Correction: Experimentally-Based Computational Investigation into Beat-To-Beat Variability in Ventricular Repolarization and Its Response to Ionic Current Inhibition.

[This corrects the article DOI: 10.1371/journal.pone.0151461.].

Effect of the intracellular calcium concentration chelator BAPTA acetoxy-methylester on action potential duration in canine ventricular myocytes.

Intracellular calcium concentration ([Ca2+]i) is often buffered by using the cell-permeant acetoxy-methylester form of the Ca2+ chelator BAPTA (BAPTA-AM) under experimental conditions. This study was designed to investigate the time-dependent actions of extracellularly applied BAPTA-AM on action potential duration (APD) in cardiac cells. Action potentials were recorded from enzymatically isolated canine ventricular myocytes with conventional sharp microelectrodes. The effect of BAPTA-AM on the rapid delayed rectifier K+ current (IKr) was studied using conventional voltage clamp and action potential voltage clamp techniques. APD was lengthened by 5 µM BAPTA-AM - but not by BAPTA - and shortened by the Ca2+ ionophore A23187 in a time-dependent manner. The APD-lengthening effect of BAPTA-AM was strongly suppressed in the presence of nisoldipine, and enhanced in the presence of BAY K8644, suggesting that a shift in the [Ca2+]i-dependent inactivation of L-type Ca2+ current may be an important underlying mechanism. However, in the presence of the IKr-blocker dofetilide or E-4031 APD was shortened rather than lengthened by BAPTA-AM. Similarly, the APD-lengthening effect of 100 nM dofetilide was halved by the pretreatment with BAPTA-AM. In line with these results, IKr was significantly reduced by extracellularly applied BAPTA-AM under both conventional voltage clamp and action potential voltage clamp conditions. This inhibition of IKr was partially reversible and was not related to the Ca2+ chelator effect BAPTA-AM. The possible mechanisms involved in the APD-modifying effects of BAPTA-AM are discussed. It is concluded that BAPTA-AM has to be applied carefully to control [Ca2+]i in whole cell systems because of its direct inhibitory action on IKr.

Dose-dependent electrophysiological effects of the myosin activator omecamtiv mecarbil in canine ventricular cardiomyocytes.

Omecamtiv mecarbil (OM) is a myosin activator agent recently developed for treatment of heart failure. Although its action on extending systolic ejection time and increasing left ventricular ejection fraction is well documented, no data is available regarding its possible side-effects on cardiac ion channels. Therefore, the present study was designed to investigate the effects of OM on action potential morphology and the underlying ion currents in isolated canine ventricular myocytes using sharp microelectrodes, conventional patch clamp, and action potential voltage clamp techniques. OM displayed a concentration-dependent action on action potential configuration: 1 µM OM had no effect, while action potential duration and phase-1 repolarization were reduced and the plateau potential was depressed progressively at higher concentrations (10 - 100 µM; P < 0.05 compared to control). Accordingly, OM (10 µM) decreased the density of the transient outward K+ current (Ito), the L-type Ca2+ current (ICa) and the rapid delayed rectifier K+ current (IKr), but failed to modify the inward rectifier K+ current (IK1). It is concluded, that although the therapeutic concentrations of OM are not likely to influence cardiac ion currents significantly, alterations of the major cardiac ion currents can be anticipated at concentrations above those clinically tolerated.

Experimentally-Based Computational Investigation into Beat-To-Beat Variability in Ventricular Repolarization and Its Response to Ionic Current Inhibition.

Beat-to-beat variability in repolarization (BVR) has been proposed as an arrhythmic risk marker for disease and pharmacological action. The mechanisms are unclear but BVR is thought to be a cell level manifestation of ion channel stochasticity, modulated by cell-to-cell differences in ionic conductances. In this study, we describe the construction of an experimentally-calibrated set of stochastic cardiac cell models that captures both BVR and cell-to-cell differences in BVR displayed in isolated canine action potential measurements using pharmacological agents. Simulated and experimental ranges of BVR are compared in control and under pharmacological inhibition, and the key ionic currents determining BVR under physiological and pharmacological conditions are identified. Results show that the 4-aminopyridine-sensitive transient outward potassium current, Ito1, is a fundamental driver of BVR in control and upon complete inhibition of the slow delayed rectifier potassium current, IKs. In contrast, IKs and the L-type calcium current, ICaL, become the major contributors to BVR upon inhibition of the fast delayed rectifier potassium current, IKr. This highlights both IKs and Ito1 as key contributors to repolarization reserve. Partial correlation analysis identifies the distribution of Ito1 channel numbers as an important independent determinant of the magnitude of BVR and drug-induced change in BVR in control and under pharmacological inhibition of ionic currents. Distributions in the number of IKs and ICaL channels only become independent determinants of the magnitude of BVR upon complete inhibition of IKr. These findings provide quantitative insights into the ionic causes of BVR as a marker for repolarization reserve, both under control condition and pharmacological inhibition.

