Search

Aims The microtubule (MT) network plays a major role in the transport of the cardiac sodium channel Nav1.5 to the membrane, where the latter associates with interacting proteins such as dystrophin. Alterations in MT dynamics are known to impact on ion channel trafficking. Duchenne muscular dystrophy (DMD), caused by dystrophin deficiency, is associated with an increase in MT detyrosination, decreased sodium current (I Na), and arrhythmias. Parthenolide (PTL), a compound that decreases MT detyrosination, has shown beneficial effects on cardiac function in DMD. We here investigated its impact on I Na and Nav1.5 subcellular distribution. Methods and results Ventricular cardiomyocytes (CMs) from wild-type (WT) and mdx (DMD) mice were incubated with either 10 µM PTL, 20 µM EpoY, or dimethylsulfoxide (DMSO) for 3–5 h, followed by patch-clamp analysis to assess I Na and action potential (AP) characteristics in addition to immunofluorescence and stochastic optical reconstruction microscopy (STORM) to investigate MT detyrosination and Nav1.5 cluster size and density, respectively. In accordance with previous studies, we observed increased MT detyrosination, decreased I Na and reduced AP upstroke velocity (V max) in mdx CMs compared to WT. PTL decreased MT detyrosination and significantly increased I Na magnitude (without affecting I Na gating properties) and AP V max in mdx CMs, but had no effect in WT CMs. Moreover, STORM analysis showed that in mdx CMs, Nav1.5 clusters were decreased not only in the grooves of the lateral membrane (LM; where dystrophin is localized) but also at the LM crests. PTL restored Nav1.5 clusters at the LM crests (but not at the grooves), indicating a dystrophin-independent trafficking route to this subcellular domain. Interestingly, Nav1.5 cluster density was also reduced at the intercalated disc (ID) region of mdx CMs, which was restored to WT levels by PTL. Treatment of mdx CMs with EpoY, a specific MT detyrosination inhibitor, also increased I Na density, while decreasing the amount of detyrosinated MTs, confirming a direct mechanistic link. Conclusion Attenuating MT detyrosination in mdx CMs restored I Na and enhanced Nav1.5 localization at the LM crest and ID. Hence, the reduced whole-cell I Na density characteristic of mdx CMs is not only the consequence of the lack of dystrophin within the LM grooves but is also due to reduced Nav1.5 at the LM crest and ID secondary to increased baseline MT detyrosination. Overall, our findings identify MT detyrosination as a potential therapeutic target for modulating I Na and subcellular Nav1.5 distribution in pathophysiological conditions. [ABSTRACT FROM AUTHOR]

Aims Opioids are associated with increased risk of sudden cardiac death. This may be due to their effects on the cardiac sodium channel (Nav1.5) current. In the present study, we aim to establish whether tramadol, fentanyl, or codeine affects Nav1.5 current. Methods and results Using whole-cell patch-clamp methodology, we studied the effects of tramadol, fentanyl, and codeine on currents of human Nav1.5 channels stably expressed in HEK293 cells and on action potential (AP) properties of freshly isolated rabbit ventricular cardiomyocytes. In fully available Nav1.5 channels (holding potential −120 mV), tramadol exhibited inhibitory effects on Nav1.5 current in a concentration-dependent manner with an IC50 of 378.5 ± 33.2 µ m. In addition, tramadol caused a hyperpolarizing shift of voltage-gated (in)activation and a delay in recovery from inactivation. These blocking effects occurred at lower concentrations in partially inactivated Nav1.5 channels: during partial fast inactivation (close-to-physiological holding potential −90 mV), IC50 of Nav1.5 block was 4.5 ± 1.1 μ m , while it was 16 ± 4.8 μ m during partial slow inactivation. The tramadol-induced changes on Nav1.5 properties were reflected by a reduction in AP upstroke velocity in a frequency-dependent manner. Fentanyl and codeine had no effect on Nav1.5 current, even when tested at lethal concentrations. Conclusion Tramadol reduces Nav1.5 currents, in particular, at close-to-physiological membrane potentials. Fentanyl and codeine have no effects on Nav1.5 current. [ABSTRACT FROM AUTHOR]

Background: SGLT2i directly inhibit the cardiac sodium-hydrogen exchanger-1 (NHE1) in isolated ventricular cardiomyocytes (CMs). However, other studies with SGLT2i have yielded conflicting results. This may be explained by methodological factors including cell isolation techniques, cell types and ambient pH. In this study, we tested whether the use of protease XIV (PXIV) may abrogate inhibition of SGLT2i on cardiac NHE1 activity in isolated rabbit CMs or rat cardiomyoblast cells (H9c2), in a pH dependent manner. Methods: Rabbit ventricular CMs were enzymatically isolated from Langendorff-perfused hearts during a 30-min perfusion period followed by a 25-min after-dissociation period, using a collagenase mixture without or with a low dose PXIV (0.009 mg/mL) present for different periods. Empagliflozin (EMPA) inhibition on NHE activity was then assessed at pH of 7.0, 7.2 and 7.4. In addition, effects of 10 min PXIV treatment were also evaluated in H9c2 cells for EMPA and cariporide NHE inhibition. Results: EMPA reduced NHE activity in rabbit CMs that were not exposed to PXIV treatment or undergoing a 35-min PXIV treatment, independent of pH levels. However, when exposure time to PXIV was extended to 55 min, NHE inhibition by Empa was completely abolished at all three pH levels. In H9c2 cells, NHE inhibition by EMPA was evident in non-treated cells but lost after 10-min incubation with PXIV. NHE inhibition by cariporide was unaffected by PXIV. Conclusion: The use of protease XIV in cardiac cell isolation procedures obliterates the inhibitory effects of SGLT2i on NHE1 activity in isolated cardiac cells, independent of pH. [ABSTRACT FROM AUTHOR]

