Protective effects of SEA0400, a novel and selective inhibitor of the Na+/Ca2+ exchanger, on myocardial ischemia-reperfusion injuries

Antiarrhythmic and Inotropic Effects of Selective Na+/Ca2+ Exchanger Inhibition: What Can We Learn from the Pharmacological Studies?

Life-long stable heart function requires a critical balance of intracellular Ca²⁺. Several ion channels and pumps cooperate in a complex machinery that controls the influx, release, and efflux of Ca²⁺. Probably one of the most interesting and most complex players of this crosstalk is the Na⁺/Ca²⁺ exchanger, which represents the main Ca²⁺ efflux mechanism; however, under some circumstances, it can also bring Ca²⁺ into the cell. Therefore, the inhibition of the Na⁺/Ca²⁺ exchanger has emerged as one of the most promising possible pharmacological targets to increase Ca²⁺ levels, to decrease arrhythmogenic depolarizations, and to reduce excessive Ca²⁺ influx. In line with this, as a response to increasing demand, several more or less selective Na⁺/Ca²⁺ exchanger inhibitor compounds have been developed. In the past 20 years, several results have been published regarding the effect of Na⁺/Ca²⁺ exchanger inhibition under various circumstances, e.g., species, inhibitor compounds, and experimental conditions; however, the results are often controversial. Does selective Na⁺/Ca²⁺ exchanger inhibition have any future in clinical pharmacological practice? In this review, the experimental results of Na⁺/Ca²⁺ exchanger inhibition are summarized focusing on the data obtained by novel highly selective inhibitors.

Calcium dysregulation in heart diseases: Targeting calcium channels to achieve a correct calcium homeostasis

Intracellular calcium signaling is a universal language source shared by the most part of biological entities inside cells that, all together, give rise to physiological and functional anatomical units, the organ. Although preferentially recognized as signaling between cell life and death processes, in the heart it assumes additional relevance considered the importance of calcium cycling coupled to ATP consumption in excitation-contraction coupling. The concerted action of a plethora of exchangers, channels and pumps inward and outward calcium fluxes where needed, to convert energy and electric impulses in muscle contraction. All this without realizing it, thousands of times, every day. An improper function of those proteins (i.e., variation in expression, mutations onset, dysregulated channeling, differential protein-protein interactions) being part of this signaling network triggers a short circuit with severe acute and chronic pathological consequences reported as arrhythmias, cardiac remodeling, heart failure, reperfusion injury and cardiomyopathies. By acting with chemical, peptide-based and pharmacological modulators of these players, a correction of calcium homeostasis can be achieved accompanied by an amelioration of clinical symptoms. This review will focus on all those defects in calcium homeostasis which occur in the most common cardiac diseases, including myocardial infarction, arrhythmia, hypertrophy, heart failure and cardiomyopathies. This part will be introduced by the state of the art on the proteins involved in calcium homeostasis in cardiomyocytes and followed by the therapeutic treatments that to date, are able to target them and to revert the pathological phenotype.

KB-R7943 Inhibits the Mitochondrial Ca2+ Uniporter but Not Na+-Ca2+ Exchanger in Cardiomyocyte-Derived H9c2 Cells

The effect of KB-R7943, an inhibitor of the plasmalemmal Na⁺-Ca²⁺ exchanger, on mitochondrial Ca²⁺ transporters was examined with membrane-permeabilized cardiomyocyte-derived H9c2 cells expressing the fluorescent Ca²⁺ indicator, yellow cameleon 3.1, in the mitochondria. KB-R7943, as well as ruthenium red, inhibited the rise in mitochondrial Ca²⁺ on increasing the extramitochondrial Ca²⁺ concentration from 0 nM to 300 nM. CGP-37157, but not KB-R7943, inhibited the decline in mitochondrial Ca²⁺on return to Ca²⁺ free extramitochondrial solution. These results indicated that KB-R7943 has inhibitory effects on the mitochondrial Ca²⁺ uniporter, but not on the mitochondrial Na⁺-Ca²⁺ exchanger.

Targeting Ca2 + Handling Proteins for the Treatment of Heart Failure and Arrhythmias

Diseases of the heart, such as heart failure and cardiac arrhythmias, are a growing socio-economic burden. Calcium (Ca²⁺) dysregulation is key hallmark of the failing myocardium and has long been touted as a potential therapeutic target in the treatment of a variety of cardiovascular diseases (CVD). In the heart, Ca²⁺ is essential for maintaining normal cardiac function through the generation of the cardiac action potential and its involvement in excitation contraction coupling. As such, the proteins which regulate Ca²⁺ cycling and signaling play a vital role in maintaining Ca²⁺ homeostasis. Changes to the expression levels and function of Ca²⁺-channels, pumps and associated intracellular handling proteins contribute to altered Ca²⁺ homeostasis in CVD. The remodeling of Ca²⁺-handling proteins therefore results in impaired Ca²⁺ cycling, Ca²⁺ leak from the sarcoplasmic reticulum and reduced Ca²⁺ clearance, all of which contributes to increased intracellular Ca²⁺. Currently, approved treatments for targeting Ca²⁺ handling dysfunction in CVD are focused on Ca²⁺ channel blockers. However, whilst Ca²⁺ channel blockers have been successful in the treatment of some arrhythmic disorders, they are not universally prescribed to heart failure patients owing to their ability to depress cardiac function. Despite the progress in CVD treatments, there remains a clear need for novel therapeutic approaches which are able to reverse pathophysiology associated with heart failure and arrhythmias. Given that heart failure and cardiac arrhythmias are closely associated with altered Ca²⁺ homeostasis, this review will address the molecular changes to proteins associated with both Ca²⁺-handling and -signaling; their potential as novel therapeutic targets will be discussed in the context of pre-clinical and, where available, clinical data.

The Effect of a Novel Highly Selective Inhibitor of the Sodium/Calcium Exchanger (NCX) on Cardiac Arrhythmias in In Vitro and In Vivo Experiments

Background

In this study the effects of a new, highly selective sodium-calcium exchanger (NCX) inhibitor, ORM-10962 were investigated on cardiac NCX current, Ca²⁺ transients, cell shortening and in experimental arrhythmias. The level of selectivity of the novel inhibitor on several major transmembrane ion currents (L-type Ca²⁺ current, major repolarizing K⁺ currents, late Na⁺ current, Na⁺/K⁺ pump current) was also determined.

Methods

Ion currents in single dog ventricular cells (cardiac myocytes; CM), and action potentials in dog cardiac multicellular preparations were recorded utilizing the whole-cell patch clamp and standard microelectrode techniques, respectively. Ca²⁺ transients and cell shortening were measured in fluorescent dye loaded isolated dog myocytes. Antiarrhythmic effects of ORM-10962 were studied in anesthetized ouabain (10 μg/kg/min i.v.) pretreated guinea pigs and in ischemia-reperfusion models (I/R) of anesthetized coronary artery occluded rats and Langendorff perfused guinea pigs hearts.

