ryanodine and Arrhythmias--Cardiac

Hypertension affects about 5% of western populations and in the majority of cases it is of unknown aetiology. It exposes the heart to greater levels of myocardial stretch as a result of increased systolic pressure and peripheral resistance. Under certain circumstances myocardial stretch may trigger arrhythmias but the mechanisms and clinical importance of this phenomenon are unclear. This article outlines the risks of sudden cardiac death conferred by hypertension and left ventricular hypertrophy, presents the results of experiments using an animal model of myocardial stretch and discusses some possible mechanisms underlying stretch-induced arrhythmias which may be important in hypertensive patients.

Topics: Animals; Arrhythmias, Cardiac; Calcium; Calcium Channel Blockers; Cardiotonic Agents; Death, Sudden, Cardiac; Dogs; Electrocardiography; Gadolinium; Heart; Heart Ventricles; Humans; Hypertension; Hypertrophy, Left Ventricular; Isoproterenol; Meta-Analysis as Topic; Models, Cardiovascular; Myocardial Contraction; Myocardium; Nifedipine; Ouabain; Quinolines; Randomized Controlled Trials as Topic; Rats; Rats, Inbred SHR; Rats, Wistar; Ryanodine; Thiadiazines; Vasodilator Agents

This is a brief review of agents that stabilize calcium release from the sarcoplasmic reticulum in cardiac muscle. An excess intracellular calcium concentration (calcium overload) is a common feature in a variety of cardiac cell injuries. Calcium overload elicits diastolic and systolic failure, and is involved in the genesis of arrhythmias. These abnormalities appear in part to be caused by the spontaneous release of calcium ions from the sarcoplasmic reticulum. Previous efforts to treat calcium overload were made with the intention to decrease the total intracellular content of calcium ions. However, such procedures would result in a decrease in contractility. Agents that stabilized calcium release from the sarcoplasmic reticulum may therefore be useful to correct abnormalities in calcium overload. In this review, after briefly describing intracellular calcium homeostasis, strategies against calcium overload, especially those involving magnesium ion, ryanodine, caffeine, dantrolene, phenytoin, R56865, KT361 and flunarizine will be discussed.

Topics: Animals; Anti-Arrhythmia Agents; Anticonvulsants; Arrhythmias, Cardiac; Benzothiazoles; Caffeine; Calcium; Calcium Channel Blockers; Dantrolene; Flunarizine; Homeostasis; Humans; Magnesium; Models, Cardiovascular; Muscle Relaxants, Central; Myocardial Contraction; Myocardium; Phenytoin; Piperidines; Ryanodine; Sarcoplasmic Reticulum; Thiazepines; Thiazoles

Calmodulin (CaM) modulates the activity of several proteins that play a key role in excitation-contraction coupling (ECC). In cardiac muscle, the major binding partner of CaM is the type-2 ryanodine receptor (RyR2) and altered CaM binding contributes to defects in sarcoplasmic reticulum (SR) calcium (Ca

Topics: Arrhythmias, Cardiac; Calcium Signaling; Calmodulin; Humans; Mutation; Ryanodine; Ryanodine Receptor Calcium Release Channel

Ryanodine receptor 2 (RyR2) hyperactivity is observed in structural heart diseases that are a result of ischemia or heart failure. It causes abnormal calcium handling and calcium leaks that cause metabolic, electrical, and mechanical dysfunction, which can trigger arrhythmias. Here, we tested the antiarrhythmic potential of dantrolene (RyR inhibitor) in human hearts. Human hearts not used in transplantation were obtained, and right ventricular outflow tract (RVOT) wedges and left ventricular (LV) slices were prepared. Pseudo-ECGs were recorded to determine premature ventricular contraction (PVC) incidences. Optical mapping was performed to determine arrhythmogenic substrates. After baseline optical recordings, tissues were treated with

Topics: Action Potentials; Anti-Arrhythmia Agents; Arrhythmias, Cardiac; Caffeine; Calcium; Dantrolene; Heart Failure; Humans; Isoproterenol; Ryanodine; Ryanodine Receptor Calcium Release Channel

Hyperactivity of cardiac sarcoplasmic reticulum (SR) ryanodine receptor (RyR2) Ca

Topics: Animals; Arrhythmias, Cardiac; Calcium; Depsipeptides; Fluorescence Resonance Energy Transfer; HEK293 Cells; Humans; Mice; Myocytes, Cardiac; Ryanodine; Ryanodine Receptor Calcium Release Channel; Swine

Topics: Arrhythmias, Cardiac; Calcium; Calcium Signaling; Heart Failure; Humans; Ryanodine; Ryanodine Receptor Calcium Release Channel

Action potential duration (APD) alternans is an established precursor or arrhythmia and sudden cardiac death. Important differences in fundamental electrophysiological properties relevant to arrhythmia exist between experimental models and the diseased in vivo human heart. To investigate mechanisms of APD alternans using a novel approach combining intact heart and cellular cardiac electrophysiology in human in vivo.. We developed a novel approach combining intact heart electrophysiological mapping during cardiac surgery with rapid on-site data analysis to guide myocardial biopsies for laboratory analysis, thereby linking repolarization dynamics observed at the organ level with underlying ion channel expression. Alternans-susceptible and alternans-resistant regions were identified by an incremental pacing protocol. Biopsies from these sites (n = 13) demonstrated greater RNA expression in Calsequestrin (CSQN) and Ryanodine (RyR) and ion channels underlying IK1 and Ito at alternans-susceptible sites. Electrical restitution properties (n = 7) showed no difference between alternans-susceptible and resistant sites, whereas spatial gradients of repolarization were greater in alternans-susceptible than in alternans-resistant sites (P = 0.001). The degree of histological fibrosis between alternans-susceptible and resistant sites was equivalent. Mathematical modelling of these changes indicated that both CSQN and RyR up-regulation are key determinants of APD alternans.. Combined intact heart and cellular electrophysiology show that regions of myocardium in the in vivo human heart exhibiting APD alternans are associated with greater expression of CSQN and RyR and show no difference in restitution properties compared to non-alternans regions. In silico modelling identifies up-regulation and interaction of CSQN with RyR as a major mechanism underlying APD alternans.

Topics: Action Potentials; Arrhythmias, Cardiac; Biopsy; Calsequestrin; Electrophysiologic Techniques, Cardiac; Female; Heart Conduction System; Humans; Ion Channels; Male; Middle Aged; Ryanodine

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Topics: Animals; Anti-Arrhythmia Agents; Arrhythmias, Cardiac; Calcium; Calcium Channel Blockers; Depsipeptides; Dimerization; Membrane Potentials; Mice; Ryanodine; Ryanodine Receptor Calcium Release Channel; Stereoisomerism

Human loss-of-function variants of ANK2 (ankyrin-B) are linked to arrhythmias and sudden cardiac death. However, their in vivo effects and specific arrhythmogenic pathways have not been fully elucidated.. We identified new ANK2 variants in 25 unrelated Han Chinese probands with ventricular tachycardia by whole-exome sequencing. The potential pathogenic variants were validated by Sanger sequencing. We performed functional and mechanistic experiments in ankyrin-B knockin (KI) mouse models and in single myocytes isolated from KI hearts.. We detected a rare, heterozygous ANK2 variant (p.Q1283H) in a proband with recurrent ventricular tachycardia. This variant was localized to the ZU5. ANK2 p.Q1283H is a disease-associated variant that confers susceptibility to stress-induced arrhythmias, which may be prevented by the administration of metoprolol or flecainide. This variant is associated with the loss of protein phosphatase 2A activity, increased phosphorylation of ryanodine receptor, exaggerated delayed afterdepolarization-mediated trigger activity, and arrhythmogenesis.

