Effective Accentuation of Voltage-Gated Sodium Current Caused by Apocynin (4'-Hydroxy-3'-methoxyacetophenone), a Known NADPH-Oxidase Inhibitor

Exploring the Effects of Tomatidine ((3β, 5α, 22β, and 25β)-Spirosolan-3-ol) on Voltage-gated Na+ currents: Insights Into Its Ionic Mechanisms of Action on Current Magnitude, Gating, and Frequency Dependence

ABSTRACT

Tomatidine, a major tomato glycoalkaloid, is effective for the prevention of skeletal muscle wasting and enhancing mitophagy. However, its effects on transmembrane ionic currents are not well explored. In this study, we explored the interactions between tomatidine and Na+ current. GH3 or Neuro-2a cells were used for recording the ion currents employing modified patch-clamp technique under whole-cell configuration. Tomatidine increased both the peak, (transient Na+ current [INa (T)]) and sustained (late Na+ current [INa (L)]) components of voltage-gated Na+ current (INa) in a concentration-dependent manner, with the concentration required for 50% stimulation values of 43.3 μM and 3.1 μM, respectively. The steady-state current-voltage relationship of INa (T) remained unchanged; however, the steady-state inactivation curve of INa (T) in the presence of 3 μM tomatidine was shifted to less depolarized potential by around 6 mV. Tomatidine enhanced the window INa (window Na+ current [INa (W)]), which were attenuated by the ranolazine (Ran) and carbamazepine (CBZ). During a train of depolarizing pulses, tomatidine slowed the exponential decay of INa (T), and this effect was reversed by Ran or dapagliflozin. Tomatidine increased both fast and slow recovery time constants from INa (T) block, affecting the recovery time course. Tomatidine increased the amplitude of persistent Na+ current in response to a sinusoidal waveform. In neuro-2a cells, tomatidine increased INa (T) amplitude and slowed its inactivation, with this effect being attenuated by Ran or CBZ. In conclusion, tomatidine enhanced magnitude and modified its gating behaviors.

Recent Advances in the Discovery of Novel Drugs on Natural Molecules

Natural products (NPs) are always a promising source of novel drugs for tackling unsolved diseases [...].

Investigating the Impact of Selective Modulators on the Renin-Angiotensin-Aldosterone System: Unraveling Their Off-Target Perturbations of Transmembrane Ionic Currents

The renin-angiotensin-aldosterone system (RAAS) plays a crucial role in maintaining various physiological processes in the body, including blood pressure regulation, electrolyte balance, and overall cardiovascular health. However, any compounds or drugs known to perturb the RAAS might have an additional impact on transmembrane ionic currents. In this retrospective review article, we aimed to present a selection of chemical compounds or medications that have long been recognized as interfering with the RAAS. It is noteworthy that these substances may also exhibit regulatory effects in different types of ionic currents. Apocynin, known to attenuate the angiotensin II-induced activation of epithelial Na+ channels, was shown to stimulate peak and late components of voltage-gated Na+ current (INa). Esaxerenone, an antagonist of the mineralocorticoid receptor, can exert an inhibitory effect on peak and late INa directly. Dexamethasone, a synthetic glucocorticoid, can directly enhance the open probability of large-conductance Ca2+-activated K+ channels. Sparsentan, a dual-acting antagonist of the angiotensin II receptor and endothelin type A receptors, was found to suppress the amplitude of peak and late INa effectively. However, telmisartan, a blocker of the angiotensin II receptor, was effective in stimulating the peak and late INa along with a slowing of the inactivation time course of the current. However, telmisartan's presence can also suppress the erg-mediated K+ current. Moreover, tolvaptan, recognized as an aquaretic agent that can block the vasopressin receptor, was noted to suppress the amplitude of the delayed-rectifier K+ current and the M-type K+ current directly. The above results indicate that these substances not only have an interference effect on the RAAS but also exert regulatory effects on different types of ionic currents. Therefore, to determine their mechanisms of action, it is necessary to gain a deeper understanding.

