Yoshinao Sugai

Electrocardiographic Characteristics of the Variants of Idiopathic Left Ventricular Outflow Tract Ventricular Tachyarrhythmias

Background: Despite similar QRS morphology, idiopathic repetitive monomorphic ventricular tachyarrhythmias (VTs) of left ventricular outflow tract (LVOT) are known to have the variants of different adjacent origins, including the aorto‐mitral continuity (AMC), anterior site around the mitral annulus (MA), aortic sinus cusps (ASC), and epicardium. However, the electrocardiographic characteristics of those variants previously have not been evaluated fully.

Sarcomere mechanics in uniform and non-uniform cardiac muscle: A link between pump function and arrhythmias

Starlings Law and the well-known end-systolic pressure-volume relationship (ESPVR) of the left ventricle reflect the effect of sarcomere length (SL) on stress (sigma) development and shortening by myocytes in the uniform ventricle. We show here that tetanic contractions of rat cardiac trabeculae exhibit a sigma-SL relationship at saturating [Ca2+] that depends on sarcomere geometry in a manner similar to skeletal sarcomeres and the existence of opposing forces in cardiac muscle shortened below slack length. The sigma-SL-[Ca2+]free relationships (sigma-SL-CaR) at submaximal [Ca2+] in intact and skinned trabeculae were similar, albeit that the sensitivity for Ca2+ of intact muscle was higher. We analyzed the mechanisms underlying the sigma-SL-CaR using a kinetic model where we assumed that the rates of Ca2+ binding by Troponin-C (Tn-C) and/or cross-bridge (XB) cycling are determined by SL, [Ca2+] or stress. We analyzed the correlation between the model results and steady state stress measurements at varied SL and [Ca2+] from skinned rat cardiac trabeculae to test the hypotheses that: (i) the dominant feedback mechanism is SL, stress or [Ca2+]-dependent; and (ii) the feedback mechanism regulates: Tn-C-Ca2+ affinity, XB kinetics or, unitary XB-force. The analysis strongly suggests that feedback of the number of strong XBs to cardiac Tn-C-Ca2+ affinity is the dominant mechanism that regulates XB recruitment. Application of this concept in a mathematical model of twitch-stress accurately reproduced the sigma-SL-CaR and the time course of twitch-stress as well as the time course of intracellular [Ca2+]i. Modeling of the response of the cardiac twitch to rapid stress changes using the above feedback model uniquely predicted the occurrence of [Ca2+]i transients as a result of accelerated Ca2+ dissociation from Tn-C. The above concept has important repercussions for the non-uniformly contracting heart in which arrhythmogenic Ca2+ waves arise from weakened areas in cardiac muscle. These Ca2+ waves can reversibly be induced in muscle with non-uniform excitation contraction coupling (ECC) by the cycle of stretch and release in the border zone between the damaged and intact regions. Stimulus trains induced propagating Ca2+ waves and reversibly induced arrhythmias. We hypothesize that rapid force loss by sarcomeres in the border zone during relaxation causes Ca2+ release from Tn-C and initiates Ca2+ waves propagated by the sarcoplasmic reticulum (SR). These observations suggest the unifying hypothesis that force feedback to Ca2+ binding by Tn-C is responsible for Starlings Law and the ESPVR in uniform myocardium and leads in non-uniform myocardium to a surge of Ca2+ released by the myofilaments during relaxation, which initiates arrhythmogenic propagating Ca2+ release by the SR.

Role of sarcomere mechanics and Ca2+ overload in Ca2+ waves and arrhythmias in rat cardiac muscle

