Blocking effect of bepridil on Na+/Ca2+ exchange current in guinea pig cardiac ventricular myocytes

Contractility measurements for cardiotoxicity screening with ventricular myocardial slices of pigs

AIMS

Cardiotoxicity is one major reason why drugs do not enter or are withdrawn from the market. Thus, approaches are required to predict cardiotoxicity with high specificity and sensitivity. Ideally, such methods should be performed within intact cardiac tissue with high relevance for humans and detect acute and chronic side effects on electrophysiological behaviour, contractility, and tissue structure in an unbiased manner. Herein, we evaluate healthy pig myocardial slices and biomimetic cultivation setups (BMCS) as a new cardiotoxicity screening approach.

METHODS AND RESULTS

Pig left ventricular samples were cut into slices and spanned into BMCS with continuous electrical pacing and online force recording. Automated stimulation protocols were established to determine the force-frequency relationship (FFR), frequency dependence of contraction duration, effective refractory period (ERP), and pacing threshold. Slices generated 1.3 ± 0.14 mN/mm2 force at 0.5 Hz electrical pacing and showed a positive FFR and a shortening of contraction duration with increasing pacing rates. Approximately 62% of slices were able to contract for at least 6 days while showing stable ERP, contraction duration-frequency relationship, and preserved cardiac structure confirmed by confocal imaging and X-ray diffraction analysis. We used specific blockers of the most important cardiac ion channels to determine which analysis parameters are influenced. To validate our approach, we tested five drug candidates selected from the Comprehensive in vitro Proarrhythmia Assay list as well as acetylsalicylic acid and DMSO as controls in a blinded manner in three independent laboratories. We were able to detect all arrhythmic drugs and their respective mode of action on cardiac tissue including inhibition of Na+, Ca2+, and hERG channels as well as Na+/Ca2+ exchanger.

CONCLUSION

We systematically evaluate this approach for cardiotoxicity screening, which is of high relevance for humans and can be upscaled to medium-throughput screening. Thus, our approach will improve the predictive value and efficiency of preclinical cardiotoxicity screening.

Proarrhythmia Risk Assessment Using Electro-Mechanical Window in Human iPS Cell-Derived Cardiomyocytes

Evaluation of drug-induced cardiotoxicity is still challenging to avoid adverse effects, such as torsade de pointes (TdP), in non-clinical and clinical studies. Numerous studies have suggested that human-induced pluripotent stem cell-derived cardiomyocytes (hiPSC-CMs) are a useful platform for detecting drug-induced TdP risks. Comprehensive in vitro Proarrhythmia Assay (CiPA) validation study suggested that hiPSC-CMs can assess clinical TdP risk more accurately than the human ether-a-go-go-related assay and QT interval prolongation. However, there were still some outliers, such as bepridil, mexiletine, and ranolazine, among the CiPA 28 compounds in the CiPA international multi-site study using hiPSC-CMs. In this study, we assessed the effects of the positive compound dofetilide, the negative compound aspirin, and several CiPA compounds (bepridil, mexiletine, and ranolazine) on the electromechanical window (E-M window), which were evaluated using multi-electrode array assay and motion analysis, in hiPSC-CMs. Similar to previous in vivo studies, dofetilide, which has a high TdP risk, decreased the E-M window in hiPSC-CMs, whereas aspirin, which has a low TdP risk, had little effect. Bepridil, classified in the high TdP-risk group in CiPA, decreased the E-M window in hiPSC-CMs, whereas ranolazine and mexiletine, which are classified in the low TdP-risk group in CiPA, slightly decreased or had little effect on the E-M window of hiPSC-CMs. Thus, the E-M window in hiPSC-CMs can be used to classify drugs into high and low TdP risk.

Blockade of the forward Na+ /Ca2+ exchanger suppresses the growth of glioblastoma cells through Ca2+ -mediated cell death

Background and Purpose

The Na⁺/Ca²⁺ exchanger (NCX) working in either forward or reverse mode participates in maintaining intracellular Ca²⁺ ([Ca²⁺]_i) homeostasis, which is essential for determining cell fate. Previously, numerous blockers targeting reverse or forward NCX have been developed and studied in ischaemic tissue injury but barely examined in glioblastoma for the purpose of anti‐tumour therapy. We assessed the effect of NCX blockers on glioblastoma growth and whether NCX can become a therapeutic target.

Experimental Approach

Patch‐clamp recording, Ca²⁺ imaging, flow cytometry, and Western blot were used to study the effects of specific and non‐specific NCX blockers on cultured glioblastoma cells. In vivo bioluminescent imaging was used to measure effects on grafted glioblastoma.

Key Results

Selectively blocking the reverse NCX with SEA0400, SN‐6, and YM‐244769 did not affect tumour cell viability. Blocking the forward NCX with bepridil, CB‐DMB, or KB‐R7943 elevated [Ca²⁺]_i and killed glioblastoma cells. Bepridil and CB‐DMB caused Ca²⁺‐dependent cell cycle arrest together with apoptosis, which were all attenuated by a Ca²⁺ chelator BAPTA‐AM. Systemic administration of bepridil inhibited growth of brain‐grafted glioblastoma. Bepridil did not appear to have a cytotoxic effect on human astrocytes, which have higher functional expression of NCX than glioblastoma cells.