Inhibition of Voltage-Gated Na+ Currents Exerted by KB-R7943 (2-[2-[4-(4-nitrobenzyloxy)phenyl]ethyl]isothiourea), an Inhibitor of Na+-Ca2+ Exchanging Process

KB-R7943, an isothiourea derivative, has been recognized as an inhibitor in the reverse mode of the Na+-Ca2+ exchanging process. This compound was demonstrated to prevent intracellular Na+-dependent Ca2+ uptake in intact cells; however, it is much less effective at preventing extracellular Na+-dependent Ca2+ efflux. Therefore, whether or how this compound may produce any perturbations on other types of ionic currents, particularly on voltage-gated Na+ current (INa), needs to be further studied. In this study, the whole-cell current recordings demonstrated that upon abrupt depolarization in pituitary GH3 cells, the exposure to KB-R7943 concentration-dependently depressed the transient (INa(T)) or late component (INa(L)) of INa with an IC50 value of 11 or 0.9 μM, respectively. Likewise, the dissociation constant for the KB-R7943-mediated block of INa on the basis of a minimum reaction scheme was estimated to be 0.97 μM. The presence of benzamil or amiloride could suppress the INa(L) magnitude. The instantaneous window Na+ current (INa(W)) activated by abrupt ascending ramp voltage (Vramp) was suppressed by adding KB-R7943; however, subsequent addition of deltamethrin or tefluthrin (Tef) effectively reversed KB-R7943-inhibted INa(W). With prolonged duration of depolarizing pulses, the INa(L) amplitude became exponentially decreased; moreover, KB-R7943 diminished INa(L) magnitude. The resurgent Na+ current (INa(R)) evoked by a repolarizing Vramp was also suppressed by adding this compound; moreover, subsequent addition of ranolazine or Tef further diminished or reversed, respectively, its reduction in INa(R) magnitude. The persistent Na+ current (INa(P)) activated by sinusoidal voltage waveform became enhanced by Tef; however, subsequent application of KB-R7943 counteracted Tef-stimulated INa(P). The docking prediction reflected that there seem to be molecular interactions of this molecule with the hNaV1.2 or hNaV1.7 channels. Collectively, this study highlights evidence showing that KB-R7943 has the propensity to perturb the magnitude and gating kinetics of INa (e.g., INa(T), INa(L), INa(W), INa(R), and INa(P)) and that the NaV channels appear to be important targets for the in vivo actions of KB-R7943 or other relevant compounds.

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.

Characterization in Effective Stimulation on the Magnitude, Gating, Frequency Dependence, and Hysteresis of INa Exerted by Picaridin (or Icaridin), a Known Insect Repellent

Picaridin (icaridin), a member of the piperidine chemical family, is a broad-spectrum arthropod repellent. Its actions have been largely thought to be due to its interaction with odorant receptor proteins. However, to our knowledge, to what extent the presence of picaridin can modify the magnitude, gating, and/or the strength of voltage-dependent hysteresis (Hys(V)) of plasmalemmal ionic currents, such as, voltage-gated Na+ current [INa], has not been entirely explored. In GH3 pituitary tumor cells, we demonstrated that with exposure to picaridin the transient (INa(T)) and late (INa(L)) components of voltage-gated Na+ current (INa) were differentially stimulated with effective EC50's of 32.7 and 2.8 μM, respectively. Upon cell exposure to it, the steady-state current versus voltage relationship INa(T) was shifted to more hyperpolarized potentials. Moreover, its presence caused a rightward shift in the midpoint for the steady-state inactivate curve of the current. The cumulative inhibition of INa(T) induced during repetitive stimuli became retarded during its exposure. The recovery time course from the INa block elicited, following the conditioning pulse stimulation, was satisfactorily fitted by two exponential processes. Moreover, the fast and slow time constants of recovery from the INa block by the same conditioning protocol were noticeably increased in the presence of picaridin. However, the fraction in fast or slow component of recovery time course was, respectively, increased or decreased with an increase in picaridin concentrations. The Hys(V)'s strength of persistent INa (INa(P)), responding to triangular ramp voltage, was also enhanced during cell exposure to picaridin. The magnitude of resurgent INa (INa(R)) was raised in its presence. Picaritin-induced increases of INa(P) or INa(R) intrinsically in GH3 cells could be attenuated by further addition of ranolazine. The predictions of molecular docking also disclosed that there are possible interactions of the picaridin molecule with the hNaV1.7 channel. Taken literally, the stimulation of INa exerted by the exposure to picaridin is expected to exert impacts on the functional activities residing in electrically excitable cells.

