Effects of pimobendan on the L-type Ca2+ current and developed tension in guinea-pig ventricular myocytes and papillary muscle: comparison with IBMX, milrinone, and cilostazol

Cellular and ionic mechanisms underlying the effects of cilostazol, milrinone, and isoproterenol to suppress arrhythmogenesis in an experimental model of early repolarization syndrome

BACKGROUND

Early repolarization syndrome (ERS) is associated with polymorphic ventricular tachycardia (PVT) and ventricular fibrillation, leading to sudden cardiac death.

OBJECTIVE

The present study tests the hypothesis that the transient outward potassium current (Ito)-blocking effect of phosphodiesterase-3 (PDE-3) inhibitors plays a role in reversing repolarization heterogeneities responsible for arrhythmogenesis in experimental models of ERS.

METHODS

Transmembrane action potentials (APs) were simultaneously recorded from epicardial and endocardial regions of coronary-perfused canine left ventricular (LV) wedge preparations, together with a transmural pseudo-electrocardiogram. The Ito agonist NS5806 (7-15 μM) and L-type calcium current (ICa) blocker verapamil (2-3 μM) were used to induce an early repolarization pattern and PVT.

RESULTS

After stable induction of arrhythmogenesis, the PDE-3 inhibitors cilostazol and milrinone or isoproterenol were added to the coronary perfusate. All were effective in restoring the AP dome in the LV epicardium, thus abolishing the repolarization defects responsible for phase 2 reentry and PVT. Arrhythmic activity was suppressed in 7 of 8 preparations by cilostazol (10 μM), 6 of 7 by milrinone (2.5 μM), and 7 of 8 by isoproterenol (0.1-1 μM). Using voltage clamp techniques applied to LV epicardial myocytes, both cilostazol (10 μM) and milrinone (2.5 μM) were found to reduce Ito by 44.4% and 40.4%, respectively, in addition to their known effects to augment ICa.

CONCLUSION

Our findings suggest that PDE-3 inhibitors exert an ameliorative effect in the setting of ERS by producing an inward shift in the balance of current during the early phases of the epicardial AP via inhibition of Ito as well as augmentation of ICa, thus reversing the repolarization defects underlying the development of phase 2 reentry and ventricular tachycardia/ventricular fibrillation.

J-wave syndromes: Brugada and early repolarization syndromes

A prominent J wave is encountered in a number of life-threatening cardiac arrhythmia syndromes, including the Brugada syndrome and early repolarization syndromes. Brugada syndrome and early repolarization syndromes differ with respect to the magnitude and lead location of abnormal J waves and are thought to represent a continuous spectrum of phenotypic expression termed J-wave syndromes. Despite two decades of intensive research, risk stratification and the approach to therapy of these 2 inherited cardiac arrhythmia syndromes are still undergoing rapid evolution. Our objective in this review is to provide an integrated synopsis of the clinical characteristics, risk stratifiers, and molecular, ionic, cellular, and genetic mechanisms underlying these 2 fascinating syndromes that have captured the interest and attention of the cardiology community in recent years.

Combination therapy of cilostazol and bepridil suppresses recurrent ventricular fibrillation related to J-wave syndromes

BACKGROUND

Brugada syndrome and idiopathic ventricular fibrillation (VF) associated with inferolateral early repolarization patterns are termed "J-wave syndromes." In such patients, an implantable cardioverter-defibrillator (ICD) is first-line therapy for prevention of sudden cardiac death. However, frequent ICD shocks due to recurrent VF remain serious problems.

OBJECTIVE

The purpose of this study was to ascertain if combination therapy of cilostazol and bepridil could suppress recurrent VF.

METHODS

We enrolled 7 patients with J-wave syndromes who experienced ICD shocks due to recurrent VF after ICD implantation. At first, cilostazol was instituted. In all subjects, palpitations due to sinus tachycardia caused by cilostazol were symptomatic. Addition of bepridil attenuated cilostazol-induced palpitations and maintained the suppressive effect of cilostazol against VF (87 ± 12 bpm to 66 ± 7 bpm, P < .01).

