Extracellular acidosis modulates drug block of Kv4.3 currents by flecainide and quinidine

Differential responses of rabbit ventricular and atrial transient outward current (Ito) to the Ito modulator NS5806

Transient outward potassium current (I_to) in the heart underlies phase 1 repolarization of cardiac action potentials and thereby affects excitation–contraction coupling. Small molecule activators of I_to may therefore offer novel treatments for cardiac dysfunction, including heart failure and atrial fibrillation. NS5806 has been identified as a prototypic activator of canine I_to. This study investigated, for the first time, actions of NS5806 on rabbit atrial and ventricular I_to. Whole cell patch‐clamp recordings of I_to and action potentials were made at physiological temperature from rabbit ventricular and atrial myocytes. 10 μmol/L NS5806 increased ventricular I_to with a leftward shift in I_to activation and accelerated restitution. At higher concentrations, stimulation of I_to was followed by inhibition. The EC₅₀ for stimulation was 1.6 μmol/L and inhibition had an IC₅₀ of 40.7 μmol/L. NS5806 only inhibited atrial I_to (IC₅₀ of 18 μmol/L) and produced a modest leftward shifts in I_to activation and inactivation, without an effect on restitution. 10 μmol/L NS5806 shortened ventricular action potential duration (APD) at APD₂₀‐APD₉₀ but prolonged atrial APD. NS5806 also reduced atrial AP upstroke and amplitude, consistent with an additional atrio‐selective effect on Na⁺ channels. In contrast to NS5806, flecainide, which discriminates between Kv1.4 and 4.x channels, produced similar levels of inhibition of ventricular and atrial I_to. NS5806 discriminates between rabbit ventricular and atrial I_to, with mixed activator and inhibitor actions on the former and inhibitor actions against the later. NS5806 may be of significant value for pharmacological interrogation of regional differences in native cardiac I_to.

Effects of neferine on Kv4.3 channels expressed in HEK293 cells and ex vivo electrophysiology of rabbit hearts

AIM

Neferine is an isoquinoline alkaloid isolated from seed embryos of Nelumbo nucifera (Gaertn), which has a variety of biological activities. In this study we examined the effects of neferine on Kv4.3 channels, a major contributor to the transient outward current (I(to)) in rabbit heart, and on ex vivo electrophysiology of rabbit hearts.

METHODS

Whole-cell Kv4.3 currents were recorded in HEK293 cells expressing human cardiac Kv4.3 channels using patch-clamp technique. Arterially perfused wedges of rabbit left ventricles (LV) were prepared, and transmembrane action potentials were simultaneously recorded from epicardial (Epi) and endocardial (Endo) sites with floating microelectrodes together with transmural electrocardiography (ECG).

RESULTS

Neferine (0.1-100 μmol/L) dose-dependently and reversibly inhibited Kv4.3 currents (the IC50 value was 8.437 μmol/L, and the maximal inhibition at 100 μmol/L was 44.12%). Neferine (10 μmol/L) caused a positive shift of the steady-state activation curve of Kv4.3 currents, and a negative shift of the steady-state inactivation curve. Furthermore, neferine (10 μmol/L) accelerated the inactivation but not the activation of Kv4.3 currents, and markedly slowed the recovery of Kv4.3 currents from inactivation. Neferine-induced blocking of Kv4.3 currents was frequency-dependent. In arterially perfused wedges of rabbit LV, neferine (1, 3, and 10 μmol/L) dose-dependently prolonged the QT intervals and action potential durations (APD) at both Epi and Endo sites, and caused dramatic increase of APD10 at Epi sites.

CONCLUSION

Neferine inhibits Kv4.3 channels likely by blocking the open state and inactivating state channels, which contributes to neferine-induced dramatic increase of APD10 at Epi sites of rabbit heart.

