Sympathetic Hyperinnervation Research Articles

Nerve Sprouting Contributes to Increased Severity of Ventricular Tachyarrhythmias by Upregulating iGluRs in Rats with Healed Myocardial Necrotic Injury

Sympathetic nerve sprouting in healed myocardial infarction (MI) has been associated with high incidences of lethal arrhythmias, but the underlying mechanisms are largely unknown. This study sought to test that sympathetic hyperinnervation and/or MI remodels the myocardial glutamate signaling and ultimately increases the severity of ventricular tachyarrhythmias. Myocardial necrotic injury (MNI) was created by liquid nitrogen freeze-thawing across an intact diaphragm to mimic MI. Cardiac sympathetic hyperinnervation was induced by chronic subcutaneous injection of 4-methylcatechol, a potent stimulator of nerve growth factor expression. The results showed that sympathetic hyperinnervation with or without MNI upregulated the myocardial expression of ionotropic glutamate receptors (iGluRs), including NMDA receptor (NMDAR) and AMPA receptor (AMPAR), and induced cardiomyocyte apoptosis. Intravenous infusion with either NMDA (12 mg/kg) or AMPA (15 mg/kg) triggered ventricular tachycardia and ventricular fibrillation in rats with healed MNI plus sympathetic hyperinnervation; these arrhythmias were prevented by respective antagonist of NMDAR or AMPAR. We conclude that MNI with sympathetic nerve sprouting upregulates the expression of NMDAR and AMPAR in the myocardium and this impact in turn enhances cardiac responses to stimulations of iGluRs and thus increases the incidence of ventricular tachyarrhythmias.

Disease modification by autonomic nerve stimulation

v 4 o m p p r One of the arrhythmogenic mechanisms in the pathophysiology of atrial fibrillation (AF) is autonomic influence. The activation of cardiac autonomic nerves results in electrophysiological changes, causing shortening of the atrial effective refractory period and action potential duration, and spatial heterogeneity of refractoriness. Such conditions favor the initiation and maintenance of atrial tachyarrhythmias such as AF. Coumel et al reported that vagus nerve might be associated with paroxysmal AF. Studies using heart rate variability analysis have confirmed that paroxysmal AF occurrence is associated with a primary increase in adrenergic tone followed by a marked modulation toward vagal predominance. Recently, direct recording of intrinsic cardiac nerve activities in ambulatory animals revealed that the nerve activities are invariable triggers of paroxysmal AF. Neural remodeling in atrial tachyarrhythmias has been introduced by several studies. Sympathetic hyperinnervation and nerve sprouting were observed in a canine AF model. Increased atrial sympathetic innervation has also been found in patients with persistent AF. In this issue of HeartRhythm, Yu et al demonstrate the ffects of low-level superior vena cava (SVC) stimulation n neural remodeling and AF inducibility in an acute canine odel of atrial tachyarrhythmia. They placed a 10-pole ircular catheter in the SVC just below the entrance of the nnominate vein. High-frequency stimulation (20 Hz, 0.1 s of pulse duration) was applied for 3 hours with a voltage hat was 50% lower than the threshold. They found that ow-level SVC stimulation restored the neural activities at he anterior right ganglionated plexus to the baseline level nd suppressed AF inducibility.

Atorvastatin attenuates atrial structural, autonomic and electrophysiologic remodelling by its anti-oxidant and anti-inflammatory action in atrial fibrillation dogs

PurposeThe protective effects of statins against atrial fibrillation (AF) have been observed in many clinical trials but the underlying mechanisms remain unclear. Therefore we investigated the role of atorvastatin on...