Conclusions and Implications

Low expression of the NCX makes glioblastoma cells sensitive to disturbance of [Ca²⁺]_i. Interventions designed to block the forward NCX can cause Ca²⁺‐mediated injury to glioblastoma thus having therapeutic potential. Bepridil could be a lead compound for developing new anti‐tumour drugs.

Ion Channels in Genetic Epilepsy: From Genes and Mechanisms to Disease-Targeted Therapies

Epilepsy is a common and serious neurologic disease with a strong genetic component. Genetic studies have identified an increasing collection of disease-causing genes. The impact of these genetic discoveries is wide reaching-from precise diagnosis and classification of syndromes to the discovery and validation of new drug targets and the development of disease-targeted therapeutic strategies. About 25% of genes identified in epilepsy encode ion channels. Much of our understanding of disease mechanisms comes from work focused on this class of protein. In this study, we review the genetic, molecular, and physiologic evidence supporting the pathogenic role of a number of different voltage- and ligand-activated ion channels in genetic epilepsy. We also review proposed disease mechanisms for each ion channel and highlight targeted therapeutic strategies.

GPR55 agonist lysophosphatidylinositol and lysophosphatidylcholine inhibit endothelial cell hyperpolarization via GPR-independent suppression of Na+-Ca2+ exchanger and endoplasmic reticulum Ca2+ refilling

Lysophosphatidylinositol (LPI) and lysophosphatidylcholine (LPC) are lipid signaling molecules that induce endothelium-dependent vasodilation. In addition, LPC suppresses acetylcholine (Ach)-induced responses. We aimed to determine the influence of LPC and LPI on hyperpolarizing responses in vitro and in situ endothelial cells (EC) and identify the underlying mechanisms. Using patch-clamp method, we show that LPI and LPC inhibit EC hyperpolarization to histamine and suppress Na⁺/Ca²⁺ exchanged (NCX) currents in a concentration-dependent manner. The inhibition is non-mode-specific and unaffected by intracellular GDPβS infusion and tempol, a superoxide dismutase mimetic. In excised mouse aorta, LPI strongly inhibits the sustained and the peak endothelial hyperpolarization induced by Ach, but not by SKA-31, an opener of Ca²⁺-dependent K⁺ channels of intermediate and small conductance. The hyperpolarizing responses to consecutive histamine applications are strongly reduced by NCX inhibition. In a Ca²⁺-re-addition protocol, bepridil, a NCX inhibitor, and KB-R7943, a blocker of reversed NCX, inhibit the hyperpolarizing responses to Ca²⁺-re-addition following Ca²⁺ stores depletion. These finding indicate that LPC and LPI inhibit endothelial hyperpolarization to Ach and histamine independently of G-protein coupled receptors and superoxide anions. Reversed NCX is critical for ER Ca²⁺ refilling in EC. The inhibition of NCX by LPI and LPC underlies diminished endothelium-dependent responses and endothelial dysfunction accompanied by increased levels of these lipids in the blood.

Role of the dysfunctional ryanodine receptor - Na(+)-Ca(2+)exchanger axis in progression of cardiovascular diseases: What we can learn from pharmacological studies?

Abnormal Ca(2+)homeostasis is often associated with chronic cardiovascular diseases, such as hypertension, heart failure or cardiac arrhythmias, and typically contributes to the basic ethiology of the disease. Pharmacological targeting of cardiac Ca(2+)handling has great therapeutic potential offering invaluable options for the prevention, slowing down the progression or suppression of the harmful outcomes like life threatening cardiac arrhythmias. In this review we outline the existing knowledge on the involvement of malfunction of the ryanodine receptor and the Na(+)-Ca(2+)exchanger in disturbances of Ca(2+)homeostasis and discuss important proof of concept pharmacological studies targeting these mechanisms in context of hypertension, heart failure, atrial fibrillation and ventricular arrhythmias. We emphasize the promising results of preclinical studies underpinning the potential benefits of the therapeutic strategies based on ryanodine receptor or Na(+)-Ca(2+)exchanger inhibition.

Short- and long-term inhibition of cardiac inward-rectifier potassium channel current by an antiarrhythmic drug bepridil

Bepridil is an effective antiarrhythmic drug on supraventricular and ventricular arrhythmias, and inhibitor of calmodulin. Recent investigations have been elucidating that bepridil exerts antiarrhythmic effects through its acute and chronic application for patients. The aim of this study was to identify the efficacy and the potential mechanism of bepridil on the inward-rectifier potassium channel in neonatal rat cardiomyocytes in acute- and long-term conditions. Bepridil inhibited inward-rectifier potassium current (I K1) as a short-term effect with IC50 of 17 μM. Bepridil also reduced I K1 of neonatal cardiomyocytes when applied for 24 h in the culture medium with IC50 of 2.7 μM. Both a calmodulin inhibitor (W-7) and an inhibitor of calmodulin-kinase II (KN93) reduced I K1 when applied for 24 h as a long-term effect in the same fashion, suggesting that the long-term application of bepridil inhibits I K1 more potently than that of the short-term application through the inhibition of calmodulin kinase II pathway in cardiomyocytes.

