The rationale for repurposing funny current inhibition for management of ventricular arrhythmia

Management of ventricular arrhythmia in structural heart disease is complicated by the toxicity of the limited antiarrhythmic options available. In others, proarrhythmia and deleterious hemodynamic and noncardiac effects prevent practical use. This necessitates new thinking in therapeutic agents for ventricular arrhythmia in structural heart disease. Ivabradine, a funny current (I_f) inhibitor, has proven safety in heart failure, angina, and inappropriate sinus tachycardia. Although it is commonly known that funny channels are primarily expressed in the sinoatrial node, atrioventricular node, and conducting system of the ventricle, ivabradine is known to exert effects on metabolism, ion homeostasis, and membrane electrophysiology of remodeled ventricular myocardium. This review considers novel concepts and evidence from clinical and experimental studies regarding this paradigm, with a potential role of ivabradine in ventricular arrhythmia.

Structural and functional remodeling of the atrioventricular node with aging in rats: The role of hyperpolarization-activated cyclic nucleotide-gated and ryanodine 2 channels

BACKGROUND

Aging is associated with an increased incidence of atrioventricular nodal (AVN) dysfunction.

OBJECTIVE

The aim of this study was to investigate the structural and functional remodeling in the atrioventricular junction (AVJ) with aging.

METHODS

Electrophysiology, histology, and immunohistochemistry experiments on male Wistar Hannover rats aged 3 months (n = 24) and 2 years (n = 15) were performed. Atrio-His (AH) interval, Wenkebach cycle length (WBCL), and AVN effective refractory period (AVNERP) were measured. Cesium (2 mM) was used to block hyperpolarization-activated cyclic nucleotide-gated (HCN) channels, while ryanodine (2 μM) was used to block ryanodine 2 (RyR2) channels. Protein expression from different regions of the AVJ was studied using immunofluorescence. The expression of connexins (connexin 43 and connexin 40), ion channels (Hyperpolarization-activated cyclic nucleotide-gated channel 4 (HCN4), voltage sensitive sodium channel (Nav1.5), and L-Type calcium channel (Cav1.3)), and calcium handling proteins (RyR2 and sarco/endoplasmic reticulum calcium ATPaset type 2a (SERCA2a)) were measured. Morphological characteristics were studied with histology.

RESULTS

Without drugs to block HCN and RyR2 channels, there was prolongation of the AH interval, WBCL, and AVNERP (P < .05) with aging. In young rats only, cesium prolonged the AH interval, WBCL, and AVNERP (P < .01). Ryanodine prolonged the AH interval and WBCL (P < .01) in both young and old rats. Immunofluorescence revealed that with aging, connexin 43, HCN4, Nav1.5, and RyR2 downregulate in the regions of the AVJ and connexin 40, SERCA2a, and Cav1.3 upregulate (P < .05). Aging results in cellular hypertrophy, loosely packed cells, a decrease in the number of nuclei, and an increase in collagen content.

CONCLUSION

Heterogeneous ion channel expression changes were observed in the AVJ with aging. For the first time, we have shown that HCN and RyR2 play an important role in AVN dysfunction with aging.

Contributions of cardiac "funny" (f) channels and sarcoplasmic reticulum Ca2+ in regulating beating rate of mouse and guinea pig sinoatrial node

The aim of this study was to investigate the effects on spontaneous beating rate of mouse atrial preparations following selective block of cardiac "funny" (f) channels, I(f), and/or suppression of sarcoplasmic reticulum (SR) function in the absence and presence of β-adrenoceptor stimulation. ZD7288 [to block I(f)] caused a substantial reduction (222 ± 13 bpm) in beating rate from 431 ± 14 to 209 ± 14 bpm, ryanodine alone (to block SR Ca(2+) release) reduced beating rate by 105 ± 11 bpm, with subsequent addition of ZD7288 further reducing rate by 57 ± 9 bpm. Cyclopiazonic acid (CPA) alone (to inhibit Ca(2+) reuptake by the SR) reduced beating rate by 148 ± 13 bpm with subsequent addition of ZD7288 further reducing rate by 79 ± 12 bpm. In additional experiments measuring Ca(2+) transients in the SA node region using Rhod-2, effects of ivabradine and ZD7288 on rate were again substantially reduced after CPA. Effects of CPA alone on rate developed much more slowly than effects on Ca(2+) transient amplitude. ZD7288, ivabradine, and CPA reduced the slope and maximum response of the log(concentration)-response curves for effects of isoprenaline on beating rate. Very little response to isoprenaline remained after treatment with CPA followed by ZD7288. Similar changes in isoprenaline log(concentration)-response curves were seen in guinea pig preparations. These observations are consistent with a role for Ca(2+) released from the SR in regulating I(f) and therefore beating rate of SA node preparations; there appear to be additional contributions of SR-derived Ca(2+) to effects of β-adrenoceptor stimulation on beating rate that are independent of I(f).