Autonomic Remodeling in the Left Atrium and Pulmonary Veins in Heart Failure

Background— Atrial fibrillation (AF) is commonly associated with congestive heart failure (CHF). The autonomic nervous system is involved in the pathogenesis of both AF and CHF. We examined the role of autonomic remodeling in contributing to AF substrate in CHF. Methods and Results— Electrophysiological mapping was performed in the pulmonary veins and left atrium in 38 rapid ventricular–paced dogs (CHF group) and 39 control dogs under the following conditions: vagal stimulation, isoproterenol infusion, β-adrenergic blockade, acetylcholinesterase (AChE) inhibition (physostigmine), parasympathetic blockade, and double autonomic blockade. Explanted atria were examined for nerve density/distribution, muscarinic receptor and β-adrenergic receptor densities, and AChE activity. In CHF dogs, there was an increase in nerve bundle size, parasympathetic fibers/bundle, and density of sympathetic fibrils and cardiac ganglia, all preferentially in the posterior left atrium/pulmonary veins. Sympathetic hyperinnervation was accompanied by increases in β 1 -adrenergic receptor R density and in sympathetic effect on effective refractory periods and activation direction. β-Adrenergic blockade slowed AF dominant frequency. Parasympathetic remodeling was more complex, resulting in increased AChE activity, unchanged muscarinic receptor density, unchanged parasympathetic effect on activation direction and decreased effect of vagal stimulation on effective refractory period (restored by AChE inhibition). Parasympathetic blockade markedly decreased AF duration. Conclusions— In this heart failure model, autonomic and electrophysiological remodeling occurs, involving the posterior left atrium and pulmonary veins. Despite synaptic compensation, parasympathetic hyperinnervation contributes significantly to AF maintenance. Parasympathetic and/or sympathetic signaling may be possible therapeutic targets for AF in CHF.

Hypoxic fetal programming of the sympathetic nervous system

The link between intra uterine growth restriction (IUGR) and increased risk of adult cardiovascular disease in the offspring is well established. Chronic hypoxia in utero (CHU) is a well established cause of IUGR, and using in vitro methods, has been shown to cause endothelial dysfunction and sympathetic hyperinnervation in the offspring. Our aim is to assess the impact of CHU upon sympathetic nerve activity. We have made direct recordings of MSNA from arteries supplying the Spinotrapezius muscle under Alfaxan anaesthesia in Normal (N, n=7) and CHU rats (n=6), discriminating single unit activity from multiunit recordings. Arterial blood pressure, measured from the femoral artery, was similar in N and CHU rats (N:123±3, CHU: 119±5, mmHg). All single units (N=17, CHU=26) had cardiac and respiratory rhythmicity. Mean firing rates were similar (N: 0.48±0.09, CHU: 0.52±0.1, Hz). Further analysis showed that high frequency doublets and triplets of activity (>20hz) were more common in CHU. This may suggest increased release of Neuropeptide Y from sympathetic varicosities, and hence, impact upon cardiovascular health. Recordings during baro- and chemoreceptor stimulation are under analysis.

Parasympathetic and substance P-immunoreactive nerve denervation in atrial fibrillation models

Recent studies demonstrated that atrial fibrillation (AF) induced heterogeneous sympathetic hyperinnervation and baroreflex impartation, but the changes of vagal and afferent nerve are not clear. Six dogs underwent atrial pacing at 600 beats/min (AF group). All paced dogs developed sustained AF by 5 weeks of pacing. Tissues from six healthy dogs were used as controls. Immunohistochemistry staining of cardiac nerves was performed using anti-growth-associated protein 43 (anti-GAP43), anti-tyrosine hydroxylase, antiacetylcholine (anti-ACh), and anti-substance P (anti-SP) antibodies. In AF group, the density of GAP43-positive in the right atrium (RA), atrial septum (AS), and left atrium (LA) was 5590.24±1417.51, 8083.22±1271.39, and 10854.56±1877.56 μm(2)/mm(2), respectively, which was significantly (P<.01) higher than the control group. Most of the newly sprouting nerves are sympathetic nerve. Sympathetic nerve density in AF group was significantly higher than that of control group (P<.001). Whereas denervation of parasympathetic and SP-immunoreactive nerve occurred in AF group. In the dogs with AF, the density of ACh-positive nerve in the RA, AS, and LA was 506.04±104.44, 317.72±84.10, and 114.9±29. 62 μm(2)/mm(2), respectively, which was lower than the control group (P<.01). At the same time, the density of SP-positive nerve in the atria of AF dogs was also significantly lower than the control tissues (P<.01). AF led to significant nerve sprouting and sympathetic hyperinnervation in the canine models, but the newly sprouting nerve did not include parasympathetic and SP-immunoreactive nerve. Heterogeneous parasympathetic and SP-immunoreactive nerve denervation occurred in the AF dogs.

