Diastolic Ca2+ overload caused by Na+/Ca2+ exchanger during the first minutes of reperfusion results in continued myocardial stunning

Proanthocyanidins Maintain Cardiac Ionic Homeostasis in Aldosterone-Induced Hypertension and Heart Failure

Excess aldosterone promotes pathological remodeling of the heart and imbalance in cardiac ion homeostasis of sodium, potassium and calcium. Novel treatment with proanthocyanidins in aldosterone-treated rats has resulted in downregulation of cardiac SGK1, the main genomic aldosterone-induced intracellular mediator of ion handling. It therefore follows that proanthocyanidins could be modulating cardiac ion homeostasis in aldosterone-treated rats. Male Wistar rats received aldosterone (1 mg kg⁻¹ day⁻¹) +1% NaCl for three weeks. Half of the animals in each group were simultaneously treated with the proanthocyanidins-rich extract (80% w/w) (PRO80, 5 mg kg⁻¹ day⁻¹). PRO80 prevented cardiac hypertrophy and decreased calcium content. Expression of ion channels (ROMK, NHE1, NKA and NCX1) and calcium transient mediators (CAV1.2, pCaMKII and oxCaMKII) were reduced by PRO80 treatment in aldosterone-treated rats. To conclude, our data indicate that PRO80 may offer an alternative treatment to conventional MR-blockade in the prevention of aldosterone-induced cardiac pathology.

The Role of Arachidonic Acid Metabolism in Myocardial Ischemia-Reperfusion Injury

Patients with myocardial ischemic diseases or who are undergoing one of various heart treatments, such as open heart surgery, coronary artery bypass grafting, percutaneous coronary artery intervention or drug thrombolysis, face myocardial ischemia-reperfusion injury (MIRI). However, no effective treatment is currently available for MIRI. To improve the prognosis of people with cardiovascular disease, it is important to research the mechanism of MIRI. Arachidonic acid (AA) is one of the focuses of current research. The various metabolic pathways of AA are closely related to the development of cardiovascular disease, and the roles of various metabolites in ischemia-reperfusion injury have gradually been confirmed. AA is mainly metabolized in the cyclooxygenase (COX) pathway, lipoxygenase (LOX) pathway, and cytochrome P450 monooxygenase (CYP) pathway. This paper summarizes the progress of research on these three major AA metabolic pathways with respect to MIRI.

Anabolic steroid excess and myocardial infarction: From ischemia to reperfusion injury

Anabolic steroids (AS) are synthetic testosterone-derivatives developed by the pharmaceutical industry to mimic testosterone biological effects. So far, AS have been implicated in the treatment of pathological conditions, such as hypogonadism, anemia, and cachexia. Since their discovery, though, AS have been illicitly used by elite and recreational athletes, bodybuilders and weightlifters in order to enhance athletic and aesthetic performance. This practice is characterized by cycles of administration and withdrawal, the combination of different AS compounds, and administration of doses 50 - 1000 times higher than those recommended for therapeutic purposes. AS excess has been correlated to cardiovascular detrimental effects, including cardiac hypertrophy, arrhythmias, and hypertension. Particularly, acute myocardial infarction (AMI) has been extensively reported by clinical and post-mortem studies. Atherosclerosis, hypercoagulability state, increased thrombogenesis and vasospasm have arisen as potential causes of myocardial ischemia in AS users. Additionally, several experimental reports have demonstrated that AS can increase the susceptibility to cardiac ischemia/reperfusion injury, whereas the cardioprotection elicited by physical exercise and ischemic postconditioning is blunted. Altogether, these factors can contribute to increased AMI morbidity and mortality during AS excess, particularly when AS are combined with other compounds, such as thyroid hormones, growth hormones, insulin, and diuretics.