Effects of tertiapin-Q and ZD7288 on changes in sinoatrial pacemaker rhythm during vagal stimulation

Heart rate slowing produced by cardiac parasympathetic (vagal) stimulation is thought to be the result of modulation of the acetylcholine-activated K(+) current (IK,ACh) and the pacemaker current (If) in sinoatrial (SAN) pacemaker cells. However, the contribution of these and other ion currents to vagal slowing is controversial. Here, we examined the contributions of IK,ACh and If to vagal slowing in 15 isolated, vagal-innervated preparations of guinea-pig atria, using 300 nM tertiapin-Q (TQ) and 2 μM ZD7288 to obtain full and substantial block of these currents, respectively. Blocking IK,ACh alone reduced atrial rate responses to 10-s trains of regular vagal stimulation (supramaximal stimulation, 2-ms duration, 1-10 Hz) by ~50% (P<0.01; N=11); blocking If alone had no effect (N=7). Blocking both IK,ACh and If produced ~90% reduction (P<0.01; N=4). Atrial cycle length response to a single burst of vagal stimuli (3 stimuli at 50 Hz), delivered at the optimum phase of the cycle was strongly suppressed by blocking IK,ACh (reduced by 98%; P<0.01; N=9), and modestly reduced by blocking If alone (by ~43%; P=0.20; N=6). The response was abolished by combined block of IK,ACh and If (P=0.04; N=4). Our data show that modulation of IK,ACh and If is sufficient to account for all the vagal slowing observed in this preparation. The vagally-induced negative shift in activation potential for If will be opposed by hyperpolarisation of SAN through activation of IK,ACh. Thus removal of IK,ACh by TQ may have exaggerated the overall contribution of If to vagal slowing.

Hydroxychloroquine reduces heart rate by modulating the hyperpolarization-activated current If: Novel electrophysiological insights and therapeutic potential

BACKGROUND

Bradycardic agents are of interest for the treatment of ischemic heart disease and heart failure, as heart rate is an important determinant of myocardial oxygen consumption.

OBJECTIVES

The purpose of this study was to investigate the propensity of hydroxychloroquine (HCQ) to cause bradycardia.

METHODS

We assessed the effects of HCQ on (1) cardiac beating rate in vitro (mice); (2) the "funny" current (If) in isolated guinea pig sinoatrial node (SAN) myocytes (1, 3, 10 µM); (3) heart rate and blood pressure in vivo by acute bolus injection (rat, dose range 1-30 mg/kg), (4) blood pressure and ventricular function during feeding (mouse, 100 mg/kg/d for 2 wk, tail cuff plethysmography, anesthetized echocardiography).

RESULTS

In mouse atria, spontaneous beating rate was significantly (P < .05) reduced (by 9% ± 3% and 15% ± 2% at 3 and 10 µM HCQ, n = 7). In guinea pig isolated SAN cells, HCQ conferred a significant reduction in spontaneous action potential firing rate (17% ± 6%, 1 μM dose) and a dose-dependent reduction in If (13% ± 3% at 1 µM; 19% ± 2% at 3 µM). Effects were also observed on L-type calcium ion current (ICaL) (12% ± 4% reduction) and rapid delayed rectifier potassium current (IKr) (35% ± 4%) at 3 µM. Intravenous HCQ decreased heart rate in anesthetized rats (14.3% ± 1.1% at 15mg/kg; n = 6) without significantly reducing mean arterial blood pressure. In vivo feeding studies in mice showed no significant change in systolic blood pressure nor left ventricular function.

CONCLUSIONS

We have shown that HCQ acts as a bradycardic agent in SAN cells, in atrial preparations, and in vivo. HCQ slows the rate of spontaneous action potential firing in the SAN through multichannel inhibition, including that of If.