Differential effects of NADPH oxidase and xanthine oxidase inhibition on sympathetic reinnervation in postinfarct rat hearts

Superoxide has been shown to play a major role in ventricular remodeling and arrhythmias after myocardial infarction. However, the source of increased myocardial superoxide production and the role of superoxide in sympathetic innervation remain to be further characterized. Male Wistar rats, after coronary artery ligation, were randomized to vehicle, allopurinol, or apocynin for 4weeks. To determine the role of peroxynitrite in sympathetic reinnervation, we also used 3-morpholinosydnonimine (a peroxynitrite generator). The postinfarction period was associated with increased oxidative stress, as measured by myocardial superoxide, nitrotyrosine, xanthine oxidase activity, NADPH oxidase activity, and dihydroethidium fluorescent staining. Measurement of myocardial norepinephrine levels revealed a significant elevation in vehicle-treated infarcted rats compared with sham. Sympathetic hyperinnervation was blunted after administration of allopurinol. Arrhythmic scores in the allopurinol-treated infarcted rats were significantly lower than those in vehicle. For similar levels of ventricular remodeling, apocynin had no beneficial effects on oxidative stress, sympathetic hyperinnervation, or arrhythmia vulnerability. Allopurinol-treated hearts had significantly decreased nerve growth factor expression, which was substantially increased after coadministration of 3-morpholinosydnonimine. These results indicate that xanthine oxidase but not NADPH oxidase largely mediates superoxide production after myocardial infarction. Xanthine oxidase inhibition ameliorates sympathetic innervation and arrhythmias possibly via inhibition of the peroxynitrite-mediated nerve growth factor pathway.

Favorable effects of resveratrol on sympathetic neural remodeling in rats following myocardial infarction

Oxidative stress and inflammatory response induced by myocardial infarction play important roles in the development of sympathetic neural remodeling. The present study was designed to investigate whether resveratrol can improve sympathetic neural remodeling and hence cause less arrhythmias via its anti-oxidant and anti-inflammatory effects. Male Sprague Dawley rats were randomly assigned to either vehicle or resveratrol (1 mg/kg) treatment for 4 weeks post myocardial infarction. Another group of sham operated rats served as controls. Cardiac electrophysiology examination was performed to evaluate the severity of ventricular arrhythmias. Sympathetic neural remodeling characterized by heterogeneous nerve sprouting and sympathetic hyperinnervation was assessed by immunohistochemistry study. Western blotting and ELISA were used to evaluate inflammatory responses and oxidative stress was also quantified. Resveratrol treatment resulted in less episodes of inducible ventricular arrhythmias which was closely associated with attenuated sympathetic neural remodeling ( P < 0.001, respectively). Decreased nerve growth factor (NGF) expression was also observed in resveratrol treated rats in the peri-infarct area at 4 weeks after myocardial infarction ( P < 0.001). Interestingly, beneficial effects of resveratrol were also associated with less inflammatory responses and oxidative stress. Our data indicated that resveratrol can suppress sympathetic neural remodeling process after myocardial infarction via attenuated inflammatory responses and oxidative stress, which in turn leads to less inducibility of ventricular arrhythmias.

β-Adrenoceptor Blockers Increase Cardiac Sympathetic Innervation by Inhibiting Autoreceptor Suppression of Axon Growth

β-Adrenoceptor antagonists are used widely to reduce cardiovascular sympathetic tone, but withdrawal is accompanied by sympathetic hyperactivity. Receptor supersensitivity accounts for some but not all aspects of this withdrawal syndrome. Therefore, we investigated effects of β-blockers on sympathetic innervation. Rats received infusions of adrenergic receptor blockers or saline for 1 week. The nonselective β-blocker propranolol and the β(1)-antagonist metoprolol both increased myocardial sympathetic axon density. At 2 d after propranolol discontinuation, β-receptor sensitivity and responsiveness to isoproterenol were similar to controls. However, tyramine-induced mobilization of norepinephrine stores produced elevated ventricular contractility consistent with enhanced sympathetic neuroeffector properties. In addition, rats undergoing discontinuation showed exaggerated increases in mean arterial pressure in response to air puff or noise startle. In sympathetic neuronal cell cultures, both propranolol and metoprolol increased axon outgrowth but the β(2)-blocker ICI 118551 did not. Norepinephrine synthesis suppression by α-methyl-p-tyrosine also increased sprouting and concurrent dobutamine administration reduced it, confirming that locally synthesized norepinephrine inhibits outgrowth via β(1)-adrenoceptors. Immunohistochemistry revealed β(1)-adrenoceptor protein on sympathetic axon terminations. In rats with coronary artery ligation, propranolol reversed heart failure-induced ventricular myocardial sympathetic axon depletion, but did not affect infarct-associated sympathetic hyperinnervation. We conclude that sympathetic neurons possess β(1)-autoreceptors that negatively regulate axon outgrowth. Chronic β-adrenoceptor blockade disrupts this feedback system, leading to ventricular sympathetic axon proliferation and increased neuroeffector gain, which are likely to contribute to β-blocker withdrawal syndrome.

