Cardiac arrhythmias and antiarrhythmic drugs: recent advances in our understanding of mechanism

Reperfusion Cardiac Injury: Receptors and the Signaling Mechanisms

It has been documented that Ca2+ overload and increased production of reactive oxygen species play a significant role in reperfusion injury (RI) of cardiomyocytes. Ischemia/reperfusion induces cell death as a result of necrosis, necroptosis, apoptosis, and possibly autophagy, pyroptosis and ferroptosis. It has also been demonstrated that the NLRP3 inflammasome is involved in RI of the heart. An increase in adrenergic system activity during the restoration of coronary perfusion negatively affected cardiac resistance to RI. Toll-like receptors are involved in RI of the heart. Angiotensin II and endothelin-1 aggravated ischemic/reperfusion injury of the heart. Activation of neutrophils, monocytes, CD4+ T-cells and platelets contributes to cardiac ischemia/reperfusion injury. Our review outlines the role of these factors in reperfusion cardiac injury.

Nupafant, a PAF-antagonist prototype for suppression of ventricular fibrillation without liability for QT prolongation?

BACKGROUND AND PURPOSE

PAF antagonists inhibit ischaemia-induced ventricular fibrillation (VF) in animals. However, unfavourable ancillary actions (on QT interval and coronary flow) have been reported with the PAF antagonist, BN-50739. If these are class actions, they would preclude development of PAF antagonists as novel anti-VF drugs. Our purpose was to examine this proposition using the hitherto untested PAF antagonist, nupafant.

EXPERIMENTAL APPROACH

Two rat heart preparations (Langendorff and 'dual coronary' perfusion) were used to assay nupafant's effects on ischaemia-induced VF, coronary flow and QT interval, and to test for the site-selectivity necessary if any effects on VF are caused by PAF antagonism.

KEY RESULTS

Global (whole-heart) delivery of 10 microM nupafant, reduced the incidence of ischaemia-induced VF and widened QT interval without affecting coronary flow. Importantly, lower concentrations (0.1 and 1 microM) had no effect on VF, yet widened QT almost identically to 10 microM nupafant. When nupafant was delivered selectively to (and entrapped within) the involved region it partially protected against VF (P<0.05). This occurred without change in QT interval. Selective nupafant delivery to the uninvolved region was without effect.

CONCLUSIONS AND IMPLICATIONS

Nupafant protects against ischaemia-induced VF primarily by site-selective actions in the ischaemic region but, unlike BN-50739, the effect is unrelated to its QT widening action, and is not compromised by any effect on coronary flow. This establishes proof of concept that VF suppression by PAF antagonism need not invariably be associated with QT prolongation or vasodilatation, justifying further development of this drug class.

Cardiac hypertrophy: a risk factor for QT-prolongation and cardiac sudden death

Cardiac hypertrophy was viewed as a compensatory response to hemodynamic stress. However, cumulative evidence obtained from studies using more advanced technologies in human patients and animal models suggests that cardiac hypertrophy is a maladaptive process of the heart in response to intrinsic and extrinsic stimuli. Although hypertrophy can normalize wall tension, it is a risk factor for QT-prolongation and cardiac sudden death. Studies using molecular biology techniques such as transgenic and knockout mice have revealed many important molecules that are involved in the development of heart hypertrophy and have demonstrated signaling pathways leading to the pathogenesis. With the same approach, the consequence of heart hypertrophy has been examined. The significance of hypertrophy in the development of overt heart failure has been demonstrated and several critical molecular pathways involved in the process were revealed. A comprehensive understanding of the threats of heart hypertrophy to patients has helped to develop novel treatment strategies. The recognition of hypertrophy as a major risk factor for QT-prolongation and cardiac sudden death is an important advance in cardiac medicine. Cellular and molecular mechanisms of this risk aspect are currently under extensively exploring. These studies would lead to more comprehensive approaches to prevention of potential life threatening arrhythmia and cardiac sudden death. The adaptation of new approaches such as functional genomics and proteomics will further advance our knowledge of heart hypertrophy.