Verkerk, Arie O., Marieke W. Veldkamp, Fabio Abbate, Gudrun Antoons, Lennart N. Bouman, Jan H. Ravesloot, and Antoni C. G. van Ginneken. Two types of action potential configuration in single cardiac Purkinje cells of sheep. Am. J. Physiol. 277 (Heart Circ. Physiol. 46): H1299-H1310, 1999.--Membrane potentials and currents of isolated sheep Purkinje and ventricular cells were compared using patch-clamp and microelectrode techniques. In ~50% of Purkinje cells, we observed action potentials that showed a prominent phase 1 repolarization and relatively negative plateau (LP cells). Action potential configuration of the remaining Purkinje cells was characterized by little phase I repolarization and relatively positive plateau (HP cells). Microelectrode impalement of Purkinje strands also revealed these two types of action potential configuration. In LP cells, the density of L-type [Ca.sup.2+] current ([I.sub.Ca,L]) was lower, whereas the density of transient outward [K.sup.+] current was higher, than in HP cells. Action potentials of HP cells strongly resembled those of ventricular cells. Densities of inward rectifier current and [I.sub.Ca],L were significantly higher in ventricular cells compared with densities in both LP and HP Purkinje cells. Differences in current densities explain the striking differences in action potential configuration and the stimulus frequency dependency thereof that we observed in LP, HP, and ventricular cells. We conclude that LP Purkinje cells, HP Purkinje cells, and ventricular cells of sheep each have a unique action potential configuration. ventricular cell; action potentials; membrane currents

The cardiac autonomic nervous system (ANS) controls normal atrial electrical function. The cardiac ANS produces various neuropeptides, among which the neurokinins, whose actions on atrial electrophysiology are largely unknown. We here demonstrate that the neurokinin substance-P (Sub-P) activates a neurokinin-3 receptor (NK-3R) in rabbit, prolonging action potential (AP) duration through inhibition of a background potassium current. In contrast, ventricular AP duration was unaffected by NK-3R activation. NK-3R stimulation lengthened atrial repolarization in intact rabbit hearts and consequently suppressed arrhythmia duration and occurrence in a rabbit isolated heart model of atrial fibrillation (AF). In human atrial appendages, the phenomenon of NK-3R mediated lengthening of atrial repolarization was also observed. Our findings thus uncover a pathway to selectively modulate atrial AP duration by activation of a hitherto unidentified neurokinin-3 receptor in the membrane of atrial myocytes. NK-3R stimulation may therefore represent an anti-arrhythmic concept to suppress re-entry-based atrial tachyarrhythmias, including AF. [ABSTRACT FROM AUTHOR]

Aims Selective inhibition of cardiac late sodium current (INaL) is an emerging target in the treatment of ventricular arrhythmias. We investigated the electrophysiological effects of GS-458967 (GS967), a potent, selective inhibitor of INaL, in an overlap syndrome model of both gain and loss of sodium channel function, comprising cardiomyocytes derived from both human SCN5A-1795insD+/- induced pluripotent stem cells (hiPSC-CMs) and mice carrying the homologous mutation Scn5a-1798ins+/-. Methods and results On patch-clamp analysis, GS967 (300 nmol/l) reduced INaL and action potential (AP) duration in isolated ventricular myocytes from wild type and Scn5a-1798ins+/- mice, as well as in SCN5A-1795ins+/- hiPSC-CMs. GS967 did not affect the amplitude of peak INa, but slowed its recovery, and caused a negative shift in voltage-dependence of INa inactivation. GS967 reduced AP upstroke velocity in Scn5a-1798ins+/- myocytes and SCN5A-1795ins+/- hiPSC-CMs. However, the same concentration of GS967 did not affect conduction velocity in Scn5a-1798ins+/- mouse isolated hearts, as assessed by epicardial mapping. GS967 decreased the amplitude of delayed after depolarizations and prevented triggered activity in mouse Scn5a-1798ins+/- cardiomyocytes. Conclusion The INNaL inhibitor GS967 decreases repolarization abnormalities and has anti-arrhythmic effects in the absence of deleterious effects on cardiac conduction. Thus, selective inhibition of INaL constitutes a promising pharmacological treatment of cardiac channelopathies associated with enhanced INaL. Our findings furthermore implement hiPSC-CMs as a valuable tool for assessment of novel pharmacological approaches in inherited sodium channelopathies. [ABSTRACT FROM AUTHOR]

An editorial is presented on the sodium-hydrogen antiporter (Na+/H++ exchanger) (NHE-1) activity. Topics mentioned include the antioxidant effect of NHE-1, the effects of insulin-like growth factor (IGF-1), and the relation of IGF-1 to heart failure. Also mentioned are the prevention of heart failure and cardiac hypertrophy and the effectiveness of Canagliflozi and Empagliflozin drugs in the reduction of cardiovascular events.