Potent activation of large-conductance Ca2+-activated K+ channels by the diphenylurea 1,3-bis-[2-hydroxy-5-(trifluoromethyl)phenyl]urea (NS1643) in pituitary tumor (GH3) cells

Rufinamide, a Triazole-Derived Antiepileptic Drug, Stimulates Ca2+-Activated K+ Currents While Inhibiting Voltage-Gated Na+ Currents

Rufinamide (RFM) is a clinically utilized antiepileptic drug that, as a triazole derivative, has a unique structure. The extent to which this drug affects membrane ionic currents remains incompletely understood. With the aid of patch clamp technology, we investigated the effects of RFM on the amplitude, gating, and hysteresis of ionic currents from pituitary GH3 lactotrophs. RFM increased the amplitude of Ca2+-activated K+ currents (IK(Ca)) in pituitary GH3 lactotrophs, and the increase was attenuated by the further addition of iberiotoxin or paxilline. The addition of RFM to the cytosolic surface of the detached patch of membrane resulted in the enhanced activity of large-conductance Ca2+-activated K+ channels (BKCa channels), and paxilline reversed this activity. RFM increased the strength of the hysteresis exhibited by the BKCa channels and induced by an inverted isosceles-triangular ramp pulse. The peak and late voltage-gated Na+ current (INa) evoked by rapid step depolarizations were differentially suppressed by RFM. The molecular docking approach suggested that RFM bound to the intracellular domain of KCa1.1 channels with amino acid residues, thereby functionally affecting BKCa channels' activity. This study is the first to present evidence that, in addition to inhibiting the INa, RFM effectively modifies the IK(Ca), which suggests that it has an impact on neuronal function and excitability.

The Evidence for Effective Inhibition of INa Produced by Mirogabalin ((1R,5S,6S)-6-(aminomethyl)-3-ethyl-bicyclo [3.2.0] hept-3-ene-6-acetic acid), a Known Blocker of CaV Channels

Mirogabalin (MGB, Tarlige®), an inhibitor of the α2δ-1 subunit of voltage-gated Ca2+ (CaV) channels, is used as a way to alleviate peripheral neuropathic pain and diabetic neuropathy. However, to what extent MGB modifies the magnitude, gating, and/or hysteresis of various types of plasmalemmal ionic currents remains largely unexplored. In pituitary tumor (GH3) cells, we found that MGB was effective at suppressing the peak (transient, INa(T)) and sustained (late, INa(L)) components of the voltage-gated Na+ current (INa) in a concentration-dependent manner, with an effective IC50 of 19.5 and 7.3 μM, respectively, while the KD value calculated on the basis of minimum reaction scheme was 8.2 μM. The recovery of INa(T) inactivation slowed in the presence of MGB, although the overall current-voltage relation of INa(T) was unaltered; however, there was a leftward shift in the inactivation curve of the current. The magnitude of the window (INa(W)) or resurgent INa (INa(R)) evoked by the respective ascending or descending ramp pulse (Vramp) was reduced during cell exposure to MGB. MGB-induced attenuation in INa(W) or INa(R) was reversed by the further addition of tefluthrin, a pyrethroid insecticide known to stimulate INa. MGB also effectively lessened the strength of voltage-dependent hysteresis of persistent INa in response to the isosceles triangular Vramp. The cumulative inhibition of INa(T), evoked by pulse train stimulation, was enhanced in its presence. Taken together, in addition to the inhibition of CaV channels, the NaV channel attenuation produced by MGB might have an impact in its analgesic effects occurring in vivo.

Molecular Activation of the Kv11.1 Channel Reprograms EMT in Colon Cancer by Inhibiting TGFβ Signaling via Activation of Calcineurin

Control of ionic gradients is critical to maintain cellular homeostasis in both physiological and pathological conditions, but the role of ion channels in cancer cells has not been studied thoroughly. In this work we demonstrated that activity of the Kv11.1 potassium channel plays a vital role in controlling the migration of colon cancer cells by reversing the epithelial-to-mesenchymal transition (EMT) into the mesenchymal-to-epithelial transition (MET). We discovered that pharmacological stimulation of the Kv11.1 channel with the activator molecule NS1643 produces a strong inhibition of colon cancer cell motility. In agreement with the reversal of EMT, NS1643 treatment leads to a depletion of mesenchymal markers such as SNAIL1, SLUG, TWIST, ZEB, N-cadherin, and c-Myc, while the epithelial marker E-cadherin was strongly upregulated. Investigating the mechanism linking Kv11.1 activity to reversal of EMT into MET revealed that stimulation of Kv11.1 produced a strong and fast inhibition of the TGFβ signaling. Application of NS1643 resulted in de-phosphorylation of the TGFβ downstream effectors R-SMADs by activation of the serine/threonine phosphatase PP2B (calcineurin). Consistent with the role of TGFβ in controlling cancer stemness, NS1643 also produced a strong inhibition of NANOG, SOX2, and OCT4 while arresting the cell cycle in G0/G1. Our data demonstrate that activation of the Kv11.1 channel reprograms EMT into MET by inhibiting TGFβ signaling, which results in inhibition of motility in colon cancer cells.

