Inhibition Of Potassium Currents Research Articles

Roscovitine differentially affects CaV2 and Kv channels by binding to the open state

Roscovitine potently inhibits cyclin-dependent kinases (CDK) and can independently slow the closing of neuronal (CaV2.2) calcium channels. We were interested if this drug could affect other ion channels similarly. Using whole cell recordings, we found that roscovitine specifically slows deactivation of all CaV2 channels (N, P/Q and R) by binding to the open state. This effect had a rapid onset and EC 50 = 54, 120 and 54 μM for N-, P/Q-, and R-type channels, respectively. Deactivation of other channel types was not slowed, including L-type calcium channels (CaV1.2, CaV1.3), potassium channels (native, Kv4.2, Kv2.1 and Kv1.3), and native sodium channels. However, most of the channels tested were inhibited by roscovitine. The inhibition was characterized by slow development and a lower affinity (EC 50 = 100–300 μM). Surprisingly, potassium channels were rapidly inhibited with an EC 50 = 23 μM, which is similar to the EC 50 for roscovitine block of cell division [Meijer, L., Borgne, A., Mulner, O., Chong, J., Blow, J., Inagaki, N., Inagaki, M., Delcros, J., Moulinoux, J., 1997. Biochemical and cellular effects of roscovitine, a potent and selective inhibitor of the cyclin-dependent kinases cdc2, cdk2 and cdk5. Eur. J. Biochem. 243, 527–536]. Potassium current inhibition seemed to result from open channel block. The high potency of these two rapid onset effects makes them complicating factors for ongoing clinical trials and research using roscovitine. Thus, the physiology and pharmacology of slow CaV2 deactivation and potassium channel block must be explored.

Postsynaptic excitation of prefrontal cortical pyramidal neurons by hypocretin‐1/orexin a through the inhibition of potassium currents

Hypocretins are crucial for the regulation of wakefulness by the excitatory actions on multiple subcortical arousal systems. To date, there is little information about the direct postsynaptic excitatory effects of hypocretins on the neurons in prefrontal cortex (PFC), which is important for higher cognitive functions and is correlated with level of wakefulness. In this study, we tested the excitatory effects of hypocretin-1 on acutely isolated PFC pyramidal neurons of rats and studied the possible ionic mechanisms by using whole-cell patch-clamp techniques. Puff application of hypocretin-1 caused a dose-dependent excitation. Further observations that perfusion of Ca2+-free artificial cerebrospinal fluid did not influence the depolarizing effects of hypocretin-1, in conjunction with the findings that hypocretin-1 could decrease net whole-cell K+ currents, demonstrate that the excitatory effects of hypocretin-1 on PFC neurons are mediated by the inhibition of K+ currents but not Ca2+ influx. Finally, the decrease in K+ currents induced by hypocretin-1 was abolished by a protein kinase C (PKC) inhibitor (BIS II) or a phospholipase C (PLC) inhibitor (D609), suggesting that PKC and PLC appear to be involved in mediating the inhibitory effects of hypocretin-1 on K+ currents. These results indicate that hypocretin-1 exerts a postsynaptic excitatory action on PFC neurons through the inhibition of K+ currents, which probably results from activation of PKC and PLC signaling pathways.

Contributions of Purkinje-Myocardial Coupling to Suppression and Facilitation of Early Afterdepolarization-Induced Triggered Activity

Electrical loading by ventricular myocardium modulates conduction system repolarization near Purkinje-ventricular junctions (PVJs). We investigated how that loading suppresses and facilitates early afterdepolarizations (EADs) under conditions where there is a high degree of functional coupling between tissue types, which is consistent with the anatomic arrangement at the peripheral conduction system-myocardial interface. Experiments were completed in eight rabbit right ventricular (RV) free wall preparations. Free-running Purkinje strands were locally superfused, and action potentials were recorded from strands. RV free walls were bathed in normal solution. Surface electrograms were recorded near strand insertions into downstream free wall myocardium. Detailed histology was performed to assemble a computer model with interspersed Purkinje and ventricular myocytes weakly coupled throughout the region. Delays from Purkinje upstrokes to downstream peripheral conduction system and myocardial activation were comparable between experiments and simulations, supporting model node-to-node electrical coupling, i.e., the functional coupling. Purkinje action potential duration (APD) prolongation with localized isoproterenol in experiments and calcium current enhancement in simulations failed to establish EADs. With myocardial APD prolongation by delayed rectifier potassium current inhibition or L-type calcium current enhancement accompanying Purkinje APD prolongation in simulations, however, EAD-induced triggered activity developed. Collectively, our findings suggest competing contributions of the myocardial sink when there is a high degree of functional coupling between tissue types, with the transition from suppression to facilitation of EAD-induced triggered activity depending critically upon myocardial APD prolongation.

