The Electrogenic Na+/K+ Pump Is a Key Determinant of Repolarization Abnormality Susceptibility in Human Ventricular Cardiomyocytes: A Population-Based Simulation Study

Abstract

Background: Cellular repolarization abnormalities occur unpredictably due to disease and drug effects, and can occur even in cardiomyocytes that exhibit normal action potentials (AP) under control conditions. Variability in ion channel densities may explain differences in this susceptibility to repolarization abnormalities. Here, we quantify the importance of key ionic mechanisms determining repolarization abnormalities following ionic block in human cardiomyocytes yielding normal APs under control conditions. Methods and Results: Sixty two AP recordings from non-diseased human heart preparations were used to construct a population of human ventricular models with normal APs and a wide range of ion channel densities. Multichannel ionic block was applied to investigate susceptibility to repolarization abnormalities. I_Kr block was necessary for the development of repolarization abnormalities. Models that developed repolarization abnormalities over the widest range of blocks possessed low Na⁺/K⁺ pump conductance below 50% of baseline, and I_CaL conductance above 70% of baseline. Furthermore, I_NaK made the second largest contribution to repolarizing current in control simulations and the largest contribution under 75% I_Kr block. Reversing intracellular Na⁺ overload caused by reduced I_NaK was not sufficient to prevent abnormalities in models with low Na⁺/K⁺ pump conductance, while returning Na⁺/K⁺ pump conductance to normal substantially reduced abnormality occurrence, indicating I_NaK is an important repolarization current. Conclusions: I_NaK is an important determinant of repolarization abnormality susceptibility in human ventricular cardiomyocytes, through its contribution to repolarization current rather than homeostasis. While we found I_Kr block to be necessary for repolarization abnormalities to occur, I_NaK decrease, as in disease, may amplify the pro-arrhythmic risk of drug-induced I_Kr block in humans.

Keywords: Na+/K+ pump; cardiac electrophysiology modeling; human; repolarization; sodium-potassium pump; variability.

Figure 1

Action potential biomarkers. Peak membrane potential (V_m Peak); time of peak membrane potential (V_m Time); AP duration (APD) at 40/50/90% repolarization; triangulation and resting membrane potential (RMP). Each biomarker was calculated as defined in the Supplementary Material.

Figure 2

Examples of action potential traces. (A) Normal repolarization. (B) An early afterdepolarization. (C) Repolarization failure.

Figure 3

Population of human ventricular cell models. (A) APs obtained from control experimental recordings (red; n = 62 including three excluded outliers, two with long time to peak and one with long APD90); simulations using the models found to be within the experimental range (blue; n = 568); and the baseline ORd model (black), at 1 Hz pacing. (B) Distributions of each biomarker across the population of models. Dashed lines indicate the experimental range used to determine whether each model is accepted into the population. (C–J) Selected pairs of conductances across the population of models. Plots show scaling values relative to the value of each conductance in the baseline ORd model. Partial correlation coefficients (PCC) are shown for the six pairs of parameters with significant correlations (p < 0.05/36).

Figure 4

APD prolongation of ventricular preparations under 0.05 μM dofetilide (n = 16) compared to population of models under 65% I_Kr block, corresponding to the effects of 0.5 μM dofetilide using IC50 and Hill coefficient determined by Kramer et al. (2013). Models displaying repolarization abnormalities (7/568 models) were excluded from the figure. Black dots are experimental data points, gray dots are outliers (>1.5 times interquartile range from the nearest inner quartile).

Figure 5

Repolarization abnormality susceptibility across the population of models, in response to simulated drug block. (A–C) Percentage of models in the population that displayed repolarization abnormalities in response to combinations of different strength blocks of I_Kr, I_Ks, I_K1, and I_CaL. No model developed repolarization abnormalities in any of the simulations in which I_Kr was not blocked, therefore these simulations are not included in the plots. (D) Biomarker distributions for NS (black), MS (red), and HS (blue) models in the population. (E) Conductance distributions for NS (black), MS (red), and HS (blue) models in the population.

Figure 6

Magnitude of I_Na, I_CaL, I_NaK, I_Kr, I_Ks, and I_K1 during repolarization for HS models in control conditions and under 75% I_Kr block. Each current was averaged from the peak of the upstroke to APD40 (early) and from APD₄₀ to APD₉₀ (late), for each model.

Figure 7

Mechanisms of repolarization abnormality formation. (A): G_CaL and G_NaK values of models in the population classified by susceptibility to repolarization abnormalities. Black: NS. Red: MS. Blue crosses: HS. (B) Incidence of repolarization abnormalities in the HS sub-population (n = 41) at 75 and 90% I_Kr block compared to the same sub-population with G_NaK = 1.0, and to the rest of the population of models. (C) Incidence of repolarization abnormalities in the HS sub-population under different conditions. Left to right: baseline I_Kr block; with intracellular Na⁺ fixed to 7 mM; with intracellular Na⁺ fixed to 20 mM and intracellular K⁺ fixed to 145 mM; and with the electrogenic effect of I_NaK on V_m removed. (D–G) Voltage and selected ionic current traces showing abnormality occurrence in a representative model with original value of G_NaK scaling factor = 0.1675 (solid black) and with G_NaK increased to a scaling factor of 1.0 (red dashes).

Figure 8

Incidence of repolarization abnormalities in the population of models under different levels of I_Kr block, following changes to extracellular potassium and intracellular calcium concentrations. (A) Control conditions ([K⁺]_o = 5.4 mM, 1 Hz pacing). (B) Reduced extracellular potassium concentration ([K⁺]_o = 4.0 mM). (C) Rapid pacing (2.5 Hz) to increase Ca²⁺ loading, followed by sudden decrease to 1 Hz.