Thrombin facilitation of voltage-gated sodium channel activation in human cardiomyocytes: implications for ischemic sodium loading

[The role of thrombin in the pathogenesis of atherosclerosis and its complications]

Thrombin is a key regulator of the homeostasis system. Also, it actively participates in progression of various systemic diseases, including atherosclerosis. There is a large amount of experimental and clinical data on the involvement of thrombin in the pathogenesis of ischemic heart disease (IHD). Thus, studying thrombin activity regulation is promising. Also, the question whether it is possible to use biomarkers of thrombin activity as predictors of cardiovascular complications in IHD patients is relevant. The present review focuses on major mechanisms of thrombin functioning, its role in development and progression of atherosclerosis, and available tests for evaluation of thrombin functional activity. Major clinical studies are discussed that evaluated the efficacy of thrombin inhibitors and protease-activated receptor antagonists.

The Sick and the Weak: Neuropathies/Myopathies in the Critically Ill

Critical illness polyneuropathies (CIP) and myopathies (CIM) are common complications of critical illness. Several weakness syndromes are summarized under the term intensive care unit-acquired weakness (ICUAW). We propose a classification of different ICUAW forms (CIM, CIP, sepsis-induced, steroid-denervation myopathy) and pathophysiological mechanisms from clinical and animal model data. Triggers include sepsis, mechanical ventilation, muscle unloading, steroid treatment, or denervation. Some ICUAW forms require stringent diagnostic features; CIM is marked by membrane hypoexcitability, severe atrophy, preferential myosin loss, ultrastructural alterations, and inadequate autophagy activation while myopathies in pure sepsis do not reproduce marked myosin loss. Reduced membrane excitability results from depolarization and ion channel dysfunction. Mitochondrial dysfunction contributes to energy-dependent processes. Ubiquitin proteasome and calpain activation trigger muscle proteolysis and atrophy while protein synthesis is impaired. Myosin loss is more pronounced than actin loss in CIM. Protein quality control is altered by inadequate autophagy. Ca(2+) dysregulation is present through altered Ca(2+) homeostasis. We highlight clinical hallmarks, trigger factors, and potential mechanisms from human studies and animal models that allow separation of risk factors that may trigger distinct mechanisms contributing to weakness. During critical illness, altered inflammatory (cytokines) and metabolic pathways deteriorate muscle function. ICUAW prevention/treatment is limited, e.g., tight glycemic control, delaying nutrition, and early mobilization. Future challenges include identification of primary/secondary events during the time course of critical illness, the interplay between membrane excitability, bioenergetic failure and differential proteolysis, and finding new therapeutic targets by help of tailored animal models.

Decreased cardiac excitability secondary to reduction of sodium current may be a significant contributor to reduced contractility in a rat model of sepsis

Introduction

Multisystem organ failure remains a poorly understood complication of sepsis. During sepsis, reduced excitability contributes to organ failure of skeletal muscle, nerves and the spinal cord. The goal of this study was to determine whether reduced excitability might also contribute to cardiac failure during sepsis.

Methods

Wistar rats were made septic by cecal ligation and puncture. One day later, action potentials were recorded from beating left ventricular papillary muscle ex vivo by impaling myocytes with sharp microelectrodes.

Results

In cardiac papillary muscle from septic rats, action potential amplitude and rate of rise were reduced, while threshold was elevated. These changes in action potential properties suggest sepsis selectively reduces sodium current. To determine the effects of selective reduction in sodium current, we applied tetrodotoxin to papillary muscle from healthy rats and found reduction in action potential amplitude and rate of rise, as well as elevation of threshold. The changes were similar to those triggered by sepsis. Blocking calcium current using nifedipine did not mimic action potential changes induced by sepsis. Contractility of healthy papillary muscle was reduced to 40% of normal following partial block of sodium current by tetrodotoxin, close to the low contractility of septic papillary muscle, which was 30% of normal.