Changes in intracellular calcium concentration influence beat-to-beat variability of action potential duration in canine ventricular myocytes.

The aim of the present work was to study the influence of changes in intracellular calcium concentration ([Ca(2+)]i) on beat-to-beat variability (short term variability, SV) of action potential duration (APD) in isolated canine ventricular cardiomyocytes. Series of action potentials were recorded from enzymatically isolated canine ventricular cells using conventional microelectrode technique. Drug effects on SV were evaluated as relative SV changes determined by plotting the drug-induced changes in SV against corresponding changes in APD and comparing these data to the exponential SV-APD function obtained with inward and outward current injections. Exposure of myocytes to the Ca(2+) chelator BAPTA-AM (5 µM) decreased, while Ca(2+) ionophore A23187 (1 µM) increased the magnitude of relative SV. Both effects were primarily due to the concomitant changes in APD. Relative SV was reduced by BAPTA-AM under various experimental conditions including pretreatment with veratridine, BAY K8644, dofetilide or E-4031. Contribution of transient changes of [Ca(2+)]i due to Ca(2+) released from the sarcoplasmic reticulum (SR) was studied using 10 µM ryanodine and 1 µM cyclopiazonic acid: relative SV was reduced by both agents. Inhibition of the Na(+)-Ca(2+) exchanger by 1 µM SEA0400 increased relative SV. It is concluded that elevation of [Ca(2+)]i increases relative SV significantly. More importantly, Ca(2+) released from the SR is an important component of this effect.

An emerging antiarrhythmic target: late sodium current.

The cardiac late sodium current (INa,L) has been in the focus of research in the recent decade. The first reports on the sustained component of voltage activated sodium current date back to the seventies, but early studies interpreted this tiny current as a product of a few channels that fail to inactivate, having neither physiologic nor pathologic implications. Recently, the cardiac INa,L has emerged as a potentially major arrhythmogenic mechanism in various heart diseases, attracting the attention of clinicians and researchers. Research activity on INa,L has exponentially increased since Ranolazine, an FDA-approved antianginal drug was shown to successfully suppress cardiac arrhythmias by inhibiting INa,L. This review aims to summarize and discuss a series of papers focusing on the cardiac late sodium current and its regulation under physiological and pathological conditions. We will discuss critical evidences implicating INa,L as a potential target for treating myocardial dysfunction and cardiac arrhythmias.

Tetrodotoxin blocks native cardiac L-type calcium channels but not CaV1.2 channels expressed in HEK cells.

Tetrodotoxin (TTX) has been believed for a long time to be a selective inhibitor of voltage-gated fast Na(+) channels in excitable tissues, including mammalian myocardium. Recently TTX has been shown to block cardiac L-type Ca(2+) current (ICa,L). Furthermore, this inhibition was ascribed to binding of TTX to the outer pore of the Ca(2+) channel, contributing to the selectivity filter region. In this study the TTX-sensitivity of Cav1.2 channels, expressed in HEK cells, was tested using the whole cell version of the patch clamp technique and compared to the TTX-sensitivity of native canine ICa,L. Cav1.2 channels mediate Ca(2+) current in ventricular myocardium of various mammalian species. Surprisingly, TTX failed to inhibit Cav1.2 current up to the concentration of 100 µM - in contrast to ICa,L - in spite of the fact that the kinetic properties of the ICa,L and Cav1.2 currents were similar. The possible reasons for this discrepancy are discussed. Present results may question the suitability of a single pore-forming channel subunit, expressed in a transfection system, for electrophysiological or pharmacological studies.