Effect of Sildenafil on Sympathetic Hyperinnervation in Post‐infarcted Rat Hearts

Sildenafil has been shown to provide cardioprotection against ischemia/reperfusion injury via activation of mitochondrial ATP‐sensitive potassium (KATP) channels. We assessed whether sildenafil attenuates cardiac sympathetic innervation after myocardial infarction through activation of KATP channels. After ligation of the anterior descending artery, male Wistar rats were randomized to either vehicle, nicorandil, pinacidil, sildenafil, glibenclamide, or a combination of nicorandil and glibenclamide, pinacidil and glibenclamide or sildenafil and glibenclamide for 4 weeks. To elucidate the role of mitochondrial KATP channels in modulating nerve growth factor, 5‐hydroxydecanoate was assessed in an in vitro model. Myocardial norepinephrine levels revealed a significant elevation in vehicle‐treated rats at the remote zone compared with sham. Immunohistochemical analysis for tyrosine hydroxylase, growth associated factor 43 and neurofilament also confirmed the change of myocardial norepinephrine. This was paralleled by a significant upregulation of nerve growth factor protein expression and mRNA in the vehicle‐treated rats, which reduced after administering either nicorandil, pinacidil or sildenafil. Arrhythmic scores during programmed stimulation in the vehicle‐treated rats were significantly higher than those treated with sildenafil. In contrast, beneficial effects of sildenafil were reversed by the addition of either glibenclamide or 5‐hydroxydecanoate. Chronic use of sildenafil after infarction, resulting in attenuated sympathetic innervation by activation of mitochondrial KATP channels, may modify the arrhythmogenic response to programmed electrical stimulation.

Norepinephrine-induced nerve growth factor depletion causes cardiac sympathetic denervation in severe heart failure

In severe congestive heart failure (CHF), sympathetic overactivity correlates with the exacerbation of cardiac performance. To test the hypothesis that the cardiac sympathetic nerve density dramatically changes with the acceleration of circulating norepinephrine (NE) concentration, we investigated the temporal association of nerve growth factor (NGF) expression in the heart and cardiac sympathetic nerve density during the development of CHF in the continuous NE-infused rats. The animals were analyzed at 0-, 1-, 3-, 7-, 14-, and 28-day after implantation of osmotic pump at a rate of 0.05 mg/kg/hr. The cardiac performance was temporally facilitated in NE-exposed rats at 3-day in accordance with the sympathetic hyper-innervation induced by the augmentation of NGF mRNA expression in the heart. In NE-treated rats, left ventricular end-diastolic pressure was significantly increased after 7-day and marked left ventricular hypertrophy and systemic fluid retention were observed at 28-day. CHF-induced sympathetic overactivity further increased plasma NE concentration in NE-treated rats and finally reached to 16.1 ± 5.6 ng/ml at 28-day (control level was 0.39 ± 0.1 ng/ml, p < 0.01). In the decompensated CHF rats at 28-day, the NGF mRNA expression was conspicuously reduced concomitant with the obvious nerve fiber loss confirmed by the immunostaining of nerve axonal marker, PGP9.5 and sympathetic neuron marker, tyrosine hydroxylase. This resulted in the attenuated tissue NE contents and the exacerbating cardiac performance. The cardiac sympathetic fiber loss was also confirmed in NE-exposed DBH (dopamine β-hydroxylase)-Cre/Floxed-EGFP (enhanced green fluorescent protein) mice with severe CHF, in which sympathetic nerve could be traced by EGFP. Our results suggest that the cardiac sympathetic nerve density is strictly regulated by the NGF expression in the heart and long-exposure of high plasma NE concentration caused myocardial NGF reduction, following sympathetic fiber loss in severe CHF animals.

