Clarithromycin reducesIsusandItocurrents in human atrial myocytes with minor repercussions on action potential duration

The macrolide antibacterial agent clarithromycin has been shown to cause QT interval prolongation on the electrocardiogram. In rabbit heart preparations clarithromycin (concentration dependently) lengthened the action potential duration and blocked the delayed rectifier current. The aim of the present study was to investigate the clarithromycin effects: (i) on the Ca2+ L-type and the main K+ repolarizing currents on human atrial myocytes, using whole-cell patch clamp recordings and (ii) on action potentials recorded from human atrial and ventricular myocardium using conventional microelectrodes. It has been found that (i) 10-30 microM clarithromycin reduced the sustained current Isus significantly and that a 100 microM concentration was needed to cause a significant reduction in the transient outward current Ito, whereas clarithomycin did not affect the calcium current and (ii) clarithromycin (10-100 microM) prolonged the action potential duration in atrial preparations but did not alter the different parameters of the ventricular action potential. It is concluded that clarithromycin exerts direct cardiac electrophysiological effects that may contribute to pro-arrythmic potential.

The birth of a channel

An ion channel protein begins life as a nascent peptide inside a ribosome, moves to the endoplasmic reticulum where it becomes integrated into the lipid bilayer, and ultimately forms a functional unit that conducts ions in a well-regulated fashion. Here, I discuss the nascent peptide and its tasks as it wends its way through ribosomal tunnels and exit ports, through translocons, and into the bilayer. We are just beginning to explore the sequence of these events, mechanisms of ion channel structure formation, when biogenic decisions are made, and by which participants. These decisions include when to exit the endoplasmic reticulum and with whom to associate. Such issues govern the expression of ion channels at the cell surface and thus the electrical activity of a cell.

Differential regulation of the slow and rapid components of guinea-pig cardiac delayed rectifier K+ channels by hypoxia

The aim of this study was to examine the effects of acute hypoxia on the slow (I(Ks)) and rapid (I(Kr)) components of the native delayed rectifier K+ channel in the absence and presence of the beta-adrenergic receptor agonist isoproterenol (isoprenaline; Iso) using the whole-cell configuration of the patch-clamp technique. Hypoxia reversibly inhibited basal I(Ks). The effect could be mimicked by exposing the cells to the thiol-specific reducing agent dithiothreitol (DTT) and attenuated upon exposure of cells to the membrane-impermeant thiol-specific oxidizing compound 5,5'-dithio-bis[2-nitrobenzoic acid] (DTNB). In the presence of hypoxia, the K(0.5) for activation of I(Ks) by Iso was significantly decreased from 18.3 to 1.9 nm. DTT mimicked the effect of hypoxia on the sensitivity of I(Ks) to Iso while DTNB had no effect. Hypoxia increased the sensitivity of I(Ks) to histamine and forskolin suggesting that the effect of hypoxia is not occurring at the beta-adrenergic receptor. The increase in sensitivity of I(Ks) to Iso could be attenuated with addition of PKCbeta peptide to the pipette solution. While hypoxia and DTT inhibited basal I(Ks) they were without effect on I(Kr.) In addition, Iso did not appear to alter the magnitude of I(Kr) in the absence or presence of hypoxia. These data suggest that hypoxia regulates native I(Ks) through two distinct mechanisms: direct inhibition of basal I(Ks) and an increase in sensitivity to Iso that occurs downstream from the beta-adrenergic receptor. Both mechanisms appear to involve redox modification of thiol groups. In contrast native I(Kr) does not appear to be regulated by Iso, hypoxia or redox state.

Hydrogen peroxide modulates the Kv1.5 channel expressed in a mammalian cell line

Reactive oxygen species have been implicated in different types of cardiac arrhythmias including human atrial fibrillation. Kv1.5, the presumed molecular correlate of I(Kur), is an important determinant of human atrial repolarization. The aim of this study was to assess the effects of H(2)O(2), at pathophysiologically relevant concentrations (20-1,000 microM), on Kv1.5 expressed in Chinese hamster ovary cell line. Kv1.5 cDNA in pcDNA3 expression vector and CD8, a surface marker protein, were cotransfected in cells by calcium phosphate precipitation. Kv1.5 activation kinetics were significantly accelerated while the activation curve was negatively shifted by 10 mV (V(1/2) changed from -9.3 to -19.0 mV) in the presence of 100 microM H(2)O(2). The shift in Kv1.5 peak current I-V curve was voltage-dependent, the current amplitude being increased for voltages <+20 mV but decreased for high depolarizing voltages. The rapid activation time constant obtained from a bi-exponential fitting was decreased from 16.1+/-3.4 ms to 8.8+/-1.5 ms for a -20 mV depolarization ( n=9; P=0.01) and from 4.3+/-2.1 ms to 2.3+/-0.4 ms when cells were depolarized to +20 mV ( P<0.05). Kv1.5 steady-state inactivation was not modified by H(2)O(2). Intracellular application of SOD or catalase reduced the H(2)O(2) induced shift of activation I-V curve and SOD significantly decreased Kv1.5 amplitude at +40 mV ( n=9; P<0.05). In conclusion, H(2)O(2) increased Kv1.5 current amplitude at voltages corresponding to the action potential repolarization phase and accelerated Kv1.5 channel opening. These changes can reduce the action potential duration, leading to a shortening of the atrial effective refractory period. H(2)O(2)-induced changes in Kv1.5 properties could thus be involved in initiation or perpetuation of AF.

Developmental anatomy of the heart: a tale of mice and man

Because of the increasing availability of tools for genetic manipulation, the mouse has become the most popular animal model for studying normal and abnormal cardiac development. However, despite the enormous advances in mouse genetics, which have led to the production of numerous mutants with cardiac abnormalities resembling those seen in human congenital heart disease, relatively little comparative work has been published to demonstrate the similarities and differences in the developmental cardiac anatomy in both species. In this review we discuss some aspects of the comparative anatomy, with emphasis on the atrial anatomy, the valvuloseptal complex, and ventricular myocardial development. From the data presented it can be concluded that, apart from the obvious differences in size, the mouse and human heart are anatomically remarkably similar throughout development. The partitioning of the cardiac chambers (septation) follows the same sequence of events, while also the maturation of the cardiac valves and myocardium is quite similar in both species. The major anatomical differences are seen in the venous pole of the heart. We conclude that, taking note of the few anatomical “variations,” the use of the mouse as a model system for the human heart is warranted. Thus the analysis of mouse mutants with impaired septation will provide valuable information on cellular mechanisms involved in valvuloseptal morphogenesis (a process often disrupted in congenital heart disease), while the study of embryonic lethal mouse mutants that present with lack of compaction of ventricular trabeculae will ultimately provide clues on the etiology of this abnormality in humans.

Review of the current management of atrial fibrillation

Atrial fibrillation (AF) is the most common sustained arrhythmia. Its prevalence is increasing and accordingly, so is its burden on healthcare systems throughout the world. The pathophysiology of AF is complex and poorly understood, which of itself presents a major challenge to the management of this important condition. AF is associated with increased morbidity and mortality, particularly in patients with underlying left ventricular dysfunction. Once AF occurs, it is often difficult to 'cure' and as such, the major focus of therapy is currently divided essentially between a rate control strategy and a need to revert to and maintain sinus rhythm. Both approaches seek to minimise the associated symptoms and complications. Over the past two decades, numerous pharmacological approaches to the management of AF have been employed, many of which have been shown to be relatively ineffective or confounded by major complications. Accordingly, recent research and interest has focused on non-pharmacological electrophysiological therapies to either cure AF or improve symptoms. This review summarises the current approaches to the management AF and provides some new insights into emerging therapies for this common clinical problem.

Age-related sensitivity to nicotine for inducible atrial tachycardia and atrial fibrillation

The influence of nicotine in modulating vulnerability to atrial tachycardia and fibrillation (AT/AF) remains ill defined. The isolated hearts of six young (2–3 mo) and six old (22–24 mo) male Fischer 344 rats were Langendorff perfused at 5 ml/min with oxygenated Tyrode solution at 37°C, and the whole heart was also super-fused with warmed oxygenated Tyrode solution at 15 ml/min. Nicotine prolonged the interatrial conduction time and effective refractory period that were significantly ( P < 0.05) higher in the old than in the young rats in a concentration-dependent manner. Nicotine had a biphasic effect on burst atrial pacing-induced AT in both groups, increasing it at 10–30 ng/ml while decreasing it at 50–100 ng/ml ( P < 0.01). Nicotine at 10–100 ng/ml increased burst atrial pacing-induced AF in the young rats but suppressed it in the old rats ( P < 0.01). Optical mapping showed the presence of multiple independent wavefronts during AF and a single periodic large wavefront during AT in both groups. Nicotine, at concentrations found in the blood of smokers (30–85 ng/ml), exerts biphasic effects on inducible AT/AF in young rats and suppresses it in the old rats by causing high degrees of interatrial conduction block.