Lessons from the heart: mirroring electrophysiological characteristics during cardiac development to in vitro differentiation of stem cell derived cardiomyocytes

The ability of human pluripotent stem cells (hPSCs) to differentiate into any cell type of the three germ layers makes them a very promising cell source for multiple purposes, including regenerative medicine, drug discovery, and as a model to study disease mechanisms and progression. One of the first specialized cell types to be generated from hPSC was cardiomyocytes (CM), and differentiation protocols have evolved over the years and now allow for robust and large-scale production of hPSC-CM. Still, scientists are struggling to achieve the same, mainly ventricular, phenotype of the hPSC-CM in vitro as their adult counterpart in vivo. In vitro generated cardiomyocytes are generally described as fetal-like rather than adult. In this review, we compare the in vivo development of cardiomyocytes to the in vitro differentiation of hPSC into CM with focus on electrophysiology, structure and contractility. Furthermore, known epigenetic changes underlying the differences between adult human CM and CM differentiated from pluripotent stem cells are described. This should provide the reader with an extensive overview of the current status of human stem cell-derived cardiomyocyte phenotype and function. Additionally, the reader will gain insight into the underlying signaling pathways and mechanisms responsible for cardiomyocyte development.

Pathological impact of hyperpolarization-activated chloride current peculiar to rat pulmonary vein cardiomyocytes

Pulmonary veins (PVs) are believed to be a crucial origin of atrial fibrillation. We recently reported that rat PV cardiomyocytes exhibit arrhythmogenic automaticity in response to norepinephrine. Herein, we further characterized the electrophysiological properties underlying the potential arrhythmogenicity of PV cardiomyocytes. Patch clamping studies revealed a time dependent hyperpolarization-activated inward current in rat PV cardiomyocytes, but not in left atrial (LA) myocytes. The current was Cs(+) resistant, and was not affected by removal of external Na(+) or K(+). The current was inhibited with Cd(2+), and the reversal potential was sensitive to changes in [Cl(-)] on either side of the membrane in a manner consistent with a Cl(-) selective channel. Cl(-) channel blockers attenuated the current, and slowed or completely inhibited the norepinephrine-induced automaticity. The biophysical properties of the hyperpolarization-activated Cl(-) current in rat PVs were different from those of ClC-2 currents previously reported: (i) the voltage-dependent activation of the Cl(-) current in rat PVs was shifted to negative potentials as [Cl(-)]i increased, (ii) the Cl(-) current was enhanced by extracellular acidification, and (iii) extracellular hyper-osmotic stress increased the current, whereas hypo-osmotic cell swelling suppressed the current. qPCR analysis revealed negligible ClC-2 mRNA expression in the rat PV. These findings suggest that rat PV cardiomyocytes possess a peculiar voltage-dependent Cl(-) channel, and that the channel may play a functional role in norepinephrine-induced automaticity.

Electronic "expression" of the inward rectifier in cardiocytes derived from human-induced pluripotent stem cells

BACKGROUND

Human-induced pluripotent stem cell (h-iPSC)-derived cardiac myocytes are a unique model in which human myocyte function and dysfunction are studied, especially those from patients with genetic disorders. They are also considered a major advance for drug safety testing. However, these cells have considerable unexplored potential limitations when applied to quantitative action potential (AP) analysis. One major factor is spontaneous activity and resulting variability and potentially anomalous behavior of AP parameters.

OBJECTIVE

To demonstrate the effect of using an in silico interface on electronically expressed I(K1), a major component lacking in h-iPSC-derived cardiac myocytes.

METHODS

An in silico interface was developed to express synthetic I(K1) in cells under whole-cell voltage clamp.

RESULTS

Electronic I(K1) expression established a physiological resting potential, eliminated spontaneous activity, reduced spontaneous early and delayed afterdepolarizations, and decreased AP variability. The initiated APs had the classic rapid upstroke and spike and dome morphology consistent with data obtained with freshly isolated human myocytes as well as the readily recognizable repolarization attributes of ventricular and atrial cells. The application of 1 µM of BayK-8644 resulted in anomalous AP shortening in h-iPSC-derived cardiac myocytes. When I(K1) was electronically expressed, BayK-8644 prolonged the AP, which is consistent with the existing results on native cardiac myocytes.

CONCLUSIONS

The electronic expression of I(K1) is a simple and robust method to significantly improve the physiological behavior of the AP and electrical profile of h-iPSC-derived cardiac myocytes. Increased stability enables the use of this preparation for a controlled quantitative analysis of AP parameters, for example, drug responsiveness, genetic disorders, and dynamic behavior restitution profiles.

