Effect of calcium on acetylstrophanthidin-induced transient depolarizations in canine Purkinje tissue

Ventricular Tachycardia Due to Triggered Activity: Role of Early and Delayed Afterdepolarizations

Most forms of sustained ventricular tachycardia (VT) are caused by re-entry, resulting from altered myocardial conduction and refractoriness secondary to underlying structural heart disease. In contrast, VT caused by triggered activity (TA) is unrelated to an abnormal structural substrate and is often caused by molecular defects affecting ion channel function or regulation of intracellular calcium cycling. This review summarizes the cellular and molecular bases underlying TA and exemplifies their clinical relevance with selective representative scenarios. The underlying basis of TA caused by delayed afterdepolarizations is related to sarcoplasmic reticulum calcium overload, calcium waves, and diastolic sarcoplasmic reticulum calcium leak. Clinical examples of TA caused by delayed afterdepolarizations include sustained right and left ventricular outflow tract tachycardia and catecholaminergic polymorphic VT. The other form of afterpotentials, early afterdepolarizations, are systolic events and inscribe early afterdepolarizations during phase 2 or phase 3 of the action potential. The fundamental defect is a decrease in repolarization reserve with associated increases in late plateau inward currents. Malignant ventricular arrhythmias in the long QT syndromes are initiated by early afterdepolarization-mediated TA. An understanding of the molecular and cellular bases of these arrhythmias has resulted in generally effective pharmacologic-based therapies, but these are nonspecific agents that have off-target effects. Therapeutic efficacy may need to be augmented with an implantable defibrillator. Next-generation therapies will include novel agents that rescue arrhythmogenic abnormalities in cellular signaling pathways and gene therapy approaches that transfer or edit pathogenic gene variants or silence mutant messenger ribonucleic acid.

Timing mechanisms to control heart rhythm and initiate arrhythmias: roles for intracellular organelles, signalling pathways and subsarcolemmal Ca2

Rhythms of electrical activity in all regions of the heart can be influenced by a variety of intracellular membrane bound organelles. This is true both for normal pacemaker activity and for abnormal rhythms including those caused by early and delayed afterdepolarizations under pathological conditions. The influence of the sarcoplasmic reticulum (SR) on cardiac electrical activity is widely recognized, but other intracellular organelles including lysosomes and mitochondria also contribute. Intracellular organelles can provide a timing mechanism (such as an SR clock driven by cyclic uptake and release of Ca²⁺, with an important influence of intraluminal Ca²⁺), and/or can act as a Ca²⁺ store involved in signalling mechanisms. Ca²⁺ plays many diverse roles including carrying electric current, driving electrogenic sodium-calcium exchange (NCX) particularly when Ca²⁺ is extruded across the surface membrane causing depolarization, and activation of enzymes which target organelles and surface membrane proteins. Heart function is also influenced by Ca²⁺ mobilizing agents (cADP-ribose, nicotinic acid adenine dinucleotide phosphate and inositol trisphosphate) acting on intracellular organelles. Lysosomal Ca²⁺ release exerts its effects via calcium/calmodulin-dependent protein kinase II to promote SR Ca²⁺ uptake, and contributes to arrhythmias resulting from excessive beta-adrenoceptor stimulation. A separate arrhythmogenic mechanism involves lysosomes, mitochondria and SR. Interacting intracellular organelles, therefore, have profound effects on heart rhythms and NCX plays a central role. This article is part of the theme issue 'The heartbeat: its molecular basis and physiological mechanisms'.

Cardiac transmembrane ion channels and action potentials: cellular physiology and arrhythmogenic behavior

Cardiac arrhythmias are among the leading causes of mortality. They often arise from alterations in the electrophysiological properties of cardiac cells and their underlying ionic mechanisms. It is therefore critical to further unravel the pathophysiology of the ionic basis of human cardiac electrophysiology in health and disease. In the first part of this review, current knowledge on the differences in ion channel expression and properties of the ionic processes that determine the morphology and properties of cardiac action potentials and calcium dynamics from cardiomyocytes in different regions of the heart are described. Then the cellular mechanisms promoting arrhythmias in congenital or acquired conditions of ion channel function (electrical remodeling) are discussed. The focus is on human-relevant findings obtained with clinical, experimental, and computational studies, given that interspecies differences make the extrapolation from animal experiments to human clinical settings difficult. Deepening the understanding of the diverse pathophysiology of human cellular electrophysiology will help in developing novel and effective antiarrhythmic strategies for specific subpopulations and disease conditions.

Mechanochemotransduction during cardiomyocyte contraction is mediated by localized nitric oxide signaling

Cardiomyocytes contract against a mechanical load during each heartbeat, and excessive mechanical stress leads to heart diseases. Using a cell-in-gel system that imposes an afterload during cardiomyocyte contraction, we found that nitric oxide synthase (NOS) was involved in transducing mechanical load to alter Ca(2+) dynamics. In mouse ventricular myocytes, afterload increased the systolic Ca(2+) transient, which enhanced contractility to counter mechanical load but also caused spontaneous Ca(2+) sparks during diastole that could be arrhythmogenic. The increases in the Ca(2+) transient and sparks were attributable to increased ryanodine receptor (RyR) sensitivity because the amount of Ca2(+) in the sarcoplasmic reticulum load was unchanged. Either pharmacological inhibition or genetic deletion of nNOS (or NOS1), but not of eNOS (or NOS3), prevented afterload-induced Ca2(+) sparks. This differential effect may arise from localized NO signaling, arising from the proximity of nNOS to RyR, as determined by super-resolution imaging. Ca(2+)-calmodulin-dependent protein kinase II (CaMKII) and nicotinamide adenine dinucleotide phosphate oxidase 2 (NOX2) also contributed to afterload-induced Ca(2+) sparks. Cardiomyocytes from a mouse model of familial hypertrophic cardiomyopathy exhibited enhanced mechanotransduction and frequent arrhythmogenic Ca(2+) sparks. Inhibiting nNOS and CaMKII, but not NOX2, in cardiomyocytes from this model eliminated the Ca2(+) sparks, suggesting mechanotransduction activated nNOS and CaMKII independently from NOX2. Thus, our data identify nNOS, CaMKII, and NOX2 as key mediators in mechanochemotransduction during cardiac contraction, which provides new therapeutic targets for treating mechanical stress-induced Ca(2+) dysregulation, arrhythmias, and cardiomyopathy.

Ca²⁺ waves in the heart

Ca(2+) waves were probably first observed in the early 1940s. Since then Ca(2+) waves have captured the attention of an eclectic mixture of mathematicians, neuroscientists, muscle physiologists, developmental biologists, and clinical cardiologists. This review discusses the current state of mathematical models of Ca(2+) waves, the normal physiological functions Ca(2+) waves might serve in cardiac cells, as well as how the spatial arrangement of Ca(2+) release channels shape Ca(2+) waves, and we introduce the idea of Ca(2+) phase waves that might provide a useful framework for understanding triggered arrhythmias.

Variability in timing of spontaneous calcium release in the intact rat heart is determined by the time course of sarcoplasmic reticulum calcium load

BACKGROUND

Abnormalities in intracellular calcium (Ca) cycling during Ca overload can cause triggered activity because spontaneous calcium release (SCR) activates sufficient Ca-sensitive inward currents to induce delayed afterdepolarizations (DADs). However, little is known about the mechanisms relating SCR and triggered activity on the tissue scale.

METHODS AND RESULTS

Laser scanning confocal microscopy was used to measure the spatiotemporal properties of SCR within large myocyte populations in intact rat heart. Computer simulations were used to predict how these properties of SCR determine DAD magnitude. We measured the average and standard deviation of the latency distribution of SCR within a large population of myocytes in intact tissue. We found that as external [Ca] is increased, and with faster pacing rates, the average and SD of the latency distribution decreases substantially. This result demonstrates that the timing of SCR occurs with less variability as the sarcoplasmic reticulum (SR) Ca load is increased, causing more sites to release Ca within each cell. We then applied a mathematical model of subcellular Ca cycling to show that a decrease in SCR variability leads to a higher DAD amplitude and is dictated by the rate of SR Ca refilling following an action potential.

CONCLUSIONS

Our results demonstrate that the variability of the timing of SCR in a population of cells in tissue decreases with SR load and is dictated by the time course of the SR Ca content.