A pharmacokinetic-pharmacodynamic model for the quantitative prediction of dofetilide clinical QT prolongation from human ether-a-go-go-related gene current inhibition data

QT prolongation is an important biomarker of the arrhythmia torsades de pointes and appears to be related mainly to blockade of delayed inward cardiac rectifier potassium currents. The aim of this study was to quantify the relationship between in vitro human ether-a-go-go-related gene (hERG) potassium channel blockade and the magnitude of QT prolongation in humans for the class III antiarrhythmic dofetilide. The in vitro affinity and activity of dofetilide were determined in recombinant cell cultures expressing the hERG channel, and the QT-prolonging effect of dofetilide was assessed in 5 clinical studies (80 healthy volunteers and 17 patients with ischemic heart disease). A population pharmacokinetic-pharmacodynamic analysis of the in vitro and in vivo data was performed in NONMEM by use of the operational model of pharmacologic agonism to estimate the efficiency of transduction from ion channel binding to Fridericia-corrected QT response. A 3-compartment pharmacokinetic model with first-order absorption characterized the time course of dofetilide concentrations. On the basis of an in vitro potency of 5.13 ng/mL for potassium current inhibition and predicted unbound dofetilide concentrations, the estimated transducer ratio (tau) of 6.2 suggests that the QT response plateaus before currents are fully blocked. In our study population, 10% hERG blockade corresponds to a QT prolongation of 20 ms (95% confidence interval, 12-32 ms). With long-term dofetilide administration, tolerance develops with a half-life of 4.7 days. The current mechanism-based pharmacokinetic-pharmacodynamic model quantified the relationship between in vitro hERG channel blockade and clinical QT prolongation for dofetilide. This model may prove valuable for assessing the risk of QT prolongation in humans for other drugs that selectively block the hERG channel on the basis of in vitro assays and pharmacokinetic properties.

Phorbol ester-induced inhibition of potassium currents in rat sensory neurons requires voltage-dependent entry of calcium.

The whole cell patch-clamp technique was used to examine the effects of protein kinase C (PKC) activation (via the phorbol ester, phorbol 12,13 dibutyrate, PDBu) on the modulation of potassium currents (I(K)) in cultured capsaicin-sensitive neurons isolated from dorsal root ganglia from embryonic rat pups and grown in culture. PDBu, in a concentration- and time-dependent manner, reduced I(K) measured at +60 mV by approximately 30% if the holding potential (V(h)) was -20 or -47 mV but had no effect if V(h) was -80 mV. The PDBu-induced inhibition of I(K) was blocked by pretreatment with the PKC inhibitor bisindolylmaleimide I and I(K) was unaffected by 4-alpha phorbol, indicating that the suppression of I(K) was mediated by PKC. The inhibition of I(K) by 100 nM PDBu at a V(h) of -50 mV was reversed over several minutes if V(h) was changed to -80 mV. In addition, intracellular perfusion with 5 mM bis-(o-aminophenoxy)-N,N,N',N'-tetraacetic acid (BAPTA) or pretreatment with omega-conotoxin GVIA or Cd(2+)-Ringer, but not nifedipine, prevented the PDBu-induced suppression of I(K) at -50 mV, suggesting that a voltage-dependent influx of calcium through N-type calcium channels was necessary for the activation of PKC. The potassium channel blockers tetraethylammonium (TEA, 10 mM) and 4-aminopyridine (4-AP, 3 mM and 30 microM) reduced I(K), but only TEA attenuated the ability of PDBu to further inhibit the current, suggesting that the I(K) modified by PDBu was sensitive to TEA. Interestingly, in the presence of 3 mM or 30 microM 4-AP, 100 nM PDBu inhibited I(K) when V(h) was -80 mV. Thus 4-AP promotes the capacity of PDBu to reduce I(K) at -80 mV. We find that activation of PKC inhibits I(K) in rat sensory neurons and that voltage-dependent calcium entry is necessary for the development and maintenance of this inhibition.