Conclusions

Our data suggest cardiac excitability is reduced during sepsis in rats. The reduction in excitability appears to be primarily due to reduction of sodium current. The reduction in sodium current may be sufficient to explain most of the reduction in cardiac contractility during sepsis.

Hyperkalemic cardioplegia for adult and pediatric surgery: end of an era?

Despite surgical proficiency and innovation driving low mortality rates in cardiac surgery, the disease severity, comorbidity rate, and operative procedural difficulty have increased. Today's cardiac surgery patient is older, has a "sicker" heart and often presents with multiple comorbidities; a scenario that was relatively rare 20 years ago. The global challenge has been to find new ways to make surgery safer for the patient and more predictable for the surgeon. A confounding factor that may influence clinical outcome is high K(+) cardioplegia. For over 40 years, potassium depolarization has been linked to transmembrane ionic imbalances, arrhythmias and conduction disturbances, vasoconstriction, coronary spasm, contractile stunning, and low output syndrome. Other than inducing rapid electrochemical arrest, high K(+) cardioplegia offers little or no inherent protection to adult or pediatric patients. This review provides a brief history of high K(+) cardioplegia, five areas of increasing concern with prolonged membrane K(+) depolarization, and the basic science and clinical data underpinning a new normokalemic, "polarizing" cardioplegia comprising adenosine and lidocaine (AL) with magnesium (Mg(2+)) (ALM™). We argue that improved cardioprotection, better outcomes, faster recoveries and lower healthcare costs are achievable and, despite the early predictions from the stent industry and cardiology, the "cath lab" may not be the place where the new wave of high-risk morbid patients are best served.

MOG1: a new susceptibility gene for Brugada syndrome

BACKGROUND

Brugada syndrome (BrS) is caused mainly by mutations in the SCN5A gene, which encodes the α-subunit of the cardiac sodium channel Na(v)1.5. However, ≈ 20% of probands have SCN5A mutations, suggesting the implication of other genes. MOG1 recently was described as a new partner of Na(v)1.5, playing a potential role in the regulation of its expression and trafficking. We investigated whether mutations in MOG1 could cause BrS.

METHODS AND RESULTS

MOG1 was screened by direct sequencing in patients with BrS and idiopathic ventricular fibrillation. A missense mutation p.Glu83Asp (E83D) was detected in a symptomatic female patient with a type-1 BrS ECG but not in 281 controls. Wild type (WT)- and mutant E83D-MOG1 were expressed in HEK Na(v)1.5 stable cells and studied using patch-clamp assays. Overexpression of WT-MOG1 alone doubled sodium current (I(Na)) density compared to control conditions (P<0.01). In contrast, overexpression of mutant E83D alone or E83D+WT failed to increase I(Na) (P<0.05), demonstrating the dominant-negative effect of the mutant. Microscopy revealed that Na(v)1.5 channels failed to properly traffic to the cell membrane in the presence of the mutant. Silencing endogenous MOG1 demonstrated a 54% decrease in I(Na) density.

CONCLUSIONS

Our results support the hypothesis that dominant-negative mutations in MOG1 can impair the trafficking of Na(v)1.5 to the membrane, leading to I(Na) reduction and clinical manifestation of BrS. Moreover, silencing MOG1 reduced I(Na), demonstrating that MOG1 is likely to be important in the surface expression of Na(v)1.5 channels. All together, our data support MOG1 as a new susceptibility gene for BrS.