Topics: Action Potentials; Animals; Ankyrins; Arrhythmias, Cardiac; Calcium; Disease Models, Animal; Electrocardiography; Female; Humans; Isoproterenol; Mice; Middle Aged; Myocytes, Cardiac; Phosphorylation; Polymorphism, Single Nucleotide; Protein Phosphatase 2; Ryanodine; Ryanodine Receptor Calcium Release Channel; Sarcoplasmic Reticulum

Topics: Alleles; Animals; Arrhythmias, Cardiac; Gene Silencing; Genetic Therapy; Mice; Mutation; Phenotype; RNA, Messenger; Ryanodine; Ryanodine Receptor Calcium Release Channel

Calcium-induced calcium release is the principal mechanism that triggers the cell-wide [Ca(2+)]i transient that activates muscle contraction during cardiac excitation-contraction coupling (ECC). Here, we characterize this process in mouse cardiac myocytes with a novel mathematical action potential (AP) model that incorporates realistic stochastic gating of voltage-dependent L-type calcium (Ca(2+)) channels (LCCs) and sarcoplasmic reticulum (SR) Ca(2+) release channels (the ryanodine receptors, RyR2s). Depolarization of the sarcolemma during an AP stochastically activates the LCCs elevating subspace [Ca(2+)] within each of the cell's 20,000 independent calcium release units (CRUs) to trigger local RyR2 opening and initiate Ca(2+) sparks, the fundamental unit of triggered Ca(2+) release. Synchronization of Ca(2+) sparks during systole depends on the nearly uniform cellular activation of LCCs and the likelihood of local LCC openings triggering local Ca(2+) sparks (ECC fidelity). The detailed design and true SR Ca(2+) pump/leak balance displayed by our model permits investigation of ECC fidelity and Ca(2+) spark fidelity, the balance between visible (Ca(2+) spark) and invisible (Ca(2+) quark/sub-spark) SR Ca(2+) release events. Excess SR Ca(2+) leak is examined as a disease mechanism in the context of "catecholaminergic polymorphic ventricular tachycardia (CPVT)", a Ca(2+)-dependent arrhythmia. We find that that RyR2s (and therefore Ca(2+) sparks) are relatively insensitive to LCC openings across a wide range of membrane potentials; and that key differences exist between Ca(2+) sparks evoked during quiescence, diastole, and systole. The enhanced RyR2 [Ca(2+)]i sensitivity during CPVT leads to increased Ca(2+) spark fidelity resulting in asynchronous systolic Ca(2+) spark activity. It also produces increased diastolic SR Ca(2+) leak with some prolonged Ca(2+) sparks that at times become "metastable" and fail to efficiently terminate. There is a huge margin of safety for stable Ca(2+) handling within the cell and this novel mechanistic model provides insight into the molecular signaling characteristics that help maintain overall Ca(2+) stability even under the conditions of high SR Ca(2+) leak during CPVT. Finally, this model should provide tools for investigators to examine normal and pathological Ca(2+) signaling characteristics in the heart.

Topics: Action Potentials; Animals; Arrhythmias, Cardiac; Calcium; Calcium Signaling; Excitation Contraction Coupling; Humans; Mice; Models, Theoretical; Myocardium; Myocytes, Cardiac; Ryanodine; Ryanodine Receptor Calcium Release Channel; Sarcolemma; Sarcoplasmic Reticulum

Left ventricular hypertrophy (LVH) is frequently associated with clinical atrial arrhythmias, but little is known about how it causes those arrhythmias. Our previous studies have shown that LVH increases the late sodium current (I(Na-L)) that plays an important role in the genesis of ventricular arrhythmias. We hypothesize that LVH may also induce an upregulation of the I(Na-L) in atrial myocytes, leading to atrial electrical abnormalities. The renovascular hypertension model was used to induce LVH in rabbits. Action potential and membrane current recordings were performed in single myocytes. At a pacing cycle length of 2,000 ms, spontaneous phase-2 early afterdepolarizations (EADs) could be recorded from the left atrial myocytes in 10 of 12 LVH rabbits, whereas no EADs could be elicited in right atrial myocytes of LVH rabbits or atrial myocytes from any of the 12 control rabbits. Spontaneous automaticity (SA) from left atrial myocytes was observed in 9 out of 12 LVH rabbits, but none in right atrial myocytes of LVH rabbits or control rabbits, at a pacing rate of 8,000 ms. The left atrial myocytes of LVH rabbits had a significantly higher density of the I(Na-L) compared with those of control rabbits (0.90 +/- 0.12 in LVH vs. 0.50 +/- 0.08 pA/pF in control, n = 8, P < 0.01). Tetrodotoxin, an I(Na-L) blocker, abolished all atrial EADs and SA at 10 microM. Our results demonstrate that LVH induction results in a significant increase of I(Na-L) in the left atrial myocytes that may render these cells susceptible to the genesis of EADs and SA. The I(Na-L) may serve as a potentially useful ionic target for antiarrhythmic drugs for the treatment of atrial arrhythmias in the setting of LVH.

Topics: Action Potentials; Animals; Arrhythmias, Cardiac; Calcium Channel Blockers; Cell Separation; Electrophysiology; Heart Atria; Hypertrophy, Left Ventricular; In Vitro Techniques; Male; Membrane Potentials; Myocytes, Cardiac; Organ Size; Rabbits; Ryanodine; Sodium Channel Blockers; Sodium Channels; Tetrodotoxin

Returning to normal pH after acidosis, similar to reperfusion after ischemia, is prone to arrhythmias. The type and mechanisms of these arrhythmias have never been explored and were the aim of the present work. Langendorff-perfused rat/mice hearts and rat-isolated myocytes were subjected to respiratory acidosis and then returned to normal pH. Monophasic action potentials and left ventricular developed pressure were recorded. The removal of acidosis provoked ectopic beats that were blunted by 1 muM of the CaMKII inhibitor KN-93, 1 muM thapsigargin, to inhibit sarcoplasmic reticulum (SR) Ca(2+) uptake, and 30 nM ryanodine or 45 muM dantrolene, to inhibit SR Ca(2+) release and were not observed in a transgenic mouse model with inhibition of CaMKII targeted to the SR. Acidosis increased the phosphorylation of Thr(17) site of phospholamban (PT-PLN) and SR Ca(2+) load. Both effects were precluded by KN-93. The return to normal pH was associated with an increase in SR Ca(2+) leak, when compared with that of control or with acidosis at the same SR Ca(2+) content. Ca(2+) leak occurred without changes in the phosphorylation of ryanodine receptors type 2 (RyR2) and was blunted by KN-93. Experiments in planar lipid bilayers confirmed the reversible inhibitory effect of acidosis on RyR2. Ectopic activity was triggered by membrane depolarizations (delayed afterdepolarizations), primarily occurring in epicardium and were prevented by KN-93. The results reveal that arrhythmias after acidosis are dependent on CaMKII activation and are associated with an increase in SR Ca(2+) load, which appears to be mainly due to the increase in PT-PLN.

