Hiroki Mani

Burst Emergence of Intracellular Ca2+ Waves Evokes Arrhythmogenic Oscillatory Depolarization via the Na+–Ca2+ Exchanger Simultaneous Confocal Recording of Membrane Potential and Intracellular Ca2+ in the Heart

Intracellular Ca2+ waves (CaWs) of cardiomyocytes are spontaneous events of Ca2+ release from the sarcoplasmic reticulum that are regarded as an important substrate for triggered arrhythmias and delayed afterdepolarizations. However, little is known regarding whether or how CaWs within the heart actually produce arrhythmogenic membrane oscillation because of the lack of data confirming direct correlation between CaWs and membrane potentials (Vm) in the heart. On the hypothesis that CaWs evoke arrhythmogenic oscillatory depolarization when they emerge synchronously and intensively in the heart, we conducted simultaneous fluorescence recording of intracellular Ca2+ ([Ca2+]i) dynamics and Vm of ventricular myocytes on subepicardial surfaces of Langendorff-perfused rat hearts using in situ dual-view, rapid-scanning confocal microscopy. In intact hearts loaded with fluo4/acetoxymethyl ester and RH237 under perfusion with cytochalasin D at room temperature, individual myocytes exhibited Ca2+ transients and action potentials uniformly on ventricular excitation, whereas low-K+–perfused (2.4 mmol/L) hearts exhibited CaWs sporadically between Ca2+ transients without discernible membrane depolarization. Further [Ca2+]i loading of the heart, produced by rapid pacing and addition of isoproterenol, evoked triggered activity and subsequent oscillatory Vm, which are caused by burst emergence of CaWs in individual myocytes. Such arrhythmogenic membrane oscillation was abolished by ryanodine or the Na+–Ca2+ exchanger inhibitor SEA0400, indicating an essential role of CaWs and resultant Na+–Ca2+ exchanger–mediated depolarization in triggered activity. In summary, we demonstrate a mechanistic link between intracellular CaWs and arrhythmogenic oscillatory depolarizations in the heart. Our findings provide a cellular perspective on abnormal [Ca2+]i handling in the genesis of triggered arrhythmias in the heart.

Generation of reentrant arrhythmias by dominant-negative inhibition of connexin43 in rat cultured myocyte monolayers

AIMS Alteration of connexin43 (Cx43)-mediated intercellular communication is known to promote susceptibility to ventricular tachyarrhythmias. However, the precise mechanism of the altered Cx43 responsible for arrhythmogenesis remains unclear. We sought to understand changes in impulse propagation of ventricular myocytes under dominant-negative (DN) inhibition of Cx43 in the development of arrhythmias. METHODS AND RESULTS Intercellular communication was inhibited in confluent monolayers of neonatal rat cultured myocytes by an adenoviral vector-mediated gene transfer for DNCx43-fused red fluorescence protein (RFP). A high-resolution, macro-zoom fluorescence imaging system was used to visualize both the fluo4- and RFP-fluorescence intensities as measures of Ca2+ transient propagation and distribution of DNCx43 inhibition, respectively, in the myocyte monolayers. DNCx43 inhibition of the monolayers resulted in not only a significant slowing of Ca2+ transient propagation velocity, but also a preferential emergence of spiral-wave reentrant arrhythmias elicited by rapid pacing. Detailed observations on the development of spiral waves revealed that the gene-transferred myocyte monolayers exhibited regional slowing of propagation and subsequent generation of wave break, resulting in reentrant arrhythmias. Furthermore, DNCx43-RFP-transferred monolayers showed higher fluorescence intensity of RFP at the break point than at the surrounding myocardium, indicating a culprit role of DNCx43 inhibition in the genesis of spiral reentry. CONCLUSION The present results indicate that regional heterogeneity in gap-junctional communication promotes, in addition to slowing of conduction velocity, susceptibility to reentrant tachyarrhythmias.