Role of action potential configuration and the contribution of C²âºa and K⁺ currents to isoprenaline-induced changes in canine ventricular cells.

BACKGROUND AND PURPOSE: Although isoprenaline (ISO) is known to activate several ion currents in mammalian myocardium, little is known about the role of action potential morphology in the ISO-induced changes in ion currents. Therefore, the effects of ISO on action potential configuration, L-type Ca²âº current (I(Ca)), slow delayed rectifier K⁺ current (I(Ks)) and fast delayed rectifier K⁺ current (I(Kr)) were studied and compared in a frequency-dependent manner using canine isolated ventricular myocytes from various transmural locations. EXPERIMENTAL APPROACH: Action potentials were recorded with conventional sharp microelectrodes; ion currents were measured using conventional and action potential voltage clamp techniques. KEY RESULTS: In myocytes displaying a spike-and-dome action potential configuration (epicardial and midmyocardial cells), ISO caused reversible shortening of action potentials accompanied by elevation of the plateau. ISO-induced action potential shortening was absent in endocardial cells and in myocytes pretreated with 4-aminopyridine. Application of the I(Kr) blocker E-4031 failed to modify the ISO effect, while action potentials were lengthened by ISO in the presence of the I(Ks) blocker HMR-1556. Both action potential shortening and elevation of the plateau were prevented by pretreatment with the I(Ca) blocker nisoldipine. Action potential voltage clamp experiments revealed a prominent slowly inactivating I(Ca) followed by a rise in I(Ks) , both currents increased with increasing the cycle length. CONCLUSIONS AND IMPLICATIONS: The effect of ISO in canine ventricular cells depends critically on action potential configuration, and the ISO-induced activation of I(Ks) - but not I(Kr) - may be responsible for the observed shortening of action potentials.

Interaction between Ca(2+) channel blockers and isoproterenol on L-type Ca(2+) current in canine ventricular cardiomyocytes.

AIM: The aim of this work was to study antagonistic interactions between the effects of various types of Ca(2+) channel blockers and isoproterenol on the amplitude of L-type Ca(2+) current in canine ventricular cells. METHODS: Whole-cell version of the patch clamp technique was used to study the effect of isoproterenol on Ca(2+) current in the absence and presence of Ca(2+) channel-blocking agents, including nifedipine, nisoldipine, diltiazem, verapamil, CoCl(2) and MnCl(2) . RESULTS: Five micromolar Nifedipine, 1 µM nisoldipine, 10 µM diltiazem, 5 µM verapamil, 3 mM CoCl(2) and 5 mM MnCl(2) evoked uniformly a 90-95% blockade of Ca(2+) current in the absence of isoproterenol. Isoproterenol (100 nM) alone increased the amplitude of Ca(2+) current from 6.8 ± 1.3 to 23.7 ± 2.2 pA/pF in a reversible manner. Isoproterenol caused a marked enhancement of Ca(2+) current even in the presence of nifedipine, nisoldipine, diltiazem and verapamil, but not in the presence of CoCl(2) or MnCl(2) . CONCLUSION: The results indicate that the action of isoproterenol is different in the presence of organic and inorganic Ca(2+) channel blockers. CoCl(2) and MnCl(2) were able to fully prevent the effect of isoproterenol on Ca(2+) current, while the organic Ca(2+) channel blockers failed to do so. This has to be born in mind when the effects of organic Ca(2+) channel blockers are evaluated either experimentally or clinically under conditions of increased sympathetic tone.

Powerful technique to test selectivity of agents acting on cardiac ion channels: the action potential voltage-clamp.

Action potential voltage-clamp (APVC) is a technique to visualize the profile of various currents during the cardiac action potential. This review summarizes potential applications and limitations of APVC, the properties of the most important ion currents in nodal, atrial, and ventricular cardiomyocytes. Accordingly, the profiles ("fingerprints") of the major ion currents in canine ventricular myocytes, i.e. in cells of a species having action potential morphology and set of underlying ion currents very similar to those found in the human heart, are discussed in details. The degree of selectivity of various compounds, which is known to be a critical property of drugs used in APVC experiments, is overviewed. Thus the specificity of agents known to block sodium (tetrodotoxin, saxitoxin), potassium (chromanol 293B, HMR 1556, E-4031, dofetilide, sotalol, 4-aminopyridine, BaCl(2)), calcium (nifedipine, nisolpidine, nicardipine, diltiazem, verapamil, gallopamil), and chloride (anthracene-9-carboxylic acid, DIDS) channels, the inhibitor of the sodium-calcium exchanger (SEA0400), and the activator of sodium current (veratridine) are accordingly discussed. Based on a theory explaining how calcium current inhibitors block calcium channels, the structural comparison of the studied substances usually confirmed the results of the literature. Using these predictions, a hypothetical super-selective calcium channel inhibitor structure was designed. APVC is a valuable tool not only for studying the selectivity of the known ion channel blockers, but is also suitable for safety studies to exclude cardiac ion channel actions of any agent under development.