Topics: Acidosis; Action Potentials; Animals; Arrhythmias, Cardiac; Benzylamines; Calcium; Calcium-Binding Proteins; Calcium-Calmodulin-Dependent Protein Kinase Type 2; Dantrolene; Disease Models, Animal; Enzyme Inhibitors; Hydrogen-Ion Concentration; Male; Mice; Mice, Transgenic; Myocytes, Cardiac; Peptides; Phosphorylation; Rats; Rats, Wistar; Ryanodine; Ryanodine Receptor Calcium Release Channel; Sarcoplasmic Reticulum; Sarcoplasmic Reticulum Calcium-Transporting ATPases; Sulfonamides; Thapsigargin; Time Factors; Ventricular Function, Left; Ventricular Pressure

A common source of arrhythmogenic spontaneous activity instigating atrial fibrillation is the myocardial tissue, or sleeves, at the base of the pulmonary veins. This study compared the properties of cells from the myocardial sleeves of the pulmonary veins (PV(m)) with cells from the normal cardiac pacemaker (the sinoatrial node) and regions of the atria. Our objective was to identify key features of these cells that predispose them to becoming the focus of cardiac arrhythmias.. Single cells were isolated from samples of rabbit PV(m), central and peripheral sinoatrial node, crista terminalis, and left and right atria. Detailed morphology of cells was assessed and intracellular calcium concentrations measured with the use of Fluo-3. Cells from the PV(m) were smaller than atrial cells and showed large elevations in diastolic calcium during activation at physiological rates, a feature the PV(m) cells shared with cells from the sinoatrial node. Unstimulated spontaneous activity was observed in a minority of cells from the PV(m), but numerous cells from this region showed spontaneous activity for a brief period immediately subsequent to stimulation at physiological rates. This was not observed in atrial cells. Assessment of calcium removal pathways showed sarcolemmal calcium extrusion in cells from the PV(m) to have a high reliance on "slow" extrusion pathways to maintain intracellular calcium homeostasis because of a low expression of sodium-calcium exchanger.. We conclude that cells from the PV(m) share some features with cells from the sinoatrial node but also have distinctly unique features that predispose them to the development of spontaneous activity.

Topics: Action Potentials; Animals; Arrhythmias, Cardiac; Biological Clocks; Caffeine; Calcium Signaling; Cardiac Pacing, Artificial; Cell Shape; Heart Atria; In Vitro Techniques; Myocytes, Cardiac; Pulmonary Veins; Rabbits; RNA, Messenger; Ryanodine; Ryanodine Receptor Calcium Release Channel; Sarcolemma; Sarcoplasmic Reticulum; Sarcoplasmic Reticulum Calcium-Transporting ATPases; Sinoatrial Node; Sodium-Calcium Exchanger

T-wave alternans, an important arrhythmogenic factor, has recently been described in human fetuses. Here we sought to determine whether alternans can be induced in the embryonic mouse hearts, despite its underdeveloped sarcoplasmic reticulum (SR) and, if so, to analyze the response to pharmacological and autonomic interventions. Immunohistochemistry confirmed minimal sarcoplasmic-endoplasmic reticulum Ca-ATPase 2a expression in embryonic mouse hearts at embryonic day (E) 10.5 to E12.5, compared with neonatal or adult mouse hearts. We optically mapped voltage and/or intracellular Ca (Ca(i)) in 99 embryonic mouse hearts (dual mapping in 64 hearts) at these ages. Under control conditions, ventricular action potential duration (APD) and Ca(i) transient alternans occurred during rapid pacing at an average cycle length of 212 +/- 34 ms in 57% (n = 15/26) of E10.5-E12.5 hearts. Maximum APD restitution slope was steeper in hearts developing alternans than those that did not (2.2 +/- 0.6 vs. 0.8 +/- 0.4; P < 0.001). Disabling SR Ca(i) cycling with thapsigargin plus ryanodine did not significantly reduce alternans incidence (44%, n = 8/18, P = 0.5), whereas isoproterenol (n = 14) increased the incidence to 100% (P < 0.05), coincident with steepening APD restitution slope. Verapamil abolished Ca(i) transients (n = 9). Thapsigargin plus ryanodine had no major effects on Ca(i)-transient amplitude or its half time of recovery in E10.5 hearts, but significantly depressed Ca(i)-transient amplitude (by 47 +/- 8%) and prolonged its half time of recovery (by 18 +/- 3%) in E11.5 and older hearts. Embryonic mouse ventricles can develop cardiac alternans, which generally is well correlated with APD restitution slope and does not depend on fully functional SR Ca(i) cycling.

Topics: Action Potentials; Adrenergic beta-Agonists; Animals; Arrhythmias, Cardiac; Calcium Channel Blockers; Calcium Signaling; Carbachol; Cardiac Pacing, Artificial; Cholinergic Agonists; Enzyme Inhibitors; Gestational Age; Heart; Heart Ventricles; Isoproterenol; Mice; Ryanodine; Sarcoplasmic Reticulum; Sarcoplasmic Reticulum Calcium-Transporting ATPases; Thapsigargin; Time Factors; Verapamil

T-wave alternans, characterized by a beat-to-beat change in T-wave morphology, amplitude, and/or polarity on the ECG, often heralds the development of lethal ventricular arrhythmias in patients with left ventricular hypertrophy (LVH). The aim of our study was to examine the ionic basis for a beat-to-beat change in ventricular repolarization in the setting of LVH. Transmembrane action potentials (APs) from epicardium and endocardium were recorded simultaneously, together with transmural ECG and contraction force, in arterially perfused rabbit left ventricular wedge preparation. APs and Ca(2+)-activated chloride current (I(Cl,Ca)) were recorded from left ventricular myocytes isolated from normal rabbits and those with renovascular LVH using the standard microelectrode and whole cell patch-clamping techniques, respectively. In the LVH rabbits, a significant beat-to-beat change in endocardial AP duration (APD) created beat-to-beat alteration in transmural voltage gradient that manifested as T-wave alternans on the ECG. Interestingly, contraction force alternated in an opposite phase ("out of phase") with APD. In the single myocytes of LVH rabbits, a significant beat-to-beat change in APD was also observed in both left ventricular endocardial and epicardial myocytes at various pacing rates. APD alternans was suppressed by adding 1 microM ryanodine, 100 microM 4,4'-diisothiocyanostilbene-2,2'-disulfonic acid (DIDS), and 100 microM 4-acetamido-4'-isothiocyanostilbene-2,2'-disulfonic acid (SITS). The density of the Ca(2+)-activated chloride currents (I(Cl,Ca)) in left ventricular myocytes was significantly greater in the LVH rabbits than in the normal group. Our data indicate that abnormal intracellular Ca(2+) fluctuation may exert a strong feedback on the membrane I(Cl,Ca), leading to a beat-to-beat change in the net repolarizing current that manifests as T-wave alternans on the ECG.