Action potential shortening rescues atrial calcium alternans

KEY POINTS

Cardiac alternans refers to a beat-to-beat alternation in contraction, action potential (AP) morphology and Ca²⁺ transient (CaT) amplitude, and represents a risk factor for cardiac arrhythmia, including atrial fibrillation. We developed strategies to pharmacologically manipulate the AP waveform with the goal to reduce or eliminate the occurrence of CaT and contraction alternans in atrial tissue. With combined patch-clamp and intracellular Ca²⁺ measurements we investigated the effect of specific ion channel inhibitors and activators on alternans. In single rabbit atrial myocytes, suppression of Ca²⁺ -activated Cl^- channels eliminated AP duration alternans, but prolonged the AP and failed to eliminate CaT alternans. In contrast, activation of K⁺ currents (I_Ks and I_Kr ) shortened the AP and eliminated both AP duration and CaT alternans. As demonstrated also at the whole heart level, activation of K⁺ conductances represents a promising strategy to suppress alternans, and thus reducing a risk factor for atrial fibrillation.

ABSTRACT

At the cellular level alternans is observed as beat-to-beat alternations in contraction, action potential (AP) morphology and magnitude of the Ca²⁺ transient (CaT). Alternans is a well-established risk factor for cardiac arrhythmia, including atrial fibrillation. This study investigates whether pharmacological manipulation of AP morphology is a viable strategy to reduce the risk of arrhythmogenic CaT alternans. Pacing-induced AP and CaT alternans were studied in rabbit atrial myocytes using combined Ca²⁺ imaging and electrophysiological measurements. Increased AP duration (APD) and beat-to-beat alternations in AP morphology lowered the pacing frequency threshold and increased the degree of CaT alternans. Inhibition of Ca²⁺ -activated Cl^- channels reduced beat-to-beat AP alternations, but prolonged APD and failed to suppress CaT alternans. In contrast, AP shortening induced by activators of two K⁺ channels (ML277 for Kv7.1 and NS1643 for Kv11.1) abolished both APD and CaT alternans in field-stimulated and current-clamped myocytes. K⁺ channel activators had no effect on the degree of Ca²⁺ alternans in AP voltage-clamped cells, confirming that suppression of Ca²⁺ alternans was caused by the changes in AP morphology. Finally, activation of Kv11.1 channel significantly attenuated or even abolished atrial T-wave alternans in isolated Langendorff perfused hearts. In summary, AP shortening suppressed or completely eliminated both CaT and APD alternans in single atrial myocytes and atrial T-wave alternans at the whole heart level. Therefore, we suggest that AP shortening is a potential intervention to avert development of alternans with important ramifications for arrhythmia prevention and therapy.

Spontaneous and Direct Transformation of N,O-Diaryl Carbamates into N,N'-Diarylureas

We have discovered a spontaneous reaction of N,O-diaryl carbamates to afford symmetrical N,N'-diarylureas. Optimization of the conditions indicated that N,N-dimethylformamide (DMF) was the best solvent and triethylamine (Et₃N) was the best additive for this transformation. The reaction requires the presence of aryl groups on the nitrogen and oxygen atoms of carbamates. Substrates bearing an electron-donating methoxy group on either of the aryl groups reacted slowly under these conditions.

Stimulatory actions of a novel thiourea derivative on large-conductance, calcium-activated potassium channels