The p75 neurotrophin receptor, semaphorins, and sympathetic traffic in the heart

the sympathetic and parasympathetic branches of the autonomic nervous system control cardiac function through the coordinated activity generated by neurons within the intracardiac autonomic ganglia and central nervous system (3). Activation of the sympathetic efferent projections to the heart increases heart rate, modifies the pattern and conduction velocity of the impulse through the heart, and augments both atrial and ventricular contractility while hastening myocardial relaxation. In the vasculature, the sympathetic nervous system (SNS) reduces venous capacitance and constricts resistance vessels, increasing blood pressure. Conversely, the parasympathetic nervous system (PSNS) decreases heart rate and cardiac impulse conduction. More recent evidence has suggested that the PSNS may also affect ventricular function, indirectly by countering β-adrenergic action or directly via inhibition of L-type Ca2+ channels (34). A transient imbalance between these two branches can lead to increased heart rate and blood pressure when cardiac output is increased. When subnormal cardiac PSNS activity concomitant with sympathetic hyperactivation persists, such as after myocardial infarction, this disequilibrium may increase the risk for cardiac adverse events, including arrhythmias, hypertension, cardiac hypertrophy, and, ultimately, heart failure. This condition is called “autonomic neural remodeling” and can greatly contribute to the induction of ventricular fibrillation, thus increasing the patient's susceptibility to sudden cardiac death (41, 47). It is plausible that neurotrophic factors, target organ-secreted proteins essential for the development and differentiation of neurons, are involved in the remarkable neuroplasticity exhibited by the SNS in the adult life, including autonomic remodeling after infarction and/or in the onset and progression of congestive heart failure (17, 30, 32).

A high-fat diet increases risk of ventricular arrhythmia in female rats: enhanced arrhythmic risk in the absence of obesity or hyperlipidemia

Obesity increases the incidence of cardiac arrhythmias and impairs wound healing. However, it is presently unknown whether a high-fat diet affects arrhythmic risk or wound healing before the onset of overt obesity or hyperlipidemia. After 8 wk of feeding a high-fat diet to adult female rats, a nonsignificant increase in body weight was observed and associated with a normal plasma lipid profile. Following ischemia/reperfusion injury, scar length (standard diet 0.29 +/- 0.09 vs. high-fat 0.32 +/- 0.13 cm), thickness (standard diet 0.047 +/- 0.02 vs. high-fat 0.059 +/- 0.01 cm), and collagen alpha(1) type 1 content (standard diet 0.21 +/- 0.04 vs. high-fat 0.20 +/- 0.04 arbitrary units/mm(2)) of infarcted hearts were not altered by the high-fat diet. However, the mortality rate was greatly increased 24 h postinfarction (from 5% to 46%, P < 0.01 for ischemia/reperfusion rats; from 20% to 89%, P < 0.0001, in complete-occlusion rats) in high-fat fed rats, in association with a higher prevalence of ventricular arrhythmias. Ventricular arrhythmia inducibility was also significantly increased in noninfarcted rats fed a high-fat diet. In the hearts of rats fed a high-fat diet, connexin-40 expression was absent, connexin-43 was hypophosphorylated and lateralized, and neurofilament-M immunoreactive fiber density (standard diet 2,020 +/- 260 vs. high-fat diet 2,830 +/- 250 microm(2)/mm(2)) and tyrosine hydroxylase protein expression were increased (P < 0.05). Thus, in the absence of overt obesity and hyperlipidemia, sympathetic hyperinnervation and an aberrant pattern of gap junctional protein expression and regulation in the heart of female rats fed a high-fat diet may have contributed in part to the higher incidence of inducible cardiac arrhythmias.