Topics: 4-Acetamido-4'-isothiocyanatostilbene-2,2'-disulfonic Acid; 4,4'-Diisothiocyanostilbene-2,2'-Disulfonic Acid; Action Potentials; Animals; Arrhythmias, Cardiac; Calcium Signaling; Cardiac Pacing, Artificial; Chloride Channels; Disease Models, Animal; Electrocardiography; Endocardium; Hypertrophy, Left Ventricular; Myocardial Contraction; Myocytes, Cardiac; Patch-Clamp Techniques; Pericardium; Rabbits; Ryanodine; Time Factors

To investigate the mechanism underlying T-wave alternans (TWA) and its hysteresis under ischemia conditions.. Transmembrane action potential (AP) from endocardial, M, and epicardial cells and monophasic AP (MAP) from four epicardial sites were recorded in ventricular wedge preparation and in isolated intact rabbit heart, respectively. The AP/MAP duration (APD/ MAPD), effective refractory period (ERP), activation time, and APD/MAPD restitution were determined under control and ischemia conditions. The effects of ryanodine (0.01 and 1 micromol x l(-1)) on TWA, and the effects of low extracellular Ca2+ and 4-aminopyridine on its hysteresis were studied.. Ischemia shortened the APD/MAPD and effective refractory period of all recording sites symmetrically, except the APD of M cells, which shortened markedly. In the ischemia group, TWA was induced within a cycle length (CL) range from 160 to 250 ms, which corresponded to a diastolic interval region of 0-70 ms. In this diastolic interval region, the repolarization restitution curve was the steepest (slope > 1.0). All TWA were accompanied by repolarization alternations. Low concentration ryanodine (0.01 micromol x l(-1)) facilitated TWA, high concentration (1 micromol x l(-1)) abolished it. Alternans of calcium transient were observed in myocytes purfused with ischemia solution during rapid stimulation. Ryanodine (0.1 micromol x l(-1)) abolished alternans of calcium transient, and ryanodine (0.01 micromol x l(-1)) facilitated them. After 60 min pacing at a CL of 200 ms, TWA persisted until the initial several beats at a CL of 300 ms at which a TWA was exceptional. The suppression of hysteresis by low extracellular Ca2+ and 4-aminopyridine indicated an underlying role of the intracellular Ca2+ overload and transient outward current (I(to)).. TWA is principally due to repolarization alternans, which is secondary to steep APD/MAPD restitution, and relates to intracellular calcium cycling. Hysteresis relates to intracellular Ca2+ overload and I(to).

Topics: 4-Aminopyridine; Animals; Arrhythmias, Cardiac; Calcium; Calcium Signaling; Electrocardiography; Endocardium; Heart Conduction System; Membrane Potentials; Myocardial Ischemia; Myocytes, Cardiac; Pericardium; Potassium Channel Blockers; Rabbits; Refractory Period, Electrophysiological; Ryanodine

Caffeine-activated, large-conductance, nonselective cation channels (LCCs) have been found in the plasma membrane of isolated cardiac myocytes in several species. However, little is known about the effects of opening these channels. To examine such effects and to further understand the caffeine-activation mechanism, we carried out studies using whole-cell patch-clamp techniques with freshly isolated cardiac myocytes from rats and mice. Unlike previous studies, thapsigargin was used so that both the effect of opening LCCs and the action of caffeine were independent of Ca(2+) release from intracellular stores. These Ca(2+)-permeable LCCs were found in a majority of the cells from atria and ventricles, with a conductance of approximately 370 pS in rat atria. Caffeine and all its direct metabolic products (theophylline, theobromine, and paraxanthine) activated the channel, while isocaffeine did not. Although they share some similarities with ryanodine receptors (RyRs, the openings of which give rise to Ca(2+) sparks), LCCs also showed some different characteristics. With simultaneous Ca(2+) imaging and current recording, the localized fluorescence increase due to Ca(2+) entry through a single opening of an LCC (SCCaFT) was detected. When membrane potential, instead of current, was recorded, SCCaFT-like fluorescence transients (indicating single LCC openings) were found to accompany membrane depolarizations. To our knowledge, this is the first report directly linking membrane potential changes to a single opening of an ion channel. Moreover, these events in cardiac cells suggest a possible additional mechanism by which caffeine and theophylline contribute to the generation of cardiac arrhythmias.

Topics: Animals; Arrhythmias, Cardiac; Caffeine; Calcium Channel Agonists; Calcium Channel Blockers; Calcium Channels; Calcium Signaling; Cell Membrane; Cresols; Enzyme Inhibitors; Heart Atria; Heart Ventricles; In Vitro Techniques; Ion Channel Gating; Membrane Potentials; Mice; Myocytes, Cardiac; Patch-Clamp Techniques; Rats; Ruthenium Red; Ryanodine; Ryanodine Receptor Calcium Release Channel; Sarcoplasmic Reticulum; Sarcoplasmic Reticulum Calcium-Transporting ATPases; Tetracaine; Thapsigargin

The 12.6-kDa FK506-binding protein (FKBP12.6) is considered to be a key regulator of the cardiac ryanodine receptor (RyR2), but its precise role in RyR2 function is complex and controversial. In the present study we investigated the impact of FKBP12.6 removal on the properties of the RyR2 channel and the propensity for spontaneous Ca(2+) release and the occurrence of ventricular arrhythmias. Single channel recordings in lipid bilayers showed that FK506 treatment of recombinant RyR2 co-expressed with or without FKBP12.6 or native canine RyR2 did not induce long-lived subconductance states. [(3)H]Ryanodine binding studies revealed that coexpression with or without FKBP12.6 or treatment with or without FK506 did not alter the sensitivity of RyR2 to activation by Ca(2+) or caffeine. Furthermore, single cell Ca(2+) imaging analyses demonstrated that HEK293 cells co-expressing RyR2 and FKBP12.6 or expressing RyR2 alone displayed the same propensity for spontaneous Ca(2+) release or store overload-induced Ca(2+) release (SOICR). FK506 increased the amplitude and decreased the frequency of SOICR in HEK293 cells expressing RyR2 with or without FKBP12.6, indicating that the action of FK506 on SOICR is independent of FKBP12.6. As with recombinant RyR2, the conductance and ligand-gating properties of single RyR2 channels from FKBP12.6-null mice were indistinguishable from those of single wild type channels. Moreover, FKBP12.6-null mice did not exhibit enhanced susceptibility to stress-induced ventricular arrhythmias, in contrast to previous reports. Collectively, our results demonstrate that the loss of FKBP12.6 has no significant effect on the conduction and activation of RyR2 or the propensity for spontaneous Ca(2+) release and stress-induced ventricular arrhythmias.