Stabilization of Kv1.5 channel protein by bepridil through its action as a chemical chaperone

While bepridil has been reported to alter the stability of ion channel proteins, the precise mechanism of action remains unclear. We examined the effect of bepridil on the stability of Kv1.5 channel proteins expressed in COS7 cells. Bepridil at 0.3-30 μM increased the protein level of Kv1.5 channels in a concentration-dependent manner. Chase experiments showed that bepridil delayed the degradation process of Kv1.5 channel proteins in the same manner as a proteasomal inhibitor, MG132, did. Bepridil increased the immunofluorescent signal of Kv1.5 channel proteins in the endoplasmic reticulum (ER) and Golgi apparatus and on the cell surface. The cell fraction experiment also showed bepridil-induced increases in Kv1.5 in the ER, Golgi apparatus, and the cell membrane. Bepridil at a lower concentration of 1 μM had no effect on the proteasome activity in vitro. A blocker of the ultrarapid delayed-rectifier K(+) channel current, 4-aminopyridine (4AP), abolished bepridil-induced increases in Kv1.5. Kv1.5-medicated membrane currents measured as 4AP-sensitive currents were increased by bepridil. Taken together, we conclude that bepridil stabilizes Kv1.5 proteins at the ER through an action as a chemical chaperone, thereby increasing the density of Kv1.5 channels in the cell membrane.

Amiodarone sensitizes human glioma cells but not astrocytes to TRAIL-induced apoptosis via CHOP-mediated DR5 upregulation

Amiodarone is a widely used anti-arrhythmic drug that inhibits diverse ion channels, including the Na(+)/Ca(2+) exchanger (NCX), L-type Ca(2+) channels, and Na(+) channels. Here, we report that subtoxic doses of amiodarone and tumor necrosis factor-related apoptosis-inducing ligand (TRAIL) synergistically induced apoptosis of various glioma cells. Treatment of U251MG glioma cells with amiodarone increased intracellular Ca(2+) levels and enhanced the expression of the endoplasmic reticulum (ER) stress-inducible transcription factor C/EBP homologous protein (CHOP). This upregulation of CHOP was followed by marked upregulation of the TRAIL receptor, DR5. Suppression of DR5 expression by small interfering (si) RNAs almost completely blocked amiodarone/TRAIL-induced apoptosis in U251MG glioma cells, demonstrating that DR5 is critical to this cell death. siRNA-mediated CHOP suppression reduced amiodarone-induced DR5 upregulation and attenuated the cell death induced by amiodarone plus TRAIL. In addition, omitting Ca(2+) from the external medium using ethylene glycol tetraacetic acid markedly inhibited this cell death, reducing the protein levels of CHOP and DR5. These results suggest that amiodarone-induced influx of Ca(2+) plays an important role in sensitizing U251MG cells to TRAIL-mediated apoptosis through CHOP-mediated DR5 upregulation. Furthermore, subtoxic doses of bepridil and cibenzoline, two other anti-arrhythmic drugs with NCX-inhibitor activity, also sensitized glioma cells to TRAIL-mediated apoptosis, via the upregulation of both CHOP and DR5. Notably, amiodarone/TRAIL cotreatment did not induce cell death in astrocytes, nor did it affect the expression of CHOP or DR5 in these cells. These results collectively suggest that a combined regimen of amiodarone plus TRAIL may offer an effective therapeutic strategy for safely and selectively treating resistant gliomas.

Bepridil facilitates early termination of spiral-wave reentry in two-dimensional cardiac muscle through an increase of intercellular electrical coupling

Bepridil is effective for conversion of atrial fibrillation to sinus rhythm and in the treatment of drug-refractory ventricular tachyarrhythmias. We investigated the effects of bepridil on electrophysiological properties and spiral-wave (SW) reentry in a 2-dimensional ventricular muscle layer of isolated rabbit hearts by optical mapping. Ventricular tachycardia (VT) induced in the presence of bepridil (1 µM) terminated earlier than in the control. Bepridil increased action potential duration (APD) by 5% - 8% under constant pacing and significantly increased the space constant. There was a linear relationship between the wavefront curvature (κ) and local conduction velocity: LCV = LCV₀ - D·κ (D, diffusion coefficient; LCV₀, LCV at κ = 0). Bepridil significantly increased D and LCV₀. The regression lines with and without bepridil crossed at κ = 20 - 40 cm⁻¹, resulting in a paradoxical decrease of LCV at κ > 40 cm⁻¹. Dye transfer assay in cultured rat cardiomyocytes confirmed that bepridil increased intercellular coupling. SW reentry in the presence of bepridil was characterized by decremental conduction near the rotation center, prominent drift, and self-termination by collision with boundaries. These results indicate that bepridil causes an increase of intercellular coupling and a moderate APD prolongation, and this combination compromises wavefront propagation near the rotation center of SW reentry, leading to its drift and early termination.