CaMKII/calpain interaction mediates ischemia/reperfusion injury in isolated rat hearts

Previous studies indicated that Ca²⁺/calmodulin-dependent kinase II (CaMKII), a kinase involved in the modulation of ryanodine receptor activity, activates Ca²⁺-regulated protease μ-calpain to promote myocardial ischemia/reperfusion injury. This study was performed to explore the underlying mechanisms in CaMKII-induced calpain activation to better understand heart injury. To examine the Ca²⁺ paradox and ischemia/reperfusion injury, isolated rat hearts were subjected to a Ca²⁺-free solution for 3 min, or left coronary artery occlusion for 40 min, prior to restoration of normal perfusion. Blockade of trans-sarcoplasmic reticulum Ca²⁺ flux using ryanodine and thapsigargin failed to prevent Ca²⁺ paradox-induced heart injury. In contrast, the Ca²⁺ paradox increased CaMKII auto-phosphorylation at Thr²⁸⁷, while the CaMKII inhibitor KN-62 and the Na⁺/Ca²⁺ exchanger inhibitor KB-R7943 alleviated heart injury and calpain activity. Intriguingly, the binding of μ-calpain large subunit calpain-1 (CAPN1) to phospho-CaMKII was blunted by both inhibitors. Thus, a Ca²⁺ leak via the ryanodine receptor is not an essential element in CaMKII-elicited calpain activation. In hearts receiving vector injection, ischemia/reperfusion caused elevated calpain activity and α-fodrin degradation, along with membrane integrity damage, similar to the effects noted in control hearts. Importantly, all these alterations were diminished with delivery of adeno-associated virus expressing mutant CaMKIIδC T287A. Ischemia/reperfusion increased CaMKII auto-phosphorylation and binding of CAPN1 to phospho-CaMKII, and facilitated the translocation of phospho-CaMKII and CAPN1 to the plasma membrane, all of which were reversed by injecting CaMKII mutant. Furthermore, the relocation capacity and the interaction of CaMKII with CAPN1 appeared to be dependent upon CaMKII autophosphorylation, as its mutant delivery increased the level of CaMKII, but did not increase membrane content of CaMKII and CAPN1, or their interactions. Together, CaMKII/calpain interaction represents a new avenue for mediating myocardial ischemia/reperfusion injury, and CaMKII likely serves as both a kinase and a carrier, thereby promoting calpain membrane translocation and activation.

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

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

The role of Levosimendan in cardiopulmonary resuscitation

Although initial resuscitation from cardiac arrest (CA) has increased over the past years, long term survival rates remain dismal. Epinephrine is the vasopressor of choice in the treatment of CA. However, its efficacy has been questioned, as it has no apparent benefits for long-term survival or favorable neurologic outcome. Levosimendan is an inodilator with cardioprotective and neuroprotective effects. Several studies suggest that it is associated with increased rates of return of spontaneous circulation as well as improved post-resuscitation myocardial function and neurological outcome. The purpose of this article is to review the properties of Levosimendan during cardiopulmonary resuscitation (CPR) and also to summarize existing evidence regarding the use of Levosimendan in the treatment of CA.

Cardio-Protection by Ginkgo biloba Extract 50 in Rats with Acute Myocardial Infarction is Related to Na+–Ca2+ Exchanger

Ginkgo biloba has been used for medical purposes for centuries in traditional Chinese medicine. Ginkgo biloba extract 50 (GBE50) is a new standardized GBE product that matches the standardized German product as EGb761. This paper is aimed at studying the cardio-protection effects of GBE50 Salvia miltiorrhiza on myocardial function, area at risk, myocardial ultra-structure, and expression of calcium handling proteins in rat ischemic myocardium. Myocardium ischemia was induced by the left anterior descending (LAD) coronary artery occlusion and myocardial function was recorded by a transducer advanced into the left ventricle on a computer system. In vitro myocardial infarction was measured by 2,3,5-triphenyltetrazolium chloride (TTC) and Evans blue staining of heart sections. Morphological change was evaluated by electric microscopy and Western blotting was used for protein expression. Hemodynamic experiments in vivo showed that postischemic cardiac contractile function was reduced in ischemic rats. Salvia miltiorrhiza (7.5 g/kg/d×7) and Ginkgo biloba extract 50 (GBE50) (100 mg/kg/d×7) improved post-schemic cardiac diastolic dysfunction while not affecting the systolic function. In hearts of GBE50 group and Salvia miltiorrhiza (SM) group, the area at risk was significantly reduced and myocardial structure was better-preserved. Moreover, Na +– Ca 2+ exchanger (NCX) expression increase and sarcoplasmic reticulum Ca 2+– ATPase 2 (SERCA2), LTCC, and ryanodine receptor 2 (RyR2) expression decreases were smaller than those in ischemia group. There was a significant difference between the GBE50 and ischemia group in NCX expression. GBE50 could improve recovery in contractile function and prevent myocardium from ischemia damage, which may be caused by attenuating the abnormal expression of NCX.

