A Na+/Ca2+ exchanger inhibitor, KB-R7943, caused negative inotropic responses and negative followed by positive chronotropic responses in isolated, blood-perfused dog heart preparations

The Ionotropic P2X4 Receptor has Unique Properties in the Heart by Mediating the Negative Chronotropic Effect of ATP While Increasing the Ventricular Inotropy

Background: Mounting evidence indicate that reducing the sinoatrial node (SAN) activity may be a useful therapeutic strategy to control of heart failure. Purines, like ATP and its metabolite adenosine, consistently reduce the SAN spontaneous activity leading to negative cardiac chronotropy, with variable effects on the force of myocardial contraction (inotropy). Apart from adenosine A₁ receptors, the human SAN expresses high levels of ATP-sensitive ionotropic P2X4 receptors (P2X4R), yet their cardiac role is unexplored. Methods: Here, we investigated the activity of P2 purinoceptors on isolated spontaneously beating atria (chronotropy) and on 2 Hz-paced right ventricular (RV, inotropy) strips from Wistar rats. Results: ATP (pEC ₅₀ = 4.05) and its stable analogue ATPγS (pEC ₅₀ = 4.69) concentration-dependently reduced atrial chronotropy. Inhibition of ATP breakdown into adenosine by NTPDases with POM-1 failed to modify ATP-induced negative chronotropy. The effect of ATP on atrial rate was attenuated by a broad-spectrum P2 antagonist, PPADS, as well as by 5-BDBD, which selectively blocks the P2X4R subtype; however, no effect was observed upon blocking the A₁ receptor with DPCPX. The P2X4R positive allosteric modulator, ivermectin, increased the negative chronotropic response of ATP. Likewise, CTP, a P2X agonist that does not generate adenosine, replicated the P2X4R-mediated negative chronotropism of ATP. Inhibition of the Na⁺/Ca²⁺ exchanger (NCX) with KB-R7943 and ORM-10103, but not blockage of the HCN channel with ZD7288, mimicked the effect of the P2X4R blocker, 5-BDBD. In paced RV strips, ATP caused a mild negative inotropic effect, which magnitude was 2 to 3-fold increased by 5-BDBD and KB-R7943. Immunofluorescence confocal microscopy studies confirm that cardiomyocytes of the rat SAN and RV co-express P2X4R and NCX1 proteins. Conclusions: Data suggest that activation of ATP-sensitive P2X4R slows down heart rate by reducing the SAN activity while increasing the magnitude of ventricular contractions. The mechanism underlying the dual effect of ATP in the heart may involve inhibition of intracellular Ca²⁺-extrusion by bolstering NCX function in the reverse mode. Thus, targeting the P2X4R activation may create novel well-tolerated heart-rate lowering drugs with potential benefits in patients with deteriorated ventricular function.

Inhibition of spontaneous activity of rabbit atrioventricular node cells by KB-R7943 and inhibitors of sarcoplasmic reticulum Ca(2+) ATPase

The atrioventricular node (AVN) can act as a subsidiary cardiac pacemaker if the sinoatrial node fails. In this study, we investigated the effects of the Na–Ca exchange (NCX) inhibitor KB-R7943, and inhibition of the sarcoplasmic reticulum calcium ATPase (SERCA), using thapsigargin or cyclopiazonic acid (CPA), on spontaneous action potentials (APs) and [Ca²⁺]_i transients from cells isolated from the rabbit AVN. Spontaneous [Ca²⁺]_i transients were monitored from undialysed AVN cells at 37 °C using Fluo-4. In separate experiments, spontaneous APs and ionic currents were recorded using the whole-cell patch clamp technique. Rapid application of 5 μM KB-R7943 slowed or stopped spontaneous APs and [Ca²⁺]_i transients. However, in voltage clamp experiments in addition to blocking NCX current (I_NCX) KB-R7943 partially inhibited L-type calcium current (I_Ca,L). Rapid reduction of external [Na⁺] also abolished spontaneous activity. Inhibition of SERCA (using 2.5 μM thapsigargin or 30 μM CPA) also slowed or stopped spontaneous APs and [Ca²⁺]_i transients. Our findings are consistent with the hypothesis that sarcoplasmic reticulum (SR) Ca²⁺ release influences spontaneous activity in AVN cells, and that this occurs via [Ca²⁺]_i-activated I_NCX; however, the inhibitory action of KB-R7943 on I_Ca,L means that care is required in the interpretation of data obtained using this compound.

