Sodium regulation during ischemia versus reperfusion and its role in injury

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.

Myocardial protection in cardiac surgery: how limited are the options? A comprehensive literature review

For patients undergoing cardiopulmonary bypass, myocardial protection is a key for successful recovery and improved outcomes following cardiac surgery that requires cardiac arrest. Different solutions, components and modes of delivery have evolved over the last few decades to optimise myocardial protection. These include cold and warm and blood and crystalloid solution through antegrade, retrograde or combined cardioplegia delivery approach. However, each method has its own advantages and disadvantages, posing a challenge to establish a gold-standard cardioplegic solution with an optimised mode of delivery for enhanced myocardial protection during cardiac surgery. The aim of this review is to provide a brief history of the development of cardioplegia, explain the electrophysiological concepts behind myocardial protection in cardioplegia, analyse the current literature and summarise existing evidence that warrants the use of varying cardioplegic techniques. We provide a comprehensive and comparative overview of the effectiveness of each technique in achieving optimal cardioprotection and propose novel techniques for optimising myocardial protection in the future.

Modulation of the cardiac Na+-Ca2+ exchanger by cytoplasmic protons: Molecular mechanisms and physiological implications

A precise temporal and spatial control of intracellular Ca²⁺ concentration is essential for a coordinated contraction of the heart. Following contraction, cardiac cells need to rapidly remove intracellular Ca²⁺ to allow for relaxation. This task is performed by two transporters: the plasma membrane Na⁺-Ca²⁺ exchanger (NCX) and the sarcoplasmic reticulum (SR) Ca²⁺-ATPase (SERCA). NCX extrudes Ca²⁺ from the cell, balancing the Ca²⁺entering the cytoplasm during systole through L-type Ca²⁺ channels. In parallel, following SR Ca²⁺ release, SERCA activity replenishes the SR, reuptaking Ca²⁺ from the cytoplasm. The activity of the mammalian exchanger is fine-tuned by numerous ionic allosteric regulatory mechanisms. Micromolar concentrations of cytoplasmic Ca²⁺ potentiate NCX activity, while an increase in intracellular Na⁺ levels inhibits NCX via a mechanism known as Na⁺-dependent inactivation. Protons are also powerful inhibitors of NCX activity. By regulating NCX activity, Ca²⁺, Na⁺ and H⁺ couple cell metabolism to Ca²⁺ homeostasis and therefore cardiac contractility. This review summarizes the recent progress towards the understanding of the molecular mechanisms underlying the ionic regulation of the cardiac NCX with special emphasis on pH modulation and its physiological impact on the heart.

Ranolazine: A true pluripotent cardiovascular drug or jack of all trades, master of none?

Cardiovascular disease (CVD) is a leading cause of morbidity and mortality worldwide. Although the majority of patients with CVD are treated with interventional procedures, a substantial number require medical therapy in terms of both prognosis and symptomatic relief. However, commonly used agents such as β-blockers and calcium channel blockers reduce blood pressure in patients whose resting pressures are often already low. Ranolazine is a promising agent that does not have significant effects on blood pressure or heart rate. Use of this drug has been documented in various cardiovascular conditions, including ischaemic heart disease, heart failure and arrhythmias. This review article aimed to examine current evidence on the use of ranolazine in various cardiovascular conditions in order to determine whether it is a true pluripotent cardiovascular agent or, on the other hand, a "jack of all trades, master of none."

VDAC1 functions in Ca2+ homeostasis and cell life and death in health and disease

In the outer mitochondrial membrane (OMM), the voltage-dependent anion channel 1 (VDAC1) serves as a mitochondrial gatekeeper, controlling the metabolic and energy cross-talk between mitochondria and the rest of the cell. VDAC1 also functions in cellular Ca²⁺ homeostasis by transporting Ca²⁺ in and out of mitochondria. VDAC1 has also been recognized as a key protein in mitochondria-mediated apoptosis, contributing to the release of apoptotic proteins located in the inter-membranal space (IMS) and regulating apoptosis via association with pro- and anti-apoptotic members of the Bcl-2 family of proteins and hexokinase. VDAC1 is highly Ca²⁺-permeable, transporting Ca²⁺ to the IMS and thus modulating Ca²⁺ access to Ca²⁺ transporters in the inner mitochondrial membrane. Intra-mitochondrial Ca²⁺ controls energy metabolism via modulating critical enzymes in the tricarboxylic acid cycle and in fatty acid oxidation. Ca²⁺ also determines cell sensitivity to apoptotic stimuli and promotes the release of pro-apoptotic proteins. However, the precise mechanism by which intracellular Ca²⁺ mediates apoptosis is not known. Here, the roles of VDAC1 in mitochondrial Ca²⁺ homeostasis are presented while emphasizing a new proposed mechanism for the mode of action of pro-apoptotic drugs. This view, proposing that Ca²⁺-dependent enhancement of VDAC1 expression levels is a major mechanism by which apoptotic stimuli induce apoptosis, position VDAC1 oligomerization at a molecular focal point in apoptosis regulation. The interactions of VDAC1 with many proteins involved in Ca²⁺ homeostasis or regulated by Ca²⁺, as well as VDAC-mediated control of cell life and death and the association of VDAC with disease, are also presented.

The Mitochondrial Voltage-Dependent Anion Channel 1, Ca2+ Transport, Apoptosis, and Their Regulation

In the outer mitochondrial membrane, the voltage-dependent anion channel 1 (VDAC1) functions in cellular Ca²⁺ homeostasis by mediating the transport of Ca²⁺ in and out of mitochondria. VDAC1 is highly Ca²⁺-permeable and modulates Ca²⁺ access to the mitochondrial intermembrane space. Intramitochondrial Ca²⁺ controls energy metabolism by enhancing the rate of NADH production via modulating critical enzymes in the tricarboxylic acid cycle and fatty acid oxidation. Mitochondrial [Ca²⁺] is regarded as an important determinant of cell sensitivity to apoptotic stimuli and was proposed to act as a "priming signal," sensitizing the organelle and promoting the release of pro-apoptotic proteins. However, the precise mechanism by which intracellular Ca²⁺ ([Ca²⁺]_i) mediates apoptosis is not known. Here, we review the roles of VDAC1 in mitochondrial Ca²⁺ homeostasis and in apoptosis. Accumulated evidence shows that apoptosis-inducing agents act by increasing [Ca²⁺]_i and that this, in turn, augments VDAC1 expression levels. Thus, a new concept of how increased [Ca²⁺]_i activates apoptosis is postulated. Specifically, increased [Ca²⁺]_i enhances VDAC1 expression levels, followed by VDAC1 oligomerization, cytochrome c release, and subsequently apoptosis. Evidence supporting this new model suggesting that upregulation of VDAC1 expression constitutes a major mechanism by which apoptotic stimuli induce apoptosis with VDAC1 oligomerization being a molecular focal point in apoptosis regulation is presented. A new proposed mechanism of pro-apoptotic drug action, namely Ca²⁺-dependent enhancement of VDAC1 expression, provides a platform for developing a new class of anticancer drugs modulating VDAC1 levels via the promoter and for overcoming the resistance of cancer cells to chemotherapy.