Diastolic transient inward current in long QT syndrome type 3 is caused by Ca2+ overload and inhibited by ranolazine

Long QT syndrome variant 3 (LQT-3) is a channelopathy in which mutations in SCN5A, the gene coding for the primary heart Na(+) channel alpha subunit, disrupt inactivation to elevate the risk of mutation carriers for arrhythmias that are thought to be calcium (Ca(2+))-dependent. Spontaneous arrhythmogenic diastolic activity has been reported in myocytes isolated from mice harboring the well-characterized Delta KPQ LQT-3 mutation but the link to altered Ca(2+) cycling related to mutant Na(+) channel activity has not previously been demonstrated. Here we have investigated the relationship between elevated sarcoplasmic reticulum (SR) Ca(2+) load and induction of spontaneous diastolic inward current (I(TI)) in myocytes expressing Delta KPQ Na(+) channels, and tested the sensitivity of both to the antianginal compound ranolazine. We combined whole-cell patch clamp measurements, imaging of intracellular Ca(2+), and measurement of SR Ca(2+) content using a caffeine dump methodology. We compared the Ca(2+) content of Delta KPQ(+/-) myocytes displaying I(TI) to those without spontaneous diastolic activity and found that I(TI) induction correlates with higher sarcoplasmic reticulum (SR) Ca(2+). Both spontaneous diastolic I(TI) and underlying Ca(2+) waves are inhibited by ranolazine at concentrations that preferentially target I(NaL) during prolonged depolarization. Furthermore, ranolazine I(TI) inhibition is accompanied by a small but significant decrease in SR Ca(2+) content. Our results provide the first direct evidence that induction of diastolic transient inward current (I(TI)) in Delta KPQ(+/-) myocytes occurs under conditions of elevated SR Ca(2+) load.

Calcium sparks

The calcium ion (Ca(2+)) is the simplest and most versatile intracellular messenger known. The discovery of Ca(2+) sparks and a related family of elementary Ca(2+) signaling events has revealed fundamental principles of the Ca(2+) signaling system. A newly appreciated "digital" subsystem consisting of brief, high Ca(2+) concentration over short distances (nanometers to microns) comingles with an "analog" global Ca(2+) signaling subsystem. Over the past 15 years, much has been learned about the theoretical and practical aspects of spark formation and detection. The quest for the spark mechanisms [the activation, coordination, and termination of Ca(2+) release units (CRUs)] has met unexpected challenges, however, and raised vexing questions about CRU operation in situ. Ample evidence shows that Ca(2+) sparks catalyze many high-threshold Ca(2+) processes involved in cardiac and skeletal muscle excitation-contraction coupling, vascular tone regulation, membrane excitability, and neuronal secretion. Investigation of Ca(2+) sparks in diseases has also begun to provide novel insights into hypertension, cardiac arrhythmias, heart failure, and muscular dystrophy. An emerging view is that spatially and temporally patterned activation of the digital subsystem confers on intracellular Ca(2+) signaling an exquisite architecture in space, time, and intensity, which underpins signaling efficiency, stability, specificity, and diversity. These recent advances in "sparkology" thus promise to unify the simplicity and complexity of Ca(2+) signaling in biology.

The missing link in the mystery of normal automaticity of cardiac pacemaker cells

Earlier studies of the initiating event of normal automaticity of the heart's pacemaker cells, inspired by classical quantitative membrane theory, focused upon ion currents (IK, I f) that determine the maximum diastolic potential and the early phase of the spontaneous diastolic depolarization (DD). These early DD events are caused by the prior action potential (AP) and essentially reflect a membrane recovery process. Events following the recovery process that ignite APs have not been recognized and remained a mystery until recently. These critical events are linked to rhythmic intracellular signals initiated by Ca2+ clock (i.e., sarcoplasmic reticulum [SR] cycling Ca2+). Sinoatrial cells, regardless of size, exhibit intense ryanodine receptor (RyR), Na+/Ca2+ exchange (NCX)-1, and SR Ca2+ ATPase-2 immunolabeling and dense submembrane NCX/RyR colocalization; Ca2+ clocks generate spontaneous stochastic but roughly periodic local subsarcolemmal Ca2+ releases (LCR). LCRs generate inward currents via NCX that exponentially accelerate the late DD. The timing and amplitude of LCR/I NCX-coupled events control the timing and amplitude of the nonlinear terminal DD and therefore ultimately control the chronotropic state by determining the timing of the I CaL activation that initiates the next AP. LCR period is precisely controlled by the kinetics of SR Ca2+ cycling, which, in turn, are regulated by 1) the status of protein kinase A-dependent phosphorylation of SR Ca2+ cycling proteins; and 2) membrane ion channels ensuring the Ca2+ homeostasis and therefore the Ca2+ available to Ca2+ clock. Thus, the link between early DD and next AP, missed in earlier studies, is ensured by a precisely physiologically regulated Ca2+ clock within pacemaker cells that integrates multiple Ca2+-dependent functions and rhythmically ignites APs during late DD via LCRs-I NCX coupling.

The effects of isoproterenol on abnormal electrical and contractile activity and diastolic calcium are attenuated in myocytes from aged Fischer 344 rats

We investigated whether the age-related decrease in sensitivity of the heart to catecholamines was accompanied by changes in Ca(2+) homeostasis and abnormal electrical and contractile activity caused by beta-adrenergic receptor (beta-AR) stimulation. Ventricular myocytes were isolated from young adult (3 months) and aged (24 months) male Fischer 344 rats. Unloaded cell shortening was measured in field-stimulated myocytes (2Hz, 37 degrees C); membrane currents and action potentials were measured with microelectrodes. Contractile responses to the non-selective beta-AR agonist, isoproterenol were significantly decreased in aged myocytes compared to younger myocytes and aged myocytes were less sensitive to isoproterenol. In contrast, Ca(2+) transients measured simultaneously with contractions were similar between groups. Isoproterenol increased sarcoplasmic reticulum Ca(2+) stores in both groups, but the increase was larger in aged cells. However, signs of Ca(2+) overload induced by isoproterenol were reduced with age. Diastolic Ca(2+) accumulation, contracture and the incidences of transient inward current, oscillatory afterpotentials (OAPs), aftertransients and aftercontractions induced by isoproterenol also were reduced with age. These results demonstrate that aged myocytes exhibit fewer signs of Ca(2+) overload in response to isoproterenol than young adult myocytes. These age-related changes in intracellular Ca(2+) may protect the aging heart against induction of arrhythmias initiated by OAPs.(1).

Altered Na+ channels promote pause-induced spontaneous diastolic activity in long QT syndrome type 3 myocytes

Long QT syndrome (LQTS) type 3 (LQT3), typified by the DeltaKPQ mutation (LQT3 mutation in which amino acid residues 1505 to 1507 [KPQ] are deleted), is caused by increased sodium entry during the action potential plateau resulting from mutation-altered inactivation of the Na(v)1.5 channel. Although rare, LQT3 is the most lethal of common LQTS variants. Here we tested the hypothesis that cellular electrical dysfunction, caused not only by action potential prolongation but also by mutation-altered Na(+) entry, distinguishes LQT3 from other LQTS variants and may contribute to its distinct lethality. We compared cellular electrical activity in myocytes isolated from mice heterozygous for the DeltaKPQ mutation (DeltaKPQ) and myocytes from wild-type littermates. Current-clamp pause protocols induced rate-dependent spontaneous diastolic activity (delayed after depolarizations) in 6 of 7 DeltaKPQ, but no wild-type, myocytes (n=11) tested. Voltage-clamp pause protocols that independently control depolarization duration and interpulse interval identified a distinct contribution of both depolarization duration and mutant Na(+) channel activity to the generation of Ca(i)(2+)-dependent diastolic transient inward current. This was found at rates and depolarization durations relevant both to the mouse model and to LQT3 patients. Flecainide, which preferentially inhibits mutation-altered late Na(+) current and is used to treat LQT3 patients, suppresses transient inward current formation in voltage-clamped DeltaKPQ myocytes. Our results demonstrate a marked contribution of mutation-altered Na(+) entry to the incidence of pause-dependent spontaneous diastolic activity in DeltaKPQ myocytes and suggest that altered Na(+) entry may contribute to the elevated lethality of LQT3 versus other LQTS variants.