Ischemia/Reperfusion injury protection by mesenchymal stem cell derived antioxidant capacity

Mesenchymal stem cell (MSC) transplantation after ischemia/reperfusion (I/R) injury reduces infarct size and improves cardiac function. We used mouse ventricular myocytes (VMs) in an in vitro model of I/R to determine the mechanism by which MSCs prevent reperfusion injury by paracrine signaling. Exposure of mouse VMs to an ischemic challenge depolarized their mitochondrial membrane potential (Ψmito), increased their diastolic Ca(2+), and significantly attenuated cell shortening. Reperfusion of VMs with Ctrl tyrode or MSC-conditioned tyrode (ConT) resulted in a transient increase of the Ca(2+) transient amplitudes in all cells. ConT-reperfused cells exhibited a decreased number early after depolarization (EADs) (ConT: 6.3% vs. Ctrl: 28.4%) and prolonged survival (ConT: 58% vs. Ctrl: 33%). Ψmito rapidly recovered in Ctrl as well as ConT-treated VMs on reperfusion; however, in Ctrl solution, an exaggerated hyperpolarization of Ψmito was determined that preceded the collapse of Ψmito. The ability of ConT to attenuate the hyperpolarization of Ψmito was suppressed on inhibition of the PI3K/Akt signaling pathway or IK,ATP. However, protection of Ψmito was best mimicked by the reactive oxygen species (ROS) scavenger mitoTEMPO. Analysis of ConT revealed a significant antioxidant capacity that was linked to the presence of extracellular superoxide dismutase (SOD3) in ConT. In conclusion, MSC ConT protects VMs from simulated I/R injury by its SOD3-mediated antioxidant capacity and by delaying the recovery of Ψmito through Akt-mediated opening of IK,ATP. These changes attenuate reperfusion-induced ROS production and prevent the opening of the permeability transition pore and arrhythmic Ca(2+) release.

Targeting reperfusion injury in acute myocardial infarction: a review of reperfusion injury pharmacotherapy

INTRODUCTION

Acute myocardial infarction (AMI) (secondary to lethal ischemia-reperfusion [IR]) contributes to much of the mortality and morbidity from ischemic heart disease. Currently, the treatment for AMI is early reperfusion; however, this itself contributes to the final myocardial infarct size, in the form of what has been termed 'lethal reperfusion injury'. Over the last few decades, the discovery of the phenomena of ischemic preconditioning and postconditioning, as well as remote preconditioning and remote postconditioning, along with significant advances in our understanding of the cardioprotective pathways underlying these phenomena, have provided the possibility of successful mechanical and pharmacological interventions against reperfusion injury.

AREAS COVERED

This review summarizes the evidence from clinical trials evaluating pharmacological agents as adjuncts to standard reperfusion therapy for ST-elevation AMI.

EXPERT OPINION

Reperfusion injury pharmacotherapy has moved from bench to bedside, with clinical evaluation and ongoing clinical trials providing us with valuable insights into the shortcomings of current research in establishing successful treatments for reducing reperfusion injury. There is a need to address some key issues that may be leading to lack of translation of cardioprotection seen in basic models to the clinical setting. These issues are discussed in the Expert opinion section.