Mitochondrial VDAC, the Na+/Ca2+ Exchanger, and the Ca2+ Uniporter in Ca2+ Dynamics and Signaling

Mitochondrial Ca²⁺ uptake and release play pivotal roles in cellular physiology by regulating intracellular Ca²⁺ signaling, energy metabolism, and cell death. Ca²⁺ transport across the inner and outer mitochondrial membranes (IMM, OMM, respectively), is mediated by several proteins, including the voltage-dependent anion channel 1 (VDAC1) in the OMM, and the mitochondrial Ca²⁺ uniporter (MCU) and Na⁺-dependent mitochondrial Ca²⁺ efflux transporter, (the NCLX), both in the IMM. By transporting Ca²⁺ across the OMM to the mitochondrial inner-membrane space (IMS), VDAC1 allows Ca²⁺ access to the MCU, facilitating transport of Ca²⁺ to the matrix, and also from the IMS to the cytosol. Intra-mitochondrial Ca²⁺ controls energy production and metabolism by modulating critical enzymes in the tricarboxylic acid (TCA) cycle and fatty acid oxidation. Thus, by transporting Ca²⁺, VDAC1 plays a fundamental role in regulating mitochondrial Ca²⁺ homeostasis, oxidative phosphorylation, and Ca²⁺ crosstalk among mitochondria, cytoplasm, and the endoplasmic reticulum (ER). VDAC1 has also been recognized as a key protein in mitochondria-mediated apoptosis, and apoptosis stimuli induce overexpression of the protein in a Ca²⁺-dependent manner. The overexpressed VDAC1 undergoes oligomerization leading to the formation of a channel, through which apoptogenic agents can be released. Here, we review the roles of VDAC1 in mitochondrial Ca²⁺ homeostasis, in apoptosis, and in diseases associated with mitochondria dysfunction.

Ranolazine, an Inhibitor of the Late Sodium Channel Current, Reduces Postischemic Myocardial Dysfunction in the Rabbit

Ranolazine is a selective inhibitor of the late sodium current relative to peak sodium channel current, and via this mechanism, it may decrease sodium-dependent intracellular calcium overload during ischemia and reperfusion. Ranolazine reduces the frequency of angina attacks, but there is little information on its effects on myocardial stunning after short-term ischemia. The objective of this study was to test the effects of ranolazine on left ventricular (LV) function and myocardial stunning after ischemia/reperfusion in rabbits. Myocardial stunning was induced in rabbits by 15 minutes of coronary artery occlusion (CAO) followed by 3 hours reperfusion. Ten minutes before CAO, rabbits were randomly assigned to vehicle (n = 15) or ranolazine (2 mg/kg bolus plus 60 μg/kg/min infusion, IV, n = 15). Myocardial stunning was assessed by LV 2-dimensional echocardiography using, as a marker of severity, ischemic free-wall fractional thickening (FWft; systolic wall thickness – diastolic wall thickness/diastolic wall thickness). Regional ejection fraction (EF) was also assessed. During CAO, FWft was depressed in both groups, indicating an ischemic insult (FWft was reduced from 0.62 ± 0.05 at baseline to 0.10 ± 0.04 in vehicle and from 0.73 ± 0.05 to 0.26 ± 0.07 in ranolazine, P < 0.05, ranolazine vs vehicle). After reperfusion, previously ischemic myocardium remained stunned; however, FWft recovered significantly better in ranolazine (0.51 ± 0.05) than in vehicle (0.35 ± 0.04, P = .027). Baseline EF was 0.65 ± 0.02 in the ranolazine and 0.68 ± 0.02 in vehicle ( P = ns). During CAO, EF was reduced by 36% ± 6% in vehicle versus only 20% ± 6% in ranolazine ( P < .05). At the end of reperfusion, EF remained depressed in both groups, but the reduction in the vehicle group (25% ± 5%) was significantly worse than in ranolazine (9% ± 4%, P = .017). Improvement in function was independent of necrosis (negligible) or differences in hemodynamics (no differences between groups). Ranolazine treatment reduced myocardial stunning following brief ischemia/reperfusion suggesting that inhibiting the late sodium channel current may be a novel approach to treating stunning independent of effects on hemodynamics.

Time-dependent activity of Na+/H+ exchanger isoform 1 and homeostasis of intracellular pH in astrocytes exposed to CoCl2 treatment

Hypoxia causes injury to the central nervous system during stroke and has significant effects on pH homeostasis. Na+/H+ exchanger isoform 1 (NHE1) is important in the mechanisms of hypoxia and intracellular pH (pHi) homeostasis. As a well-established hypoxia-mimetic agent, CoCl2 stabilizes and increases the expression of hypoxia inducible factor‑1α (HIF-1α), which regulates several genes involved in pH balance, including NHE1. However, it is not fully understood whether NHE1 is activated in astrocytes under CoCl2 treatment. In the current study, pHi and NHE activity were analyzed using the pHi‑sensitive dye BCECF‑AM. Using cariporide (an NHE1‑specific inhibitor) and EIPA (an NHE nonspecific inhibitor), the current study demonstrated that it was NHE1, not the other NHE isoforms, that was important in regulating pHi homeostasis in astrocytes during CoCl2 treatment. Additionally, the present study observed that, during the early period of CoCl2 treatment (the first 2 h), NHE1 activity and pHi dropped immediately, and NHE1 mRNA expression was reduced compared with control levels, whereas expression levels of the NHE1 protein had not yet changed. In the later period of CoCl2 treatment, NHE1 activity and pHi significantly increased compared with the control levels, as did the mRNA and protein expression levels of NHE1. Furthermore, the cell viability and injury of astrocytes was not changed during the initial 8 h of CoCl2 treatment; their deterioration was associated with the higher levels of pHi and NHE1 activity. The current study concluded that NHE1 activity and pHi homeostasis are regulated by CoCl2 treatment in a time-dependent manner in astrocytes, and may be responsible for the changes in cell viability and injury observed under hypoxia-mimetic conditions induced by CoCl2 treatment.

Membrane Repair: Mechanisms and Pathophysiology

Eukaryotic cells have been confronted throughout their evolution with potentially lethal plasma membrane injuries, including those caused by osmotic stress, by infection from bacterial toxins and parasites, and by mechanical and ischemic stress. The wounded cell can survive if a rapid repair response is mounted that restores boundary integrity. Calcium has been identified as the key trigger to activate an effective membrane repair response that utilizes exocytosis and endocytosis to repair a membrane tear, or remove a membrane pore. We here review what is known about the cellular and molecular mechanisms of membrane repair, with particular emphasis on the relevance of repair as it relates to disease pathologies. Collective evidence reveals membrane repair employs primitive yet robust molecular machinery, such as vesicle fusion and contractile rings, processes evolutionarily honed for simplicity and success. Yet to be fully understood is whether core membrane repair machinery exists in all cells, or whether evolutionary adaptation has resulted in multiple compensatory repair pathways that specialize in different tissues and cells within our body.

BNIP3 promotes calcium and calpain-dependent cell death

AIMS

Loss of cardiac muscle by programmed cell death contributes to the progression of ischemic heart disease. Hypoxia, metabolite waste buildup and energy depletion are components of ischemia which may initiate caspase dependent and independent cell death pathways. Previous work from our laboratory has shown that combined hypoxia with acidosis, a hallmark of ischemia promotes cardiac myocyte injury with increasing severity as the pH declines. Hypoxia-acidosis was demonstrated to activate the pro-apoptotic Bcl-2 protein BNIP3 which initiated opening of the mitochondrial permeability transition pore and cell death in the absence of caspase activation. Because calpains are known to contribute to ischemic myocardial damage in some models, we hypothesized that they are intermediates in the BNIP3-mediated death caused by hypoxia-acidosis.

MAIN METHODS

Neonatal rat cardiac myocytes were subjected to hypoxia with and without acidosis and the contribution of calpains to hypoxia-acidosis cell death determined.

