Block of cloned Kv4.3 potassium channels by dapoxetine

Calcium/calmodulin-dependent protein kinase II associates with the K+ channel isoform Kv4.3 in adult rat optic nerve

Spikes are said to exhibit "memory" in that they can be altered by spikes that precede them. In retinal ganglion cell axons, for example, rapid spiking can slow the propagation of subsequent spikes. This increases inter-spike interval and, thus, low-pass filters instantaneous spike frequency. Similarly, a K+ ion channel blocker (4-aminopyridine, 4AP) increases the time-to-peak of compound action potentials recorded from optic nerve, and we recently found that reducing autophosphorylation of calcium/calmodulin-dependent protein kinase II (CaMKII) does too. These results would be expected if CaMKII modulates spike propagation by regulating 4AP-sensitive K+ channels. As steps toward identifying a possible substrate, we test whether (i) 4AP alters optic nerve spike shape in ways consistent with reducing K+ current, (ii) 4AP alters spike propagation consistent with effects of reducing CaMKII activation, (iii) antibodies directed against 4AP-sensitive and CaMKII-regulated K+ channels bind to optic nerve axons, and (iv) optic nerve CaMKII co-immunoprecipitates with 4AP-sensitive K+ channels. We find that, in adult rat optic nerve, (i) 4AP selectively slows spike repolarization, (ii) 4AP slows spike propagation, (iii) immunogen-blockable staining is achieved with anti-Kv4.3 antibodies but not with antibodies directed against Kv1.4 or Kv4.2, and (iv) CaMKII associates with Kv4.3. Kv4.3 may thus be a substrate that underlies activity-dependent spike regulation in adult visual system pathways.

The selective serotonin reuptake inhibitor dapoxetine inhibits voltage-dependent K+ channels in rabbit coronary arterial smooth muscle cells

We investigated the inhibitory effect of dapoxetine, a selective serotonin reuptake inhibitor (SSRI), on voltage-dependent K⁺ (Kv) channels using native smooth muscle cells from rabbit coronary arteries. Dapoxetine inhibited Kv channel currents in a concentration-dependent manner, with an IC₅₀ value of 2.68±0.94 μmol/L and a slope value (Hill coefficient) of 0.63±0.11. Application of 10 μmol/L dapoxetine accelerated the rate of inactivation of Kv currents. Although dapoxetine did not modify current activation kinetics, it caused a significant negative shift in the inactivation curves. Application of train step (1 or 2 Hz) progressively increased the inhibitory effect of dapoxetine on Kv channels. In addition, the recovery time constant was extended in its presence, suggesting that the longer recovery time constant from inactivation underlies a use-dependent inhibition of the channel. From these results, we conclude that dapoxetine inhibits Kv channels in a dose-, time-, use-, and state (open)-dependent manner, independent of serotonin reuptake inhibition.

Dapoxetine induces neuroprotective effects against glutamate-induced neuronal cell death by inhibiting calcium signaling and mitochondrial depolarization in cultured rat hippocampal neurons