Bepridil up-regulates cardiac Na+ channels as a long-term effect by blunting proteasome signals through inhibition of calmodulin activity

BACKGROUND AND PURPOSE

Bepridil is an anti-arrhythmic agent with anti-electrical remodelling effects that target many cardiac ion channels, including the voltage-gated Na+ channel. However, long-term effects of bepridil on the Na+ channel remain unclear. We explored the long-term effect of bepridil on the Na+ channel in isolated neonatal rat cardiomyocytes and in a heterologous expression system of human Na(v)1.5 channel.

EXPERIMENTAL APPROACH

Na+ currents were recorded by whole-cell voltage-clamp technique. Na+ channel message and protein were evaluated by real-time RT-PCR and Western blot analysis.

KEY RESULTS

Treatment of cardiomyocytes with 10 micromol.L(-1) bepridil for 24 h augmented Na+ channel current (I(Na)) in a dose- and time-dependent manner. This long-term effect of bepridil was mimicked or masked by application of W-7, a calmodulin inhibitor, but not KN93 [2-[N-(2-hydroxyethyl)-N-(4-methoxy benzenesulphonyl)]-amino-N-(4-chlorocinnamyl)-N-methylbenzylamine], a Ca2+/calmodulin-dependent kinase inhibitor. During inhibition of protein synthesis by cycloheximide, the I(Na) increase due to bepridil was larger than the increase without cycloheximide. Bepridil and W-7 significantly slowed the time course of Na(v)1.5 protein degradation in neonatal cardiomyocytes, although the mRNA levels of Na(v)1.5 were not modified. Bepridil and W-7 did not increase I(Na) any further in the presence of the proteasome inhibitor MG132 [N-[(phenylmethoxy)carbonyl]-L-leucyl-N-[(1S)-1-formyl-3-methylbutyl]-L-leucinamide]. Bepridil, W-7 and MG132 but not KN93 significantly decreased 20S proteasome activity in a concentration-dependent manner.

CONCLUSIONS AND IMPLICATIONS

We conclude that long-term exposure of cardiomyocytes to bepridil at therapeutic concentrations inhibits calmodulin action, which decreased degradation of the Na(v)1.5 alpha-subunit, which in turn increased Na+ current.

SEA0400 fails to alter the magnitude of intracellular Ca2+ transients and contractions in Langendorff-perfused guinea pig heart

SEA0400 is a recently developed inhibitor of the Na+/Ca2+ exchanger (NCX) shown to suppress both forward and reverse mode operation of NCX. Present experiments were designed to study the effect of partial blockade of NCX on Ca handling and contractility in Langendorff-perfused guinea pig hearts loaded with the fluorescent Ca-sensitive dye fura-2. Left ventricular pressure and intracellular calcium concentration ([Ca2+]i) were synchronously recorded before and after cumulative superfusion with 0.3 and 1 muM SEA0400. SEA0400 caused no significant change in the systolic and diastolic values of left ventricular pressure and [Ca2+]i. Accordingly, pulse pressure and amplitude of the [Ca2+]i transient also remained unchanged in the presence of SEA0400. SEA0400 had no influence either on the time required to reach peak values of pressure and [Ca2+)]i or on half relaxation time. On the other hand, both 0.3 and 1 microM SEA0400 significantly increased the decay time constant of [Ca2+]i transients, obtained by fitting its descending limb between 30% and 90% of relaxation, from 127 +/- 7 to 165 +/- 7 and 177 +/- 14 ms, respectively (P < 0.05, n=6). In contrast to the guinea pig hearts, rat hearts responded to SEA0400 treatment with increased [Ca2+]i transients and contractility. These interspecies differences observed in the effect of SEA0400 can be explained by the known differences in calcium handling between the two species.

Acute inhibitory effect of dronedarone, a noniodinated benzofuran analogue of amiodarone, on Na+/Ca2+ exchange current in guinea pig cardiac ventricular myocytes

Using the whole-cell voltage-clamp method, we examined an acute effect of dronedarone, a noniodinated benzofuran analogue of amiodarone, on Na+/Ca2+ exchange current (INCX) in guinea pig cardiac ventricular cells. The INCX was recorded by ramp pulses with a holding potential of -60 mV using a pipette solution containing 226 nM free Ca2+ (20 mM 1,2-bis(2-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid and 10 mM Ca2+) and 20 mM Na+. The external solution contained 140 mM Na+, 1 mM Ca2+, and blockers of other currents and pumps such as Cs+, nifedipine, ryanodine, and ouabain. A selective potent NCX inhibitor, KB-R7943 (100 microM), was used to completely inhibit INCX. Dronedarone inhibited INCX in a concentration-dependent manner. The IC50 values for the outward and inward INCX inhibition were about 33 and 28 microM, respectively, with the Hill coefficient of 1 for both. The inhibitory effect of dronedarone at 50 microM on INCX did not change in the presence of trypsin in the pipette solution. Therefore, dronedarone is classified as a trypsin-insensitive NCX inhibitor and distinct from amiodarone which is a trypsin sensitive. We conclude that dronedarone inhibits INCX but the potency is tenfold less than that of amiodarone. Dronedarone may modestly inhibit INCX in a therapeutic concentration range.