RESULTS

Six patients remained free of VF. Three patients underwent replacement of the ICD generator 4-5 years after ICD placement. Cilostazol was discontinued 2 days before replacement because of its antiplatelet effects. In all 3 patients, temporary discontinuation of cilostazol led to the reappearance of J waves, culminating in VF and an appropriate ICD shock in 1 patient. J waves disappeared with reinstitution of cilostazol.

CONCLUSION

These data suggest that combination therapy of cilostazol and bepridil may be effective and safe in suppressing VF recurrence in some cases of J-wave syndromes.

Comparative evaluation of calcium-sensitizing agents, pimobendan and SCH00013, on the myocardial function of canine pacing-induced model of heart failure

Pimobendan and SCH00013 are calcium sensitizers that possess dual action of calcium sensitization and phosphodiesterase-III inhibition. This study was conducted to comparatively evaluate the effect of these medications on the myocardial function of the canine pacing-induced heart failure model using echocardiography. Heart failure was induced in 20 dogs, to which pimobendan and two different doses of SCH00013 were administered orally to 15 dogs for 3 weeks, and the remaining 5 dogs served as the control. Cardiac evaluations were performed at baseline, week 1, week 2, and week 3. Significant thinning and dilation of the left ventricles, with systolic dysfunction, indicated by reduction of fractional shortening (FS) and strain values, were observed with a low dose of SCH00013. Whereas, although systolic dysfunction was observed with reduction of FS and radial strain, significant dilation and thinning of the left ventricles and reduction of circumferential strain were not observed with pimobendan. Pimobendan had a potent positive inotropic effect, with little effect on synchronicity, while low-dose SCH00013 had a weaker positive inotropic effect but was able to sustain synchronicity. Although, it failed to show significant statistical differences, the results of this study allow speculations that administration of pimobendan and SCH00013 may have differing effect on the myocardial function in the canine pacinginduced heart failure model.

Cellular mechanisms underlying the effects of milrinone and cilostazol to suppress arrhythmogenesis associated with Brugada syndrome

BACKGROUND

Brugada syndrome is an inherited disease associated with vulnerability to ventricular tachycardia and sudden cardiac death in young adults. Milrinone and cilostazol, oral phosphodiesterase (PDE) type III inhibitors, have been shown to increase L-type calcium channel current (ICa) and modestly increase heart rate by elevating the level of intracellular cyclic adenosine monophosphate.

OBJECTIVE

To examine the effectiveness of these PDE inhibitors to suppress arrhythmogenesis in an experimental model of Brugada syndrome.

METHODS

Action potential (AP) and electrocardiographic recordings were obtained from epicardial and endocardial sites of coronary-perfused canine right ventricular wedge preparations. The Ito agonist NS5806 (5 μM) and Ca(2+) channel blocker verapamil (2 μM) were used to pharmacologically mimic Brugada phenotype.

RESULTS

The combination induced all-or-none repolarization at some epicardial sites but not others, leading to ST-segment elevation as well as an increase in both epicardial and transmural dispersion of repolarization. Under these conditions, phase 2 reentry developed as the epicardial AP dome propagated from sites where it was maintained to sites at which it was lost, generating closely coupled extrasystoles and ventricular tachycardia. The addition of the PDE inhibitor milrinone (2.5 μM) or cilostazol (5-10 μM) to the coronary perfusate restored the epicardial AP dome, reduced dispersion, and abolished phase 2 reentry-induced extrasystoles and ventricular tachycardia.

CONCLUSIONS

Our study identifies milrinone as a more potent alternative to cilostazol for reversing the repolarization defects responsible for the electrocardiographic and arrhythmic manifestations of Brugada syndrome. Both drugs normalize ST-segment elevation and suppress arrhythmogenesis in experimental models of Brugada syndrome.