KEY FINDINGS

Here we report that the death pathway activated by hypoxia-acidosis is driven by a combination of calcium-activated calpains and pro-death factors (DNases) secreted by the mitochondria. Cytochrome c accumulated in the cytoplasm during hypoxia-acidosis but caspase activity was repressed through a calpain-dependent process that prevents the cleavage of procaspase 3. Calpain inhibitors provide vigorous protection against hypoxia-acidosis-induced programmed death. Knockdown of BNIP3 with siRNA prevented calpain activation confirming a central role of BNIP3 in this pathway.

SIGNIFICANCE

The results implicate BNIP3 and calpain as dependent components of cardiac myocyte death caused by hypoxia-acidosis.

Effects of Ranolazine in Patients With Chronic Angina in Patients With and Without Percutaneous Coronary Intervention for Acute Coronary Syndrome: Observations From the MERLIN-TIMI 36 Trial

BACKGROUND

Ranolazine, a piperazine derivative with anti-ischemic effects, reduces the frequency of angina and improves exercise performance in patients with chronic angina. The effects of ranolazine in patients with established ischemic heart disease and chronic angina undergoing percutaneous coronary intervention (PCI) for acute coronary syndromes (ACS) is not well described. We hypothesized that ranolazine would reduce ischemic events, regardless of revascularization.

METHODS

We examined the 1-year incidence of recurrent cardiovascular (CV) events in the subgroup of patients with prior chronic angina (n = 3565) enrolled in the randomized, double-blind, placebo-controlled Metabolic Efficiency with Ranolazine for Less Ischemia in Non-ST-Elevation ACS (MERLIN)-Thrombolysis In Myocardial Infarction (TIMI) 36 trial who did or did not have a PCI within 30 days of the index event.

RESULTS

Ranolazine reduced the risk of recurrent ischemia following admission regardless of whether patients had (hazard ratio [HR], 0.69; 95% confidence interval [CI] 0.51-0.92] or did not have PCI (HR, 0.81; 95% CI, 0.66-0.99; P interaction = 0.39). CV death, myocardial infarction, and recurrent ischemia were similarly lower with ranolazine in the PCI group (HR, 0.71; 95% CI, 0.55-0.91) vs the non-PCI group (HR, 0.91; 95% CI, 0.78-1.06; P interaction = 0.10), with a nominally significant decrease in CV death (HR, 0.39; 95% CI, 0.16-0.93) in the PCI group vs no difference in the non-PCI group (HR, 1.19; 95% CI, 0.89-1.59; P interaction = 0.02).

CONCLUSIONS

In patients with chronic angina, ranolazine reduced recurrent ischemic events, regardless of whether patients did or did not receive PCI within 30 days of a non-ST-segment ACS.

Update on ranolazine in the management of angina

Mortality rates attributable to coronary heart disease have declined in recent years, possibly related to changes in clinical presentation patterns and use of proven secondary prevention strategies. Chronic stable angina (CSA) remains prevalent, and the goal of treatment is control of symptoms and reduction in cardiovascular events. Ranolazine is a selective inhibitor of the late sodium current in myocytes with anti-ischemic and metabolic properties. It was approved by the US Food and Drug Administration in 2006 for use in patients with CSA. Multiple, randomized, placebo-controlled trials have shown that ranolazine improves functional capacity and decreases anginal episodes in CSA patients, despite a lack of a significant hemodynamic effect. Ranolazine did not improve cardiovascular mortality or affect incidence of myocardial infarction in the MERLIN (Metabolic Efficiency with Ranolazine for Less Ischemia in Non-ST-Elevation Acute Coronary Syndrome)-TIMI (Thrombolysis In Myocardial Infarction) 36 trial, but significantly decreased the incidence of recurrent angina. More recently, ranolazine has been shown to have beneficial and potent antiarrhythmic effects, both on supraventricular and ventricular tachyarrhythmias, largely due to its inhibition of the late sodium current. Randomized controlled trials testing these effects are underway. Lastly, ranolazine appears to be cost-effective due to its ability to decrease angina-related hospitalizations and improve quality of life.

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.

Anandamide reduces intracellular Ca2+ concentration through suppression of Na+/Ca2+ exchanger current in rat cardiac myocytes

Purpose

Anandamide, one of the endocannabinoids, has been reported to exhibit cardioprotective properties, particularly in its ability to limit the damage produced by ischemia reperfusion injury. However, the mechanisms underlying the effect are not well known. This study is to investigate whether anandamide alter Na⁺/Ca²⁺ exchanger and the intracellular free Ca²⁺ concentration ([Ca²⁺]_i).

Methods

Na⁺/Ca²⁺ exchanger current (I_NCX) was recorded and analysed by using whole-cell patch-clamp technique and [Ca²⁺]_i was measured by loading myocytes with the fluorescent Ca²⁺ indicator Fura-2/AM.

Results

We found that I_NCX was enhanced significantly after perfusion with simulated ischemic external solution; [Ca²⁺]_i was also significantly increased by simulated ischemic solution. The reversal potential of I_NCX was shifted to negative potentials in simulated ischemic external solution. Anandamide (1–100 nM) failed to affect I_NCX and [Ca²⁺]_i in normal solution. However, anandamide (1–100 nM) suppressed the increase in I_NCX in simulated ischemic external solution concentration-dependently and normalized I_NCX reversal potential. Furthermore, anandamide (100 nM) significantly attenuated the increase in [Ca²⁺]_i in simulated ischemic solution. Blocking CB1 receptors with the specific antagonist AM251 (500 nM) failed to affect the effects of anandamide on I_NCX and [Ca²⁺]_i in simulated ischemic solution. CB2 receptor antagonist AM630 (100 nM) eliminated the effects of anandamide on I_NCX and [Ca²⁺]_i in simulated ischemic solution, and CB2 receptor agonist JWH133 (100 nM) simulated the effects of anandamide that suppressed the increase in I_NCX and [Ca²⁺]_i in simulated ischemic solution. In addition, pretreatment with the Gi/o-specific inhibitor pertussis toxin (PTX, 500 ng/ml) eliminated the effects of anandamide and JWH133 on I_NCX in simulated ischemic solution.

Conclusions

Collectively, these findings suggest that anandamide suppresses calcium overload through inhibition of I_NCX during perfusion with simulated ischemic solution; the effects may be mediated by CB2 receptor via PTX-sensitive Gi/o proteins. This mechanism is importantly involved in the anti-ischemia injury caused by endocannabinoids.

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.

Emerging clinical role of ranolazine in the management of angina

Chronic stable angina is an exceedingly prevalent condition with tremendous clinical, social, and financial implications. Traditional medical therapy for angina consists of beta-blockers, calcium channel blockers, and nitrates. These agents decrease myocardial oxygen demand and ischemia by reducing heart rate, lowering blood pressure, and/or optimizing ventricular loading characteristics. Unique in its mechanism of action, ranolazine is the first new antianginal agent approved for use in the US for chronic angina in over 25 years. By inhibiting the late inward sodium current (I(Na)), ranolazine prevents pathologic intracellular calcium accumulation that leads to ischemia, myocardial dysfunction, and electrical instability. Ranolazine has been proven in multiple clinical trials to reduce the symptoms of angina safely and effectively and to improve exercise tolerance in patients with symptomatic coronary heart disease. These benefits occur without reduction in heart rate and blood pressure or increased mortality. Although ranolazine prolongs the QT(c), experimental data indicate that ranolazine may actually be antiarrhythmic. In a large acute coronary syndrome clinical trial, ranolazine reduced the incidence of supraventricular tachycardia, ventricular tachycardia, new-onset atrial fibrillation, and bradycardic events. Additional benefits of ranolazine under investigation include reductions in glycosylated hemoglobin levels and improved left ventricular function. Ranolazine is a proven antianginal medication in patients with symptomatic coronary heart disease, and should be considered as an initial antianginal agent for those with hypotension or bradycardia.