Inhibition of cardiac Kv1.5 and Kv4.3 potassium channels by the class Ia anti-arrhythmic ajmaline: mode of action

Ajmaline is a class Ia anti-arrhythmic compound that is widely used for the diagnosis of Brugada syndrome and the acute treatment of atrial or ventricular tachycardia. For ajmaline, inhibitory effects on a variety of cardiac K(+) channels have been observed, including cardiac Kv1 and Kv4 channels. However, the exact pharmacological properties of channel blockade have not yet been addressed adequately. Using two different expression systems, we analysed pharmacological effects of ajmaline on the potassium channels Kv1.5 and Kv4.3 underlying cardiac I Kur and I to current, respectively. When expressed in a mammalian cell line, we find that ajmaline inhibits Kv1.5 and Kv4.3 with an IC50 of 1.70 and 2.66 μM, respectively. Pharmacological properties were further analysed using the Xenopus expression system. We find that ajmaline is an open channel inhibitor of cardiac Kv1.5 and Kv4.3 channels. Whereas ajmaline results in a mild leftward shift of Kv1.5 activation curve, no significant effect on Kv4.3 channel activation could be observed. Ajmaline did not significantly affect channel inactivation kinetics. Onset of block was fast. For Kv4.3 channels, no significant effect on recovery from inactivation or channel deactivation could be observed. Furthermore, there was no use-dependence of block. Taken together, we show that ajmaline inhibits cardiac Kv1.5 and Kv4.3 channels at therapeutic concentrations. These data add to the current understanding of the electrophysiological basis of anti-arrhythmic action of ajmaline.

Modulation of ventricular transient outward K⁺ current by acidosis and its effects on excitation-contraction coupling

The contribution of transient outward current (Ito) to changes in ventricular action potential (AP) repolarization induced by acidosis is unresolved, as is the indirect effect of these changes on calcium handling. To address this issue we measured intracellular pH (pHi), Ito, L-type calcium current (ICa,L), and calcium transients (CaTs) in rabbit ventricular myocytes. Intracellular acidosis [pHi 6.75 with extracellular pH (pHo) 7.4] reduced Ito by ~50% in myocytes with both high (epicardial) and low (papillary muscle) Ito densities, with little effect on steady-state inactivation and activation. Of the two candidate α-subunits underlying Ito, human (h)Kv4.3 and hKv1.4, only hKv4.3 current was reduced by intracellular acidosis. Extracellular acidosis (pHo 6.5) shifted Ito inactivation toward less negative potentials but had negligible effect on peak current at +60 mV when initiated from -80 mV. The effects of low pHi-induced inhibition of Ito on AP repolarization were much greater in epicardial than papillary muscle myocytes and included slowing of phase 1, attenuation of the notch, and elevation of the plateau. Low pHi increased AP duration in both cell types, with the greatest lengthening occurring in epicardial myocytes. The changes in epicardial AP repolarization induced by intracellular acidosis reduced peak ICa,L, increased net calcium influx via ICa,L, and increased CaT amplitude. In summary, in contrast to low pHo, intracellular acidosis has a marked inhibitory effect on ventricular Ito, perhaps mediated by Kv4.3. By altering the trajectory of the AP repolarization, low pHi has a significant indirect effect on calcium handling, especially evident in epicardial cells.

Volume regulation in the human airway epithelial cell line Calu-3

Cells regulate their volume in response to changes in the osmolarity of both their extracellular and their intracellular environments. We investigated the ability of the human airway epithelial cell line Calu-3 to respond to changes in extracellular osmolarity. Although switching Calu-3 cells from an isosmotic to a hyperosmotic environment resulted in cell shrinkage, there was no compensatory mechanism for the cells to return to their original volume. In contrast, switching to a hyposmotic environment resulted in an initial cell swelling response, followed by a regulatory volume decrease (RVD). Pharmacologic studies demonstrate that the voltage-activated K+ channels Kv4.1 and (or) Kv4.3 play a crucial role in mediating this RVD response, and we demonstrated expression of these channel types at the mRNA and protein levels. Furthermore, inhibition of the large- and intermediate-conductance Ca2+-activated K+ channels KCa1.1 (maxi-K) and KCa3.1 (hIK) also implicated these channels as playing a role in volume recovery in Calu-3 cells. This report describes the nature of volume regulation in the widely used model cell line Calu-3.