Involvement of protein kinase C and RhoA in protease-activated receptor 1-mediated F-actin reorganization and cell growth in rat cardiomyocytes

Protease-activated receptor 1 (PAR1) that can be activated by serine proteinases such as thrombin has been demonstrated to contribute to the development of cardiac remodeling and hypertrophy after myocardial injury. Here, we investigated the mechanisms by which PAR1 leads to hypertrophic cardiomyocyte growth using cultured rat neonatal ventricular myocytes. PAR1 stimulation with thrombin (1 U/ml) or a synthetic agonist peptide (TFLLR-NH(2), 50 µM) for 48 h induced an increase in cell size and myofibril formation associated with BNP (brain natriuretic peptide) production. This actin reorganization assessed by fluorescein isothiocyanate (FITC)-conjugated phalloidin staining appeared at 1 h after PAR1 stimulation, and this response was reduced by a protein kinase C (PKC) inhibitor, chelerythrine, inhibitors of Rho (simvastatin) and Rho-associated kinase (ROCK) (Y-27632), but not by pertussis toxin (PTX). By Western blot analysis, translocation of PKCα or PKCε from the cytosol to membrane fractions was observed in cells stimulated with thrombin or TFLLR-NH(2) for 2 - 5 min. In addition, PAR1 stimulation for 3 - 5 min increased the level of active RhoA. Furthermore, inhibitors of PKC and ROCK and Rho abrogated PAR1-mediated increase in cell size. Depletion of PKCα or PKCε by specific small interfering RNA also suppressed both actin reorganization and cell growth. These results suggest that PAR1 stimulation of cardiomyocytes induces cell hypertrophy with actin cytoskeletal reorganization through activation of PKCα and PKCε isoforms and RhoA via PTX-insensitive G proteins.

Blocking effect of methylflavonolamine on human Na(V)1.5 channels expressed in Xenopus laevis oocytes and on sodium currents in rabbit ventricular myocytes

AIM

To investigate the blocking effects of methylflavonolamine (MFA) on human Na(V)1.5 channels expressed in Xenopus laevis oocytes and on sodium currents (I(Na)) in rabbit ventricular myocytes.

METHODS

Human Na(V)1.5 channels were expressed in Xenopus oocytes and studied using the two-electrode voltage-clamp technique. I(Na) and action potentials in rabbit ventricular myocytes were studied using the whole-cell recording.

RESULTS

MFA and lidocaine inhibited human Na(V)1.5 channels expressed in Xenopus oocytes in a positive rate-dependent and concentration-dependent manner, with IC(50) values of 72.61 micromol/L and 145.62 micromol/L, respectively. Both of them markedly shifted the steady-state activation curve of I(Na) toward more positive potentials, shifted the steady-state inactivation curve of I(Na) toward more negative potentials and postponed the recovery of the I(Na) inactivation state. In rabbit ventricular myocytes, MFA inhibited I(Na) with a shift in the steady-state inactivation curve toward more negative potentials, thereby postponing the recovery of the I(Na) inactivation state. This shift was in a positive rate-dependent manner. Under current-clamp mode, MAF significantly decreased action potential amplitude (APA) and maximal depolarization velocity (V(max)) and shortened action potential duration (APD), but did not alter the resting membrane potential (RMP). The demonstrated that the kinetics of sodium channel blockage by MFA resemble those of class I antiarrhythmic agents such as lidocaine.

CONCLUSION

MFA protects the heart against arrhythmias by its blocking effect on sodium channels.

Glycyrretinic acid blocks cardiac sodium channels expressed in Xenopus oocytes

ETHNOPHARMACOLOGICAL RELEVANCE

Licorice has been used to treat many ailments including cardiovascular disorders in China for long time. Recent studies have shown that the cardiac actions of licorice have been attributed to its active component, glycyrretinic acid (GA). However, its mechanism remains poorly understood.

AIM OF THE STUDY

The effects of GA on the cardiac sodium currents (I(Na)), L-type calcium currents (I(Ca,L)) and hyperpolarization-activated inward currents (I(f)) were investigated.

MATERIALS AND METHODS

Human isoforms of wild-type and DeltaKPQ-mutant type sodium channels were expressed in Xenopus oocytes, and the resulting currents (peak and late I(Na)) were recorded using a two-microelectrode voltage-clamp technique. A perforated patch clamp technique was employed to record I(Ca,L) and I(f) from isolated rabbit sinoatrial node pacemaker cells.