Combined acetaldehyde and nicotine exposure depresses cardiac contraction in ventricular myocytes: prevention by folic acid

Chronic alcoholism can progress to alcoholic cardiomyopathy characterized by ventricular dilation and impaired ventricular contractility. Nicotine abuse continues to remain a serious risk factor for cardiovascular diseases. However, little is know regarding the combined effects. This study was designed to examine the role of short-term combined exposure of the main alcohol metabolite acetaldehyde (ACA) and nicotine on cardiac contractile function in adult rat ventricular myocytes, and if folate exposure attenuates the effect of ACA/nicotine. Mechanical and intracellular Ca2+ properties were evaluated by an IonOptix SoftEdge system. Short-term (4-6 h) culture of myocytes with ACA (10 microM), nicotine (100 microM), or combination of the two in sealed vials with silicone septa depressed maximal velocity of shortening/relengthening, without affecting duration of shortening/relengthening and intracellular Ca2+ handling. Interestingly, the toxin-induced defects on myocyte mechanical properties were ablated with cotreatment of folate (100 microM), an essential vitamin required for DNA synthesis and repair. Collectively, these data provided evidence that ACA and nicotine, either alone or in combination, directly depressed cardiomyocyte mechanical function possibly through mechanisms related to DNA damage.

Comparison of time- and voltage-dependent K+ currents in myocytes from left and right atria of adult mice

Consistent differences in K+ currents in left and right atria of adult mouse hearts have been identified by the application of current- and voltage-clamp protocols to isolated single myocytes. Left atrial myocytes had a significantly (P < 0.05) larger peak outward K+ current density than myocytes from the right atrium. Detailed analysis revealed that this difference was due to the rapidly activating sustained K+ current, which is inhibited by 100 muM 4-aminopyridine (4-AP); this current was almost three times larger in the left atrium than in the right atrium. Accordingly, 100 muM 4-AP caused a significantly (P < 0.05) larger increase in action potential duration in left than in right atrial myocytes. Inward rectifier K+ current density was also significantly (P < 0.05) larger in left atrial myocytes. There was no difference in the voltage-dependent L-type Ca2+ current between left and right atria. As expected from this voltage-clamp data, the duration of action potentials recorded from single myocytes was significantly (P < 0.05) shorter in myocytes from left atria, and left atrial tissue was found to have a significantly (P < 0.05) shorter effective refractory period than right atrial tissue. These results reveal similarities between mice and other mammalian species where the left atrium repolarizes more quickly than the right, and provide new insight into cellular electrophysiological mechanisms responsible for this difference. These findings, and previous results, suggest that the atria of adult mice may be a suitable model for detailed studies of atrial electrophysiology and pharmacology under control conditions and in the context of induced atrial rhythm disturbances.

Mesenteric Lymph From Rats With Thermal Injury Prolongs the Action Potential and Increases Ca2+ Transient in Rat Ventricular Myocytes

Although gut-derived mesenteric lymph from animals with thermal injury appears to lead to myocardial contractile dysfunction, the cellular mechanisms remain unclear. We examined the direct effects of intestinal lymph on excitation-contraction coupling in rat ventricular myocytes. Lymph from rats receiving burn injury (burn lymph), but not from sham-burned rats, rapidly enhanced myocyte contraction and the amplitude of Ca2+ transient; the average percentage of shortening was increased from 5.5 +/- 0.3% to 10.5 +/- 0.9%. 90% and the Ca2+ transients increased by 80% +/- 20%. Burn lymph had no effect on the amplitude of L-type Ca2+ current (ICa) or the inward rectifier K+ current, but the transient outward K+ currents (Ito) were reduced significantly by burn lymph. Inhibition of Ito was not altered by an alpha1-adrenergic receptor (AR) antagonist, prazosin, indicating that the block was not mediated via alpha1-AR signaling pathway. Action potential (AP) duration, measured at 50% and 90% repolarization, was prolonged by burn lymph. Stimulation of myocytes with AP voltage-clamp waveforms derived from prolonged AP induced by burn lymph revealed a 1.7-fold increase in Ca2+ influx via ICa compared with the Ca2+ influx induced by control AP. Blocking of Ito by 4-aminopyridine prolonged AP duration and increased Ca2+ transients, mimicking the effects of burn lymph. Burn lymph did not affect Na+/Ca2+ exchange currents or caffeine-induced SR Ca2+ release. Thus, acute exposure of normal cardiac myocytes to burn lymph increases Ca2+ transients by a prolongation of AP as a result of a reduction of Ito with no intrinsic change in ICa or exchanger. The electrophysiological changes are similar to those that occur during compensated cardiac hypertrophy, suggesting a common mechanistic link between burn lymph- and hypertrophy-induced cardiac dysfunction.

Evaluation of cardiac rhythm in dogs treated with levamisole hydrochloride using 24-hour ambulatory electrocardiography

The objective of this study was to evaluate the electrocardiographic alterations in the cardiac rhythm in dogs treated with levamisole hydrochloride over a period of 24 hours. Thirty-six mixed-breed dogs, both male and female, all clinically healthy, were used in the experiment. The dogs were divided into 6 groups with 6 dogs in each group, according to dosage and route of administration. The Holter test was initiated immediately after the treatment, and was maintained for 24 hours. In the group treated with 10 mg/kg by way of subcutaneous injection, one of them showed ventricular premature complexes, sometimes isolated and other times in pairs, and ventricular tachycardia, concentrated mainly in the first hour after administration of the drug. In the group of 6 animals treated subcutaneously with 25mg/kg, four showed isolated ventricular premature complexes, ventricular bigeminy and trigeminy, mainly during the first 2 hours after administration of the drug. All the animals in the other groups showed sinus arrhythmia followed by sinus arrest. The disturbances in the cardiac rhythm observed in clinically healthy animals treated with levamisole hydrochloride, indicate that it is preferable to avoid subcutaneous administration of levamisole hydrochloride and that the oral administration of the drug should be done with caution.

Identification of a heteromeric interaction that influences the rectification, gating, and pH sensitivity of Kir4.1/Kir5.1 potassium channels

Heteromultimerization between different potassium channel subunits can generate channels with novel functional properties and thus contributes to the rich functional diversity of this gene family. The inwardly rectifying potassium channel subunit Kir5.1 exhibits highly selective heteromultimerization with Kir4.1 to generate heteromeric Kir4.1/Kir5.1 channels with unique rectification and kinetic properties. These novel channels are also inhibited by intracellular pH within the physiological range and are thought to play a key role in linking K+ and H+ homeostasis by the kidney. However, the mechanisms that control heteromeric K+ channel assembly and the structural elements that generate their unique functional properties are poorly understood. In this study we identify residues at an intersubunit interface between the cytoplasmic domains of Kir5.1 and Kir4.1 that influence the novel rectification and gating properties of heteromeric Kir4.1/Kir5.1 channels and that also contribute to their pH sensitivity. Furthermore, this interaction presents a structural mechanism for the functional coupling of these properties and explains how specific heteromeric interactions can contribute to the novel functional properties observed in heteromeric Kir channels. The highly conserved nature of this structural association between Kir subunits also has implications for understanding the general mechanisms of Kir channel gating and their regulation by intracellular pH.

KvLQT1 modulates the distribution and biophysical properties of HERG. A novel alpha-subunit interaction between delayed rectifier currents

Cardiac repolarization is under joint control of the slow (IKs) and rapid (IKr) delayed rectifier currents. Experimental and clinical evidence indicates important functional interactions between these components. We hypothesized that there might be more direct interactions between the KvLQT1 and HERG alpha-subunits of IKs and IKr and tested this notion with a combination of biophysical and biochemical techniques. Co-expression of KvLQT1 with HERG in a mammalian expression system significantly accelerated HERG current deactivation at physiologically relevant potentials by increasing the contribution of the fast component (e.g. upon repolarization from +20 mV to -50 mV: from 20 +/- 3 to 32 +/- 5%, p < 0.05), making HERG current more like native IKr. In addition, HERG current density was approximately doubled (e.g. tail current after a step to +10 mV: 18 +/- 3 versus 39 +/- 7 pA/picofarad, p < 0.01) by co-expression with KvLQT1. KvLQT1 co-expression also increased the membrane immunolocalization of HERG by approximately 2-fold (p < 0.05). HERG and KvLQT1 co-immunolocalized in canine ventricular myocytes and co-immunoprecipitated in cultured Chinese hamster ovary cells as well as in native cardiac tissue, indicating physical interactions between HERG and KvLQT1 proteins in vitro and in vivo. Protein interaction assays also demonstrated binding of KvLQT1 (but not another K+ channel alpha-subunit, Kv3.4) to a C-terminal HERG glutathione S-transferase fusion protein. Co-expression with HERG did not affect the membrane localization or ionic current properties of KvLQT1. This study shows that the alpha-subunit of IKs can interact with and modify the localization and current-carrying properties of the alpha-subunit of IKr, providing potentially novel insights into the molecular function of the delayed rectifier current system.

