Divergent action potential morphologies reveal nonequilibrium properties of human cardiac Na channels

Late Sodium Current of the Heart: Where Do We Stand and Where Are We Going?

Late sodium current has long been linked to dysrhythmia and contractile malfunction in the heart. Despite the increasing body of accumulating information on the subject, our understanding of its role in normal or pathologic states is not complete. Even though the role of late sodium current in shaping action potential under physiologic circumstances is debated, it’s unquestioned role in arrhythmogenesis keeps it in the focus of research. Transgenic mouse models and isoform-specific pharmacological tools have proved useful in understanding the mechanism of late sodium current in health and disease. This review will outline the mechanism and function of cardiac late sodium current with special focus on the recent advances of the area.

Late Sodium Current Inhibitors as Potential Antiarrhythmic Agents

Based on recent findings, an increased late sodium current (INa,late) plays an important pathophysiological role in cardiac diseases, including rhythm disorders. The article first describes what is INa,late and how it functions under physiological circumstances. Next, it shows the wide range of cellular mechanisms that can contribute to an increased INa,late in heart diseases, and also discusses how the upregulated INa,late can play a role in the generation of cardiac arrhythmias. The last part of the article is about INa,late inhibiting drugs as potential antiarrhythmic agents, based on experimental and preclinical data as well as in the light of clinical trials.

Forces from the Portal Govern the Late-Stage DNA Transport in a Viral DNA Packaging Nanomotor

In the Phi29 bacteriophage, the DNA packaging nanomotor packs its double-stranded DNA genome into the virus capsid. At the late stage of DNA packaging, the negatively charged genome is increasingly compacted at a higher density in the capsid with a higher internal pressure. During the process, two Donnan effects, osmotic pressure and Donnan equilibrium potentials, are significantly amplified, which, in turn, affect the channel activity of the portal protein, GP10, embedded in the semipermeable capsid shell. In the research, planar lipid bilayer experiments were used to study the channel activities of the viral protein. The Donnan effect on the conformational changes of the viral protein was discovered, indicating GP10 may not be a static channel at the late stage of DNA packaging. Due to the conformational changes, GP10 may generate electrostatic forces that govern the DNA transport. For the section of the genome DNA that remains outside of the connector channel, a strong repulsive force from the viral protein would be generated against the DNA entry; however, for the section of the genome DNA within the channel, the portal protein would become a Brownian motor, which adopts the flash Brownian ratchet mechanism to pump the DNA against the increasingly built-up internal pressure (up to 20 atm) in the capsid. Therefore, the DNA transport in the nanoscale viral channel at the late stage of DNA packaging could be a consequence of Brownian movement of the genomic DNA, which would be rectified and harnessed by the forces from the interior wall of the viral channel under the influence of the Donnan effect.

Electrophysiological properties and calcium handling of embryonic stem cell-derived cardiomyocytes

Embryonic stem cell-derived cardiomyocytes (ESC-CMs) hold great interest in many fields of research including clinical applications such as stem cell and gene therapy for cardiac repair or regeneration. ESC-CMs are also used as a platform tool for pharmacological tests or for investigations of cardiac remodeling. ESC-CMs have many different aspects of morphology, electrophysiology, calcium handling, and bioenergetics compared with adult cardiomyocytes. They are immature in morphology, similar to sinus nodal-like in the electrophysiology, higher contribution of trans-sarcolemmal Ca²⁺ influx to Ca²⁺ handling, and higher dependence on anaerobic glycolysis. Here, I review a detailed electrophysiology and Ca²⁺ handling features of ESC-CMs during differentiation into adult cardiomyocytes to gain insights into how all the developmental changes are related to each other to display cardinal features of developing cardiomyocytes.

The late sodium current in heart failure: pathophysiology and clinical relevance

Large and growing body of data suggest that an increased late sodium current (I_Na,late ) can have a significant pathophysiological role in heart failure and other heart diseases. The first goal of this article is to describe how I_Na,late functions under physiological circumstances. The second goal is to show the wide range of cellular mechanisms that can increase I_Na,late in cardiac disease, and also to describe how the up-regulated I_Na,late contributes to the pathophysiology of heart failure. The final section of the article discusses the possible use of I_Na,late -modifying drugs in heart failure, on the basis of experimental and preclinical data.

