Identification and characterization of atrioventricular parasympathetic innervation in humans

INTRODUCTION: We hypothesized that in humans there is an epicardial fat pad from which parasympathetic ganglia supply the AV node. We also hypothesized that the parasympathetic nerves innervating the AV node also innervate the right atrium, and the greatest density of innervation is near the AV nodal fat pad. METHODS AND RESULTS: An epicardial fat pad near the junction of the left atrium and right inferior pulmonary vein was identified during cardiac surgery in seven patients. A ring electrode was used to stimulate this fat pad intraoperatively during sinus rhythm to produce transient complete heart block. Subsequently, temporary epicardial wire electrodes were sutured in pairs on this epicardial fat pad, the high right atrium, and the right ventricle by direct visualization during coronary artery bypass surgery in seven patients. Experiments were performed in the electrophysiology laboratory 1 to 5 days after surgery. Programmed atrial stimulation was performed via an endocardial electrode catheter advanced to the right atrium. The catheter tip electrode was moved in 1-cm concentric zones around the epicardial wires by fluoroscopic guidance. Atrial refractoriness at each catheter site was determined in the presence and absence of parasympathetic nerve stimulation (via the epicardial wires). In all seven patients, an AV nodal fat pad was identified. Fat pad stimulation during and after surgery caused complete heart block but no change in sinus rate. Fat pad stimulation decreased the right atrial effective refractory period at 1 cm (280 +/- 42 msec to 242 +/- 39 msec) and 2 cm (235 +/- 21 msec to 201 +/- 11 msec) from the fat pad (P = 0.04, compared with baseline). No significant change in atrial refractoriness occurred at distances >2 cm. The response to stimulation decreased as the distance from the fat pad increased. CONCLUSION: For the first time in humans, an epicardial fat pad was identified from which parasympathetic nerve fibers selectively innervate the AV node but not the sinoatrial node. Nerves in this fat pad also innervate the surrounding right atrium.     

Endocardial Stimulation of Efferent Parasympathetic Nerves to the Atrioventricular Node in Humans: Optimal Stimulation Sites and the Effects of Digoxin

The purposes of this study were to identify optimal sites of stimulation of efferent parasympathetic nerve fibers to the human atrioventricular node via an endocardial catheter and to investigate the interaction between digoxin and vagal activation at the end organ. Methods: The ventricular rate was measured during atrial fibrillation, prior to and during parasympathetic nerve stimulation, in 8 patients taking digoxin and in 10 controls. High frequency electrical stimuli were delivered via an hexapolar or quadripolar electrode catheter, placed at the posteroseptal right atrium near the atrioventricular node (n=18 patients) or in the coronary sinus (n=12 of 18 patients). In 4 patients, stimulation was repeated after intravenous administration of 1 to 2[emsp4 ]mg of atropine. Results: Nerve stimulation prolonged the R-R interval in all patients. Stimulation close to the posteroseptal right atrium led to maximal atrioventricular nodal slowing. The mean R-R intervals at baseline and during parasympathetic nerve stimulation (60[emsp4 ]mA) from the posteroseptal right atrium and the proximal coronary sinus were 581±79[emsp4 ]ms, 2440±466, and 900±228[emsp4 ]ms respectively (p=0.0001). The response to nerve stimulation was greater in patients taking digoxin than in patients not taking the drug (p=0.02). Junctional rhythm occurred during nerve stimulation in 8/8 patients taking digoxin and 0/10 not taking the drug (p=0.0001). The response to stimulation was eliminated after atropine (p=0.01). Conclusions: Parasympathetic nerves to the atrioventricular node were stimulated from the proximal coronary sinus as well as the posteroseptal right atrium. Stimulation at the posteroseptal right atrium resulted in the greatest response, and digoxin enhanced this response. The augmented response suggests that an interaction may exist between parasympathetic stimulation and digoxin at the end organ.     

