Kazuyuki Mitsui

Pacing-Induced Spontaneous Activity in Myocardial Sleeves of Pulmonary Veins After Treatment With Ryanodine

Background—Recent clinical electrophysiology studies and successful results of radiofrequency catheter ablation therapy suggest that high-frequency focal activity in the pulmonary veins (PVs) plays important roles in the initiation and perpetuation of atrial fibrillation, but the mechanisms underlying the focal arrhythmogenic activity are not understood. Methods and Results—Extracellular potential mapping of rabbit right atrial preparations showed that ryanodine (2 &mgr;mol/L) caused a shift of the leading pacemaker from the sinoatrial node to an ectopic focus near the right PV-atrium junction. The transmembrane potential recorded from the isolated myocardial sleeve of the right PV showed typical atrial-type action potentials with a stable resting potential under control conditions. Treatment with ryanodine (0.5 to 2 &mgr;mol/L) resulted in a depolarization of the resting potential and a development of pacemaker depolarization. These changes were enhanced transiently after an increase in the pacing rate: a self-terminating burst of spontaneous action potentials (duration, 33.6±5.0 s; n=32) was induced by a train of rapid stimuli (3.3 Hz) applied after a brief rest period. The pacing-induced activity was attenuated by either depletion of the sarcoplasmic reticulum of Ca2+ or blockade of the sarcolemmal Na+-Ca2+ exchanger or Cl− channels and potentiated by &bgr;-adrenergic stimulation. Conclusions—PV myocardial sleeves have the potential to generate spontaneous activity, and such arrhythmogenic activity is uncovered by modulation of intracellular Ca2+ dynamics.

Sarcoplasmic Reticulum Ca2+ Release Is Not a Dominating Factor in Sinoatrial Node Pacemaker Activity

Abstract— Recent work on isolated sinoatrial node cells from rabbit has suggested that sarcoplasmic reticulum Ca2+ release plays a dominant role in the pacemaker potential, and ryanodine at a high concentration (30 &mgr;mol/L blocks sarcoplasmic reticulum Ca2+ release) abolishes pacemaking and at a lower concentration abolishes the chronotropic effect of &bgr;-adrenergic stimulation. The aim of the present study was to test this hypothesis in the intact sinoatrial node of the rabbit. Spontaneous activity and the pattern of activation were recorded using a grid of 120 pairs of extracellular electrodes. Ryanodine 30 &mgr;mol/L did not abolish spontaneous activity or shift the position of the leading pacemaker site, although it slowed the spontaneous rate by 18.9±2.5% (n=6). After ryanodine treatment, &bgr;-adrenergic stimulation still resulted in a substantial chronotropic effect (0.3 &mgr;mol/L isoproterenol increased spontaneous rate by 52.6±10.5%, n=5). In isolated sinoatrial node cells from rabbit, 30 &mgr;mol/L ryanodine slowed spontaneous rate by 21.5±2.6% (n=13). It is concluded that sarcoplasmic reticulum Ca2+ release does not play a dominating role in pacemaking in the sinoatrial node. The full text of this article is available at http://www.circresaha.org.

Pacemaker shift in the rabbit sinoatrial node in response to vagal nerve stimulation

Effects of brief postganglionic vagal nerve stimulation on the activation sequence of the rabbit sinoatrial (SA) node were investigated. Activation sequences in a small area (7 mm × 7 mm) on the epicardial surface were measured in a beat‐to‐beat manner using an extracellular potential mapping system composed of 64 modified bipolar electrodes with high‐gain and low‐frequency band‐pass filtering. The leading pacemaker site was recognised clearly from both the activation sequence and the characteristic morphology of the potentials. Vagal stimulation resulted in a short‐lasting initial slowing of spontaneous rate followed by a long‐lasting secondary slowing; a brief period of relative or absolute acceleration was interposed between the two slowing phases. During these changes of spontaneous rate, the leading pacemaker site shifted in a complex beat‐to‐beat manner by 1‐6 mm alongside the crista terminalis in the superior or inferior direction. For the first spontaneous excitation following stimulation, the greater the slowing, the larger the distance of the pacemaker shift. There was no such linear relationship between the extent of slowing and the distance of pacemaker shift for the subsequent beats. These changes in the leading pacemaker site in response to vagal stimulation may be the result of the functional and morphological heterogeneity of the mammalian SA node in terms of innervation, receptor distribution and ion channel densities.