Effects of inhibition of calcium and potassium currents in guinea-pig cardiac contraction: comparison of beta-caryophyllene oxide, eugenol, and nifedipine.

1. To investigate the effects of the clove oil constituents beta-caryophyllene oxide and eugenol on the heart muscle, experiments were performed on isolated papillary muscles and on ventricular myocytes of the guinea-pig. The results were compared with those obtained with the dihydropyridine, nifedipine. 2. All three substances exerted negative inotropic effects in heart muscle although with different potencies and different influences on the time course of the contraction curve. 3. They all reduced rested-state contractions (RSCs) in the presence of isoprenaline which are thought to be due to the Ca(2+) current (I(Ca)). 4. beta-Caryophyllene oxide, eugenol and nifedipine inhibited the I(Ca) in single cells from the guinea-pig ventricle in a concentration-dependent, reversible way, but with different potencies. 5. In addition to the I(Ca)-inhibiting effect, beta-caryophyllene oxide strongly inhibited and eugenol slightly inhibited the potassium current. 6. The action potential of papillary muscles at a 1 Hz contraction frequency was greatly shortened by nifedipine, slightly shortened by eugenol, but not changed by beta-caryophyllene oxide. 7. The inhibition of the potassium current by beta-caryophyllene oxide obviously prevents a shortening of the action potential due to the diminution of the I(Ca). Accordingly, the negative inotropic effect of beta-caryophyllene oxide is closely related to the inhibition of I(Ca). In contrast, eugenol and nifedipine, which shorten the action potential, exert stronger negative inotropic effects than expected from their influence on I(Ca). 8. The results show that the negative inotropic effect of a calcium channel blocker can be attenuated by an additional inhibition of potassium channels.

The cAMP transduction cascade mediates the PGE2-induced inhibition of potassium currents in rat sensory neurones.

1. The role of the cyclic AMP (cAMP) transduction cascade in mediating the prostaglandin E2 (PGE2)-induced decrease in potassium current (IK) was investigated in isolated embryonic rat sensory neurones using the whole-cell patch-clamp recording technique. 2. Exposure to 100 microM chlorophenylthio-adenosine cyclic 3', 5'-monophosphate (cpt-cAMP) or 1 microM PGE2 caused a slow suppression of the whole-cell IK by 34 and 36 %, respectively (measured after 20 min), without a shift in the voltage dependence of activation for this current. Neither of these agents altered the shape of the voltage-dependent inactivation curve indicating that the suppression of IK did not result from alterations in the inactivation properties. 3. To determine whether the PGE2-mediated suppression of IK depended on activation of the cAMP pathway, cells were exposed to this prostanoid in the presence of the protein kinase A (PKA) inhibitor, PKI. The PGE2-induced suppression of IK was prevented by PKI. In the absence of PGE2, PKI had no significant effect on the magnitude of IK. 4. Results obtained from protocols using different conditioning prepulse voltages indicated that the extent of cpt-cAMP- and PGE2-mediated suppression of IK was independent of the prepulse voltage. The subtraction of control and treated currents revealed that the cpt-cAMP- and PGE2-sensitive currents exhibited little time-dependent inactivation. Taken together, these results suggest that the modulated currents may be delayed rectifier-like IK. 5. Exposure to the inhibitors of IK, tetraethylammonium (TEA) or 4-aminopyridine (4-AP), reduced the control current elicited by a voltage step to +60 mV by 40-50 %. In the presence of 10 mM TEA, treatment with cpt-cAMP did not result in any further inhibition of IK. In contrast, cpt-cAMP reduced IK by an additional 25-30 % in the presence of 1 mM 4-AP. This effect was independent of the conditioning prepulse voltage. 6. These results establish that PGE2 inhibits an outward IK in sensory neurones via activation of PKA and are consistent with the idea that the PGE2-mediated sensitization of sensory neurones results, in part, from an inhibition of delayed rectifier-like IK.

