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) 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 (I_Kr), contains the methanesulfonamide group. Reduced I_Kr 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 I_Kr and an early outward one corresponding to transient outward potassium current (I_to). 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 can reach the low μM range, ABT-333 application carries a risk of cardiac side effects especially in case of coadministration with strong inhibitors of CYP2C8.

The present study was designed to test the hypothesis that the selectivity of blocking the late Na⁺ current (I_NaL) over the peak Na⁺ current (I_NaP) is related to the fast offset kinetics of the Na⁺ channel inhibitor. Therefore, the effects of 1 µM GS967 (I_NaL inhibitor), 20 µM mexiletine (I/B antiarrhythmic) and 10 µM quinidine (I/A antiarrhythmic) on I_NaL and I_NaP were compared in canine ventricular myocardium. I_NaP was estimated as the maximum velocity of action potential upstroke (V⁺_max). Equal amounts of I_NaL were dissected by the applied drug concentrations under APVC conditions. The inhibition of I_NaL 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 I_NaL over I_NaP inhibition without any influence of the onset rate constant. It is concluded that the selective inhibition of I_NaL over I_NaP is related to the fast offset kinetics of the Na⁺ channel inhibitor.

Several ion currents in the mammalian ventricular myocardium are substantially regulated by the sympathetic nervous system via β-adrenergic receptor activation, including the slow delayed rectifier K⁺ current and the L-type calcium current. This study investigated the downstream mechanisms of β-adrenergic receptor stimulation by isoproterenol (ISO) on the inward rectifier (I_K1) and the rapid delayed rectifier (I_Kr) K⁺ currents using action potential voltage clamp (APVC) and conventional voltage clamp techniques in isolated canine left ventricular cardiomyocytes. I_K1 and I_Kr were dissected by 50 µM BaCl₂ and 1 µM E-4031, respectively. Acute application of 10 nM ISO significantly increased I_K1 under the plateau phase of the action potential (0–+20 mV) using APVC, and similar results were obtained with conventional voltage clamp. However, β-adrenergic receptor stimulation did not affect the peak current density flowing during terminal repolarization or the overall I_K1 integral. The ISO-induced enhancement of I_K1 was blocked by the calcium/calmodulin kinase II (CaMKII) inhibitor KN-93 (1 µM) but not by the protein kinase A inhibitor H-89 (3 µM). Neither KN-93 nor H-89 affected the I_K1 density under baseline conditions (in the absence of ISO). In contrast, parameters of the I_Kr current were not affected by β-adrenergic receptor stimulation with ISO. These findings suggest that sympathetic activation enhances I_K1 in canine left ventricular cells through the CaMKII pathway, while I_Kr remains unaffected under the experimental conditions used.

Our review focuses on methanesulfonamide-containing compounds, a well-characterized class of high-affinity blockers of the hERG potassium channel, which plays a critical role in cardiac repolarization by mediating the cardiac I_Kr. These compounds, which include notable class III antiarrhythmic drugs such as dofetilide and d-sotalol, block the hERG channel in its open state by binding within the inner vestibule. This interaction is particularly strong with some residues and the compounds form hydrogen bonds with others. This binding results in high-affinity inhibition with slow dissociation kinetics, frequently leading to drug trapping and prolonged action potential duration (APD). This can predispose patients to arrhythmias, including Torsades de Pointes. Beyond cardiac drugs, there are several non-cardiac methanesulfonamide drugs that also block the hERG channel. This causes pro-arrhythmic side effects despite their primary indications. The clinical significance of these effects, especially in patients with impaired drug elimination, is that accumulation increases the risk of arrhythmia. The objective of forthcoming research endeavors is to mitigate hERG affinity, with the aim of reducing pro-arrhythmic risks while maintaining therapeutic efficacy. This means structural modifications that seek to remove or modify the methanesulfonamide group. Machine learning also emerged as promising tool for exploring drug–protein interactions. It is evident that the methanesulfonamide moiety plays a pivotal role in the structural basis of hERG blockade. However, it should be noted that this moiety does not necessarily represent a universal pharmacophore. This observation underscores the necessity for a nuanced approach in drug development, aimed at achieving a balance between efficacy and safety.