Molecular basis for Kv1.5 channel block: conservation of drug binding sites among voltage-gated K+ channels

Kv1.5 channels conduct the ultrarapid delayed rectifier current (IKur) that contributes to action potential repolarization of human atrial myocytes. Block of these channels has been proposed as a treatment for atrial arrhythmias. Here we report a novel and potent inhibitor of Kv1.5 potassium channels, N-benzyl-N-pyridin-3-yl-methyl-2-(toluene-4-sulfonylamino)-benzamide hydrochloride (S0100176), which exhibits features consistent with preferential block of the open state. The IC50 of S0100176 for Kv1.5 expressed in Xenopus oocytes was 0.7 microm. Ala-scanning mutagenesis within the pore helix and the S6 segment, regions that form the walls of the central cavity, was combined with voltage clamp analysis to identify point mutations that altered drug affinity. This approach identified Thr-479, Thr-480, Val-505, Ile-508, and Val-512 as the most important residues for block by S0100176. Mutations of these key residues to Ala or other amino acids caused marked changes in the IC50 of S0100176 (p<0.01). For example, the IC50 of S0100176 increased 362-fold for T480A, 26-fold for V505A, 150-fold for I508A, and 99-fold for V512A. We used modeling to dock S0100176 into the inner cavity of a Kv1.5 pore homology model that was generated based on the crystal structure of KcsA. The docking predicted that the five residues identified by the Ala scan were positioned less than 4.5 A from the compound. Based on the homology models, the positions of the five amino acids identified to interact with S0100176 face toward the central cavity and overlap with putative binding sites for other blockers and voltage-gated potassium channels.

Electrophysiological properties of human mesenchymal stem cells

Human mesenchymal stem cells (hMSC) have gained considerable interest due to their potential use for cell replacement therapy and tissue engineering. One strategy is to differentiate these bone marrow stem cells in vitro into cardiomyocytes prior to implantation. In this context ion channels can be important functional markers of cardiac differentiation. At present there is little information about the electrophysiological behaviour of the undifferentiated hMSC. We therefore investigated mRNA expression of 26 ion channel subunits using semiquantitative RT-PCR and recorded transmembrane ion currents with the whole-cell voltage clamp technique. Bone marrow hMSC were obtained from healthy donors. The cells revealed a distinct pattern of ion channel mRNA with high expression levels for some channel subunits (e.g. Kv4.2, Kv4.3, MaxiK, HCN2, and alpha1C of the L-type calcium channel). Outward currents were recorded in almost all cells. The most abundant outward current rapidly activated at potentials positive to +20 mV. This current was identified as a large-conductance voltage- and Ca(2+)-activated K(+) current, conducted by MaxiK channels, due to its high sensitivity to tetraethylammonium (IC(50)= 340 microm) and its inhibition by 100 nm iberiotoxin. A large fraction of cells also demonstrated a more slowly activating current at potentials positive to -30 mV. This current was selectively inhibited by clofilium (IC(50)= 0.8 microm). Ba(2+) inward currents, stimulated by 1 microm BayK 8644 were found in a few cells, indicating the expression of functional L-type Ca(2+) channels. Other inward currents such as sodium currents or inward rectifier currents were absent. We conclude that undifferentiated hMSC express a distinct pattern of ion channel mRNA and functional ion channels that might contribute to physiological cell function.

Aldosteronism and peripheral blood mononuclear cell activation: a neuroendocrine-immune interface

Aldosteronism eventuates in a proinflammatory/fibrogenic vascular phenotype of the heart and systemic organs. It remains uncertain whether peripheral blood mononuclear cells (PBMCs) are activated before tissue invasion by monocytes/macrophages and lymphocytes, as is the case for responsible pathogenic mechanisms. Uninephrectomized rats treated for 4 weeks with dietary 1% NaCl and aldosterone (ALDOST, 0.75 microg/h) with or without spironolactone (Spi, 100 mg/kg per daily gavage) were compared with unoperated/untreated and uninephrectomized/salt-treated controls. Before intramural coronary vascular lesions appeared at week 4 of ALDOST, we found (1) a reduction of PBMC cytosolic free [Mg2+]i, together with intracellular Mg2+ and Ca2+ loading, whereas plasma and cardiac tissue Mg2+ were no different from controls; (2) increased H2O2 production by monocytes and lymphocytes together with upregulated PBMC gene expression of oxidative stress-inducible tyrosine phosphatase and Mn2+-superoxide dismutase and the presence of 3-nitrotyrosine in CD4+ and ED-1-positive inflammatory cells that had invaded intramural coronary arteries; (3) B-cell activation, including transcription of immunoglobulins, intracellular adhesion molecule-1, and CC and CXC chemokines and their receptors; (4) expansion of B lymphocyte subset and myosin heavy chain class II-expressing lymphocytes; and (5) autoreactivity with gene expression for antibodies to acetylcholine receptors and a downregulation of RT-6.2, which is in keeping with cell activation and associated with autoimmunity. Spi cotreatment attenuated the rise in intracellular Ca2+, the appearance of oxidative/nitrosative stress in PBMCs and invading inflammatory cells, and alterations in PBMC transcriptome. Thus, aldosteronism is associated with an activation of circulating immune cells induced by iterations in PBMC divalent cations and transduced by oxidative/nitrosative stress. ALDO receptor antagonism modulates this neuroendocrine-immune interface. The full text of this article is available online at http://www.circresaha.org.

Frequency Measures Obtained from the Surface Electrocardiogram in Atrial Fibrillation Research and Clinical Decision‐Making

INTRODUCTION

Frequency analysis of fibrillation (FAF) and time-frequency analysis (TFA) were developed recently in order to quantify atrial electrical remodeling in atrial fibrillation (AF) from the surface ECG. This article describes the experience with these two different frequency analysis techniques in consecutive AF patients and discusses possible applications in AF research and clinical decision-making.

METHODS AND RESULTS

Baseline 2-minute, high-gain, high-resolution ECG recordings using three bipolar leads were obtained from 80 consecutive patients with AF lasting > 24 hours. A power spectrum was obtained using Fourier analysis following spatiotemporal QRST cancellation. The dominant fibrillatory rate (in fibrillations per minute [fpm]) was derived (FAF). Stability of the instantaneous fibrillatory rate measured in overlapping 1-second segments was expressed as the segment proportion with consecutive rate differences < 6 fpm (TFA). An adequate power spectrum that could be submitted for determination of fibrillatory rate was obtained in all patients. Dominant atrial rates ranged between 288 and 534 fpm and showed a high correlation (R = 0. 878-0.911, P < 0.001) when assessed from the three different leads. The average instantaneous fibrillatory rate was inversely related with its stability (R = -0.417, P < 0.001). It was closely related with the dominant fibrillatory rate obtained from FAF (R = 0.948, P < 0.001). A literature review revealed that pharmacologic or electrical cardioversion and AF pace termination success rates were highly dependent on fibrillatory rate.

CONCLUSION

Atrial fibrillatory rate and its variability can be reliable obtained from the surface ECG in AF patients. These parameters exhibit a significant interindividual variability allowing individual quantification of the atrial electrical remodeling process and might prove useful for predicting therapy efficacy.

The Electrical Restitution Curve Revisited:

The electrical restitution curve (ERC) traditionally describes the recovery of action potential duration (APD) as a function of the interbeat interval or, more correctly, the diastolic interval (DI). Often overlooked in modeling studies, the normal ventricular ERC is triphasic, starting with a steep initial recovery at the shortest DIs, a transient decline, and a final asymptotic rise to a plateau phase reached at long DIs. Recent studies have proposed that it would be advantageous to lower the slope of the ERC by drug intervention, as this might reduce the potential for electrical alternans and ventricular fibrillation. This review discusses the pros and cons of a flat versus steep slope of the ERC and draws attention to mechanisms thatjustify the (physiologically) steep slope, rather than a flat slope, as a better design against arrhythmias. Five potential mechanisms are discussed, which allows for a different interpretation of the effect of the slope on arrhythmogenicity. The most important appears to be the physiologic rate adaptive shortening of APD that, by reciprocal lengthening of the DI, allows the subsequent APD to move more quickly from the steep initial ERC phase onto the flat phase. A less steep initial ERC phase would protract the transition toward more fully recovered APD and, in fact, may perpetuate electrical alternans. The triphasic ERC time course in normal myocardium cannot be explained by or fitted to single exponentials or single ion channel recovery kinetics. A simple tri-ionic model is suggested that may help explain the shape of the ERC at various repolarization levels and place APD recovery into perspective with intracellular calcium recycling and recovery of contractile force.