Recent advances in the management of chronic stable angina II. Anti-ischemic therapy, options for refractory angina, risk factor reduction, and revascularization

The objectives in treating angina are relief of pain and prevention of disease progression through risk reduction. Mechanisms, indications, clinical forms, doses, and side effects of the traditional antianginal agents - nitrates, β-blockers, and calcium channel blockers - are reviewed. A number of patients have contraindications or remain unrelieved from anginal discomfort with these drugs. Among newer alternatives, ranolazine, recently approved in the United States, indirectly prevents the intracellular calcium overload involved in cardiac ischemia and is a welcome addition to available treatments. None, however, are disease-modifying agents. Two options for refractory angina, enhanced external counterpulsation and spinal cord stimulation (SCS), are presented in detail. They are both well-studied and are effective means of treating at least some patients with this perplexing form of angina. Traditional modifiable risk factors for coronary artery disease (CAD) - smoking, hypertension, dyslipidemia, diabetes, and obesity - account for most of the population-attributable risk. Individual therapy of high-risk patients differs from population-wide efforts to prevent risk factors from appearing or reducing their severity, in order to lower the national burden of disease. Current American College of Cardiology/American Heart Association guidelines to lower risk in patients with chronic angina are reviewed. The Clinical Outcomes Utilizing Revascularization and Aggressive Drug Evaluation (COURAGE) trial showed that in patients with stable angina, optimal medical therapy alone and percutaneous coronary intervention (PCI) with medical therapy were equal in preventing myocardial infarction and death. The integration of COURAGE results into current practice is discussed. For patients who are unstable, with very high risk, with left main coronary artery lesions, in whom medical therapy fails, and in those with acute coronary syndromes, PCI is indicated. Asymptomatic patients with CAD and those with stable angina may defer intervention without additional risk to see if they will improve on optimum medical therapy. For many patients, coronary artery bypass surgery offers the best opportunity for relieving angina, reducing the need for additional revascularization procedures and improving survival. Optimal medical therapy, percutaneous coronary intervention, and surgery are not competing therapies, but are complementary and form a continuum, each filling an important evidence-based need in modern comprehensive management.

Ranolazine (Ranexa) in the treatment of chronic stable angina

Ischemic heart disease is the major cause of morbidity and mortality in the Western world. Patients often suffer a reduction in quality of life due to chronic stable angina, but therapeutic options can be limited due to concerns for heart rate and blood pressure, as well as side effect profiles. Even revascularization therapy has its limitations and newer agents are required to help in this battle for symptomatic relief. Ranolazine (Ranexa(R), A. Menarini Pharma UK, High Wycombe, UK) is a drug with a novel mechanism of action that has been shown in several large trials to be an efficacious adjunctive agent in reducing symptoms of chronic stable angina. It is thought to work by inhibiting the late sodium current in cardiac myocytes, thereby reducing sodium and calcium overload that follows ischemia. This improves myocardial relaxation and reduces left ventricular diastolic stiffness, which in turn enhances myocardial contractility and perfusion. The drug is generally well tolerated and the evidence so far is encouraging, with a clear clinical benefit achieved in the target groups. Its main strength is that it does not appear to affect either heart rate or blood pressure. This review provides an insight into this treatment option, describes the clinical trials evidence, proposed mechanism of action, and pharmacokinetics, and outlines the indications for its use in chronic stable angina.

Myocardial protection by F 15845, a persistent sodium current blocker, in an ischemia-reperfusion model in the pig

The specific persistent sodium current blocker F 15845 was tested in two myocardial ischemia-reperfusion models in the pig in order to evaluate its cardioprotective effects. In the first protocol, the left circumflex coronary artery was ligated for 60-min and then reperfused for 48-h. F 15845 (2.5+2.5 and 5+5mg/kg) was administered by i.v. infusion, starting before ischemia to the beginning of reperfusion. The second protocol attempted to evaluate F 15845 (5+5mg/kg) response in a more pathological state of the heart. To this end, a non necrotic ligation of the left circumflex coronary artery was applied for 15 min one week before the actual 60 min occlusion. For both protocols, infarct size was determined at the end of the reperfusion period and was assessed by histochemistry (tetrazolium staining). Plasma levels of biochemical markers (myoglobin and troponin I) were also evaluated. In protocol 1, F 15845 significantly reduced the infarct size by 27+/-3 and 43+/-5% at 2.5+2.5 and 5+5mg/kg, respectively. At 5+5mg/kg, F 15845 decreased plasma levels of myoglobin and cardiac troponin I. In protocol 2, F 15845 (5+5mg/kg) significantly reduced myocardial infarct size by 54+/-15% and lowered the plasma myoglobin and troponin I levels relative to vehicle-treated animals. In conclusion, the highly effective persistent sodium current blocker F 15845 exerts remarkable cardioprotective activities. It reduces both myocardial infarct size and the release of biochemical markers in healthy pigs as well in pigs previously exposed to an ischemic episode.

3-(R)-[3-(2-methoxyphenylthio-2-(S)-methylpropyl]amino-3,4-dihydro-2H-1,5-benzoxathiepine bromhydrate (F 15845) prevents ischemia-induced heart remodeling by reduction of the intracellular Na+ overload

The present study investigates whether 3-(R)-[3-(2-methoxyphenylthio-2-(S)-methylpropyl]amino-3,4-dihydro-2H-1,5-benzoxathiepine bromhydrate (F 15845), a new, persistent sodium current blocker, can reduce the ischemic Na(+) accumulation and exert short- and long-term cardioprotection after myocardial infarction. First, F 15845 concentration-dependently reduced veratrine-induced diastolic contracture (IC(50) = 0.14 microM) in isolated atria. Second, F 15845 from 1 microM preserved viability in 54.2 +/- 12.5% of isolated cardiomyocytes exposed to lysophosphatidylcholine. Third, the effect of F 15845 on intracellular Na(+) of isolated hearts from control and diabetic db/db mice was monitored using (23)Na-nuclear magnetic resonance spectroscopy. F 15845 (0.3 microM) significantly counteracted [Na(+)](i) increase during no-flow ischemia in control mouse hearts. In diabetic db/db mouse hearts, the reduction in [Na(+)](i) was delayed relative to control. However, it was more marked and maintained upon reperfusion. The cardioprotective properties after myocardial infarction associated with short- (24-h) and long-term (14-day) reperfusion were measured in anesthetized rats. After 24-h reperfusion, F 15845 (5 mg/kg) significantly reduced infarct size (32.4 +/- 1.7% with vehicle and 24.2 +/- 3.4% with F 15845; P < 0.05) and decrease of troponin I levels (524 +/- 93 microg/l with vehicle versus 271 +/- 63 microg/l with F 15845; P < 0.05). It is important that F 15845 limits the long-term expansion of infarct size (35.2 +/- 2.6%, n = 19 versus 46.7 +/- 1.6%, n = 27 in the vehicle group; P < 0.001). Overall, F 15845 attenuates [Na(+)](i) and prevents (or reverses) contractile and biochemical dysfunction in ischemic and remodeling heart. F 15845 constitutes a new generation of cardioprotective agent.