Parasympathetic Inhibition of Sympathetic Effects on Pacemaker Location and Rate in Hearts of Anesthetized Dogs

INTRODUCTION: The site of impulse origin in the right atrium generally is considered to be a single static locus within the sinoatrial (SA) node. Previous investigators showed that the pacemaker site may shift due to changes in sympathetic or parasympathetic neural activity. We investigated the interactions between sympathetic and parasympathetic influences on the site of impulse initiation in the right atrium in anesthetized dogs. METHODS AND RESULTS: We determined the site of impulse initiation and the spread of excitation over the anterior and posterior regions of the right atrium by a matrix of 48 unipolar recording electrodes. We assessed the spread of excitation at 3-msec intervals by constructing isochronal activation sequence maps. Sympathetic stimulation increased the frequency of atrial excitation (i.e., the heart rate), but also shifted the earliest activation region (EAR) from a locus in the SA node to a locus in the superior vena cava (the superior pacemaker site). Vagus stimulation decreased the heart rate and shifted the EAR to a lower site in the SA node or a site in the inferior right atrium along the sulcus terminalis (the inferior pacemaker site). A short period of vagus stimulation during a more prolonged sympathetic stimulation elicited a larger decrease in rate than did vagus stimulation alone and shifted the EAR from the superior site to the SA node or to the inferior site. After atropine, combined stimulation shifted the EAR to the superior site, but propranolol did not change EAR location. CONCLUSION: Our results suggest that parasympathetic activity predominates over sympathetic activity not only on heart rate, but also on the location of the EAR in the anesthetized dog.     

Cardiac Parasympathetic Stimulation via QRS-Synchronous Low-Energy Shocks In Humans

Parasympathetic Stimulation via Intracardiac Shocks. Introduction: In patients receiving test shocks to verify lead connections at implantation, we anecdotally have observed postshock delay. The purpose of this study was to determine whether QRS-synchronous low-energy shocks delivered by implantable defibrillators result in postshock cycle length prolongation, and to determine the mechanism of this phenomenon.
Methods and Results: Twenty-five patients undergoing defibrillator testing were studied, three with epicardial patches and 22 with transvenous leads. Each patient received QRSsynchronous shocks of 0.2, 0.4, 0.6, and 2.0 J in random order. Patients were further randomized to receive either saline or 2.0 mg atropine intravenously, and then given a second sequence of shocks. At baseline, the postshock cycle length (1, 035 ± 245 msec) was significantly longer than the preshock cycle length (968 ± 177 msec, P = 0.01). In patients with a coronary sinus (CS) or superior vena cava (SVC) lead, the mean prolongation was 91 ± 160 msec, compared with 12 ± 106 msec for patients without such a lead (P < 0.0001). All energy levels resulted in significant postshock prolongation compared with preshock cycle lengths (P < 0.05). Postshock prolongation before atropine was 76 ± 162 msec, compared with −13 ± 52 msec afterward (P < 0.00001). Biphasic shocks resulted in greater postshock prolongation than monophasic shocks of equal energy.
Conclusion: Low-energy shocks delivered during the QRS complex cause postshock cycle length prolongation in man. This effect required the presence of a CS or SVC lead. Atropine inhibited this effect, suggesting the phenomenon was mediated by direct cardiac parasympathetic nerve stimulation by the intracardiac shock.     

Parasympathetic neurotransmission in rabbit isolated bronchus is modulated at prejunctional sites via endothelinB receptor stimulation