The effects of hydrocortisone on rat heart muscarinic and adrenergic alpha 1, beta 1 and beta 2 receptors, propranolol-resistant binding sites and on some subsequent steps in intracellular signalling

Glucocorticoids affect the expression and density of neurotransmitter receptors in many tissues but data concerning the heart are contradictory and incomplete. We injected rats with hydrocortisone for 1-12 days and measured the densities of cardiac muscarinic receptors, alpha(1)-, beta(1)- and beta(2)-adrenoceptors and propranolol-resistant binding sites (formerly assumed to be the putative beta(4)-adrenoceptor). Some aspects of intracellular signalling were also evaluated: we measured adenylyl cyclase activity (basal, isoprenaline- and forskolin-stimulated and carbachol-inhibited), the coupling between muscarinic receptors and G proteins and basal and isoprenaline-stimulated heart rate. The density of cardiac muscarinic receptors increased (in both the atria and the ventricles). The density of beta(1)-adrenoceptors increased in the atria and was little changed in the ventricles. The density of beta(2)-adrenoceptors increased in both the atria and the ventricles. The number of alpha(1)-adrenoceptors decreased initially, followed by a transient increase in the atria and did not change in the ventricles. The density of propranolol-resistant binding sites first increased and then diminished in the atria and did not change in the ventricles. Although there were noticeable changes in receptor densities, the stimulatory and inhibitory effects on adenylyl cyclase, basal and isoprenaline-stimulated heart rate and the coupling between muscarinic receptors and G proteins were not significantly altered. This may indicate that changes in receptor densities might be one of the mechanisms maintaining stable functional output.

Urocortin protects cardiac myocytes from ischemia/reperfusion injury by attenuating calcium insensitive phospholipase A2gene expression

We have used Affymetrix gene chip technology to look for changes in gene expression caused by a 24 h exposure of rat primary neonatal cardiac myocytes to the cardioprotective agent urocortin. We observed a 2.5-fold down-regulation at both the mRNA and protein levels of a specific calcium-insensitive phospholipase A2 enzyme. Levels of lysophosphatidylcholine, a toxic metabolite of phospholipase A2, were lowered by 30% in myocytes treated with urocortin for 24 h and by 50% with the irreversible iPLA2 inhibitor bromoenol lactone compared with controls. Both 4 h ischemia and ischemia followed by 24 h reperfusion caused a significant increase in lysophosphatidylcholine concentration compared with controls. When these myocytes were pretreated with urocortin, the ischemia-induced increase in lysophosphatidylcholine concentration was significantly lowered. Moreover, co-incubation of cardiac myocytes with urocortin, or the specific phospholipase A2 inhibitor bromoenol lactone, reduces the cytotoxicity produced by lysophosphatidylcholine or ischemia/reperfusion. Similarly, in the intact heart ex vivo we found that cardiac damage measured by infarct size was significantly increased when lysophoshatidylcholine was applied during ischemia, compared with ischemia alone, and that pre-treatment with both urocortin and bromoenol lactone reversed the increase in infarct size. This, to our knowledge, is the first study linking the cardioprotective effect of urocortin to a decrease in a specific enzyme protein and a subsequent decrease in the concentration of its cardiotoxic metabolite.

Topogenesis of two transmembrane type K+ channels, Kir 2.1 and KcsA

Potassium channels, which control the passage of K+ across cell membranes, have two transmembrane segments, M1 and M2, separated by a hydrophobic P region containing a highly conserved signature sequence. Here we analyzed the membrane topogenesis characteristics of the M1, M2, and P regions in two animal and bacterial two-transmembrane segment-type K+ channels, Kir 2.1 and KcsA, using an in vitro translation and translocation system. In contrast to the equivalent transmembrane segment, S5, in the voltage-dependent K+ channel, KAT1, the M1 segment in KcsA, was found to have a strong type II signal-anchor function, which favors the Ncyt/Cexo topology. The N-terminal cytoplasmic region was required for efficient, correctly orientated integration of M1 in Kir 2.1. Analysis of N-terminal modification by in vitro metabolic labeling showed that the N terminus in Kir 2.1 was acetylated. The hydrophobic P region showed no topogenic function, allowing it to form a loop, but not a transmembrane structure in the membrane; this region was transiently exposed in the endoplasmic reticulum lumen during the membrane integration process. M2 was found to possess a stop-transfer function and a type I signal-anchor function, enabling it to span the membrane. The C-terminal cytoplasmic region in KcsA was found to affect the efficiency with which the M2 achieved their final structure. Comparative topogenesis studies of Kir 2.1 and KcsA allowed quantification of the relative contributions of each segment and the cytoplasmic regions to the membrane topology of these two proteins. The membrane topogenesis of the pore-forming structure is discussed using results for Kir 2.1, KcsA, and KAT1.

Phenotypic differences in transient outward K+ current of human and canine ventricular myocytes: insights into molecular composition of ventricular Ito

The Ca(2+)-independent transient outward K(+) current (I(to)) plays an important electrophysiological role in normal and diseased hearts. However, its contribution to ventricular repolarization remains controversial because of differences in its phenotypic expression and function across species. The dog, a frequently used model of human cardiac disease, exhibits altered functional expression of I(to). To better understand the relevance of electrical remodeling in dogs to humans, we studied the phenotypic differences in ventricular I(to) of both species with electrophysiological, pharmacological, and protein-chemical techniques. Several notable distinctions were elucidated, including slower current decay, more rapid recovery from inactivation, and a depolarizing shift of steady-state inactivation in human vs. canine I(to). Whereas recovery from inactivation of human I(to) followed a monoexponential time course, canine I(to) recovered with biexponential kinetics. Pharmacological sensitivity to flecainide was markedly greater in human than canine I(to), and exposure to oxidative stress did not alter the inactivation kinetics of I(to) in either species. Western blot analysis revealed immunoreactive bands specific for Kv4.3, Kv1.4, and Kv channel-interacting protein (KChIP)2 in dog and human, but with notable differences in band sizes across species. We report for the first time major variations in phenotypic properties of human and canine ventricular I(to) despite the presence of the same subunit proteins in both species. These data suggest that differences in electrophysiological and pharmacological properties of I(to) between humans and dogs are not caused by differential expression of the K channel subunit genes thought to encode I(to), but rather may arise from differences in molecular structure and/or posttranslational modification of these subunits.

MinK-related peptide 2 modulates Kv2.1 and Kv3.1 potassium channels in mammalian brain

Delayed rectifier potassium current diversity and regulation are essential for signal processing and integration in neuronal circuits. Here, we investigated a neuronal role for MinK-related peptides (MiRPs), membrane-spanning modulatory subunits that generate phenotypic diversity in cardiac potassium channels. Native coimmunoprecipitation from rat brain membranes identified two novel potassium channel complexes, MiRP2-Kv2.1 and MiRP2-Kv3.1b. MiRP2 reduces the current density of both channels, slows Kv3.1b activation, and slows both activation and deactivation of Kv2.1. Altering native MiRP2 expression levels by RNAi gene silencing or cDNA transfection toggles the magnitude and kinetics of endogenous delayed rectifier currents in PC12 cells and hippocampal neurons. Computer simulations predict that the slower gating of Kv3.1b in complexes with MiRP2 will broaden action potentials and lower sustainable firing frequency. Thus, MiRP2, unlike other known neuronal beta subunits, provides a mechanism for influence over multiple delayed rectifier potassium currents in mammalian CNS via modulation of alpha subunits from structurally and kinetically distinct subfamilies.

Selective elimination of I(K,slow1) in mouse ventricular myocytes expressing a dominant negative Kv1.5alpha subunit

Although previous studies have revealed a role for the voltage-gated K+ channel alpha-subunit Kv1.5 (KCNA5) in the generation of the 4-aminopyridine (4-AP)-sensitive component of delayed rectification in mouse ventricles (IK,slow1), the phenotypic consequences of manipulating IK,slow1 expression in vivo in different (mouse) models are distinct. In these experiments, point mutations were introduced in the pore region of Kv1.5 to change the tryptophan (W) at position 461 to phenylalanine (F) to produce a nonconducting subunit, Kv1.5W461F, that is shown to function as a Kv1 subfamily-specific dominant negative (Kv1.5DN). With the use of the alpha-myosin heavy chain promoter to direct cardiac-specific expression, three lines of Kv1.5DN-expressing (C57BL6) transgenic mice were generated and characterized. Electrophysiological recordings from Kv1.5-DN-expressing left ventricular myocytes revealed that the micromolar 4-AP sensitive IK,slow1 is selectively eliminated. The attenuation of IK,slow1 is accompanied by increased ventricular action potential durations and marked QT prolongation. In contrast to previous findings in mice expressing a truncated (DN) Kv1.1 transgene; however, no electrical remodeling is evident in Kv1.5DN-expressing ventricular myocytes, and the (Kv1.5DN-induced) elimination of IK,slow1 does not result in spontaneous ventricular arrhythmias.

The cholinomimetic agent bethanechol activates IK(ACh) in feline atrial myocytes

The effect of the cholinomimetic agent, bethanechol on macroscopic membrane currents was studied in dispersed cat atrial myocytes, using the whole-cell patch-clamp technique. Bethanechol activated an inward rectifying potassium current similar to I(K(ACh)), and a delayed rectifying-like outward current, similar to I(KM3) activated by pilocarpine, choline, and tetramethylammonium, and I(KM4) activated by 4-aminopyridine. The relatively specific muscarinic receptors subtype antagonists methoctramine (M(2)), and tropicamide (M(4)) inhibited both current components induced by bethanechol, suggesting a lack of specificity of these antagonists on cat atrial myocytes. The specific antagonist of M(3) receptors, para-fluoro-hexahydro-siladifenidol did not significantly inhibit the bethanechol-induced currents. In addition, pretreatment with PTX prevented activation of the bethanechol-induced inward and outward currents, suggesting that M(3) receptors are probably not involved in the bethanechol action. The I(K(ACh)) specific blocker tertiapin inhibited both inward rectifying- and delayed rectifying-like currents. These results suggest that both current components result from activation of a single channel type, likely I(K(ACh)).