Ryanodine receptors and ventricular arrhythmias: emerging trends in mutations, mechanisms and therapies

It has been six years since the first reported link between mutations in the cardiac ryanodine receptor Ca(2+) release channel (RyR2) and catecholaminergic polymorphic ventricular tachycardia (CPVT), a malignant stress-induced arrhythmia. In this time, rapid advances have been made in identifying new mutations, and in understanding how these mutations disrupt normal channel function to cause VT that frequently degenerates into ventricular fibrillation (VF) and sudden death. Functional characterisation of these RyR2 Ca(2+) channelopathies suggests that mutations alter the ability of RyR2 to sense its intracellular environment, and that channel modulation via covalent modification, Ca(2+)- and Mg(2+)-dependent regulation and structural feedback mechanisms are catastrophically disturbed. This review reconciles the current status of RyR2 mutation-linked etiopathology, the significance of mutational clustering within the RyR2 polypeptide and the mechanisms underlying channel dysfunction. We will also review new data that explores the link between abnormal Ca(2+) release and the resultant cardiac electrical instability in VT and VF, and how these recent developments impact on novel anti-arrhythmic therapies. Finally, we evaluate the concept that mechanistic differences between CPVT and other arrhythmogenic disorders may preclude a common therapeutic strategy to normalise RyR2 function in cardiac disease.

The emergence of a general theory of the initiation and strength of the heartbeat

Sarcoplasmic reticulum (SR) Ca(2+) cycling, that is, the Ca(2+) clock, entrained by externally delivered action potentials has been a major focus in ventricular myocyte research for the past 5 decades. In contrast, the focus of pacemaker cell research has largely been limited to membrane-delimited pacemaker mechanisms (membrane clock) driven by ion channels, as the immediate cause for excitation. Recent robust experimental evidence, based on confocal cell imaging, and supported by numerical modeling suggests a novel concept: the normal rhythmic heart beat is governed by the tight integration of both intracellular Ca(2+) and membrane clocks. In pacemaker cells the intracellular Ca(2+) clock is manifested by spontaneous, rhythmic submembrane local Ca(2+) releases from SR, which are tightly controlled by a high degree of basal and reserve PKA-dependent protein phosphorylation. The Ca(2+) releases rhythmically activate Na(+)/Ca(2+) exchange inward currents that ignite action potentials, whose shape and ion fluxes are tuned by the membrane clock which, in turn, sustains operation of the intracellular Ca(2+) clock. The idea that spontaneous SR Ca(2+) releases initiate and regulate normal automaticity provides the key that reunites pacemaker and ventricular cell research, thus evolving a general theory of the initiation and strength of the heartbeat.

Calcium overload and cardiac function

The changes in cardiac function caused by calcium overload are reviewed. Intracellular Ca(2+) may increase in different structures [e.g. sarcoplasmic reticulum (SR), cytoplasm and mitochondria] to an excessive level which induces electrical and mechanical abnormalities in cardiac tissues. The electrical manifestations of Ca(2+) overload include arrhythmias caused by oscillatory (V(os)) and non-oscillatory (V(ex)) potentials. The mechanical manifestations include a decrease in force of contraction, contracture and aftercontractions. The underlying mechanisms involve a role of Na(+) in electrical abnormalities as a charge carrier in the Na(+)-Ca(2+) exchange and a role of Ca(2+) in mechanical toxicity. Ca(2+) overload may be induced by an increase in [Na(+)](i) through the inhibition of the Na(+)-K(+) pump (e.g. toxic concentrations of digitalis) or by an increase in Ca(2+) load (e.g. catecholamines). The Ca(2+) overload is enhanced by fast rates. Purkinje fibers are more susceptible to Ca(2+) overload than myocardial fibers, possibly because of their greater Na(+) load. If the SR is predominantly Ca(2+) overloaded, V(os) and fast discharge are induced through an oscillatory release of Ca(2+) in diastole from the SR; if the cytoplasm is Ca(2+) overloaded, the non-oscillatory V(ex) tail is induced at negative potentials. The decrease in contractile force by Ca(2+) overload appears to be associated with a decrease in high energy phosphates, since it is enhanced by metabolic inhibitors and reduced by metabolic substrates. The ionic currents I(os) and I(ex) underlie V(os) and V(ex), respectively, both being due to an electrogenic extrusion of Ca(2+) through the Na(+)-Ca(2+) exchange. I(os) is an oscillatory current due to an oscillatory release of Ca(2+) in early diastole from the Ca(2+)-overloaded SR, and I(ex) is a non-oscillatory current due to the extrusion of Ca(2+) from the Ca(2+)-overloaded cytoplasm. I(os) and I(ex) can be present singly or simultaneously. An increase in [Ca(2+)](i) appears to be involved in the short- and long-term compensatory mechanisms that tend to maintain cardiac output in physiological and pathological conditions. Eventually, [Ca(2+)](i) may increase to overload levels and contribute to cardiac failure. Experimental evidence suggests that clinical concentrations of digitalis increase force in Ca(2+)-overloaded cardiac cells by decreasing the inhibition of the Na(+)-K(+) pump by Ca(2+), thereby leading to a reduction in Ca(2+) overload and to an increase in force of contraction.

Calcium and cardiac arrhythmias: DADs, EADs, and alternans

Rapid progress has been made in understanding the molecular mechanisms by which calcium ions mediate certain cardiac arrhythmias. Principal advances include imaging of cytosolic calcium in isolated cells and in intact tissues, use of fluorescent indicators and monophasic action potentials to record membrane potentials in isolated tissue, and sequencing of the genes that encode critical ion channel proteins. In this review, five types of arrhythmias are discussed where calcium ion currents, or currents controlled by calcium, appear to be responsible for arrythmogenesis. These include: (1) the delayed afterpotential that occurs in conditions of intracellular calcium overload such as digitalis toxicity; (2) the early afterdepolarization that occurs when action potential duration is prolonged; (3) the slowly conducted calcium-dependent action potential (the slow response) in the SA and AV nodes; (4) the phenomenon of calcium transient alternans during ischemia, which is related to action potential duration alternans and t-wave alternans; (5) catecholamine-induced cardiac arrhythmias in families with mutations of the sarcoplasmic reticulum calcium-release channel. For each type of arrhythmia, the clinical implications of emerging knowledge are discussed. An especially important issue is whether ventricular fibrillation during acute coronary artery occlusion is due to calcium transient alternans. Ventricular fibrillation due to acute ischemia is an important subset of the 400,000 sudden cardiac deaths that occur annually in the U.S. Certain drugs, including beta blockers, fish oils, verapamil, and diltiazem, seem to specifically prevent ventricular fibrillation in this setting, and in most cases an effect of the drug on cytosolic calicum appears to be involved.

Spontaneous secondary spiking in excitable cells

Kepler & Marder (1993, Biol. Cybern.68, 209-214) proposed a model describing the electrical activity of a crab neuron in which a train of directly induced action potentials is sometimes followed by one or more spontaneous action potentials, referred to as spontaneous secondary spikes. We reduce their five-dimensional model to three dimensions in two different ways in order to gain insight into the mechanism underlying the spontaneous spikes. We then treat a slowly varying current as a parameter in order to give a qualitative explanation of the phenomenon using phase-plane and bifurcation analysis. We demonstrate that a three-dimensional model, consisting of a two-dimensional excitable system plus a slow inward current, is sufficient to produce the behaviour observed in the original model. The exact dynamics of the excitable system are not important, but the relative time constant and amplitude of the slow inward current are crucial. Using the numerical bifurcation analysis package AUTO (Doedel & Kernevez, 1986, AUTO: Software for Continuation and Bifurcation Problems in Ordinary Differential Equations. California Institute of Technology), we compute bifurcation diagrams using the maximum amplitude of the slow inward current as the bifurcation parameter. The full and reduced models have a stable resting potential for all values of the bifurcation parameter. At a critical value of the bifurcation parameter, a stable tonic firing mode arises via a saddle-node of periodics bifurcation. Whether or not the models can exhibit transient or continuous spontaneous spiking depends on their position in parameter space relative to this saddle-node of periodics.

Clinical relevance of cardiac arrhythmias generated by afterdepolarizations. Role of M cells in the generation of U waves, triggered activity and torsade de pointes

Recent findings point to an important heterogeneity in the electrical behavior of cells spanning the ventricular wall as well as important differences in the response of the various cell types to cardioactive drugs and pathophysiologic states. These observations have permitted a fine tuning and, in some cases, a reevaluation of basic concepts of arrhythmia mechanisms. This brief review examines the implications of some of these new findings within the scope of what is already known about early and delayed afterdepolarizations and triggered activity and discusses the possible relevance of these mechanisms to clinical arrhythmias.