Calcium-mediated cell death during myocardial reperfusion

Reperfusion may induce additional cell death in patients with acute myocardial infarction receiving primary angioplasty or thrombolysis. Altered intracellular Ca(2+) handling was initially considered an essential mechanism of reperfusion-induced cardiomyocyte death. However, more recent studies have demonstrated the importance of Ca(2+)-independent mechanisms that converge on mitochondrial permeability transition (MPT) and are shared by cardiomyocytes and other cell types. This article analyses the importance of Ca(2+)-dependent cell death in light of these new observations. Altered Ca(2+) handling includes increased cytosolic Ca(2+) levels, leading to activation of calpain-mediated proteolysis and sarcoplasmic reticulum-driven oscillations; this can induce hypercontracture, but also MPT due to the privileged Ca(2+) transfer between sarcoplasmic reticulum and mitochondria through cytosolic Ca(2+) microdomains. In the opposite direction, permeability transition can worsen altered Ca(2+) handling and favour hypercontracture. Ca(2+) appears to play an important role in cell death during the initial minutes of reperfusion, particularly after brief periods of ischaemia. Developing effective and safe treatments to prevent Ca(2+)-mediated cardiomyocyte death in patients with transient ischaemia, by targeting Ca(2+) influx, intracellular Ca(2+) handling, or Ca(2+)-induced cell death effectors, is an unmet challenge with important therapeutic implications and large potential clinical impact.

Suppression of rat Frizzled-2 attenuates hypoxia/reoxygenation-induced Ca2+ accumulation in rat H9c2 cells

Growing evidence suggests that Ca(2+) overload is one of the major contributors of myocardial ischemia/reperfusion-induced injury. Since Frizzled-2 receptor, a seven transmembrane protein, transduces downstream signaling by specialized binding of Wnt5a to increase intracellular Ca(2+) release, this work aimed to investigate the effect of Frizzled-2 on Ca(2+) accumulation in H9c2 cells, which were subjected to hypoxia/reoxygenation to mimic myocardial ischemia/reperfusion. After exposing H9c2 cells to hypoxia/reoxygenation, we observed higher expression of Frizzled-2 and Wnt5a as compared to control group cells. Hypoxia/reoxygenation-induced intracellular Ca(2+) accumulation approached that of cells transfected with frizzled-2 plasmid. In cells treated with RNAi specifically designed against frizzled-2, intracellular Ca(2+) in both hypoxia/reoxygenation-treated cells and plasmid-treated cells were decreased. Rats that underwent ischemia/reperfusion injury exhibited increased intracellular Ca(2+) with high expression levels of Frizzled-2 and Wnt5a as compared to the sham group. Our data indicates that upon binding to Wnt5a, increased Frizzled-2 expression after hypoxia/reoxygenation treatment activated intracellular calcium release in H9c2 cells. Our findings provide a new perspective in understanding calcium overload in myocardial ischemia/reperfusion.

Ablation of junctin or triadin is associated with increased cardiac injury following ischaemia/reperfusion

AIMS

Junctin and triadin are calsequestrin-binding proteins that regulate sarcoplasmic reticulum (SR) Ca(2+) release by interacting with the ryanodine receptor. The levels of these proteins are significantly down-regulated in failing human hearts. However, the significance of such decreases is currently unknown. Here, we addressed the functional role of these accessory proteins in the heart's responses to ischaemia/reperfusion (I/R) injury.

METHODS AND RESULTS

Isolated mouse hearts were subjected to global I/R, and contractile parameters were assessed in wild-type (WT), junctin-knockout (JKO), and triadin-knockout (TKO) hearts. Both JKO and TKO were associated with significantly depressed post-I/R contractile recovery. However, ablation of triadin resulted in the most severe post-I/R phenotype. The additional contractile impairment of TKO hearts was not related to a mitochondrial death pathway, but attributed to endoplasmic reticulum (ER) stress-mediated apoptosis. Activation of the X-box-binding protein-1 and transcriptional up-regulation of C/EBP-homologous protein (CHOP) provided a molecular mechanism of caspase-12-dependent apoptosis in myocytes. In addition, elevation of cytosolic Ca(2+) during reperfusion was associated with the activation of calpain proteases and troponin I breakdown. Accordingly, treatment with the calpain inhibitor MDL-28170 significantly ameliorated post-I/R impairment of contractile recovery in intact hearts.