Modified cAMP derivatives: powerful tools in heart research.

Receptor-mediated changes in intracellular cyclic AMP concentration play critical role in the autonomic control of the heart, including regulation of a variety of ion channels via mechanisms involving protein kinase A, EPAC, or direct actions on cyclic nucleotide gated ion channels. In case of any ion channel, the actual signal transduction cascade can be identified by using properly modified cAMP derivatives with altered binding and activating properties. In this study we focus to structural modifications of cAMP resulting in specific activator and blocking effects on PKA or EPAC. Involvement of the cAMP-dependent signal transduction pathway in controlling rapid delayed rectifier K(+ ) current was studied in canine ventricular myocytes using these specific cAMP analogues. Adrenergic stimulation increased the density of I(Kr) in canine ventricular cells, which effect was mediated by a PKA-dependent but EPAC-independent pathway. It was also shown that intracellular application of large concentrations of cAMP failed to fully activate PKA comparing to the effect of isoproterenol, forskolin, or PDE-resistant cAMP derivatives. This difference was fully abolished following inhibition of phosphodiesterase by IBMX. These results are in line with the concept of compartmentalized release, action, and degradation of cAMP within signalosomes.

Long term regulation of cardiac L-type calcium channel by small G proteins.

Calcium ions are crucial elements of excitation-contraction coupling in cardiac myocytes. The intracellular Ca(2+ ) concentration changes continously during the cardiac cycle, but the Ca(2+ ) entering to the cell serves as an intracellular second messenger, as well. The Ca(2+ ) as a second messenger influences the activity of many intracellular signalling pathways and regulates gene expression. In cardiac myocytes the major pathway for Ca(2+ ) entry into cells is L-type calcium channel (LTCC). The precise control of LTCC function is essential for maintaining the calcium homeostasis of cardiac myocytes. Dysregulation of LTCC may result in different diseases like cardiac hypertrophy, arrhytmias, heart failure. The physiological and pathological structural changes in the heart are induced in part by small G proteins. These proteins are involved in wide spectrum of cell biological functions including protein transport, regulation of cell proliferation, migration, apoptosis, and cytoskeletal rearrangement. Understanding the crosstalk between small G proteins and LTCC may help to understand the pathomechanism of different cardiac diseases and to develop a new generation of genetically-encoded Ca(2+ ) channel inhibitors.

Cardiac calmodulin kinase: a potential target for drug design.

Therapeutic strategy for cardiac arrhythmias has undergone a remarkable change during the last decades. Currently implantable cardioverter defibrillator therapy is considered to be the most effective therapeutic method to treat malignant arrhythmias. Some even argue that there is no room for antiarrhythmic drug therapy in the age of implantable cardioverter defibrillators. However, in clinical practice, antiarrhythmic drug therapies are frequently needed, because implantable cardioverter defibrillators are not effective in certain types of arrhythmias (i.e. premature ventricular beats or atrial fibrillation). Furthermore, given the staggering cost of device therapy, it is economically imperative to develop alternative effective treatments. Cardiac ion channels are the target of a number of current treatment strategies, but therapies based on ion channel blockers only resulted in moderate success. Furthermore, these drugs are associated with an increased risk of proarrhythmia, systemic toxicity, and increased defibrillation threshold. In many cases, certain ion channel blockers were found to increase mortality. Other drug classes such as ßblockers, angiotensin-converting enzyme inhibitors, aldosterone antagonists, and statins appear to have proven efficacy for reducing cardiac mortality. These facts forced researchers to shift the focus of their research to molecular targets that act upstream of ion channels. One of these potential targets is calcium/calmodulin-dependent kinase II (CaMKII). Several lines of evidence converge to suggest that CaMKII inhibition may provide an effective treatment strategy for heart diseases. (1) Recent studies have elucidated that CaMKII plays a key role in modulating cardiac function and regulating hypertrophy development. (2) CaMKII activity has been found elevated in the failing hearts from human patients and animal models. (3) Inhibition of CaMKII activity has been shown to mitigate hypertrophy, prevent functional remodeling and reduce arrhythmogenic activity. In this review, we will discuss the structural and functional properties of CaMKII, the modes of its activation and the functional consequences of CaMKII activity on ion channels.