RGS4 Inhibits Signaling by Group I Metabotropic Glutamate Receptors

Metabotropic glutamate receptors (mGluRs) couple to heterotrimeric G-proteins and regulate cell excitability and synaptic transmission in the CNS. Considerable effort has been focused on understanding the cellular and biochemical mechanisms that underlie regulation of signaling by G-proteins and their linked receptors, including the mGluRs. Recent findings demonstrate that regulators of G-protein signaling (RGS) proteins act as effector antagonists and GTPase-activating proteins for Galpha subunits to inhibit cellular responses by G-protein-coupled receptors. RGS4 blocks Gq activation of phospholipase Cbeta and is expressed broadly in rat brain. The group I mGluRs (mGluRs 1 and 5) couple to Gq pathways to regulate several effectors in the CNS. We examined the capacity of RGS4 to regulate group I mGluR responses. In Xenopus oocytes, purified RGS4 virtually abolishes the mGluR1a- and mGluR5a-mediated but not the inositol trisphospate-mediated activation of a calcium-dependent chloride current. Additionally, RGS4 markedly attenuates the mGluR5-mediated inhibition of potassium currents in hippocampal CA1 neurons. This inhibition is dose-dependent and occurs at concentrations that are virtually identical to those required for inhibition of phospholipase C activity in NG108-15 membranes and reconstituted systems using purified proteins. These findings demonstrate that RGS4 can modulate mGluR responses in neurons, and they highlight a previously unknown mechanism for regulation of G-protein-coupled receptor signaling in the CNS.

Diversity of response in vascular smooth muscle cells to changes in oxygen tension

Hypoxia causes pulmonary vasoconstriction (HPV), but also dilation of systemic vessels and the ductus arteriosus. In the adult animal. HPV is initiated by inhibition of potassium current (IK) in the smooth muscle cells of small resistance arteries, which results in membrane depolarization and calcium entry through voltage-gated calcium channels. The oxygen-sensitive channels that initiate HPV are 4-aminopyridine (4-AP)-sensitive delayed rectifier channels (KDR), the most prominent of which has a conductance of 37 pS. In the fetus, hypoxia causes pulmonary vasoconstriction through inhibition of a calcium-sensitive potassium channel (KCa). In smooth muscle cells from the rabbit ductus arteriosus, which dilates in response to hypoxia, whole-cell potassium current is reversibly enhanced, rather than inhibited, by hypoxia. The principal oxygen-sensitive channel is inhibited by 4-AP and has a conductance of about 58 pS. There are morphological and electrophysiological differences between individual pulmonary artery smooth muscle cells, for example, in some cells IK is predominantly carried by KDR channels and in others by KCa channels. KDR cells are more common in the resistance pulmonary arteries and KCa in the conduit arteries. Responses of specific vessels (conduit, resistance; pulmonary, systemic, ductus) at different stages of development (fetal, neonatal and adult) to changes in oxygen tension may be determined by the distribution of a variety of ion channels in the smooth muscle cells.

Potassium current inhibition by nonselective cation channel-mediated sodium entry in rat pheochromocytoma (PC-12) cells

Under physiological conditions, nonselective cation (NSC) channels mediate the entry of cations into cells, the most important being Na+ and Ca2+. In contrast to the Ca(2+)-dependent signaling mechanisms, little is known about the consequences and the spatial distribution of intracellular [Na+] elevation. In this study we demonstrate that Na+ entry, during the opening of ATP-activated NSC channels, leads to an inhibition of voltage-dependent K+ currents (IK) in cromaffin-like undifferentiated PC-12 cells. The effect was dependent on the charge carrier as well as on the density of the ATP-activated current. Extracellular alkali cations (Na+, Li+) were more efficient than NH4+ in suppressing IK. Intracellular infusion of Na+ had the same effect as Na+ influx through ATP-activated NSC channels. The inhibition of IK persisted when the total ATP-induced Na+ entry was reduced by membrane depolarization, suggesting a spatial restriction of the required Na+ accumulation. Our results indicate that NSC channels influence the function of other ion channels by changing local intracellular ion concentrations.

Inhibition of potassium currents by the antiarrhythmic drug E4031 in rat taste receptor cells

The effect of the class III antiarrhythmic agent E4031 was investigated on a non-cardiac preparation as a potential tool for studying potassium currents. Patch clamp recordings in the whole cell configuration were performed on dissociated rat taste cells. These cells possess a variety of potassium currents; they also conduct action potentials. Unlike its more specific action on a type of delayed rectifier channel in cardiac cells, three types of potassium currents were reversibly diminished in taste cells in the presence of E4031. These included transient, sustained, and inwardly-rectifying potassium currents. Activation properties were not altered but the inactivation curve was shifted to the left by approximately 10 mV. Inhibitions of whole cell currents were voltage-dependent, larger at depolarized potentials, but were never complete. E4031 significantly broadened the gustatory action potential and, at higher concentrations, inhibited spike height, suggesting an additional inhibitory effect on sodium channels that was evident in voltage-clamp records. We conclude that E4031 is an effective inhibitor of potassium currents in the micromolar range and that it likely acts at a conserved segment of the potassium channel.