Influence of lidocaine on ouabain-induced inotropic response in rat atria

In this paper we demonstrated that lidocaine broadens the therapeutic range of ouabain action having a protective effect on ouabain-induced toxicity on rat atria. The lidocaine effect on therapeutic ouabain action was associated with the increase in the sensitivity of Na(+)-K(+)-ATPase related to a decreased in the equilibrium dissociation constant (K(d)) of high affinity binding sites. Lidocaine suppressed the ouabain-induced tonotropic effect and arrhythmias, decreasing the number of low affinity binding sites (B(max)) without changes in K(d). Blockade of Na(+)-Ca(2+) exchange with KB-R7943 or dual Na(+)-Ca(2+) channel with flunarizine, mimicked lidocaine effect increasing ouabain therapeutic action, extending its concentration range tolerated, delaying the onset of contracture. Lidocaine itself triggered negative inotropic response at high concentration. This effect was increased in the presence of flunarizine and verapamil but not by the inhibition of calcium/calmodulin with W-7. The mechanism underlying the lidocaine-induced negative inotropic response, appears to be different that underlying the positive inotropic effect on ouabain action. This study provides evidence that lidocaine can interact with the same or similar binding sites for ouabain in rat atrial tissue, providing a protective effect on ouabain-induced changes in contractility. The contribution of Na(+)-Ca(2+) exchange and/or Ca(2+) overload on lidocaine effect is discussed.

Evidence for the binding of β-adrenoceptor blockers to microsomal Na+/K+-ATPase in guinea pig heart preparations

We reported in a previous study that β-adrenoceptor blockers inhibit the Mg2+-dependent ATP-hydrolytic function of Na+/K+-ATPase. To determine if this action is a result of binding of β-blockers to the receptor sites that bind the digitalis glycosides, we performed displacement binding assays of eight β-blockers with [3H]-ouabain (OUA) in guinea pig myocardial microsomal preparations. In the first series of experiments, 10-200 µM of the β-blockers were displaced with 250 nM OUA. In the second set of experiments, 10-500 nM of OUA was displaced using 200 µM of the β-blockers. The drugs showed concentration-dependent receptor occupancy at the different OUA levels. Propranolol (PPN), metoprolol (MTP), and sotalol (STL) showed the strongest binding; nadolol (NDL), indenolol (IDN), and atenolol (ATN) had intermediate binding; carazolol (CRZ) and celiprolol (CLP) had the weakest binding properties. The results suggest that β-blockers may compete for the same binding sites with ouabain in their inhibition of the Na+/K+-ATPase. These actions may contribute to the mechanism for some of their cardiac effects, especially their proarrhythmic and arrhythmogenic actions.Key words: β-adrenoceptor blockers, antiarrhythmic agents, arrhythmogenic effects, Na+/K+-ATPase, ouabain binding.

Spatial correlation analysis of the pharmacological conversion of sustained atrial fibrillation in conscious goats by cibenzoline

The nonlinear spatial redundancy and the linear spatial correlation function were used to investigate to what extent non-linearity was involved in the coupling of atrial regions and how organization in activation patterns of sustained atrial fibrillation (AF) had been modified by administration of the class IC agent cibenzoline in the experimental model of sustained AF in instrumented conscious goats. Electrograms were measured in five goats during sustained AF and when the fibrillation interval had been prolonged to about 25%, 50% and 85% (CIB25, CIB50, CIB85) with respect to control. The nonlinear association length and linear correlation length were estimated along the principal axes of two-dimensional correlation maps estimated from the spatial redundancy and the spatial correlation function, respectively. The estimated short axis association length in the right atrium increased already shortly after the start of infusion (CIB25, +61%), and remained significantly different from control during the experiment, including the effects of non-simultaneous interaction. At CIB85 the association length had almost become twice as long with respect to control (increase from 16 to 29 mm, 89%), while in the left atrium changes were less pronounced (increase from 9 to 12 mm, +32%). The linearized association length which was estimated using multivariate surrogate data increased more gradually and was less sensitive to changes in spatial organization. The results of the spatial correlation analysis suggest that the drug-induced nonlinearity in the spatio-temporal dynamics of sustained AF is related to activation patterns which are characterized by extended uniformly propagating fibrillation wavefronts (AF type I). We conclude that cibenzoline enhanced the spatial organization of sustained AF associated with a transition from type II to type I AF activation patterns. This may destabilize the perpetuation of AF since an increase in association length is equivalent to a reduction of atrial tissue mass available to support reentrant circuits. The results are consistent with the hypothesis that larger association lengths result from fewer and larger reentrant circuits. It is argued that effects of diminished curvature of fibrillation wavefronts are anti-arrhythmic under conditions of suppressed excitability imposed by cibenzoline. Termination of AF may be mediated by a mechanism resembling a bifurcation of the dynamics which sets in when the ends of fractionated wavefronts cannot sufficiently curve anymore to maintain a positive balance of newly generated wavelets needed to sustain AF.