Results

ORM-10962 significantly reduced the inward/outward NCX currents with estimated EC50 values of 55/67 nM, respectively. The compound, even at a high concentration of 1 μM, did not modify significantly the magnitude of I_CaL in CMs, neither had any apparent influence on the inward rectifier, transient outward, the rapid and slow components of the delayed rectifier potassium currents, the late and peak sodium and Na⁺/K⁺ pump currents. NCX inhibition exerted moderate positive inotropic effect under normal condition, negative inotropy when reverse, and further positive inotropic effect when forward mode was facilitated. In dog Purkinje fibres 1 μM ORM-10962 decreased the amplitude of digoxin induced delayed afterdepolarizations (DADs). Pre-treatment with 0.3 mg/kg ORM-10962 (i.v.) 10 min before starting ouabain infusion significantly delayed the development and recurrence of ventricular extrasystoles (by about 50%) or ventricular tachycardia (by about 30%) in anesthetized guinea pigs. On the contrary, ORM-10962 pre-treatment had no apparent influence on the time of onset or the severity of I/R induced arrhythmias in anesthetized rats and in Langendorff perfused guinea-pig hearts.

Conclusions

The present study provides strong evidence for a high efficacy and selectivity of the NCX-inhibitory effect of ORM-10962. Selective NCX inhibition can exert positive as well as negative inotropic effect depending on the actual operation mode of NCX. Selective NCX blockade may contribute to the prevention of DAD based arrhythmogenesis, in vivo, however, its effect on I/R induced arrhythmias is still uncertain.

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.

Selective Na(+) /Ca(2+) exchanger inhibition prevents Ca(2+) overload-induced triggered arrhythmias

BACKGROUND AND PURPOSE

Augmented Na(+) /Ca(2+) exchanger (NCX) activity may play a crucial role in cardiac arrhythmogenesis; however, data regarding the anti-arrhythmic efficacy of NCX inhibition are debatable. Feasible explanations could be the unsatisfactory selectivity of NCX inhibitors and/or the dependence of the experimental model on the degree of Ca(2+) i overload. Hence, we used NCX inhibitors SEA0400 and the more selective ORM10103 to evaluate the efficacy of NCX inhibition against arrhythmogenic Ca(2+) i rise in conditions when [Ca(2+) ]i was augmented via activation of the late sodium current (INaL ) or inhibition of the Na(+) /K(+) pump.

EXPERIMENTAL APPROACH

Action potentials (APs) were recorded from canine papillary muscles and Purkinje fibres by microelectrodes. NCX current (INCX ) was determined in ventricular cardiomyocytes utilizing the whole-cell patch clamp technique. Ca(2+) i transients (CaTs) were monitored with a Ca(2+) -sensitive fluorescent dye, Fluo-4.

KEY RESULTS

Enhanced INaL increased the Ca(2+) load and AP duration (APD). SEA0400 and ORM10103 suppressed INCX and prevented/reversed the anemone toxin II (ATX-II)-induced [Ca(2+) ]i rise without influencing APD, CaT or cell shortening, or affecting the ATX-II-induced increased APD. ORM10103 significantly decreased the number of strophanthidin-induced spontaneous diastolic Ca(2+) release events; however, SEA0400 failed to restrict the veratridine-induced augmentation in Purkinje-ventricle APD dispersion.

CONCLUSIONS AND IMPLICATIONS

Selective NCX inhibition - presumably by blocking rev INCX (reverse mode NCX current) - is effective against arrhythmogenesis caused by [Na(+) ]i -induced [Ca(2+) ]i elevation, without influencing the AP waveform. Therefore, selective INCX inhibition, by significantly reducing the arrhythmogenic trigger activity caused by the perturbed Ca(2+) i handling, should be considered as a promising anti-arrhythmic therapeutic strategy.

Efficacy of selective NCX inhibition by ORM-10103 during simulated ischemia/reperfusion

In this study we evaluated the effects of selective Na+/Ca2+ exchanger (NCX) inhibition by ORM-10103 on the [Ca2+]i transient (CaT), action potential (AP), and cell viability in isolated canine ventricular cardiomyocytes exposed to a simulated ischemia/reperfusion protocol performed either alone (modeling moderate low-flow ischemia) or with simultaneous strophantidine challenge (modeling more severe low-flow ischemia). CaTs were monitored using a Ca2+-sensitive fluorescent dye, APs were recorded by intracellular microelectrodes, and anaerobic shifts in cellular metabolism were verified via monitoring native NADH fluorescence. Simulated ischemia increased the NADH fluorescence, reduced the amplitudes of the AP and CaT and induced membrane depolarization. APs moderately shortened, CaTs prolonged. Diastolic [Ca2+]i ([Ca2+]iD) level increased significantly during ischemia and further elevated following strophantidine application. Reperfusion normalized the NADH level, the amplitude of the AP and duration of the [Ca2+]i transient, but only partially restored action potential triangulation and the amplitude of the CaT. [Ca2+]iD decreased in untreated, but further increased in strophantidine-treated cells. 10 µM ORM-10103 significantly reduced the ischemic [Ca2+]i raise in both untreated and strophantidine-treated cells. During reperfusion ORM-10103 decreased [Ca2+]i and eliminated its diastolic elevation in untreated and strophantidine-treated cardiomyocytes. Following the application of ORM-10103 the detrimental effect of ischemia/reperfusion on cell viability and the reperfusion-induced increase in AP and CaT variabilities were substantially reduced, but ischemia-induced shifts in AP morphology were barely influenced. In conclusion, selective NCX inhibition by ORM-10103 is highly effective against ischemia/reperfusion induced pathologic alterations in [Ca2+]i homeostasis, however, it fails to normalize untoward arrhythmogenic changes in AP morphology.

KB-R7943, a plasma membrane Na(+)/Ca(2+) exchanger inhibitor, blocks opening of the mitochondrial permeability transition pore

The isothiourea derivative, KB-R7943, inhibits the reverse-mode of the plasma membrane sodium/calcium exchanger and protects against ischemia/reperfusion injury. The mechanism through which KB-R7943 confers protection, however, remains controversial. Recently, KB-R7943 has been shown to inhibit mitochondrial calcium uptake and matrix overload, which may contribute to its protective effects. While using KB-R7943 for this purpose, we find here no evidence that KB-R7943 directly blocks mitochondrial calcium uptake. Rather, we find that KB-R7943 inhibits opening of the mitochondrial permeability transition pore in permeabilized cells and isolated liver mitochondria. Furthermore, we find that this observation correlates with protection against calcium ionophore-induced mitochondrial membrane potential depolarization and cell death, without detrimental effects to basal mitochondrial membrane potential or complex I-dependent mitochondrial respiration. Our data reveal another mechanism through which KB-R7943 may protect against calcium-induced injury, as well as a novel means to inhibit the mitochondrial permeability transition pore.

The Na+/Ca2+ exchange inhibitor SEA0400 limits intracellular Ca2+ accumulation and improves recovery of ventricular function when added to cardioplegia

Background

The Na⁺/Ca²⁺ exchange inhibitor SEA0400 prevents myocardial injury in models of global ischemia and reperfusion. We therefore evaluated its potential as a cardioplegia additive.

Methods

Isolated rat cardiomyocytes were exposed to hypoxia (45 min) followed by reperfusion. During hypoxia, cells were protected using cardioplegia with (n = 25) or without (n = 24) SEA0400 (1 μM), or were not protected with cardioplegia (hypoxic control, n = 8). Intracellular Ca²⁺ levels were measured using Ca²⁺ sensitive dye (fura-2 AM). Isolated rat hearts were arrested using cardioplegia with (n = 7) or without (n = 6) SEA0400 (1 μM) then reperfused after 45 min of ischemia. Left ventricular (LV) function, troponin release, and mitochondrial morphology were evaluated.