Sodium-pump injury and arrhythmogenic transient depolarizations in catecholamine-induced cardiac hypertrophy

The pathogenesis of arrhythmogenic transient depolarizations (TDs) was studied by means of electrophysiological and cytochemical methods in normal and hypertrophied left ventricular myocardium of the rat. In hypertrophy induced by administration of 5 mg/kg isoprenaline once daily for 7 days, the myocardial membrane was depolarized, the action potential duration was prolonged and the Vmax was decreased, as compared with those of age-matched normal controls. TDs induced by a train of action potentials could be observed in hypertrophied myocardium, but not in normal control myocardium. Ryanodine completely abolished TDs, but the beta-adrenoceptor agonist noradrenaline and the adenylate cyclase activator forskolin were without effect. In cytochemical studies, the Na+,K(+)-ATPase activity was localized in the sarcolemma, and three times as much reaction product, which appeared on the inner side of the cell membrane, was found in the normal myocardium than in the hypertrophied myocardium. The results suggest that catecholamine-induced cardiac hypertrophy damages the membrane-bound Na+,K(+)-ATPase and causes a cAMP-independent intracellular Ca overload and TDs, thereby permitting abnormal impulse formation, which predisposes the diseased myocardium to develop arrhythmias.

Mechanisms by which calcium modulates diastolic depolarization in sheep cardiac Purkinje fibers

The mechanisms by which calcium modulates diastolic depolarization (DD) in sheep cardiac Purkinje fibers were studied in vitro. Increasing [Ca]o from 2.7 mM to 10.8 mM increased both the slope and amplitude of DD, induced oscillatory potentials (V(os)), and prolonged depolarization (V(ex)). The steepening of DD occurred even in the absence of an obvious V(os). The increase in DD amplitude was due both to an increase in the maximum diastolic potential and to a less negative steady-state level. At constant [Ca]o, increasing the driving rate had effects similar to those induced by increasing [Ca]o. The increase in DD slope and amplitude was least at the slowest rates and leveled off at the fastest rates in high [Ca]o. Lowering [Ca]o decreased DD slope and amplitude, but spontaneous activity could be present during interruption of the drive. In slowly driven fibers, increasing [Ca]o to 10.8 mM initially shifted the maximum diastolic potential and steady state DD to more negative values, and subsequently shifted the latter (but not the former) to less negative values. On recovery, a transient depolarization occurred. Quiescent fibers exposed to high [Ca]o also underwent a transient hyperpolarization and a subsequent depolarization, whereas reciprocal effects occurred when [Ca]o was lowered. It is concluded that [Ca]o modulates DD through several different mechanisms and that most (but not all) modifications induced are brought about by changes in [Ca]i.

Ca2+ dependence of alpha-adrenergic effects on the contractile properties and Ca2+ homeostasis of cardiac myocytes

alpha-Adrenergic stimulation is known to enhance myocardial contractility. Adult rat left ventricular myocytes bathed in 1 mM [Ca2+] (Ca0) and electrically stimulated at 0.2 Hz responded to alpha-adrenergic stimulation with 50 microM phenylephrine and 1 microM propranolol with an increase in twitch amplitude to 177.1 +/- 25.6% of control (mean +/- SEM). In contrast, when cell Ca2+ loading was increased by bathing cells in 5 mM Ca0, alpha-adrenergic stimulation decreased twitch amplitude to 68.6 +/- 8.2% of control. Time-averaged cytosolic [Ca2+] of cells in 1.0 mM Ca0 is enhanced via an increase in the frequency of electrical stimulation. When myocytes were stimulated at 2 Hz in 1 mM Ca0, alpha-adrenergic stimulation did not increase twitch amplitude (103.8 +/- 12.4% of control). In myocytes loaded with the Ca2+ probe into-1, alpha-adrenergic effects during stimulation at 0.2 Hz (an increase in twitch amplitude in 1 mM Ca0 and a decrease in twitch amplitude in 5 mM Ca0) were associated with similar changes in the indo-1 transient. In 5 mM Ca0, spontaneous Ca2+ releases from the sarcoplasmic reticulum (SR) occurred in the diastolic interval between twitches (2.9 +/- 1.4 spontaneous SR Ca2+ oscillations/min; n = 7); alpha-adrenergic stimulation abolished these oscillations in six of seven cells. Thus, an increase in the frequency of spontaneous diastolic SR Ca2+ release (i.e., Ca2+ overload) is not the mechanism for the negative inotropic effect of alpha-adrenergic stimulation in 5 mM Ca0. In experiments with unstimulated myocytes, we determined whether the effect of alpha-adrenergic stimulation on cell Ca2+ homeostasis and oscillatory SR Ca2+ release observed in 5 mM Ca0 occurs only during electrical stimulation, when voltage-dependent currents are operative, or also at rest. Unstimulated rat ventricular myocytes in 5 mM Cao exhibit oscillatory SR Ca2+ release; alpha-adrenergic stimulation decreased the frequency of these oscillations to 53.9 +/- 8.9% of control, and this effect was blocked by 1 microM prazosin. In unstimulated indo-1-loaded myocytes alpha-adrenergic stimulation decreased the resting indo-1 fluorescence ratio in 5 mM Ca0, whereas it had no effect in 1 mM Ca0. Additional experiments were aimed at defining a role for Ca(2+)-activated, phospholipid-dependent protein kinase C (PKC) for the negative inotropic effect of alpha-adrenergic stimulation in 5 mM Ca0. Short-term preexposure to 0.1 microM 4 beta-phrobol 12-myristate 13-acetate (PMA) has been shown to maximally activate PKC.(ABSTRACT TRUNCATED AT 400 WORDS)

Effects of thiopental and halothane on spontaneous contractile activity induced in isolated ventricular muscles of the rabbit

To see if the known properties of thiopental of reducing Ca2+ and K+ fluxes across the myocardial sarcolemma account for its arrhythmogenic action, we have evaluated the effect of the anesthetic on spontaneous contractile activity induced in isolated rabbit papillary muscles. Thiopental (20 mg/l) prolonged the duration of sustained automaticity induced by stimulation at 1-2 Hz in the presence of 1 mumol/l isoproterenol. Thiopental (10, 20 mg/l) shortened the delay before the onset of Ba(2+)-induced automaticity, which involves a decrease in a K+ current. The minimum concentration of Ba2+ required to induce automaticity was lowered by thiopental. Whether spontaneous activities were induced by high frequency stimulation in the presence of isoproterenol or by Ba2+, thiopental lowered the frequency of spontaneous beats. Thus, thiopental appears to have both arrhythmogenic and antiarrhythmic actions, and the former may be unmasked when catecholamines counteract the latter by increasing Ca2+ influx. Like thiopental, halothane (1.0%) decreased the frequency and force of Ba(2+)-induced automatic beats but, unlike thiopental, prolonged the delay before the onset of Ba(2+)-induced automaticity, indicating that halothane acts as a purely antiarrhythmic agent in this type of automaticity.

Deleterious effects of digitalis on reperfusion-induced arrhythmias and myocardial injury in ischemic rat hearts: possible involvements of myocardial Na+ and Ca2+ imbalance

Isolated rat hearts were made ischemic for 25 min after an initial recirculating perfusion, followed by 30 min of reperfusion. In some hearts, interventions including administration of ouabain and/or high [K+] in the buffer were performed during the first 10 min of reperfusion. During ischemia, intracellular Na+ (Nai) increased from 15 to 64 mumol/g dry weight (dwt). During reperfusion, Nai declined rapidly (at 10 min of reperfusion: 48 mumol/g dwt, at 30 min: 25 mumol/g dwt) and regular rhythm was recovered within 10 min in hearts without any intervention during reperfusion. 45Ca2+ uptake increased from 0.8 to 7.5 mumol/g dwt after 30 min of reperfusion. Ventricular function recovered by 45%. A 10-min perfusion with 10 or 50 microM of ouabain increased Nai (17 to 21 or 27 mumol/g dwt) with increased left-ventricular (LV) contractile function, but these effects were reversed by combination of high perfusate [K+] (20 mM) in non-ischemic hearts. A 10-min reperfusion with ouabain retarded or stopped the decline in Nai (at 10 min of reperfusion: 54 or 63 mumol/g dwt, at 30 min: 32 or 40 mumol/g dwt). These amounts of ouabain also increased the incidence of ventricular tachyarrhythmias during reperfusion to 30% or 50%, and increased the duration of ventricular fibrillation from 6.5 to 11.5 or 18.0 min. 45Ca2+ uptake reached to 8.8 or 10.0 mumol/g dwt, and function recovered only 35% or 28%. When high perfusate [K+] was combined with ouabain during reperfusion, the retarded decline in Nai, augmented 45Ca2+ uptake, and reduced recovery of function caused by ouabain alone were attenuated. These results suggest that digitalis has toxic effects on reperfused ischemic hearts by inhibition of rapid active outward transport of previously elevated Nai and potentiation of Ca2+ overload.