Regulation of intracellular and mitochondrial sodium in health and disease

The transmembrane sodium gradient is essential for both excitability of the cardiac cell and the regulation of the cytoplasmic concentrations of Ca and protons. In addition, movements of Na across the mitochondrial membrane affect matrix protons and calcium. In the first part of the review, we discuss the most important pathways responsible for sarcolemmal and mitochondrial sodium movements. The bulk of the review considers the changes of intracellular Na concentration ([Na(+)](i)) that occur in disease, specifically, ischemia, reperfusion, and heart failure. We review evidence implicating the increase of intracellular sodium to either increased influx of sodium (via either sodium channels or sodium/hydrogen exchange) or, alternatively, to decreased efflux on the Na/K pump. Although much has been learned about sodium regulation in the heart, there are still many unanswered questions, particularly concerning mitochondrial Na regulation.

Cardiomyocytes from postinfarction failing rat hearts have improved ischemia tolerance

Altered myocardial Ca(2+) and Na(+) handling in congestive heart failure (CHF) may be expected to decrease the tolerance to ischemia by augmenting reperfusion Ca(2+) overload. The aim of the present study was to investigate tolerance to hypoxia-reoxygenation by measuring enzyme release, cell death, ATP level, and cell Ca(2+) and Na(+) in cardiomyocytes from failing rat hearts. CHF was induced in Wistar rats by ligation of the left coronary artery during isoflurane anesthesia, after which cardiac failure developed within 6 wk. Isolated cardiomyocytes were cultured for 24 h and subsequently exposed to 4 h of hypoxia and 2 h of reoxygenation. Cell damage was measured as lactate dehydrogenase (LD) release, cell death as propidium iodide uptake, and ATP by firefly luciferase assay. Cell Ca(2+) and Na(+) were determined with radioactive isotopes, and free intracellular Ca(2+) concentration ([Ca(2+)](i)) with fluo-3 AM. CHF cells showed less increase in LD release and cell death after hypoxia-reoxygenation and had less relative reduction in ATP level after hypoxia than sham cells. CHF cells accumulated less Na(+) than sham cells during hypoxia (117 vs. 267 nmol/mg protein). CHF cells maintained much lower [Ca(2+)](i) than sham cells during hypoxia (423 vs. 1,766 arbitrary units at 4 h of hypoxia), and exchangeable Ca(2+) increased much less in CHF than in sham cells (1.4 vs. 6.7 nmol/mg protein) after 120 min of reoxygenation. Ranolazine, an inhibitor of late Na(+) current, significantly attenuated both the increase in exchangeable Ca(2+) and the increase in LD release in sham cells after reoxygenation. This supports the suggestion that differences in Na(+) accumulation during hypoxia cause the observed differences in Ca(2+) accumulation during reoxygenation. Tolerance to hypoxia and reoxygenation was surprisingly higher in CHF than in sham cardiomyocytes, probably explained by lower hypoxia-mediated Na(+) accumulation and subsequent lower Ca(2+) accumulation in CHF after reoxygenation.

F 15845 inhibits persistent sodium current in the heart and prevents angina in animal models

BACKGROUND AND PURPOSE

Activation of the persistent sodium current in ischaemic myocardium results in calcium overload which is toxic for the cardiomyocyte. Thus, the activity of 3-(R)-[3-(2-methoxyphenylthio-2-(S)-methylpropyl]amino-3,4-dihydro-2H-1,5 benzoxathiepine bromhydrate (F 15845), a new selective persistent sodium current blocker, in protecting against the effects of cardiac ischaemia was examined, in both in vitro and in vivo models.

EXPERIMENTAL APPROACH

Electrophysiological studies using patch-clamp and conventional microlelectrode techniques, isolated perfused hearts and models of angina in anaesthetized animals were used to assess the protection afforded by F 15845 against ischaemia-induced changes.

KEY RESULTS

F 15845 reduced the persistent sodium current activated by veratridine (IC(50) 1.58 x 10(-6) mol.L(-1)). F 15845 blocked voltage-gated human cardiac sodium channels in a novel, voltage-dependent manner, selectively affecting steady-state inactivation. F 15845 did not affect action potential shape and basal function of guinea pig isolated perfused hearts but did reduce ischaemia-induced diastolic contracture in this model (IC(50) 0.64 x 10(-6) mol.L(-1)). In rabbits, F 15845 given i.v. (ED(50) 0.05 mg.kg(-1)) or orally (ED(50) 0.13 mg.kg(-1)) dose-dependently and powerfully inhibited regional myocardial ischaemia-induced ST segment elevation in the absence of haemodynamic effects, implying direct cardiac activity. In dogs, F 15845 dose-dependently inhibited epicardial ST segment changes (70 +/- 8% at 0.63 mg.kg(-1)) in an experimental angina model of demand ischaemia, again without haemodynamic effects, confirming a direct anti-anginal activity.

CONCLUSIONS AND IMPLICATIONS

F 15845 is a selective, potent blocker of the persistent sodium current, generated by the human Na(v)1.5 channel isoforms, and prevents cardiac angina in animal models.

Ranolazine for chronic stable angina

Ranolazine is a new and unique antianginal drug that has been approved for the treatment of chronic stable angina pectoris. The drug is administered as a sustained-release formulation. Although the drug's mechanism of action has not been fully elucidated, current thinking is that ranolazine, a selective inhibitor of late sodium influx, attenuates the abnormalities of ventricular repolarisation and contractility associated with ischaemia. Three randomised trials have shown efficacy for ranolazine in increasing exercise testing or reducing anginal episodes or use of glyceryl trinitrate. Side-effects include dizziness, constipation, nausea, and the potential for prolongation of the QT(c) interval. Ranolazine seems to be a safe addition to current traditional drugs for chronic stable angina, especially in aggressive multidrug regimens.

Ion transport and energetics during cell death and protection

During ischemia, ATP and phosphocreatine (PCr) decline, whereas intracellular hydrogen ion, intracellular sodium (Na(+)), calcium (Ca(2+)), and magnesium (Mg(2+)) concentrations all rise. If the ischemia is relatively short and there is little irreversible injury (cell death), PCr, pH, Na(+), Mg(2+), and Ca(2+) all recovery quickly on reperfusion. ATP recovery can take up to 24 h because of loss of adenine base from the cell and the need for de novo synthesis. There are correlative data showing that a sustained rise in Ca(2+) during ischemia and/or lack of recovery during reperfusion is associated with irreversible cell injury. Interventions that reduce the rise in Ca(2+) during ischemia and reperfusion have been shown to reduce cell death. Therefore, a better understanding of the mechanisms responsible for the rise in Ca(2+) during ischemia and early reperfusion could have important therapeutic implications. This review will discuss mechanisms involved in alterations in ions and high energy phosphate metabolites in perfused or intact heart during ischemia and reperfusion.

The Antianginal Agent, Ranolazine, Reduces Myocardial Infarct Size but Does Not Alter Anatomic No-Reflow or Regional Myocardial Blood Flow in Ischemia/Reperfusion in the Rabbit

It has been suggested that ranolazine protects the ischemic/reperfused heart by reducing diastolic wall pressure during ischemia. However, there is limited information regarding the effect of ranolazine on the anatomic zone of no-flow in a model of acute myocardial occlusion/reperfusion. Before coronary artery occlusion (CAO), open-chest anesthetized rabbits were assigned to vehicle or ranolazine. Hearts received 60 minutes of CAO and 3 hours reperfusion. Ischemic risk zone was comparable in the 2 groups. Ranolazine significantly reduced infarct size. There was a non-significant trend for the no-reflow defect to be smaller in the ranolazine group. Regional myocardial blood flow was similar in both groups in the risk zone during ischemia and at 3 hours reperfusion. Heart rates were similar in both groups, whereas mean arterial pressure was reduced in the ranolazine group. While ranolazine was effective in reducing myocardial infarct size, the mechanism by which it did this was independent of improving perfusion during either ischemia or reperfusion, suggesting that ranolazine's effect of reducing infarct size involves alternative mechanisms.