CONCLUSION

These findings indicate that deficiency of either junctin or triadin impairs the contractile recovery in post-ischaemic hearts, which appears to be primarily attributed to increased ER stress and activation of calpain.

Differential hemodynamic effects of levosimendan in a porcine model of neonatal hypoxia-reoxygenation

BACKGROUND

Neonatal asphyxia can be complicated by myocardial dysfunction with secondary alterations in pulmonary and regional hemodynamics. Levosimendan is a calcium-sensitizing inotrope that may support cardiac output, but little is known regarding its differential hemodynamic effects in asphyxiated neonates.

METHODS

Mixed breed piglets (1-4 days old, weight 1.6-2.3 kg) were acutely instrumented. Normocapnic alveolar hypoxia (10-15% oxygen) was induced for 2 h, followed by reoxygenation with 100% (1 h) and then 21% oxygen (3 h). At 2 h of reoxygenation, after volume loading (Ringer's lactate 10 ml/kg), either levosimendan (0.1 or 0.2 μg/kg/min) or D(5)W (placebo) was infused for 2 h in a blinded, block-randomized fashion (n = 7-8/group). The systemic, pulmonary and regional (carotid, superior mesenteric and renal) hemodynamics were compared.

RESULTS

At 0.1 and 0.2 μg/kg/min, levosimendan significantly increased cardiac output (121 and 123% of pretreatment, respectively) and heart rate, and decreased systemic vascular resistance without causing hypotension. Pulmonary arterial pressure and estimated pulmonary vascular resistance were significantly increased from pretreatment baseline in 0.1 but not 0.2 μg/kg/min levosimendan. Levosimendan infusion had no effects on regional hemodynamics. Myocardial efficiency but not oxygen consumption increased with 0.1 μg/kg/min levosimendan without significant effects on plasma troponin and myocardial lactate levels.

CONCLUSIONS

In newborn piglets following hypoxia-reoxygenation injury, levosimendan improves cardiac output but has no marked effects in carotid, superior mesenteric and renal perfusion. It appears that various doses of levosimendan increase the cardiac output through different mechanisms. Further investigations are needed to examine the effectiveness of levosimendan as a cardiovascular supportive therapy either alone or in conjunction with other inotropes in asphyxiated neonates.

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.

Antioxidant network expression abrogates oxidative posttranslational modifications in mice

Antioxidant enzymatic pathways form a critical network that detoxifies ROS in response to myocardial stress or injury. Genetic alteration of the expression levels of individual enzymes has yielded mixed results with regard to attenuating in vivo myocardial ischemia-reperfusion injury, an extreme oxidative stress. We hypothesized that overexpression of an antioxidant network (AON) composed of SOD1, SOD3, and glutathione peroxidase (GSHPx)-1 would reduce myocardial ischemia-reperfusion injury by limiting ROS-mediated lipid peroxidation and oxidative posttranslational modification (OPTM) of proteins. Both ex vivo and in vivo myocardial ischemia models were used to evaluate the effect of AON expression. After ischemia-reperfusion injury, infarct size was significantly reduced both ex vivo and in vivo, ROS formation, measured by dihydroethidium staining, was markedly decreased, ROS-mediated lipid peroxidation, measured by malondialdehyde production, was significantly limited, and OPTM of total myocardial proteins, including fatty acid-binding protein and sarco(endo)plasmic reticulum Ca(²+)-ATPase (SERCA)2a, was markedly reduced in AON mice, which overexpress SOD1, SOD3, and GSHPx-1, compared with wild-type mice. These data demonstrate that concomitant SOD1, SOD3, and GSHPX-1 expression confers marked protection against myocardial ischemia-reperfusion injury, reducing ROS, ROS-mediated lipid peroxidation, and OPTM of critical cardiac proteins, including cardiac fatty acid-binding protein and SERCA2a.