Positive inotropic effect of porcine left ventricular extract on canine ventricular muscle

1. We previously isolated an extract from porcine left ventricle that possessed digitalis-like properties such as inhibition of cardiac and kidney Na+, K(+)-ATPase, displacement of [3H]-ouabain from its binding sites and cross reactivity with digoxin antibodies. The extract also had a positive inotropic effect on the guinea-pig heart. 2. In the present study the positive inotropic response of the extract was characterized in canine right ventricular trabeculae. Maximum inotropic response (501 +/- 20%) was produced by 300 microliters and the half maximal increase occurred with 125 microliters of the extract. 3. Ouabagenin produced aftercontractions in rapidly paced trabeculae. Equipotent and even greater amounts of the extract did not produce aftercontractions. 4. The extract increased the amplitude of the delayed component (P2) of biphasic contractions produced by replacing about 92-96% of the external Ca with Sr. A smaller increase in the size of the early component (P1) was also seen. 5. The extract decreased post-rest potentiation after rest for 30s and 2 min. After 8 min of rest, post-rest potentiation was converted to post-rest depression. 6. The extract (20 microliters) produced a decrease in the amplitude of the post-rest rapid cooling contracture (RCC) at all rest intervals. The steady state RCC, although greater than that in the control muscle, was increased to a lesser extent than the size of the steady state electrically driven contractions. 7. It is suggested that the extract from porcine left ventricle produces a positive inotropic response by increasing the trans-sarcolemmal influx of Ca. It also has additional effect(s) on the sarcoplasmic reticulum in that it may facilitate the loss of Ca from the sarcoplasmic reticulum and/or inhibit the uptake of Ca by the organelle.

In vivo induction of "focal" triggered ventricular arrhythmias and responses to overdrive pacing in the canine heart

Delayed afterdepolarizations and triggered activity were evoked in focal areas of myocardium in vivo by local exposure of endocardium to ouabain by means of a catheter electrode system capable of recording monophasic action potentials (MAPs) and delivering ouabain to the recording site. MAPs were recorded from the septum and the posterior wall of the left ventricle with silver-silver chloride electrode catheters. Ouabain (10(-5) M) was infused through the MAP recording catheter onto the endocardial surface of the septum. After infusion of 10 micrograms/kg ouabain, the amplitude of MAPs recorded from the septum (the site of ouabain infusion) decreased from 37.4 +/- 11.8 to 32.0 +/- 10.1 mV (p less than 0.01), MAP duration at 50% repolarization shortened from 160 +/- 29 to 148 +/- 34 msec (p less than 0.01), and MAP duration at 90% repolarization shortened from 198 +/- 38 to 189 +/- 46 msec (p less than 0.01). MAPs recorded from the posterior wall (the reference site) were unchanged. Delayed afterdepolarizations were recorded at the site of ouabain infusion, but not at the reference site, when the heart was paced at cycle lengths of 200-600 msec. Additional infusion of ouabain induced sustained monomorphic ventricular tachycardia (VT) (mean cycle length, 369 +/- 12 msec) in all 15 dogs studied. The mean concentration of ouabain required to induce VT was 20.9 +/- 10.0 micrograms/kg. Paced QRS complexes when stimulated at the site of ouabain infusion had the same morphology as those of spontaneous VT. Local perfusion of verapamil, 0.015-0.034 mg/kg, through the MAP recording catheter onto the site of ouabain infusion completely eliminated VT and premature ventricular contractions. After perfusion of verapamil, delayed afterdepolarizations could no longer be induced by pacing. These observations indicate that induced VT originated from the site of ouabain infusion, and the presence of delayed afterdepolarizations before development of VT strongly suggests that the induced VT was due to triggered activity. Using this model, we examined the responses to rapid ventricular pacing of "focal" triggered VT. The first beat of the reinitiated tachycardia displayed the same morphology as the spontaneous VT. Local perfusion of verapamil, 0.015-0.034 mg/kg, through the MAP recording catheter onto the site of ouabain infusion completely eliminated VT and premature ventricular contractions. After perfusion of verapamil, delayed afterdepolarizations could no longer be induced by pacing. These observations indicate that induced VT originated from the site of ouabain infusion, and the presence of delayed afterdepolarizations before development of VT strongly suggests that the induced VT was due to triggered activity.(ABSTRACT TRUNCATED AT 400 WORDS)

Cardiac glycosides and congestive heart failure

Although the cardiac glycosides are universally acknowledged to be important agents in the drug therapy of advanced congestive heart failure (CHF), their role in the treatment of more moderate CHF, particularly in patients in sinus rhythm, remains controversial. Over the past decade, several randomized clinical trials have been undertaken to help clarify the appropriate use of the cardiac glycosides in these patients. Although the data are not conclusive, the available evidence indicates that digoxin is efficacious and relatively safe in patients with CHF whether given alone or in combination with vasodilators. Ongoing myocardial ischemia, hypokalemia and reduced drug clearance due to renal disease or drug interactions remain the clinical parameters most closely associated with digitalis toxicity. However, the recent introduction and widespread availability of a safe and rapidly effective antidote to digitalis preparations--Fab fragments of antidigoxin antibodies--offers the clinician a greater margin of safety in the use of the cardiac glycosides than has been available in the past.

Comparison of sodium-calcium exchanger and transient inward currents in single cells from rabbit ventricle

1. Whole-cell voltage-clamp measurements have been made in rabbit ventricular myocytes under conditions in which both Na(+)-Ca2+ exchanger currents (IEX, slow tails) and transient inward currents (ITI or TI) can be recorded. A number of experimental manoeuvres have been used in an attempt to separate or dissociate these two currents. 2. As expected, partial inhibition of the Na(+)-K+ pump by application of 0.54 mM [K+] Tyrode solution or 10(-5) M-strophanthidin induced TI currents which were recorded in the presence of IEX slow tails. 3. Complete inhibition of the Na(+)-K+ pump with zero [K+] Tyrode solution resulted in larger and more frequent TIs but smaller IEX tails. 4. A somewhat similar dissociation between ITI and IEX was observed when NaCl was reduced to 37.5 mM by using LiCl to replace NaCl. This inhibited the Na(+)-Ca2+ exchanger current, but induced ITI. 5. Transient inward currents and IEX tails could also be separated by selected patterns of stimulation (voltage-clamp depolarizations): following the second pulse of a pair of stimuli, IEX was significantly reduced whereas the TIs increased in size and frequency. 6. Additional experimental tests involving changes in external divalent ions could also separate these two currents. Increasing [Ca2+]o 3-fold increased the TIs without changing IEX. Shortly after [Ca2+]o was replaced with either [Ba2+]o or [Sr2+]o the TIs were blocked but IEX was unchanged. Application of MnCl2 (1 mM) and elevation of [K+]o inhibited IEX but did not significantly change the TI currents. 7. Application of caffeine (5-10 mM) or ryanodine (2 x 10(-6) M) blocked the TI currents at times when the IEX tails were not changed. 8. In combination these results suggest that even though both IEX and ITI are triggered (activated) by increases in [Ca2+]i, these two currents are distinct. IEX is generated by electrogenic Na(+)-Ca2+ exchange, while the TI currents may be due to Ca2(+)-activated cation-selective channels in the sarcolemma.