Regulation of murine cardiac function by phosphodiesterases type 3 and 4

Cyclic nucleotide phosphodiesterases (PDEs) encompass a large group of enzymes that regulate intracellular levels of two-second messengers, cAMP and cGMP, by controlling the rates of their degradation. More than 60 isoforms, subdivided into 11 gene families (PDE1-11), exist in mammals with at least six families (PDE1-5 and PDE8) identified in mammalian hearts. The two predominant families implicated in regulating contraction strength of the heart are PDE3 and PDE4. Studies using transgenic models in combination with family-specific PDE inhibitors have demonstrated that PDE3A, PDE4B, and PDE4D isoforms regulate cardiac contractility by modulating cAMP levels in various subcellular compartments. These studies have further uncovered contributions of PDE4B and PDE4D in preventing ventricular arrhythmias.

Cardiac myosin activation part 1: from concept to clinic

Decreased cardiac contractility is a central feature of systolic heart failure and yet we have no effective drugs to improve cardiac contractility. Existing drugs that increase cardiac contractility do so indirectly through signaling cascades and their use is limited by their mechanism-related adverse effects. Direct activation of the cardiac sarcomere to increase cardiac contractility may provide a means to avoid these limitations. Using a reconstituted version of the cardiac sarcomere, we screened a small molecule library and identified several chemical classes that directly activate cardiac myosin. One compound class has been optimized extensively using an iterative process; omecamtiv mecarbil, a small-molecule, selective, cardiac myosin activator is the most advanced exemplar of this novel mechanistic class. It accelerates the transition of myosin into the force-generating state without affecting cardiac myocyte calcium homeostasis. In animal models, omecamtiv mecarbil increases cardiac function by increasing the duration of ejection without changing the rates of contraction. Initial clinical studies have demonstrated the translation of this mechanism into humans, and further clinical studies of its use in acute and chronic heart failure are planned. Cardiac myosin activation may provide a new therapeutic approach for systolic heart failure. This article is part of a special issue entitled "Key Signaling Molecules in Hypertrophy and Heart Failure."

Drugs and Brugada syndrome patients: review of the literature, recommendations, and an up-to-date website (www.brugadadrugs.org)

BACKGROUND

Worldwide, the Brugada syndrome has been recognized as an important cause of sudden cardiac death in individuals at a relatively young age. Importantly, many drugs have been reported to induce the characteristic Brugada syndrome-linked ECG abnormalities and/or (fatal) ventricular tachyarrhythmias.

OBJECTIVE

The purpose of this study was to review the literature on the use of drugs in Brugada syndrome patients, to make recommendations based on the literature and on expert opinion regarding drug safety, and to ensure worldwide online and up-to-date availability of this information to all physicians who treat Brugada syndrome patients.

METHODS

We performed an extensive review of the literature, formed an international expert panel to produce a consensus recommendation to each drug, and initiated a website (www.brugadadrugs.org).

RESULTS

The literature search yielded 506 reports for consideration. Drugs were categorized into one of four categories: (1) drugs to be avoided (n = 18); (2) drugs preferably avoided (n = 23); (3) antiarrhythmic drugs (n = 4); and (4) diagnostic drugs (n = 4). Level of evidence for most associations was C (only consensus opinion of experts, case studies, or standard-of-care) as there are no randomized studies and few nonrandomized studies in Brugada syndrome patients.

CONCLUSION

Many drugs have been associated with adverse events in Brugada syndrome patients. We have initiated a website (www.brugadadrugs.org) to ensure worldwide availability of information on safe drug use in Brugada syndrome patients.