Role of the small GTPase Rho in modulation of the inwardly rectifying potassium channel Kir2.1

The inwardly rectifying potassium channel Kir2.1 is inhibited by a variety of G-protein-coupled receptors (GPCRs). However, the mechanisms underlying the inhibition have not been fully elucidated. In this study the role of the small GTPase, Rho, in mediating this inhibition was determined. Stimulation of the m1 muscarinic receptor inhibited Kir2.1, when both receptor and channel were coexpressed in tsA201 cells. The inhibition of Kir2.1 by carbachol was reversible and atropine-sensitive. Cotransfection with a dominant-negative mutant of the small GTPase Rho abolished the inhibition of Kir2.1 with current amplitudes remaining at control levels in the presence of carbachol. Conversely, cotransfection with the constitutively activated mutant of Rho resulted in a reduction in basal Kir2.1 current amplitudes, suggesting that Rho inhibits Kir2.1. To further confirm the involvement of Rho in the signal transduction pathway, cotransfection with C3 transferase (EFC3), a selective inhibitor of Rho, abolished the reduction in Kir2.1 currents noted upon application of carbachol under control conditions. Preincubation with the phosphatidylinositol 3-kinase inhibitor wortmannin or the Rho kinase inhibitor (R)-(+)-trans-N-(4-pyridyl)-4-(1-aminoethyl)-cyclohexanecarboxamide, 2 HCl (Y-27632) had no effect on agonist-induced inhibition of Kir2.1, precluding these kinases as downstream effectors of Rho in mediation of the signal. In addition, 2'-amino-3'-methoxyflavone (PD98059), an inhibitor of mitogen-activated protein (MAP) kinase kinase (MEK), had no effect on the m1 receptor-induced inhibition of Kir2.1, suggesting that MAP kinases are not involved in the signaling pathway. In conclusion, these data indicate that the small GTPase, Rho, transduces the m1 muscarinic receptor-induced inhibition of Kir2.1 via an unidentified mechanism.

Chronic atrial fibrillation does not further decrease outward currents. It increases them

Rapid atrial pacing causes electrical remodeling that leads to atrial fibrillation (AF). AF can further remodel atrial electrophysiology to maintain AF. Our previous studies showed that there was a marked difference in the duration of AF in dogs that have been atrial paced at 400 beats/min for 6 wk. We hypothesized that this difference is based on the changes in the degree of electrical remodeling caused by rapid atrial pacing versus that by AF. Right atrial cells were isolated from control dogs (Con, N = 28), from dogs with chronic AF (cAF dogs, N = 13, episodes lasting at least 6 days), or from dogs with nonsustained or brief episodes of AF (nAF dogs, N = 10, episodes lasting minutes to hours). Both transient outward (Ito) and sustained outward K+ current (Isus) densities/functions were determined using whole cell voltage-clamp techniques. In nAF cells, Ito density was reduced by 69% at +40 mV: from 7.1 +/- 0.5 pA/pF (Con, n = 59) to 2.2 +/- 0.2 pA/pF (nAF, n = 24) (P < 0.05). The voltage dependence of inactivation of Ito was shifted positively and decay kinetics were changed; however, recovery from inactivation was not altered in nAF cells. In contrast, Ito density in cAF cells was both significantly different from Con cells and larger than that in nAF cells [at +40 mV, 3.5 +/- 0.3 pA/pF (cAF, n = 29), P < 0.05]. In cAF cells, recovery from inactivation and decay of Ito were both slow; yet, voltage dependence inactivation of Ito approached that of Con cells. Furthermore, "recovered" Ito of cAF cells was more sensitive to tetraethylammonium than currents of Con and nAF cells. Isus densities of nAF and cAF cells did not differ. Both nAF and cAF cells have reduced Ito versus Con cells, but Ito remodeling of nAF cells differed from that of cAF cells. Ito in cAF dogs was likely remodeled by AF per se, whereas that in nAF dogs was likely the consequence of the rapid rate in the absence of sustained AF.

Molecular basis of species-specific expression of repolarizing K+ currents in the heart

There are important species-specific differences in K+ current profiles and arrhythmia susceptibility, but interspecies comparisons of K+ channel subunit expression are lacking. We quantified voltage-gated K+ channel (Kv) subunit mRNA and protein in rabbits, guinea pigs, and humans. Kv1.4, Kv4.2, and Kv4.3 mRNA was present in rabbits but undetectable in guinea pigs. MinK mRNA concentration in guinea pigs was almost threefold greater versus humans and 20-fold versus rabbits. MinK protein expression in guinea pigs was almost twofold that in humans and sixfold that in rabbits. KvLQT1 mRNA concentration was greatest in humans, and protein expression in humans was increased by approximately 2- and approximately 7-fold compared with values in rabbits and guinea pigs, respectively. The ether-a-go-go-related gene (ERG1) mRNA was more concentrated in humans, but ERG1 protein expression could not be compared across species because of epitope sequence differences. We conclude that important interspecies differences in cardiac K+ channel subunit expression exist and may contribute to the following: 1) lack of a transient outward current in the guinea pig (alpha-subunit transcription absent in the guinea pig heart); 2) small slow delayed rectifier current and torsades de pointes susceptibility in the rabbit (low-level minK expression); and 3) large slow component of the delayed rectifier current in the guinea pig (strong minK expression).

Kv1.5 is an important component of repolarizing K+ current in canine atrial myocytes

Although the canine atrium has proven useful in several experimental models of atrial fibrillation and for studying the effects of rapid atrial pacing on atrial electrical remodeling, it may not fully represent the human condition because of reported differences in functional ionic currents and ion channel subunit expression. In this study, we reassessed the molecular components underlying one current, the ultrarapid delayed rectifier current in canine atrium [IKur(d)], by evaluating the mRNA, protein, immunofluorescence, and currents of the candidate channels. Using reverse transcriptase-polymerase chain reaction, we found that Kv1.5 mRNA was expressed in canine atrium whereas message for Kv3.1 was not detected. Western analysis on cytosolic and membrane fractions of canine tissues, using selective antibodies, showed that Kv3.1 was only detectable in the brain preparations, whereas Kv1.5 was expressed at high levels in both atrial and ventricular membrane fractions. Confocal imaging performed on isolated canine atrial myocytes clearly demonstrated the presence of Kv1.5 immunostaining, whereas that of Kv3.1 was equivocal. Voltage- and current-clamp studies showed that 0.5 mmol/L tetraethylammonium had variable effects on sustained K+ currents, whereas a compound with demonstrated selectivity for hKv1.5 versus Kv3.1, hERG or the sodium channel, fully suppressed canine atrial IKur tail currents and depressed sustained outward K+ current. This agent also increased action potential plateau potentials and action potential duration at 20% and 50% repolarization. These results suggest that in canine atria, as in other species including human, Kv1.5 protein is highly expressed and contributes to IKur.

Cellular electrophysiology of canine pulmonary vein cardiomyocytes: action potential and ionic current properties

Pulmonary vein (PV) cardiomyocytes play an important role in atrial fibrillation; however, little is known about their specific cellular electrophysiological properties. We applied standard microelectrode recording and whole-cell patch-clamp to evaluate action potentials and ionic currents in canine PVs and left atrium (LA) free wall. Resting membrane potential (RMP) averaged -66 +/- 1 mV in PVs and -74 +/- 1 mV in LA (P < 0.0001) and action potential amplitude averaged 76 +/- 2 mV in PVs vs. 95 +/- 2 mV in LA (P < 0.0001). PVs had smaller maximum phase 0 upstroke velocity (Vmax: 98 +/- 9 vs. 259 +/- 16 V s(-1), P < 0.0001) and action potential duration (APD): e.g. at 2 Hz, APD to 90% repolarization in PVs was 84 % of LA (P < 0.05). Na+ current density under voltage-clamp conditions was similar in PV and LA, suggesting that smaller Vmax in PVs was due to reduced RMP. Inward rectifier current density in the PV cardiomyocytes was approximately 58% that in the LA, potentially accounting for the less negative RMP in PVs. Slow and rapid delayed rectifier currents were greater in the PV (by approximately 60 and approximately 50 %, respectively), whereas transient outward K+ current and L-type Ca2+ current were significantly smaller (by approximately 25 and approximately 30%, respectively). Na(+)-Ca(2+)-exchange (NCX) current and T-type Ca2+ current were not significantly different. In conclusion, PV cardiomyocytes have a discrete distribution of transmembrane ion currents associated with specific action potential properties, with potential implications for understanding PV electrical activity in cardiac arrhythmias.