Topics: Animals; Arrhythmias, Cardiac; Calcium; Dogs; Electrocardiography; Humans; Lipid Bilayers; Mice; Mice, Nude; Models, Biological; Muscle Cells; Rats; Ryanodine; Ryanodine Receptor Calcium Release Channel; Sarcoplasmic Reticulum; Tacrolimus Binding Proteins

Catecholaminergic polymorphic ventricular tachycardia (CPVT) is an inherited disease characterized by life threatening arrhythmias and mutations in the gene encoding the ryanodine receptor (RyR2). Disagreement exists on whether (1) RyR2 mutations induce abnormal calcium transients in the absence of adrenergic stimulation; (2) decreased affinity of mutant RyR2 for FKBP12.6 causes CPVT; (3) K201 prevent arrhythmias by normalizing the FKBP12.6-RyR2 binding. We studied ventricular myocytes isolated from wild-type (WT) and knock-in mice harboring the R4496C mutation (RyR2(R4496C+/-)). Pacing protocols did not elicit delayed afterdepolarizations (DADs) (n=20) in WT but induced DADs in 21 of 33 (63%) RyR2(R4496C+/-) myocytes (P=0.001). Superfusion with isoproterenol (30 nmol/L) induced small DADs (45%) and no triggered activity in WT myocytes, whereas it elicited DADs in 87% and triggered activity in 60% of RyR2(R4496C+/-) myocytes (P=0.001). DADs and triggered activity were abolished by ryanodine (10 micromol/L) but not by K201 (1 micromol/L or 10 micromol/L). In vivo administration of K201 failed to prevent induction of polymorphic ventricular tachycardia (VT) in RyR2(R4496C+/-) mice. Measurement of the FKBP12.6/RyR2 ratio in the heavy sarcoplasmic reticulum membrane showed normal RyR2-FKBP12.6 interaction both in WT and RyR2(R4496C+/-) either before and after treatment with caffeine and epinephrine. We suggest that (1) triggered activity is the likely arrhythmogenic mechanism of CPVT; (2) K201 fails to prevent DADs in RyR2(R4496C+/-) myocytes and ventricular arrhythmias in RyR2(R4496C+/-) mice; and (3) RyR2-FKBP12.6 interaction in RyR2(R4496C+/-) is identical to that of WT both before and after epinephrine and caffeine, thus suggesting that it is unlikely that the R4496C mutation interferes with the RyR2/FKBP12.6 complex.

Topics: Animals; Arrhythmias, Cardiac; Caffeine; Cells, Cultured; Epinephrine; Membrane Potentials; Mice; Mice, Mutant Strains; Mutation, Missense; Myocytes, Cardiac; Protein Binding; Ryanodine; Ryanodine Receptor Calcium Release Channel; Tachycardia, Ventricular; Tacrolimus Binding Proteins; Thiazepines

We investigated the inward rectifier potassium current (I(K1)), which can be blocked by intracellular Ca(2+), in heart failure (HF).. We used the whole-cell patch-clamp technique to record I(K1) from single rat ventricular myocytes in voltage-clamp conditions. Fluorescence measurements of diastolic Ca(2+) were performed with Indo-1 AM. HF was examined 8 weeks after myocardial infarction (coronary artery ligation).. I(K1) was reduced and diastolic Ca(2+) was increased in HF cells. The reduction of I(K1) was attenuated when EGTA was elevated from 0.5 to 10 mM in the patch pipette and prevented with high BAPTA (20 mM). Ryanodine (100 nM) and FK506 (10 microM), both of which promote spontaneous SR Ca(2+) release from ryanodine receptor (RyR2) during diastole, reproduced the effect of HF on I(K1) in normal cells but had no effect in HF cells. The effects of ryanodine and FK506 were not additive and were prevented by BAPTA. Rapamycin (10 microM), which removes FKBP binding proteins from RyR2 with no effect on calcineurin, mimicked the effect of FK506 on I(K1). Cyclosporine A (10 microM), which inhibits calcineurin via cyclophilins, had no effect. In both HF cells and normal cells treated by FK506, the protein kinase C (PKC) inhibitor staurosporine totally restored the inward component of I(K1), but only partially restored its outward component at potentials corresponding to the late repolarizing phase of the action potential (-80 to -40 mV).. I(K1) is reduced by elevated diastolic Ca(2+)in HF, which involves in parallel PKC-dependent and PKC-independent mechanisms. This regulation provides a novel paradigm for Ca(2+)-dependent modulation of membrane potential in HF. Since enhanced RyR2-mediated Ca(2+)release also reduces I(K1), this paradigm might be relevant for arrhythmias related to acquired or inherited RyR2 dysfunction.

Topics: Action Potentials; Animals; Arrhythmias, Cardiac; Calcium; Depression, Chemical; Egtazic Acid; Heart Failure; Immunosuppressive Agents; Male; Myocardial Infarction; Myocardium; Patch-Clamp Techniques; Potassium Channels, Inwardly Rectifying; Protein Kinase C; Rats; Rats, Wistar; Ryanodine; Ryanodine Receptor Calcium Release Channel; Sirolimus; Staurosporine; Tacrolimus

Perturbations of the trans-sarcolemmal and sarcoplasmic Ca2+ transport contribute to the abnormal myocardial activity provoked by anoxia and reoxygenation. Whether Ca2+ pools of the extracellular compartment and sarcoplasmic reticulum (SR) are involved to the same extent in the dysfunction of the anoxic-reoxygenated immature heart has not been investigated. Spontaneously contracting hearts isolated from 4-day-old chick embryos were submitted to repeated anoxia (1 min) followed by reoxygenation (5 min). Heart rate, atrioventricular propagation velocity, ventricular shortening, velocities of contraction and relaxation, and incidence of arrhythmias were studied, recorded continuously. Addition of verapamil (10 nM), which blocks selectively sarcolemmal L-type Ca2+ channels, was expected to protect against excessive entry of extracellular Ca2+, whereas addition of ryanodine (10 nM), which opens the SR Ca2+ release channel, was expected to increase cytosolic Ca2+ concentration. Verapamil (a) had no dromotropic effect by contrast to adult heart, (b) attenuated ventricular contracture induced by repeated anoxia, (c) shortened cardioplegia induced by reoxygenation, and (d) had remarkable antiarrhythmic properties during reoxygenation specially. On the other hand, ryanodine potentiated markedly arrhythmias both during anoxia and at reoxygenation. Thus despite its immaturity, the SR seems to be functional early in the developing chick heart and involved in the reversible dysfunction induced by anoxia-reoxygenation. Moreover, Ca2+ entry through L-type channels appears to worsen arrhythmias especially during reoxygenation. These findings show that the Ca2+-handling systems involved in irregular activity in immature heart, such as the embryonic chick heart, may differ from those in the adult.

