On the genesis of myocardial ischemia

Reduced Cell Excitability of Cardiac Postganglionic Parasympathetic Neurons Correlates With Myocardial Infarction-Induced Fatal Ventricular Arrhythmias in Type 2 Diabetes Mellitus

Objective

Withdrawal of cardiac vagal activity is considered as one of the important triggers for acute myocardial infarction (MI)-induced ventricular arrhythmias in type 2 diabetes mellitus (T2DM). Our previous study demonstrated that cell excitability of cardiac parasympathetic postganglionic (CPP) neurons was reduced in T2DM rats. This study investigated whether cell excitability of CPP neurons is associated with cardiac vagal activity and MI-induced ventricular arrhythmias in T2DM rats.

Methods

Rat T2DM was induced by a high-fat diet plus streptozotocin injection. MI-evoked ventricular arrhythmia was achieved by surgical ligation of the left anterior descending coronary artery. Twenty-four-hour, continuous ECG recording was used to quantify ventricular arrhythmic events and heart rate variability (HRV) in conscious rats. The power spectral analysis of HRV was used to evaluate autonomic function. Cell excitability of CPP neurons was measured by the whole-cell patch-clamp technique.

Results

Twenty-four-hour ECG data demonstrated that MI-evoked fatal ventricular arrhythmias are more severe in T2DM rats than that in sham rats. In addition, the Kaplan-Meier analysis demonstrated that the survival rate over 2 weeks after MI is significantly lower in T2DM rats (15% in T2DM+MI) compared to sham rats (75% in sham+MI). The susceptibility to ventricular tachyarrhythmia elicited by programmed electrical stimulation was higher in anesthetized T2DM+MI rats than that in rats with MI or T2DM alone (7.0 ± 0.58 in T2DM+MI group vs. 3.5 ± 0.76 in sham+MI). Moreover, as an index for vagal control of ventricular function, changes of left ventricular systolic pressure (LVSP) and the maximum rate of increase of left ventricular pressure (LV dP/dt_max) in response to vagal efferent nerve stimulation were blunted in T2DM rats. Furthermore, T2DM increased heterogeneity of ventricular electrical activities and reduced cardiac parasympathetic activity and cell excitability of CPP neurons (current threshold-inducing action potentials being 62 ± 3.3 pA in T2DM rats without MI vs. 27 ± 1.9 pA in sham rats without MI). However, MI did not alter vagal control of the ventricular function and CPP neuronal excitability, although it also induced cardiac autonomic dysfunction and enhanced heterogeneity of ventricular electrical activities.

Conclusion

The reduction of CPP neuron excitability is involved in decreased cardiac vagal function, including cardiac parasympathetic activity and vagal control of ventricular function, which is associated with MI-induced high mortality and malignant ventricular arrhythmias in T2DM.

Is cervical region tightness related to vagal function and stomach symptoms?

The vagal nerve is a cranial nerve that carries mainly parasympathetic fibers (average 75%) with both sensory and motor functions. The vagal nerve contains a complex neuro-endocrine-immune network. The majority, at least 66%, of the gastric myenteric neurons receive direct cholinergic excitatory stimulation from the pre-enteric vagal nerve. Changes in vagal function may cause stomach problems, although the mechanisms that change the vagal function have not yet been fully illuminated. Considering the course of the vagal nerve in the cervical region, it is thought that conditions such as stiffness, tightness and decreased elasticity in this region may compress the vagal nerve andmay affect vagal function. According to this hypothesis, neuroinflammation and hyperalgesia may occur in the vagal nerve under mechanical pressure, resulting in increased complaints of pain and burning in the stomach increases. However, as the vagal nerve has various effects on the motility of the stomach and vagal dysfunction affects the motor function of the stomach, relaxation techniques applied to the soft tissues of the cervical region will provide mechanical relief in the nerve. Thus, the vagal nerve will be decompressed and be able to function optimally. According to our clinical observations, in patients whose soft tissues in the cervical region are relaxed, gastric symptoms are decreased. Based on research results and clinical experience, cervical region tightness can be considered to cause stomach problems through the vagal nerve, and soft tissue relaxation of the cervical region can be a promising treatment method for stomach symptoms.