Phosphorylation of ARC is a critical element in the antiapoptotic effect of anesthetic preconditioning

BACKGROUND

Transient exposure to volatile anesthetics before cardiac ischemia/reperfusion (I/R), termed anesthetic preconditioning, limits myocardial injury and inhibits apoptosis. Apoptosis repressor with caspase recruitment domain (ARC) is a novel protein that has been demonstrated to protect cardiomyocytes from apoptosis induced by I/R and is regulated by phosphorylation. We therefore hypothesized that the antiapoptotic effect of anesthetic preconditioning is, in part, mediated by phosphorylation of ARC.

METHODS

In the experiments we used a perfused rat heart model of sevoflurane anesthetic preconditioning and I/R. In addition to measures of left ventricular function, phosphorylation of ARC was measured with and without anesthetic preconditioning. Because the phosphorylation status of ARC is determined by calcineurin and protein kinase CK2, the role of ARC was defined by measuring calcineurin activity and using the calcineurin inhibitor FK506 and the ARC phosphorylation inhibitor 4,5,6,7-tetrabromobenzotrizole (TBB).

RESULTS

I/R without anesthetic preconditioning increased calcineurin and reduced ARC phosphorylation levels, whereas anesthetic preconditioning significantly improved functional recovery, decreased ischemic injury, limited the increase in calcineurin activity, increased the phosphorylation level of ARC, reduced cytochrome c release, and blocked the increase in caspase-8 after I/R. The effects of anesthetic preconditioning were mirrored by FK506 and abolished by TBB.

CONCLUSION

This study has identified a novel cardiac pathway in which anesthetic preconditioning prevents the increase in calcineurin after I/R, resulting in increased phosphorylated ARC and decreased markers of apoptosis.

The Na+/Ca2+ exchange inhibitor 2-(2-(4-(4-nitrobenzyloxy)phenyl)ethyl)isothiourea methanesulfonate (KB-R7943) also blocks ryanodine receptors type 1 (RyR1) and type 2 (RyR2) channels

Na(+)/Ca(2+) exchanger (NCX) is a plasma membrane transporter that moves Ca(2+) in or out of the cell, depending on membrane potential and transmembrane ion gradients. NCX is the main pathway for Ca(2+) extrusion from excitable cells. NCX inhibitors can ameliorate cardiac ischemia-reperfusion injury and promote high-frequency fatigue of skeletal muscle, purportedly by inhibiting the Ca(2+) inward mode of NCX. Here we tested two known NCX inhibitors, 2-(2-(4-(4-nitrobenzyloxy)phenyl)ethyl)-isothiourea methanesulfonate (KB-R7943) and the structurally related 2-[[4-[(4-Nitrophenyl)methoxy]phenyl]methyl]-4-thiazoli dinecarboxylic acid ethyl ester (SN-6), for their influence on electrically or caffeine-evoked Ca(2+) transients in adult dissociated flexor digitorum brevis (FDB) skeletal muscle fibers and human embryonic kidney (HEK) 293 cells that have stable expression of type 1 ryanodine receptor (RyR1). KB-R7943 (< or = 10 microM) reversibly attenuates electrically evoked Ca(2+) transients in FDB and caffeine-induced Ca(2+) release in HEK 293, whereas the structurally related NCX inhibitor SN-6 does not, suggesting that KB-R7943 directly inhibits RyR1. In support of this interpretation, KB-R7943 inhibits high-affinity binding of [(3)H]ryanodine to RyR1 (IC(50) = 5.1 +/- 0.9 microM) and the cardiac isoform RyR2 (IC(50) = 13.4 +/- 1.8 microM). KB-R7943 interfered with the gating of reconstituted RyR1 and RyR2 channels, reducing open probability (P(o)), shortening mean open time, and prolonging mean closed time. KB-R7943 was more effective at blocking RyR1 with cytoplasmic conditions favoring high P(o) compared with those favoring low P(o). SN-6 has negligible activity toward altering [(3)H]ryanodine binding of RyR1 and RyR2. Our results identify that KB-R7943 is a reversible, activity-dependent blocker of the two most broadly expressed RyR channel isoforms and contributes to its pharmacological and therapeutic activities.