Characterization of SN-6, a novel Na+/Ca2+ exchange inhibitor in guinea pig cardiac ventricular myocytes

We examined the effect of SN-6, a new benzyloxyphenyl Na(+)/Ca(2+) exchange (NCX) inhibitor on the Na(+)/Ca(2+) exchange current (I(NCX)) and other membrane currents in isolated guinea pig ventricular myocytes using the whole-cell voltage-clamp technique. SN-6 suppressed I(NCX) in a concentration-dependent manner. The IC(50) values of SN-6 were 2.3 microM and 1.9 microM for the outward and inward components of the bi-directional I(NCX), respectively. On the other hand, SN-6 suppressed the outward uni-directional I(NCX) more potently (IC(50) value of 0.6 microM) than the inward uni-directional I(NCX). SN-6 at 10 microM inhibited the uni-directional inward I(NCX) by only 22.4+/-3.1%. SN-6 and KB-R7943 suppressed I(NCX) more potently when intracellular Na(+) concentration was higher. Thus, both drugs inhibit NCX in an intracellular Na(+) concentration-dependent manner. Intracellular application of trypsin via a pipette solution did not change the blocking effect of SN-6 on I(NCX). Therefore, SN-6 is categorized as an intracellular-trypsin-insensitive NCX inhibitor. SN-6 at 10 microM inhibited I(Na), I(Ca), I(K) and I(K1) by about 13%, 34%, 33% and 13%, respectively. SN-6 at 10 microM shortened the action potential duration at 50% repolarization (APD(50)) by about 34%, and that at 90% repolarization (APD(90)) by about 25%. These results indicate that SN-6 inhibits NCX in a similar manner to that of KB-R7943. However, SN-6 at 10 microM affected other membrane currents less potently than KB-R7943.

Electrophysiological effects of SN-6, a novel Na+/Ca2+ exchange inhibitor on membrane currents in guinea pig ventricular myocytes

We examined the effect of SN-6 on the Na+/Ca2+ exchanger (NCX) current (I(NCX)) and other membrane currents in isolated guinea pig ventricular myocytes using the whole-cell voltage clamp technique. SN-6 suppressed the bidirectional I(NCX) in a concentration-dependent manner. The IC50 values of SN-6 were 2.3 microM and 1.9 microM for the outward and inward components of the bidirectional I(NCX), respectively. On the other hand, SN-6 suppressed the unidirectional outward I(NCX) more potently than the inward I(NCX), with an IC(50) value of 0.6 microM. SN-6 at 10 microM inhibited the unidirectional inward I(NCX) by only 22.4 +/- 3.1%. SN-6 suppressed I(NCX) more potentially when intracellular Na+ concentration became higher. SN-6 inhibited I(Na), I(Ca), I(Kr), I(Ks), and I(K1) by about 13%, 34%, 33%, 18%, and 13%, respectively. SN-6 shortened the action potential duration (APD) by about 34% and 25% at APD(50) and APD(90), respectively. These results indicate that SN-6 inhibits NCX in a similar manner to that of KB-R7943. SN-6 and KB-R7943 inhibit the unidirectional outward I(NCX) more potently than the unidirectional inward I(NCX). Both drugs inhibit NCX in an intracellular Na+ concentration-dependent manner. However, SN-6 affected other membrane currents less potently than KB-R7943.

Neuronal preconditioning with the antianginal drug, bepridil

It has recently been shown that the antianginal drug bepridil (BEP) activates mitochondrial ATP-sensitive potassium (mitoK(ATP)) channels and thus confers cardioprotection. Our aim was to investigate whether BEP could induce preconditioning in cultured rat cortical neurons. Although BEP depolarized isolated and in situ mitochondria and increased reactive oxygen species generation, no acute protection was observed. However, a 3-day BEP-treatment elicited dose-dependent delayed neuroprotection against 180 min of oxygen-glucose deprivation (cell viability: untreated, 52.5 +/- 0.85%; BEP 1 micromol/L, 59.6 +/- 1.53%*; BEP 2.5 micromol/L, 71.9 +/- 1.23%*; BEP 5 micromol/L, 95.3 +/- 0.89%*; mean +/- SEM; *p < 0.05 vs. untreated) and 60 min of glutamate excitotoxicity (200 micromol/L; cell viability: untreated, 54.1 +/- 0.69%; BEP 1 micromol/L, 61.2 +/- 1.19%*; BEP 2.5 micromol/L, 78.1 +/- 1.67%*; BEP 5 micromol/L, 91.2 +/- 1.20%*; mean +/- SEM; *p < 0.05 vs. untreated), and inhibited the reactive oxygen species surge upon glutamate exposure. The protection was antagonized with co-application of the superoxide dismutase mimetic M40401, but not with reduced glutathione, catalase, or with the mitoK(ATP) blocker 5-hydroxydecanoate. Furthermore, BEP treatment resulted in increased levels of phosphorylated protein kinase C, manganese-dependent superoxide dismutase, glutathione peroxidase, and Bcl-2. Our results indicate that BEP induces delayed neuronal preconditioning which is dependent on superoxide generation but perhaps not on direct mitoK(ATP) activation.