Results

Cardiomyocytes exposed to hypoxia without cardioplegia had poor survival (13%). Survival was significantly improved when cells were protected with cardioplegia containing SEA0400 (68%, p = 0.009); cardioplegia without SEA0400 was associated with intermediate survival (42%). Cardiomyocytes exposed to hypoxia alone had a rapid increase in intracellular Ca²⁺ (305 ± 123 nM after 20 minutes of ischemia). Increases in intracellular Ca²⁺ were reduced in cells arrested with cardioplegia without SEA0400; however cardioplegia containing SEA0400 was associated with the lowest intracellular Ca²⁺ levels (110 ± 17 vs. 156 ± 42 nM after 45 minutes of ischemia, p = 0.004). Hearts arrested with cardioplegia containing SEA0400 had better recovery of LV work compared to cardioplegia without SEA0400 (23140 ± 2264 vs. 7750 ± 929 mmHg.μl, p = 0.0001). Troponin release during reperfusion was lower (0.6 ± 0.2 vs. 2.4 ± 0.5 ng/mL, p = 0.0026), and there were more intact (41 ± 3 vs. 22 ± 5%, p < 0.005), and fewer disrupted mitochondria (24 ± 2 vs. 33 ± 3%, p < 0.05) in the SEA0400 group.

Conclusions

SEA0400 added to cardioplegia limits accumulation of intracellular Ca²⁺ during ischemic arrest in isolated cardiomyocytes and prevents myocardial injury and improves recovery of LV function in isolated hearts.

Regulation of the Na+/Ca2+ exchanger by pyridine nucleotide redox potential in ventricular myocytes

The cardiac Na(+)/Ca(2+) exchanger (NCX) is the major Ca(2+) efflux pathway on the sarcolemma, counterbalancing Ca(2+) influx via L-type Ca(2+) current during excitation-contraction coupling. Altered NCX activity modulates the sarcoplastic reticulum Ca(2+) load and can contribute to abnormal Ca(2+) handling and arrhythmias. NADH/NAD(+) is the main redox couple controlling mitochondrial energy production, glycolysis, and other redox reactions. Here, we tested whether cytosolic NADH/NAD(+) redox potential regulates NCX activity in adult cardiomyocytes. NCX current (INCX), measured with whole cell patch clamp, was inhibited in response to cytosolic NADH loaded directly via pipette or increased by extracellular lactate perfusion, whereas an increase of mitochondrial NADH had no effect. Reactive oxygen species (ROS) accumulation was enhanced by increasing cytosolic NADH, and NADH-induced INCX inhibition was abolished by the H2O2 scavenger catalase. NADH-induced ROS accumulation was independent of mitochondrial respiration (rotenone-insensitive) but was inhibited by the flavoenzyme blocker diphenylene iodonium. NADPH oxidase was ruled out as the effector because INCX was insensitive to cytosolic NADPH, and NADH-induced ROS and INCX inhibition were not abrogated by the specific NADPH oxidase inhibitor gp91ds-tat. This study reveals a novel mechanism of NCX regulation by cytosolic NADH/NAD(+) redox potential through a ROS-generating NADH-driven flavoprotein oxidase. The mechanism is likely to play a key role in Ca(2+) homeostasis and the response to alterations in the cytosolic pyridine nucleotide redox state during ischemia-reperfusion or other cardiovascular diseases.

The Na+/Ca²+ exchanger in cardiac ischemia/reperfusion injury

The Na⁺/Ca²⁺ exchanger (NCX) is an important electrogenic transporter in maintaining Na⁺ and Ca²⁺ homeostasis in a variety of mammalian organs, and is involved in the physiological and pathophysiological regulation of Ca²⁺ concentration in the myocardium. It can affect cardial structure, electrophysiology and contractile properties. The role of the NCX in heart cells following ischemia/reperfusion (IR) has been investigated using a number of in vitro and in vivo models. During ischemia, ionic disturbances favor Ca²⁺-influx mode activity as excess Na⁺ is extruded in exchange for Ca²⁺, giving rise to increased intracellular Ca²⁺ levels (Cai). This rise in Cai contributes to reversible cellular dysfunction upon reperfusion, such as myocardial necrosis, arrhythmia, systolic dysfunction and heart failure. We have reviewed the major in vivo and in vitro cardiac IR-related NCX studies in an attempt to clarify the functions of NCX in IR and conclude that recent studies suggest blockage of NCX has potential therapeutic applications. Although the use of different IR models, application of NCX stimulators and inhibitors, and development of NCX transgenic animals do help elucidate the role of this ion exchanger in heart cells, related mechanisms are not completely understood and clinically effective specific NCX inhibitors need further research.

Combined Na(+)/Ca(2+) exchanger and L-type calcium channel block as a potential strategy to suppress arrhythmias and maintain ventricular function

BACKGROUND

L-type calcium channel (LTCC) and Na(+)/Ca(2+) exchanger (NCX) have been implicated in repolarization-dependent arrhythmias, but also modulate calcium and contractility. Although LTCC inhibition is negative inotropic, NCX inhibition has the opposite effect. Combined block may, therefore, offer an advantage for hemodynamics and antiarrhythmic efficiency, particularly in diseased hearts. In a model of proarrhythmia, the dog with chronic atrioventricular block, we investigated whether combined inhibition of NCX and LTCC with SEA-0400 is effective against dofetilide-induced torsade de pointes arrhythmias (TdP), while maintaining calcium homeostasis and hemodynamics.

METHODS AND RESULTS

Left ventricular pressure (LVP) and ECG were monitored during infusion of SEA-0400 and verapamil in anesthetized dogs. Different doses were tested against dofetilide-induced TdP in chronic atrioventricular block dogs. In ventricular myocytes, effects of SEA-0400 were tested on action potentials, calcium transients, and early afterdepolarizations. In cardiomyocytes, SEA-0400 (1 μmol/L) blocked 66±3% of outward NCX, 50±2% of inward NCX, and 33±9% of LTCC current. SEA-0400 had no effect on systolic calcium, but slowed relaxation, despite action potential shortening, and increased diastolic calcium. SEA-0400 stabilized dofetilide-induced lability of repolarization and suppressed early afterdepolarizations. In vivo, SEA-0400 (0.4 and 0.8 mg/kg) had no effect on left ventricular pressure and suppressed dofetilide-induced TdPs dose dependently. Verapamil (0.3 mg/kg) also inhibited TdP, but caused a 15±8% drop of left ventricular pressure. A lower dose of verapamil without effects on left ventricular pressure (0.06 mg/kg) was not antiarrhythmic.

CONCLUSIONS

In chronic atrioventricular block dogs, SEA-0400 treatment is effective against TdP. Unlike specific inhibition of LTCC, combined NCX and LTCC inhibition has no negative effects on cardiac hemodynamics.

Selective inhibition of the reverse mode of Na(+)/Ca(2+) exchanger attenuates contrast-induced cell injury

BACKGROUND

The precise mechanisms underlying radiocontrast nephropathy (RCN) are not well understood. Intracellular Ca(2+) overload is considered to be a key factor in RCN. The Na(+)/Ca(2+) exchanger (NCX) system is one of the main pathways of intracellular Ca(2+) overload. We investigated whether intracellular Ca(2+) overload via the NCX system was involved in contrast-induced renal tubular cytotoxicity.

METHODS

NRK-52E cells were exposed to ioversol (100 mg iodine/ml) for 4 h. KB-R7943 (inhibitor of reverse mode of NCX, 4 × 10(-5), 4 × 10(-6)M) was added 1 h before incubation with ioversol. Cell viability and permeability were determined by 3-(4,5-dimethyldiazol-2-yl)-2,5-diphenyl tetrazolium bromide and lactate dehydrogenase assay. Apoptosis was determined by flow cytometry. Intracellular Ca(2+) concentration ([Ca(2+)](i)] and reactive oxygen species (ROS) were detected by confocal microscopy. The expression of NCX1 mRNA and caspase-3 protein was evaluated by reverse transcription-polymerase chain reaction and Western blot, respectively.