The Role of Cardiac Ganglia in the Prevention of Coronary Atherosclerosis: An Analytical Examination of Cholesterol-fed Rabbits

Background

The heart is innervated by the autonomic nervous system, which contributes to the control of the heart’s rhythm and coronary circulation. It has been suggested that the cardiac fibers of the vagus nerve play important roles in controlling circulatory functions and in protecting against atherosclerotic pathologies in coronary arteries.

Aims

To investigate the presence of atherosclerotic differences in the coronary arteries of cholesterol-fed rabbits by measuring the density of cardiac ganglia neurons.

Study Design

Animal experiment.

Methods

This study was conducted using 45 male rabbits. Over a period of 16 weeks, they were kept on an atherogenic diet of water ad libitum and high fat (8.6%) containing saturated fatty acids with 205 mg/kg of cholesterol (1%) per day. Then, their hearts were removed and examined by histopathological methods. Atherosclerotic plaques of the main coronary arteries were examined using the Cavalieri method. Atherosclerosis index values (AIVs) were estimated as the wall surface area/plaque surface area, and the results were analyzed with the Kruskal-Wallis and Mann-Whitney U tests.

Results

While the average atherosclerosis index value was estimated to be ≤8% in 21 animals, the atherosclerosis index value was 9-20% in animals with minor plaque detection (n=11) and ≥20% in animals with major plaque detection (n=10). Increased atherosclerosis index values were more common in animals with low neuron densities than in animals with high neuron densities (p<0.017).

Conclusion

The low neuron density of the cardiac ganglia in cholesterol-fed rabbits is associated with an increased atherosclerotic plaque incidence and volume.

Identification of lncRNA and mRNA expression profiles in rat spinal cords at various time‑points following cardiac ischemia/reperfusion

The identification of the expression patterns of long non-coding RNAs (lncRNAs) and mRNAs in the spinal cord under normal and cardiac ischemia/reperfusion (I/R) conditions is essential for understanding the genetic mechanisms underlying the pathogenesis of cardiac I/R injury. The present study used high-throughput RNA sequencing to investigate differential gene and lncRNA expression patterns in the spinal cords of rats during I/R-induced cardiac injury. Male Sprague Dawley rats were assigned to the following groups: i) Control; ii) 2 h (2 h post-reperfusion); and iii) 0.5 h (0.5 h post-reperfusion). Further mRNA/lncRNA microarray analysis revealed that the expression profiles of lncRNA and mRNA in the spinal cords differed markedly between the control and 2 h groups, and in total 7,980 differentially expressed (>2-fold) lncRNAs (234 upregulated, 7,746 downregulated) and 3,428 mRNAs (767 upregulated, 2,661 downregulated) were identified. Reverse transcription-quantitative polymerase chain reaction analysis was performed to determine the expression patterns of several lncRNAs. The results indicated that the expression levels of lncRNA NONRATT025386 were significantly upregulated in the 2 and 0.5 h groups when compared with those in the control group, whereas the expression levels of NONRATT016113, NONRATT018298 and NONRATT018300 were elevated in the 2 h group compared with those in the control group; however, there was no statistically significant difference between the 0.5 h and control groups. Furthermore, the expression of lncRNA NONRATT002188 was significantly downregulated in the 0.5 and 2 h groups when compared with the control group. The present study determined the expression pattern of lncRNAs and mRNAs in rat spinal cords during cardiac I/R. It was suggested that lncRNAs and mRNAs from spinal cords may be novel therapeutic targets for the treatment of I/R-induced cardiac injury.