Class I antiarrhythmic drug effects on ouabain binding to guinea pig cardiac Na+-K+ATPase

The notion that the inhibition of the Mg2+-dependent ATP-hydrolytic function of the myocardial Na+-K+ATPase by class I antiarrhythmic agents occurs as a result of their binding to the same receptor sites as the digitalis glycosides was tested by performing competitive binding assays of [3H]ouabain (OUA) with eight drugs: disopyramide, encainide, lidocaine, lorcainide, phenytoin, procainamide, quinidine, and tocainide in guinea pig heart microsomal preparations. In the first set of experiments, 10-200 µM concentrations of the drugs were preincubated with the enzyme and displacement assays performed with 250 nM OUA. The drugs showed receptor occupancy of 19-32% at 50 µM, 25-44% at 100 µM, and 37-56% at 200 µM. Then, 10-500 nM concentrations of OUA were preincubated with the enzyme, and competitive assays were performed using 200 µM concentrations of the drugs. OUA occupied 39-51% of the receptor sites at 100 nM, 44-67% at 250 nM, and 62-82% at 500 nM, displacing the drugs in a concentration-dependent fashion. The results show that antiarrhythmic drugs interact with the same or similar receptor sites as ouabain on the Na+-K+ATPase, pointing to a possible contribution of these interactions to the mechanism for their inhibitory actions on the enzyme, and perhaps their arrhythmogenic effects.Key words: class I antiarrhythmic agents, proarrhythmias, Na+-K+ATPase, ouabain binding.

Protection against ventricular fibrillation by the PAF antagonist, BN-50739, involves an ischaemia-selective mechanism

The platelet-activating factor (PAF) antagonist BN-50739 can suppress certain cardiac arrhythmias. PAF is released from ischaemic myocardium and may contribute to initiation of ischaemia-induced ventricular fibrillation (VF). In this study we characterised the action of BN-50739 on left regional ischaemia-induced VF and examined whether effects are mediated within the ischaemic territory, or are nonspecific. In rat isolated Langendorff perfused hearts (n = 12/group), 10 microM BN-50739 reduced the incidence of ischaemia-induced VF from 75 to 17% (p<0.05). This was accompanied by QT widening and an increase in coronary flow. Heart rate and PR interval were not affected by the drug. In separate studies, isolated rat hearts were perfused by using a dual-lumen tube that allows independent delivery of solution to the left and right coronary beds. Successful regional localisation of drug delivery was confirmed by observing, before ischaemia, a regionally selective increase in coronary flow (p<0.05), measured by using two in-line flow meters. Protection against ischaemia-induced VF (p<0.05) was achieved by pretreatment with BN-50739, delivered selectively and entrapped within the involved region, but not when the drug was delivered to the uninvolved region. In conclusion, BN-50739 protects against ischaemia-induced VF by eliciting a pharmacologic action in the involved (ischaemic) myocardium. This supports the hypothesis that BN-50739 suppresses an arrhythmogenic effect of endogenous PAF released within the ischaemic tissue.