RESULTS

Ioversol exposure induced significantly increased lactate dehydrogenase release and decreased 3-(4,5-dimethyldiazol-2-yl)-2,5-diphenyl tetrazolium bromide conversion in NRK-52E cells. Significantly increased apoptosis and caspase-3 protein expression were observed in the NRK-52E cells exposed to ioversol for 4 h. Ioversol treatment induced a significant increase in [Ca(2+)](i) and intracellular ROS. KB-R7943 dose-dependently and significantly suppressed the increase in [Ca(2+)](i), intracellular ROS and caspase-3 overexpression induced by ioversol and attenuated the contrast-induced NRK-52E cell apoptosis. No significant changes in NCX1 mRNA expression were observed following contrast exposure.

CONCLUSION

Intracellular Ca(2+) overload via the reverse mode of NCX, followed by ROS overproduction and caspase-3 overexpression played an important role in the contrast-induced renal tubular cytotoxicity. The reverse mode of the NCX inhibitor KB-R7943 attenuated contrast-induced renal tubular cytotoxicity.

Different roles for contracture and calpain in calcium paradox-induced heart injury

The Ca(2+) paradox represents a good model to study Ca(2+) overload injury in ischemic heart diseases. We and others have demonstrated that contracture and calpain are involved in the Ca(2+) paradox-induced injury. This study aimed to elucidate their roles in this model. The Ca(2+) paradox was elicited by perfusing isolated rat hearts with Ca(2+)-free KH media for 3 min or 5 min followed by 30 min of Ca(2+) repletion. The LVDP was measured to reflect contractile function, and the LVEDP was measured to indicate contracture. TTC staining and the quantification of LDH release were used to define cell death. Calpain activity and troponin I release were measured after Ca(2+) repletion. Ca(2+) repletion of the once 3-min Ca(2+) depleted hearts resulted in almost no viable tissues and the disappearance of contractile function. Compared to the effects of the calpain inhibitor MDL28170, KB-R7943, an inhibitor of the Na(+)/Ca(2+) exchanger, reduced the LVEDP level to a greater extent, which was well correlated with improved contractile function recovery and tissue survival. The depletion of Ca(2+) for 5 min had the same effects on injury as the 3-min Ca(2+) depletion, except that the LVEDP in the 5-min Ca(2+) depletion group was lower than the level in the 3-min Ca(2+) depletion group. KB-R7943 failed to reduce the level of LVEDP, with no improvement in the LVDP recovery in the hearts subjected to the 5-min Ca(2+) depletion treatment; however, KB-R7943 preserved its protective effects in surviving tissue. Both KB-R7943 and MDL28170 attenuated the Ca(2+) repletion-induced increase in calpain activity in 3 min or 5 min Ca(2+) depleted hearts. However, only KB-R7943 reduced the release of troponin I from the Ca(2+) paradoxic heart. These results provide evidence suggesting that contracture is the main cause for contractile dysfunction, while activation of calpain mediates cell death in the Ca(2+) paradox.

Preferential involvement of Na⁺/Ca²⁺ exchanger type-1 in the brain damage caused by transient focal cerebral ischemia in mice

The Na(+)/Ca(2+) exchanger (NCX), an ion-transporter located in the plasma membrane of neuronal cells, contributes to intracellular Ca(2+) homeostasis. Within the brain, three isoforms (NCX1, NCX2, and NCX3) are widely distributed. However, it is not clear to what extent these isoforms are involved in ischemic brain damage in mammals. We therefore used genetically altered mice and isoform-selective NCX inhibitors in a model of transient focal ischemia to investigate the role of each NCX isoform in ischemic brain damage. NCX isoform-mutant mice (NCX1(+/-), NCX2(+/-), and NCX3(+/-)) and wild-type mice were subjected to 90min of middle cerebral artery occlusion (MCAO) followed by 24h of reperfusion. One of three NCX inhibitors [SN-6, KB-R7943, or SEA0400 (3 or 10mgkg(-1), i.p.)] was administered to ddY mice at 30min before more prolonged (4-h) MCAO followed by 24h of reperfusion. After transient MCAO reperfusion, the cerebral infarcts in NCX1(+/-) mice, but not those in NCX2(+/-) or NCX3(+/-) mice, were significantly smaller than those in wild-type mice. SN-6 and SEA0400, which are more selective for the NCX1 isoform, significantly reduced the infarct volume at 10mg/kg. In contrast, KB-R7943, which is more selective for NCX3, did not. These results suggest that the NCX1 isoform may act preferentially (vs. the NCX2 and NCX3 isoforms) to exacerbate the cerebral damage caused by ischemic insult in mice, and that NCX1-selective inhibitors warrant investigation as a potential therapeutic agents for stroke.

Alternative strategies in arrhythmia therapy: evaluation of Na/Ca exchange as an anti-arrhythmic target

The search for alternative anti-arrhythmic strategies is fueled by an unmet medical need as well as by the opportunities arising from identification of novel targets and novel drugs. Na/Ca exchange is a potential target involved in several types of arrhythmias, such as those related to ischemia-reperfusion, heart failure and also some forms of genetic arrhythmias. Inhibition of Na/Ca exchange is theoretically not only anti-arrhythmic but also increases cellular Ca(2+) content. This could be an advantage in conditions of low inotropy, such as in heart failure, but may also worsen conditions such as the recovery from ischemia or relaxation abnormalities. With the available drugs such as KB-R7943 and SEA-0400 these theories have now been tested in a number of cellular and in vivo models. Experience is overall rather positive and seems less hampered by the potential drawbacks than expected. This may be because the currently available drugs are not highly selective, with additional benefit derived from concurrent effects. While this precludes a definite answer regarding the benefit of a pure NCX inhibitor, they indicate that Na/Ca exchange inhibition as part of a multi-target strategy is an avenue to be considered. Such studies will need further 'bench' work and testing in relevant preclinical models, including chronic disease.

Triple threat: the Na+/Ca2+ exchanger in the pathophysiology of cardiac arrhythmia, ischemia and heart failure

The Na(+)/Ca(2+) exchanger (NCX) is the main Ca(2+) extrusion mechanism of the cardiac myocyte and thus is crucial for maintaining Ca(2+) homeostasis. It is involved in the regulation of several parameters of cardiac excitation contraction coupling, such as cytosolic Ca(2+) concentration, repolarization and contractility. Increased NCX activity has been identified as a mechanism promoting heart failure, cardiac ischemia and arrhythmia. Transgenic mice as well as pharmacological interventions have been used to support the idea of using NCX inhibition as a future pharmacological strategy to treat cardiovascular disease.