RESULTS

GA inhibited peak I(Na) (33% at 90 microM) and late I(Na) (72% at 90 microM), but caused no significant effects on I(Ca,L) and I(f).

CONCLUSION

GA blocked cardiac sodium currents, particularly late I(Na.) Our findings might help to understand the traditional use of licorice in the treatment of cardiovascular disorders, because reduction of sodium currents (particularly late I(Na)) would be expected to provide protection from Na(+)-induced Ca(2+) overload and cell damage.

Thrombin receptor and ventricular arrhythmias after acute myocardial infarction

The mechanism mediating the development of ventricular arrhythmia (VA) after acute myocardial infarction (AMI) is still uncertain. Thrombin receptor (TR) activation has been proven to be arrhythmogenic in many other situations, and we hypothesize that it may participate in the genesis of post-AMI VA. Using a left coronary artery ligation rat model of AMI, we found that a local injection of hirudin into the left ventricle (LV) significantly reduced the ratio of VA durations to infarction sizing, whereas injection of thrombin receptor-activating peptide (TRAP) increased the ratios of VA duration to infarction sizing. The effects of TR activation on whole-cell currents were investigated in isolated myocytes. TRAP increased a glibenclamide-sensitive outward current. Pretreatment of rats with glibenclamide (4 mg/kg intraperitoneally) eliminated the effects of a local injection of TRAP on the ratios of VA durations to infarction sizing. TR mRNA and protein expression in the ischemic left ventricle had reached its peak by 20 min postligation in the rat AMI model (P < 0.05). TR-immunoreactive myocytes were observed in infarcted LV but were seldom seen in the right ventricle or in the normal heart. By 60 min, TR transcript levels had returned to control levels. We conclude that increased TR activation and expression in the infarcted LV after AMI may contribute to VA through a mechanism involving glibenclamide-sensitive potassium channels.

Protease-activated receptor-1 mediates thrombin-induced persistent sodium current in human cardiomyocytes

After the thrombus formation in cardiac cavities or coronaries, the serine protease thrombin is produced and can therefore reach the myocardial tissue by the active process of extravasation and binds to the G protein-coupled protease-activated receptor-1 (PAR1) expressed in human myocardium. The role of PAR1 was investigated in the thrombin effect on sodium current (I(Na)). I(Na) was recorded in freshly isolated human atrial myocytes by the whole-cell patch-clamp method. Action potentials (AP) were recorded in guinea pig ventricular tissue by the conventional glass microelectrode technique. Thrombin-activated PAR1 induced a tetrodotoxin-blocked persistent sodium current, I(NaP), in a concentration-dependent manner with an apparent EC(50) of 28 U/ml. The PAR1 agonist peptide SFLLR-NH(2) (50 microM) was able to mimic PAR1-thrombin action, whereas PAR1 antagonists N(3)-cyclopropyl-7-((4-(1-methylethyl)-phenyl)methyl)-7H-pyrrolo(3,2-f)quinazoline-1,3-diamine (SCH 203099; 10 microM) and 1-(3,5-di-tert-butyl-4-hydroxy-phenyl)-2-[3-(3-ethyl-3-hydroxy-pentyl)-2-imino-2,3-dihydro-imidazol-1-yl]-ethanone (ER 112787) (1 microM), completely inhibited it. The activated PAR1 involves the calcium-independent phospholipase-A(2) signaling pathway because two inhibitors of this cascade, bromoenol lactone (50 microM) and haloenol lactone suicide substrate (50 microM), block PAR1-thrombin-induced I(NaP).Asa consequence of I(NaP) activation, in guinea pig right ventricle papillary muscle, action potential duration (APD) were significantly increased by 20% and 15% under the respective action of 32 U/ml thrombin and 50 microM SFLLR-NH(2), and these increases in APD were prevented by 1 microM tetrodotoxin or markedly reduced by application of 1 microM SCH 203099 or ER 112787. Thrombin, through PAR1 activation, increases persistent component of the Na(+) current resulting in an uncontrolled sodium influx into the cardiomyocyte, which can contribute to cellular injuries observed during cardiac ischemia.