OBJECTIVE: The aim of this study was to investigate the mechanism involved in endothelin-induced potentiation of the response to parasympathetic nerve stimulation. METHODOLOGY: We used autoradiographic and functional studies in rabbit isolated bronchi. RESULTS: Autoradiography revealed dense binding sites for radiolabelled endothelin-3 over bronchial parasympathetic ganglia. The contractile response of the bronchus to electrical field stimulation was significantly potentiated by endothelin-3, endothelin-1, sarafotoxin S6c and BQ-3020 to 326+/-53%, 293+/-63%, 514+/-119% and 655+/-178%, respectively, of control values. The endothelin-3-induced potentiation of neurally evoked responses was not affected by the presence of propranolol, phentolamine or hexamethonium. The potentiation was also unaltered by pretreatment with the endothelinA receptor antagonist BQ-123 (3 micromol/L), but was significantly reduced in the presence of the combined endothelinA/endothelinB receptor antagonist PD 145065, indicating that the potentiation was mediated via endothelinB receptors. Confirmation of endothelinB receptor involvement in the neuropotentiation was obtained by demonstration of a significant amelioration of the potentiation in the presence of the endothelinB receptor selective antagonist BQ-788, and after endothelinB receptor desensitization by the endothelin, receptor selective agonist sarafotoxin S6b. CONCLUSIONS: These results suggest that the endothelin-induced potentiation of parasympathetic neural responses in the rabbit bronchus is mediated via endothelinB receptor activation.     

阿托品静脉注射对食管心脏电生理的影响

目的　再评价迷走高敏症对窦房结和房室结功能的影响。方法　用食管电生理检测阿托品用药前后各电生理参数 ,对比 2 4小时动态心电图及常规心电图 ,并进行长期随访。结果　 48例迷走高敏症阻断迷走神经前后电生理各参数差异均有显著性 (P <0 0 1)。迷走阻断后 48例房室结不应期 (AVN ERP)平均缩短率 48 0 1± 10 2 1,大于窦房结恢复时间 (SNRT)平均缩短率 41 5 3± 9 9(P <0 0 5 )。结论　提示迷走神经对房室结功能影响大于对窦房结功能影响。部分迷走高敏症在动态心电图上可貌似各种心律失常 ,阿托品食管电生理检查可有助于对其鉴别 ,这对临床治疗具有重要意义     

Origin of the Sinoatrial Node and Atrioventricular Node Arteries in Right,Mixed, and Left Inferior Emphasis Systems

The origin of the sinoatrial node artery (SAN) and atrioventricular node artery (AVN) was determined for 118 patients with normal coronary arteriograms. The coronary arteriograms of the 118 patients were divided into right (66.1%), mixed (26.3%), and left (7.6%) inferior emphasis systems. The SAN arose from the right coronary artery in 53%, the left coronary artery in 35%, and had a dual origin in 11%. The proximal right coronary artery was the origin of the SAN in 48.7% of right, 60.5% of mixed, and 55.6% of left inferior emphasis systems. The left coronary artery was the origin of the SAN in 38.5% of right, 25.8% of mixed, and 44.4% of left inferior emphasis systems. The AVN arose from the right coronary artery in 84%, the left coronary artery in 8%, and from both in 8% of the 118 patients. The right coronary artery was the origin of the AVN in 98.7% of right, 74.2% of mixed, and 0% of left inferior emphasis systems. The left coronary artery was the origin of the AVN in 0% of right, 25.8% of mixed, and 100% of left inferior emphasis systems.     

Intracardiac Stimulation of Human Parasympathetic Nerve Fibers Induces Negative Dromotropic Effects:

Intracardiac Stimulation and Dromotropic Effect. Introduction : The dromotropic effects of intracardiac parasympathetic nerve stimulation have not been well studied; furthermore, the effects of radiofrequency ablation lesions on parasympathetic nerve stimulation are not clear.
Methods and Results : Group I: intracardiac electrical stimulation in the right posteroseptal and anteroseptal areas under different stimulation strengths; group II: intracardiac electrical stimulation before and 10 minutes after intravenous propranolol; group III; intracardiac electrical stimulation before and 5 minutes after intravenous atropine. Among the 10 patients with AV nodal reentrant tachycardia (group IV) and the 10 patients with atrial flutter (group V), atrial fibrillation was induced before and after successful ablation, and intracardiac electrical stimulation in the right posteroseptal area was performed before and after successful ablation. The maximal response and complete decay of the response occurred within 2 to 6 seconds of initiation or termination of parasympathetic nerve stimulation. This negative dromotropic effect disappeared after atropine was administered, but not after propranolol. After successful ablation, parasympathetic stimulation still induced negative dromotropic effects.
Conclusion : Electrical stimulation of parasympathetic nerve fibers near the posteroseptal and anteroseptal areas could induce a negative dromotropic effect, and this effect was preserved after successful radiofrequency ablation of slow pathway and isthmus conduction.     