Electrophysiological abnormalities and enhanced reperfusion arrhythmias in the isolated hearts of hyperthyroid rats

1. The influence of hyperthyroidism on electrophysiological characteristics and on reperfusion arrhythmias was examined in rat hearts. 2. Electrophysiological studies were performed with glass microelectrodes, and the experiments on reperfusion arrhythmias were done in isolated perfused hearts. 3. Ventricular muscle from hyperthyroid rats was more prone than that from euthyroid rats to develop triggered activity under conditions believed to cause myoplasmic Ca2+ overload. 4. The severity of reperfusion arrhythmias was significantly enhanced in hyperthyroid preparations as compared with euthyroid ones. 5. The enhanced reperfusion arrhythmias in hyperthyroid rats were significantly reduced by propranolol (3 x 10(-7) M), lignocaine (1 x 10(-5) M) and verapamil (3 x 10(-8) M), but not by nadolol (3 x 10(-7) M) or prazosin (3 x 10(-7) M). 6. These results suggest that increased heart rate due to hyperthyroidism and responses mediated via either alpha- or beta-adrenoceptors were not dominant causes of enhanced reperfusion arrhythmias in hyperthyroid hearts. 7. The increased tendency to develop triggered activity which was observed in the electrophysiological study, may be one possible explanation of enhanced reperfusion arrhythmias in hyperthyroid hearts.

Fluctuations of membrane potential in isolated single ventricular myocytes of guinea-pig upon resumption of oxidative phosphorylation

Electrical and mechanical activities of guinea-pig single ventricular myocytes were investigated under conditions simulating hypoxia-reoxygenation. The localized movement of sarcomere was recorded simultaneously with membrane potential, and analyzed using microcomputer-based image processing. Exposure to 5 mM CN- caused progressive shortening of action potential duration and attenuation of twitch contraction. The myocytes became inexcitable about 30 to 70 min after the CN- treatment. On removal of CN-, the myocytes exhibited periodic miniature membrane depolarizations from the resting potential level (-95 mV). When depolarizations were smaller than 6 mV in amplitude and longer than 500 ms in duration, they were accompanied by localized sarcomere shortening like a propagating contractile wave (unifocal oscillation). Membrane depolarizations of larger amplitude and shorter duration were associated with a more uniform pattern of localized sarcomere shortening (multifocal oscillation). Trains of electrical stimuli applied during the washing out period caused transient augmentation of potential fluctuation and enhancement of synchronization of sarcomere shortening. These results suggest that non-uniform elevation of intracellular calcium concentration on the resumption of oxidative phosphorylation may initiate oscillatory fluctuations of membrane potential leading to abnormal spontaneous excitation.

Antiarrhythmic effects of alpha-adrenoceptor antagonists in guinea pig ventricular myocardium

Antiarrhythmic effects of alpha-adrenoceptor antagonists were assessed in the reserpinized guinea pig ventricular myocardium. Both bunazosin (1 to 3 x 10(-7) M), a new alpha 1-adrenoceptor antagonist, and yohimbine (1 to 3 x 10(-7) M), another adrenoceptor antagonist, suppressed the transient depolarization and triggered activity induced by a train of rapid stimuli in the solution containing low potassium ion (K+), high calcium ion (Ca2+) and strophanthidin (1 to 5 x 10(-7) M). Bunazosin (3 x 10(-6) M) abolished the facilitatory effect of hypoxia on beta-adrenoceptor mediated abnormal automaticity. To clarify the mechanisms underlying the antiarrhythmic properties of alpha-adrenoceptor antagonists, their electrophysiologic effects on the fast and slow action potentials were investigated. Alpha-adrenoceptor antagonists (bunazosin, yohimbine and phentolamine) suppressed the slow response in a dose-related manner. The voltage-dependent block and use-dependent block of the maximal rate of rise (Vmax) of action potentials by bunazosin (10(-5) to 10(-4) M) and yohimbine (10(-6) to 10(-5) M) were studied. The analysis of the onset and recovery kinetics from the use-dependent block of drugs showed that both bunazosin and yohimbine act as slow kinetic drugs. It is concluded that alpha-adrenoceptor antagonists seem to have an antiarrhythmic effect through the inhibition of fast sodium ion (Na+) and slow Ca2+ currents of the cell membrane independently of blockade of myocardial alpha-adrenoceptors.

Electrical and mechanical effects of new aminosteroids on guinea-pig isolated ventricular muscle

1. LND 623 and LND 796 are two aminosteroid derivatives which exert similar positive inotropic effects to digitalis. Their electrophysiological, toxic and inotropic effects were investigated in both normal and partially K+-depolarized ventricular muscle. 2. In guinea-pig myocardial fibres, LND 623 and LND 796 required tenfold higher concentrations than digoxin to induce the same signs of toxicity; e.g. triggered activities generated from delayed afterdepolarizations, leading to the marked depression of action potential characteristics and inexcitability. These abnormal rhythms and delayed afterdepolarizations were abolished by 1 mM caffeine. The toxic effects were reversed by washout, particularly in the case of LND 796. 3. In normal-K+ solution, LND 623 and LND 796 exhibited concentration-dependent positive inotropic effects on guinea-pig papillary muscle and increased concomitantly resting membrane potential and action potential amplitude. The range of active concentrations (0.1 to 1 microM) of LND 623 was larger than that of digoxin (0.3 to 1 microM). Like digoxin, LND 796 exerted negative inotropic effects at the lowest concentrations (0.01 to 0.03 microM) and positive inotropic effects at high concentrations (1 and 3 microM). 4. In partially K+-depolarized papillary muscle, in the presence of 2 microM histamine, LND 623 (3 and 10 microM) and LND 796 (10 and 30 microM) enhanced the two components P1 and P2 of the contraction and increased slow action potential amplitude, resting potential and maximal rate of depolarization. Low concentrations (0.03 to 0.3 microM) of LND 796 induced negative inotropic effects. beta-Adrenoceptor blockade with atenolol (1 microM) did not modify the activity of LND 623 but significantly enhanced the negative inotropic effect on P2 induced by 1 and 3 microM LND 796 and reduced the positive inotropic effect on P1 and P2 of the highest concentration (30 microM) studied. 5. In the presence of either caffeine (1 mM) or Ca2+-free, Sr2+-rich (3.6 mM) solution, LND 623 and LND 796 produced a positive inotropic effect which was stronger with LND 623. 6. It is suggested that two mechanisms are involved in the inotropic effects of these aminosteroids: (i) an enhanced Ca2 + entry via the slow calcium channels partially brought about by a local release of endogenous catecholamines in the case of LND 796, (ii) an inhibitory effect on Na+-K+ ATPase which, at the highest concentrations, lead to similar signs of cellular toxicity to those described for digitalis drugs. Because of their enlarged positive inotropic range, both aminosteroids may be of interest in the treatment of congestive heart failure.

Calcium-activated non-selective cation channel in ventricular cells isolated from adult guinea-pig hearts

1. A class of Ca2+-activated non-selective cation channel was identified in ventricular cells, which were dissociated from adult guinea-pig hearts using collagenase. 2. Under cell-attached conditions the patch electrode filled with a Na+-rich solution recorded no obvious single-channel current at the resting membrane potential. Subsequent superfusion of the ventricular cell with a Na+-free Tyrode solution induced an inward-going single-channel current as well as contracture of the cell. Kinetics of this channel were not affected by varying the membrane potential. 3. Single-channel currents showing a conductance similar to those observed in the cell-attached patches were recorded in isolated inside-out membrane patches when the inner side of the membrane was exposed to a free Ca2+ concentration ([Ca2+]i) higher than 0.3 microM. The slope conductance of the channel was 14.8 +/- 2.9 pS (mean +/- S.D., n = 17) at 20-25 degrees C. 4. The reversal potential examined in the inside-out patch was about 0 mV irrespective of the Na+-rich, K+-rich, Li+-rich or Cs+-rich solutions on either side of the membrane, thereby indicating that the channel was almost equally permeable to these cations. 5. The open probability of the channel was increased by raising [Ca2+]i with the maximum value of 0.93 +/- 0.17 (n = 4) at about 10 microM [Ca2+]i. The dose-response relation was fitted to the saturation kinetics with a Hill coefficient of 3.0 and a half-maximum concentration of 1.2 microM [Ca2+]i. 6. The gating kinetics were complex; both the open and closed time histograms showed at least two exponential components with time constants of 3.8 +/- 1.3 ms and 140 +/- 110 ms for open time and 1.8 +/- 1.1 ms and 14.9 +/- 5.3 ms for closed time (n = 4) at 10 microM [Ca2+]i. Reduction of [Ca2+]i resulted in both a decrease of the time constant of the slow component in the open time histogram and an increase of the two time constants of the closed time histogram. 7. Contribution of the channel to the whole-cell current was discussed based on an estimation of the channel density, presumably about 0.04 approximately 0.4/microns 2. Maximum activation of the channel would produce 7.2 approximately 72 nS of membrane conductance, which would explain the reported magnitude of the Ca2+-mediated background conductance of the single myocyte. The channel may also contribute, at least in part, to the transient inward current which develops in Ca2+-overloaded cardiac cells.

