Relationship between ion currents and membrane capacitance in canine ventricular myocytes

Current density, the membrane current value divided by membrane capacitance (Cm), is widely used in cellular electrophysiology. Comparing current densities obtained in different cell populations assume that Cm and ion current magnitudes are linearly related, however data is scarce about this in cardiomyocytes. Therefore, we statistically analyzed the distributions, and the relationship between parameters of canine cardiac ion currents and Cm, and tested if dividing original parameters with Cm had any effect. Under conventional voltage clamp conditions, correlations were high for IK1, moderate for IKr and ICa,L, while negligible for IKs. Correlation between Ito1 peak amplitude and Cm was negligible when analyzing all cells together, however, the analysis showed high correlations when cells of subepicardial, subendocardial or midmyocardial origin were analyzed separately. In action potential voltage clamp experiments IK1, IKr and ICa,L parameters showed high correlations with Cm. For INCX, INa,late and IKs there were low-to-moderate correlations between Cm and these current parameters. Dividing the original current parameters with Cm reduced both the coefficient of variation, and the deviation from normal distribution. The level of correlation between ion currents and Cm varies depending on the ion current studied. This must be considered when evaluating ion current densities in cardiac cells.

Selective Inhibition of Cardiac Late Na+ Current Is Based on Fast Offset Kinetics of the Inhibitor

The present study was designed to test the hypothesis that the selectivity of blocking the late Na+ current (INaL) over the peak Na+ current (INaP) is related to the fast offset kinetics of the Na+ channel inhibitor. Therefore, the effects of 1 µM GS967 (INaL inhibitor), 20 µM mexiletine (I/B antiarrhythmic) and 10 µM quinidine (I/A antiarrhythmic) on INaL and INaP were compared in canine ventricular myocardium. INaP was estimated as the maximum velocity of action potential upstroke (V+max). Equal amounts of INaL were dissected by the applied drug concentrations under APVC conditions. The inhibition of INaL by mexiletine and quinidine was comparable under a conventional voltage clamp, while both were smaller than the inhibitory effect of GS967. Under steady-state conditions, the V+max block at the physiological cycle length of 700 ms was 2.3% for GS967, 11.4% for mexiletine and 26.2% for quinidine. The respective offset time constants were 110 ± 6 ms, 456 ± 284 ms and 7.2 ± 0.9 s. These results reveal an inverse relationship between the offset time constant and the selectivity of INaL over INaP inhibition without any influence of the onset rate constant. It is concluded that the selective inhibition of INaL over INaP is related to the fast offset kinetics of the Na+ channel inhibitor.

Conductance Changes of Na+ Channels during the Late Na+ Current Flowing under Action Potential Voltage Clamp Conditions in Canine, Rabbit, and Guinea Pig Ventricular Myocytes

Late sodium current (I_Na,late) is an important inward current contributing to the plateau phase of the action potential (AP) in the mammalian heart. Although I_Na,late is considered as a possible target for antiarrhythmic agents, several aspects of this current remained hidden. In this work, the profile of I_Na,late, together with the respective conductance changes (G_Na,late), were studied and compared in rabbit, canine, and guinea pig ventricular myocytes using the action potential voltage clamp (APVC) technique. In canine and rabbit myocytes, the density of I_Na,late was relatively stable during the plateau and decreased only along terminal repolarization of the AP, while G_Na,late decreased monotonically. In contrast, I_Na,late increased monotonically, while G_Na,late remained largely unchanged during the AP in guinea pig. The estimated slow inactivation of Na⁺ channels was much slower in guinea pig than in canine or rabbit myocytes. The characteristics of canine I_Na,late and G_Na,late were not altered by using command APs recorded from rabbit or guinea pig myocytes, indicating that the different shapes of the current profiles are related to genuine interspecies differences in the gating of I_Na,late. Both I_Na,late and G_Na,late decreased in canine myocytes when the intracellular Ca²⁺ concentration was reduced either by the extracellular application of 1 µM nisoldipine or by the intracellular application of BAPTA. Finally, a comparison of the I_Na,late and G_Na,late profiles induced by the toxin of Anemonia sulcata (ATX-II) in canine and guinea pig myocytes revealed profound differences between the two species: in dog, the ATX-II induced I_Na,late and G_Na,late showed kinetics similar to those observed with the native current, while in guinea pig, the ATX-II induced G_Na,late increased during the AP. Our results show that there are notable interspecies differences in the gating kinetics of I_Na,late that cannot be explained by differences in AP morphology. These differences must be considered when interpreting the I_Na,late results obtained in guinea pig.