Characterization in Inhibitory Effectiveness of Carbamazepine in Voltage-Gated Na+ and Erg-Mediated K+ Currents in a Mouse Neural Crest-Derived (Neuro-2a) Cell Line

Carbamazepine (CBZ, Tegretol®) is an anticonvulsant used in the treatment of epilepsy and neuropathic pain; however, several unwanted effects of this drug have been noticed. Therefore, the regulatory actions of CBZ on ionic currents in electrically excitable cells need to be reappraised, although its efficacy in suppressing voltage-gated Na+ current (INa) has been disclosed. This study was undertaken to explore the modifications produced by CBZ on ionic currents (e.g., INa and erg-mediated K+ current [IK(erg)]) measured from Neuro-2a (N2a) cells. In these cells, we found that this drug differentially suppressed the peak (transient, INa(T)) and sustained (late, INa(L)) components of INa in a concentration-dependent manner with effective IC50 of 56 and 18 μM, respectively. The overall current-voltage relationship of INa(T) with or without the addition of CBZ remained unchanged; however, the strength (i.e., ∆area) in the window component of INa (INa(W)) evoked by the short ascending ramp pulse (Vramp) was overly lessened in the CBZ presence. Tefluthrin (Tef), a synthetic pyrethroid, known to stimulate INa, augmented the strength of the voltage-dependent hysteresis (Hys(V)) of persistent INa (INa(P)) in response to the isosceles-triangular Vramp; moreover, further application of CBZ attenuated Tef-mediated accentuation of INa(P)'s Hys(V). With a two-step voltage protocol, the recovery of INa(T) inactivation seen in Neuro-2a cells became progressively slowed by adding CBZ; however, the cumulative inhibition of INa(T) evoked by pulse train stimulation was enhanced during exposure to this drug. Neuro-2a-cell exposure to CBZ (100 μM), the magnitude of erg-mediated K+ current measured throughout the entire voltage-clamp steps applied was mildly inhibited. The docking results regarding the interaction of CBZ and voltage-gate Na+ (NaV) channel predicted the ability of CBZ to bind to some amino-acid residues in NaV due to the existence of a hydrogen bond or hydrophobic contact. It is conceivable from the current investigations that the INa (INa(T), INa(L), INa(W), and INa(P)) residing in Neuro-2a cells are susceptible to being suppressed by CBZ, and that its block on INa(L) is larger than that on INa(T). Collectively, the magnitude and gating of NaV channels produced by the CBZ presence might have an impact on its anticonvulsant and analgesic effects occurring in vivo.

Inhibitory Effectiveness in Delayed-Rectifier Potassium Current Caused by Vortioxetine, Known to Be a Novel Antidepressant

Vortioxetine (VOR) is recognized to exert antidepressant actions. However, whether this drug modifies ionic currents in excitable cells remains unclear. The aim of this study was to explore the electrophysiological effects of VOR and other related compounds in pituitary GH3 cells and in Neuro-2a cells. VOR suppressed the delayed-rectifier K+ current (IK(DR)) in a concentration-, time-, and state-dependent manner. Effective IC50 values needed to inhibit peak and sustained IK(DR) were computed to be 31.2 and 8.5 μM, respectively, while the KD value estimated from minimal binding scheme was 7.9 μM. Cell exposure to serotonin (10 μM) alone failed to alter IK(DR), while fluoxetine (10 μM), a compound structurally similar to VOR, mildly suppressed current amplitude. In continued presence of VOR, neither further addition of propranolol nor risperidone reversed VOR-mediated inhibition of IK(DR). Increasing VOR concentration not only depressed IK(DR) conductance but also shifted toward the hyperpolarized potential. As the VOR concentration was raised, the recovery of IK(DR) block became slowed. The IK(DR) activated by a downsloping ramp was suppressed by its presence. The inhibition of IK(DR) by a train pulse was enhanced during exposure to VOR. In Neuro-2a cells, this drug decreased IK(DR). Overall, inhibitory effects of VOR on ionic currents might constitute another underlying mechanism of its actions.