Delayed afterdepolarizations elicited in vivo by left stellate ganglion stimulation

Activation of cardiac sympathetic nerves is recognized as a triggering factor for cardiac arrhythmias. However, the mechanisms involved have only been speculated. Because evidence from studies in vitro has established a relation between catecholamines, delayed afterdepolarizations (DAD), and triggered rhythms, it seemed possible that in vivo adrenergic activation also might lead to the development of DAD. Because very little evidence was available for DAD in vivo, we have evaluated whether monophasic action potential (MAP) recording with a contact electrode could be a suitable technique for the detection of DAD from the endocardium of anesthetized cats. In six animals, atrial pacing and graded aortic constriction were performed during MAP recording to assess MAP stability during hemodynamic changes, and in no cases were modifications of the baseline observed. In 11 cats, calcium gluconate (0.5 g) and G-strophanthin (100 micrograms) were administered. Action potential duration at 50% (APD50) and 90% (APD90) repolarization were reduced (from 138 +/- 16 to 122 +/- 18 msec, p less than 0.02, and from 163 +/- 23 to 149 +/- 20 msec, p less than 0.025, respectively). In eight of 11 (73%) animals, DAD were elicited with a mean amplitude of 1.2 +/- 0.4 mV. In 14 cats, the left stellate ganglion was stimulated for 45 seconds. APD50 and APD90 decreased (from 153 +/- 15 to 145 +/- 16 msec, p less than 0.005, and from 176 +/- 18 to 165 +/- 13 msec, p less than 0.001, respectively). DAD were induced in 10 of 14 animals (71%) with a mean amplitude of 1.2 +/- 0.3 mV. These results show that DAD can be induced in vivo by administration of calcium and digitalis and by activation of the cardiac sympathetic nerves. This latter finding further strengthens the existing link between adrenergic activation and ventricular arrhythmogenesis and suggests triggered activity as a likely mechanism.

Transient inward current in guinea-pig atrial myocytes reflects a change of sodium-calcium exchange current

1. Enzymatically isolated, cultured myocytes from hearts of adult guinea-pigs were voltage clamped with a whole-cell patch-clamp technique. The pipette-filling solution for internal dialysis contained 65 mM-citrate and 50 microM-EGTA as Ca2+-chelating agents and 20 mM-Na+. Potassium channel currents were blocked by replacing this ion on both sides of the membrane by Cs+. 2. In the above conditions myocytes develop spontaneous transient inward currents (Iti) at constant negative membrane holding potentials. At a given membrane potential Iti can be recorded with constant amplitude and frequency for periods of up to ca. 40 min. A membrane current with similar properties can be evoked by superfusion of the cell with caffeine-containing (5-10 mM) solution. 3. Depolarization results in a reduction of Iti amplitude and a prolongation of its duration. After a step change of the membrane potential to ca. -10 mV or a less-negative level only one inward current change is observed. Thereafter the membrane current remains inward with regard to the instantaneous current at this membrane potential. Complete relaxation of Iti then is only observed after repolarization to a more-negative membrane potential. 4. The current change caused by sarcoplasmic Ca2+ release is inward in a range of membrane potentials between -90 and +75 mV. A reversal of Iti was never detected. 5. Both the instantaneous current-voltage (I-V) relation and voltage dependence of peak Iti display distinct outward rectification. Both I-V relations can be described by a formalism suggested for a membrane current caused by electrogenic Na+-Ca2+ exchange (INa, Ca) assuming a 3:1 stoichiometry and a single energy barrier in the electric field of the membrane. 6. An increase of the time integral of Iti at the holding potential is observed after depolarizations to positive membrane potentials, where the outward-rectifying current component is prominent. This supports the view that the outward current represents INa, Ca in the 'reverse mode', carrying Ca2+ ions into the cell. 7. After prolonged cell dialysis a run-down of Iti is observed. Since strong depolarizations in this condition still can cause inward currents upon repolarization, run-down is likely to reflect an impairment of sarcoplasmic reticulum function rather than an effect of cell dialysis on the exchanger. 8. We conclude that under the present conditions a membrane current is measured, which to a large extent determines the 'passive' I-V curve of the myocytes. This current is modified by a rise in Ca2+(i) following sarcoplasmic Ca2+ release.(ABSTRACT TRUNCATED AT 400 WORDS)

Calcium oscillations in digitalis-induced ventricular fibrillation: pathogenetic role and metabolic consequences in isolated ferret hearts

The pathophysiology of the ventricular fibrillation that complicates digitalis intoxication was investigated. In this and other calcium-overload states, oscillations of the intracellular free calcium concentration ([Ca2+]i) have been implicated as the cause of ventricular tachyarrhythmias. We addressed two questions: 1) Are [Ca2+]i oscillations obligatory in the pathogenesis of ventricular fibrillation during digitalis toxicity? 2) What are the metabolic consequences of [Ca2+]i oscillations? Ferret hearts (n = 20) were Langendorff-perfused at constant flow with oxygenated HEPES-buffered Tyrode's solution at 37 degrees C. Isovolumic left ventricular pressure was measured along with the extracellular electrogram or with simultaneous phosphorus nuclear magnetic resonance spectra. When strophanthidin (20 microM) was added during pacing at 3 Hz, the positive inotropic effect soon gave way to a decrease in developed force. The decrease in force was accompanied by an increase in inorganic phosphate concentration, a decrease in phosphocreatine concentration, and a slight acidosis. The rhythm changed to ventricular fibrillation after 12-25 minutes. This change was initially accompanied by further metabolic deterioration, but all metabolites reached steady state within 12-18 minutes of the onset of ventricular fibrillation. Fast Fourier transformation revealed the existence of periodic oscillations at 7-10 Hz in both the extracellular electrogram and the ventricular pressure during ventricular fibrillation. Ryanodine, an inhibitor of [Ca2+]i oscillations, abolished the pressure oscillations but not the voltage oscillations. Exposure to ryanodine significantly decreased the inorganic phosphate concentration and increased the phosphocreatine concentration (p less than 0.05) despite continuing exposure to strophanthidin. The results indicate that oscillations of [Ca2+]i are not required to sustain ventricular fibrillation, but when present, such oscillations contribute importantly to metabolic deterioration.

Effect of tetrodotoxin, lidocaine, and quinidine on the transient inward current of sheep Purkinje fibres

The effect of tetrodotoxin (TTX), lidocaine, and quinidine on the transient inward current (TI) was studied in voltage-clamped sheep cardiac Purkinje fibres. The TI was induced by elevation of extracellular Ca or addition of strophanthidin. Reduction of external Na had a biphasic effect on the steady state TI magnitude; a moderate (less than 50%) reduction of external Na had an enhancing effect on the TI; a further decrease of extracellular Na was accompanied by a decline of TI amplitude. The TI could not be induced in Na-free medium (external Ca less than or equal to 9.0 mM). TTX, lidocaine, and quinidine reduced the magnitude of the TI in a dose-dependent way. The blocking effect of these agents could be compensated for by a moderate (less than 50%) reduction of external Na or an elevation of extracellular Ca. It is suggested that the blocking effect of TTX, lidocaine, and quinidine on the TI is due to a reduction of intracellular Na, which causes a decay of intracellular Ca via the Na-Ca exchange mechanism.

Mechanism of inhibition by SUN 1165, a new Na channel blocking antiarrhythmic agent, of cardiac glycoside-induced triggered activity

The mechanism of the antiarrhythmic action of SUN 1165, a selective Na channel blocker, in digitalis-induced arrhythmias was investigated by means of conventional microelectrode and suction pipette methods with isolated canine Purkinje fibers and guinea-pig single ventricular cells, respectively. SUN 1165 decreased acetylstrophanthidin-induced delayed afterdepolarization and completely blocked the initiation of triggered activity by acetylstrophanthidin in Purkinje fibers. Delayed afterdepolarization, which was completely abolished either by intracellular dialysis with EGTA or by extracellular superfusion with caffeine, was decreased by SUN 1165 in a concentration-dependent manner in single ventricular cells. These results suggest that an increase in intracellular Ca2+ concentration is a requirement for delayed afterdepolarization. Furthermore, the antiarrhythmic action of SUN 1165 in cardiac glycoside-induced arrhythmias in dogs could be mediated not only by an inhibition of sodium channels and subsequent reduction in the intracellular sodium activity, but also by a reduction of intracellular calcium activity due to the Na+-Ca2+ exchange mechanism.