Can the electrophysiological action of rosiglitazone explain its cardiac side effects?

Recent large clinical trials found an association between the antidiabetic drug rosiglitazone therapy and increased risk of cardiovascular adverse events. The aim of this report is to elucidate the cardiac electrophysiological properties of rosiglitazone (R) on isolated rat and murine ventricular papillary muscle cells and canine ventricular myocytes using conventional microelectrode, whole cell voltage clamp, and action potential (AP) voltage clamp techniques. In histidine-decarboxylase knockout mice as well as in their wild types R (1-30 µM) shortened AP duration at 90% level of repolarization (APD(90)) and increased the AP amplitude (APA) in a concentration-dependent manner. In rat ventricular papillary muscle cells R (1-30 µM) caused a significant reduction of APA and maximum velocity of depolarization (V(max)) which was accompanied by lengthening of APD(90). In single canine ventricular myocytes at concentrations ≥10 µM R decreased the amplitude of phase-1 repolarization, the plateau potential and reduced V(max). R suppressed several ion currents in a concentration-dependent manner under voltage clamp conditions. The EC(50) value for this inhibition was 25.2±2.7 µM for the transient outward K(+ ) current (I(to)), 72.3±9.3 µM for the rapid delayed rectifier K(+ ) current (I(Kr)), and 82.5±9.4 µM for the L-type Ca(2+ ) current (I(Ca)) with Hill coefficients close to unity. The inward rectifier K(+ ) current (I(K1)) was not affected by R up to concentrations of 100 µM. Suppression of I(to), I(Kr), and I(Ca) has been confirmed under action potential voltage clamp conditions as well. The observed alterations in the AP morphology and densities of ion currents may predict serious proarrhythmic risk in case of intoxication with R as a consequence of overdose or decreased elimination of the drug, particularly in patients having multiple cardiovascular risk factors, such as elderly diabetic patients.

Xanthine derivatives in the heart: blessed or cursed?

Methylxanthines, such as theophylline, have been used to treat cardiorespiratory disorders, whereas caffeine is the most widely consumed psychoactive agent in various soft drinks. Because of the worldwide use of these drugs and the recently synthesized xanthine derivatives, an intensive research on the cardiac actions of these substances is under progress. This review focuses on the molecular mechanisms involved in the actions of xanthine derivatives with special reference to their adenosine receptor antagonistic properties. The main basic and human studies on the action of xanthines on impulse initiation and conduction, as well as the electrophysiological and mechanical activity of the working myocardium will be overviewed. The potential beneficial and harmful actions of the methylxanthines will be discussed in light of the recent experimental and clinical findings. The pharmacological features and clinical observations with adenosine receptor subtype-specific xanthine antagonists are also the subject of this paper. Based on the adenosine receptor-antagonistic activity of these compounds, it can be raised that xanthine derivatives might inhibit the cardioprotective action of endogenous adenosine on various subtypes (A(1), A(2A), A(2B) and A(3)) of adenosine receptors. Adenosine is an important endogenous substance with crucial role in the regulation of cardiac function under physiological and pathological conditions (preconditioning, postconditioning, ischemia/reperfusion injury). Recent clinical studies show that acute administration of caffeine or theophylline can inhibit various types of preconditioning in human subjects. There are no human studies, however, for the cardiovascular actions of long-term administration of these drugs. Upregulation of adenosine receptors and increased effectiveness of adenosine receptor-related cardiovascular functions have been observed after long-lasting treatment with methylxanthines. In addition, there are data indicating that blood adenosine level increases after long-term caffeine administration. Since the salutary actions (and also the adverse reactions) of a number of xanthine derivatives are repeatedly shown, the main goal is the development of novel structures that mimic the actions of the conventional methylxanthines as lead compounds, but their adenosine receptor subtype-specificity is higher, their water solubility is optimal, and the unwanted reactions are minimized.