窦房折返性心动过速的射频消融治疗(附二例报告)

报道２例窦房折返性心动过速（ＳＮＲＴ）的电生理特点及射频消融结果。男、女各１例,两例患者心动过速发作时体表心电图１２导联Ｐ波形态与窦性心律时相同,心内电生理检查证实为ＳＮＲＴ。采用激动顺序标测,心动过速发作时于右房高侧壁记录到心房最早激动,且与窦性心律时激动顺序相同,成功消融靶点部位Ａ波分别早于体表心电图Ｐ波５０和３０ｍｓ。以１５～３０Ｗ输出功率消融６０～１２０ｓ均成功。随访２～６个月无心动过速发作,窦房结功能正常。比较有效消融和无效消融的靶点特征,提示提前、增宽及碎裂的Ａ波可作为消融靶点。根据笔者初步经验认为射频消融治疗ＳＮＲＴ是安全有效的。     

心脏血管内迷走神经丛刺激与阵发性心房颤动的动物模型制作

探讨心脏血管内迷走神经丛刺激与阵发性心房颤动 (简称房颤 )的动物模型制作。 32条Mongrel狗活体心脏大血管 :冠状窦、左右肺动脉、左房、上下腔静脉等处插入 7F蓝状电极进行迷走神经丛刺激 ,刺激频率为 2 0Hz,刺激间期 0 .1ms,刺激电压 1～ 4 0V ,刺激时间 30～ 5 0s。为了避免神经丛刺激直接对心房的影响 ,于刺激迷走神经丛的同时在P波后发放 2 0 0Hz、2 0～ 5 0ms的PS2 心房高频刺激 ,使迷走神经刺激落入心房的不应期。在这些心脏血管迷走神经丛刺激时减慢窦性心律 ,且减慢速度呈电压依赖。在一定的刺激强度下 ,窦性心律能够达到最大减低 (从75 0± 10 2ms至 15 6 0± 2 30ms) ,心房肌不应期显著缩短 (从 175± 13ms缩至 96± 2 3ms) ,同时出现房性早搏、房性心动过速和房颤 ,且重复性很好。应用 β 阻断剂 (esmolol1mg/kg)时 ,提高了房颤诱发域值 ;迷走神经阻断剂 (atropine1～ 2mg/kg)可以完全阻断房颤的诱发。结论 :蓝状电极非常有利于快速在静脉血管腔内找到迷走神经丛刺激位点 ;心脏大血管处存在迷走神经丛 ,刺激这些神经丛能够复制出与临床灶性阵发性房颤非常类同的房颤 ,迷走神经阻断剂可阻断这类房颤的诱发。     

Differential Effects of Parasympathetic Blockade and Parasympathetic Withdrawal on Heart Rate Variability