Pivotal effects of phosphodiesterase inhibitors on myocyte contractility and viability in normal and ischemic hearts

Phosphodiesterases (PDEs) are enzymes that degrade cellular cAMP and cGMP and are thus essential for regulating the cyclic nucleotides. At least 11 families of PDEs have been identified, each with a distinctive structure, activity, expression, and tissue distribution. The PDE type-3, -4, and -5 (PDE3, PDE4, PDE5) are localized to specific regions of the cardiomyocyte, such as the sarcoplasmic reticulum and Z-disc, where they are likely to influence cAMP/cGMP signaling to the end effectors of contractility. Several PDE inhibitors exhibit remarkable hemodynamic and inotropic properties that may be valuable to clinical practice. In particular, PDE3 inhibitors have potent cardiotonic effects that can be used for short-term inotropic support, especially in situations where adrenergic stimulation is insufficient. Most relevant to this review, PDE inhibitors have also been found to have cytoprotective effects in the heart. For example, PDE3 inhibitors have been shown to be cardioprotective when given before ischemic attack, whereas PDE5 inhibitors, which include three widely used erectile dysfunction drugs (sildenafil, vardenafil and tadalafil), can induce remarkable cardioprotection when administered either prior to ischemia or upon reperfusion. This article provides an overview of the current laboratory and clinical evidence, as well as the cellular mechanisms by which the inhibitors of PDE3, PDE4 and PDE5 exert their beneficial effects on normal and ischemic hearts. It seems that PDE inhibitors hold great promise as clinically applicable agents that can improve cardiac performance and cell survival under critical situations, such as ischemic heart attack, cardiopulmonary bypass surgery, and heart failure.

Antiplatelet agents sarpogrelate and cilostazol affect experimentally-induced ventricular arrhythmias and mortality

Antiplatelet agents, sarpogrelate (SAR), a 5-hydroxy tryptamine 2A receptor antagonist and cilostazol (CIL), a phosphodiesterase-III inhibitor, were observed to be beneficial in attenuating cardiac remodeling and improving cardiac function in congestive heart failure due to myocardial infarction in rats; however, CIL increased ventricular tachycardia and mortality. In order to study the effects of these antiplatelet agents on arrhythmias, Sprague-Dawley rats were pretreated with either SAR or CIL (5 mg/kg/day) for 2 weeks and were then either injected cumulative doses of epinephrine (Epi) or subjected to coronary occlusion. Saline-treated animals served as controls. Electrocardiographic analysis revealed that SAR pretreatment decreased the incidence and severity of ventricular arrhythmias (time of onset of arrhythmias as well as the occurrence of premature ventricular contractions, salvos, tachycardia, and fibrillations), whereas CIL treatment augmented the incidence of cardiac arrhythmias due to both Epi and coronary occlusion. None of the drugs affected the corrected QT interval significantly. Furthermore, the levels of cyclic adenosine monophosphate (cAMP) in left ventricle were markedly higher in CIL-pretreated rats when compared to SAR-pretreated or control rats. It is suggested that an excessive level of cAMP may contribute to increase incidence of ventricular arrhythmias and mortality in animals pretreated with CIL, unlike the SAR-pretreated rats.

Myocardial phosphodiesterases and regulation of cardiac contractility in health and cardiac disease