Enhanced repolarization capacity: new potential antiarrhythmic strategy based on HERG channel activation.

The delayed rectifier potassium current (I(K)) is the major outward current responsible for ventricular repolarization in cardiac tissues. Based on kinetic properties and drug sensitivity it is composed of a slow (I(Ks)) and a rapid (I(Kr)) component, the latter is mediated by hERG channels. Suppression of IKr is the common mechanism of action of all class III antiarrhythmics, causing prolongation of the refractory period. However, lengthening of repolarization - either by a pathological factor or due to a pharmacological intervention - threatens with an increased risk of EAD generation and the concomitant sudden cardiac death. Therefore, a new potential anti-arrhythmic strategy, based on augmentation of the repolarization reserve, has been emerged. Recently a new class of compounds has been introduced as activators of the hERG channel. In this article we systematically review the chemical structures found to enhance IKr. Since the majority of previous experiments were performed in expression systems or in rodent cardiac preparations (neither is relevant to the human heart), in the second part of this article we present some results obtained with NS1643, the best examined hERG activator, in canine ventricular cardiomyocytes. This preparation is believed to have electrophysiological parameters most resembling those of human. NS1643 shortened the duration of canine ventricular action potential and was shown to interact with several transmembrane ion currents, including I(Ca), I(Kr), I(Ks), and I(to). However, the action potential shortening effect of NS1643 is likely related to inhibition of ICa, in addition to the enhancement of IKr. Although the multiple ion channel activity of NS1643 may carry proarrhythmic risk, the rationale of antiarrhythmic strategy based on I(Kr) activation is not questioned.

Role of Ca²+-sensitive K+ currents in controlling ventricular repolarization: possible implications for future antiarrhytmic drug therapy.

Normal heart function and repolarization of the cardiac action potential (AP) is to a high extent subjective to synchronized activity of sarcolemmal K(+) channels, expressed in both ventricular and atrial myocardium, largely contributing to regulation of the resting potential, the pacemaker activity, and the shape and duration of the AP. Clinical observations and experimental studies in cardiomyocytes and multicellular preparations provided firm evidence for the sensitivity of some major outward K+ currents and the corresponding ion channels to shifts in intracellular Ca(2+) concentration ([Ca(2+)](i)). Direct regulation via interaction between [Ca(2+ )](i) and the channel protein or indirect modulation via Ca(2+ ) signaling pathways of these currents have strong implications to mechanical and electrical performance of the heart, and its physiological adaptation to altered load. It may also lead to severe cardiac dysfunction, if [Ca(2+ )](i) handling is disturbed in a variety of pathological conditions. In this review we attempt to summarize the present state of the topic on two ubiquitous repolarizing K(+) currents (I(to1) and I(K1)) with documented Ca(2+)-sensitivity and critical significance in cellular antiarrhythmic defense, to highlight fields where clue data are missing, and discuss the apparently unsolved "mystery" of the cardiac small conductance Ca(2+ )-activated K(+ ) (SK) channels. We have collected the available information on the known novel, although usually still not enough selective inhibitors and activators of these currents justifying the need for more selective ones. Finally, we emphasize a few related therapeutical perspectives to be considered for future experimental research and particularly in pharmaceutical development.

Mechanism of reverse rate-dependent action of cardioactive agents.

Class 3 antiarrhythmic agents exhibit reverse rate-dependent lengthening of the action potential duration (APD), i.e. changes in APD are greater at longer than at shorter cycle lengths. In spite of the several theories developed to explain this reverse rate-dependency, its mechanism has been clarified only recently. The aim of the present study is to elucidate the mechanisms responsible for reverse rate-dependency in mammalian ventricular myocardium. Action potentials were recorded using conventional sharp microelectrodes from human, canine, rabbit, guinea pig, and rat ventricular myocardium in a rate-dependent manner. Rate-dependent drug-effects of various origin were studied using agents known to lengthen or shorten action potentials allowing thus to determine the drug-induced changes in APD as a function of the cycle length. Both drug-induced lengthening and shortening of action potentials displayed reverse rate-dependency in human, canine, and guinea pig preparations, but not in rabbit and rat myocardium. Similar results were obtained when repolarization was modified by injection of inward or outward current pulses in isolated canine cardiomyocytes. In contrast to reverse rate-dependence, drug-induced changes in APD well correlated with baseline APD values (i.e. that measured before the superfusion of drug or injection of current) in all of the preparations studied. Since the net membrane current (I(net)), determined from the action potential waveform at the middle of the plateau, was inversely proportional to APD, and consequently to cycle length, it is concluded that that reverse rate-dependency may simply reflect the inverse relationship linking I(net) to APD. In summary, reverse rate-dependency is an intrinsic property of drug action in the hearts of species showing positive APD - cycle length relationship, including humans. This implies that development of a pure K(+) channel blocking agent without reverse rate-dependent effects is not likely to be successful.