Selective serotonin reuptake inhibitors (SSRIs) have an inhibitory effect on various ion channels including Ca²⁺ channels. We used fluorescent dye-based digital imaging, whole-cell patch clamping and cytotoxicity assays to examine whether dapoxetine, a novel rapid-acting SSRI, affect glutamate-induced calcium signaling, mitochondrial depolarization and neuronal cell death in cultured rat hippocampal neurons. Pretreatment with dapoxetine for 10min inhibited glutamate-induced intracellular free Ca²⁺ concentration ([Ca²⁺]_i) increases in a concentration-dependent manner (Half maximal inhibitory concentration=4.79µM). Dapoxetine (5μM) markedly inhibited glutamate-induced [Ca²⁺]_i increases, whereas other SSRIs such as fluoxetine and citalopram only slightly inhibited them. Dapoxetine significantly inhibited the glutamate-induced [Ca²⁺]_i responses following depletion of intracellular Ca²⁺ stores by treatment with thapsigargin. Dapoxetine markedly inhibited the metabotropic glutamate receptor agonist, (S)-3,5-dihydroxyphenylglycine-induced [Ca²⁺]_i increases. Dapoxetine significantly inhibited the glutamate and α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA)-induced [Ca²⁺]_i responses in either the presence or absence of nimodipine. Dapoxetine also significantly inhibited AMPA-evoked currents. However, dapoxetine slightly inhibited N-methyl-D-aspartate (NMDA)-induced [Ca²⁺]_i increases. Dapoxetine markedly inhibited 50mMK⁺-induced [Ca²⁺]_i increases. Dapoxetine significantly inhibited glutamate-induced mitochondrial depolarization. In addition, dapoxetine significantly inhibited glutamate-induced neuronal cell death and its neuroprotective effect was significantly greater than fluoxetine. These data suggest that dapoxetine reduces glutamate-induced [Ca²⁺]_i increases by inhibiting multiple pathways mainly through AMPA receptors, voltage-gated L-type Ca²⁺ channels and metabotropic glutamate receptors, which are involved in neuroprotection against glutamate-induced cell death through mitochondrial depolarization.

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.

Auxiliary KChIP4a suppresses A-type K+ current through endoplasmic reticulum (ER) retention and promoting closed-state inactivation of Kv4 channels

In the brain and heart, auxiliary Kv channel-interacting proteins (KChIPs) co-assemble with pore-forming Kv4 α-subunits to form a native K(+) channel complex and regulate the expression and gating properties of Kv4 currents. Among the KChIP1-4 members, KChIP4a exhibits a unique N terminus that is known to suppress Kv4 function, but the underlying mechanism of Kv4 inhibition remains unknown. Using a combination of confocal imaging, surface biotinylation, and electrophysiological recordings, we identified a novel endoplasmic reticulum (ER) retention motif, consisting of six hydrophobic and aliphatic residues, 12-17 (LIVIVL), within the KChIP4a N-terminal KID, that functions to reduce surface expression of Kv4-KChIP complexes. This ER retention capacity is transferable and depends on its flanking location. In addition, adjacent to the ER retention motif, the residues 19-21 (VKL motif) directly promote closed-state inactivation of Kv4.3, thus leading to an inhibition of channel current. Taken together, our findings demonstrate that KChIP4a suppresses A-type Kv4 current via ER retention and enhancement of Kv4 closed-state inactivation.

Dapoxetine: a new option in the medical management of premature ejaculation

Premature ejaculation (PE) is a common male sexual disorder which is associated with substantial personal and interpersonal negative psychological consequences. Pharmacotherapy of PE with off-label antidepressant selective serotonin reuptake inhibitors (SSRIs) is common, effective and safe. Development and regulatory approval of drugs specifically for the treatment of PE will reduce reliance on off-label treatments and serve to fill an unmet treatment need. The objective of this article is to review evidence supporting the efficacy and safety of dapoxetine in the treatment of PE. MEDLINE, Web of Science, PICA, EMBASE and the proceedings of major international and regional scientific meetings were searched for publications or abstracts published during the period 1993-2012 that used the word 'dapoxetine' in the title, abstract or keywords. This search was then manually cross referenced for all papers. This review encompasses studies of dapoxetine pharmacokinetics, animal studies, human phase I, II and III studies, independent postmarketing and pharmacovigilance efficacy and safety studies and drug-interaction studies. Dapoxetine is a potent SSRI which is administered on demand 1-3 h prior to planned sexual contact. It is rapidly absorbed and eliminated, resulting in minimal accumulation, and has dose-proportional pharmacokinetics which are unaffected by multiple dosing. Dapoxetine 30 mg and 60 mg has been evaluated in five industry-sponsored randomized, double-blind, placebo-controlled studies in 6081 men aged at least 18 years. Outcome measures included stopwatch-measured intravaginal ejaculatory latency time (IELT), Premature Ejaculation Profile (PEP) inventory items, Clinical Global Impression of Change (CGIC) in PE, and adverse events. Mean IELT, all PEP items and CGIC improved significantly with both doses of dapoxetine versus placebo (all p <0.001). The most common treatment-related adverse effects included nausea (11.0% for 30 mg, 22.2% for 60 mg), dizziness (5.9% for 30 mg, 10.9% for 60 mg), and headache (5.6% for 30 mg, 8.8% for 60 mg), and evaluation of validated rated scales demonstrated no SSRI class-related effects with dapoxetine use. Dapoxetine, as the first drug developed for PE, is an effective and safe treatment for PE and represents a major advance in sexual medicine.