Reduced N-Type Ca2+ Channels in Atrioventricular Ganglion Neurons Are Involved in Ventricular Arrhythmogenesis

Background

Attenuated cardiac vagal activity is associated with ventricular arrhythmogenesis and related mortality in patients with chronic heart failure. Our recent study has shown that expression of N‐type Ca²⁺ channel α‐subunits (Ca_v2.2‐α) and N‐type Ca²⁺ currents are reduced in intracardiac ganglion neurons from rats with chronic heart failure. Rat intracardiac ganglia are divided into the atrioventricular ganglion (AVG) and sinoatrial ganglion. Ventricular myocardium receives projection of neuronal terminals only from the AVG. In this study we tested whether a decrease in N‐type Ca²⁺ channels in AVG neurons contributes to ventricular arrhythmogenesis.

Methods and Results

Lentiviral Ca_v2.2‐α shRNA (2 μL, 2×10⁷ pfu/mL) or scrambled shRNA was in vivo transfected into rat AVG neurons. Nontransfected sham rats served as controls. Using real‐time single‐cell polymerase chain reaction and reverse‐phase protein array, we found that in vivo transfection of Ca_v2.2‐α shRNA decreased expression of Ca_v2.2‐α mRNA and protein in rat AVG neurons. Whole‐cell patch‐clamp data showed that Ca_v2.2‐α shRNA reduced N‐type Ca²⁺ currents and cell excitability in AVG neurons. The data from telemetry electrocardiographic recording demonstrated that 83% (5 out of 6) of conscious rats with Ca_v2.2‐α shRNA transfection had premature ventricular contractions (P<0.05 versus 0% of nontransfected sham rats or scrambled shRNA‐transfected rats). Additionally, an index of susceptibility to ventricular arrhythmias, inducibility of ventricular arrhythmias evoked by programmed electrical stimulation, was higher in rats with Ca_v2.2‐α shRNA transfection compared with nontransfected sham rats and scrambled shRNA‐transfected rats.

Conclusions

A decrease in N‐type Ca²⁺ channels in AVG neurons attenuates vagal control of ventricular myocardium, thereby initiating ventricular arrhythmias.

Role of endothelial nitric oxide synthase and vagal activity in the endothelial protection of atorvastatin in ischemia/reperfusion injury

Vascular endothelial dysfunction plays a pivotal role in the development and maintenance of ischemia/reperfusion (I/R) injury. Statins, developed as lipid-lowering drugs, partially restore vagal activity and exhibit pleiotropic effects. This study was aimed at determining the effect of atorvastatin (ATV) on endothelial dysfunction in peripheral resistance arteries after I/R injury. After pretreatment with ATV (10 mg·kg·d) or its vehicle for 3 days, the superior mesenteric artery was occluded for 60 minutes and reperfusion for 90 minutes or the rats were anesthetized without being subjected to ischemia. In the ATV-treated I/R group, the increased contractions to KCl and 5-hydroxytryptamine induced by I/R were ameliorated, and attenuated endothelium-dependent relaxations to acetylcholine (ACh) were normalized. The restored relaxation to ACh was abolished by N-nitro-L-arginine methyl ester. ATV prevented the structural damage of vascular endothelial cells. Furthermore, the activities of phosphatidylinositol-3-kinase, Akt, and endothelial nitric oxide synthase were elevated in mesenteric arteries after ATV treatment. In addition, I/R-induced increment of endothelial cells apoptosis was also attenuated by ATV. Intriguingly, ATV also increased baroreflex sensitivity and serum ACh content after I/R. In conclusion, the endothelial protective effect of ATV in peripheral arteries is associated with the activated phosphatidylinositol-3-kinase/Akt/endothelial nitric oxide synthase pathway and restored vagal activity.