Effects of MiRP1 and DPP6 beta-subunits on the blockade induced by flecainide of Kv4.3/KChIP2 channels

BACKGROUND AND PURPOSE

The human cardiac transient outward potassium current (Ito) is believed to be composed of the pore-forming Kv4.3 alpha-subunit, coassembled with modulatory beta-subunits as KChIP2, MiRP1 and DPP6 proteins. beta-Subunits can alter the pharmacological response of Ito; therefore, we analysed the effects of flecainide on Kv4.3/KChIP2 channels coassembled with MiRP1 and/or DPP6 beta-subunits.

EXPERIMENTAL APPROACH

Currents were recorded in Chinese hamster ovary cells stably expressing K(V)4.3/KChIP2 channels, and transiently transfected with either MiRP1, DPP6 or both, using the whole-cell patch-clamp technique.

KEY RESULTS

In control conditions, Kv4.3/KChIP2/MiRP1 channels exhibited the slowest activation and inactivation kinetics and showed an 'overshoot' in the time course of recovery from inactivation. The midpoint values (Vh) of the activation and inactivation curves for Kv4.3/KChIP2/DPP6 and Kv4.3/KChIP2/MiRP1/DPP6 channels were approximately 10 mV more negative than Vh values for Kv4.3/KChIP2 and Kv4.3/KChIP2/MiRP1 channels. Flecainide (0.1-100 microM) produced a similar concentration-dependent blockade of total integrated current flow (IC50 approximately 10 microM) in all the channel complexes. However, the IC50 values for peak current amplitude and inactivated channel block were significantly different. Flecainide shifted the Vh values of both the activation and inactivation curves to more negative potentials and apparently accelerated inactivation kinetics in all channels. Moreover, flecainide slowed recovery from inactivation in all the channel complexes and suppressed the 'overshoot' in Kv4.3/KChIP2/MiRP1 channels.

CONCLUSIONS AND IMPLICATIONS

Flecainide directly binds to the Kv4.3 alpha-subunit when the channels are in the open and inactivated state and the presence of the beta-subunits modulates the blockade by altering the gating function.

[New antiarrhythmic drugs for therapy of atrial fibrillation: I. Ion channel blockers]

During the last ten years we have made substantial progress in our understanding of the underlying mechanisms of atrial fibrillation. The high rate associated alterations in electrical and structural properties of the atria, referred to as atrial remodeling, promote the progression of atrial fibrillation. The development of new therapeutic approaches addresses three different directions: (i) prevention of atrial remodeling, especially of structural remodeling; (ii) increase of long-term efficacy of currently used drugs and improvement of their side-effect profile; and (iii) design of atria- and pathology-specific antiarrhythmic drugs without concomitant proarrhythmic effects in the ventricles. The current review outlines the pathophysiology of atrial fibrillation and focuses on electrical remodeling. The properties of new antiarrhythmic drugs for atrial fibrillation are discussed in detail.

Blocking Characteristics of hKv1.5 and hKv4.3/hKChIP2.2 After Administration of the Novel Antiarrhythmic Compound AZD7009

AZD7009 is a novel antiarrhythmic compound in early clinical development for management of atrial fibrillation. Electrophysiological studies in animals have shown high antiarrhythmic efficacy, predominant action on atrial electrophysiology, and low proarrhythmic activity. AZD7009 has previously been shown to inhibit hERG and hNav1.5 currents. The main objective of the present study was to characterize the effects of AZD7009 on hKv1.5 and hKv4.3/hKChIP2.2 currents to get a deeper understanding of the ion channel-blocking properties of the compound. hKv1.5 and hKv4.3/hKChIP2.2 currents were expressed in CHO cells. Currents were measured using the whole-cell configuration of the voltage-clamp technique. AZD7009 inhibited hKv1.5 and hKv4.3/hKChIP2.2 currents with equal potency: the IC50 for hKv1.5 block was 27.0 +/- 1.6 muM (n = 6), and the IC50 for hKv4.3/hKChIP2.2 block was 23.7 +/- 4.4 muM (n = 5). Block of the hKv4.3/hKChIP2.2 current was frequency dependent with larger block at higher frequency, whereas block of the hKv1.5 current was slightly decreased at higher frequency. In conclusion, AZD7009 inhibits both the hKv1.5 and the hKv4.3/hKChIP2.2 currents. These effects likely contribute to the effects described in animals in vivo.