Implication of the TRPM4 nonselective cation channel in mammalian sinus rhythm

BACKGROUND

The transient receptor potential melastatin 4 (TRPM4) channel is expressed in the sinoatrial node, but its physiologic roles in this tissue with cardiac pacemaker properties remain unknown. This Ca(2+)-activated nonselective cation channel (NSCCa) induces cell depolarization at negative potentials. It is implicated in burst generation in neurons and participates in induction of ectopic beating in cardiac ventricular preparations submitted to hypoxia/reoxygenation. Accordingly, TRPM4 may participate in action potential (AP) triggering in the sinoatrial node.

OBJECTIVE

The purpose of this study was to investigate the influence of TRPM4 on spontaneous heart beating.

METHODS

Spontaneous APs were recorded using intracellular microelectrodes in mouse, rat, and rabbit isolated right atria.

RESULTS

In the spontaneously beating mouse atrium, superfusion of the TRPM4-specific inhibitor 9-phenanthrol produced a concentration-dependent reduction in AP rate (maximal reduction = 62% that of control; EC50 = 8 × 10(-6) mol●L(-1)) without affecting other AP parameters. These effects were absent in TRPM4(-/-) mice. 9-Phenanthrol exerted a rate-dependent reduction with a higher effect at low rates. Similar results were obtained in rat. Moreover, application of 9-phenanthrol produced a reduction in diastolic depolarization slope in rabbit sinus node pacemaker cells.

CONCLUSION

These data showed that TRPM4 modulates beating rate. Pacemaker activity in the sinoatrial node results from the slow diastolic depolarization slope due to the "funny" current, Na/Ca exchange, and a Ca(2+)-activated nonselective cation current, which can be attributable in part to TRPM4 that may act against bradycardia.

Effect of If-channel inhibition on hemodynamic status and exercise tolerance in heart failure with preserved ejection fraction: a randomized trial. J Am Coll Cardiol. 2013;62:1330-1338. [PMID: 23916925 DOI: 10.1016/j.jacc.2013.06.043]

OBJECTIVES

The aim of this study was to test the effects of treatment with ivabradine on exercise capacity and left ventricular filling in patients with heart failure with preserved ejection fraction (HFpEF).

BACKGROUND

Because symptoms of HFpEF are typically exertional, optimization of diastolic filling time by controlling heart rate may delay the onset of symptoms.

METHODS

Sixty-one patients with HFpEF were randomly assigned to ivabradine 5 mg twice daily (n = 30) or placebo (n = 31) for 7 days in this double-blind trial. Cardiopulmonary exercise testing with echocardiographic assessment of myocardial function and left ventricular filling were undertaken at rest and after exercise.

RESULTS

The ivabradine group demonstrated significant improvement between baseline and follow-up exercise capacity (4.2 ± 1.8 METs vs. 5.7 ± 1.9 METs, p = 0.001) and peak oxygen uptake (14.0 ± 6.1 ml/min/kg vs. 17.0 ± 3.3 ml/min/kg, p = 0.001), with simultaneous reduction in exercise-induced increase in the ratio of peak early diastolic mitral flow velocity to peak early diastolic mitral annular velocity (3.1 ± 2.7 vs. 1.3 ± 2.0, p = 0.004). Work load-corrected chronotropic response (the difference in heart rate at the same exercise time at the baseline and follow-up tests) showed a slower increase in heart rate during exercise than in the placebo-treated group. Therapy with ivabradine (β = 0.34, p = 0.04) and change with treatment in exertional increase in the ratio of peak early diastolic mitral flow velocity to peak early diastolic mitral annular velocity (β = -0.30, p = 0.02) were independent correlates of increase in exercise capacity, and therapy with ivabradine (β = 0.32, p = 0.007) was independently correlated with increase in peak oxygen uptake.

CONCLUSIONS

In patients with HFpEF, short-term treatment with ivabradine increased exercise capacity, with a contribution from improved left ventricular filling pressure response to exercise as reflected by the ratio of peak early diastolic mitral flow velocity to peak early diastolic mitral annular velocity. Because this patient population is symptomatic on exertion, therapeutic treatments targeting abnormal exercise hemodynamic status may prove useful. (Use of Exercise and Medical Therapies to Improve Cardiac Function Among Patients With Exertional Shortness of Breath Due to Lung Congestion; ACTRN12610001087044).