Autonomic cardiac innervation: development and adult plasticity

Autonomic cardiac neurons have a common origin in the neural crest but undergo distinct developmental differentiation as they mature toward their adult phenotype. Progenitor cells respond to repulsive cues during migration, followed by differentiation cues from paracrine sources that promote neurochemistry and differentiation. When autonomic axons start to innervate cardiac tissue, neurotrophic factors from vascular tissue are essential for maintenance of neurons before they reach their targets, upon which target-derived trophic factors take over final maturation, synaptic strength and postnatal survival. Although target-derived neurotrophins have a central role to play in development, alternative sources of neurotrophins may also modulate innervation. Both developing and adult sympathetic neurons express proNGF, and adult parasympathetic cardiac ganglion neurons also synthesize and release NGF. The physiological function of these “non-classical” cardiac sources of neurotrophins remains to be determined, especially in relation to autocrine/paracrine sustenance during development.

 

Cardiac autonomic nerves are closely spatially associated in cardiac plexuses, ganglia and pacemaker regions and so are sensitive to release of neurotransmitter, neuropeptides and trophic factors from adjacent nerves. As such, in many cardiac pathologies, it is an imbalance within the two arms of the autonomic system that is critical for disease progression. Although this crosstalk between sympathetic and parasympathetic nerves has been well established for adult nerves, it is unclear whether a degree of paracrine regulation occurs across the autonomic limbs during development. Aberrant nerve remodeling is a common occurrence in many adult cardiovascular pathologies, and the mechanisms regulating outgrowth or denervation are disparate. However, autonomic neurons display considerable plasticity in this regard with neurotrophins and inflammatory cytokines having a central regulatory function, including in possible neurotransmitter changes. Certainly, neurotrophins and cytokines regulate transcriptional factors in adult autonomic neurons that have vital differentiation roles in development. Particularly for parasympathetic cardiac ganglion neurons, additional examinations of developmental regulatory mechanisms will potentially aid in understanding attenuated parasympathetic function in a number of conditions, including heart failure.

Protection against ischemia-induced oxidative stress conferred by vagal stimulation in the rat heart: involvement of the AMPK-PKC pathway

Reactive oxygen species (ROS) production is an important mechanism in myocardial ischemia and nicotinamide adenine dinucleotide phosphate (NADPH) oxidase is one of major sources of ROS in the heart. Previous studies showed that vagus nerve stimulation (VNS) is beneficial in treating ischemic heart diseases. However, the effect of VNS on ROS production remains elusive. In this study, we investigated the role of VNS onischemia-induced ROS production. Our results demonstrated that VNS alleviated the myocardial injury, attenuated the cardiac dysfunction, reserved the antioxidant enzyme activity and inhibited the formation of ROS as evidenced by the decreased NADPH oxidase (Nox) activity and superoxide fluorescence intensity as well as the expression of p67phox, Rac1 and nitrotyrosine. Furthermore, VNS resulted in the phosphorylation and activation of adenosine monophosphate activated protein kinase (AMPK), which in turn led to an inactivation of Nox by protein kinase C (PKC); however, the phenomena were repressed by the administration of a muscarinic antagonist atropine. Taken together, these data indicate that VNS decreases ROS via AMPK-PKC-Nox pathway; this may have potential importance for the treatment of ischemic heart diseases.

Ouabain--the key to cardioprotection?

Based on a wealth of mechanistic evidence supported by the fact that ouabain mimics the spleen-liver effect in this article, the hypothesis is established that the endogenous hormone ouabain not only mimics the effects of ischemic preconditioning but also may be an ideal drug for the prevention of ischemic diseases. Moreover, it is argued that the spleen-liver effect may represent a general protective mechanism for the protection of organisms against oxygen deficiency. Investigating the spleen-liver mechanism offers a new approach to decipher the secrets of ischemic conditioning. Preconditioning represents a basic mechanism to protect a wide variety of cells against stressful stimuli such as ischemia. The ability to undergo preconditioning is almost ubiquitous in tissues and is highly conserved across species. Reinvestigation of the "spleen-liver mechanism" will allow the study of metabolic inhibitors and hormone mimics that all could help to transform ischemic preconditioning into a cure of the epidemic ischemic heart disease. Ouabain mimics the effects of the spleen factor. Cardioprotection induced by ouabain is due to the activation of pathways that are also activated in ischemic preconditioning. Just like ischemic preconditioning, ouabain activates the reperfusion injury salvage kinase pathway. Activation of nuclear factor kappa B and other transcription factors contribute to the long lasting effects of ouabain. The endogenous hormone ouabain just like preconditioning offers multiorgan protection based on innate mechanisms, which warrants clinical investigation. Clinical studies with ouabain that correspond to current standards are warranted.