Topics on the Na+/Ca2+ Exchanger: Pharmacological Characterization of Na+/Ca2+ Exchanger Inhibitors

Using the whole-cell voltage clamp, we examined acute effects of various agents on Na(+)/Ca(2+) exchange current (I(NCX)) in guinea-pig cardiac ventricular cells and transfected cells. Among the antiarrhythmic drugs, amiodarone, bepridil, dronedarone, cibenzoline, azimilide, and aprindine inhibited I(NCX) in a concentration-dependent manner. We also investigated the effects on NCX of 2,3-buanedione monoxim (BDM) and selective NCX inhibitors such as KB-R7943, SEA0400, and SN-6. The presence of trypsin in the pipette solution attenuated the inhibitory effects on NCX of amiodarone, bepridil, and BDM, suggesting that these drugs inhibit NCX from the cytosolic side. In contrast, the trypsin-insensitive NCX inhibitors were aprindine, azimilide, dronedarone, cibenzoline, KB-R7943, SEA0400, and SN-6. KB-R7943, SEA0400, and SN-6 suppressed the uni-directional outward I(NCX) more potently than the uni-directional inward I(NCX). The mechanism of this mode-dependency is unknown, but is suggested to be related to intracellular Na(+) concentration.

Effects of SEA0400 and KB-R7943 on Na+/Ca2+ exchange current and L-type Ca2+ current in canine ventricular cardiomyocytes

SEA0400 and KB-R7943 are compounds synthesised to block transsarcolemmal Na+/Ca2+ exchange current (I(Na/Ca)); however, they have also been shown to inhibit L-type Ca2+ current (I(Ca)). The potential value of these compounds depends critically on their relative selectivity for I(Na/Ca) over I(Ca). In the present work, therefore, the concentration-dependent effects of SEA0400 and KB-R7943 on I(Na/Ca) and I(Ca) were studied and compared in canine ventricular cardiomyocytes using the whole-cell configuration of the patch clamp technique. SEA0400 and KB-R7943 decreased I(Na/Ca) in a concentration-dependent manner, having EC50 values of 111+/-43 nM and 3.35+/-0.82 microM, when suppressing inward currents, while the respective EC50 values were estimated at 108+/-18 nM and 4.74+/-0.69 microM in the case of outward current block. SEA0400 and KB-R7943 also blocked I(Ca), having comparable EC50 values (3.6 microM and 3.2 microM, respectively). At higher concentrations (10 microM) both drugs accelerated inactivation of I(Ca), retarded recovery from inactivation and shifted the voltage dependence of inactivation towards more negative voltages. The voltage dependence of activation was slightly modified by SEA0400, but not by KB-R7943. Based on the relatively good selectivity of submicromolar concentrations of SEA0400--but not KB-R7943--for I(Na/Ca) over I(Ca), SEA0400 appears to be a suitable tool to study the role of I(Na/Ca) in Ca2+ handling in canine cardiac cells. At concentrations higher than 1 microM, however, I(Ca) is progressively suppressed by the compound.

Voltage-dependent and frequency-independent inhibition of recombinant Cav3.2 T-type Ca2+ channel by bepridil

Effects of bepridil on the low voltage-activated T-type Ca2+ channel (CaV3.2) current stably expressed in human embryonic kidney (HEK)-293 cells were examined using patch-clamp techniques. Bepridil potently inhibited ICa,T with a markedly voltage-dependent manner; the IC50 of bepridil was 0.4 micromol/l at the holding potential of -70 mV, which was 26 times as potent as that at -100 mV (10.6 micromol/l). Steady-state inactivation curve (8.4 +/- 1.7 mV) and conductance curve (5.9 +/- 1.9 mV) were shifted to the hyperpolarized potential by 10 micromol/l bepridil. Bepridil exerted the tonic blocking action but not the use-dependent block. Bepridil had no effect on the recovery from inactivation of T-type Ca2+ channels. Thus, high efficacy of bepridil for terminating atrial fibrillation and atrial flutter may be considered to be attributed, at least in a part, to the T-type Ca2+ channel-blocking actions.

Effects of amiodarone on mutant Na+/Ca2+ exchangers expressed in CCL 39 cells

Using the whole cell voltage clamp, we reported previously that amiodarone acutely inhibits Na+/Ca2+ exchange current (INCX) in guinea pig cardiac ventricular myocytes. Intracellular application of trypsin via the patch pipette attenuated the blocking effect of amiodarone, suggesting that amiodarone affects the Na+/Ca2+ exchanger (NCX) from the cytoplasmic side. Here, we attempted to detect the site of amiodarone inhibition using wild type NCX1, mutants, and NCX3 expressed in CCL39 fibroblasts. INCX was recorded by ramp pulses. Amiodarone at 30 microM inhibited INCX by 80% in cells expressing wild type NCX1. However, 30 microM amiodarone inhibited INCX by about 55% in cells expressing mutant NCX1 with amino acids 217-671 (DeltaXIP) or 247-671 (Delta247-671) deleted in the long intracellular loop between the transmembrane segments (TM) 5 and 6. INCXs from NCX mutants deleted of cytoplasmic TM1-2, TM3-4 or the C-terminus were inhibited by amiodarone to a similar extent as the wild type. Amiodarone also inhibited INCX of NCX3 by 76%. These results suggest that a long intracellular loop may be involved in the inhibition of NCX1 by amiodarone, but that other intracellular loops, XIP region or C terminus are not involved in the amiodarone inhibition of NCX1.