Antiarrhythmic drugs: a reorientation in light of recent developments in the control of disorders of rhythm

Numerous developments in our knowledge of arrhythmias during the past decade or so have had a major influence on antiarrhythmic drug therapy. It has become increasingly evident that arrhythmias merit treatment not only for the relief of symptoms, with improvement in quality of life, but also for the prolongation of survival by decreasing arrhythmic deaths. No longer can mere suppression of arrhythmias, symptomatic or asymptomatic, be equated with prolonged survival. We now know that antiarrhythmic drugs that act by blocking sodium channels can increase mortality and that the most important determinants of arrhythmia mortality are the degree and nature of ventricular dysfunction. To these considerations must be added the advances in nonpharmacologic approaches to controlling cardiac arrhythmias. There has been a shift to the use of implantable devices and of drugs with alternative modes of action, such as beta blockers and class III drugs (e.g., sotalol, amiodarone). However, the side-effect profiles of these 2 classes of compounds have led to the synthesis and characterization of agents that act simply by blocking > or = 1 membrane ion channels. The isolated block of the rapid component of the delayed rectifier potassium current (IKr) has been associated with potent antifibrillatory activity in the atria, with a neutral (e.g., with dofetilide) or deleterious (with d-sotalol) effect on mortality in postinfarct survivors. Therefore, the focus now is on compounds that can block > 1 ion channel (e.g., tedisamil and azimilide). Azimilide is the first of the class III agents that blocks both components of the delayed rectifier potassium current. The drug's overall action is associated with a spectrum of electrophysiologic properties that hold promise in the control of atrial and ventricular arrhythmias, with potential for improving survival in patients at risk for cardiac arrest.

General principles of antiarrhythmic therapy for ventricular tachyarrhythmias

Sudden cardiac death due to ventricular arrhythmias is a significant cause of mortality in patients with structural heart disease. Over the past several decades, the introduction of new pharmacologic and nonpharmacologic therapy has expanded the treatment options available. This article will focus on the use of antiarrhythmic medication for the treatment of ventricular arrhythmias and will review the following: (1) treatment goals for various clinical populations, (2) the mechanisms of antiarrhythmic and proarrhythmic actions of antiarrhythmic medications, and (3) empiric versus guided pharmacologic therapy.

Altered Ca2+ mobilization during excitation-contraction in cultured cardiac myocytes exposed to antimony

Industrial exposure and pharmaceutical use of antimony compounds have been linked to altered cardiovascular function and pathology. Antimony compounds induce hypotension, bradycardia, and cardiac arrhythmias, all of which can arise from aberrations in myocyte regulation of intracellular free calcium concentration ([Ca2+]i). To determine if trivalent antimony affects [Ca2+]i during excitation-contraction, we developed an in vitro cardiac myocyte model that was exposed for 24 hr to potassium antimonyl tartrate (PAT) at 0-10 microM. Control myocytes received sodium potassium tartrate. Concentrations of up to 10 microM PAT were without effect on total DNA and protein content of cultures, indicating that PAT exposures were not overtly toxic. However, spontaneous beating rates of myocytes were significantly reduced by 5 and 10 microM PAT. Myocytes were paced by electric field stimulation at 0.5 Hz, and the effect of PAT on [Ca2+]i transients during excitation-contraction was monitored with fura-2. PAT (2-8 microM) significantly reduced systolic [Ca2+]i in a concentration-dependent fashion, but was without effect on diastolic [Ca2+]i or on the first derivative of the transient rise (d[Ca2+]i/dt). Myocytes from control cells responded to epinephrine (10(-8)-10(-5) m) in concentration-dependent fashion with elevated systolic [Ca2+]i and an increase in the rate of decay of transients. In PAT-exposed myocytes, the systolic response was blunted while the decay rate was enhanced. PAT-exposed cells also exhibited a reduced basal [Ca2+]i when depolarized by 90 mm KCl and a reduced caffeine-releasable Ca2+ pool of the sarcoplasmic reticulum. Both control and PAT-treated cells responded to ryanodine in a comparable fashion. Results indicate that a nonlethal exposure to PAT reduces Ca2+ availability during excitation-contraction. Decreased influx of Ca2+ across the sarcolemma and enhanced removal of Ca2+ appear to be responsible.