The role of muscarinic receptors in the beneficial effects of adenosine against myocardial reperfusion injury in rats

Adenosine, a catabolite of ATP, displays a wide variety of effects in the heart including regulation of cardiac response to myocardial ischemia and reperfusion injury. Nonetheless, the precise mechanism of adenosine-induced cardioprotection is still elusive. Isolated Sprague-Dawley rat hearts underwent 30 min global ischemia and 120 min reperfusion using a Langendorff apparatus. Both adenosine and acetylcholine treatment recovered the post-reperfusion cardiac function associated with adenosine and muscarinic receptors activation. Simultaneous administration of adenosine and acetylcholine failed to exert any additive protective effect, suggesting a shared mechanism between the two. Our data further revealed a cross-talk between the adenosine and acetylcholine receptor signaling in reperfused rat hearts. Interestingly, the selective M(2) muscarinic acetylcholine receptor antagonist methoctramine significantly attenuated the cardioprotective effect of adenosine. In addition, treatment with adenosine upregulated the expression and the maximal binding capacity of muscarinic acetylcholine receptor, which were inhibited by the selective A(1) adenosine receptor antagonist 8-Cyclopentyl-1,3-dipropylxanthine (DPCPX) and the nitric oxide synthase inhibitor N(ω)-nitro-L-arginine methyl ester (L-NAME). These data suggested a possible functional coupling between the adenosine and muscarinic receptors behind the observed cardioprotection. Furthermore, nitric oxide was found involved in triggering the response to each of the two receptor agonist. In summary, there may be a cross-talk between the adenosine and muscarinic receptors in ischemic/reperfused myocardium with nitric oxide synthase might serve as the distal converging point. In addition, adenosine contributes to the invigorating effect of adenosine on muscarinic receptor thereby prompting to regulation of cardiac function. These findings argue for a potentially novel mechanism behind the adenosine-mediated cardioprotection.

Acetylcholine inhibits hypoxia-induced tumor necrosis factor-α production via regulation of MAPKs phosphorylation in cardiomyocytes

Recent findings have reported that up-regulation of tumor necrosis factor-alpha (TNF-α) induced by myocardial hypoxia aggravates cardiomyocyte injury. Acetylcholine (ACh), the principle vagal neurotransmitter, protects cardiomyocytes against hypoxia by inhibiting apoptosis. However, it is still unclear whether ACh regulates TNF-α production in cardiomyocytes after hypoxia. The concentration of extracellular TNF-α was increased in a time-dependent manner during hypoxia. Furthermore, ACh treatment also inhibited hypoxia-induced TNF-α mRNA and protein expression, caspase-3 activation, cell death and the production of reactive oxygen species (ROS) in cardiomyocytes. ACh treatment prevented the hypoxia-induced increase in p38 mitogen-activated protein kinase (MAPK) and c-Jun N-terminal kinase (JNK) phosphorylation, and increased extracellular signal-regulated kinase (ERK) phosphorylation. Co-treatment with atropine, a non-selective muscarinic acetylcholine receptor antagonist, or methoctramine, a selective type-2 muscarinic acetylcholine (M(2) ) receptor antagonist, abrogated the effects of ACh treatment in hypoxic cardiomyocytes. Co-treatment with hexamethonium, a non-selective nicotinic receptor antagonist, and methyllycaconitine, a selective alpha7-nicotinic acetylcholine receptor antagonist, had no effect on ACh-treated hypoxic cardiomyocytes. In conclusion, these results demonstrate that ACh activates the M(2) receptor, leading to regulation of MAPKs phosphorylation and, subsequently, down-regulation of TNF-α production. We have identified a novel pathway by which ACh mediates cardioprotection against hypoxic injury in cardiomyocytes.