Pharmacology of Brain Na+/Ca2+Exchanger: From Molecular Biology to Therapeutic Perspectives

In the last two decades, there has been a growing interest in unraveling the role that the Na+/Ca2+ exchanger (NCX) plays in the function and regulation of several cellular activities. Molecular biology, electrophysiology, genetically modified mice, and molecular pharmacology have helped to delve deeper and more successfully into the physiological and pathophysiological role of this exchanger. In fact, this nine-transmembrane protein, widely distributed in the brain and in the heart, works in a bidirectional way. Specifically, when it operates in the forward mode of operation, it couples the extrusion of one Ca2+ ion with the influx of three Na+ ions. In contrast, when it operates in the reverse mode of operation, while three Na+ ions are extruded, one Ca2+ enters into the cells. Different isoforms of NCX, named NCX1, NCX2, and NCX3, have been described in the brain, whereas only one, NCX1, has been found in the heart. The hypothesis that NCX can play a relevant role in several pathophysiological conditions, including hypoxia-anoxia, white matter degeneration after spinal cord injury, brain trauma and optical nerve injury, neuronal apoptosis, brain aging, and Alzheimer's disease, stems from the observation that NCX, in parallel with selective ion channels and ATP-dependent pumps, is efficient at maintaining intracellular Ca2+ and Na+ homeostasis. In conclusion, although studies concerning the involvement of NCX in the pathological mechanisms underlying brain injury during neurodegenerative diseases started later than those related to heart disease, the availability of pharmacological agents able to selectively modulate each NCX subtype activity and antiporter mode of operation will provide a better understanding of its pathophysiological role and, consequently, more promising approaches to treat these neurological disorders.

Evidence for a protective role played by the Na+/Ca2+ exchanger in cerebral ischemia induced by middle cerebral artery occlusion in male rats

In the present paper, the role played by Na+/Ca2+ exchanger (NCX) in focal cerebral ischemia was investigated. To this aim, permanent middle cerebral artery occlusion (pMCAO) was performed in male rats. The effects on the infarct volume of some inhibitors, such as tyrosine-6 glycosylated form of the exchanger inhibitory peptide (GLU-XIP), benzamil derivative (CB-DMB) and diarylaminopropylamine derivative (bepridil), and of the NCX activator, FeCl3, were examined. FeCl3, CB-DMB, bepridil and GLU-XIP, a modified peptide synthesized in our laboratory in order to facilitate its entrance into the cells through the glucose transporter, were intracerebroventricularly (i.c.v.) infused. FeCl3 (10 microg/kg) was able to reduce the extension of brain infarct volume. This effect was counteracted by the concomitant icv administration of CB-DMB (120 microg/kg). All NCX inhibitors, GLU-XIP, CB-DMB and bepridil, caused a worsening of the brain infarct lesion. These results suggest that a stimulation of NCX activity may help neurons and glial cells that are not irreversibly damaged in the penumbral zone to survive, whereas its pharmacological blockade can compromise their survival.

Vasorelaxant Activity of N-Caffeoylamino Acids

Twelve N-caffeoylamino acids and N-cinnamoylamino acids were synthesized and their vasorelaxation activity against norepinephrine (NE)-induced contraction of rat aorta was examined. The following structure-activity relationships were found. 1) On the benzene ring, the caffeoyl structure is effective for vasorelaxation, while the cinnamoyl structure reduced vasorelaxation activity. 2) Four to six carbons are more effective as the carbon chain connecting the acylamino and carboxyl terminal groups. N-Caffeoyl-beta-alanine and N-caffeoyltranexamic acid were used to investigate the action mechanism of vasorelaxing activities. It is believed that these compounds antagonize NE-induced vasocontraction by inhibiting receptor-operated calcium channels.

Sodium-calcium exchange influences the response to endothelin-1 in lens epithelium

Studies were conducted to examine the possible involvement of Na+-Ca2+ exchanger in determining the magnitude of the endothelin-1 (ET-1)-receptor-mediated calcium signal in porcine lens epithelial cells. Cytoplasmic calcium concentration was measured in primary cultured cells loaded with Fura-2. ET-1 (100 nM) caused cytoplasmic calcium to increase transiently to approximately 250 nM from a baseline of approximately 65 nM. The calcium increase decayed to a sustained plateau 35-45 nM above the baseline. Both the peak and plateau component of the ET-1 calcium response were abolished by PD145065, an ET receptor antagonist, and by cyclopiazonic acid (CPA) (10 microM). In calcium-free bathing solution, only the plateau was abolished. In the presence of ouabain, low-sodium bathing solution or bepridil, a sodium-calcium exchange inhibitor, peak height more than doubled. Bepridil also increased the peak height of the calcium response to ATP. The half-time for decay of the ET-1 and ATP calcium peak was increased several folds by bepridil, ouabain and low-sodium conditions. Measurements of ionomycin-releasable calcium suggested calcium store size was not increased in bepridil-treated cells. Taken together findings suggest inhibition of sodium-calcium exchange increases the magnitude of the receptor-initiated store-release phase of the ET-1 calcium signaling response as the result of impaired calcium clearance from the cytoplasm.