Myocardial ischemia and ventricular fibrillation: pathophysiology and clinical implications

Ventricular fibrillation (VF) and myocardial ischemia are inseparable. The first clinical manifestation of myocardial ischemia or infarction may be sudden cardiac death in 20-25% of patients. The occurrence of potentially lethal arrhythmia is the end result of a cascade of pathophysiological abnormalities that result from complex interactions between coronary vascular events, myocardial injury, and changes in autonomic tone, metabolic conditions and ionic state of the myocardium. It is also related to the time from the onset of ischemia. Within the first few minutes there is abundant ventricular arrhythmogenesis usually lasting for 30 min. Triggers for ischemic VF occur at the border zone or regionally ischemic heart. The border zone of ischemia is the predominant site of fragmentation. Acute ischemia opens K(ATP) channels and causes acidosis and hypoxia of myocardial cells leading to a large dispersion in repolarization across the border zone. Abnormalities of intracellular Ca2+ handling also occur in the first few minutes of acute myocardial ischemia and may be an important cause of arrhythmias in human coronary artery disease. Substrate on the other hand transforms triggers into VF and serves to maintain it through fragmentation of waves in the ischemic zone. Thrombin levels, stretch, catecholamine, genetic predisposition, etc. are some of these factors. Reentry models described are spiral wave reentry, 3 dimensional rotors, reentry around 'M' cells and figure-of-eight reentry. Continuing efforts to better understand these arrhythmias will help identify patients of myocardial ischemia prone to arrhythmias.

Assessment of markers of thrombin generation in patients with acute myocardial infarction complicated by ventricular fibrillation

BACKGROUND

In most cases, sudden cardiac death is triggered by ischemia-related ventricular tachyarrhythmias and accounts for 50% of deaths from cardiovascular disease in developed countries. Chronic elevation of indicators of coagulation activation has been found in patients with coronary heart disease, but a role of coagulation activation as a potential risk factor for ventricular fibrillation (VF) during acute myocardial infarction (MI) has not been investigated.

METHODS

We enrolled 50 patients with a history of MI, of whom 26 presented with VF in the acute phase of myocardial ischemia; 24 patients had an acute MI without ventricular tachyarrhythmias. Levels of thrombin-antithrombin complexes (TAT), prothrombin fragment F1 + 2 (F1 + 2), fibrinopeptide A (FPA), plasmin-antiplasmin complexes (PAP), protein C, antithrombin, activated partial thromboplastin time (aPTT), thromboplastin time, D-Dimer, fibrinogen, and high-sensitivity C-reactive protein (hs-CRP) were measured in plasma samples of all patients. Blood collection was obtained sequentially in two separate settings. Patients were studied at a median of 351 days after the acute coronary event.

RESULTS

Higher levels of TAT complexes (13.4 +/- 22.2 vs. 3.03 +/- 4.3 microg/l; p = 0.02), FPA (79.7 +/- 132.3 vs. 24.04 +/- 41.3 ng/ml; p = 0.04), and F1+2 (1.89 +/- 1.3 vs. 1.16 +/- 0.5 nmol/l; p = 0.01) were observed in patients with VF compared with patients without ventricular tachyarrhythmias during the acute phase of MI. D-Dimer levels displayed a trend without reaching statistical significance (0.69 +/- 0.48 vs. 0.48 +/- 0.24 mg/l; p = 0.06). No differences were found in hs-CRP (3.25 +/- 4.5 vs. 4.4 +/- 8.8 mg/l; p = 0.5) and fibrinogen (2.8 +/- 0.9 vs. 2.7 +/- 0.9 g/l; p = 0.6) measurements. Repeat assessment of markers of coagulation activation at a median of 847 days revealed a highly significant decrease in patients with VF.