Dynamics of the late Na(+) current during cardiac action potential and its contribution to afterdepolarizations

The objective of this work is to examine the contribution of late Na(+) current (INa,L) to the cardiac action potential (AP) and arrhythmogenic activities. In spite of the rapidly growing interest toward this current, there is no publication available on experimental recording of the dynamic INa,L current as it flows during AP with Ca(2+) cycling. Also unknown is how the current profile changes when the Ca(2+)-calmodulin dependent protein kinase II (CaMKII) signaling is altered, and how the current contributes to the development of arrhythmias. In this study we use an innovative AP-clamp Sequential Dissection technique to directly record the INa,L current during the AP with Ca(2+) cycling in the guinea pig ventricular myocytes. First, we found that the magnitude of INa,L measured under AP-clamp is substantially larger than earlier studies indicated. CaMKII inhibition using KN-93 significantly reduced the current. Second, we recorded INa,L together with IKs, IKr, and IK1 in the same cell to understand how these currents counterbalance to shape the AP morphology. We found that the amplitude and the total charge carried by INa,L exceed that of IKs. Third, facilitation of INa,L by Anemone toxin II prolonged APD and induced Ca(2+) oscillations that led to early and delayed afterdepolarizations and triggered APs; these arrhythmogenic activities were eliminated by buffering Ca(2+) with BAPTA. In conclusion, INa,L contributes a significantly large inward current that prolongs APD and unbalances the Ca(2+) homeostasis to cause arrhythmogenic APs.

Proton-dependent inhibition of the cardiac sodium channel Nav1.5 by ranolazine

Ranolazine is clinically approved for treatment of angina pectoris and is a potential candidate for antiarrhythmic, antiepileptic, and analgesic applications. These therapeutic effects of ranolazine hinge on its ability to inhibit persistent or late Na⁺ currents in a variety of voltage-gated sodium channels. Extracellular acidosis, typical of ischemic events, may alter the efficiency of drug/channel interactions. In this study, we examined pH modulation of ranolazine's interaction with the cardiac sodium channel, Na_v1.5. We performed whole-cell path clamp experiments at extracellular pH 7.4 and 6.0 on Na_v1.5 transiently expressed in HEK293 cell line. Consistent with previous studies, we found that ranolazine induced a stable conformational state in the cardiac sodium channel with onset/recovery kinetics and voltage-dependence resembling intrinsic slow inactivation. This interaction diminished the availability of the channels in a voltage- and use-dependent manner. Low extracellular pH impaired inactivation states leading to an increase in late Na⁺ currents. Ranolazine interaction with the channel was also slowed 4–5 fold. However, ranolazine restored the voltage-dependent steady-state availability profile, thereby reducing window/persistent currents at pH 6.0 in a manner comparable to pH 7.4. These results suggest that ranolazine is effective at therapeutically relevant concentrations (10 μM), in acidic extracellular pH, where it compensates for impaired native slow inactivation.

Kinetic model of Nav1.5 channel provides a subtle insight into slow inactivation associated excitability in cardiac cells

Voltage-gated sodium channel Nav1.5 has been linked to the cardiac cell excitability and a variety of arrhythmic syndromes including long QT, Brugada, and conduction abnormalities. Nav1.5 exhibits a slow inactivation, corresponding to a duration-dependent bi-exponential recovery, which is often associated with various arrhythmia syndromes. However, the gating mechanism of Nav1.5 and the physiological role of slow inactivation in cardiac cells remain elusive. Here a 12-state two-step inactivation Markov model was successfully developed to depict the gating kinetics of Nav1.5. This model can simulate the Nav1.5 channel in not only steady state processes, but also various transient processes. Compared with the simpler 8-state model, this 12-state model is well-behaved in simulating and explaining the processes of slow inactivation and slow recovery. This model provides a good framework for further studying the gating mechanism and physiological role of sodium channel in excitable cells.

Multiple arrhythmic syndromes in a newborn, owing to a novel mutation in SCN5A

BACKGROUND

Mutations in the SCN5A gene have been linked to Brugada syndrome (BrS), conduction disease, Long QT syndrome (LQT3), atrial fibrillation (AF), and to pre- and neonatal ventricular arrhythmias.

OBJECTIVE

The objective of this study is to characterize a novel mutation in Na(v)1.5 found in a newborn with fetal chaotic atrial tachycardia, post-partum intraventricular conduction delay, and QT interval prolongation.

METHODS

Genomic DNA was isolated and all exons and intron borders of 15 ion-channel genes were sequenced, revealing a novel missense mutation (Q270K) in SCN5A. Na(v)1.5 wild type (WT) and Q270K were expressed in CHO-K1 with and without the Na(v)β1 subunit. Results. Patch-clamp analysis showed ∼40% reduction in peak sodium channel current (I(Na)) density for Q270K compared with WT. Fast and slow decay of I(Na) were significantly slower in Q270K. Steady-state activation and inactivation of Q270K channels were shifted to positive potentials, and window current was increased. The tetrodotoxin-sensitive late I(Na) was increased almost 3-fold compared with WT channels. Ranolazine reduced late I(Na) in WT and Q270K channels, while exerting minimal effects on peak I(Na).

CONCLUSION

The Q270K mutation in SCN5A reduces peak I(Na) while augmenting late I(Na), and may thus underlie the development of atrial tachycardia, intraventricular conduction delay, and QT interval prolongation in an infant.