Duloxetine blocks cloned Kv4.3 potassium channels

The effects of duloxetine were examined on cloned Kv4.3 channels stably expressed in CHO cells using the whole-cell patch-clamp technique. Duloxetine decreased the peak amplitude of Kv4.3 currents with an acceleration of the decay rate of current inactivation in a concentration-dependent manner. The IC(50) values required for the blocking effects of duloxetine on the peak amplitude and the integral of currents were 8.4 and 2.1μM, respectively. Duloxetine accelerated the rate of inactivation of Kv4.3 currents and thereby decreased the time-to-peak in a concentration-dependent manner. Analysis of the time dependence of the drug block produced estimates of 21.9μM(-1)s(-1) and 165.9s(-1), for the respective association (k(+1)) and dissociation (k(-1)) rate constants. The K(d) value (k(-1)/k(+1)) yielded 7.5μM, which approximates the experimental IC(50) value obtained from the concentration-response curve. The block of Kv4.3 by duloxetine was voltage-dependent at a membrane potential coinciding with the activation of the channels. At a more positive potential, however, the block was relieved. Duloxetine produced a hyperpolarizing shift in the voltage dependence of the steady-state inactivation of Kv4.3, and accelerated the closed-state inactivation of Kv4.3 in the subthreshold voltage range. Duloxetine induced a significant use-dependent block at frequencies of 1 and 2Hz. In the presence of duloxetine, the recovery from inactivation was slower than under control conditions. These results demonstrate that duloxetine exerts a concentration-dependent block of Kv4.3 by binding to the channels in the open and inactivated states and these actions may contribute to its analgesic effect in neuropathic pain.

Effects of dapoxetine on cloned Kv1.5 channels expressed in CHO cells

The effects of dapoxetine were examined on cloned Kv1.5 channels stably expressed in Chinese hamster ovary cells using the whole-cell patch clamp technique. Dapoxetine decreased the peak amplitude of Kv1.5 currents and accelerated the decay rate of current inactivation in a concentration-dependent manner with an IC ( 50 ) of 11.6 μM. Kinetic analysis of the time-dependent effects of dapoxetine on Kv1.5 current decay yielded the apparent association (k (+1 )) and dissociation (k (-1 )) rate constants of 2.8 μM(-1) s(-1) and 34.2 s(-1), respectively. The theoretical K ( D ) value, derived by k (-1 )/k (+1 ), yielded 12.3 μM, which was reasonably similar to the IC ( 50 ) value obtained from the concentration-response curve. Dapoxetine decreased the tail current amplitude and slowed the deactivation process of Kv1.5, which resulted in a tail crossover phenomenon. The block by dapoxetine is voltage-dependent and steeply increased at potentials between -10 and +10 mV, which correspond to the voltage range of channel activation. At more depolarized potentials, a weaker voltage dependence was observed (δ=0.31). Dapoxetine had no effect on the steady-state activation of Kv1.5 but shifted the steady-state inactivation curves in a hyperpolarizing direction. Dapoxetine produced a use-dependent block of Kv1.5 at frequencies of 1 and 2 Hz and slowed the time course for recovery of inactivation. These effects were reversible after washout of the drug. Our results indicate that dapoxetine blocks Kv1.5 currents by interacting with the channel in both the open and inactivated states of the channel.