Indirectly probing Ca(2+) handling alterations following myocardial infarction in a murine model using T(1)-mapping manganese-enhanced magnetic resonance imaging

Prolonged ischemia causes cellular necrosis and myocardial infarction (MI) via intracellular calcium (Ca(2+)) overload. Manganese-enhanced MRI indirectly assesses Ca(2+) influx movement in vivo as manganese (Mn(2+)) is a Ca(2+) analog. To characterize myocardial Mn(2+) efflux properties, T(1)-mapping manganese-enhanced MRI studies were performed on adult male C57Bl/6 mice in which Ca(2+) efflux was altered using pharmacological intervention agents or MI-inducing surgery. Results showed that (1) Mn(2+) efflux rate increased exponentially with increasing Mn(2+) doses; (2) SEA0400 (a sodium-calcium exchanger inhibitor) decreased the rate of Mn(2+) efflux; and (3) dobutamine (a positive inotropic agent) increased the Mn(2+) efflux rate. A novel analysis technique also delineated regional features in the MI mice, which showed an increased Mn(2+) efflux rate in the necrosed and peri-infarcted tissue zones. The T(1)-mapping manganese-enhanced MRI technique characterized alterations in myocardial Mn(2+) efflux rates following both pharmacologic intervention and an acute MI. The Mn(2+) efflux results were consistent with those in ex vivo studies showing an increased Ca(2+) concentration under similar conditions. Thus, T(1)-mapping manganese-enhanced MRI has the potential to indirectly identify and quantify intracellular Ca(2+) handling in the peri-infarcted tissue zones, which may reveal salvageable tissue in the post-MI myocardium.

Assessing manganese efflux using SEA0400 and cardiac T1-mapping manganese-enhanced MRI in a murine model

The sodium-calcium exchanger (NCX) is one of the transporters contributing to the control of intracellular calcium (Ca(2+)) concentration by normally mediating net Ca(2+) efflux. However, the reverse mode of the NCX can cause intracellular Ca(2+) concentration overload, which exacerbates the myocardial tissue injury resulting from ischemia. Although the NCX inhibitor SEA0400 has been shown to therapeutically reduce myocardial injury, no in vivo technique exists to monitor intracellular Ca(2+) fluctuations produced by this drug. Cardiac manganese-enhanced MRI (MEMRI) may indirectly assess Ca(2+) efflux by estimating changes in manganese (Mn(2+)) content in vivo, since Mn(2+) has been suggested as a surrogate marker for Ca(2+). This study used the MEMRI technique to examine the temporal features of cardiac Mn(2+) efflux by implementing a T(1)-mapping method and inhibiting the NCX with SEA0400. The change in (1)H(2)O longitudinal relaxation rate, Delta R(1), in the left ventricular free wall, was calculated at different time points following infusion of 190 nmol/g manganese chloride (MnCl(2)) in healthy adult male mice. The results showed 50% MEMRI signal attenuation at 3.4 +/- 0.6 h post-MnCl(2) infusion without drug intervention. Furthermore, treatment with 50 +/- 0.2 mg/kg of SEA0400 significantly reduced the rate of decrease in Delta R(1). At 4.9-5.9 h post-MnCl(2) infusion, the average Delta R(1) values for the two groups treated with SEA0400 were 2.46 +/- 0.29 and 1.72 +/- 0.24 s(-1) for 50 and 20 mg/kg doses, respectively, as compared to the value of 1.27 +/- 0.28 s(-1) for the control group. When this in vivo data were compared to ex vivo absolute manganese content data, the MEMRI T(1)-mapping technique was shown to effectively quantify Mn(2+) efflux rates in the myocardium. Therefore, combining an NCX inhibitor with MEMRI may be a useful technique for assessing Mn(2+) transport mechanisms and rates in vivo, which may reflect changes in Ca(2+) transport.

The role of the Na+/Ca2+ exchanger, I(Na) and I(CaL) in the genesis of dofetilide-induced torsades de pointes in isolated, AV-blocked rabbit hearts

BACKGROUND AND PURPOSE

The Na+/Ca2+ exchanger (NCX) may contribute to triggered activity and transmural dispersion of repolarization, which are substrates of torsades de pointes (TdP) type arrhythmias. This study examined the effects of selective inhibition of the NCX by SEA0400 on the occurrence of dofetilide-induced TdP.

EXPERIMENTAL APPROACH

Effects of SEA0400 (1 micromol x L(-1)) on dofetilide-induced TdP was studied in isolated, Langendorff-perfused, atrioventricular (AV)-blocked rabbit hearts. To verify the relevance of the model, lidocaine (30 micromol x L(-1)) and verapamil (750 nmol x L(-1)) were also tested against dofetilide-induced TdP.

KEY RESULTS

Acute AV block caused a chaotic idioventricular rhythm and strikingly increased beat-to-beat variability of the RR and QT intervals. SEA0400 exaggerated the dofetilide-induced increase in the heart rate-corrected QT interval (QTc) and did not reduce the incidence of dofetilide-induced TdP [100% in the SEA0400 + dofetilide group vs. 75% in the dofetilide (100 nmol x L(-1)) control]. In the second set of experiments, verapamil further increased the dofetilide-induced QTc prolongation and neither verapamil nor lidocaine reduced the dofetilide-induced increase in the beat-to-beat variability of the QT interval. However, lidocaine decreased and verapamil prevented the development of dofetilide-induced TdP as compared with the dofetilide control (TdP incidence: 13%, 0% and 88% respectively).

CONCLUSIONS AND IMPLICATIONS

Na+/Ca2+ exchanger does not contribute to dofetilide-induced TdP, whereas Na+ and Ca2+ channel activity is involved in TdP genesis in isolated, AV-blocked rabbit hearts. Neither QTc prolongation nor an increase in the beat-to-beat variability of the QT interval is a sufficient prerequisite of TdP genesis in rabbit hearts.

Involvement of protein kinase C in the regulation of Na+/Ca2+ exchanger in bovine adrenal chromaffin cells

1. The Na(+)/Ca(2+) exchanger (NCX) exchanges Na+ and Ca(2+) bidirectionally through the forward mode (Ca(2+) extrusion) or the reverse mode (Ca(2+) influx). The present study was undertaken to clarify the role of protein kinase C (PKC) in the regulation of NCX in bovine adrenal chromaffin cells. The Na(+)-loaded cells were prepared by treatment with 100 micromol/L ouabain and 50 micromol/L veratridine. Incubation of Na(+)-loaded cells with Na(+)-free solution in the presence of the Ca(2+) channel blockers nicardipine (3 micromol/L) and omega-conotoxin MVIIC (0.3 micromol/L) caused Ca(2+) uptake and catecholamine release. 2. The Na(+)-dependent Ca(2+) uptake and catecholamine release were inhibited by 2-[4-[(2,5-difluorophenyl)methoxy]phenoxy]-5-ethoxyaniline (SEA0400; 1 micromol/L) and 2-[2-[4-(4-nitrobenzyloxy)phenyl]isothiourea (KB-R7943; 10 micromol/L), both NCX inhibitors. These results indicate that the Na(+)-dependent responses are mostly due to activation of the NCX working in the reverse mode. 3. In addition, we examined the effects of PKC inhibitors and an activator on the NCX-mediated Ca(2+) uptake and catecholamine release. Bisindolylmaleimide I (0.3-10 micromol/L) and chelerythrine (3-100 micromol/L), both PKC inhibitors, inhibited NCX-mediated responses. In contrast, phorbol 12,13-dibutyrate (0.1-10 micromol/L), a PKC activator, enhanced the responses. Bisindolylmaleimide I and chelerythrine, at effective concentrations for inhibition of Na(+)-dependent catecholamine release, had a little or no effect on high K(+)-induced catecholamine release in intact cells or on Ca(2+)-induced catecholamine release in beta-escin-permeabilized cells. 4. These results suggest that PKC is involved in the activation of NCX in bovine adrenal chromaffin cells.

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.