Antiarrhythmic and Arrhythmogenic Actions of Varying Levels of Extracellular Magnesium: Possible Cellular Basis for the Differences in the Efficacy of magnesium and Lidocaine in Torsade de Pointes

BACKGROUND: In recent years, there has been an increasing use of antiarrhythmic drugs that act predominantly by prolonging myocardial repolarization. An inevitable electrophysiologic consequence of these drugs is the development of torsade de pointes as a proarrhythmic reaction. Both intravenous lidocaine and magnesium sulphate have been used in the acute control of such a proarrhythmia. Their electrophysiologic mechanisms in this setting are not well defined. METHODS AND RESULTS: Using the standard microelectrode techniques, the effects of magnesium (Mg) and lidocaine on action potential duration (APD), and on barium-induced spontaneous action potentials, were studied in canine Purkinje fiber preparations. The objective was to clarify the direct and indirect effects of magnesium on triggered activities due to early afterdepolarizations. Superfusion in media with 0.1 mM Mg and 2.5 mM K produced more pronounced increases in APD measured at -20mV repolarization time [APD(20)] than those in a solution with 5 mM K. This effect was further enhanced at lower stimulation frequencies. The striking prolongation of APD(20) by solutions with low potassium concentrations diminished as the Mg concentration was increased. In solutions with 2.5 mM K, Mg produced concentration-dependent decreases in APD(20). This effect was greater at lower stimulation frequencies. Lidocaine at 4.0 x 10(_5) M produced a marked shortening of the APD in the entire firing frequency of the abnormal automaticity in a concentration-dependent manner. With 10 mM Mg, such action potentials appeared only sporadically. Magnesium also decreased the amplitude and the maximum upstroke velocity of these action potentials. In contrast, lidocaine at 4.0 x 10(-5) M exhibited no significant effects on action potentials due to barium-induced abnormal automaticity, or on additional depolarizations developing from the repolarization phase of these action potentials. CONCLUSIONS: The data indicate that (i) hypomagnesemia may be arrhythmogenic when combined with hypokalemia and bradycardia leading to a prolongation of the plateau phase of the action potential, (ii) magnesium administration may suppress triggered activities mainly by a direct inhibition of the development of triggered potentials, and (iii) lidocaine may suppress triggered potentials only indirectly by preventing the development of early afterdepolarizations due to the shortening effect on the APD. These findings are consistent with the clinical observation of a high incidence of torsade de pointes in the setting of hypokalemia and hypomagnesemia introduced by a chronic diuretic therapy. They are also consistent with the marked effectiveness of intravenous Mg relative to the inconsistent clinical effects of lidocaine in controlling torsade de pointes.

The coming of age of the class III antiarrhythmic principle: retrospective and future trends

Antiarrhythmic drug therapy is in a state of continuous flux. In the last decade or so, numerous experimental and clinical studies have revealed that drugs that act by delaying conduction, while markedly suppressing ventricular arrhythmias, have the proclivity to increase mortality in subsets of patients with significant cardiac disease. The adverse impact on mortality was confirmed by placebo-controlled randomized trials as well as meta-analysis of smaller randomized clinical trials. The latter indicated that beta blockers exert a beneficial effect on mortality. Benefit from drugs that lengthen repolarization, especially drugs that have the additional property of blocking sympathetic excitation, was also seen in relatively small numbers of patients. Sotalol and amiodarone fell into this category of antiarrhythmic drugs. There were 2 major consequences that stemmed from the results of these trials. First, the endpoint of clinical trials shifted to total mortality from surrogates such as defined degree of suppression of ventricular arrhythmias. Second, concern regarding increases in mortality produced by class I drugs engendered a shift in favor of drugs that prolong repolarization. Such a shift was bolstered by the growing body of data that established the efficacy of sotalol and amiodarone as potent agents for the control of life-threatening ventricular arrhythmias. They were both found to be superior to class I agents. The perception that the critical factor that mediates their efficacy is the homogeneous prolongation of repolarization has led to the synthesis and characterization of so-called pure class III agents, which include d-sotalol and other lKr blockers such as dofetilide, sematilide, E-4031, and almokalant, among numerous others. The increase in mortality produced by d-sotalol in patients with myocardial infarction and lowered ejection fraction and in patients with and without heart failure has led researchers to question how to design future antiarrhythmic molecules. In the search for an ideal antifibrillatory agent, should emphasis be placed on simple molecules such as pure class III agents or on those with more complex profiles, such as sotalol and amiodarone, which exhibit antiadrenergic actions and the ability to prolong cardiac repolarization? The available data are in favor of the latter approach.