CONCLUSIONS

Markers of thrombin generation are transiently increased in patients with VF during the acute phase of MI. These findings have implications for risk assessment and genetic screening of patients prone to VF during acute myocardial ischemia.

Intracellular sodium increase and susceptibility to ischaemia in hearts from type 2 diabetic db/db mice

AIMS/HYPOTHESIS

An important determinant of sensitivity to ischaemia is altered ion homeostasis, especially disturbances in intracellular Na(+) (Na(i)(+)) handling. As no study has so far investigated this in type 2 diabetes, we examined susceptibility to ischaemia-reperfusion in isolated hearts from diabetic db/db and control db/+ mice and determined whether and to what extent the amount of (Na(i)(+)) increase during a transient period of ischaemia could contribute to functional alterations upon reperfusion.

METHODS

Isovolumic hearts were exposed to 30-min global ischaemia and then reperfused. (23)Na nuclear magnetic resonance (NMR) spectroscopy was used to monitor[Formula: see text] and (31)P NMR spectroscopy to monitor intracellular pH (pH(i)).

RESULTS

A higher duration of ventricular tachycardia and the degeneration of ventricular tachycardia into ventricular fibrillation were observed upon reperfusion in db/db hearts. The recovery of left ventricular developed pressure was reduced. The increase in[Formula: see text] induced by ischaemia was higher in db/db hearts than in control hearts, and the rate of pH(i) recovery was increased during reperfusion. The inhibition of Na(+)/H(+) exchange by cariporide significantly reduced (Na(i)(+)) gain at the end of ischaemia. This was associated with a lower incidence of ventricular tachycardia in both heart groups, and with an inhibition of the degeneration of ventricular tachycardia into ventricular fibrillation in db/db hearts.

CONCLUSIONS/INTERPRETATION

These findings strongly support the hypothesis that increased (Na(i)(+)) plays a causative role in the enhanced sensitivity to ischaemia observed in db/db diabetic hearts.

Head-tail interactions in numerical simulations of reentry in a ring of cardiac tissue

BACKGROUND

The relationships between action potential duration (APD) and conduction velocity (CV) to the previous diastolic interval (DI) are known as the APD and CV restitution relationships. There is considerable debate regarding the importance of these relationships in the development and stability of reentry.

OBJECTIVES

The purpose of this study was to increase the understanding of the ionic basis for restitution during reentry.

METHODS

APD and CV were studied numerically during one-dimensional reentry as ring length (L) was shortened. A three-state variable model (u, v, w) was used to analyze the effect of gating variables of the fast (v) and slow (w) currents on the spatial and temporal dynamics of transmembrane potential (u). Three parameter sets were used corresponding to three APD and CV restitution curves.

RESULTS

Sustained spatial oscillations of APD and CV larger than the ring length were observed in two of the parameter sets (cytochalasin-D model [CYTO] and model 3 [M3]) before block occurred at L = 6 cm. The last model (diacetyl monoxime [DAM]) resulted in uniform APD and CV for all L until block occurred at L = 3 cm. Multivalued APD and CV restitution relationships due to "dephasing" of w and v with DI were observed in M3 and CYTO simulations. Overall, these dynamics could be explained by the wavelength-to-ring length ratio and the sensitivity of APD on the value of the gating variables w and v.

CONCLUSION

Propagation stability is mostly controlled by APD sensitivity to w, but the APD restitution slope does not always reflect this sensitivity. The interaction of the dynamic history (i.e., memory) of the fast and slow currents and electrotonic effects resulted in multivalued restitution curves.

Enhanced Coagulation Activation by In Vitro Lipopolysaccharide Challenge in Patients with Ventricular Fibrillation Complicating Acute Myocardial Infarction

BACKGROUND

Indicators of coagulation and inflammation are elevated in patients with coronary heart disease. A role of coagulation activation in ventricular fibrillation during acute myocardial infarction has not been described.