Mechanisms underlying acute protection from cardiac ischemia-reperfusion injury

Mitochondria play an important role in cell death and cardioprotection. During ischemia, when ATP is progressively depleted, ion pumps cannot function resulting in a rise in calcium (Ca(2+)), which further accelerates ATP depletion. The rise in Ca(2+) during ischemia and reperfusion leads to mitochondrial Ca(2+) accumulation, particularly during reperfusion when oxygen is reintroduced. Reintroduction of oxygen allows generation of ATP; however, damage to the electron transport chain results in increased mitochondrial generation of reactive oxygen species (ROS). Mitochondrial Ca(2+) overload and increased ROS can result in opening of the mitochondrial permeability transition pore, which further compromises cellular energetics. The resultant low ATP and altered ion homeostasis result in rupture of the plasma membrane and cell death. Mitochondria have long been proposed as central players in cell death, since the mitochondria are central to synthesis of both ATP and ROS and since mitochondrial and cytosolic Ca(2+) overload are key components of cell death. Many cardioprotective mechanisms converge on the mitochondria to reduce cell death. Reducing Ca(2+) overload and reducing ROS have both been reported to reduce ischemic injury. Preconditioning activates a number of signaling pathways that reduce Ca(2+) overload and reduce activation of the mitochondrial permeability transition pore. The mitochondrial targets of cardioprotective signals are discussed in detail.

Na+/Ca2+ exchangers: three mammalian gene families control Ca2+ transport

Mammalian Na+/Ca2+ exchangers are members of three branches of a much larger family of transport proteins [the CaCA (Ca2+/cation antiporter) superfamily] whose main role is to provide control of Ca2+ flux across the plasma membranes or intracellular compartments. Since cytosolic levels of Ca2+ are much lower than those found extracellularly or in sequestered stores, the major function of Na+/Ca2+ exchangers is to extrude Ca2+ from the cytoplasm. The exchangers are, however, fully reversible and thus, under special conditions of subcellular localization and compartmentalized ion gradients, Na+/Ca2+ exchangers may allow Ca2+ entry and may play more specialized roles in Ca2+ movement between compartments. The NCX (Na+/Ca2+ exchanger) [SLC (solute carrier) 8] branch of Na+/Ca2+ exchangers comprises three members: NCX1 has been most extensively studied, and is broadly expressed with particular abundance in heart, brain and kidney, NCX2 is expressed in brain, and NCX3 is expressed in brain and skeletal muscle. The NCX proteins subserve a variety of roles, depending upon the site of expression. These include cardiac excitation-contraction coupling, neuronal signalling and Ca2+ reabsorption in the kidney. The NCKX (Na2+/Ca2+-K+ exchanger) (SLC24) branch of Na+/Ca2+ exchangers transport K+ and Ca2+ in exchange for Na+, and comprises five members: NCKX1 is expressed in retinal rod photoreceptors, NCKX2 is expressed in cone photoreceptors and in neurons throughout the brain, NCKX3 and NCKX4 are abundant in brain, but have a broader tissue distribution, and NCKX5 is expressed in skin, retinal epithelium and brain. The NCKX proteins probably play a particularly prominent role in regulating Ca2+ flux in environments which experience wide and frequent fluctuations in Na+ concentration. Until recently, the range of functions that NCKX proteins play was generally underappreciated. This situation is now changing rapidly as evidence emerges for roles including photoreceptor adaptation, synaptic plasticity and skin pigmentation. The CCX (Ca2+/cation exchanger) branch has only one mammalian member, NCKX6 or NCLX (Na+/Ca2+-Li+ exchanger), whose physiological function remains unclear, despite a broad pattern of expression.

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.

NHE1 inhibition by amiloride- and benzoylguanidine-type compounds. Inhibitor binding loci deduced from chimeras of NHE1 homologues with endogenous differences in inhibitor sensitivity

The interaction of the ubiquitous Na(+)/H(+) exchanger, NHE1, with its commonly used inhibitors, amiloride- and benzoylguanidine (Hoechst type inhibitor (HOE))-type compounds, is incompletely understood. We previously cloned NHE1 from Amphiuma tridactylum (AtNHE1) and Pleuronectes americanus (PaNHE1). Although highly homologous to the amiloride- and HOE-sensitive human NHE1 (hNHE1), AtNHE1 is insensitive to HOE-type and PaNHE1 to both amiloride- and HOE-type compounds. Here we generated chimeras to "knock in" amiloride and HOE sensitivity to PaNHE1, and we thereby identified several NHE1 regions involved in inhibitor interaction. The markedly different inhibitor sensitivities of hNHE1, AtNHE1, and PaNHE1 could not be accounted for by differences in transmembrane (TM) region 9. Replacing TM10 through the C-terminal tail of PaNHE1 with the corresponding region of AtNHE1 partially restored sensitivity to amiloride and the related compound 5'-(N-ethyl-N-isopropyl)amiloride (EIPA) but not to HOE694. This effect was not due to the tail region, but it was dependent on TM10-11, because replacing only this region with that of AtNHE1 also partially restored amiloride and EIPA but not HOE sensitivity. The converse mutant (TM10-11 of AtNHE1 replaced with those of PaNHE1) exhibited even higher amiloride and EIPA sensitivity and was also HOE-sensitive. Replacing an LFFFY motif in TM region 4 of PaNHE1 with the corresponding residues of hNHE1 (VFFLF) or AtNHE1 (TFFLF) greatly increased sensitivity to both amiloride- and HOE-type compounds, despite the fact that AtNHE1 is HOE694-insensitive. Gain of amiloride sensitivity appeared to correlate with increased Na(+)/H(+) exchange rates. It is concluded that regions within TM4 and TM10-11 contribute to amiloride and HOE sensitivity, with both regions imparting partial inhibitor sensitivity to NHE1.

Ranolazine: a novel agent that improves dysfunctional sodium channels

Treatment for coronary heart disease is usually directed at either increasing myocardial oxygen supply or decreasing myocardial oxygen demand. Although combination therapy with beta-blockers, calcium-channel blockers and nitrates are effective, many patients suffer from adverse effects of hypotension and bradycardia. Ranolazine is a novel medication that reduces ischaemia by preventing sodium induced calcium overload in myocardial cells without adversely affecting haemodynamic parameters. This agent is the first in the USA to be approved to treat angina in over 10 years. The purpose of this review is to evaluate the pharmacology, pharmacokinetics, clinical trials for safety and efficacy, precautions, adverse effects, drug interactions, and dosage and administration of ranolazine in the treatment of chronic stable angina and acute coronary syndrome.