ABT-333 (Dasabuvir) Increases Action Potential Duration and Provokes Early Afterdepolarizations in Canine Left Ventricular Cells via Inhibition of IKr

ABT-333 (dasabuvir) is an antiviral agent used in hepatitis C treatment. The molecule, similarly to some inhibitors of hERG channels, responsible for the delayed rectifier potassium current (IKr), contains the methanesulfonamide group. Reduced IKr current leads to long QT syndrome and early afterdepolarizations (EADs), therefore potentially causing life-threatening arrhythmias and sudden cardiac death. Our goal was to investigate the acute effects of ABT-333 in enzymatically isolated canine left ventricular myocardial cells. Action potentials (APs) and ion currents were recorded with a sharp microelectrode technique and whole-cell patch clamp, respectively. Application of 1 μM ABT-333 prolonged the AP in a reversible manner. The maximal rates of phases 0 and 1 were irreversibly decreased. Higher ABT-333 concentrations caused larger AP prolongation, elevation of the early plateau potential, and reduction of maximal rates of phases 0, 1, and 3. EADs occurred in some cells in 3–30 μM ABT-333 concentrations. The 10 μM ABT-333-sensitive current, recorded with AP voltage clamp, contained a late outward component corresponding to IKr and an early outward one corresponding to transient outward potassium current (Ito). ABT-333 reduced hERG-channel-mediated ion current in a concentration-dependent, partially reversible manner with a half-inhibitory concentration of 3.2 μM. As the therapeutic plasma concentration of ABT-333 is 1 nM, the arrhythmic risk of ABT-333 is very low, even in the case of drug overdose.

Exploring the Coordination of Cardiac Ion Channels With Action Potential Clamp Technique

The patch clamp technique underwent continual advancement and developed numerous variants in cardiac electrophysiology since its introduction in the late 1970s. In the beginning, the capability of the technique was limited to recording one single current from one cell stimulated with a rectangular command pulse. Since that time, the technique has been extended to record multiple currents under various command pulses including action potential. The current review summarizes the development of the patch clamp technique in cardiac electrophysiology with special focus on the potential applications in integrative physiology.

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.

Pharmacological Modulation and (Patho)Physiological Roles of TRPM4 Channel—Part 1: Modulation of TRPM4

Transient receptor potential melastatin 4 is a unique member of the TRPM protein family and, similarly to TRPM5, is Ca²⁺-sensitive and permeable to monovalent but not divalent cations. It is widely expressed in many organs and is involved in several functions by regulating the membrane potential and Ca²⁺ homeostasis in both excitable and non-excitable cells. This part of the review discusses the pharmacological modulation of TRPM4 by listing, comparing, and describing both endogenous and exogenous activators and inhibitors of the ion channel. Moreover, other strategies used to study TRPM4 functions are listed and described. These strategies include siRNA-mediated silencing of TRPM4, dominant-negative TRPM4 variants, and anti-TRPM4 antibodies. TRPM4 is receiving more and more attention and is likely to be the topic of research in the future.

Late Na+ Current Is [Ca2+]i-Dependent in Canine Ventricular Myocytes

Enhancement of the late sodium current (I_NaL) increases arrhythmia propensity in the heart, whereas suppression of the current is antiarrhythmic. In the present study, we investigated I_NaL in canine ventricular cardiomyocytes under action potential voltage-clamp conditions using the selective Na⁺ channel inhibitors GS967 and tetrodotoxin. Both 1 µM GS967 and 10 µM tetrodotoxin dissected largely similar inward currents. The amplitude and integral of the GS967-sensitive current was significantly smaller after the reduction of intracellular Ca²⁺ concentration ([Ca²⁺]_i) either by superfusion of the cells with 1 µM nisoldipine or by intracellular application of 10 mM BAPTA. Inhibiting calcium/calmodulin-dependent protein kinase II (CaMKII) by KN-93 or the autocamtide-2-related inhibitor peptide similarly reduced the amplitude and integral of I_NaL. Action potential duration was shortened in a reverse rate-dependent manner and the plateau potential was depressed by GS967. This GS967-induced depression of plateau was reduced by pretreatment of the cells with BAPTA-AM. We conclude that (1) I_NaL depends on the magnitude of [Ca²⁺]_i in canine ventricular cells, (2) this [Ca²⁺]_i-dependence of I_NaL is mediated by the Ca²⁺-dependent activation of CaMKII, and (3) I_NaL is augmented by the baseline CaMKII activity.