Anandamide, an Endogenous Cannabinoid, Inhibits Shaker-related Voltage-gated K+ Channels

Anandamide has been identified in porcine brain as an endogenous cannabinoid receptor ligand and is believed to be a counterpart to the psychoactive component of marijuana, Δ9-tetrahydrocannabinol (Δ9-THC). Here we report that anandamide directly inhibits (IC50, 2.7 μM) Shaker-related Kv1.2 K+ channels that are found ubiquitously in the mammalian brain. Δ9-THC also inhibited Kv1.2 channels with comparable potency (IC50, 2.4 μM), as did several N-acyl-ethanolamides with cannabinoid receptor binding activity. Potassium current inhibition occurred through a pertussis toxin-insensitive mechanism and was not prevented by the cannabinoid receptor antagonist SR141716A. Utilizing excised patches of Kv1.2 channel-rich membrane as a rapid and sensitive bioassay, we found that phospholipase D stimulated the release of an endogenous anandamide-like K+ channel blocker from rat brain slices. Structure-activity studies were consistent with the possibility that the released blocker was either anandamide or another N-acyl-ethanolamide. Copyright © 1996 Elsevier Science Ltd

Thrombin‐induced inhibition of potassium currents in human retinal glial (Müller) cells.

1. Glial cells are known to play a role in regulating the microenvironment of the nervous system. While earlier considerations of glial function assumed a passive, static physiology for these cells, this is not likely to be the case. In this study, we begin to examine how the physiology of Müller glial cells changes in response to molecules in the microenvironment. 2. Perforated-path recordings and intracellular calcium measurements were performed on human retinal Müller cells in vitro. 3. Analysis of whole-cell currents revealed that the human Müller glial cells have an inwardly rectifying K+ current (IK(IR) which is active near the resting membrane potential. This IK(IR) is significantly inhibited when the Müller cell is exposed to thrombin, a molecule that is likely to enter the retina with a breakdown of the blood-retinal barrier and may be endogenous to the nervous system. 4. A variety of experiments point to a role for Ca2+ as a second messenger mediating the inhibitory effect of thrombin on the IK(IR) of Müller cells. Specifically, thrombin evokes an increase in intracellular [Ca2+] in the Müller cells; the Ca2+ chelator BAPTA blocks the effects of thrombin on both the inhibition of IK(IR) and the rise in intracellular [Ca2+]; exposure to ionomycin, a calcium ionophore, induces a reduction in the IK(IR) of Müller cells. 5. A thrombin- induced inhibition in the IK(IR) of Müller cells is likely to have significant functional consequences for the retina since these ion channels are involved in K+ homeostasis. 6. Our experiments support the idea that the physiology of Müller glial cells is dynamic and can be markedly affected by molecules in the microenvironment.

Evidence for an external location of the dihydropyridine agonist receptor site on smooth muscle and skeletal muscle calcium channels.

1. The location of the binding domain for agonist dihydropyridines (DHP) has been studied by comparing the action of (+)-202,791 and (-)-Bay K 8644 on Ba2+ currents (IBa) in whole cell patch clamp experiments. Drug effects were examined upon internal and external (extracellular) application in A7r5 smooth muscle cells and BC3H1 cells, a cell line expressing Ca channels of the skeletal muscle type. 2. Efficiency of internal drug application in the whole cell studies was demonstrated by inhibition of potassium currents and barium currents (IBa) upon internal perfusion with tetraethylammonium (TEA+) (10 mM) and the permanently charged phenylalkylamine, D 890 (100 microM) respectively. The uncharged DHP, (-)-STBODIPY-DHP (2 microM) was used to estimate the time course of internal perfusion by monitoring its fluorescence. 3. Intracellular application of (+)-202,791 and (-)-Bay K 8644 (5 microM) in patch clamp experiments was ineffective in stimulating Ca2+ channel currents in both cell lines. In contrast a 50 fold lower agonist concentration (0.1 microM (-)-Bay K 8644) applied to the external face of the membrane induced typical changes in tail currents and a current increase under conditions when up to 10 microM of the agonist was present in the intracellular perfusion solution. 4. In cell-attached patches in A7r5 cells, (-)-Bay K 8644 increased and (+)-PN 200,110 inhibited single channel activity when applied via the bath solution. This suggests partitioning and lateral diffusion of the DHPs in the lipid of the plasma membrane. 5. We conclude that the binding site for agonist DHPs on Ca2+ channels in A7r5 and BC3H1 cells is located close to the external surface of the membrane. The DHP binding domain can be reached by agonists and antagonists from the extracellular but not from the intracellular face of the membrane.