Na(+)/Ca(2+) exchanger inhibition exerts a positive inotropic effect in the rat heart, but fails to influence the contractility of the rabbit heart

BACKGROUND AND PURPOSE

The Na(+)/Ca(2+) exchanger (NCX) may play a key role in myocardial contractility. The operation of the NCX is affected by the action potential (AP) configuration and the intracellular Na(+) concentration. This study examined the effect of selective NCX inhibition by 0.1, 0.3 and 1.0 microM SEA0400 on the myocardial contractility in the setting of different AP configurations and different intracellular Na(+) concentrations in rabbit and rat hearts.

EXPERIMENTAL APPROACH

The concentration-dependent effects of SEA0400 on I(Na/Ca) were studied in rat and rabbit ventricular cardiomyocytes using a patch clamp technique. Starling curves were constructed for isolated, Langendorff-perfused rat and rabbit hearts. The cardiac sarcolemmal NCX protein densities of both species were compared by immunohistochemistry.

KEY RESULTS

SEA0400 inhibited I(Na/Ca) with similar efficacy in the two species; there was no difference between the inhibitions of the forward or reverse mode of the NCX in either species. SEA0400 increased the systolic and the developed pressure in the rat heart in a concentration-dependent manner, for example, 1.0 microM SEA0400 increased the maximum systolic pressures by 12% relative to the control, whereas it failed to alter the contractility in the rabbit heart. No interspecies difference was found in the cardiac sarcolemmal NCX protein densities.

CONCLUSIONS AND IMPLICATIONS

NCX inhibition exerted a positive inotropic effect in the rat heart, but it did not influence the contractility of the rabbit heart. This implies that the AP configuration and the intracellular Na(+) concentration may play an important role in the contractility response to NCX inhibition.

S1P1-selective agonist SEW2871 exacerbates reperfusion arrhythmias

Sphingosine-1-phosphate (S1P) has been considered to play an important role in ischemia/reperfusion (I/R) injury. We used SEW2871 (SEW), a novel receptor-selective agonist for S1P1, to elucidate the role of S1P1 in myocardial I/R. Isolated perfused rat hearts exposed to S1P (1 and 10 mM) or SEW (1 and 0.1 mM) were subjected to 30 minutes of global no-flow ischemia and 2 hours of reperfusion. S1P at 1 and 10 mM significantly reduced infarct size and CK release compared with vehicle-control. The effect of 0.1 microM SEW on infarct size was modest. After I/R, S1P at both doses and SEW at 0.1 microM improved developed pressure (LVDP). SEW at 1 mM significantly prolonged the duration of ventricular tachycardia and ventricular fibrillation, leading to irreversible reperfusion tachyarrhythmias in 60% of the hearts. This is the first demonstration of the critical role of the S1P1 receptor in I/R injury.

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.

[Molecular pharmacologic approaches to functional analysis of new biological target molecules for drug discovery]

This review focuses on two pharmacologic approaches to the functional evaluation of new target molecules for drug discovery. One is the development of a novel specific antagonist of the Na(+)-Ca(++) exchanger (NCX) SEA0400. The other is a comprehensive analysis of the functions of pituitary adenylate cyclase-activating polypeptide (PACAP), a neuropeptide ligand for G protein-coupled receptors. NCX is the one of the last target molecules regulating the cellular Ca(++) concentration. There was no efficient way to address the pathophysiologic roles of NCX until a specific antagonist, 2-[4-[(2,5-difluorophenyl)methoxy]phenoxy]-5-ethoxyaniline (SEA0400), was developed. Our recent studies using SEA0400 clearly showed the possible roles of NCX in several pathologic states of cardiovascular and nervous tissues. In our second approach including gene-targeting methods, we found new, unexpected roles of PACAP in higher brain functions, such as psychomotor, cognition, photoentrainment, and nociception. Based on these experimental findings, a genetic association study in schizophrenia patients revealed that the single-nucleotide polymorphisms of the PACAP gene are significantly associated with the hypofunction of the hippocampus. Regarding the peripheral roles of PACAP, we found that PACAP is involved not only in the regulation of insulin secretion in pancreatic islets, but also in the regulation of islet turnover. In subsequent phenotypic analysis of PACAP transgenic mice, we identified novel candidate genes that probably have promising functional roles.

Preservation of mitochondrial function may contribute to cardioprotective effects of Na+/Ca2+ exchanger inhibitors in ischaemic/reperfused rat hearts

BACKGROUND AND PURPOSE

Na+/Ca2+ exchanger (NCX) inhibitors are known to attenuate myocardial reperfusion injury. However, the exact mechanisms for the cardioprotection remain unclear. The present study was undertaken to examine the mechanism underlying the cardioprotection by NCX inhibitors against ischaemia/reperfusion injury.

EXPERIMENTAL APPROACH

Isolated rat hearts were subjected to 35-min ischaemia/60-min reperfusion or 20-min ischaemia/60-min reperfusion. NCX inhibitors (3-30 microM KB-R7943 (KBR) or 0.3-1 microM SEA0400 (SEA)) were given for 5 min prior to ischaemia (pre-ischaemic treatment) or for 10 min after the onset of reperfusion (post-ischaemic treatment).

KEY RESULTS

With 35-min ischaemia/60-min reperfusion, pre- or post-ischaemic treatment with KBR or SEA neither enhanced post-ischaemic contractile recovery nor attenuated ischaemia- or reperfusion-induced Na+ accumulation and damage to mitochondrial respiratory function. With the milder model (20-min ischaemia/reperfusion), pre- or post-ischaemic treatment with 10 microM KBR or 1 microM SEA significantly enhanced the post-ischaemic contractile recovery, associated with reductions in reperfusion-induced Ca2+ accumulation, damage to mitochondrial function, and decrease in myocardial high-energy phosphates. Furthermore, Na+ influx to mitochondria in vitro was enhanced by increased concentrations of NaCl. KBR (10 microM) and 1 microM SEA partially decreased the Na+ influx.

CONCLUSIONS AND IMPLICATIONS

The NCX inhibitors exerted cardioprotective effects during relatively mild ischaemia. The mechanism may be attributable to prevention of mitochondrial damage, possibly mediated by attenuation of Na+ overload in cardiac mitochondria during ischaemia and/or Ca2+ overload via the reverse mode of NCX during reperfusion.

Effect of partial blockade of the Na(+)/Ca(2+)-exchanger on Ca(2+) handling in isolated rat ventricular myocytes

SEA0400 is a selective inhibitor of the Na(+)/Ca(2+) exchanger having equal potencies to suppress both the forward and reverse mode operation of the Na(+)/Ca(2+) exchanger. Present experiments were designed to study the effect of partial blockade of Na(+)/Ca(2+) exchanger on Ca(2+) handling in isolated rat ventricular myocytes. Intracellular Ca(2+) transient and cell shortening were measured in ventricular myocytes loaded with Fura-2-AM fluorescent dye. Partial blockade of Na(+)/Ca(2+) exchanger was induced by superfusion of the cells with SEA0400 at a concentration of 0.3 microM. Amplitude of the intracellular Ca(2+) transient and cell shortening was significantly increased by SEA0400 in both field stimulated and voltage clamped myocytes, without significant elevation of diastolic Ca(2+) level and the decay time constant of the Ca(2+) transient. In patch clamped myocytes the SEA0400 induced increase in the Ca(2+) transient and cell shortening was accompanied by significant reduction of peak L-type Ca(2+) current. These effects can be explained by the autoregulative nature of cardiac Ca(2+) handling, as the reduced Ca(2+) efflux from the cell results in an increased Ca(2+) load to the sarcoplasmic reticulum leading to increased Ca(2+) release, which in turn may decrease the L-type Ca(2+) current by accelaration of Ca(2+) dependent inactivation of L-type Ca(2+) current. Our results suggest that complex changes in the Ca(2+) cycling can occur after selective pharmacological inhibition of the Na(+)/Ca(2+) exchanger.