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.

SEA0400: A Novel Sodium-Calcium Exchange Inhibitor with Cardioprotective Properties1

The cardiac sodium-calcium exchanger (NCX) plays an important role in calcium homeostasis. It is the primary mechanism for removing calcium ions that enter myocytes through L-type calcium channels on a beat-to-beat basis. Its direction of transport is determined by the membrane potential and the ionic concentrations of Na+ and Ca2+, with the forward (or Ca2+-efflux) mode of transport being the dominant mode under physiological conditions. In contrast, the Ca2+-influx mode (or reverse mode) of NCX becomes important in certain pathophysiological conditions, such as myocardial ischemia and reperfusion. Recent discovery of compounds that inhibit the Ca2+-influx mode (or reverse mode) of NCX has generated intense research interest in the pharmacology of NCX. Among the newer NCX inhibitors described to date, 2-[4-[(2,5-difluorophenyl)methoxy]-phenoxy]-5-ethoxyaniline (SEA0400) appears particularly promising in attenuating cardiac, renal, and cerebral ischemia/reperfusion injuries in various experimental models. Moreover, the mixed results that have emerged from clinical trials evaluating the efficacy and safety of inhibitors of the sodium-hydrogen exchanger (an upstream target in relation to the sodium-calcium exchanger) in myocardial protection stimulated interest in evaluating NCX as an alternative therapeutic target. This article reviews the pharmacological profile of SEA0400, as presented in the published literature, and discusses the therapeutic potential of this compound in attenuating myocardial ischemia/reperfusion injury.

Effects of caffeine, verapamil, lithium, and KB-R7943 on mechanics and energetics of rat myocardial bigeminies

We examined the effects of pharmacological alteration of Ca2+sources on mechanical and energetic properties of paired-pulse (“bigeminic”) contractions. The fraction of heat release that is related to pressure development and pressure-independent heat release were measured during isovolumic contractions in arterially perfused rat ventricles. The heat released by regular and bigeminic contractions showed two brief pressure-independent components (H1 and H2) and a pressure-dependent component (H3). We used the ratio of active heat (H′a) to pressure-time integral (PtI) and the ratio of H3 to PtI to estimate the energetic cost of muscle contraction (overall economy) and pressure maintenance (contractile economy), respectively. Neither of these ratios was affected by stimulation pattern. Caffeine (an inhibitor of sarcoplasmic reticulum function) significantly decreased mechanical responses and increased the energetic cost of contraction (Δ = 101 ± 12.6%). Verapamil (an L-type Ca2+channel blocker) decreased pressure maintenance of extrasystolic (Δ = 43.4 ± 3.7%) and postextrasystolic (Δ = 37.5 ± 3.5%) contractions without affecting postextrasystolic potentiation, suggesting that a verapamil-insensitive fraction is responsible for potentiation. The verapamil-insensitive fraction was further studied in the presence of lithium (45 mM) and KB-R7943 (5 μM), inhibitors of the Na+/Ca2+exchanger. Both agents decreased all mechanical responses, including postextrasystolic potentiation (Δ = 67.3 ± 3.3%), without altering overall or contractile economies, suggesting an association of the verapamil-insensitive Ca2+fraction to the sarcolemmal Na+/Ca2+exchanger. The effect of the inhibitors of the Na+/Ca2+exchanger on potentiation suggests an increased participation of extracellular Ca2+(and, thus, a redistribution of the relative participation of the Ca2+pools) during bigeminic contractions in rat myocardium.

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.

Sodium–calcium exchange inhibitors: therapeutic potential in cardiovascular diseases

Intracellular calcium ions (Ca2+) are the key regulators in cardiac and arterial functions during the contraction–relaxation cycle. Myocyte Ca2+ imbalance thus produces mechanical dysfunction, electrical instability (arrhythmia) and muscle remodeling. The sodium–calcium exchanger (NCX) is one of the major Ca2+-handling proteins in myocytes. Evidence is currently accumulating to suggest that NCX1 is upregulated in various cardiovascular diseases. Recently developed benzyloxyphenyl NCX inhibitors effectively prevent myocardial ischemia/reperfusion injury and salt-sensitive hypertension in animal models. Furthermore, several experiments with genetically engineered mice provide compelling evidence that these diseases are triggered by pathologic Ca2+ entry through NCX1 in cardiac and arterial myocytes, respectively. Thus, NCX inhibitors may have therapeutic potential as novel cardiovascular drugs for myocardial reperfusion injury and salt-sensitive hypertension. However, the efficacy of NCX inhibitors, as well as the role of NCX1, in heart failure or arrhythmias requires more detailed study.