Differential inhibition of potassium currents in rat ventricular myocytes by capsaicin.

Capsaicin is a pungent irritant present in peppers of the Capsicum family. Its major target of action is believed to be sensory neurones. Capsaicin has also been shown to prolong cardiac action potential in atrial muscle, perhaps by local release of calcitonin gene related peptide which in turn enhances inward calcium currents. However, capsaicin has been shown to inhibit K+ current in neurones. Since such an action could contribute to action potential prolonging activity of capsaicin in heart, the aim of the study was to examine the effects of capsaicin on cardiac K+ currents. Ionic currents and action potentials were examined in isolated adult rat ventricular myocytes using the whole cell variant of the patch clamp technique at 25 degrees C. Capsaicin (10 microM) increased the action potential duration (APD50) from 45 ms to 166 ms. This effect was associated with an inhibition of three distinct K+ currents. The decreasing rank order of potency was: transient outward K+ current (ITO, IC50 = 6.4 microM), a voltage dependent non-inactivating outward current (IK, IC = 11.5 microM), and the inward rectifier K+ current (IK1, IC50 = 46.9 microM). Capsaicin induced block of ITO was characterised by a decrease in the peak current amplitude and an increase in the rate of inactivation. The inactivation of ITO in the absence of capsaicin was well described by a single exponential [tau = 77 (SEM 2) ms at +40 mV, n = 10]. However, in the presence of 10 microM capsaicin inactivation was best described by the sum of two exponentials [tau FAST = 4.4(0.5) ms; tau SLOW = 92.4(3.0) ms, n = 10] with the fast component contributing 46(2)% of the total decay. A small but consistent hyperpolarising shift (approximately 3 mV) in the steady state voltage dependence of inactivation of ITO was induced by 10 microM capsaicin. Capsaicin had no effect on the rate of ITO recovery from inactivation (tau = 49 ms and 48 ms for control and drug respectively). The capsaicin analogue, resiniferatoxin, which as an irritant is up to 10(4)-fold more potent than capsaicin, had no effect on any of the K+ currents when present at concentrations of up to 10 microM. In contrast another capsaicin analogue, zingerone (30 microM) blocked ITO by 52(12)% and IK by 35%. Capsaicin produces a prolongation of the rat ventricular action potential, an effect which is associated with inhibition of potassium currents.

Multiple subunits of a voltage-dependent potassium channel contribute to the binding site for tetraethylammonium

RNAs encoding a wild-type (RBK1) and a mutant (RBK1(Y379V,V381 T); RBK1 ∗) subunit of voltage-dependent potassium channels were injected into Xenopus oocytes. When expressed separately, they made homotetrameric channels that differed about 100-fold in sensitivity to tetraethylammonium (TEA). Mixtures of channels having one, two, or three low affinity subunits were expressed by injecting various proportions of RBK1 and RBK1 ∗ RNAs. The affinity for TEA of these three channel species was deduced by fitting concentration-response curves for the inhibition of potassium currents. DNAs were also concatenated to construct a sequence that encoded two connected subunits, and channels that contained four, two, or no TEA-sensitive subunits were expressed. The results suggest that bound TEA interacts simultaneously with all four subunits.