Roles of Na+/H+ exchange in regulation of p38 mitogen-activated protein kinase activity and cell death after chemical anoxia in NIH3T3 fibroblasts

Activation of Na(+)/H(+) exchange (NHE) plays a major role in cell death following ischemia/hypoxia in many cell types, yet counteracts apoptotic cell death after other stimuli. To address the role of NHE activity in regulation of cell death/survival, we examined the causal relationship between NHE, p38 mitogen-activated protein kinase (MAPK), ERK1/2, p53, and Akt activity, and cell death, after chemical anoxia in NIH3T3 fibroblasts. The NHE1 inhibitor 5'-(N-ethyl-N-isopropyl) amiloride (EIPA) (5 muM), as well as removal of extracellular Na(+) [replaced by N-methyl-D: -glucamine (NMDG(+))], prevented recovery of intracellular pH (pH(i)) during chemical anoxia (10 mM NaN(3) +/- 10 mM glucose), indicating that activation of NHE was the dominating mechanism of pH(i) regulation under these conditions. NHE activation by chemical anoxia was unaffected by inhibitors of p38 MAPK (SB203580) and extracellular signal-regulated kinase (ERK) (PD98059). In contrast, chemical anoxia activated p38 MAPK in an NHE-dependent manner, while ERK1/2 activity was unaffected. Anoxia-induced cell death was caspase-3-independent, mildly attenuated by EIPA, potently exacerbated by SB203580, and unaffected by PD98059. Ser(15) phosphorylation of p53 was increased by anoxia in an NHE- and p38 MAPK-independent manner, while Akt activity was unaffected. It is suggested that after chemical anoxia in NIH3T3 fibroblasts, NHE activity is required for activation of p38 MAPK, which in turn protects the cells against anoxia-induced death. In spite of this, NHE inhibition slightly attenuates anoxia-induced cell death, likely due to the involvement of NHE in other anoxia-induced death pathways.

Caldaret, an intracellular Ca2+ handling modulator, limits infarct size of reperfused canine heart

The cardioprotective effect of caldaret, a novel intracellular Ca(2+) handling modulator that acts through reverse-mode Na(+)/Ca(2+) exchanger inhibition and potential sarcoplasmic reticulum (SR) Ca(2+) uptake enhancement, against reperfusion injury was investigated. We employed a canine model of myocardial infarction induced by 90-min occlusion of left circumflex (LCX) coronary artery followed by 4 h of reperfusion. Intravenously infused caldaret (3 or 30 microg/kg per hour) for 30 min at LCX-reperfusion markedly reduced infarct size (by 51.3% or 71.9%, respectively). This cardioprotection was accompanied by an acceleration of left ventricular (LV) contraction and relaxation during reperfusion, but not by an increase in ischemic regional transmural myocardial blood flow (TMBF) or endocardial/epicardial blood flow ratio (Endo/Epi ratio) or a reduction in double-product throughout the protocol. Diltiazem (2000 microg/kg per hour) also reduced infarct size (by 36.1%), but unlike caldaret, was accompanied by the significant increase in Endo/Epi ratio in the ischemic region and decrease in double-product. There were significant inverse relationships between infarct size and ischemic regional TMBF in all groups. Caldaret, but not diltiazem shifted the regression line downward with a flatter slope. These results suggest that the amelioration of intracellular Ca(2+) handling dysfunction achieved by caldaret leads to cardioprotective effects against reperfusion injury following prolonged ischemia.

Differential effects of postconditioning on myocardial stunning and infarction: a study in conscious dogs and anesthetized rabbits

Postconditioning, i.e., brief intermittent episodes of myocardial ischemia-reperfusion performed at the onset of reperfusion, reduces infarct size after prolonged ischemia. Our goal was to determine whether postconditioning is protective against myocardial stunning. Accordingly, conscious chronically instrumented dogs (sonomicrometry, coronary balloon occluder) were subjected to a control sequence (10 min coronary artery occlusion, CAO, followed by coronary artery reperfusion, CAR) and a week apart to postconditioning with four cycles of brief CAR and CAO performed at completion of the 10 min CAO. Three postconditioning protocols were investigated, i.e., 15 s CAR/15 s CAO ( n = 5), 30 s CAR/30 s CAO ( n = 7), and 1 min CAR/1 min CAO ( n = 6). Left ventricular wall thickening was abolished during CAO and similarly reduced during subsequent stunning in control and postconditioning sequences (e.g., at 1 h CAR, 33 ± 4 vs. 34 ± 4%, 30 ± 4 vs. 30 ± 4%, and 33 ± 4 vs. 32 ± 4% for 15 s postconditioning, 30 s postconditioning, and 1 min postconditioning vs. corresponding control, respectively). We confirmed this result in anesthetized rabbits by demonstrating that shortening of left ventricular segment length was similarly depressed after 10 min CAO in control and postconditioning sequences (4 cycles of 30 s CAR/30 s CAO). In additional rabbits, the same postconditioning protocol significantly reduced infarct size after 30 min CAO and 3 h CAR (39 ± 7%, n = 6 vs. 56 ± 4%, n = 7 of the area at risk in postconditioning vs. control, respectively). Thus, contrasting to its beneficial effects on myocardial infarction, postconditioning does not protect against myocardial stunning in dogs and rabbits. Conversely, additional episodes of ischemia-reperfusion with postconditioning do not worsen myocardial stunning.

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.

Evaluation of a novel anti-ischemic agent in acute coronary syndromes: design and rationale for the Metabolic Efficiency with Ranolazine for Less Ischemia in Non-ST-elevation acute coronary syndromes (MERLIN)-TIMI 36 trial

BACKGROUND

Despite advances in antithrombotic therapies and invasive technology, the risk of recurrent ischemic complications in patients with non-ST-elevation acute coronary syndromes (NSTE-ACSs) remains substantial. Ranolazine is a novel agent that inhibits the late sodium current thereby reducing cellular sodium and calcium overload and has been shown to reduce ischemia in patients with chronic stable angina.

STUDY DESIGN

MERLIN-TIMI 36 is a phase III, randomized, double-blind, parallel-group, placebo-controlled, multinational clinical trial to evaluate the efficacy and safety of ranolazine during long-term treatment of patients with NSTE-ACS receiving standard therapy (N = 6500). Eligible patients are randomized 1:1 to ranolazine or matched placebo, initiated as 200 mg intravenously over 1 hour, followed by an 80-mg/h infusion (40 mg/h for patients with severe renal insufficiency) for up to 96 hours and oral ranolazine ER 1000 mg BID or matched placebo until the end of study. The primary end point is the time to first occurrence of any element of the composite of cardiovascular death, myocardial infarction, or recurrent ischemia. Secondary end points include ischemia on Holter monitoring, hospitalization for new or worsening heart failure, quality of life measures, and exercise performance. The evaluation of long-term safety will include death from any cause and symptomatic documented arrhythmia. Recruitment began in October 2004. The trial will continue until 730 major cardiovascular events and 310 deaths are recorded with expected completion in 24 to 28 months.

CONCLUSIONS

MERLIN-TIMI 36 will evaluate the role of ranolazine in the acute and chronic management of patients presenting with NSTE-ACS.

Modelling Cl- homeostasis and volume regulation of the cardiac cell

We aim at introducing a Cl- homeostasis to the cardiac ventricular cell model (Kyoto model), which includes the sarcomere shortening and the mitochondria oxidative phosphorylation. First, we examined mechanisms underlying the cell volume regulation in a simple model consisting of Na+/K+ pump, Na+-K+-2Cl- cotransporter 1 (NKCC1), cystic fibrosis transmembrane conductance regulator, volume-regulated Cl- channel and background Na+, K+ and Cl- currents. The high intracellular Cl- concentration of approximately 30 mM was achieved by the balance between the secondary active transport via NKCC1 and passive currents. Simulating responses to Na+/K+ pump inhibition revealed the essential role of Na+/K+ pump in maintaining the cellular osmolarity through creating the negative membrane potential, which extrudes Cl- from a cell, confirming the previous model study in the skeletal muscle. In addition, this model well reproduced the experimental data such as the responses to hypotonic shock in the presence or absence of beta-adrenergic stimulation. Finally, the volume regulation via Cl- homeostasis was successfully incorporated to the Kyoto model. The steady state was well established in the comprehensive cell model in respect to both the intracellular ion concentrations and the shape of the action potential, which are all in the physiological range. The source code of the model, which can reproduce every result, is available from http://www.sim-bio.org/.