Abstract:  Ca2+ release from the sarcoplasmic reticulum (SR) depends on the sarcoplasmic reticulum (SR) Ca2+ load and the cytosolic Ca2+ level. Arrhythmogenic Ca2+ waves underlying triggered propagated contractions arise from Ca2+ overloaded regions near damaged areas in the cardiac muscle. Ca2+ waves can also be induced in undamaged muscle, in regions with nonuniform excitation–contraction (EC) coupling by the cycle of stretch and release in the border zone between the damaged and intact regions. We hypothesize that rapid shortening of sarcomeres in the border zone during relaxation causes Ca2+ release from troponin C (TnC) on thin filaments and initiates Ca2+ waves. Elimination of this shortening will inhibit the initiation of Ca2+ waves, while SR Ca2+ overload will enhance the waves. Force, sarcomere length (SL), and [Ca2+]i were measured and muscle length was controlled. A small jet of Hepes solution with an extracellular [Ca2+] 10 mM (HC), or HC containing BDM, was used to weaken a 300 μm long muscle segment. Trains of electrical stimuli were used to induce Ca2+ waves. The effects of small exponential stretches on triggered propagatory contraction (TPC) amplitude and propagation velocity of Ca2+ waves (Vprop) were studied. Sarcomere shortening was uniform prior to activation. HC induced spontaneous diastolic sarcomere contractions in the jet region and attenuated twitch sarcomere shortening; HC+ butanedione monoxime (BDM) caused stretch only in the jet region. Stimulus trains induced Ca2+ waves, which started inside the HC jet region during twitch relaxation. Ca2+ waves started in the border zone of the BDM jet. The initial local [Ca2+]i rise of the waves by HC was twice that by BDM. The waves propagated at a Vprop of 2.0 ± 0.2 mm/sec. Arrhythmias occurred frequently in trabeculae following exposure to the HC jet. Stretch early during relaxation, which reduced sarcomere shortening in the weakened regions, substantially decreased force of the TPC (FTPC) and delayed Ca2+ waves, and reduced Vprop commensurate with the reduction FTPC. These results are consistent with the hypothesis that Ca2+ release from the myofilaments initiates arrhythmogenic propagating Ca2+ release. Prevention of sarcomere shortening, by itself, did not inhibit Ca2+ wave generation. SR Ca2+ overload potentiated initiation and propagation of Ca2+ waves.

Spatial Non-uniformity of Excitation-Contraction Coupling Can Enhance Arrhythmogenic Delayed Afterdepolarizations in Rat Cardiac Muscle

AIMS We examined whether non-uniform muscle contraction affects delayed afterdepolarizations (DADs) by dissociating Ca(2+) from myofilaments within the border zone (BZ) between contracting and stretched regions. METHODS AND RESULTS Force, sarcomere length (SL), membrane potential, and [Ca(2+)](i) dynamics were measured in 31 ventricular trabeculae from rat hearts. Non-uniform muscle contraction was produced by exposing a restricted region of muscle to a jet of solution containing 20 mmol/L 2,3-butanedione monoxime (BDM). DADs were induced by 7.5 s-2 Hz stimulus trains at an SL of 2.0 microm (24 degrees C, [Ca(2+)](o) 2.0 mmol/L). The BDM jet enhanced DADs (n = 6, P < 0.05) and aftercontractions (n = 6, P < 0.05) with or without 100 micromol/L streptomycin and occasionally elicited an action potential. A stretch pulse from an SL of 2.0 microm to 2.1 or 2.2 microm during the last stimulated twitch of the trains accelerated Ca(2+) waves in proportion to the increment of force by the stretch (P < 0.01) with or without streptomycin. In the presence of 1 mmol/L caffeine, rapid shortening of the muscle after the stretch pulse increased [Ca(2+)](i) within the BZ, whose amplitude correlated with the increment of force by the stretch (n = 15, P < 0.01). CONCLUSION These results suggest that non-uniform muscle contraction can enhance DADs by dissociating Ca(2+) from myofilaments within the BZ and thereby cause triggered arrhythmias.

Contribution of Na+/Ca2+ exchange current to the formation of delayed afterdepolarizations in intact rat ventricular muscle.