In this study, we examine whether an anti-inflammatory thiourea derivative, compound #326, actions on ion channels. The effects of compound #326 on Ca²⁺ -activated K⁺ channels were evaluated by patch-clamp recordings obtained in cell-attached, inside-out or whole-cell configuration. In pituitary GH₃ cells, compound #326 increased the amplitude of Ca²⁺ -activated K⁺ currents (I_K(Ca) ) with an EC₅₀ value of 11.6 μM, which was reversed by verruculogen, but not tolbutamide or TRAM-34. Under inside-out configuration, a bath application of compound #326 raised the probability of large-conductance Ca²⁺ -activated K⁺ (BK_Ca ) channels. The activation curve of BK_Ca channels was shifted to less depolarised potential with no modification of the gating charge of the curve; consequently, the difference of free energy was reduced in the presence of this compound. Compound #326-stimulated activity of BK_Ca channels is explained by a shortening of mean closed time, despite its inability to alter single-channel conductance. Neither delayed-rectifier nor erg-mediated K⁺ currents was modified. Compound #326 decreased the peak amplitude of voltage-gated Na⁺ current with no clear change in the overall current-voltage relationship of this current. In HEK293T cells expressing α-hSlo, compound #326 enhanced BK_Ca channels effectively. Intriguingly, the inhibitory actions of compound #326 on interleukin 1β in lipopolysaccharide-activated microglia were significantly reversed by verruculogen, whereas BK_Ca channel inhibitors suppressed the expressions of inducible nitric oxide synthase. The BK_Ca channels could be an important target for compound #326 if similar in vivo results occur, and the multi-functionality of BK_Ca channels in modulating microglial immunity merit further investigation.

Actions of KMUP-1, a xanthine and piperazine derivative, on voltage-gated Na(+) and Ca(2+) -activated K(+) currents in GH3 pituitary tumour cells

BACKGROUND AND PURPOSE

7-[2-[4-(2-Chlorophenyl)piperazinyl]ethyl]-1,3-dimethylxanthine (KMUP-1) is a xanthine-based derivative. It has soluble GC activation and K(+) -channel opening activity. Effects of this compound on ion currents in pituitary GH3 cells were investigated in this study.

EXPERIMENTAL APPROACH

The aim of this study was to evaluate effects of KMUP-1 on the amplitude and gating of voltage-gated Na(+) current (INa ) in pituitary GH3 cells and in HEKT293T cells expressing SCN5A. Both the amplitude of Ca(2+) -activated K(+) current and the activity of large-conductance Ca(2+) -activated K(+) (BKCa ) channels were also studied.

KEY RESULTS

KMUP-1 depressed the transient and late components of INa with different potencies. The IC50 values required for its inhibitory effect on transient and late INa were 22.5 and 1.8 μM respectively. KMUP-1 (3 μM) shifted the steady-state inactivation of INa to a hyperpolarized potential by -10 mV, despite inability to alter the recovery of INa from inactivation. In cell-attached configuration, KMUP-1 applied to bath increased BKCa -channel activity; however, in inside-out patches, this compound applied to the intracellular surface had no effect on it. It prolonged the latency in the generation of action currents elicited by triangular voltage ramps. Additionally, KMUP-1 decreased the peak INa with a concomitant increase of current inactivation in HEKT293T cells expressing SCN5A.

CONCLUSIONS AND IMPLICATIONS

Apart from activating BKCa channels, KMUP-1 preferentially suppresses late INa . The effects of KUMP-1 on ion currents presented here constitute an underlying ionic mechanism of its actions.

Ability of naringenin, a bioflavonoid, to activate M-type potassium current in motor neuron-like cells and to increase BKCa-channel activity in HEK293T cells transfected with α-hSlo subunit

BACKGROUND

Naringenin (NGEN) is a citrus bioflavonoid known to have beneficial health properties; however, the ionic mechanism of its actions remains largely unclear. In this study, we attempted to evaluate the possible effects of NGEN on K(+) currents in NSC-34 neuronal cells and in HEK293T cells expressing α-hSlo.

RESULTS

NGEN increased M-type K(+) current (I(K(M))) in a concentration-dependent manner with an EC50 value of 9.8 μM in NSC-34 cells. NGEN shifted the activation curve of I(K(M)) conductance to the more negative potentials. In cell-attached recordings, NGEN or flupirtine enhanced the activity of M-type K(+) (K(M)) channels with no changes in single-channel amplitude. NGEN (10 μM) had minimal effect on erg-mediated K(+) currents. Under cell-attached voltage-clamp recordings, NGEN decreased the frequency of spontaneous action currents and further application of linopirdine can reverse NGEN-induced inhibition of firing. In HEK293T cells expressing α-hSlo, this compound increased the amplitude of Ca(2+)-activated K(+) current (I(K(Ca))). Under inside-out recordings, NGEN applied to the intracellular side of the detached patch enhanced the activity of large-conductance Ca(2+)-activated K(+) (BK(Ca)) channels. Moreover, from the study of a modeled neuron, burst firing of simulated action potentials (APs) was reduced in the presence of the increased conductances of both K(M) and K(Ca) channels. Fast-slow analysis of AP bursting from this model also revealed that as the conductances of both K(M) and BK(Ca) channels were increased by two-fold, the voltage nullcline was shifted in an upward direction accompanied by the compression of burst trajectory.

