

Role of I_NaL in Determining AP Duration
The clinical importance of AP prolongation was recently shown in the SWORD trial.³⁶ It was demonstrated that d-sotalol, a potassium channel blocker, increases the incidence of sudden death in patients with left ventricular dysfunction. The ventricular AP plateau is maintained by a delicate balance between inward and outward currents.³⁷ In concert with the diminished K⁺ currents in HF,^{3 38} I_NaL would be expected to prolong AP duration by shifting this balance in favor of inward currents (Figure 1). We showed that I_NaL can modulate the duration of AP over a broad range of pacing rates and that it may have a greater impact in HF (Table 3; Figure 8). Accordingly, I_NaL can be implicated in repolarization impairments shown in human HF.¹ Also, I_NaL may play a role in acquired long-QT syndrome and in instances of severe bradycardia.

In conclusion, data from this study demonstrate for the first time the existence of a late Na⁺ current in ventricular cardiomyocytes of normal donor and explanted failing human hearts. The current is characterized by an ultraslow, voltage-independent inactivation and reactivation.

	Acknowledgments

 
The study was supported in part by grants from the National Heart, Lung, and Blood Institute (HL-53819 to Dr Undrovinas and HL-49090 to Dr Sabbah).

Received June 1, 1998; revision received August 7, 1998; accepted August 20, 1998.

	References

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