INTRODUCTION: Low heart rate variability (HRV) has been shown to have important prognostic significance in multiple settings. Although this is believed to reflect reduced parasympathetic tone, the physiology of reduced parasympathetic tone has not been elucidated. METHODS AND RESULTS: To evaluate whether parasympathetic withdrawal and partial parasympathetic blockade result in similar changes in HRV, 27 normal volunteers underwent complete beta-adrenergic blockade and then were given (1) graded doses of nitroprusside to achieve baroreflex-mediated parasympathetic withdrawal and (2) low-dose atropine (0.01 mg/kg) to achieve partial parasympathetic blockade. Five-minute ECG recordings were obtained for HRV analysis. In 19 subjects, paired 5-minute recordings from each condition were available with mean RR intervals that differed by < 50 msec (low-dose atropine: 869 +/- 96 msec and nitroprusside 875 +/- 99 msec). The root mean square of the successive RR interval differences was lower following low-dose atropine than following parasympathetic withdrawal with nitroprusside (16 +/- 11 msec vs 22 +/- 15 msec; P < 0.02). During parasympathetic withdrawal, the low-frequency (LF) power was 0.917 +/- 0.602 bpm2 and the high-frequency (HF) power was 0.501 +/- 0.521 bpm2. During partial parasympathetic blockade, the LF and HF powers were significantly lower (0.443 +/- 0.474 bpm2, P < 0.005; and 0.198 +/- 0.207 bpm2, P < 0.02). CONCLUSION: These data confirm that HRV reflects the character of parasympathetic modulation of the heart rate rather than parasympathetic tone per se. Furthermore, this study identifies two distinct physiologic explanations for the finding of low HRV, namely, diminished vagal discharge and resistance of cardiac muscarinic receptors to vagal discharge. Further delineation of the relationships between parasympathetic tone and HRV will allow for better understanding of the pathophysiologic derangements associated with low HRV.     

室房逆传对窦房结功能及心房肌电活动影响及其意义的实验研究

目的:探讨室房逆传(VAC)对兔窦房结功能低下动物模型窦房结功能及心房肌电活动的影响.方法:选用40只健康新西兰大耳白家兔,其中32只成功制作窦房结功能低下动物模型,以200次/分的起搏频率起搏右心室,将家兔分为1:1VAC组(22只)、非1:1VAC组(10只).观察心室起搏1 h,2 h,4 h,7 d后窦房结功能低下家兔模型右心房压、心房有效不应期、心房激动时间、心肌波长指数、校正窦房结恢复时间的变化,并比较两组上述指标的差别.结果:①1:1VAC组心室起搏1 h后右心房压明显升高(P＜0.01),心房有效不应期、心房激动时间、心肌波长指数、校正窦房结恢复时间无明显变化(P＞0.05);2 h后右心房压继续升高(P＜0.01),校正窦房结恢复时间、心房激动时间延长(P＜0.01),心房有效不应期缩短(P＜0.01),心肌波长指数减小(P＜0.01);4 h后上述指标变化更明显(P＜0.01);7 d后右心房压恢复至原来水平(P＞0.05),心房有效不应期、心房激动时间、心肌波长指数、校正窦房结恢复时间变化更明显(P＜0.01).②非1:1VAC组心室起搏1 h后右心房压明显升高(P＜0.01),校正窦房结恢复时间、心房有效不应期、心房激动时间、心肌波长指数无明显变化(P＞0.05);2 h、4 h后右心房压进一步升高(P＜0.01),校正窦房结恢复时间、心房有效不应期、心房激动时间、心肌波长指数无明显变化(P＞0.05);7 d后右心房压恢复至原来水平(P＞0.05),心房有效不应期、心房激动时间缩小(P＜0.05),校正窦房结恢复时间、心肌波长指数无明显变化(P＞0.05).③1:1VAC组与非1:1VAC组比较:1 h时两组间右心房压、校正窦房结恢复时间、心房有效不应期无明显变化(P＞0.05),但1:1VAC组心房激动时间延长(P＜0.05)、心肌波长指数减小(P＜0.05);2 h时右心房压、心房有效不应期无明显变化(P＞0.05),1:1VAC组校正窦房结恢复时间、心房激动时间明显延长(P＜0.01),心肌波长指数明显减少(P＜0.01);心室起搏4 h,7 d后右心房压无明显变化(P＞0.05),但1:1VAC组心房有效不应期、校正窦房结恢复时间、心房激动时间、心肌波长指数变化更明显(P＜0.01).结论:VAC对窦房结功能及心房肌电活动能产生不良影响.病态窦房结综合征患者应尽量避免使用VVI起搏器,最好安装生理性起搏器.     