Role of prostaglandins in the arrhythmogenic effects of ouabain on isolated guinea pig hearts

We examined the hypothesis that endogenous prostaglandins participate in the arrhythmogenic influence of ouabain in guinea pig hearts. Addition of ouabain (10 ng/ml) resulted in a 5-fold increase in the release of 6-keto-prostaglandin F1 alpha in the coronary effluent. Ten of 13 hearts studied (77%) demonstrated arrhythmic activity with a mean time to the onset of arrhythmias of approximately 35 min. The nonsteroidal antiinflammatory drugs indomethacin and acetylsalicylic acid which significantly inhibited the efflux of 6-keto-prostaglandin F1 alpha also reduced the incidence of arrhythmias to 10 of 30 hearts studied. In those hearts in which arrhythmias occurred, the time to onset was significantly increased to approximately 50 and 55 min for acetylsalicylic acid and indomethacin, respectively. In contrast, exogenous prostaglandin F2 alpha (0.1 and 1 ng/ml) and prostacyclin (0.1 and 10 ng/ml) increased the incidence of arrhythmias to 100% (10 of 10 hearts studied) and decreased the time to onset to approximately 10 min. These prostaglandin pretreatments were also able to reverse the protective actions of both acetylsalicylic acid and indomethacin. Other concentrations (10 ng/ml prostaglandin F2 alpha and 1 ng/ml prostacyclin) had no influence either on the incidence of arrhythmias or their time to onset. Prostaglandin E2 (0.1 ng/ml) produced a modest but not significant decrease in the time to onset of arrhythmias although this concentration was significantly effective in reversing the nonsteroidal antiinflammatory drug effects. The inotropic, chronotropic and coronary constricting actions of ouabain were unaffected either by nonsteroidal antiinflammatory drug or prostaglandin pretreatment. These studies suggest that prostaglandins are involved, at least in part, in the arrhythmogenic actions of ouabain in the isolated guinea pig heart.

Automaticity, triggered activity, and responses to adrenergic stimulation in cat subendocardial Purkinje fibers after healing of myocardial infarction

We studied automaticity, triggered activity, and responses to alpha- and beta-adrenergic stimulation in subendocardial Purkinje fibers overlying healed infarct scars (infarct preparation) and from remote normal zones (noninfarct preparation) of cat left ventricles. The preparations were studied 2 to 4 months after ligation of multiple distal tributaries of the left anterior descending and circumflex arteries. Subendocardial Purkinje fibers from corresponding areas of normal hearts served as control samples (control preparation). Transmembrane action potential characteristics and rates of automaticity (spontaneous phase 4 depolarization) did not differ among control, noninfarct, and infarct preparations. However, overdrive at cycle lengths of less than 400 msec suppressed automaticity to a greater degree in Purkinje fibers of infarct preparations than those of control and noninfarct preparations. Changes in automatic rate during superfusion with isoproterenol (10(-10)M to 10(-6)M) were not different among the three groups of preparations, but exposure to phenylephrine (10(-9)M to 10(-5)M) in the presence of 5 X 10(-7)M propranolol reduced the automatic rate to a greater degree in Purkinje fibers of infarct preparations than those of control or noninfarct preparations. Triggered activity arising from delayed afterdepolarizations was recorded in 10 of 29 infarct preparations (34%), but not in 12 control and 10 noninfarct preparations. These afterpotentials were augmented by increasing extracellular Ca++ concentration, 10(-7)M isoproterenol, and 10(-5)M phenylephrine in the presence of 5 X 10(-7)M propranolol. We conclude that Purkinje fibers overlying healed infarct scars have altered physiology of spontaneous automaticity, enhanced responses to alpha-adrenergic interventions, and a tendency to triggered activity, and that both alpha- and beta-adrenergic effects may result in worsening of arrhythmias by augmentation of afterpotentials in healed myocardial infarction.

Cardiac dysrhythmias in the acute setting: pathophysiology or anyone can understand cardiac dysrhythmias

Cardiac dysrhythmias are easy. Unlike the lung (which has formidable neuroendocrine, metabolic, and respiratory responsibilities), the heart is simple. It is an innervated muscular pump. A resting Purkinje or ventricular muscle cell membrane maintains a charge of about 90 millivolts. The five phases of a cardiac action potential are similar to the action potential in skeletal muscle, however, the cardiac action potential lasts a hundred times longer. When sodium specific "fast" channels and calcium specific "slow" channels open, positive ions rush into the myocardial cell, thus causing rapid membrane depolarization. In order to produce an action potential, some stimulus must decrease the membrane potential from -90 millivolts to "threshold" or -60 millivolts. Purkinje fibers do not have a stable phase for diastolic potential. These fibers continuously depolarize during diastole. Hypoxemia or hypokalemia may exacerbate this diastolic depolarization, thus promoting "hyperexcitability" or "automatic" ectopy. When myocardium is damaged, characteristically with myocardial ischemia, rapid conduction of cardiac impulses may be slowed dramatically. Very slow impulses may course through muscle such that by the time the activation wave front returns to the initiating site, this origin has had a chance to repolarize. This is the basis for re-entrant dysrhythmias. All cardiac dysrhythmias are automatic, re-entrant or both.

Triggered rhythms in atrial muscle

Intracellular and extracellular recording techniques were used to study triggered activity in tissues isolated from the inferior right atrium of the dog. In the presence of norepinephrine (greater than 10(-7) M) single stimuli or short rapid pacing elicited action potentials with delayed after-depolarizations, leading to single or repetitive non-driven beats. Most commonly, during sustained triggered activity action potential and electrogram configurations remained constant. In addition, cycle length changed gradually, initially decreasing and then increasing before activity stopped. However in some preparations, once sustained triggered activity was initiated there were abrupt spontaneous changes in action potential and electrogram configurations that were coincident with abrupt spontaneous changes in cycle length. When quiescent, short rapid pacing initiated triggered activity with a relatively short cycle length that abruptly shifted to a longer cycle length before stopping. During sustained triggered activity, short rapid pacing caused an abrupt decrease in cycle length that was coincident with changes in action potential and electrogram configurations. The present results indicate that abrupt changes in cycle length can occur during bouts of triggered activity resulting in unique patterns of arrhythmias. These rhythms may be due to interactions and shifts between multiple sites of triggered pacemaker activity.

Direct and indirect effects of ciguatoxin on guinea-pig atria and papillary muscles

The mode of action of ciguatoxin (CTX) on the isolated atrial and papillary muscle of the guinea-pig heart was investigated using conventional methods for the measurement of mechanical and electrophysiological parameters. CTX induced positive inotropic and positive klinotropic responses in atrial and papillary muscles. Each response consisted of two phases. The initial positive inotropic response developed rapidly and resulted from the previously reported indirect action of CTX. The second phase of positive inotropy developed more slowly and was well maintained at doses of CTX up to 0.15 mouse units/ml in atria and up to 0.8 M.U./ml in papillary muscles. This phase was found to result from a direct action of CTX on the myocardium which was not reversed by washing. Tetrodotoxin (TTX) reversed the positive inotropic effects stemming from the direct action of CTX. The (-) and (+) enantiomers of propranolol were equally effective in inhibiting the direct effect of CTX. These antagonists did not displace CTX from the myocardium. CTX induced a TTX-sensitive depolarization of stimulated or quiescent atrial cells. All the effects of CTX on the atrial action potential were reversed by TTX. It was therefore concluded that CTX opens voltage dependent Na+ channels. CTX bound equally to resting and K+-depolarized Na+ channels but there were indications that electrical stimulation enhanced the rate of CTX binding. CTX overrides the positive staircase effect of increasing stimulation frequency. Na+ channels found in the atria which were particularly sensitive to TTX did not play a prominent role in mediating the CTX effect. CTX appeared to have little effect on the normal Na+ channel inactivation process. CTX did not restore contractions in the K+-depolarized cardiac muscles examined. The sensitivity of CTX action to TTX distinguished it from cardiac glycoside activity. Established mechanisms of Na+/Ca2+ exchange and Ca2+-induced release of Ca2+ can explain the link between CTX-induced increase of intracellular [Na+] and the positive inotropic response.