Analysis of the contribution of I(to) to repolarization in canine ventricular myocardium.

BACKGROUND AND PURPOSE: The contribution of the transient outward potassium current (I(to)) to ventricular repolarization is controversial as it depends on the experimental conditions, the region of myocardium and the species studied. The aim of the present study was therefore to characterize I(to) and estimate its contribution to repolarization reserve in canine ventricular myocardium. EXPERIMENTAL APPROACH: Ion currents were recorded using conventional whole-cell voltage clamp and action potential voltage clamp techniques in canine isolated ventricular cells. Action potentials were recorded from canine ventricular preparations using microelectrodes. The contribution of I(to) to repolarization was studied using 100 µM chromanol 293B in the presence of 0.5 µM HMR 1556, which fully blocks I(Ks). KEY RESULTS: The high concentration of chromanol 293B used effectively suppressed I(to) without affecting other repolarizing K(+) currents (I(K1), I(Kr), I(p)). Action potential clamp experiments revealed a slowly inactivating and a 'late' chromanol-sensitive current component occurring during the action potential plateau. Action potentials were significantly lengthened by chromanol 293B in the presence of HMR 1556. This lengthening effect induced by I(to) inhibition was found to be reverse rate-dependent. It was significantly augmented after additional attenuation of repolarization reserve by 0.1 µM dofetilide and this caused the occurrence of early afterdepolarizations. The results were confirmed by computer simulation. CONCLUSIONS AND IMPLICATIONS: The results indicate that I(to) is involved in regulating repolarization in canine ventricular myocardium and that it contributes significantly to the repolarization reserve. Therefore, blockade of I(to) may enhance pro-arrhythmic risk.

Novel trends in the treatment of cardiovascular disorders: site- and event- selective adenosinergic drugs.

This review focuses on the potential role of site- and event-selective adenosinergic drugs in the treatment of cardiovascular diseases. Adenosine is released from the myocardium and vessels in response to various forms of stress and acts on four receptor subtypes (A1, A2A, A2B and A3). Adenosine is an important endogenous substance with important homeostatic activity in the regulation of cardiac function and circulation. Adenosine receptors are also involved in the modulation of various cellular events playing crucial role in physiological and pathological processes of the cardiovascular system. These actions are associated to activation of distinct adenosine receptor subtypes, therefore drugs targeting specific adenosine receptors might be promising therapeutic tools in treatment of several disorders including various forms of cardiac arrhythmia, myocardial ischemia-reperfusion injury, angina pectoris, chronic heart failure, etc. Recently, in addition to subtype-specific adenosine receptor agonists and antagonists, a number of substances that enhance adenosine receptor activation locally at the site where the release of endogenous adenosine is the most intensive have been developed. Thus global actions of adenosine receptor agonists and antagonists, as well as desensitization or down-regulation following chronic administration of these orthosteric compounds can possibly be avoided. We discuss the chemical, pharmacological and clinical features of these compounds: (1) inhibitors of membrane adenosine transporters (NBTI, dipyridamole), (2) inhibitors of adenosine deaminase (coformycin, EHNA), (3) inhibitors of adenosine kinase (tubercidin, aristeromycin), (4) inhibitors of AMP deaminase (GP3269), (5) activators of 5'-nucleotidase (methotrexate), (6) adenosine regulators (acadesine) and (7) allosteric adenosine receptor modulators (PD81723, LUF6000). The development of this type of substances might offer a novel therapeutic approach for treating cardiovascular diseases in the near future.