Life-long stable heart function requires a critical balance of intracellular Ca²⁺. Several ion channels and pumps cooperate in a complex machinery that controls the influx, release, and efflux of Ca²⁺. Probably one of the most interesting and most complex players of this crosstalk is the Na⁺/Ca²⁺ exchanger, which represents the main Ca²⁺ efflux mechanism; however, under some circumstances, it can also bring Ca²⁺ into the cell. Therefore, the inhibition of the Na⁺/Ca²⁺ exchanger has emerged as one of the most promising possible pharmacological targets to increase Ca²⁺ levels, to decrease arrhythmogenic depolarizations, and to reduce excessive Ca²⁺ influx. In line with this, as a response to increasing demand, several more or less selective Na⁺/Ca²⁺ exchanger inhibitor compounds have been developed. In the past 20 years, several results have been published regarding the effect of Na⁺/Ca²⁺ exchanger inhibition under various circumstances, e.g., species, inhibitor compounds, and experimental conditions; however, the results are often controversial. Does selective Na⁺/Ca²⁺ exchanger inhibition have any future in clinical pharmacological practice? In this review, the experimental results of Na⁺/Ca²⁺ exchanger inhibition are summarized focusing on the data obtained by novel highly selective inhibitors.

The health benefits of regular physical exercise are well known. Even so, there is increasing evidence that the exercise regimes of elite athletes can evoke cardiac arrhythmias including ventricular fibrillation and even sudden cardiac death (SCD). The mechanism of exercise-induced arrhythmia and SCD is poorly understood. Here, we show that chronic training in a canine model (12 sedentary and 12 trained dogs) that mimics the regime of elite athletes induces electrophysiological remodeling (measured by ECG, patch-clamp, and immunocytochemical techniques) resulting in increases of both the trigger and the substrate for ventricular arrhythmias. Thus, 4 months sustained training lengthened ventricular repolarization (QTc: 237.1±3.4 ms vs. 213.6±2.8 ms, n=12; APD90: 472.8±29.6 ms vs. 370.1±32.7 ms, n=29 vs. 25), decreased transient outward potassium current (6.4±0.5 pA/pF vs. 8.8±0.9 pA/pF at 50 mV, n=54 vs. 42), and increased the short-term variability of repolarization (29.5±3.8 ms vs. 17.5±4.0 ms, n=27 vs. 18). Left ventricular fibrosis and HCN4 protein expression were also enhanced. These changes were associated with enhanced ectopic activity (number of escape beats from 0/hr to 29.7±20.3/hr) in vivo and arrhythmia susceptibility (elicited ventricular fibrillation: 3 of 10 sedentary dogs vs. 6 of 10 trained dogs). Our findings provide in vivo, cellular electrophysiological and molecular biological evidence for the enhanced susceptibility to ventricular arrhythmia in an experimental large animal model of endurance training.

BACKGROUND: Cardiovascular disease is the leading global cause of morbidity and mortality. New technologies are needed to improve mechanistic understanding and inform therapeutic strategies. Human-centric cardiac simulations show great promise; however, existing cellular models can reproduce only a few arrhythmia-driving behaviors and show important discrepancies with experimental data. We aimed to develop a new model overcoming this lack of generality, which markedly limits the predictivity and translational utility of virtual cardiomyocytes. METHODS: We developed T-World, a novel virtual human cardiomyocyte, using data-driven differential equations to describe sex-specific excitation-contraction coupling, mechanical contraction, β-adrenergic signaling, and its effects on cellular targets. The model contains several key innovations, including a new approach to coupling L-type calcium channels and ryanodine receptors, with updated calcium-dependent-inactivation of the former and novel calcium-induced refractoriness and complete reparameterization of the latter. We also redeveloped the sodium-potassium pump and made major improvements to the sodium-calcium exchanger formulation. RESULTS: T-World shows broad agreement with experimental data on rate-dependent action potential (AP), calcium handling, and contraction properties. Extensively validated on independent data, T-World demonstrates strong predictive performance, for example, in drug-induced AP changes. The model reproduces the effects of sympathetic stimulation, including AP duration shortening and increased calcium-transient amplitude and contractility. Importantly, it recapitulates for the first time all key cellular mechanisms driving life-threatening arrhythmias (early and delayed afterdepolarizations, alternans, and steep S1-S2 restitution), including experimentally observed responses to interventions such as sympathetic activation, SERCA (sarco/endoplasmic reticulum Ca ²⁺ ATPase) inhibition, and AP prolongation. Combined with the model’s ability to simulate physiological sex-specific differences in electrophysiology, this revealed increased proclivity of female cardiomyocytes to early afterdepolarizations and steep restitution of AP duration. CONCLUSIONS: T-World is a highly general and predictive open-source computer model of a human ventricular cardiomyocyte, suitable for multiscale research studies investigating determinants of arrhythmogenesis.