β-Adrenergic and Muscarinic Cholinergic Receptor Densities in the Human Sinoatrial Node:

Autonomic Receptors in Human Sinus Node. The objective of this study was to measure autonomic receptor densities in the human sinoatrial node and adjacent atrial myocardium to gain further insights into autonomic regulation of sinoatrial node function in the human heart. Sinoatrial nodes (n = 9) were acquired from human donors. Quantitative light microscopic autoradiography of radioligand binding sites in tissue sections was used to compare β-adrenergic and muscarinic cholinergic receptor densities within specific tissue compartments of the sinoatrial node and adjacent myocardium. Total β-adrenergic receptors were measured with the nonsubtype selective radioligand [¹²⁵I] iodocyanopindolol. β₂-Adrenergic receptors were determined by measuring the amount of radioactivity bound to sections incubated with radioligand in the presence of the highly β₁-selective antagonist CGP-20712A. Specific autoradiographic grain densities were normalized to myocyte area/unit tissue area. Myocytes in the sinoatrial node occupied 47.7%± 0.1% of the total tissue area compared with 92.8%± 0.1% in myocardium (P < 0.001). Total specific β-adrenergic receptor density per unit myocyte area was 3.5 ± 0.9 times greater in the sinoatrial node than in myocardium (P < 0.001). The relative densities of β₁- (4.2. P < 0.002), β₂- (2.6, P < 0.002), and muscarinic (3.3, P < 0.001) receptors were significantly greater in the sinoatrial node than in the atrium. Thus, total β-adrenergic and muscarinic cholinergic receptor densities are > 3-fold higher in the sinoatrial node than adjacent atrial myocardium, reflecting their specialized roles in regulating cardiac rate and rhythm. The β₂-subtype is predominant in both regions. the β₂-subtype, however, is > 2.5-fold more abundant in the sinoatrial node than in atrial myocardium. The relatively high β₂-receptor density in the human sinoatrial node is consistent with physiologic studies that implicate this receptor in regulating cardiac chronotropism.     

Endocardial Transcatheter Stimulation of the AV Nodal Fat Pad: Stabilization of Rapid Ventricular Rate Response During Atrial Fibrillation in Left Ventricular Failure

Introduction: Recent acute studies demonstrated that atrioventricular (AV) node vagal stimulation during atrial fibrillation (AF) decreases the mean ventricular rate, thus improving hemodynamics.
Methods and Results: We report a case of a woman with acute heart failure (HF), chronic AF with untreatable ventricular rapid response, in severe hypotensive state due to a cardiogenic shock. The patient underwent left ventricular (LV) pacemaker implantation and received 50 Hz AV node stimulation, delivered through a posteroseptal atrial lead, thus allowing a 100% pacing. Hemodynamics improvements allowed carvedilol titration; the rate was below 85 bpm after 4 days, then the atrial lead was removed.
Conclusions: This novel strategy may allow controlling the rapid AV response in patients undergoing pacemaker implantation.     

Ionic Current in Sinoatrial Node Cells

Ionic Current in Sinoatrial Node Cells. In this article, we report our current understanding of ionic mechanisms of the spontaneous rhythm of the mammalian sinoatrial pacemaker cells obtained from whole-cell voltage clamp studies. Various ionic currents that underlie the pacemaker potential are discussed. Calcium (Ca) current, delayed-rectifier potassium (K) current, and hyperpolarization-activated inward current were found to be the major time-dependent currents responsible for the pacemaker depolarization. Together with these time-dependent currents, a time-independent background current may also contribute to generation of pacemaker activity.     