Accumulation of slowly activating delayed rectifier potassium current (IKs) in canine ventricular myocytes

In guinea-pig ventricular myocytes, in which the deactivation of slowly activating delayed rectifier potassium current (IKs) is slow, IKs can be increased by rapid pacing as a result of incomplete deactivation and subsequent current accumulation. Whether accumulation of IKs occurs in dogs, in which the deactivation is much faster, is still unclear. In this study the conditions under which accumulation occurs in canine ventricular myocytes were studied with regard to its physiological relevance in controlling action potential duration (APD). At baseline, square pulse voltage clamp experiments revealed that the accumulation of canine IKs could occur, but only at rather short interpulse intervals (< 100 ms). With action potential (AP) clamp commands of constant duration (originally recorded at rate of 2 Hz), an accumulation was only found at interpulse intervals close to 0 ms. Transmembrane potential recordings with high-resistance microelectrodes revealed, however, that at the fastest stimulation rates with normally captured APs (5 Hz) the interpulse interval exceeded 50 ms. This suggested that no IKs accumulation occurs, which was supported by the lack of effect of an IKs blocker, HMR 1556 (500 nM), on APD. In the presence of the beta-adrenergic receptor agonist isoproterenol (isoprenaline, 100 nM) the accumulation with AP clamp commands of constant duration was much more pronounced and a significant accumulating current was found at a relevant interpulse interval of 100 ms. HMR 1556 prolonged APD, but this lengthening was reverse rate dependent. AP clamp experiments in a physiologically relevant setting (short, high rate APs delivered at a corresponding rate) revealed a limited accumulation of IKs in the presence of isoproterenol. In conclusion, a physiologically relevant accumulation of IKs was only observed in the presence of isoproterenol. Block of IKs, however, led to a reverse rate-dependent prolongation of APD indicating that IKs does not have a dominant role at short cycle lengths.

MinK-related peptide 2 modulates Kv2.1 and Kv3.1 potassium channels in mammalian brain

Delayed rectifier potassium current diversity and regulation are essential for signal processing and integration in neuronal circuits. Here, we investigated a neuronal role for MinK-related peptides (MiRPs), membrane-spanning modulatory subunits that generate phenotypic diversity in cardiac potassium channels. Native coimmunoprecipitation from rat brain membranes identified two novel potassium channel complexes, MiRP2-Kv2.1 and MiRP2-Kv3.1b. MiRP2 reduces the current density of both channels, slows Kv3.1b activation, and slows both activation and deactivation of Kv2.1. Altering native MiRP2 expression levels by RNAi gene silencing or cDNA transfection toggles the magnitude and kinetics of endogenous delayed rectifier currents in PC12 cells and hippocampal neurons. Computer simulations predict that the slower gating of Kv3.1b in complexes with MiRP2 will broaden action potentials and lower sustainable firing frequency. Thus, MiRP2, unlike other known neuronal beta subunits, provides a mechanism for influence over multiple delayed rectifier potassium currents in mammalian CNS via modulation of alpha subunits from structurally and kinetically distinct subfamilies.

Global repolarization sequence of the right atrium: monphasic action potential mapping in health pigs

The aim of the study was to explore the global sequence of atrial repolarization and its correlation to that of activation. Endocardial monophasic action potentials (MAPs) were sequentially recorded from 51 +/- 14 sites in the right atrium of ten healthy pigs using the CARTO electroanatomic mapping system. Local activation time (AT), MAP duration, and 90% repolarization time (RT) were obtained, and from these data, color coded three-dimensional maps of AT and RT sequences and spatial distribution of MAP duration were reconstructed. The results of the study were: (1) An activation sequence was recognizable in all maps, starting from the posterosuperior wall and ending in the posteroinferior wall near the tricuspid annulus. (2) The repolarization sequence was also recognizable in all maps, and mainly followed the sequence of activation. (3) A significant positive correlation between the RT and AT was observed in all maps with an average r value being 0.571 +/- 0.159 (P < 0.01 - 0.0001), suggesting that progressively later AT associates with progressively longer RT. (4) No consistent correlation between the MAP duration and AT was found. In conclusion, repolarization gradients exist over the atrial endocardium in healthy pigs. The repolarization sequence follows the same sequence as the activation, suggesting that the spatiotemporal pattern of activation is an important determinant of the characteristics of the repolarization sequence.

Hallmarks of ion channel gene expression in end-stage heart failure

Electrical conductance is greatly altered in end-stage heart failure, but little is known about the underlying events. We therefore investigated the expression of genes coding for major inward and outward ion channels, calcium binding proteins, ion receptors, ion exchangers, calcium ATPases, and calcium/calmodulin-dependent protein kinases in explanted hearts (n=13) of patients diagnosed with end-stage heart failure. With the exception of Kv11.1 and Kir3.1 and when compared with healthy controls, major sodium, potassium, and calcium ion channels, ion transporters, and exchangers were significantly repressed, but expression of Kv7.1, HCN4, troponin C and I, SERCA1, and phospholamban was elevated. Hierarchical gene cluster analysis provided novel insight into regulated gene networks. Significant induction of the transcriptional repressor m-Bop and the translational repressor NAT1 coincided with repressed cardiac gene expression. The statistically significant negative correlation between repressors and ion channels points to a mechanism of disease. We observed coregulation of ion channels and the androgen receptor and propose a role for this receptor in ion channel regulation. Overall, the reversal of repressed ion channel gene expression in patients with implanted assist devices exemplifies the complex interactions between pressure load/stretch force and heart-specific gene expression.

Signals for death and differentiation: a two-step mechanism for in vitro transformation of adult islets of Langerhans to duct epithelial structures

Phenotypic change of adult pancreatic islets has been implicated in the development of certain pancreatic cancers and in islet transplant failure. The aim of this study was to characterize intracellular events that mediate changes in adult islet phenotype. Using an in vitro islet-to-duct transformation model, canine islets were induced to undergo phenotypic transformation to duct-like epithelial structures through a two-stage process. Stage one was characterized by widespread islet cell apoptosis associated with the formation of cavitary spaces within the islets. During this stage, c-Jun N-terminal regulated kinase (JNK) and caspase-3 activities were elevated, while extracellular signal-regulated kinase (ERK) and Akt activities were decreased. The second stage of the process was characterized by an inversion in the balance in activity between these signal transduction pathways and by a concomitant decrease in apoptosis. The transformed islets were no longer immunoreactive for islet cell hormones, but expressed the duct epithelial cell marker CK-AE1/AE3. In contrast to islet cells, these duct epithelial cells were highly proliferative. To clarify the role of the identified changes in signal transduction events, we performed additional studies using pharmacological inhibitors of enzyme activity and demonstrated that inhibition of JNK and caspase-3 activity prevented cystic transformation. Our results indicate that the balance in signaling activity between ERK/Akt and JNK/caspase-3 appears to be an important regulator of islet cell death and differentiation.

Oxidant preconditioning protects human proximal tubular cells against lethal oxidant injury via p38 MAPK and heme oxygenase-1

Ischemic preconditioning protects the kidney from subsequent ischemic injury but the signal transduction pathways involved are unknown. Human proximal tubular (HK-2) cells were protected from injury with 2.5 mM H(2)O(2) by preconditioning with a single 15-min exposure to 500 microM H(2)O(2) followed by 16 h of recovery (oxidant preconditioning). To identify the signaling pathways involved in oxidant preconditioning, we utilized inhibitors of several signaling intermediates (MAPK/ERK kinase I, p38 mitogen-activated protein kinase (MAPK), protein kinase C and tyrosine kinase). A rapid but transient increase in p38 MAPK was observed following oxidant preconditioning and an inhibitor of p38 MAPK (SB203580) abolished the protection provided by oxidant preconditioning. Oxidant preconditioning was also associated with heat shock protein-27 phosphorylation (by p38 MAPK) and an increased synthesis of heme oxygenase-1 (HO-1). Stimulation or inhibition of HO-1 with hemin or Zn(II) protoporphyrin IX, respectively, mimicked or abolished oxidant preconditioning-mediated cytoprotection. Inhibitors of new protein synthesis (cycloheximide) and gene transcription (actinomycin D) also blocked the cytoprotection by oxidant preconditioning. We conclude that oxidant preconditioning protects HK-2 cells against more severe oxidant injury via activation of signaling pathways that include p38 MAPK and increased synthesis of HO-1.

Recent advances in drug therapy for atrial fibrillation

Despite the major new insights into our knowledge of the mechanisms underlying initiation and perpetuation of atrial fibrillation (AF) gained in the last decade, the treatment of this common arrhythmia remains unsatisfactory in many patients. Although several new treatment modalities (e.g., internal cardioversion, pulmonary vein ablation, preventive pacing) have been developed, pharmacologic therapy remains the first-line therapy in most patients with AF. As illustrated by recent trials comparing rhythm control and rate control, current antifibrillatory drugs are hampered by a relatively low success rate in maintaining long-term sinus rhythm and the occurrence of proarrhythmic and other adverse events. This article discusses currently available antiarrhythmic drugs for rhythm and rate control, with special emphasis on more recently developed drugs and drugs still under development. Selective blockers of atrial ion channels (IKur and IK.ACh), multi-ion channel blockers, and selective A1-adenosine receptor antagonists are examples of the newer antiarrhythmic drugs that are expected to be more effective and safer than those currently available.