Activation of Voltage-Gated Na+ Current by GV-58, a Known Activator of CaV Channels

GV-58 ((2R)-2-[(6-{[(5-methylthiophen-2-yl)methyl]amino}-9-propyl-9H-purin-2-yl)amino]butan-1-ol) is recognized to be an activator of N- and P/Q-type Ca2+ currents. However, its modulatory actions on other types of ionic currents in electrically excitable cells remain largely unanswered. This study was undertaken to explore the possible modifications caused by GV-58 in ionic currents (e.g., voltage-gated Na+ current [INa], A-type K+ current [IK(A)], and erg-mediated K+ current [IK(erg)]) identified from pituitary GH3 lactotrophs. GH3 cell exposure to GV-58 enhanced the transient and late components of INa with varying potencies; consequently, the EC50 values of GV-58 required for its differential increase in peak and late INa in GH3 cells were estimated to be 8.9 and 2.6 μM, respectively. The INa in response to brief depolarizing pulse was respectively stimulated or suppressed by GV-58 or tetrodotoxin, but it failed to be altered by ω-conotoxin MVIID. Cell exposure to this compound increased the recovery of INa inactivation evoked by two-pulse protocol based on a geometrics progression; however, in its presence, there was a slowing in the inactivation rate of current decay evoked by a train of depolarizing pulses. The existence of GV-58 also resulted in an increase in the amplitude of ramp-induced resurgent and window INa. The presence of this compound inhibited IK(A) magnitude, accompanied by a shortening in inactivation time course of the current; however, it mildly decreased IK(erg). Under current-clamp conditions, GV-58 increased the frequency of spontaneous action potentials in GH3 cells. Moreover, in NSC-34 motor neuron-like cells, the presence of GV-58 not only raised INa amplitude but also reduced current inactivation. Taken together, the overall work provides a noticeable yet unidentified finding which implies that, in addition to its agonistic effect on Ca2+ currents, GV-58 may concertedly modify the amplitude and gating kinetics of INa in electrically excitable cells, hence modifiying functional activities in these cells.

The Evidence for Sparsentan-Mediated Inhibition of INa and IK(erg): Possibly Unlinked to Its Antagonism of Angiotensin II or Endothelin Type a Receptor

Sparsentan is viewed as a dual antagonist of endothelin type A (ET_A) receptor and angiotensin II (AngII) receptor and it could be beneficial in patients with focal segmental glomerulosclerosis. Moreover, it could improve glomerular filtration rate and augment protective tissue remodeling in mouse models of focal segmental glomerulosclerosis. The ionic mechanisms through which it interacts with the magnitude and/or gating kinetics of ionic currents in excitable cells were not thoroughly investigated. Herein, we aimed to examine the effects of varying sparsentan concentrations on ionic currents residing in pituitary GH₃ somatolactotrophs. From whole-cell current recordings made in GH₃ cells, sparsentan (0.3–100 μM) differentially inhibited the peak and late components of voltage-gated Na⁺ current (I_Na). The IC₅₀ value of sparsentan required to exert a reduction in peak and late I_Na in GH₃ cells was 15.04 and 1.21 μM, respectively; meanwhile, the K_D value estimated from its shortening in the slow component of I_Na inactivation time constant was 2.09 μM. The sparsentan (10 μM) presence did not change the overall current–voltage relationship of I_Na; however, the steady-state inactivation curve of the current was shifted to more negative potential in its presence (10 μM), with no change in the gating charge of the curve. The window I_Na activated by a brief upsloping ramp was decreased during exposure to sparsentan (10 μM); moreover, recovery of peak I_Na became slowed in its presence. The Tefluthrin (Tef)-stimulated resurgent I_Na activated in response to abrupt depolarization followed by the descending ramp pulse was additionally attenuated by subsequent application of sparsentan. In continued presence of Tef (3 μM) or β-pompilidotoxin (3 μM), further application of sparsentan (3 μM) reversed their stimulation of I_Na. However, sparsentan-induced inhibition of I_Na failed to be overcome by subsequent application of either endothelin 1 (1 μM) or angiotensin II (1 μM); moreover, in continued presence of endothelin (1 μM) or angiotensin II (1 μM), further addition of sparsentan (3 μM) effectively decreased peak I_Na. Additionally, the application of sparsentan (3 μM) inhibited the peak and late components of erg-mediated K⁺ current in GH₃ cells, although it mildly decreased the amplitude of delayed-rectifier K⁺ current. Altogether, this study provides a distinct yet unidentified finding that sparsentan may perturb the amplitude or gating of varying ionic currents in excitable cells.