Topics: Animals; Anti-Arrhythmia Agents; Arrhythmias, Cardiac; Calcium Channel Blockers; Chick Embryo; Contracture; Heart; Heart Arrest; Heart Rate; Hypoxia; In Vitro Techniques; Myocardial Contraction; Oxygen; Ryanodine; Verapamil

1. Effects of ryanodine on the action potentials and the ionic currents in spontaneously beating single rabbit sinoatrial (SA) nodal cells were examined using current-clamp and whole-cell voltage-clamp techniques. 2. Cumulative administrations of ryanodine (10(-8) to 10(-4) M) caused a negative chronotropic effect in a concentration-dependent manner; the effect was not modified by atropine (10(-7) M). At 10(-6) M, ryanodine increased the action potential amplitude and the maximum rate of depolarization, and prolonged the duration of action potentials, significantly. The maximum diastolic potential was unaffected. 3. No arrhythmia occurred in the presence of ryanodine (10(-6) M) alone, but addition of either caffeine (10 mM) or high Ca2+ (10.8 mM) elicited arrhythmias. The incidence increased with an increase in extracellular Ca2+ concentration. 4. Ryanodine, at 10(-6) M, enhanced the Ca2+ current but, at 10(-5) M, inhibited it. Ryanodine inhibited the delayed rectifier K+ current and the hyperpolarization-activated inward current in a concentration-dependent manner. 5. In addition, ryanodine actually elevated the cytosolic Ca2+ level in the SA nodal cells loaded with Ca(2+)-sensitive fluorescent dye (fura-2). 6. These results indicate that ryanodine modulates the ionic currents (presumably dependent on cellular Ca2+ concentration), suggesting similar pharmacological properties to caffeine.

Topics: Action Potentials; Animals; Arrhythmias, Cardiac; Caffeine; Calcium; Calcium Channels; Drug Synergism; In Vitro Techniques; Patch-Clamp Techniques; Potassium Channels; Rabbits; Ryanodine; Sinoatrial Node

Intracellular calcium plays an essential role in regulation of many cellular processes, but increases in internal calcium levels can also exacerbate pathophysiologic or pharmacologic responses, in particular myocardial arrhythmias. Pharmacologic increases in intracellular calcium may be obtained by opening calcium channels, either directly or indirectly, or by increasing calcium release from intracellular stores. In this study, cesium chloride administered intracoronarily (i.c.) through the left anterior descending coronary artery (LAD) dose-dependently elicited ventricular arrhythmias. Glyburide (3 micrograms/kg/min i.c.), clofilium (1 micrograms/kg/min i.c.) or ryanodine (0.03 micrograms/kg/min i.c.) exacerbated arrhythmias. Specifically, the ED50 values for cesium were shifted from 0.56 mM in the vehicle group to 0.17, 0.27, and 0.20 mM in the glyburide, clofilium, and ryanodine groups, respectively. Coronary blood flow (CBF) and blood pressure (BP) did not change significantly in any treatment group. Effects of glyburide were not mediated by either insulin or decreased glucose levels, since infusions of insulin (decreasing blood glucose to 20 mg/dl) did not exacerbate arrhythmias. In vitro electrophysiologic studies showed that glyburide (1 microM) and ryanodine (1 microM) did not significantly affect action potential durations (APD). In contrast, clofilium (1 microM) significantly prolonged APD. These results demonstrate that glyburide, clofilium, and ryanodine tend to exacerbate cesium-induced arrhythmias. We suggest that glyburide and ryanodine may exacerbate arrhythmias by increasing internal calcium from intracellular stores, whereas clofilium may increase internal calcium by increasing influx of calcium across the sarcolemma.

Topics: Anesthesia; Animals; Anti-Arrhythmia Agents; Arrhythmias, Cardiac; Calcium; Cesium; Coronary Circulation; Coronary Vessels; Dogs; Dose-Response Relationship, Drug; Electrophysiology; Extracellular Space; Female; Glyburide; Guinea Pigs; In Vitro Techniques; Injections, Intra-Arterial; Insulin; Male; Quaternary Ammonium Compounds; Ryanodine; Sarcoplasmic Reticulum

Ventricular arrhythmias can be initiated by a mechanism of transient diastolic dilation. To test the hypothesis that Ca2+ release from sarcoplasmic reticulum (SR) is important in initiation of such stretch-induced arrhythmias (SIAs), we studied effects of ryanodine in an isolated canine heart model. Arrhythmias were induced by a computerized ventricular volume servo-pump system that transiently increased left ventricular volume by precise amounts (delta V) during diastole. The probability of eliciting an SIA (PSIA) was compared at the minimum delta V that resulted in PSIA of > or = 90% under baseline conditions. Block of SR Ca2+ release with 10(-5) M ryanodine in 11 ventricles produced mild inhibition of SIAs, reducing PSIA by 19.4% (P = 0.039). Because ryanodine produces leakage of SR Ca2+ at low concentration and block of SR Ca2+ release at high concentration, ryanodine concentration was varied from 10(-9) to 10(-5) M in six ventricles. Ryanodine had minimal effect on PSIA over this concentration range. In six ventricles with elevated intracellular Ca2+ produced by pretreatment with 0.1-0.3 microM strophanthidin, 10(-5) M ryanodine did not significantly reduce PSIA. Probability of inducing ventricular pairs or nonsustained ventricular tachycardia was greater in strophanthidin-treated ventricles than in controls, but induction of these repetitive ventricular beats in the strophanthidin group was virtually abolished by addition of 10(-5) M ryanodine.(ABSTRACT TRUNCATED AT 250 WORDS)

Topics: Animals; Arrhythmias, Cardiac; Blood Pressure; Calcium; Cardiac Complexes, Premature; Dogs; Heart; Heart Rate; Heart Ventricles; In Vitro Techniques; Perfusion; Probability; Ryanodine; Sarcoplasmic Reticulum; Stress, Mechanical; Strophanthidin; Time Factors

The effect of extracellular ATP on transient inward current (Iti), delayed afterdepolarization (DAD), early afterdepolarization (EAD), and triggered activity were investigated in guinea pig isolated ventricular myocytes. ATP alone did not induce afterdepolarizations nor did it significantly alter the resting membrane potentials and action potentials. However, when it was applied with drugs known to increase intracellular Ca2+, ATP facilitated the induction of afterdepolarizations and triggered activity in approximately 60% of the cells. In the presence of isoproterenol, ATP increased the amplitude of Iti and DADs by 55 and 206%, respectively, and caused increases in the amplitude of L-type Ca2+ current (ICa) and EADs, which occasionally led to triggered activity. Similarly, addition of ATP increased the amplitude of Iti and DADs induced by elevated extracellular Ca2+ by 110 and 83%, respectively. Ryanodine inhibited the ATP-induced increase in Iti but not the increase in ICa. In the presence of BAY K 8644 or quinidine, ATP not only further prolonged the action potential durations by 18 +/- 4 and 17 +/- 4%, respectively, but also increased the amplitude of EADs. The present results show a novel arrhythmogenic effect of extracellular ATP, which facilitates the genesis of triggered arrhythmias when Ca2+ influx is increased, probably by further increasing Ca2+ influx from extracellular medium and Ca2+ release from intracellular stores.