Importance of gradients in membrane properties and electrical coupling in sinoatrial node pacing.

The sinoatrial node (SAN) is heterogeneous in terms of cell size, ion channels, current densities, connexins and electrical coupling. For example, Nav1.5 (responsible for I Na) and Cx43 (responsible for electrical coupling) are absent from the centre of the SAN (normally the leading pacemaker site), but present in the periphery (at SAN-atrial muscle junction). To test whether the heterogeneity is important for the functioning of the SAN, one- and two-dimensional models of the SAN and surrounding atrial muscle were created. Normal functioning of the SAN (in terms of cycle length, position of leading pacemaker site, conduction times, activation and repolarization sequences and space constants) was observed when, from the centre to the periphery, (i) cell characteristics (cell size and ionic current densities) were changed in a gradient fashion from a central-type (lacking I Na) to a peripheral-type (possessing I Na) and (ii) coupling conductance was increased in a gradient fashion. We conclude that the heterogeneous nature of the node is important for its normal functioning. The presence of Nav1.5 and Cx43 in the periphery may be essential for the node to be able to drive the atrial muscle: Nav1.5 provides the necessary depolarizing current and Cx43 delivers it to the atrial muscle.

Simulation study of complex action potential conduction in atrioventricular node

The atrioventricular (AV) node, which is located between the atria and ventricles of the heart, acts as important roles in cardiac excitation conduction between the two chambers. Although there are multiple conduction pathways in the AV node, the structure of the AV node has not been clarified. In this study, we constructed a one-dimensional model of the AV node and simulated excitation conduction between the right atrium and the bundle of His via the AV node. We also investigated several characteristics of the AV node: (1) responses of the AV node to high-rate excitation in the right atrium, (2) the AV nodal reentrant beat induced by premature stimulus, and (3) ventricular rate control during atrial fibrillation with various methods. Our simulation results suggest that multiple conduction pathways act as important roles in controlling the ventricular rate. The one-dimensional model constructed in this study may be useful to analyze complex conduction patterns in the AV node.

Simulation of ventricular rate control during atrial fibrillation using ionic channel blockers

The atrioventricular (AV) node is the only compartment that conducts an electrical impulse between the atria and the ventricles. The main role of the AV node is to facilitate efficient pumping by conducting excitation slowly between the two chambers as well as reduce the ventricular rate during atrial fibrillation (AF).

A study of the blood flow restriction pressure of a tourniquet system to facilitate development of a system that can prevent musculoskeletal complications

BACKGROUND After an emergency or disaster, subsequent trauma can cause severe bleeding and this can often prove fatal, so promptly stopping that bleeding is crucial to preventing avoidable trauma deaths. A tourniquet is often used to restrict blood flow to an extremity. In operation and hospital, the tourniquet systems currently in use are pneumatically actuated by an air compressor, so they must have a steady power supply. These devices have several drawbacks: they vibrate and are noisy since they are pneumatically actuated and they are far from portable since they are large and heavy. INTRODUCTION Presumably, the drawbacks of pneumatic tourniquets could be overcome by developing a small, lightweight, vibration-free, quiet, and battery-powered tourniquet system. The current study built a small, vibration-free electrohydrodynamic (EHD) pump and then used that pump to restrict blood flow to the leg of rats in an experiment. This study explored the optimal conditions for effective restriction of blood flow by assessing biochemical and musculoskeletal complications following the restriction of blood flow, and this study also examined whether or not an EHD pump could be used to actuate a tourniquet system. METHODS A tourniquet cuff (width 12 mm × length 150 mm, material: polyolefin) was placed on the thigh of Wistar rats and pressure was applied for 2 hours by a device that uses EHD phenomena to generate pressure (an EHD pump). Animals were divided into four groups based on how much compressive pressure was applied with a tourniquet: 40 kPa (300 mm Hg, n = 13), 30 kPa (225 mm Hg, n = 12), 20 kPa (150 mm Hg, n = 15), or 0 kPa (controls, n = 25). Tissue oxygen saturation (regional oxygen saturation, denoted here as rSO2) was measured to assess the restriction of blood flow. To assess behavior once blood flow resumed, animal activity was monitored for third day and the amount of movement was counted with digital counters. Body weight was measured before and after the behavioral experiment, and changes in body weight were determined. Blood was sampled after a behavioral experiment and biochemically assessed and creatine kinase (CK) levels were measured. RESULTS Tissue oxygen saturation decreased significantly in each group. When a tourniquet was applied at a pressure of 30 kPa or more, tissue oxygen saturation decreased significantly. The amount of movement (the count) over third day decreased more when a tourniquet was applied at a higher pressure. The control group resumed the same amount of movement per day second after blood flow resumed. Animals to which a tourniquet was applied at a pressure of 20 or 30 kPa resumed the same amount of movement third day after blood flow resumed. In contrast, animals to which a tourniquet was applied at a pressure of 40 kPa did not resume the same amount of movement third day after blood flow resumed. After the behavioral experiment, animals to which a tourniquet was applied at a pressure of 40 kPa had a significantly lower body weight in comparison to the control group. After the behavioral experiment, animals to which a tourniquet was applied at a pressure of 40 kPa had significantly elevated CK levels in comparison to the control group. DISCUSSION AND CONCLUSION A relationship between blood flow restriction pressure and tissue oxygen saturation was noted. rSO2 measurement can be used to assess the restriction of blood flow during surgery. On the basis of the decrease in rSO2, blood flow was effectively restricted at a pressure of 30 kPa or more. When, however, blood flow was restricted at a pressure of 40 kPa, weight loss and decreased movement were noted and CK levels increased after the behavioral experiment. Thus, complications had presumably developed due to damage to muscle tissue. These findings indicate that blood flow was effectively restricted in this experiment and they also indicate the existence of an optimal blood flow restriction pressure that does not cause musculoskeletal complications. The pressure in question was around 30 kPa. The tourniquet system that was developed here is actuated with an EHD pump that is still in the trial stages. That said, its pressure can readily be controlled and this pump could be used in a tourniquet system since it is quiet, vibration-free, and small. The pressure of this pump can be finely adjusted to prevent musculoskeletal complications.