Extracellular proton depression of peak and late Na⁺ current in the canine left ventricle

Cardiac ischemia reduces excitability in ventricular tissue. Acidosis (one component of ischemia) affects a number of ion currents. We examined the effects of extracellular acidosis (pH 6.6) on peak and late Na(+) current (I(Na)) in canine ventricular cells. Epicardial and endocardial myocytes were isolated, and patch-clamp techniques were used to record I(Na). Action potential recordings from left ventricular wedges exposed to acidic Tyrode solution showed a widening of the QRS complex, indicating slowing of transmural conduction. In myocytes, exposure to acidic conditions resulted in a 17.3 ± 0.9% reduction in upstroke velocity. Analysis of fast I(Na) showed that current density was similar in epicardial and endocardial cells at normal pH (68.1 ± 7.0 vs. 63.2 ± 7.1 pA/pF, respectively). Extracellular acidosis reduced the fast I(Na) magnitude by 22.7% in epicardial cells and 23.1% in endocardial cells. In addition, a significant slowing of the decay (time constant) of fast I(Na) was observed at pH 6.6. Acidosis did not affect steady-state inactivation of I(Na) or recovery from inactivation. Analysis of late I(Na) during a 500-ms pulse showed that the acidosis significantly reduced late I(Na) at 250 and 500 ms into the pulse. Using action potential clamp techniques, application of an epicardial waveform resulted in a larger late I(Na) compared with when an endocardial waveform was applied to the same cell. Acidosis caused a greater decrease in late I(Na) when an epicardial waveform was applied. These results suggest acidosis reduces both peak and late I(Na) in both cell types and contributes to the depression in cardiac excitability observed under ischemic conditions.

The cardiac persistent sodium current: an appealing therapeutic target?

The sodium current in the heart is not a single current with a mono-exponential decay but rather a mixture of currents with different kinetics. It is not clear whether these arise from distinct populations of channels, or from modulation of a single population. A very slowly inactivating component, [(INa(P))] I(Na(P)) is usually about 1% of the size of the peak transient current [I(Na(T))], but is enhanced by hypoxia. It contributes to Na(+) loading and cellular damage in ischaemia and re-perfusion, and perhaps to ischaemic arrhythmias. Class I antiarrhythmic agents such as flecainide, lidocaine and mexiletine generally block I(NA(P)) more potently than block of I(Na(T)) and have been used clinically to treat LQT3 syndrome, which arises because mutations in SCN5A produce defective inactivation of the cardiac sodium channel. The same approach may be useful in some pathological situations, such as ischaemic arrhythmias or diastolic dysfunction, and newer agents are being developed with this goal. For example, ranolazine blocks I(Na(P)) about 10 times more potently than I(Na(T)) and has shown promise in the treatment of angina. Alternatively, the combination of I(Na(P)) block with K(+) channel block may provide protection from the induction of Torsades de Pointe when these agents are used to treat atrial arrhythmias (eg Vernakalant). In all of these scenarios, an understanding of the role of I(Na(P)) in cardiac pathophysiology, the mechanisms by which it may affect cardiac electrophysiology and the potential side effects of blocking I(Na(P)) in the heart and elsewhere will become increasingly important.

Long-QT syndrome-related sodium channel mutations probed by the dynamic action potential clamp technique

Long-QT3 syndrome (LQT3) is linked to cardiac sodium channel gene (SCN5A) mutations. In this study, we used the 'dynamic action potential clamp' (dAPC) technique to effectively replace the native sodium current (I(Na)) of the Priebe-Beuckelmann human ventricular cell model with wild-type (WT) or mutant I(Na) generated in a human embryonic kidney (HEK)-293 cell that is voltage clamped by the free-running action potential of the ventricular cell. We recorded I(Na) from HEK cells expressing either WT or LQT3-associated Y1795C or A1330P SCN5A at 35 degrees C, and let this current generate and shape the action potential (AP) of subepicardial, mid-myocardial and subendocardial model cells. The HEK cell's endogenous background current was completely removed by a real-time digital subtraction procedure. With WT I(Na), AP duration (APD) was longer than with the original Priebe-Beuckelmann model I(Na), due to a late I(Na) component of approximately 30 pA that could not be revealed with conventional voltage-clamp protocols. With mutant I(Na), this late component was larger ( approximately 100 pA), producing a marked increase in APD ( approximately 70-80 ms at 1 Hz for the subepicardial model cell). The late I(Na) magnitude showed reverse frequency dependence, resulting in a significantly steeper APD-frequency relation in the mutant case. AP prolongation was more pronounced for the mid-myocardial cell type, resulting in increased APD dispersion for each of the mutants. For both mutants, a 2 s pause following rapid (2 Hz) pacing resulted in distorted AP morphology and beat-to-beat fluctuations of I(Na). Our dAPC data directly demonstrate the arrhythmogenic nature of LQT3-associated SCN5A mutations.