Disruption of mitochondria–sarcoplasmic reticulum microdomain connectomics contributes to sinus node dysfunction in heart failure

The sinoatrial node (SAN), the leading pacemaker region, generates electrical impulses that propagate throughout the heart. SAN dysfunction with bradyarrhythmia is well documented in heart failure (HF). However, the underlying mechanisms are not completely understood. Mitochondria are critical to cellular processes that determine the life or death of the cell. The release of Ca²⁺ from the ryanodine receptors 2 (RyR2) on the sarcoplasmic reticulum (SR) at mitochondria–SR microdomains serves as the critical communication to match energy production to meet metabolic demands. Therefore, we tested the hypothesis that alterations in the mitochondria–SR connectomics contribute to SAN dysfunction in HF. We took advantage of a mouse model of chronic pressure overload–induced HF by transverse aortic constriction (TAC) and a SAN-specific CRISPR-Cas9–mediated knockdown of mitofusin-2 (Mfn2), the mitochondria–SR tethering GTPase protein. TAC mice exhibited impaired cardiac function with HF, cardiac fibrosis, and profound SAN dysfunction. Ultrastructural imaging using electron microscope (EM) tomography revealed abnormal mitochondrial structure with increased mitochondria–SR distance. The expression of Mfn2 was significantly down-regulated and showed reduced colocalization with RyR2 in HF SAN cells. Indeed, SAN-specific Mfn2 knockdown led to alterations in the mitochondria–SR microdomains and SAN dysfunction. Finally, disruptions in the mitochondria–SR microdomains resulted in abnormal mitochondrial Ca²⁺ handling, alterations in localized protein kinase A (PKA) activity, and impaired mitochondrial function in HF SAN cells. The current study provides insights into the role of mitochondria–SR microdomains in SAN automaticity and possible therapeutic targets for SAN dysfunction in HF patients.

Heart failure (HF) is a progressive condition that occurs when the heart no longer generates sufficient cardiac output to meet the metabolic demands of the body (1). Despite the current armamentarium in HF therapies, the 5-y mortality in HF patients remains greater than 50% (2). One of the known complications in HF is bradyarrhythmia from sinoatrial node (SAN) dysfunction, which significantly increases the morbidity and mortality of HF patients (3). Patients diagnosed with HF and SAN dysfunction have an increased risk of sudden cardiac death (2, 3). Hence, it is imperative to understand the mechanistic underpinning of SAN dysfunction in HF to improve clinical outcomes.The SAN is a highly complex structure consisting of specialized cells that spontaneously fire action potentials (APs), propagating throughout the heart. Its automaticity is orchestrated by ion channels and transporters that contribute to the membrane and Ca²⁺ clocks, collectively known as the “coupled clock” (4, 5). These two cyclical processes are significantly impaired in HF, leading to SAN dysfunction (6), with documented remodeling of ion channels, gap junction channels, Ca²⁺-, Na⁺-, and H⁺-handling proteins, and receptors (7). There is a documented reduction in the hyperpolarization-activated “pacemaker” current (I_f) from a decrease in hyperpolarization-activated and cyclic nucleotide–gated (HCN)2 and HCN4 channel expression (8), and a reduction in the slow component of the delayed rectifier K⁺ current (I_Ks) (9).Given the critical roles of the mitochondria in energy production and determination of cell survival, the remodeling of the coupled clock may be secondary to or potentiated by alterations in mitochondria in HF. Beat-to-beat alterations in electrochemical gradients within the SAN need to be reestablished by the energy-dependent exchangers, primarily fueled by aerobic respiration from the mitochondria.Similar to ventricular myocytes, the SAN is endowed with a dense mitochondrial network with a high basal respiratory rate (10). To accomplish their role in energy production, they require constant feedback on the cell’s energetic state (11). Organelle connectomics, or close communication between mitochondria and the sarcoplasmic reticulum (SR), occurs at microdomains, physically established, in part, by mitofusin-2 (Mfn2). Mfn2, a dynamin-like GTPase embedded in the outer mitochondrial membrane, is a key protein involved in tethering the mitochondria and SR, ensuring sufficient energy production for cellular bioenergetics (12). Indeed, a critical interorganelle communication occurs within the mitochondria–SR contact sites, mediated in part by microdomains of reactive oxygen species (ROS), Ca²⁺, and cyclic adenosine monophosphate (cAMP). The SAN’s automaticity is highly dependent on the cyclic changes in Ca²⁺ within the cell (13). Specifically, the release of Ca²⁺ from the ryanodine receptors 2 (RyR2) on the SR at mitochondria–SR microdomains—regions of high, localized Ca²⁺—serves as a critical communication to match energy production and demand (11, 14). Additionally, these microdomains serve as crucial communication hubs for cAMP and ROS signaling (15–17).Therefore, proper communications with the mitochondria is essential for cellular survival, ensuring adequate energy production to meet the metabolic demands of the SAN. Impaired mitochondrial connectomics, either through injury to the mitochondria or disruption of their microdomains, can significantly affect SAN function. However, to date, little is known regarding the mitochondria–SR cross-talk in SAN cells (SANCs) and the critical roles of this communication in regulating SAN automaticity and dysfunction commonly seen in HF.The objective of this study is to investigate the structural remodeling that occurs in the SAN mitochondria–SR contact sites and how this influences functional outcome in HF. We used a well-established pressure overload murine model where HF was induced by transverse aortic constriction (TAC) (18). Concurrent with the development of HF, the animals show evidence of SAN dysfunction with sinus bradycardia. High-resolution imaging, transmission electron microscopy (TEM), and electron microscope (EM) tomography demonstrate structurally altered mitochondria with impaired Ca²⁺ and cAMP signaling at the mitochondria–SR microdomains. The expression of Mfn2 is significantly down-regulated and shows reduced colocalization with RyR2 in HF SANCs. Importantly, SAN-specific CRISPR-Cas9–mediated Mfn2 knockdown (KD) recapitulates SAN dysfunction in HF. The data support the critical roles of mitochondria–SR connectomics in regulating SAN automaticity. Structural and functional remodeling of the mitochondria and their microdomains contribute to the pathogenesis of SAN dysfunction in HF.     