Aim: To evaluate the role of the Na+-Ca2+ exchange current in the induction of arrhythmias during Ca2+ waves, we investigated the relationship between Ca2+ waves and delayed afterdepolarizations (DADs) and further investigated the effect of KB-R7943, an Na+-Ca2+ exchange inhibitor, on such relationship in multicellular muscle. Methods: Force, sarcomere length, membrane potential, and [Ca2+]i dynamics were measured in 32 ventricular trabeculae from rat hearts. After the induction of Ca2+ waves by trains of electrical stimuli (400, 500, or 600 ms intervals) for 7.5 seconds, 23 Ca2+ waves in the absence of KB-R7943 and cilnidipine ([Ca2+]o = 2.3 ± 0.2 mmol/L), 11 Ca2+ waves in the presence of 10 μmol/L KB-R7943 ([Ca2+]o = 2.5 ± 0.5 mmol/L), and 8 Ca2+ waves in the presence of 1 μmol/L cilnidipine ([Ca2+]o = 4.1 ± 0.3 mmol/L) were measured at a sarcomere length of 2.1 μm (23.9 ± 0.8°C). Results: The amplitude of DADs correlated with the velocity (r = 0.90) and the amplitude (r = 0.90) of Ca2+ waves. The amplitude of DADs was significantly decreased to ~40% of the initial value by 10 μmol/L KB-R7943. Conclusions: These results suggest that the velocity and the amplitude of Ca2+ waves determine the formation of DADs principally through the activation of the Na+-Ca2+ exchange current, thereby inducing triggered arrhythmias in multicellular ventricular muscle.

Reduced Inotropic Effect of Nifekalant in Failing Hearts in Rats

Class III antiarrhythmic agents have been widely used to suppress ventricular tachyarrhythmias in patients with heart failure because they have been shown to have positive inotropic effects as well. However, it remains to be examined whether those agents also exert positive inotropic effects in failing hearts. We addressed this important issue in a rat model of heart failure. We used Nifekalant as a representative class III antiarrhythmic agent. Four weeks after a s.c. injection of 60 mg/kg monocrotaline (MCT) or vehicle (Ctr) into rats, we obtained trabeculae from right ventricles and measured the developed force and intracellular Ca2+ ([Ca2+]i) by the fura-2 microinjection method. The sarcoplasmic reticulum (SR) Ca2+ content was assessed by the rapid-cooling contracture (RCC) technique. MCT rats exhibited right ventricular hypertrophy induced by pressure overload. The protein expression of SR Ca2+ ATPase type 2 (SERCA2) and the SERCA2/phospholamban ratio in MCT rats was lower with a slower decline of Ca2+ transients and a reduced amplitude of RCCs. Nifekalant concentration-dependently increased the force, peak [Ca2+]i, and the amplitude of RCCs in Ctr rats but not in MCT rats with identical prolongation of the action potential. Under the SR inhibited with cyclopiazonic acid and ryanodine, Nifekalant increased the force in Ctr rats but not in MCT rats. These results indicate that the positive inotropic effects of Nifekalant is reduced in failing hearts, probably due to the depressed SR Ca2+ uptake and reduced reserve of the trans-sarcolemmal Ca2+ transport, warranting a caution in the antiarrhythmic therapy with a class III antiarrhythmic agent in heart failure.