Fluctuation of the blood pressure during head-up tilt test in neurally mediated syncope patients

Maintaining the blood pressure is crucial to perfuse the blood to the vital organs in order to preserve the life. In neurally mediated syncope (NMS), the failure of blood pressure control causes the ischemia of the brain, which induces syncope. We tested whether the failure of the compensation of the blood pressure predisposes the syncope in NMS. Six healthy volunteers (Group H), 5 syncope patients with HUT negative (Group N), and 6 syncope patients with HUT positive (Group P) were included in the study. The changes, as well as the fluctuation of the systolic blood pressure (SBP), were compared among the three groups. The time domain analysis of the SBP was evaluated using mean and standard deviation. The frequency domain of SBP was evaluated using frequency analysis. In Group P slight decrease in SBP (106 [mmHg]) starts between 4 and 2 minutes before syncope. Then, significant decrease (88 [mmHg]) follows at 2minutes later. These changes in BP indicate that deterioration of the BP homeostasis may starts around 2 to 4 minutes and accelerate around 1 minute before syncope. The SBP-SD at 4 to 1 minutes before syncope is smaller (3.9~4.8 [mmHg]) than in Group H and N (4.0~6.5 [mmHg]). Low component of the frequency domain power during 6 to 2 minutes before syncope in Group P (2200~3400 [mmHg2]) is rather lower than Group H and N (3000~13000 [mmHg2]). The failure of maintaining BP might be the etiology of syncope episode. Decreased LF power of fSBP might have some relationship to the mechanism of SBP homeostasis failure.

Role of Ca^ Release from Sarcoplasmic Reticulum in Pacemaker Activity of the Sinoatrial Node

Recent studies using confocal microscopy combined with patch clamping on single sinoatrial (SA) node pacemaker cells suggest that Ca 2 + release from the sarcoplasmic reticulum (SR) during diastole may play a prominent role in the late phase of pacemaker depolarization. The present study was designed to test this hypothesis in the intact SA node. We investigated the effects of a high concentration of ryanodine, which is known to disable SR Ca 2 + release, on spontaneous activity of isolated rabbit right atrium including the whole SA node by using an extracellular potential mapping technique. Inhibition of SR Ca 2 + release by 30 μM ryanodine caused only a moderate reduction of the spontaneous firing rate (by 20.0′2.8 %, n=4) of the intact SA node. This observation is inconsistent with previous data obtained from single pacemaker cells. Physiological significance of SR Ca 2 + release in the regulation of SA node pacemaker activity is still unsettled.