Clinical Significance of Preserving Spontaneous QRS Wave in the Therapy of DDD Pacing for Sick Sinus Syndrome

The aim of this clinical crossover study was to elucidate the effects of atrioventricular (AV) synchronous pacing on cardiac function in patients with sick sinus syndrome (SSS). Thirty SSS patients, each with dual chamber pacemaker (DDD), were enrolled and divided into two groups based on echocardiographic findings. Group A (n = 16) had hypertensive heart disease (wall thickness 11 ∼ 12 mm) or mitral or aortic regurgitation (Grade I or II). Group B (n = 14) had no organic heart disease. Three successive 3‐month pacing periods were tested. For the first 3 months, long AV delay that achieved > 80% ventricular sensing was chosen. For the next 3 months, AV delay was abbreviated to achieve > 80% ventricular pacing at an optimal AV interval. For the final 3 months, the first setting was resumed. At the end of each period, M mode echocardiography, pulsed‐Doppler study, and measurement of plasma brain natriuretic peptide (BNP) level were conducted. In both groups, echocardiographic parameters were not significantly changed during the evaluation. In group A, plasma BNP level was significantly higher at the end of the short AV delay period than at the long AV delay period (P = 0.009), while in group B it did not differ during each period. AV synchronous pacing (>80% ventricular pacing) in the SSS patients with a DDD pacemaker implanted could increase the ventricular load, and it is better to preserve the spontaneous QRS with the DDD mode with prolonged AV delay in patients with mild hypertensive or valvular disease.

How Does the Ca 2+ -paradox Injury Induce Contracture in the Heart?—A Combined Study of the Intracellular Ca 2+ Dynamics and Cell Structures in Perfused Rat Hearts—

The calcium (Ca2+)-paradox injury of the heart, induced by restoration of extracellular Ca2+ after its short-term depletion, is known to provoke cardiomyocyte contracture. However, undetermined is how the Ca2+-paradox provokes such a distinctive presentation of myocytes in the heart. To address this, we imaged sequential intracellular Ca2+ dynamics and concomitant structures of the subepicardial ventricular myocytes in fluo3-loaded, Langendorff-perfused rat hearts produced by the Ca2+ paradox. Under rapid-scanning confocal microscopy, repletion of Ca2+ following its depletion produced high-frequency Ca2+ waves in individual myocytes with asynchronous localized contractions, resulting in contracture within 10 min. Such alterations of myocytes were attenuated by 5-mM NiCl2, but not by verapamil, SEA0400, or combination of ryanodine and thapsigargin, indicating a contribution of non-specific transmembrane Ca2+ influx in the injury. However, saponin-induced membrane permeabilization of Ca2+ showed no apparent contracture despite the emergence of high-frequency Ca2+ waves, indicating an essential role of myocyte-myocyte and myocyte-extracellular matrix (ECM) mechanical connections in the Ca2+ paradox. In immunohistochemistry Ca2+ depletion produced separation of the intercalated disc that expresses cadherin and dissipation of β-dystroglycan located along the sarcolemma. Taken together, along with the trans-sarcolemmal Ca2+ influx, disruption of cell-cell and cell-ECM connections is essential for contracture in the Ca2+-paradox injury.

Exercise-induced Atrioventricular Block with Coronary Artery Stenosis that Appeared Five Years After Bypass Surgery

A 68-year-old man with a history of coronary artery bypass surgery was referred to our hospital because of pre-syncope on effort. During a treadmill exercise electrocardiogram test, the patient developed advanced atrioventricular block associated with dizziness. Coronary angiography revealed significant stenosis of the right coronary artery, which had not existed at the time of the bypass surgery. We implanted drug-eluting stents in the stenotic lesion, and an exercise test showed resolution of the atrioventricular block. Exercise-induced atrioventricular block is rare, and it is necessary to distinguish it from ischemic heart disease, especially in patients with a history of coronary artery disease.

Postextrasystolic potentiation on quantitative gated SPECT.

A longer filling interval induces better ventricular filling and subsequently stronger contraction. This phenomenon is often observed after a premature ventricular contraction and is called postextrasystolic potentiation. The following three-dimensional perfusion images show visualization of postextrasystolic potentiation induced by ventricular trigeminy.