Topics: 3-Pyridinecarboxylic acid, 1,4-dihydro-2,6-dimethyl-5-nitro-4-(2-(trifluoromethyl)phenyl)-, Methyl ester; Action Potentials; Adenosine Triphosphate; Animals; Arrhythmias, Cardiac; Calcium; Calcium Channels; Cells, Cultured; Electric Conductivity; Female; Guinea Pigs; Heart; Heart Ventricles; Isoproterenol; Male; Membrane Potentials; Potassium Channel Blockers; Quinidine; Ryanodine; Time Factors

Intracellular calcium ([Ca2+]i) elevation may mediate cardiac arrhythmias. However, direct measurement of the rapid alterations of [Ca2+]i on a beat-to-beat basis using fast temporal resolution and without signal averaging in the spontaneously beating in vivo heart is lacking. Furthermore, data from an isolated spontaneously beating myocyte preparation that develops arrhythmia similar to that in the in vivo heart are unavailable. We measured rapid changes of [Ca2+]i with fast temporal resolution in isolated spontaneously beating neonatal rat ventricular myocytes with cell-to-cell communication and characterized the interrelation between [Ca2+]i and arrhythmia. An elevated extracellular calcium ([Ca2+]o) concentration of 10.8 mM induced premature beats, a rapid beating rate (tachyarrhythmia), and chaotic or fibrillatory beating activity in a small group of myocytes. [Ca2+]i levels during systole increased from the nanomolar to micromolar concentration range before arrhythmia development. Spontaneous oscillations of [Ca2+]i during diastole could evoke a spontaneous tachyarrhythmia. In the presence of [Ca2+]i elevation, a spontaneous tachyarrhythmia could induce severe [Ca2+]i overload. Reduction of [Ca2+]i with 1,2-bis(2-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid AM (5 microM) in the presence of 10.8 mM [Ca2+]o reversed the arrhythmia. In single ventricular myocytes superfused with 10.8 mM [Ca2+]o, oscillations of membrane potential characteristic of transient inward current occurred that were prevented by ryanodine (0.1 microM), an inhibitor of Ca2+ flux across the sarcoplasmic reticulum. This study characterizes 1) an isolated multicellular myocyte model of arrhythmia similar to that evident in in vivo hearts, 2) elevation of [Ca2+]i with systolic [Ca2+]i levels of 1-3 microM and diastolic [Ca2+]i oscillations before the initiation of arrhythmia, 3) tachyarrhythmia as a cause of severe [Ca2+]i overload, which may be important in the perpetuation and degeneration of arrhythmias, and 4) reversal of arrhythmia with reduction of [Ca2+]i. The results in the isolated myocyte model may have relevance to the generation and perpetuation of certain cardiac arrhythmias associated with calcium overload.

Topics: Animals; Animals, Newborn; Arrhythmias, Cardiac; Calcium; Cells, Cultured; Electrophysiology; Heart; Membrane Potentials; Myocardium; Rats; Ryanodine; Sarcoplasmic Reticulum

The effects of ryanodine on ventricular arrhythmias in guinea-pigs in vivo, on delayed after potentials and after contractions, and on spontaneous oscillations of the membrane potential (SOP) and of resting tension (SOT) of guinea-pig papillary muscle under ouabain intoxication were studied. After addition of ouabain (1 microM) the after potentials, after contractions, and SOP and SOT amplitude were significantly increased. The power spectra of SOT and SOP under these conditions had a resonance harmonic with the frequency of about 5 Hz. Three to 5 mins after the addition of ryanodine (0.1-0.5 microM), the after potentials, after contractions, and SOP and SOT were abolished, suggesting a close relationship between these oscillations and the oscillatory activity of sarcoplasmic reticulum. In in vivo experiments, ouabain-induced (75-115 micrograms/kg) ventricular arrhythmias were terminated 4 to 5 min after intravenous injection of ryanodine (15 micrograms/kg); within 8-10 min, sinus rhythm was completely restored. We attribute the antiarrhythmic effect of ryanodine to a cellular effect and alteration of SR function, rather than to effects that are secondary to this.

Topics: Animals; Anti-Arrhythmia Agents; Arrhythmias, Cardiac; Cricetinae; Heart; In Vitro Techniques; Membrane Potentials; Myocardial Contraction; Ouabain; Ryanodine; Sarcoplasmic Reticulum

This study investigates the possible role of oscillatory release of calcium from sarcoplasmic reticulum in the genesis of ventricular arrhythmias during acute myocardial ischemia and reperfusion in isolated rat hearts. We used ryanodine and caffeine, which are known to modulate the oscillatory release of calcium from sarcoplasmic reticulum. During 30 minutes of left main coronary artery ligation, all 13 control hearts developed ventricular premature beats (number of beats, 225 +/- 51) and ventricular tachycardia (duration, 123 +/- 21 seconds); five hearts developed ventricular fibrillation. In a separate series of experiments, reperfusion after 15 minutes of coronary artery ligation caused ventricular fibrillation to occur within 15 seconds in all 12 hearts. Ryanodine (10(-9) to 10(-7) M) abolished ventricular arrhythmias during coronary artery ligation and prevented reperfusion ventricular fibrillation. Ryanodine (10(-9), 10(-8), and 10(-7) M) caused 15%, 23%, and 74% decreases in the maximal rate of rise of left ventricular pressure development and 20%, 32%, and 85% decreases in the maximal rate of fall of left ventricular pressure development, respectively, prior to coronary artery ligation. During acute myocardial ischemia, ryanodine 10(-9) M maintained and 10(-8) M impaired left ventricular function; 10(-7) M caused left ventricular failure. Coronary perfusion rate did not increase during ischemia. Antiarrhythmic activity occurred independent of preservation of high energy phosphates, reduction in tissue lactate, or tissue cyclic adenosine monophosphate in the ischemic myocardium. Caffeine 10(2) M decreased the incidence of ventricular arrhythmias during ischemia and upon reperfusion; protection occurred coincident with development of diastolic contracture. Caffeine increased ischemic tissue cyclic adenosine monophosphate content and worsened tissue energy status.(ABSTRACT TRUNCATED AT 250 WORDS)

Topics: Alkaloids; Animals; Arrhythmias, Cardiac; Caffeine; Calcium; Coronary Circulation; Cyclic AMP; Energy Metabolism; Heart; Myocardial Infarction; Rats; Ryanodine; Sarcoplasmic Reticulum

The effects of ryanodine on (1) ventricular arrhythmias in guinea-pigs in vivo, (2) delayed afterpotentials and aftercontractions and (3) spontaneous oscillations of the membrane potential (SOP) and of resting tension (SOT) of guinea-pig papillary muscle under ouabain intoxication have been studied. After addition of ouabain (1 microM), the afterpotentials, aftercontractions and the amplitude of SOP and SOT were significantly increased. The power spectra of SOT and SOP under these conditions had a resonance harmonic with a frequency of about 5 Hz. The afterpotentials, aftercontractions, SOP and SOT were abolished 3 to 5 min after ryanodine addition (0.1 to 0.5 microM), suggesting a close relationship between these oscillations and the oscillatory activity of sarcoplasmic reticulum. During in vivo experiments, ouabain-induced (75 to 115 micrograms/kg) ventricular arrhythmias were terminated 4 to 5 min after the intravenous injection of ryanodine (15 micrograms/kg) and within 8 to 10 min, the sinus rhythm was completely restored. We conclude that the antiarrhythmic effect of ryanodine is related to the inhibition of the diastolic fluctuations of the membrane potential.