Na+/Ca2+ exchange inhibition protects the rat heart from ischemia-reperfusion injury by blocking energy-wasting processes

We have recently reported that exposure of rat hearts to high Ca2+ produces a Ca2+ overload-induced contractile failure in rat hearts, which was associated with proteolysis of α-fodrin. We hypothesized that contractile failure after ischemia-reperfusion (I/R) is similar to that after high Ca2+ infusion. To test this hypothesis, we investigated left ventricular (LV) mechanical work and energetics in the cross-circulated rat hearts, which were subjected to 15 min global ischemia and 60 min reperfusion. Sixty minutes after I/R, mean systolic pressure-volume area (PVA; a total mechanical energy per beat) at midrange LV volume (mLVV) (PVAmLVV) was significantly decreased from 5.89 ± 1.55 to 3.83 ± 1.16 mmHg·ml·beat−1·g−1 ( n = 6). Mean myocardial oxygen consumption per beat (Vo2) intercept of (Vo2-PVA linear relation was significantly decreased from 0.21 ± 0.05 to 0.15 ± 0.03 μl O2·beat−1·g−1 without change in its slope. Initial 30-min reperfusion with a Na+/Ca2+ exchanger (NCX) inhibitor KB-R7943 (KBR; 10 μmol/l) significantly reduced the decrease in mean PVAmLVV and Vo2 intercept ( n = 6). Although Vo2 for the Ca2+ handling was finally decreased, it transiently but significantly increased from the control for 10–15 min after I/R. This increase in Vo2 for the Ca2+ handling was completely blocked by KBR, suggesting an inhibition of reverse-mode NCX by KBR. α-Fodrin proteolysis, which was significantly increased after I/R, was also significantly reduced by KBR. Our study shows that the contractile failure after I/R is similar to that after high Ca2+ infusion, although the contribution of reverse-mode NCX to the contractile failure is different. An inhibition of reverse-mode NCX during initial reperfusion protects the heart against reperfusion injury.

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.

Clinical potential of sodium-calcium exchanger inhibitors as antiarrhythmic agents

The Na(+)/Ca(2+) exchanger (NaCaX) plays an important role in calcium handling in myocytes, but in the setting of calcium overload NaCaX can also contribute to the activation of an arrhythmogenic transient inward current (I(ti)). Therefore, approaches to inhibit NaCaX could have potential antiarrhythmic effects in pathophysiological states such as heart failure (HF) or myocardial ischaemia and reperfusion. NaCaX typically functions in a forward (Ca(2+) extrusion) mode but can also function in a reverse (Ca(2+) influx) mode. The determining factors for the directionality of NaCaX ion movement are the electrochemical gradients of calcium and sodium, and membrane potential (E(m)). In HF, upregulated NaCaX plays a dual role: it decreases sarcoplasmic reticulum (SR) calcium load, which leads to contractile dysfunction, and it underlies the I(ti) responsible for delayed after-depolarisations (DADs) and ventricular arrhythmias. In myocardial ischaemia and reperfusion, increases in [Na(+)](i) (as a result of acidosis and activation of the Na(+)/H(+) exchanger [NHE]) lead to calcium overload via the NaCaX and arrhythmogenesis is probably mediated by I(ti) activation due to NaCaX. As such, inhibition of NaCaX could provide a novel therapeutic approach to the prevention and treatment of arrhythmias. Unfortunately, it is difficult to assess the efficacy of such an approach since there are no specific NaCaX inhibitors. Currently available agents are hampered by their nonspecific effects on other ion channels and carriers. The potential utility of specific inhibition of forward or reverse mode NaCaX as an antiarrhythmic approach in the settings of HF and ischaemia/ reperfusion is discussed within the context of current knowledge of myocyte calcium and sodium handling. NaCaX is a challenging and complex therapeutic target because of the delicate balance of SR calcium load (too little contributes to contractile dysfunction and too much leads to calcium overload and arrhythmogenesis). Further understanding of NaCaX function, [Na(+)](i) and [Ca(2+)](i) in HF and ischaemia/reperfusion, combined with the development and assessment of specific NaCaX inhibitors, will ultimately define the potential role of NaCaX inhibition in the prevention and treatment of ventricular arrhythmias.