To understand the large inter-species variations in drug effects on repolarization, the properties of the rapid (I_Kr) and the slow (I_Ks) components of the delayed rectifier potassium currents were compared in myocytes isolated from undiseased human donor (HM), dog (DM), rabbit (RM) and guinea pig (GM) ventricles by applying the patch clamp and conventional microelectrode techniques at 37 °C. The amplitude of the E-4031-sensitive I_Kr tail current measured at −40 mV after a 1 s long test pulse of 20 mV, which was very similar in HM and DM but significant larger in RM and GM. The L-735,821-sensitive I_Ks tail current was considerably larger in GM than in RM. In HM, the I_Ks tail was even smaller than in DM. At 30 mV, the I_Kr component was activated extremely rapidly and monoexponentially in each studied species. The deactivation of the I_Kr component in HM, DM, and RM measured at −40 mV. After a 30 mV pulse, it was slow and biexponential, while in GM, the I_Kr tail current was best fitted triexponentially. At 30 mV, the I_Ks component activated slowly and had an apparent monoxponential time course in HM, DM, and RM. In contrast, in GM, the activation was clearly biexponential. In HM, DM, and RM, I_Ks component deactivation measured at −40 mV was fast and monoexponential, while in GM, in addition to the fast component, another slower component was also revealed. These results suggest that the I_K in HM resembles that measured in DM and RM and considerably differs from that observed in GM. These findings suggest that the dog and rabbit are more appropriate species than the guinea pig for preclinical evaluation of new potential drugs expected to affect cardiac repolarization.

Sinus node (SN) pacemaking is based on a coupling between surface membrane ion-channels and intracellular Ca²⁺-handling. The fundamental role of the inward Na⁺/Ca²⁺ exchanger (NCX) is firmly established. However, little is known about the reverse mode exchange. A simulation study attributed important role to reverse NCX activity, however experimental evidence is still missing. Whole-cell and perforated patch-clamp experiments were performed on rabbit SN cells supplemented with fluorescent Ca²⁺-tracking. We established 2 and 8 mM pipette NaCl groups to suppress and enable reverse NCX. NCX was assessed by specific block with 1 μM ORM-10962. Mechanistic simulations were performed by Maltsev–Lakatta minimal computational SN model. Active reverse NCX resulted in larger Ca²⁺-transient amplitude with larger SR Ca²⁺-content. Spontaneous action potential (AP) frequency increased with 8 mM NaCl. When reverse NCX was facilitated by 1 μM strophantin the Ca²⁺_i and spontaneous rate increased. ORM-10962 applied prior to strophantin prevented Ca²⁺_i and AP cycle change. Computational simulations indicated gradually increasing reverse NCX current, Ca²⁺_i and heart rate with increasing Na⁺_i. Our results provide further evidence for the role of reverse NCX in SN pacemaking. The reverse NCX activity may provide additional Ca²⁺-influx that could increase SR Ca²⁺-content, which consequently leads to enhanced pacemaking activity.

Vagal nerve stimulation (VNS) has a meaningful basis as a potentially effective treatment for heart failure with reduced ejection fraction. There is an ongoing VNS randomized study, and four studies are completed. However, relatively little is known about the effect of acetylcholine (ACh) on repolarization in human ventricular cardiomyocytes, as well as the effect of ACh on the rapid component of the delayed rectifier K⁺ current (I_Kr). Here, we investigated the effect of ACh on the action potential parameters in human ventricular preparations and on I_Kr in human induced pluripotent stem-cell-derived cardiomyocytes (hiPSC-CMs). Using standard microelectrode technique, we demonstrated that ACh (5 µM) significantly increased the action potential duration in human left ventricular myocardial slices. ACh (5 µM) also prolonged repolarization in a human Purkinje fiber and a papillary muscle. Optical mapping revealed that ACh increased the action potential duration in human left ventricular myocardial slices and that the effect was dose-dependent. Perforated patch clamp experiments demonstrated action potential prolongation and a significant decrease in I_Kr by ACh (5 µM) in hiPSC-CMs. Computer simulations of the electrical activity of a human ventricular cardiomyocyte showed an increase in action potential duration upon implementation of the experimentally observed ACh-induced changes in the fully activated conductance and steady-state activation of I_Kr. Our findings support the hypothesis that ACh can influence the repolarization in human ventricular cardiomyocytes by at least changes in I_Kr.