Effect of N-acetylcysteine on sympathetic hyperinnervation in post-infarcted rat hearts

The purpose of this study was to determine whether N-acetylcysteine (NAC) attenuates cardiac sympathetic hyperinnervation through replenishment of glutathione in infarcted rats. After ligation of the coronary artery, male Wistar rats were randomized to either vehicle, NAC, or vitamins C + E groups for 4 weeks. Post-infarction was associated with increased oxidant release, as measured by tissue isoprostane and myocardial glutathione. Measurement of myocardial norepinephrine levels revealed a significant elevation in vehicle-treated infarcted rats compared with sham-operated rats. Sympathetic hyperinnervation was blunted after administering NAC, as assessed by immunofluorescent analysis of tyrosine hydroxylase and western blotting and real-time quantitative RT-PCR of nerve growth factor. Arrhythmic scores during programmed stimulation in the vehicle-treated infarcted rats were significantly higher than those in animals treated with NAC. Although NAC and vitamins showed similar effects on ventricular remodelling, only NAC demonstrated beneficial effects on sympathetic hyperinnervation. Furthermore, the effects of NAC on nerve growth factor were abolished by administering l-buthionine sulfoximinem, an inhibitor of gamma-glutamylcysteine ligase. Chronic use of NAC, but not vitamins, after infarction is associated with down-regulation of nerve growth factor proteins, probably through a glutathione-dependent pathway, and thus plays a critical role in the beneficial effect on the arrhythmogenic response to programmed electrical stimulation.

A novel peptide ghrelin inhibits neural remodeling after myocardial infarction in rats

Ghrelin is a newly discovered peptide as an endogenous ligand for the growth hormone secretagogue receptor, and has been demonstrated to exert beneficial effect in the cardiovascular system. In the present study, we investigated whether ghrelin administration could inhibit cardiac neural remodeling and sympathetic hyperinnervation after myocardial infarction. Sprague–Dawley rats underwent coronary ligation to induce myocardial infarction and receiving ghrelin chronically (100 µg/kg s.c., twice daily) or saline control for 4 weeks after onset of ischemia. Four weeks after treatment, rats were sacrificed. We examined the expression of nerve growth factor and never markers as well as the mRNA expressions of inflammatory mediators in the infarcted border and non-infarcted left ventricular free wall. We also examined the NF-κBp65 protein and I-κBα protein levels by Western blot analysis. Compared to the control group, ghrelin administration significantly decreased the density of nerve fibers with positive immunostaining for GAP43 and TH, and decreased NGF mRNA and protein levels in the infarcted border and the non-infarcted area. Ghrelin also significantly suppressed interleukin-1β, tumor necrosis factor-α, and endothelin-l mRNA expression, and inhibited NF-κB activation. In conclusion, treatment with ghrelin inhibited neural remodeling and sympathetic hyperinnervation, the process that may be associated with the inhibition of proinflammatory response and NGF signaling.

The arrythmogenic potential of post-myocardial infarction cytokine treatment

a front page story in The New York Times ([22][1]) at the end of March 2001 reported that “the basis of a possible revolution in treating heart attack patients has been laid by three reports of using stem cells from bone marrow to repair heart tissue in animals.” Those three reports ([11][2], [

Remodelling sympathetic innervation in rat pancreatic islets ontogeny

BackgroundPancreatic islets are not fully developed at birth and it is not clear how they are vascularised and innervated. Nerve Growth Factor (NGF) is required to guide sympathetic neurons that innervate peripheral organs and also in cardiovascular system and ovary angiogenesis. Pancreatic beta cells of a transgenic mouse that over-expressed NGF in attracts sympathetic hyper-innervation towards them. Moreover, we have previously demonstrated that adult beta cells synthesize and secrete NGF; however, we do not know how is NGF secreted during development, nor if it might be trophic for sympathetic innervation and survival in the pancreas.We analyzed sympathetic innervation and vasculature development in rat pancreatic islets at different developmental stages; foetal (F19), early postnatal (P1), weaning period (P20) and adults. We temporarily correlated these events to NGF secretion by islet cells.ResultsSympathetic fibres reached pancreatic islets in the early postnatal period, apparently following blood vessels. The maximal number of sympathetic fibres (TH immunopositive) in the periphery of the islets was observed at P20, and then fibres entered the islets and reached the core where beta cells are mainly located. The number of fibres decreased from that stage to adulthood. At all stages studied, islet cells secreted NGF and also expressed the high affinity receptor TrkA. Foetal and neonatal isolated islet cells secreted more NGF than adults. TrkA receptors were expressed at all stages in pancreatic sympathetic fibres and blood vessels. These last structures were NGF–immunoreactive only at early stages (foetal and P0).ConclusionThe results suggest that NGF signalling play an important role in the guidance of blood vessels and sympathetic fibres toward the islets during foetal and neonatal stages and could also preserve innervation at later stages of life.

