Chronic heart failure slows late sodium current in human and canine ventricular myocytes: implications for repolarization variability

Abstract

Background: Late Na(+) current (I(NaL)) in human and dog hearts has been implicated in abnormal repolarization associated with heart failure (HF). HF slows inactivation gating of late Na(+) channels, which could contribute to these abnormalities.

Aims: To test how altered gating affects I(NaL) time course, Na(+) influx, and action potential (AP) repolarization.

Methods: I(NaL) and AP were measured by patch clamp in left ventricular cardiomyocytes from normal and failing hearts of humans and dogs. Canine HF was induced by coronary microembolization.

Results: I(NaL) decay was slower and I(NaL) density was greater in failing hearts than in normal hearts at 24 degrees C (human hearts: tau=659+/-16 vs. 529+/-21 ms; n=16 and 4 hearts, respectively; mean+/-SEM; p<0.002; dog hearts: 561+/-13 vs. 420+/-17 ms; and 0.307+/-0.014 vs. 0.235+/-0.019 pA/pF; n=25 and 14 hearts, respectively; p<0.005) and at 37 degrees C this difference tended to increase. These I(NaL) changes resulted in much greater (53.6%) total Na(+) influx in failing cardiomyocytes. I(NaL) was sensitive to cadmium but not to cyanide and exhibited low sensitivity to saxitoxin (IC(50)=62 nM) or tetrodotoxin (IC(50)=1.2 muM), tested in dogs. A 50% I(NaL) inhibition by toxins or passing current opposite to I(NaL), decreased beat-to-beat AP variability and eliminated early afterdepolarizations in failing cardiomyocytes.

Conclusions: Chronic HF leads to larger and slower I(NaL) generated mainly by the cardiac-type Na(+) channel isoform, contributing to larger Na(+) influx and AP duration variability. Interventions designed to reduce/normalize I(NaL) represent a potential cardioprotective mechanism in HF via reduction of related Na(+) and Ca(2+) overload and improvement of repolarization.

Figure 1

Changes in whole-cell late Na⁺ current (I_NaL) in left ventricular cardiomyocytes from normal and failing human and canine hearts. A, representative superimposed whole-cell traces recorded in normal and failing (patient #16, Table 1) human hearts (upper traces) and dog hearts (lower traces) using a voltage-clamp protocol (inset). Current was sent through a low-pass filter (200 Hz) and truncated at -150 pA. Dashed lines represent zero current. Solid lines show exponential fits to the experimental recordings, with decay time constants (τ) of 435 and 929 ms (human) or 351 and 739 ms (canine) for normal and failing myocardium, respectively. B & C, summarized data on τ for human and canine normal and failing hearts. D, average data on I_NaL density in normal and failing canine hearts. Plots show mean ± SEM; n=number of hearts (with number of tested cells in parenthesis); *p < 0.005, normal vs. failing hearts (ANOVA).

Figure 2

Idealized total I_NaL time course calculated using an exponential decay function: I(t)=C*d200ms*exp[- (t-200ms)/τ] with mean time constants (τ) and I_NaL densities, d=I_NaL/C at 24°C (A) and 37°C (B) taken from Fig.1C,D for normal (solid) and failing canine ventricular cardiomyocytes (dashed line) having the same electric capacitance C=200 pF. The curves for 37°C were calculated using Q₁₀ factors: $d_{0ms,37}=d_{0ms,24}*Q_{10d}^{\frac{37-24}{10}}$ and ${\tau }_{37}={\tau }_{24}*Q_{10\tau }^{\frac{24-37}{10}}$ with Q_10d=1.5 for Na_v conductance and Q_10τ=2.2 for late Na_v gating [11, 38]. The absolute difference between normal and failing cells is shown by the dotted lines. The inset shows the relative difference (per cent change) between normal and failing cells. C and D, total electrical charge transferred by I_NaL, assessed as an I_NaL integral at 24°C and 37°C (shown in A and B, respectively).

Figure 3

Pharmacological characterization of I_NaL. A-C: Dose-response curve fitting percent of I_NaL blockade by TTX (A) and STX (B) in canine failing ventricular cardiomyocytes and by Cd²⁺ in human VCs (C, patient #3, Table 1) with a one-binding-site model (see Methods). Insets in A&B: superimposed whole-cell traces recorded in failing canine ventricular cardiomyocytes with different concentrations of tetrodotoxin (TTX) and saxitoxin (STX) in the bath (indicated at the traces), respectively. Traces were truncated at –200 pA, Vh= -140mV, Vt= -30mV, low-pass filter 50 Hz The inset in panel C shows superimposed I_NaL current trace fragments from 200 to 2000 ms before (control) and after application of 1 mM Cd²⁺. D, summarized data on I_NaL current density recorded in control and cyanide-treated cardiomyocytes from human failing hearts. The inset shows representative raw I_NaL traces before (left) and after (right) exposure of cardiomyocytes isolated from a failing heart (patient #2, Table 1) to cyanide for 2 hours. Room temperature was 22–23°C in all experiments.

Figure 4

Chronic HF increases beat-to-beat action potential duration variability (APDV) in canine ventricular myocytes that can be rescued by a reduction of I_NaL. A : Superimposed consecutive APs recorded by perforated patch clamp in representative normal and failing dog ventricular myocytes at 37°C and 0.25 Hz pacing rate. B: Average data on APDV assessed as SD/mean. The figure shows the mean±SEM for 18 failing hearts (61 cells, 908 APs) and 4 normal hearts (7 cells, 141 APs). P was evaluated by ANOVA. C and D: Beat-to-beat APDV in failing canine ventricular myocytes was decreased by electrical neutralization of the late Na⁺ current with a delayed external current (200 ms after AP upstroke) at 37°C and 0.25 Hz pacing rate. C: A representative example of a delayed external current effect on the consecutive APs (overlapped traces) in one cell. D: Average APDV data (mean±SEM) assessed as SD/mean in 3 cells. Statistical comparison was made by Student’s paired t test.