METHODS AND RESULTS

Whole blood samples of 21 patients with a history of acute myocardial infarction complicated by ventricular fibrillation and whole blood samples of 18 patients without ventricular fibrillation were incubated with lipopolysaccharide (LPS). In both groups, the in vitro blood coagulation time was measured with the ReoRox, a viscometric whole blood coagulometer. CD62P expression on platelets, tissue-factor binding on monocytes, and platelet-monocyte aggregates were measured with flow cytometry. Without LPS, no difference in the coagulation times were observed in both patient groups. After incubation with LPS, patients with a history of ventricular fibrillation showed a significantly decreased coagulation time compared to patients without ventricular fibrillation. The decrease of coagulation time after incubation with LPS also differed significantly in both groups. Expression of CD62P on platelets was significantly higher in patients with a history of ventricular fibrillation after incubation with LPS. Although in each patient group incubation with LPS induced a significantly increased amount of tissue factor on monocytes and a significantly increased the number of platelet-monocyte aggregates, the two groups did not differ significantly concerning tissue factor binding on monocytes and the amount of platelet-monocyte aggregates.

CONCLUSIONS

After in vitro LPS challenge, patients with a history of ventricular fibrillation during myocardial infarction show an enhanced coagulation activation, which may partly be due to an enhanced platelet activation.

Head-tail interactions in numerical simulations of reentry in a ring of cardiac tissue

BACKGROUND

The relationships between action potential duration (APD) and conduction velocity (CV) to the previous diastolic interval (DI) are known as the APD and CV restitution relationships. There is considerable debate regarding the importance of these relationships in the development and stability of reentry.

OBJECTIVES

The purpose of this study was to increase the understanding of the ionic basis for restitution during reentry.

METHODS

APD and CV were studied numerically during one-dimensional reentry as ring length (L) was shortened. A three-state variable model (u, v, w) was used to analyze the effect of gating variables of the fast (v) and slow (w) currents on the spatial and temporal dynamics of transmembrane potential (u). Three parameter sets were used corresponding to three APD and CV restitution curves.

RESULTS

Sustained spatial oscillations of APD and CV larger than the ring length were observed in two of the parameter sets (cytochalasin-D model [CYTO] and model 3 [M3]) before block occurred at L = 6 cm. The last model (diacetyl monoxime [DAM]) resulted in uniform APD and CV for all L until block occurred at L = 3 cm. Multivalued APD and CV restitution relationships due to "dephasing" of w and v with DI were observed in M3 and CYTO simulations. Overall, these dynamics could be explained by the wavelength-to-ring length ratio and the sensitivity of APD on the value of the gating variables w and v.

CONCLUSION

Propagation stability is mostly controlled by APD sensitivity to w, but the APD restitution slope does not always reflect this sensitivity. The interaction of the dynamic history (i.e., memory) of the fast and slow currents and electrotonic effects resulted in multivalued restitution curves.

KC 12291: an atypical sodium channel blocker with myocardial antiischemic properties

KC 12291 was designed as a voltage-gated sodium channel (VGSC) blocker with cardioprotective properties. KC 12291 has moderate inhibitory effects on peak (or rapid) Na+ current, and markedly reduces sustained (or slowly or non-inactivating) Na+ current. This distinguishes KC 12291 from conventional VGSC blockers such as local anesthetics or antiarrhythmics, which have little or no cardioprotective properties. Since VGSCs represent the main pathway for ischemic Na+ loading by failing to inactivate fully, KC 12291 exerts pronounced antiischemic activity principally by reducing the amplitude of sustained Na+ current. In isolated atria and Langendorff-perfused hearts, KC 12291 inhibits diastolic contracture, renowned for its resistance to pharmacological inhibition, reduces ischemic Na+ loading and preserves cardiac energy status. KC 12291 exerts oral antiischemic activity in vivo in the absence of major hemodynamic effects. Cardiac VGSC blockers such as KC 12291, which block cardiac VGSCs in atypical fashion by effectively inhibiting the sustained component of Na+ current, represent, therefore, promising potential antiischemic and cardioprotective drugs.