Phosphodiesterase (PDE) inhibitors are potent cardiotonic agents used for parenteral inotropic support in heart failure. Contractile effects of these agents are mediated through cAMP-protein kinase A-induced stimulation of I (Ca2+) which ultimately results in increased Ca(2+)-induced sarcoplasmic reticulum Ca(2+) release. A number of additional effects such as increases in sarcoplasmic reticulum Ca(2+) stores, stimulation of reverse mode Na(+)-Ca(2+) exchange, direct or cAMP-mediated effects on sarcoplasmic reticulum ryanodine receptor, stimulation of the voltage-sensitive sarcoplasmic reticulum Ca(2+) release mechanism, as well as A(1) adenosine receptor blockade could contribute to positive inotropic responses to PDE inhibitors. Moreover, some PDE inhibitors exhibit Ca(2+) sensitizer properties as they could increase the affinity of troponin C Ca(2+)-binding sites as well as reduce Ca(2+) threshold for thin myofilament sliding and facilitate cross-bridge cycling. Inotropic responses to PDE inhibitors are significantly reduced in cardiac disease, an effect largely attributed to downregulation of cAMP-mediated signalling due to sustained sympathetic activation. Four PDE isoenzymes (PDE1, PDE2, PDE3 and PDE4) are present in myocardial tissue of various mammalian species, of which PDE3 and PDE4 are particularly involved in regulation of cardiac myocyte contraction. PDE cAMP-hydrolysing activity is preserved in compensated cardiac hypertrophy but significantly reduced in animal models of heart failure. However, clinical studies have not revealed any changes in distribution profile as well as kinetic and regulatory properties of myocardial PDEs in failing human hearts. A reduction of PDE inhibitors-induced contractile responses in heart failure has therefore been ascribed to reduced cAMP synthesis due to uncoupling of adenylyl cyclase from beta-adrenoreceptor. In cardiac myocytes, PDEs are targeted to distinct subcellular compartments by scaffolding proteins such as myomegalin, mAKAP and beta-arrestins. Over subcellular microdomains, cAMP hydrolysis by PDE3 and PDE4 allows to control the activity of local pools of protein kinase A and therefore the extent of protein kinase A-mediated phosphorylation of cellular proteins.

Ca2+-sensitisers--a promising option to treat heart failure?

Because the number of transplants is still fewer than the number of patients waiting for a donor organ, new concepts of therapy are needed that allow patients to bridge the time gap until heart transplantation or even to improve symptoms while on treatment. Ca(2+)-sensitisers are agents that directly influence myofilaments and/or the cross-bridge-cycle. Depending on the molecular mechanisms underlying their action, Ca(2+)-sensitisers have been divided into three classes. While, a number of Ca(2+)-sensitising drugs have been described, currently only the Ca(2+)-sensitisers pimobendan and levosimendan are in clinical use. This review provides a survey on the molecular mechanisms and the therapeutic effectiveness of Ca(2+)-sensitisers for the treatment of human heart failure.

High-frequency potentials developed in wavelet-transformed electrocardiogram as a novel indicator for detecting Brugada syndrome

BACKGROUND

A reliable alternative method for detecting Brugada syndrome is desirable because the diagnosis of Brugada syndrome using 12-lead ECG is not optimal.

OBJECTIVES

The purpose of this study was to assess the usefulness of the wavelet-transformed ECG in detecting Brugada syndrome.

METHODS

The study consisted of 15 patients with Brugada syndrome and 15 healthy subjects (control group). The parameters on the signal-averaged ECG and the frequency components recorded from the wavelet-transformed ECG were compared between the two groups. Measurements were repeated after pilsicainide infusion in the two groups of patients, after an isoproterenol infusion following pilsicainide injection, and after administration of cilostazol in the group of patients with Brugada syndrome.

RESULTS

The positive rate of late potentials was 80% in the Brugada syndrome group and 0% in the control group (P <.01). The high-frequency components (80-150 Hz) were developed in the Brugada syndrome group to a greater extent than in the control group, but the low-frequency components (10-50 Hz) did not differ (mean peak power at 80 Hz; 713 +/- 36 vs 488 +/- 60, P <.001). After pilsicainide injection, high-frequency components significantly increased in both groups. However, after isoproterenol and cilostazol administration, high-frequency components significantly decreased but remained higher than in the control group (80 Hz; 655 +/- 40 vs 488 +/- 60, P <.001). The sensitivity of the development of high-frequency components in detecting Brugada syndrome was higher than that of signal-averaged ECG (100% vs 80%), but specificity remained high and similar (100% for both methods).

CONCLUSION

Abnormally high-frequency components recorded from the wavelet-transformed ECG might be a novel factor in detecting Brugada syndrome.