Sarcomere Mechanics in Uniform and Nonuniform Cardiac Muscle

Starlings law and the end‐systolic pressure–volume relationship (ESPVR) reflect the effect of sarcomere length (SL) on the development of stress (σ) and shortening by myocytes in the uniform ventricle. We show here that tetanic contractions of rat cardiac trabeculae exhibit a σ–SL relationship at saturating [Ca2+] that depends on sarcomere geometry in a manner similar to that of skeletal sarcomeres and the existence of opposing forces in cardiac muscle shortened below slack length. The σ−SL −[Ca2+]free relationships (σ–SL–Ca relationships) at submaximal [Ca2+] in intact and skinned trabeculae were similar, although the sensitivity for Ca2+ of intact muscle was higher. We analyzed the mechanisms underlying the σ–SL–Ca relationship by using a kinetic model assuming that the rates of Tn‐C Ca2+ binding and/or cross‐bridge (XB) cycling are determined by either the SL, [Ca2+], or σ. We analyzed the correlation between the model results and steady‐state σ measurements at varied SL at [Ca2+] from skinned rat cardiac trabeculae to test the hypotheses that the dominant feedback mechanism is SL‐, σ‐, or [Ca2+]‐dependent, and that the feedback mechanism regulates Tn‐C Ca2+ affinity, XB kinetics, or the unitary XB force. The analysis strongly suggests that the feedback of the number of strong XBs to cardiac Tn‐C Ca2+ affinity is the dominant mechanism regulating XB recruitment. Using this concept in a model of twitch‐σ accurately reproduced the σ–SL–Ca relationship and the time courses of twitch σ and the intracellular [Ca2+]i. The foregoing concept has equally important repercussions for the nonuniformly contracting heart, in which arrhythmogenic Ca2+ waves arise from weakened areas in the cardiac muscle. These Ca2+ waves can reversibly be induced with nonuniform excitation–contraction coupling (ECC) by the cycle of stretch and release in the border zone between the damaged and intact regions. Stimulus trains induced propagating Ca2+ waves and reversibly induced arrhythmias. We hypothesize that rapid force loss by the sarcomeres in the border zone during relaxation causes Ca2+ release from Tn‐C and initiates Ca2+ waves propagated by the sarcoplasmic reticulum (SR). Modeling of the response of the cardiac twitch to rapid force changes using the feedback concept uniquely predicts the occurrence of [Ca2+]i transients as a result of accelerated Ca2+ dissociation from Tn‐C. These results are consistent with the hypothesis that a force feedback to Ca2+ binding by Tn‐C is responsible for Starlings law and the ESPVR in the uniform myocardium and leads to a surge of Ca2+ released by the myofilaments during relaxation in the nonuniform myocardium, which initiates arrhythmogenic propagating Ca2+ release by the SR.

Effect of transient stretch on intracellular Ca2+ during triggered propagated contractions in intact trabeculae

Transient stretch of cardiac muscle during a twitch contraction may dissociate Ca2+ from myofilaments into the cytosol at the moment of quick release of the muscle. We studied the effect of stretch and quick release of trabeculae on changes in intracellular Ca2+ ([Ca2+]i) during triggered propagated contractions (TPCs). Trabeculae were dissected from the right ventricle of 9 rat hearts. [Ca2+]i was measured using electrophoretically injected fura-2. Force was measured using a silicon strain gauge and sarcomere length was measured using laser diffraction techniques. Reproducible TPCs (n = 13) were induced by trains of electrical stimuli (378 +/- 19 ms interval) for 7.5 s at [Ca2+]o of 2.0 mM (27.9 +/- 0.2 degrees C). The latency of the TPC force and the underlying increase in [Ca2+]i was calculated from the time (TimeF) between the last stimulus and the peak of TPC force (PeakF), or the time (TimeCa) between the last stimulus and the peak of the increase in [Ca2+]i during the TPCs (PeakCa). As a result of a 10% increase in muscle length for 150-200 ms during the last stimulated twitches, TimeF and TimeCa decreased and PeakF and PeakCa increased significantly (n = 13). In addition, transient stretch sometimes induced a twitch contraction subsequent to the accelerated TPC and its underlying increase in [Ca2+]i. These results suggest that Ca2+ binding and dissociation from the myofilaments by the stretch and quick release of muscle may modulate the TPC force and the underlying increases in [Ca2+]i and play an important role in the induction of arrhythmias.

Evaluation of the atrial substrate based on low-voltage areas and dominant frequencies after pulmonary vein isolation in nonparoxysmal atrial fibrillation

This study aimed to evaluate the atrial substrate in the left atrium (LA) by low‐voltage areas (LVAs) and high‐dominant frequencies (DFs) after circumferential pulmonary vein isolation (PVI) in nonparoxysmal atrial fibrillation (AF).

Association between epicardial adipose tissue and coronary artery vasospasm during pulmonary vein isolation

Coronary artery vasospasms (CAVs) during pulmonary vein isolation have been reported, but the mechanism remains unclear. We experienced a rare case of CAVs caused by radiofrequency (RF) applications to sites with massive epicardial adipose tissue (EAT) attached. Because EAT contains ganglionated plexuses, RF application may have caused an autonomic nervous system imbalance, which thereby provoked severe CAVs.