Na+/Ca2+ exchange as a drug target--insights from molecular pharmacology and genetic engineering

The Na+/Ca2+ exchanger (NCX) is an ion transporter that exchanges Na+ and Ca2+ in either Ca2+-efflux or Ca2+-influx mode, depending on membrane potential and transmembrane ion gradients. In myocytes, neurons, and renal tubular cells, NCX is thought to play an important role in the regulation of intracellular Ca2+ concentration. So far the benzyloxyphenyl derivatives (KB-R7943, SEA0400, SN-6, and YM-244769) have been developed as selective NCX inhibitors. These inhibitors possess different isoform selectivities, although they have similar properties, such as Ca2+-influx mode selectivity and I1 inactivation-dependence. Site-directed mutageneses have revealed that these inhibitors possess some molecular determinants (Phe-213, Val-227, Tyr-228, Gly-833, and Asn-839) for interaction with NCX1. These benzyloxyphenyl derivatives are expected to be useful tools to study the physiological roles of NCX. Interestingly, benzyloxyphenyl NCX inhibitors effectively prevent several ischemia-reperfusion injuries and salt-dependent hypertension in animal models. Furthermore, several experiments with genetically engineered mice provide compelling evidence that these diseases are triggered by pathological Ca2+ entry through NCX1. Thus, NCX inhibitors may have therapeutic potential as novel drugs for reperfusion injury and salt-dependent hypertension.

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.

Potential role and mechanisms of subcellular remodeling in cardiac dysfunction due to ischemic heart disease

Several studies have revealed varying degrees of changes in sarcoplasmic reticular and myofibrillar activities, protein content, gene expression and intracellular Ca-handling during cardiac dysfunction due to ischemia-reperfusion (I/R); however, relatively little is known about the sarcolemmal and mitochondrial alterations, as well as their mechanisms in the I/R hearts. Because I/R is associated with oxidative stress and intracellular Ca-overload, it has been indicated that changes in subcellular activities, protein content and gene expression due to I/R are related to both oxidative stress and Ca-overload. Intracellular Ca-overload appears to induce changes in subcellular activities, protein contents and gene expression (subcellular remodeling) by activation of proteases and phospholipases, as well as by affecting the genetic apparatus, whereas oxidative stress is considered to cause oxidation of functional groups of different subcellular proteins in addition to modifying the genetic machinery. Ischemic preconditioning, which is known to depress the development of both intracellular Ca-overload and oxidative stress due to I/R, was observed to attenuate the I/R-induced subcellular remodeling and improve cardiac performance. It is suggested that a combination therapy with antioxidants and interventions, which reduce the development of intracellular Ca-overload, may improve cardiac function by preventing or attenuating the occurrence of subcellular remodeling due to ischemic heart disease. It is proposed that defects in the activities of subcellular organelles may serve as underlying mechanisms for I/R-induced cardiac dysfunction under acute conditions, whereas subcellular remodeling due to alterations in gene expression may explain the impaired cardiac performance under chronic conditions of I/R.

Involvement of the Na+/Ca2+ Exchanger in Ouabain-Induced Inotropy and Arrhythmogenesis in Guinea-Pig Myocardium as Revealed by SEA0400

Involvement of the Na+/Ca2+ exchanger in ouabain-induced inotropy and arrhythmogenesis was examined with a specific inhibitor, SEA0400. In right ventricular papillary muscle isolated from guinea-pig ventricle, 1 microM SEA0400, which specifically inhibits the Na+/Ca2+ exchanger by 80%, reduced the ouabain (1 microM)-induced positive inotropy by 40%, but had no effect on the inotropy induced by 100 microM isobutyl methylxantine. SEA0400 significantly inhibited the contracture induced by low Na+ solution. In HEK293 cells expressing the Na+/Ca2+ exchanger, 1 microM ouabain induced an increase in intracellular Ca2+, which was inhibited by SEA0400. The arrhythmic contractions induced by 3 microM ouabain were significantly reduced by SEA0400. These results provide pharmacological evidence that the Na+/Ca2+ exchanger is involved in ouabain-induced inotropy and arrhythmogenesis.

Effects of purified endogenous inhibitor of the Na+/Ca2+ exchanger on ouabain-induced arrhythmias in the atria and ventricle strips of guinea pig

Previous studies demonstrated that the purified endogenous inhibitor (NCX(IF)) of the cardiac Na(+)/Ca(2+) exchanger (NCX1) has the capacity to modulate cardiac muscle contractility. Here, we tested the effects of purified NCX(IF) on arrhythmias induced by ouabain in the atria and ventricle strips of guinea pig. For the sake of comparison NCX(IF) was compared to lidocaine and KB-R7943. In the ventricle strip, NCX(IF) ( approximately 10 U/ml) results in rapid, complete and stable inhibition of ouabain-induced arrhythmias (the inhibition of arrhythmia is not followed by revival of irregular contractions). Under similar experimental conditions the atria strips require somewhat higher doses of NCX(IF) (25-50 U/ml) for complete suppression of arrhythmia. In the atria strip, NCX(IF) (10-25 U/ml) increases the threshold dose (1 microM) of ouabain for arrhythmia onset 2.2+/-0.5-fold (n=5, p<0.05) as well as prolongs the lag-phase for arrhythmia appearance 4.0+/-0.5-fold (n=5, p<0.01). The lag period for arrhythmia onset was also lengthened (2.0+/-0.4-fold) by NCX(IF) in the ventricle strips (n=6, p<0.002). At low frequency of pacing (1 Hz), all three tested substances, lidocaine, KB-R7943, and NCX(IF) can effectively suppress the ouabain-induced arrhythmia. However, at higher frequency (2 Hz), lidocaine is ineffective in suppressing arrhythmia, whereas KB-R7943 becomes pro-arrhythmic. In contrast to reference drugs, NCX(IF) retains its anti-arrhythmic capacity at high frequencies, either in the atria (n=6, p<0.01) or ventricle (n=5, p<0.05) strips. In conclusion, NCX(IF) results in rapid, effective and stable suppression of arrhythmia both in the atria and ventricle preparations under conditions at which the reference drugs become ineffective.

Na+/Ca2+ exchanger inhibitors modify the accumulation of tumor-diagnostic PET tracers in cancer cells

AIM

To establish the effects of Na(+)/Ca(2+) exchanger (NCX) blockers on 2-[(18)F]fluoro-2-deoxy-D-glucose ((18)FDG) and (11)C-choline accumulation in different cancer cells.

METHODS

The tumor cells were incubated with NCX inhibitors, and the uptakes of (18)FDG and (11)C-choline were measured. Flow cytometric measurements of intracellular Ca(2+) and Na(+) concentrations were carried out. The presence of the NCX antigen in the cancer cells was proved by Western blotting, flow cytometry and confocal laser scanning microscopy.

RESULTS

The NCX is expressed at a noteworthy level in the cytosol and on the cytoplasmic membrane of the examined cells. Incubation of the cells with three chemically unrelated NCX blockers (bepridil, KB-R7943 or 3',4'-dichlorobenzamil hydrochloride) resulted in an increase in the intracellular Ca(2+) concentration, with a simultaneous decrease in the intracellular Na(+) concentration. The treatment with the NCX inhibitors increased the energy consumption of the tumor cells by 50-100%. Thapsigargin abolished the NCX-induced (18)FDG accumulation in the cells. The NCX blockers applied decreased the (11)C-choline accumulation of all the investigated cancer cells by 60-80% relative to the control.