Topics: Action Potentials; Alkaloids; Animals; Arrhythmias, Cardiac; Guinea Pigs; Heart Ventricles; In Vitro Techniques; Myocardial Contraction; Ouabain; Papillary Muscles; Ryanodine

This study examines the arrhythmogenic action of alpha 1 and alpha 2-adrenoceptor stimulation in the isolated perfused rat heart. The alpha 1-agonist methoxamine in the presence of the beta 1-antagonist atenolol 10(-6) M decreased the ventricular fibrillation threshold in the normoxic rat ventricular myocardium: VFT values (mA): Control 11.2 +/- 0.5; methoxamine 10(-6) M 4.9 +/- 0.9 (P less than 0.01 vs control); methoxamine 10(-5) M 3.5 +/- 0.5 (P less than 0.01 vs control). The alpha 1-antagonist prazosin 10(-8) M prevented the methoxamine-induced fall in ventricular fibrillation threshold. The alpha 2-agonist BHT 933 (azepexole) in the presence of atenolol 10(-6) M produced no alteration in the ventricular fibrillation threshold. Methoxamine 10(-6) M to 10(-5) M had a positive inotropic effect with increased left ventricular pressure development, myocardial oxygen consumption and QT-interval; however, tissue levels of cyclic AMP remained unchanged. Methoxamine 10(-6) M did not alter heart rate, coronary flow rate or deplete tissue levels of adenosine triphosphate, phosphocreatine or glycogen. The enhanced vulnerability to ventricular fibrillation induced by methoxamine could be demonstrated only at supraphysiological extracellular calcium concentrations (2.5 mM) but not at physiological calcium concentrations (1.25 mM). The arrhythmogenic and inotropic effect of methoxamine 10(-6) M was prevented by inhibition of transsarcolemmal Ca2+ ion influx by nisoldipine 10(-8) M or by inhibition of release of Ca2+ from sarcoplasmic reticulum by ryanodine 10(-9) M to 10(-8) M. Thus in isolated normoxic rat heart preparations, activity of the alpha 1-receptor appears to mediate ventricular arrhythmogenesis but only in the setting of myocardial calcium overload. The arrhythmogenic effect of alpha 1-stimulation may be due to increased transsarcolemmal calcium influx and enhanced release of calcium from the sarcoplasmic reticulum; increased myocardial oxygen consumption secondary to greater left ventricular pressure development may contribute in part.

Topics: Adrenergic alpha-Agonists; Alkaloids; Animals; Arrhythmias, Cardiac; Azepines; Calcium; Calcium Channel Blockers; Heart; Male; Methoxamine; Nifedipine; Nisoldipine; Prazosin; Rats; Rats, Inbred Strains; Ryanodine

The effect of ryanodine on membrane potential oscillations in vitro (in isolated guinea-pig papillary muscle) and on ventricular arrhythmias in vivo (in glycoside intoxication) were studied. 3-5 minutes after ryanodine (0.5 microM) addition the membrane potential oscillations induced by ouabain (1 microM) were abolished. 4-5 minutes after intravenous ryanodine infusion (15 micrograms/kg) ventricular arrhythmias induced by ouabain intoxication (75-115 micrograms/kg) disappeared and 8-10 minutes later sinus rhythm was restored. It is suggested that antiarrhythmic effect of ryanodine is a result of the inhibition of diastolic membrane potential oscillations.

Topics: Action Potentials; Alkaloids; Animals; Anti-Arrhythmia Agents; Arrhythmias, Cardiac; Drug Evaluation, Preclinical; Electrocardiography; Guinea Pigs; In Vitro Techniques; Muscle Tonus; Ouabain; Papillary Muscles; Ryanodine

Drug-induced triggered arrhythmias in heart muscle involve oscillations of membrane potential known as delayed or early afterdepolarizations (DADs or EADs). We examined the mechanism of DADs and EADs in ferret ventricular muscle. Membrane potential, tension and aequorin luminescence were measured during exposure to elevated [Ca2+]0, strophanthidin and/or isoproterenol (to induce DADs), or cesium chloride (to induce EADs). Ryanodine (10(-9)-10(-6) M), an inhibitor of Ca2+ release from the sarcoplasmic reticulum, rapidly suppressed DADs and triggered arrhythmias. When cytoplasmic Ca2+-buffering capacity was enhanced by loading cells with the Ca2+ chelators BAPTA or quin2, DADs were similarly inhibited, as were contractile force and aequorin luminescence. In contrast to DADs, EADs induced by Cs were not suppressed by ryanodine or by loading with intracellular Ca2+ chelators. The possibility that transsarcolemmal Ca2+ entry might produce EADs was evaluated with highly specific dihydropyridine Ca channel agonists and antagonists. Bay K8644 (100-300 nM) potentiated EADs, whereas nitrendipine (3-20 microM) abolished EADs. We conclude that DADs and DAD-related triggered arrhythmias are activated by an increase in intracellular free Ca2+ concentration, whereas EADs do not require elevated [Ca2+]i but rather arise as a direct consequence of Ca2+ entry through sarcolemmal slow Ca channels.

Topics: Animals; Arrhythmias, Cardiac; Carnivora; Cesium; Chelating Agents; Chlorides; Ferrets; Heart; Heart Rate; Heart Ventricles; Isoproterenol; Membrane Potentials; Ryanodine; Strophanthidin; Ventricular Function

In addition to its role in myocardial excitation-contraction coupling, the sarcoplasmic reticulum may be involved in arrhythmogenic actions of cardiotonic steroids. This study in isolated cardiac muscle demonstrates that ryanodine can prevent the arrhythmias and reduce the positive inotropic effect produced by cardiotonic steroids without affecting their specific binding to sites on Na,K-ATPase. These data suggest that the antagonism is mediated by ryanodine-induced alterations in Ca2+ release from the sarcoplasmic reticulum.

Topics: Alkaloids; Animals; Arrhythmias, Cardiac; Cardiac Glycosides; Digoxin; Guinea Pigs; In Vitro Techniques; Kinetics; Myocardial Contraction; Receptors, Drug; Ryanodine; Sodium-Potassium-Exchanging ATPase; Strophanthidin

Topics: Alkaloids; Arrhythmias, Cardiac; Blood Circulation; Blood Pressure; Digitalis; Digitalis Glycosides; Dogs; Electrocardiography; Hemodynamics; Humans; Pharmacology; Research; Ryanodine; Toxicology