Improved left ventricular function and reduced necrosis after myocardial ischemia/reperfusion in rabbits treated with ranolazine, an inhibitor of the late sodium channel

Ranolazine is an inhibitor of the late sodium current and, via this mechanism, decreases sodium-dependent intracellular calcium overload during ischemia and reperfusion. Ranolazine reduces angina, but there is little information on its effects in acute myocardial infarction. The aim of this study was to test the effects of ranolazine on left ventricular (LV) function and myocardial infarct size after ischemia/reperfusion in rabbits. Ten minutes before coronary artery occlusion (CAO), anesthetized rabbits were assigned to vehicle (n=15) or ranolazine (2 mg/kg i.v. bolus plus 60 microg/kg/min i.v. infusion; n=15). Hearts received 60 min of CAO and 3 h of reperfusion. CAO caused LV dysfunction associated with necrosis. However, at the end of reperfusion, rabbits treated with ranolazine had better global LV ejection fraction (0.42+/-0.02 versus 0.33+/-0.02; p<0.007) and stroke volume (1.05+/-0.08 versus 0.78+/-0.07 ml; p<0.01) compared with vehicle. The fraction of the LV wall that was akinetic or dyskinetic was significantly less in the ranolazine group at 0.23+/-0.03 versus 0.34+/-0.03 in vehicle-treated group; p<0.02. The ischemic risk region was similar in both groups; however, infarct size was significantly smaller in the treated group (44+/-5 versus 57+/-4% vehicle; p<0.04). There were no significant differences among groups in heart rate, arterial pressure, LV end-diastolic pressure, or maximum-positive or -negative first time derivative of LV pressure (dP/dt). In conclusion, the results of this study show that ranolazine provides protection during acute myocardial infarction in this rabbit model of ischemia/reperfusion. Ranolazine treatment led to better ejection fraction, stroke volume and less wall motion abnormality after reperfusion, and less myocardial necrosis.

Effects of cold cardioplegia on pH, Na, and Ca in newborn rabbit hearts

Many studies suggest myocardial ischemia-reperfusion (I/R) injury results largely from cytosolic proton (Hi)-stimulated increases in cytosolic Na (Nai), which cause Na/Ca exchange-mediated increases in cytosolic Ca concentration ([Ca]i). Because cold, crystalloid cardioplegia (CCC) limits [H]i, we tested the hypothesis that in newborn hearts, CCC diminishes Hi, Nai, and Cai accumulation during I/R to limit injury. NMR measured intracellular pH (pHi), Nai, [Ca]i, and ATP in isolated Langendorff-perfused newborn rabbit hearts. The control ischemia protocol was 30 min for baseline perfusion, 40 min for global ischemia, and 40 min for reperfusion, all at 37°C. CCC protocols were the same, except that ice-cold CCC was infused for 5 min before ischemia and heart temperature was lowered to 12°C during ischemia. Normal potassium CCC solution (NKCCC) was identical to the control perfusate, except for temperature; the high potassium (HKCCC) was identical to NKCCC, except that an additional 11 mmol/l KCl was substituted isosmotically for NaCl. NKCCC and HKCCC were not significantly different for any measurement. The following were different ( P < 0.05). End-ischemia pHi was higher in the CCC than in the control group. Similarly, CCC limited increases in Nai during I/R. End-ischemia Nai values (in meq/kg dry wt) were 115 ± 16 in the control group, 49 ± 13 in the NKCCC group, and 37 ± 12 in the HKCCC group. CCC also improved [Ca]i recovery during reperfusion. After 40 min of reperfusion, [Ca]i values (in nmol/l) were 302 ± 50 in the control group, 145 ± 13 in the NKCCC group, and 182 ± 19 in the HKCCC group. CCC limited ATP depletion during ischemia and improved recovery of ATP and left ventricular developed pressure and decreased creatine kinase release during reperfusion. Surprisingly, CCC did not significantly limit [Ca]i during ischemia. The latter is explained as the result of Ca release from intracellular buffers on cooling.

Physiology and pathophysiology of Na+/H+ exchange and Na+ -K+ -2Cl- cotransport in the heart, brain, and blood

Maintenance of a stable cell volume and intracellular pH is critical for normal cell function. Arguably, two of the most important ion transporters involved in these processes are the Na+/H+ exchanger isoform 1 (NHE1) and Na+ -K+ -2Cl- cotransporter isoform 1 (NKCC1). Both NHE1 and NKCC1 are stimulated by cell shrinkage and by numerous other stimuli, including a wide range of hormones and growth factors, and for NHE1, intracellular acidification. Both transporters can be important regulators of cell volume, yet their activity also, directly or indirectly, affects the intracellular concentrations of Na+, Ca2+, Cl-, K+, and H+. Conversely, when either transporter responds to a stimulus other than cell shrinkage and when the driving force is directed to promote Na+ entry, one consequence may be cell swelling. Thus stimulation of NHE1 and/or NKCC1 by a deviation from homeostasis of a given parameter may regulate that parameter at the expense of compromising others, a coupling that may contribute to irreversible cell damage in a number of pathophysiological conditions. This review addresses the roles of NHE1 and NKCC1 in the cellular responses to physiological and pathophysiological stress. The aim is to provide a comprehensive overview of the mechanisms and consequences of stress-induced stimulation of these transporters with focus on the heart, brain, and blood. The physiological stressors reviewed are metabolic/exercise stress, osmotic stress, and mechanical stress, conditions in which NHE1 and NKCC1 play important physiological roles. With respect to pathophysiology, the focus is on ischemia and severe hypoxia where the roles of NHE1 and NKCC1 have been widely studied yet remain controversial and incompletely elucidated.

Effects of [5-(2-methoxy-5-fluorophenyl)furan-2-ylcarbonyl]guanidine (KR-32560), a novel sodium/hydrogen exchanger-1 inhibitor, on myocardial infarct size and ventricular arrhythmias in a rat model of ischemia/reperfusion heart injury

The cardioprotective effects of the novel sodium/hydrogen exchanger-1 (NHE-1) inhibitor KR-32560 {[5-(2-methoxy-5-fluorophenyl)furan-2-ylcarbonyl]guanidine} were studied in an anesthetized rat model of 30-min ischemia / 2.5-h reperfusion heart injury. KR-32560 (0.01 - 1 microM) dose-dependently inhibited NHE-1-mediated rabbit platelet swelling induced by intracellular acidification. KR-32560 at 0.1 and 1.0 mg/kg (i.v. bolus, given 10 min before ischemia) reduced infarct size from 65.9% (control) to 49.7% and 32.7%, respectively, while reducing the extension of myocardial injury (mm(3)/g of left heart weight) from 405.1 (control) to 302.9 and 185.4, respectively (all P<0.05 vs control). KR-32560 dose-dependently reduced the total number of ventricular premature beats (VPBs) during ischemia from 510.2 (control) to 353.8 and 134.2 beats (all P<0.05, n = 6), while reducing ventricular tachycardia (VT) incidence from 49.3 (control) to 26.8 and 4.3 and VT duration from 249.2 s (control) to 150.5 and 26.7 s (all P<0.05, n = 6). KR-32560 dose-dependently reduced ventricular fibrillation (VF) incidence from 19.0 (control) to 9.2 and 1.2 and VF duration from 88.0 s to 34.5 and 2.8 s (all P<0.05, n = 6). KR-32560 also exerted similar effects on reperfusion arrhythmias, except for VPBs. These results indicate that KR-32560 may exert significant cardioprotective effects in ischemia/reperfusion heart injury.