CONCLUSIONS

The present results demonstrate that activation of both K(M) and BK(Ca) channels caused by NGEN might combine to influence neuronal activity if similar channels were functionally co-expressed in central neurons in vivo.

The human ether-a-go-go-related gene activator NS1643 enhances epilepsy-associated KCNQ channels

Human ether-a-go-go-related gene (hERG) and KCNQ channels are two classes of voltage-gated potassium channels. Specific mutations have been identified that are causal for type II long QT (LQT2) syndrome, neonatal epilepsy, and benign familial neonatal convulsions. Increasing evidence from clinical studies suggests that LQT2 and epilepsy coexist in some patients. Therefore, an integral approach to investigating and treating the two diseases is likely more effective. In the current study, we found that NS1643 [1,3-bis-(2-hydroxy-5-trifluoromethyl-phenyl)-urea], a previously reported hERG activator, is also an activator of KCNQ channels. It potentiates the neuronal KCNQ2, KCNQ4, and KCNQ2/Q3 channels, but not the cardiac KCNQ1. The effects of NS1643 on the KCNQ2 channel include left shifting of voltage for reaching 50% of the maximum conductance and slowing of deactivation. Analysis of the dose-response curve of NS1643 revealed an EC50 value of 2.44 ± 0.25 μM. A hydrophobic phenylalanine (F137) located at the middle region of the voltage-sensing domain was identified as critical for NS1643 activity on KCNQ2. When testing NS1643 effects in rescuing LQT2 hERG mutants and the KCNQ2 BFNC mutants, we found it is particularly efficacious in some cases. Considering the substantial relationship between LQT2 and epilepsy, these findings reveal that NS1643 is a useful compound to elucidate the causal connection of LQT2 and epilepsy. More generally, this may provide a strategy in the development of therapeutics for LQT2 and epilepsy.

Characterization of TRPM8-like channels activated by the cooling agent icilin in the macrophage cell line RAW 264.7

Icilin is recognized as a chemical agonist of nociceptors and can activate TRPM8 channels. However, whether this agent has any effects on immune cells remains unknown. In this study, the effects of icilin on ion currents were investigated in RAW 264.7 murine macrophage-like cells. Icilin (1-100 μM) increased the amplitude of nonselective (NS) cation current (INS) in a concentration-dependent manner with an EC50 value of 8.6 μM. LaCl3 (100 μM) or capsazepine (30 μM) reversed icilin-induced INS; however, neither apamin (200 nM) nor iberiotoxin (200 nM) had any effects on it. In cell-attached configuration, when the electrode was filled with icilin (30 μM), a unique population of NS cation channels were activated with single-channel conductance of 158 pS. With the use of a long-lasting ramp pulse protocol, increasing icilin concentration produced a left shift in the activation curve of NS channels, with no change in the slope factor of the curve. The probability of channel opening enhanced by icilin was increased by either elevated extracellular Ca2+ or application of ionomycin (10 μM), while it was reduced by BAPTA-AM (10 μM). Icilin-stimulated activity is associated with an increase in mean open time and a reduction in mean closed time. Under current-clamp conditions, icilin caused membrane depolarization. Therefore, icilin interacts with the TRPM8-like channel to increase INS and depolarizes the membrane in these cells.

Ion channels and signaling in the pituitary gland

Endocrine pituitary cells are neuronlike; they express numerous voltage-gated sodium, calcium, potassium, and chloride channels and fire action potentials spontaneously, accompanied by a rise in intracellular calcium. In some cells, spontaneous electrical activity is sufficient to drive the intracellular calcium concentration above the threshold for stimulus-secretion and stimulus-transcription coupling. In others, the function of these action potentials is to maintain the cells in a responsive state with cytosolic calcium near, but below, the threshold level. Some pituitary cells also express gap junction channels, which could be used for intercellular Ca(2+) signaling in these cells. Endocrine cells also express extracellular ligand-gated ion channels, and their activation by hypothalamic and intrapituitary hormones leads to amplification of the pacemaking activity and facilitation of calcium influx and hormone release. These cells also express numerous G protein-coupled receptors, which can stimulate or silence electrical activity and action potential-dependent calcium influx and hormone release. Other members of this receptor family can activate calcium channels in the endoplasmic reticulum, leading to a cell type-specific modulation of electrical activity. This review summarizes recent findings in this field and our current understanding of the complex relationship between voltage-gated ion channels, ligand-gated ion channels, gap junction channels, and G protein-coupled receptors in pituitary cells.