The influence of extracellular acidosis on the effect of IKr blockers

BACKGROUND

Myocardial infarction causes the acidification of the cellular environment and the resultant acidosis maybe arrhythmogenic. The effect of acidosis on the action of antiarrhythmic drugs, an important issue in the antiarrhythmic drug therapy after myocardial infarction, remains to be studied.

METHODS

To evaluate the effect of acidosis on rectifier potassium current (Ikr) blockers, the human ether-a-go-go-related gene (HERG), which encodes IKr, was expressed in Xenopus laevis oocytes. The two electrodes voltage clamp technique was used and the experiments were performed at room temperature.

RESULTS

Quinidine (10 microM) inhibited HERG tail current by 37% +/- 5% at pH7.4. The block decreased to 5% +/- 2% with extracellular pH at 6.2. Dofetilide (0.3 microM) inhibited HERG tail current by 34% +/- 3% and 1% +/- 2% at extracellular pH 7.4 and 6.2, respectively. Azimilide (10 microM) inhibited HERG tail current by 59% +/- 3% and 17% +/- 3% at extracellular pH 7.4 and 6.2. There were significant differences in the HERG inhibition by quinidine, dofetilide, and azimilide between pH 7.4 and pH 6.2 (P < .01). The drug concentration blocking 50% of current (IC50) was 5.8 +/- 0.3 microM for azimilide, 9.9 +/- 1.0 microM for quinidine, and 0.5 +/- 0.02 microM for dofetilide at pH 7.4. When extracellular pH was decreased from 7.4 to 6.2, the IC50 increased to 95.5 +/- 11.3 microM for azimilide, 203.2 +/- 15.7 microM for quinidine, and 12.6 +/- 1.2 microM for dofetilide. Unlike quinidine, dofetilide, and azimilide, there was no significant difference in the percentage of current block by amiodarone between pH 6.2 and 7.4. For amiodarone, the IC50 was 38.3 +/- 8.5 microM at pH 7.4 and 27.3 +/- 1.6 microM at pH 6.2.

CONCLUSION

Our data show that the Ikr blocking effect of azimilide, dofetilide, and quinidine was attenuated at acid pH, whereas this was not the case for amiodarone. These observations may explain the efficacy of amiodarone in reducing arrhythmic death in patients after a myocardial infarction compared with other IKr blockers.

A pH-sensitive potassium conductance (TASK) and its function in the murine gastrointestinal tract

The excitability of smooth muscles is regulated, in part, by background K+ conductances that determine resting membrane potential. However, the K+ conductances so far described in gastrointestinal (GI) muscles are not sufficient to explain the negative resting potentials of these cells. Here we describe expression of two-pore K+ channels of the TASK family in murine small and large intestinal muscles. TASK-2, cloned from murine intestinal muscles, resulted in a pH-sensitive, time-dependent, non-inactivating K+ conductance with slow activation kinetics. A similar conductance was found in native intestinal myocytes using whole-cell patch-clamp conditions. The pH-sensitive current was blocked by local anaesthetics. Lidocaine, bupivacaine and acidic pH depolarized circular muscle cells in intact muscles and decreased amplitude and frequency of slow waves. The effects of lidocaine were not blocked by tetraethylammonium chloride, 4-aminopyridine, glibenclamide, apamin or MK-499. However, depolarization by acidic pH was abolished by pre-treatment with lidocaine, suggesting that lidocaine-sensitive K+ channels were responsible for pH-sensitive changes in membrane potential. The kinetics of activation, sensitivity to pH, and pharmacology of the conductance in intestinal myocytes and the expression of TASK-1 and TASK-2 in these cells suggest that the pH-sensitive background conductance is encoded by TASK genes. This conductance appears to contribute significantly to resting potential and may regulate excitability of GI muscles.