Pharmacological evidence for involvement of phospholipase D, protein kinase C, and sodium-calcium exchanger in alpha-adrenoceptor-mediated negative inotropy in adult mouse ventricle

The intracellular signalling pathway for alpha-adrenoceptor-mediated negative inotropy was studied pharmacologically in isolated adult mouse ventricle. The negative inotropy was inhibited by GF-109203X, a nonselective protein kinase C inhibitor. Phorbol 12-myristate 13-acetate also produced sustained negative inotropy, which was inhibited by KB-R7943, a Na(+)/Ca(2+) exchanger inhibitor. The alpha-adrenoceptor-mediated negative inotropy was augmented by RHC-80267, a diacylglycerol lipase inhibitor, but was inhibited either by C(2)-ceramide, a phospholipase D inhibitor, and high concentration of propranolol (50 micro M), which inhibits phosphatidate phosphohydrolase. The inotropy was not affected by U-73122, a phospholipase C inhibitor. Lavendustin-A, a tyrosine kinase inhibitor, also inhibited the negative inotropy. These findings suggest that alpha-adrenoceptor-mediated negative inotropy in adult mouse ventricle is mediated by activation of tyrosine kinase, the phospholipase D-phosphatidate phosphohydrolase pathway, and protein kinase C.

Pharmacology of Na+/Ca2+ exchanger

KB-R7943 inhibits the Na(+)/Ca(2+) exchanger in an independent manner or in a manner dependent on the direction of the current. This effect may be due to the experimental protocols bawed on the competition between the drug and external substrate ions. Some antiarrhythmic drugs inhibit NCX. A new column of NCX was added in Sicilian Gambit.

Chronic administration of amiodarone does not affect Na+/Ca2+ exchange current in guinea pig cardiac ventricular myocytes

We investigated chronic effects of amiodarone on Na+/Ca2+ exchange current (INCX) and on the level of Na+/Ca2+ exchanger (NCX1) mRNA in guinea pig ventricular myocytes using the whole-cell clamp technique and RT-PCR analysis, respectively. Guinea pigs were intraperitoneally injected with 80 mg/kg per day of amiodarone or the vehicle (saline) for 1 or 4 weeks. Single ventricular cells were isolated from the hearts of both groups of animals. Action potential duration at 90% repolarization level was prolonged to 143% and 165% of the control values by treatment with amiodarone for 1 and 4 weeks, respectively. INCX density and the level of NCX1 mRNA were not significantly changed by chronic treatment with amiodarone. The level of thyroid hormone (T4) within the blood was not changed by the treatments. These results suggest that chronic treatment with amiodarone does not affect the Na+/Ca2+ exchanger, with respect to the level of its mRNA and current density in guinea pig ventricular myocytes.

Inhibitory effect of aprindine on Na+/Ca2+ exchange current in guinea-pig cardiac ventricular myocytes

1. Using the whole-cell voltage clamp technique, the effect of aprindine on Na+/Ca2+ exchange current (I(NCX)) was examined in guinea-pig single cardiac ventricular myocytes and CCL39 fibroblasts expressing a dog cardiac Na+/Ca2+ exchanger (NCX1). 2. I(NCX) was recorded by ramp pulses from the holding potential of -60 mV with the external solution containing 140 mM Na+ and 1 mM Ca2+, and the pipette solution containing 20 mM Na+, 20 mM BAPTA and 13 mM Ca2+ (433 nM free Ca2+). 3. External application of aprindine suppressed I(NCX) in a concentration-dependent manner. The IC50 values of outward (measured at 50 mV) and inward (measured at -100 mV) I(NCX) components were 48.8 and 51.8 microM with Hill coefficients of 1.3 and 1, respectively. 4. Intracellular application of trypsin via the pipette solution did not change the blocking effect of aprindine, suggesting that aprindine does not affect the exchanger from the cytoplasmic side. 5. Aprindine inhibited I(NCX) of a mutant NCX1 with a deletion of amino acids 247 - 671 in the large intracellular domain between the transmembrane segments 5 and 6 in a similar manner to that of the wild-type, suggesting that the site of aprindine inhibition is not in the large intracellular domain of NCX1. 6. A kinetic study indicated that aprindine was cooperatively competitive with KB-R7943, another inhibitor of NCX and that aprindine was a competitive inhibitor with respect to external Ca2+. 7. We conclude that aprindine may modestly inhibit I(NCX) in a therapeutic range of concentrations (around 2.5 approximately 6.9 microM) possibly at an external or intra-membranous site of the exchanger.