Stimulation of kappa-opioid receptor reduces isoprenaline-induced cardiac hypertrophy and fibrosis

The aim of the present study was to determine whether U50,488H (a selective kappa-opioid receptor agonist) inhibits cardiac hypertrophy and fibrosis induced by beta-adrenoceptor stimulation in a rat model. Cardiac hypertrophy and fibrosis were developed by intraperitoneal administration of isoprenaline (ip. 3.0 mg/kg/day,14 days). In the isoprenaline-treated group, heart weight and heart-to-body ratio increased significantly. Hypertrophic alterations were observed in light micrographs of tissue and transmission electron micrographs of myocardial ultra structures. Increases in heart weight, heart-to-body ratio, diameter of cardiomyocytes, and morphological hypertrophic alterations induced by isoprenaline were significantly attenuated by U50,488H(i.p. 1.25 mg/kg/day). Growth of cardiomyocytes was induced by incubating with isoprenaline (10(-6) mol/l), which resulted in an increase in optical density (OD) values. The increased OD value was attenuated by U50,488H(10(-7) mol/l-10(-5) mol/l) in a dose dependent manner. Animals receiving administration of isoprenaline displayed significant fibrosis. The extent of isoprenaline induced left ventricular fibrosis was dramatically reduced in U50,488H treated animals. Increased cardiac fibroblast proliferation and collagen synthesis induced by isoprenaline, as evidenced by increased OD value, (3)H-thymidine, and (3)H-proline incorporation, were significantly reduced in the U50,488H treated group. The specific extracellular matrix proteins, including type I, type III collagen and fibronectin, which increased after administration of isoproterenol, were also attenuated by U50,488H. The abovementioned effects of U50,488H were completely abolished by nor-BNI (nor-binaltorphimine), a selective kappa-opioid receptor antagonist. The enhanced intracellular Ca(2+) transient and L-type Ca(2+) current elicited by isoprenaline in cardiomyocytes were significantly inhibited by U50,488H. This study provides the first morphological evidence of the inhibitory effect of U50,488H on isoprenaline-induced cardiac hypertrophy and fibrosis via kappa-opioid receptor stimulation.

Diastolic calcium is elevated in metabolic recovery of cardiomyocytes expressing elevated levels of the Na+/H+ exchanger

In the myocardium, the Na+/H+ exchanger isoform 1 (NHE1) plays a pivotal role in mediating ischemia–reperfusion (I/R) injury by causing intracellular Na+ accumulation that results in a subsequent increase in intracellular calcium (Ca2+ overload). One of the major clinical correlates of I/R injury is contractile dysfunction, in which Ca2+ overload via increased Na+/Ca2+ exchange is a major contributor. To better understand the cellular role of NHE1 during I/R injury, contractile function and calcium transients were measured during metabolic inhibition and recovery in single ventricular myocytes from transgenic mice with elevated NHE1 expression. During normoxic conditions, no differences were seen between NHE1-overexpressing cardiomyocytes and wild-type (WT) cardiomyocytes with respect to fractional cell shortening (FCS), rate of shortening (+dL/dt), and rate of relaxation (–dL/dt). When metabolic recovery followed metabolic inhibition, NHE1-overexpressing ventricular myocytes exhibited a significant increase in FCS (130.2% ± 11.77% baseline) and ±dL/dt (146.93% ± 12.27% baseline). This correlated with a significant increase in the concentration of diastolic intracellular calcium, which was attenuated by the NHE1 inhibitor HOE694. These results indicate that in normoxic conditions, elevated NHE1 expression does not alter contractile function. During metabolic recovery, however, elevated NHE1 expression increased diastolic Ca2+ loading that led to augmented cell contractility.