Electrophysiological Effects of the Transient Receptor Potential Melastatin 4 Channel Inhibitor (4-Chloro-2-(2-chlorophenoxy)acetamido) Benzoic Acid (CBA) in Canine Left Ventricular Cardiomyocytes

Transient receptor potential melastatin 4 (TRPM4) plays an important role in many tissues, including pacemaker and conductive tissues of the heart, but much less is known about its electrophysiological role in ventricular myocytes. Our earlier results showed the lack of selectivity of 9-phenanthrol, so CBA ((4-chloro-2-(2-chlorophenoxy)acetamido) benzoic acid) was chosen as a new, potentially selective inhibitor. Goal: Our aim was to elucidate the effect and selectivity of CBA in canine left ventricular cardiomyocytes and to study the expression of TRPM4 in the canine heart. Experiments were carried out in enzymatically isolated canine left ventricular cardiomyocytes. Ionic currents were recorded with an action potential (AP) voltage-clamp technique in whole-cell configuration at 37 °C. An amount of 10 mM BAPTA was used in the pipette solution to exclude the potential activation of TRPM4 channels. AP was recorded with conventional sharp microelectrodes. CBA was used in 10 µM concentrations. Expression of TRPM4 protein in the heart was studied by Western blot. TRPM4 protein was expressed in the wall of all four chambers of the canine heart as well as in samples prepared from isolated left ventricular cells. CBA induced an approximately 9% reduction in AP duration measured at 75% and 90% of repolarization and decreased the short-term variability of APD₉₀. Moreover, AP amplitude was increased and the maximal rates of phase 0 and 1 were reduced by the drug. In AP clamp measurements, CBA-sensitive current contained a short, early outward and mainly a long, inward current. Transient outward potassium current (I_to) and late sodium current (I_Na,L) were reduced by approximately 20% and 47%, respectively, in the presence of CBA, while L-type calcium and inward rectifier potassium currents were not affected. These effects of CBA were largely reversible upon washout. Based on our results, the CBA induced reduction of phase-1 slope and the slight increase of AP amplitude could have been due to the inhibition of I_to. The tendency for AP shortening can be explained by the inhibition of inward currents seen in AP-clamp recordings during the plateau phase. This inward current reduced by CBA is possibly I_Na,L, therefore, CBA is not entirely selective for TRPM4 channels. As a consequence, similarly to 9-phenanthrol, it cannot be used to test the contribution of TRPM4 channels to cardiac electrophysiology in ventricular cells, or at least caution must be applied.

Is selective late sodium current inhibition different from class I/B antiarrhythmic action? Comparison of the effects of GS967 to mexiletine in canine ventricular myocardium

Funding Acknowledgements

Type of funding sources: Public grant(s) – EU funding. Main funding source(s): National Research, Development and Innovation Office New National Excellence Programme

Enhancement of the late Na+ current (INa,late) increases arrhythmia propensity in the heart, while suppression of the current is antiarrhythmic. GS-458967 (GS) is an agent considered to be a selective blocker of INa,late. In the present study, effects of GS967 on INa,late, on L-type calcium current (ICaL), and on action potential (AP) morphology were studied in canine ventricular myocytes by using conventional voltage clamp, action potential voltage clamp and sharp microelectrode techniques. These effects of GS were compared to tetrodotoxin (TTX) and to the class I/B antiarrhythmic compound mexiletine.

GS (1 μM), mexiletine (40 μM) and TTX (10 μM) dissected largely similarly shaped inward currents under action potential voltage clamp conditions. In case of GS and mexiletine, the amplitude and integral of this inward current was significantly smaller when measured in the presence of 1 μM nisoldipine, while no difference was observed in case of TTX. Under conventional voltage clamp conditions, INa,late was significantly reduced by 1 μM GS and 40 μM mexiletine (about 79% and 63% reduction of current integrals, respectively). The integral of ICa,L was moderately but significantly decreased by both drugs (reduction of 9% and 14%, respectively). These changes were associated with a faster inactivation of ICa,L.

Drug effects on early Na+ current (INa,early) were assessed by analyzing the maximal rate of depolarization (V + max) in multicellular preparations. Both GS and mexiletine showed fast onset and offset kinetics: 110 ms and 289 ms offset time constants, respectively, as determined from V + max measurements in right ventricular papillary muscles, while the onset kinetics was characterized by 5.3 AP and 2.6 AP lengths, respectively, at 2.5 Hz.