CONCLUSION

A possible masking effect of NCX medication must be taken into consideration during the diagnostic interpretation of PET scans.

YM-244769, a novel Na+/Ca2+ exchange inhibitor that preferentially inhibits NCX3, efficiently protects against hypoxia/reoxygenation-induced SH-SY5Y neuronal cell damage

We investigated the pharmacological properties and interaction domains of N-(3-aminobenzyl)-6-{4-[(3-fluorobenzyl)oxy]phenoxy} nicotinamide (YM-244769), a novel potent Na(+)/Ca(2+) exchange (NCX) inhibitor, using various NCX-transfectants and neuronal and renal cell lines. YM-244769 preferentially inhibited intracellular Na(+)-dependent (45)Ca(2+) uptake via NCX3 (IC(50) = 18 nM); the inhibition was 3.8- to 5.3-fold greater than for the uptake via NCX1 or NCX2, but it did not significantly affect extracellular Na(+)-dependent (45)Ca(2+) efflux via NCX isoforms. We searched for interaction domains with YM-244769 by NCX1/NCX3-chimeric analysis and determined that the alpha-2 region in NCX1 is mostly responsible for the differential drug response between NCX1 and NCX3. Further cysteine scanning mutagenesis in the alpha-2 region identified that the mutation at Gly833 markedly reduced sensitivity to YM-244769. Mutant exchangers that display either undetectable or accelerated Na(+)-dependent inactivation, had markedly reduced sensitivity or hypersensitivity to YM-244769, respectively. YM-244769, like 2-[2-[4-(4-nitrobenzyloxyl)phenyl]ethyl]isothiourea methanesulfonate (KB-R7943), protected against hypoxia/reoxygenation-induced cell damage in neuronal SH-SY5Y cells, which express NCX1 and NCX3, more efficiently than that in renal LLC-PK(1) cells, which exclusively express NCX1, whereas 2-[4-(4-nitrobenzyloxy)benzyl]thiazolidine-4-carboxylic acid ethyl ester (SN-6) suppressed renal cell damage to a greater degree than neuronal cell damage. These protective potencies consistently correlated well with their inhibitory efficacies for the Ca(2+) uptake via NCX isoforms existing in the corresponding cell lines. Antisense knockdown of NCX1 and NCX3 in SH-SY5Y cells confirmed that NCX3 contributes to the neuronal cell damage more than NCX1. Thus, YM-244769 is not only experimentally useful as a NCX inhibitor that preferentially inhibits NCX3, but also has therapeutic potential as a new neuroprotective drug.

Metabolic regulation of sodium-calcium exchange by intracellular acyl CoAs

The sodium-calcium exchanger (NCX) is a critical mediator of calcium homeostasis. In the heart, NCX1 predominantly operates in forward mode to extrude Ca(2+); however, reverse-mode NCX1 activity during ischemia/reperfusion (IR) contributes to Ca(2+) loading and electrical and contractile dysfunction. IR injury has also been associated with altered fat metabolism and accumulation of long-chain acyl CoA esters. Here, we show that acyl CoAs are novel, endogenous activators of reverse-mode NCX1 activity, exhibiting chain length and saturation dependence, with longer chain saturated acyl moieties being the most effective NCX1 activators. These results implicate dietary fat composition as a plausible determinant of IR injury. We further show that acyl CoAs may interact directly with the XIP (exchanger inhibitory peptide) sequence, a known region of anionic lipid modulation, to dynamically regulate NCX1 activity and Ca(2+) homeostasis. Additionally, our findings have broad implications for the coupling of Ca(2+) homeostasis to fat metabolism in a variety of tissues.

A selective inhibitor of Na+/Ca2+ exchanger, SEA0400, preserves cardiac function and high-energy phosphates against ischemia/reperfusion injury

The Ca2+ overload by Ca2+ influx via Na+/Ca2+ exchanger (NCX) is a critical mechanism in myocardial ischemia/reperfusion injury. We investigated protective effects of a novel selective inhibitor of NCX, SEA0400, on cardiac function and energy metabolism during ischemia and reperfusion. Langendorff-perfused rat hearts were exposed to 35 minutes global ischemia and 40 minutes reperfusion. Using 31P nuclear magnetic resonance spectroscopy, cardiac phosphocreatine (PCr), ATP, and pHi were monitored. SEA0400 did not change the basic cardiac function, but improved the recovery of left ventricular developed pressure (LVDP) after reperfusion (27.6 +/- 4.9 mm Hg in control, 101.2 +/- 19.3 mm Hg in 0.1 microM, and 115.5 +/- 13.3 mm Hg in 1 microM SEA0400, means +/- SE, n = 6, P < 0.05). SEA0400 reduced left ventricular end-diastolic pressure and increased coronary flow after reperfusion. SEA0400 improved the recoveries of cardiac phosphocreatine and ATP after reperfusion, but did not affect pHi. There were significant linear correlations between left ventricular developed pressure and cardiac phosphocreatine (r = 0.79, P < 0.05), and left ventricular developed pressure and ATP (r = 0.80, P < 0.05). However, SEA0400 increased the incidence and duration of reperfusion ventricular arrhythmias. SEA0400 added only after reperfusion also improved both the contractile function and energy metabolism. It is concluded that the selective inhibition of NCX may be effective to preserve high-energy phosphates and to improve cardiac function after reperfusion, but may not be able to prevent fatal arrhythmias.

Reduction by SEA0400 of myocardial ischemia-induced cytoplasmic and mitochondrial Ca2+ overload

The cardioprotective effects of SEA0400, a novel Na(+)-Ca(2+) exchanger inhibitor, were examined in isolated guinea pig myocardial tissue and ventricular myocytes. In a coronary-perfused right ventricular tissue preparation, SEA0400 had no cardiosuppressive effect during normoxia and experimental ischemia, but enhanced the recovery of contractile force during reperfusion. SEA0400 had no effect on tissue ATP content during normoxia, but attenuated its decrease during ischemia. Treatment of ventricular myocytes with an ischemia mimetic solution (high K(+), glucose free, pH 6.0, gassed with N(2)) resulted in the depolarization of the mitochondrial membrane potential and an increase in cytoplasmic and mitochondrial Ca(2+) concentration, which had a similar time course. SEA0400 significantly delayed these changes. These results suggest that SEA0400 maintains mitochondrial function and tissue ATP content during ischemia through the inhibition of cytoplasmic and mitochondrial Ca(2+) overload.

Pharmacology of KB-R7943: A Na+-Ca2+ exchange inhibitor

The Na+-Ca2+ exchange (NCX) system plays a pivotal role in regulating intracellular Ca2+ concentration in cardiomyocytes, neuronal cells, kidney and a variety of other cells. It performs a particularly important function in regulating cardiac contractility and electrical activity. One of the leading NCX inhibitors is KB-R9743 (KBR) that appears to exhibit selectivity for Ca2+-influx-mode NCX activity (reverse mode of NCX). In this article we reviewed pharmacology of KBR and provide a brief summary of studies with other NCX inhibitors, such as SEA0400 (SEA) and SN-6 (SN). Potential clinical usefulness of KBR and other NCX inhibitors is still controversial but the reviewed findings may be helpful in designing more selective and clinically useful NCX inhibitors for the treatment of cardiac, neuronal and kidney diseases.