Differing cardioprotective efficacy of the Na+/Ca2+ exchanger inhibitors SEA0400 and KB-R7943

KB-R7943 and SEA0400 are Na(+)/Ca(2+) exchanger (NCX) inhibitors with differing potency and selectivity. The cardioprotective efficacy of these NCX inhibitors was examined in isolated rabbit hearts (Langendorff perfused) subjected to regional ischemia (coronary artery ligation) and reperfusion. KB-R7943 and SEA0400 elicited concentration-dependent reductions in infarct size (SEA0400 EC(50): 5.7 nM). SEA0400 was more efficacious than KB-R7943 (reduction in infarct size at 1 microM: SEA0400, 75%; KB-R7943, 40%). Treatment with either inhibitor yielded similar reductions in infarct size whether administered before or after regional ischemia. SEA0400 (1 microM) improved postischemic recovery of function (+/-dP/dt), whereas KB-R7943 impaired cardiac function at >/=1 microM. At 5-20 microM, KBR-7943 elicited rapid and profound depressions of heart rate, left ventricular developed pressure, and +/-dP/dt. Thus the ability of KB-R7943 to provide cardioprotection is modest and limited by negative effects on cardiac function, whereas the more selective NCX inhibitor SEA0400 elicits marked reductions in myocardial ischemic injury and improved +/-dP/dt. NCX inhibition represents an attractive approach for achieving clinical cardioprotection.

KB-R7943, a Na+/Ca2+ exchange inhibitor, does not suppress ischemia/reperfusion arrhythmias nor digitalis arrhythmias in dogs

KB-R7943 (2-[2-[4-(4-nitrobenzyloxy)phenyl]ethyl]isothiourea methanesulfonate) has been used as a pharmacological tool to block the Ca(2+) influx-mode of the Na(+)/Ca(2+) exchanger, which is thought to contribute to ischemia/reperfusion and digitalis arrhythmias. We examined effects of KB-R7943 on ischemia/reperfusion arrhythmias in beagle dogs anesthetized with sodium pentobarbital. Lead II ECG and BP were measured. KB-R7943 or the solvent (10% DMSO) was injected i.v. as a bolus, and 5 min later, the left anterior descending coronary artery was occluded for 30 min followed by reperfusion. KB-R7943 at 5 or 10 mg/kg increased BP without changing ECG parameters including the heart rate. Although 5 mg/kg KB-R7943 deceased the number of arrhythmic beats during the ischemic period, mortality due to ischemia/reperfusion was not decreased by KB-R7943 (5 and 10 mg/kg). KB-R7943 at 5 mg/kg also did not suppress the ouabain-induced arrhythmias. These negative results suggest that Na(+)/Ca(2+) exchange inhibition may not be a useful strategy of suppressing arrhythmias.

Inhibition of Na+/Ca2+ exchange by KB-R7943: transport mode selectivity and antiarrhythmic consequences

The Na+/Ca2+ exchanger plays a prominent role in regulating intracellular Ca2+ levels in cardiac myocytes and can serve as both a Ca2+ influx and efflux pathway. A novel inhibitor, KB-R7943, has been reported to selectively inhibit the reverse mode (i.e., Ca2+ entry) of Na+/Ca2+ exchange transport, although many aspects of its inhibitory properties remain controversial. We evaluated the inhibitory effects of KB-R7943 on Na+/Ca2+ exchange currents using the giant excised patch-clamp technique. Membrane patches were obtained from Xenopus laevis oocytes expressing the cloned cardiac Na+/Ca2+ exchanger NCX1.1, and outward, inward, and combined inward-outward currents were studied. KB-R7943 preferentially inhibited outward (i.e., reverse) Na+/Ca2+ exchange currents. The inhibitory mechanism consists of direct effects on the transport machinery of the exchanger, with additional influences on ionic regulatory properties. Competitive interactions between KB-R7943 and the transported ions were not observed. The antiarrhythmic effects of KB-R7943 were then evaluated in an ischemia-reperfusion model of cardiac injury in Langendorff-perfused whole rabbit hearts using electrocardiography and measurements of left ventricular pressure. When 3 microM KB-R7943 was applied for 10 min before a 30-min global ischemic period, ventricular arrhythmias (tachycardia and fibrillation) associated with both ischemia and reperfusion were almost completely suppressed. The observed electrophysiological profile of KB-R7943 and its protective effects on ischemia-reperfusion-induced ventricular arrhythmias support the notion of a prominent role of Ca2+ entry via reverse Na+/Ca2+ exchange in this process.