Decrease in activity of smooth muscle L-type Ca2+ channels and its reversal by NF-kappaB inhibitors in Crohn's colitis model

We investigated the mechanisms of dysmotility of the colonic circular muscle of the Crohn's disease rat model. Contractions induced by KCl, carbachol, and Bay K 8644 were decreased in circular smooth muscles isolated from 2,4,6-trinitrobenzenesulfonic acid (TNBS)-induced colitis rat colon. However, the absolute force and Ca2+ sensitivity of contractile proteins were not affected as assessed in alpha-toxin permeabilized smooth muscle. The current density of the L-type Ca2+ channel in circular smooth muscle cells was significantly decreased in the TNBS-treated colonic cells. However, expressions of the L-type Ca2+ channel mRNA and protein did not differ between control and TNBS-treated preparations. Pretreatment with the NF-kappaB inhibitors pyrrolidinedithiocarbamate and sulfasalazine partially recovered the decreased contractility and current density of the L-type Ca2+ channel by TNBS treatment. These results suggest that the decrease in the contraction of circular smooth muscle isolated from TNBS-induced colitis rat colon, which may be related to gut dysmotility in Crohn's disease, is attributable to the decreased activity of the L-type Ca2+ channel. The dysfunction of the L-type Ca2+ channel may be mediated by NF-kappaB-dependent pathways.

Thyroid hormone regulates mRNA expression and currents of ion channels in rat atrium

Atrial fibrillation is one of the common arrhythmias associated with hyperthyroidism. This study examined the effects of thyroid hormone (T3) on mRNA expression and currents of major ionic channels determining the action potential duration (APD) in the rat atrium using the RNase protection assay and the whole-cell patch-clamp technique, respectively. T3 increased the Kv1.5 mRNA expression and decreased the L-type calcium channel mRNA expression, while the Kv4.2 mRNA expression did not change. APD was shorter in hyperthyroid than in euthyroid myocytes. The ultrarapid delayed rectifier potassium currents were remarkably increased in hyperthyroid than in euthyroid myocytes, whereas the transient outward potassium currents were unchanged. L-type calcium currents were decreased in hyperthyroid than in euthyroid myocytes. T3 shifted the current-voltage relationship for calcium currents negatively. In conclusion, T3 increased the outward currents and decreased the inward currents. The resultant changes of ionic currents shortened APD, providing a substrate for atrial fibrillation.

Acute effects of cigarette smoking on the QT interval in healthy smokers

This study assesses the effect of a first cigarette smoked in the morning on the QT interval duration in healthy habitual smokers after not smoking overnight. This study demonstrates that although after cigarette smoking the QT(Bc) values increase, the QT(Fc) and QT(Lc) values remained unchanged, and an increased sympathetic response was evident. The discrepancies between the results seen by different QT correction formulas emphasize the importance of selecting an accurate method for heart rate correction when evaluating and interpreting alterations in QT values.

Canine ventricular KCNE2 expression resides predominantly in Purkinje fibers

Mutations in minK-related peptide 1 (MiRP1), the product of the KCNE2 gene, have been associated with malignant ventricular arrhythmia syndromes related to impaired repolarization. MiRP1 interacts with a variety of ion-channel alpha-subunits, dysfunction of which could account for arrhythmia syndromes; however, the observation of very low-level expression of MiRP1 in ventricular tissue has led to doubts about its relevance. The specialized His-Purkinje system plays a key role in cardiac electrophysiology and is an important contributor to ventricular arrhythmias related to abnormal repolarization. We examined the relative abundance of MiRP1 in canine Purkinje versus ventricular tissue and found much greater expression at both mRNA and protein levels in Purkinje tissue. Thus, the cardiac Purkinje system is a strong candidate to play a role in arrhythmic syndromes due to MiRP1 abnormalities.

Beta-adrenoceptor activation plays a role in the reverse rate-dependency of effective refractory period lengthening by dofetilide in the guinea-pig atrium, in vitro

1. Blockers of the rapid component of the delayed rectifier potassium current (I(Kr)) prolong cardiac action potential duration (APD) and effective refractory period (ERP) in a reverse rate-dependent manner. Since activation of beta-adrenoceptors attenuates prolongation of APD evoked by I(Kr) blockers, rate-dependent neuronal noradrenaline liberation in the myocardium may contribute to the reverse rate-dependent nature of the effects of I(Kr) blockers. In order to test this hypothesis, we studied the effects of dofetilide, a pure I(Kr) blocker, on ERP after activation or blockade of beta-adrenoceptors and after catecholamine depletion in guinea-pig left atrial myocardium paced at 3, 2 and 1 Hz, in vitro. 2. Dofetilide (100 nM) lengthened ERP in a reverse rate-dependent manner in the left atrial myocardium of guinea-pigs. Strong activation of beta-adrenoceptors using 10 nM isoproterenol abolished the dofetilide-induced lengthening of ERP at all pacing rates. 3. Blockade of the beta-adrenoceptors with metoprolol (1 micro M), atenolol (3 micro M) or propranolol (300 nM) increased the dofetilide-evoked prolongation of ERP at 3 and 2 Hz, but not at 1 Hz. As a consequence, metoprolol attenuated while propranolol and atenolol fully eliminated the reverse rate-dependent nature of the dofetilide-induced ERP lengthening. In catecholamine-depleted atrial preparations of the guinea-pig (24 h pretreatment with 5 mg kg(-1) reserpine i.p.), the effect of dofetilide on ERP was not frequency dependent, and propranolol did not alter the effects of dofetilide. 4. In contrast to results obtained in guinea-pig atrial preparations, propranolol failed to change the reverse rate-dependent effect of dofetilide on ERP in the right ventricular papillary muscles of rabbits and guinea-pigs. 5. As an indication of the functional consequences of rate-dependent noradrenaline liberation, propranolol decreased twitch tension at 3 and 2 Hz but not at 1 Hz in the atrial myocardium of control guinea-pigs, whereas no such effect was detected in catecholamine-depleted atrial preparations. Propranolol failed to change contractility of ventricular myocardium in guinea-pigs and rabbits. 6. It is concluded that rate-dependent noradrenaline release and the ensuing beta-adrenoceptor activation contributed to the reverse rate-dependent nature of ERP prolongation caused by I(Kr) blockers in isolated guinea-pig atrial myocardium.

Antiarrhythmic effects of JTV-519, a novel cardioprotective drug, on atrial fibrillation/flutter in a canine sterile pericarditis model

Effect of JTV-519 on AF.

INTRODUCTION

A new cardioprotective drug, JTV-519, blocks Na+ current and inwardly rectifying K+ current and inhibits Ca2+ current. However, its role in atrial electrophysiology is unknown. We investigated the antiarrhythmic effects of JTV-519 on atrial fibrillation/flutter in the canine sterile pericarditis model.

METHODS AND RESULTS

In nine dogs with sterile pericarditis, 38 episodes of sustained (>30 sec) atrial fibrillation (8 dogs) and 24 episodes of sustained atrial flutter (7 dogs) were induced by rapid atrial pacing. When atrial fibrillation or atrial flutter was sustained >15 minutes, it was cardioverted and reinduced. The inducibility of atrial fibrillation/flutter, the atrial effective refractory period, and the intra-atrial conduction time were compared before and after the continuous infusion of JTV-519 (0.03 mg/kg/min). JTV-519 significantly decreased the mean number of sustained atrial fibrillation episodes (from 4.2 +/- 2.9 to 0 +/- 0, P < 0.01). In contrast, atrial flutter was still inducible in 4 dogs after JTV-519 (from 2.7 +/- 2.5 to 1.6 +/- 2.1, P = NS). JTV-519 significantly prolonged effective refractory period (from 123 +/- 18 to 143 +/- 14 msec, from 127 +/- 18 to 151 +/- 12 msec, and from 132 +/- 13 to 159 +/- 9 msec at basic cycle lengths of 200, 300, and 400 msec, respectively, P < 0.01), but it did not affect the intra-atrial conduction time (from 47 +/- 11 msec to 48 +/- 11 msec, P = NS).

CONCLUSION

JTV-519 had significant protective effects on atrial fibrillation in the canine sterile pericarditis model, mainly by increasing effective refractory period, suggesting that it may have potential as a novel antiarrhythmic agent for atrial fibrillation.