The therapeutic potential of novel cardiotonic agents

During the course of treatment of heart failure patients, cardiotonic agents are inevitable for improvement of myocardial dysfunction. Clinically available agents, such as beta-adrenoceptor agonists and selective phosphodiesterase 3 inhibitors, act mainly via cyclic AMP/protein kinase A-mediated facilitation of Ca(2+) mobilisation (upstream mechanism). These agents are associated with the risk of Ca(2+) overload leading to arrhythmias, myocardial cell injury and premature cell death. In addition, they are energetically disadvantageous because of an increase in activation energy and metabolic effects. Cardiac glycosides act also via an upstream mechanism and readily elicit Ca(2+) overload with a narrow safety margin. No currently available agents act primarily via an increase in the myofilament sensitivity to Ca(2+) ions (central and/or downstream mechanisms). Novel Ca(2+) sensitisers under basic research may deserve clinical trials to examine the therapeutic potential to replace currently employed agents in acute and chronic heart failure patients. Molecular mechanisms of action of Ca(2+) sensitisers are divergent. In addition, they show a wide range of discrete pharmacological profiles due to additional actions associated with individual compounds. Therefore, the outcome of clinical trials has to be explained carefully based on these mechanisms of actions.

Mechanisms of action of novel cardiotonic agents

Regulation of myocardial contractility by cardiotonic agents is achieved by an increase in intracellular Ca2+ mobilization (upstream mechanism), an increase in Ca2+ binding affinity to troponin C (central mechanism), or facilitation of the process subsequent to Ca2+ binding to troponin C (downstream mechanism). cAMP mediates the regulation induced by Ca2+ mobilizers such as beta-adrenoceptor agonists and selective phosphodiesterase III inhibitors acting through the upstream mechanism. These agents act likewise on the central mechanism to decrease Ca2+ sensitivity of troponin C in association with the cAMP-mediated phosphorylation of troponin I. In addition to such a well-known action of cAMP, recent experimental findings have revealed that Ca2+ sensitizers, such as levosimendan, OR-1896, and UD-CG 212 Cl, require the cAMP-mediated signaling for induction of Ca2+ sensitizing effect. These agents shift the [Ca2+] -force relationship to the left, but their positive inotropic effect (PIE) is inhibited by carbachol, which suppresses selectively the cAMP-mediated PIE. These findings imply that cAMP may play a crucial role in increasing the myofilament Ca2+ sensitivity by cross-talk with the action of individual cardiotonic agents. No clinically available cardiotonic agents act primarily via Ca2+ sensitization, but the PIE of pimobendan and levosimendan is partly mediated by an increase in myofilament Ca2+ sensitivity. Evidence is accumulating that cardiotonic agents with Ca2+ sensitizing action are more effective than agents that act purely via the upstream mechanism in clinical settings. Further clinical trials are required to establish the effectiveness of Ca2+ sensitizers in long-term therapy for congestive heart failure patients.

Molecular actions of drugs that sensitize cardiac myofilaments to Ca2+

Ca(2+)-sensitizers are inotropic agents that modify the response of myofilaments to Ca2+, and are potentially valuable drugs in the treatment of heart failure. These agents have diverse chemical structures, and in some cases also have effects as inhibitors of phosphodiesterase activity. Advantages of their actions include vasodilation combined with inotropic effects. Reduction in the amounts of Ca2+ required to activate the myofilaments also lowers the oxygen consumption required for Ca2+ transport, lowers the threat of arrhythmias, and may blunt Ca(2+)-dependent transcriptional and translational mechanisms leading to hypertrophy and failure. Although diastolic abnormalities and impaired relaxation were thought to be potential undesirable effects of Ca(2+)-sensitizers, studies of hearts beating in situ indicate that this may not be a major problem. We focus here on Ca(2+)-sensitizers that act on cardiac troponin C, the Ca2+ receptor that triggers activation of the actin-myosin interaction. Structural studies have identified a unique mode of Ca2+ signaling in cardiac troponin C that should aid in targeting drugs to the heart. Moreover, identification of docking sites of Ca(2+)-sensitizers on troponin C suggest new directions for rational drug design.