Effect of ATP-sensitive potassium channel agonists on sympathetic hyperinnervation in postinfarcted rat hearts

Although the acute administration of ATP-sensitive potassium (K(ATP)) channel agonists provides a neuroprotection, it is unclear whether similar benefits are found by modulating sympathetic innervation in chronic settings after myocardial infarction. We assessed whether K(ATP) channel agonists can attenuate the sprouting of cardiac sympathetic nerves after infarction. Male Wistar rats after ligating coronary artery were randomized to either saline, nicorandil, pinacidil, glibenclamide, or a combination of 1) nicorandil and glibenclamide or 2) pinacidil and glibenclamide for 4 wk. To elucidate the role of mitochondrial K(ATP) channels in modulating nerve growth factor, 5-hydroxydecanoate was assessed in an in vitro model. The measurement of myocardial norepinephrine levels revealed a significant elevation in saline-treated infarcted rats compared with sham-operated rats, consistent with excessive sympathetic innervation. Excessive sympathetic innervation was blunted after giving the rats either nicorandil or pinacidil, compared with saline, as assessed by the immunohistochemical analysis of tyrosine hydroxylase, growth associated protein-43, and neurofilament and Western blot analysis and real-time quantitative RT-PCR of nerve growth factor. The arrhythmic scores during programmed stimulation in the saline- or glibenclamide-treated infarcted rats were significantly higher than those of rats treated with K(ATP) channel agonists. In contrast, the beneficial effects of nicorandil and pinacidil were abolished by administering either glibenclamide or 5-hydroxydecanoate. The sympathetic hyperinnervation after infarction is attenuated by the activation of mitochondrial K(ATP) channels. The chronic use of mitochondrial K(ATP) channel agonists after infarction may attenuate the arrhythmogenic response to programmed electrical stimulation.

Macrophage depletion suppresses sympathetic hyperinnervation following myocardial infarction

Myocardial infarction induces sympathetic axon sprouting adjacent to the necrotic region, and this has been implicated in the etiology of arrhythmias resulting in sudden cardiac death. Previous studies show that nerve growth factor (NGF) is essential for enhanced post-infarct sympathetic sprouting, but the cell types necessary to supply this neurotrophic protein are unknown. The objective of the present study was to determine whether macrophages, which are known to synthesize NGF, are necessary for post-infarct cardiac sympathetic sprouting. Ovariectomized female rats received left coronary artery ligation or sham operation, followed by intravenous injection of liposomes containing saline vehicle or clodronate, which kills macrophages. Sham-operated myocardium contained some sympathetic axons, few myofibroblasts and T cells and no CD-68-positive macrophages. In rats receiving saline liposomes through 7 days post-ligation, the posterolateral infarct border contained numerous myofibroblasts, macrophages and T cells, and sympathetic innervation was increased twofold. Treatment with clodronate liposomes reduced macrophage numbers by 69%, while myofibroblast area was reduced by 23% and T cell number was unaffected. Clodronate liposome treatment reduced sympathetic axon density to levels comparable to the uninfarcted heart. NGF protein content measured in western blots was reduced to 33% of that present in infarcts where rats received saline-containing liposomes. Tissue morphometry confirmed that NGF immunostaining was dramatically reduced, and this was attributable primarily to reduced macrophage content. These results show that macrophage destruction markedly reduces post-infarction levels of NGF and that the presence of elevated numbers of macrophages is obligatory for development of sympathetic hyperinnervation following myocardial infarction.

What have we learned about the contribution of autonomic nervous system to human arrhythmia?

Myocardial infarction results in denervation, followed by neural remodeling characterized by nerve sprouting and heterogeneous sympathetic hyperinnervation throughout the myocardium. There is an association between the density of sympathetic nerves and occurrence of cardiac arrhythmia in humans. Autonomic nerve recording in ambulatory dogs showed a close association between autonomic nerve activity and paroxysmal atrial and ventricular arrhythmias. Cryoablation of the stellate ganglion prevented paroxysmal atrial tachycardia and atrial fibrillation in canine models. Further studies are needed to determine if these same methods can be used to control atrial arrhythmias in humans.