Effects of U50,488H on transient outward and ultra-rapid delayed rectifier K+ currents in young human atrial myocytes

The effects of trans-(+/-)-3,4-dichloro-N-methyl-N-[2-(1-pyrrolidinyl)-cyclohexyl]-benzeneacetamide methanesulfonate salt (U50,488H), a selective kappa-opioid receptor agonist, on transient outward K+ current (Ito1) and ultra-rapid delayed rectifier K+ current (IKur) in young human atrial myocytes were evaluated with a whole-cell patch-clamp technique. At +10 mV, U50,488H decreased Ito1 in a concentration-dependent manner (IC50=12.4+/-3.5 microM), while at +50 mV, U50,488H produced biphasic effects on Ito1-increasing and decreasing the current at 1-3 and 10-30 microM, respectively. U50,488H at 10 microM shifted the midpoint (V0.5) of Ito1 activation in a depolarizing direction by approximately 5 mV, accelerated the inactivation, and slowed the recovery from inactivation of Ito1. In addition, U50,488H inhibited IKur in a concentration-dependent manner (IC50=3.3+/-0.6 microM). The effects of U50,488H on the two types of K+ currents were not antagonized by either 5 microM nor-binaltorphimine or 300 nM naloxone. These results indicate that U50,488H affects both Ito1 and IKur in young human atrial myocytes in an opioid receptor-independent manner.

Involvement of MEK/ERK pathway in cephaloridine-induced injury in rat renal cortical slices

We have previously reported that free radical-mediated injury induced by cephaloridine (CER) is enhanced by phorbol 12-myristate 13-acetate (PMA), a protein kinase C (PKC) activator, in rat renal cortical slices. We have also shown that PKC activation in mitochondria is involved in CER-induced nephrotoxicity in rats. We investigated the role of a downstream PKC pathway, a MEK/ERK pathway, in free radical-induced injury in rat renal cortical slices exposed to CER. Immediately after preparing slices from rat renal cortex, the slices were incubated in the medium containing MEK inhibitors. ERK1/2 activation was determined by Western blot analysis for phosphorylated ERK (pERK) 1/2 protein in nucleus fraction prepared from the slices exposed to CER. Prominently, CER caused not only increases in lipid peroxidation as an index of free radical generation and in LDH leakage as that of cell injury in the slices, but also marked activation of ERK1/2 in nucleus fraction. PD98059 and U0126, MEK1/2 inhibitors, significantly attenuated CER-induced increases in lipid peroxidation and LDH leakage in the slices. PD98059 also suppressed ERK1/2 activation in nucleus fraction prepared from the slices treated with CER. Inhibition of other MAP kinase pathways, p38 MAP kinase and c-Jun N-terminal kinase (JNK) had no effect on CER-induced increases in lipid peroxidation level and LDH leakage in the slices. The present results suggest that a MEK/ERK pathway down stream of a PKC pathway is probably involved in free radical-induced injury in rat renal cortical slices exposed to CER.

High resolution mapping of the pulmonary vein and the vein of Marshall during induced atrial fibrillation and atrial tachycardia in a canine model of pacing-induced congestive heart failure

OBJECTIVES

The study examined the activations in the pulmonary veins (PVs) and the vein of Marshall (VOM) during atrial fibrillation (AF) in dogs with congestive heart failure (CHF).

BACKGROUND

The patterns of activation within the PVs and the VOM during AF in CHF are unclear.

METHODS

We induced CHF in nine dogs by rapid ventricular pacing. The patterns of activation during induced AF were studied one week after ceasing ventricular pacing.

RESULTS

The duration of induced AF averaged 80.7 +/- 177.3 s. The termination of low-amplitude fractionated activity in the PVs preceded the termination of AF in 25 of 29 episodes. High-density mapping (1-mm resolution) showed that the PV was activated by a focal wave front independent of left atrial (LA) activation in 22 AF episodes. Frequent intra-PV conduction blocks and multiple wave fronts in the PVs were recorded during 10 AF episodes. Focal activations were observed within the VOM in 4 of 12 episodes of AF. Three atrial tachycardia (AT) episodes originated from a focus within a PV. Histological studies showed extensive fibrosis in the PVs and in the atria. The PVs in five normal dogs did not have focal or fractionated activity during induced AF.

CONCLUSIONS

Atrial fibrillation in canine CHF is associated with independent focal activations in the PVs and the VOM, and with complex wave fronts within the PVs. The PVs may also serve as the origin of AT. These findings suggest that electrical and anatomical remodeling of the PVs and the VOM are important in the maintenance of AF and AT in dogs with CHF.

Fibrosis of the left atria during progression of heart failure is associated with increased matrix metalloproteinases in the rat

OBJECTIVES

The purpose of this study was to determine the pathogenic factors and molecular mechanisms involved in fibrosis of the atria.

BACKGROUND

Fibrosis is an important component of the pathophysiology of atrial fibrillation, especially when the arrhythmia is associated with heart failure (HF) or atrial dilation.

METHODS

We used a rat model of myocardial infarction (MI) complicated by various degrees of left ventricular dysfunction and atrial dilation to study fibrosis and matrix metalloproteinase (MMP) activity in the left atrial (LA) myocardium by means of histologic, Western blot, zymographic, and immunohistologic techniques.

RESULTS

Three months after surgical ligature of the left coronary artery, 27 rats had a large MI, 12 were in mild HF, and 15 in severe HF. Both groups had LA enlargement at the echocardiography. Masson's trichrome and picrosirius staining of tissue sections revealed marked fibrosis at the periphery of trabeculae and also surrounding myolytic myocytes, in both mild and severe HF. In mild HF, the activity and expression of the matrilysin MMP-7 were increased (122%), whereas in severe HF, both MMP-7 (211%) and the gelatinase MMP-2 (187%) were up-regulated. There were no changes in the expression or activity of MMP inhibitors, TIMP-1, -2, and -4. Immunostaining of cryosections showed that MMP-2 was present in the interstitial spaces, whereas MMP-7 accumulated in myolytic myocytes.

CONCLUSIONS

Hemodynamic overload of the atria is an important pathogenic factor of fibrosis; MMP-7 appears to be involved in the early stage of this tissue remodeling process.

Molecular dissection of the inward rectifier potassium current (IK1) in rabbit cardiomyocytes: evidence for heteromeric co-assembly of Kir2.1 and Kir2.2

Cardiac inward rectifier K+ currents (IK1) play an important role in maintaining resting membrane potential and contribute to late phase repolarization. Members of the Kir2.x channel family appear to encode for IK1. The purpose of this study was to determine the molecular composition of cardiac IK1 in rabbit ventricle. Western blots revealed that Kir2.1 and Kir2.2, but not Kir2.3, are expressed in rabbit ventricle. Culturing rabbit myocytes resulted in an approximately 50% reduction of IK1 density after 48 or 72 h in culture which was associated with an 80% reduction in Kir2.1, but no change in Kir2.2, protein expression. Dominant-negative (DN) constructs of Kir2.1, Kir2.2 and Kir2.3 were generated and tested in tsA201 cells. Adenovirus-mediated over-expression of Kir2.1dn, Kir2.2dn or Kir2.1dn plus Kir2.2dn in cultured rabbit ventricular myocytes reduced IK1 density equally by 70% 72 h post-infection, while AdKir2.3dn had no effect, compared to green fluorescent protein (GFP)-infected myocytes. Previous studies indicate that the [Ba2+] required for half-maximum block (IC50) differs significantly between Kir2.1, Kir2.2 and Kir2.3 channels. The dependence of IK1 on [Ba2+] revealed a single binding isotherm which did not change with time in culture. The IC50 for block of IK1 was also unaffected by expression of the different DN genes after 72 h in culture. Taken together, these results demonstrate functional expression of Kir2.1 and Kir2.2 in rabbit ventricular myocytes and suggest that macroscopic IK1 is predominantly composed of Kir2.1 and Kir2.2 heterotetramers.

In vivo canine cardiac electrophysiologic profile of 1,4-benzodiazepine IKs blockers

Previous cardiac electrophysiologic studies of blockers of the slowly activating delayed rectifier (IKs) current have focused primarily on ventricular repolarization. This report summarizes an extensive in vivo cardiac electrophysiologic profile of four 1,4-benzodiazepine IKs blocker analogues (L-761334, L-763540, L-761710, and L-768673) in dogs. At 3.0 mg/kg intravenously, all four analogues elicited 14.5%-21.4% increases in ventricular refractoriness and 19.2%-22.6% increases in QTc interval. Concomitant 11.1%-13.5% increases in atrial refractoriness were noted with all four analogues. Decreases in sinus heart rate of 8.4%-17.3% were noted with all four compounds. No effects on atrial, His Purkinje, ventricular conduction or atrial and ventricular excitation were observed. One analogue, L-761710, significantly delayed atrioventricular (AV) nodal conduction (40.7+/-17.4% increase in atrial-to-His interval) and increased the AV conduction system functional refractory period 19.9+/-6.2%. The lack of effect of the other three 1,4-benzodiazepine IKs blockers on AV nodal function at dosages producing comparable effects on atrial and ventricular refractoriness suggest that the AV nodal effects of L-761710 were unrelated to IKs blockade. These findings indicate IKs plays important roles in both atrial and ventricular refractoriness as well as pacemaker function in the dog heart, suggesting potential utility for IKs blockers in the treatment of atrial and ventricular arrhythmias.