Characterization of the interaction of R 56865 with cardiac Na‐ and L‐type Ca channels

1. In isolated cardiac muscle, submicromolar concentrations of R 56865 (N-[1-[4-(4-fluorophenoxy)-butyl]-4-piperidinyl]-N-methyl-2- benzothiazolamine) have been shown to attenuate the toxicity of cardiac glycosides. 2. We studied the influence of R 56865 on calcium and sodium currents in single isolated ventricular cardiomyocytes. The effect of R 56865 on action potential and contractile force in the presence of increased sodium load was also tested by exposing papillary muscles to veratridine or Anemonia sulcata toxin ATX II. 3. The calcium current was not affected by R 56865 as assessed in slow action potentials of papillary muscles and current measurements in ventricular cardiomyocytes. 4. In papillary muscles, R 56865 (1 mumol l-1) abolished veratridine-induced aftercontractions and afterdepolarizations without affecting the profound prolongation of the action potential. When pretreated with R 56865, the occurrence of afterdepolarizations was prevented and the decline of the resting membrane potential was attenuated. 5. Pretreatment with R 56865 (1 mumol l-1) did not counteract the ATX II-induced prolongation of the action potential. 6. The sodium current (Nao 30 mmol l-1) was concentration-dependently decreased by R 56865 (0.1-10 mumol l-1). The blocking effect was more pronounced at less negative holding potentials. 7. Our results demonstrate that the protective effect of R 56865 against veratridine-induced electrical and mechanical oscillations is not due to a direct effect on the calcium current. A potential-dependent inhibition of the sodium current may contribute. Additional sites of action, like interference with intracellular calcium release and inhibition of potassium currents, remain to be investigated.

Effects of E4080 on action potentials and ionic currents in guinea pig hearts: activation and inhibition of potassium currents

Effects of E4080 on action potentials and ionic currents in guinea pig hearts: activation and inhibition of potassium currents

Inhibition of potassium currents by the sigma receptor ligand (+)-3-(3-hydroxyphenyl)-N-(1-propyl)piperidine in sympathetic neurons of the mouse isolated hypogastric ganglion

The actions of (+)-3-(3-hydroxyphenyl)-N-(1-propyl)piperidine [(+)3-PPP] on sympathetic neurons of the mouse isolated hypogastric ganglion were studied using the current clamp and single electrode voltage clamp techniques. In neurons studied under current clamp (+)3-PPP ( 10 −5 to 3 × 10 −4 M ) evoked a concentration-dependent depolarization, which was fully reversible on washout of the drug. The depolarization was associated with an increase in membrane input resistance. At membrane potentials between −43 and −65mV the amplitude of the depolarization was inversely related to the membrane potential. (+)3-PPP had no effect on membrane potential at potentials negative to −65mV. The effect of (+)3-PPP on the M-current was studied in cells voltage clamped at −40 mV and stepped to −60 mV for 300–500 ms. The slow current relaxations seen during and after the voltage step are largely due to the M-current. (+)3-PPP ( 3 × 10 −5 to 3 × 10 −4M ) inhibited the M-current and produced an inward current in a concentration-dependent manner. (−)3-PPP ( 3 × 10 −5 M ) had similar effects, but was less potent than (+)3-PPP. (+)3-PPP ( 3 × 10 −5 M ) also inhibited the A-current and a calcium-dependent potassium current, but to a lesser degree than the M -current. Inhibition of the M-current by (+)3-PPP was not antagonized by atropine ( 10 −6M ), sulpiride ( 10 −6M ), or by the putative sigma antagonists 1, 3-di-O-tolylguanidine ( 3 × 10 −6and10 −5M ) and rimcazole ( 3 × 10 −6Mand10 −5M ). It is concluded that (+)3-PPP depolarizes sympathetic neurons of the mouse isolated hypogastric ganglion by inhibiting the M-current. (+)3-PPP also inhibits the A-current and calcium-dependent potassium current. Through these actions (+)3-PPP may increase evoked neurotransmitter release and potentiate neurogenic twitch contractions of the mouse isolated vas deferens, as previously reported.

Tetrahydroaminoacridine blocks voltage-dependent ion channels in hippocampal neurons

Tetrahydroaminoacridine (THA) is a centrally active anticholinesterase that may produce functional improvement in patients with Alzheimer's disease. Because of its structural similarity to the potassium channel (A-current) blocking drug 4-aminopyridine (4-AP), it has been speculated that some of the therapeutic benefit of THA might result from inhibition of potassium currents. In the present study, THA produced a dose-dependent, reversible block of the A-current in cultured hippocampal neurons (IC 50, 30 μM). However, at similar concentrations it also inhibited other voltage-dependent currents.