Determinants and effects of electrical stimulation of the inferior interatrial parasympathetic plexus during atrial fibrillation

INTRODUCTION: Catheter stimulation of the inferior interatrial ganglionated parasympathetic plexus decreases the ventricular rate during atrial fibrillation (AF) in humans. However, the relatively high stimulation voltages might prevent implementation of neurostimulation in chronic implantable devices. From myocardial electrostimulation it is known that the required impulse energy and charge is lowest at the chronaxie time. In order to lower energy requirements for cardiac neurostimulation, the present study evaluates the impulse-strength versus impulse-duration relationship for a neurostimulation lead that was implanted into the inferior interatrial ganglionated plexus. METHODS AND RESULTS: In nine dogs, permanent epicardial bipolar screw-in electrodes were fixed in the inferior interatrial ganglionated plexus. AF was maintained via rapid atrial pacing. During AF, neural stimulation was performed at various frequencies (1-100 Hz), impulse durations (0.05-2 msec), and voltages (0.02-11.5 V). There was a linear correlation between R-R interval lengthening and stimulus voltage (R = 0.99; P < 0.001) and a bell-shaped relationship between stimulation frequency and negative dromotropic effect with maximum rate slowing at 30-50 Hz. The rheobase for a 50% R-R interval prolongation during AF was 1.81 V and 2.72 V for high-grade AVB yielding a chronaxie time of 0.14 msec and 0.18 msec, respectively. The impulse energy (charge) at the chronaxie time was 4-6 microJ (6-8 microC). CONCLUSIONS: Cardiac neurostimulation follows a chronaxie/rheobase behavior. Energy, charge, and voltage values needed to achieve significant negative dromotropic effects are within the limits of conventional cardiac pacemaker outputs, which may allow implementation of neurostimulation capabilities in current pacemaker technology.