Rate dependence and regulation of action potential and calcium transient in a canine cardiac ventricular cell model

Abstract

Background: Computational biology is a powerful tool for elucidating arrhythmogenic mechanisms at the cellular level, where complex interactions between ionic processes determine behavior. A novel theoretical model of the canine ventricular epicardial action potential and calcium cycling was developed and used to investigate ionic mechanisms underlying Ca2+ transient (CaT) and action potential duration (APD) rate dependence.

Methods and results: The Ca2+/calmodulin-dependent protein kinase (CaMKII) regulatory pathway was integrated into the model, which included a novel Ca2+-release formulation, Ca2+ subspace, dynamic chloride handling, and formulations for major ion currents based on canine ventricular data. Decreasing pacing cycle length from 8000 to 300 ms shortened APD primarily because of I(Ca(L)) reduction, with additional contributions from I(to1), I(NaK), and late I(Na). CaT amplitude increased as cycle length decreased from 8000 to 500 ms. This positive rate-dependent property depended on CaMKII activity.

Conclusions: CaMKII is an important determinant of the rate dependence of CaT but not of APD, which depends on ion-channel kinetics. The model of CaMKII regulation may serve as a paradigm for modeling effects of other regulatory pathways on cell function.

Figure 1

A, Canine ventricular cell model. Symbols are defined in text and in online-only Data Supplement Table I. B, Ratio of peak SR Ca²⁺ release flux to peak LTCC flux vs test potential (V_test) in model (line in each panel) and experiment (circles in each panel). Model was clamped for 50 ms to −40 mV holding potential, followed by 50-ms test pulse. C, I_Ca(L) I–V relation compared with canine ventricular data. [Ca²⁺]_o = 2.0 mmol/L. D, Left, I_Ca(L) voltage-dependent inactivation compared with experiment. Variable 1-second prepulse (V_pre) was followed by 10-ms holding interval at −50 mV and +80 mV test pulse. Right, I_Ca(L) recovery from voltage-dependent inactivation compared with canine ventricular data.⁵² Prepulse of 350 ms to +20 mV was followed by varying interpulse interval at −40 mV and +20 mV test pulse. Model Ca²⁺-dependent inactivation gates were held constant to isolate voltage-dependent inactivation. E, Peak I_Ks and I_Kr tail currents on repolarization to −40 mV holding potential after 5-second test pulse, compared with canine epicardial data. F, I_to1 I–V relation compared with canine epicardial data.

Figure 2

A, Steady-state AP simulated (top) and measured in canine epicardial myocyte (bottom) for CLs of 8000, 4000, 2000, 1000, 500, and 300 ms. B, Steady-state APD vs CL (adaptation curve) in canine epicardial myocyte (circles) and in model under control conditions (bold line), in presence of CaMKII inhibition (thin line), without I_to1 (dashed line), and without I_Ca(L) (long dashed line). Abbreviations are as defined in text.

Figure 3

A, AP (arrow identifies rapid phase 1 repolarization); B, I_Ca(L) (arrow indicates augmentation); C, I_Kr; and D, I_NaCa computed for steady-state pacing at CL = 8000 ms. Values were computed with (= 0.19 mS/μF, thick line) and without (= 0 mS/μF, thin line) I_to1. Abbreviations are as defined in text.

Figure 4

Currents during the AP in HRd canine (left panels) and LRd guinea pig (right panels) cell models. Steady-state values are shown at CLs of 300 ms (thin line) and 2000 ms (thick line). A, AP; B, I_Ca(L); C, I_Ks (arrow indicates I_Ks accumulation); D, I_Kr. Abbreviations are as defined in text.

Figure 5

A, Simulated (bottom) and measured (top) steady-state CaT for 0.25-, 0.5-, 1-, and 2-Hz pacing. Adapted from Sipido et al. B, CaT_amp-frequency relation for experiment (circles), model under control conditions (line), and in presence of CaMKII inhibition (dashed line). C, Minimal diastolic CaMKII activity (normalized to 3.3 Hz) and ECC gain. ECC gain =time integral of F_rel/F_Ca(L), where F_rel and F_Ca(L) are fluxes through RyRs and LTCCs, respectively, and integration interval, A, equals 1 cycle. Gain is shown for control model (thin line) and in presence of CaMKII inhibition (dashed line). D, PLB phosphorylation vs pacing frequency compared with experimental data. Abbreviations are as defined in text.

Figure 6

Effect of CaMKII inhibition on CaT. A, CaMKII activity; B, [Ca²⁺]_I; C, I_up; D, [Ca²⁺]_JSR (arrows indicate loading); E, I_Ca(L) (arrow indicates peak); and F, I_rel for steady-state pacing at CL = 500 ms (2.0 Hz) in model under control conditions (thick line) and in presence of CaMKII inhibition (thin line). Quantities are shown during AP. Abbreviations are as defined in text.