Effects of ranolazine, a novel anti-anginal drug, on ion currents and membrane potential in pituitary tumor GH(3) cells and NG108-15 neuronal cells

Ranolazine, a piperazine derivative, is currently approved for the treatment of chronic angina. However, its ionic mechanisms in other types of cells remain unclear, although it is thought to be a selective blocker of late Na(+) current. This study was conducted to evaluate the possible effects of ranolazine on Na(+) current (I(Na)), L-type Ca(2+) current (I(Ca,L)), inwardly rectifying K(+) current (I(K(IR))), delayed-rectifier K(+) current (I(K(DR))), and Ca(2+)-activated K(+) current (I(K(Ca))) in pituitary tumor (GH(3)) cells. Ranolazine depressed the transient and late components of I(Na) with different potencies. This drug exerted an inhibitory effect on I(K(IR)) with an IC(50) value of 0.92 microM, while it slightly inhibited I(K(DR)) and I(K(Ca)). It shifted the steady-state activation curve of I(K(IR)) to more positive potentials with no change in the gating charge of the channel. Ranolazine (30 microM) also reduced the activity of large-conductance Ca(2+)-activated K(+) channels in HEK293T cells expressing alpha-hSlo. Under current-clamp conditions, low concentrations (e.g., 1 microM) of ranolazine increased the firing of action potentials, while at high concentrations (>or=10 microM), it diminished the firing discharge. The exposure to ranolazine also suppressed I(Na) and I(K(IR)) effectively in NG108-15 neuronal cells. Our study provides evidence that ranolazine could block multiple ion currents such as I(Na) and I(K(IR)) and suggests that these actions may contribute to some of the functional activities of neurons and endocrine or neuroendocrine cells in vivo.

Underlying mechanism of actions of tefluthrin, a pyrethroid insecticide, on voltage-gated ion currents and on action currents in pituitary tumor (GH3) cells and GnRH-secreting (GT1-7) neurons

Tefluthrin is a synthetic pyrethroid and involved in acute neurotoxic effects. How this compound affects ion currents in endocrine or neuroendocrine cells remains unclear. Its effects on membrane ion currents in pituitary tumor (GH3) cells and in hypothalamic (GT1-7) neurons were investigated. Application of Tef (10 microM) increased the amplitude of voltage-gated Na+ current (INa), along with a slowing in current inactivation and deactivation in GH3 cells. The current-voltage relationship of INa was shifted to more negative potentials in the presence of this compound. Tef increased INa with an EC50 value of 3.2 +/- 0.8 microM. It also increased the amplitude of persistent INa. Tef reduced the amplitude of L-type Ca2+ current. This agent slightly inhibited K+ outward current; however, it had no effect on the activity of large-conductance Ca2+-activated K+ channels. Under cell-attached voltage-clamp recordings, Tef (10 microM) increased amplitude and frequency of spontaneous action currents, along with appearance of oscillatory inward currents. Tef-induced inward currents were suppressed after further application of tetrodotoxin, riluzole or ranolazine. In GT1-7 cells, Tef also increased the amplitude and frequency of action currents. Taken together, the effects of Tef and its structural related pyrethroids on ion currents can contribute to the underlying mechanisms through which they affect endocrine or neuroendocrine function in vivo.

Loss of functional K+ channels encoded by ether-à-go-go-related genes in mouse myometrium prior to labour onset

There is a growing appreciation that ion channels encoded by the ether-à-go-go-related gene family have a functional impact in smooth muscle in addition to their accepted role in cardiac myocytes and neurones. This study aimed to assess the expression of ERG1-3 (KCNH1-3) genes in the murine myometrium (smooth muscle layer of the uterus) and determine the functional impact of the ion channels encoded by these genes in pregnant and non-pregnant animals. Quantitative RT-PCR did not detect message for ERG2 and 3 in whole myometrial tissue extracts. In contrast, message for two isoforms of mERG1 were readily detected with mERG1a more abundant than mERG1b. In isometric tension studies of non-pregnant myometrium, the ERG channel blockers dofetilide (1 microM), E4031 (1 microM) and Be-KM1 (100 nM) increased spontaneous contractility and ERG activators (PD118057 and NS1643) inhibited spontaneous contractility. In contrast, neither ERG blockade nor activation had any effect on the inherent contractility in myometrium from late pregnant (19 days gestation) animals. Moreover, dofetilide-sensitive K(+) currents with distinctive 'hooked' kinetics were considerably smaller in uterine myocytes from late pregnant compared to non-pregnant animals. Expression of mERG1 isoforms did not alter throughout gestation or upon delivery, but the expression of genes encoding auxillary subunits (KCNE) were up-regulated considerably. This study provides the first evidence for a regulation of ERG-encoded K(+) channels as a precursor to late pregnancy physiological activity.