Beta-adrenoceptor stimulation exacerbates detrimental effects of ischemia and reperfusion in isolated guinea pig ventricular myocytes

We investigated whether beta-adrenoceptor stimulation exacerbates detrimental effects of ischemia and reperfusion on electrical and contractile function and on intracellular Ca(2+) homeostasis in isolated guinea pig ventricular myocytes. Myocytes were exposed to 20 min of simulated ischemia (37 degrees C) in the absence or presence of isoproterenol (10 nM, applied prior to and during ischemia) and reperfused with Tyrode's solution for 30 min. Unloaded cell shortening, Ca(2+) transients (fura-2), and cell viability were recorded at 5 min intervals in field-stimulated cells (2 Hz). In experiments using microelectrodes, membrane potentials, contractions, and transmembrane currents also were recorded at 5 min intervals. In the absence of ischemia, 10 nM isoproterenol had little effect on either contractile function or Ca(2+) homeostasis. In contrast, when cells were exposed to ischemia, isoproterenol increased the size of contractions and Ca(2+) transients and augmented the increase in diastolic Ca(2+) concentration during ischemia in field-stimulated myocytes. Exposure to isoproterenol also promoted contractile depression in reperfusion. In voltage clamp experiments, isoproterenol abolished the decrease in the magnitude of L-type Ca(2+) current caused by ischemia. Isoproterenol also increased the incidence of abnormal contractile activity and induced delayed afterdepolarizations and the arrhythmogenic transient inward current in ischemia. Additionally, the decline in cell viability in ischemia and reperfusion was exacerbated by isoproterenol. This study shows that beta-adrenoceptor stimulation strongly potentiates adverse effects of ischemia and reperfusion on electrical and contractile function. These adverse effects of isoproterenol are likely caused by an increase in intracellular Ca(2+) accumulation during ischemia.

Endothelial nitric oxide attenuates Na+/Ca2+ exchanger-mediated vasoconstriction in rat aorta

BACKGROUND AND PURPOSE

The Na+/Ca2+ exchanger (NCX) may be an important modulator of Ca2+ entry and exit. The present study investigated whether NCX was affected by prostacyclin and nitric oxide (NO) released from the vascular endothelium, as NCX contains phosphorylation sites for PKA and PKG.

EXPERIMENTAL APPROACH

Rat aortic rings were set up in organ baths. Tension was measured across the ring with a force transducer.

KEY RESULTS

Lowering extracellular [Na+] ([Na+]o) to 1.18 mM induced vasoconstriction in rat endothelium-denuded aortic rings. This effect was blocked by the NCX inhibitor KB-R7943 (2-2-[4-(4-nitrobenzyloxy)phenyl] ethyl isothiourea methanesulphonate; 1 microM). In endothelium-intact aortic rings, decreasing [Na+]o did not constrict the aortic rings significantly, but after treatment with the guanylate cyclase inhibitor ODQ (1H-[1,2,4]oxadiazolo[4,3-a]quinoxalin-1-one; 1 microM) or the NOS inhibitor L-NAME (N(omega)-nitro-L-arginine methyl ester; 50 microM), a vasoconstriction that was similar in size to that in endothelium-denuded preparations was evident. The vasorelaxation induced by the NO donor sodium nitroprusside sodium nitroprusside dihydrate (30 nM) was the same in the endothelium-denuded aortic rings preconstricted with either low Na+ (1.18 mM), the thromboxane A2 agonist U46619 (9,11-dideoxy-9alpha, 11alpha-methanoepoxy prostaglandin F(2alpha); 0.1 microM) or high K+ (80 mM).

CONCLUSIONS AND IMPLICATIONS

The results suggest that the endothelium inhibits NCX operation via guanylate cyclase/NO. This is stronger than for other constrictors such as phenylephrine and may relate to concomitant NCX-stimulated NO release from the endothelium. This finding may be important where NCX operates in reverse mode, such as during ischaemia, and highlights a new mechanism whereby the endothelium modulates Ca2+ homoeostasis in vascular smooth muscle.