Effects on beat-to-beat variability of AP duration (APD) was studied in isolated myocytes. Short-term variability was significantly decreased by both GS and mexiletine (average reduction of 42% and 24%, respectively) while they caused similar shortening of the APD.

The electrophysiological effects of GS are similar to those of mexiletine, but with a somewhat faster offset kinetics of V + max block. However, since GS reduced V+ max and INa,late in the same concentration, the currently accepted view that GS that selectively blocks INa,late has to be questioned and it is suggested that GS should be classified as a class I/B (or I/B + IV) antiarrhythmic agent.

Mexiletine-like cellular electrophysiological effects of GS967 in canine ventricular myocardium

Enhancement of the late Na⁺ current (I_NaL) increases arrhythmia propensity in the heart, while suppression of the current is antiarrhythmic. GS967 is an agent considered as a selective blocker of I_NaL. In the present study, effects of GS967 on I_NaL and action potential (AP) morphology were studied in canine ventricular myocytes by using conventional voltage clamp, action potential voltage clamp and sharp microelectrode techniques. The effects of GS967 (1 µM) were compared to those of the class I/B antiarrhythmic compound mexiletine (40 µM). Under conventional voltage clamp conditions, I_NaL was significantly suppressed by GS967 and mexiletine, causing 80.4 ± 2.2% and 59.1 ± 1.8% reduction of the densities of I_NaL measured at 50 ms of depolarization, and 79.0 ± 3.1% and 63.3 ± 2.7% reduction of the corresponding current integrals, respectively. Both drugs shifted the voltage dependence of the steady-state inactivation curve of I_NaL towards negative potentials. GS967 and mexiletine dissected inward I_NaL profiles under AP voltage clamp conditions having densities, measured at 50% of AP duration (APD), of −0.37 ± 0.07 and −0.28 ± 0.03 A/F, and current integrals of −56.7 ± 9.1 and −46.6 ± 5.5 mC/F, respectively. Drug effects on peak Na⁺ current (I_NaP) were assessed by recording the maximum velocity of AP upstroke (V⁺_max) in multicellular preparations. The offset time constant was threefold faster for GS967 than mexiletine (110 ms versus 289 ms), while the onset of the rate-dependent block was slower in the case of GS967. Effects on beat-to-beat variability of APD was studied in isolated myocytes. Beat-to-beat variability was significantly decreased by both GS967 and mexiletine (reduction of 42.1 ± 6.5% and 24.6 ± 12.8%, respectively) while their shortening effect on APD was comparable. It is concluded that the electrophysiological effects of GS967 are similar to those of mexiletine, but with somewhat faster offset kinetics of V⁺_max block. However, since GS967 depressed V⁺_max and I_NaL at the same concentration, the current view that GS967 represents a new class of drugs that selectively block I_NaL has to be questioned and it is suggested that GS967 should be classified as a class I/B antiarrhythmic agent.

Transient receptor potential melastatin 4 channel inhibitor 9-phenanthrol inhibits K+ but not Ca2+ currents in canine ventricular myocytes

The role of transient receptor potential melastatin 4 (TRPM4) channels has been frequently tested using their inhibitor 9-phenanthrol in various cardiac preparations; however, the selectivity of the compound is uncertain. Therefore, in the present study, the concentration-dependent effects of 9-phenanthrol on major ionic currents were studied in canine isolated ventricular cells using whole-cell configuration of the patch-clamp technique and 10 mM BAPTA-containing pipette solution to prevent the Ca²⁺-dependent activation of TRPM4 channels. Transient outward (I_to1), rapid delayed rectifier (I_Kr), and inward rectifier (I_K1) K⁺ currents were suppressed by 10 and 30 μM 9-phenanthrol with the blocking potency for I_K1 < I_Kr < I_to1 and partial reversibility. L-type Ca²⁺ current was not affected up to the concentration of 30 μM. In addition, a steady outward current was detected at voltages positive to -40 mV in 9-phenanthrol, which was larger at more positive voltages and larger 9-phenanthrol concentrations. Action potentials were recorded using microelectrodes. Maximal rate of depolarization, phase-1 repolarization, and terminal repolarization were decreased and the plateau potential was depressed by 9-phenanthrol (3-30 μM), congruently with the observed alterations of ionic currents. Significant action potential prolongation was observed by 9-phenanthrol in the majority of the studied cells, but only at 30 μM concentration. In conclusion, 9-phenanthrol is not selective to TRPM4 channels in canine ventricular myocardium; therefore, its application as a TRPM4 blocker can be appropriate only in expression systems but not in native cardiac cells.