Heart-rate variability in women during 40-hour prolonged wakefulness

Heart-rate variability patterns of 18 women during a 40-h constant routine of prolonged wakefulness under controlled laboratory conditions were analyzed. The authors tested the circadian timing of the autonomic nervous system and the relationship between the sympathetic and vagal branches in women with both a functional disorder of vascular regulation (main symptom: cold hands and feet) and prolonged sleep onset and controls without these symptoms. Spectral analysis of R-R intervals during paced breathing episodes revealed significantly lower power values in the high-frequency band (HF; 0.15-0.4 Hz) but not in the low-frequency band (LF; 0.04-0.15 Hz), leading to a significantly elevated LF/HF ratio in the former group. A significant circadian rhythm in LF power and heart rate occurred in both groups, and a significant correlation was found between sleepiness and sympathovagal balance (r = .53, p < .05). These findings indicate not only an autonomic imbalance in the first group compared with controls, but also two strategies of the autonomic nervous system to fight against fatigue in women. One implies circadian control and the other homeostatic control, and both are reflected by the LF/HF ratio.

Adenine sulfate improves cardiac function and the cardiac cholinergic system after myocardial infarction in rats

Recent studies have shown that vagal activation may have an important therapeutic implication for myocardial infarction (MI), but effective strategies remain unexplored. Here, we investigate whether adenine sulfate can preserve cardiac function and the cholinergic system against MI. Rats were treated with adenine sulfate for three weeks after coronary ligation. Cardiac function was assessed by hemodynamics. The muscarinic M(2) receptor and cholinesterase-positive nerves were semi-quantified by immunochemical and histochemical staining. The maximal binding capacity (B(max)) of muscarinic receptors, determined by radioligand binding assay, showed that cardiac function was impaired in MI rats. Adenine sulfate reversed MI-induced reduction of mean artery pressure and left ventricular systolic pressure and elevation of left ventricular end-diastolic pressure. Moreover, adenine sulfate also increased nitric oxide (NO) and nitric oxide synthase (NOS) activity. The amelioration was accompanied by a reversal of the infarction-induced reduction of cholinesterase-positive nerves and M(2)-receptor expression and B(max) in the adenine sulfate high dose group. Meanwhile, adenine sulfate treatment corrected the disorder of cardiac redox state by reduction in maleic dialdehyde and increase in superoxide dismutase. In conclusion, adenine sulfate exerts cardioprotection against MI and ameliorates NO production. Changes in cardiac vagal distribution density and M(2)-receptor expression raise the possibility that improvement of the cardiac cholinergic system is involved in adenine sulfate-induced cardioprotective effects.

Modulation of rat parasympathetic cardiac ganglion phenotype and NGF synthesis by adrenergic nerves

Cardiac function is regulated by interactions among intrinsic and extrinsic autonomic neurons, and the mechanisms responsible for organizing these circuits are poorly understood. Parasympathetic neurons elsewhere synthesize the neurotrophin NGF, which may promote postganglionic axonal associations where parasympathetic axons inhibit sympathetic transmitter release. Previous studies have shown that parasympathetic NGF content and neurochemical phenotype are regulated by sympathetic innervation. In this study we assessed contributions of sympathetic input on cardiac ganglion neuronal phenotype and NGF expression. Because cardiac ganglia are reported to contain putative noradrenergic neurons, we eliminated sympathetic input both surgically (extrinsic) and chemically (extrinsic plus intrinsic). In controls, most cardiac ganglion neurons expressed vesicular acetylcholine transporter, frequently colocalized with vesicular monoamine transporter, but lacked catecholamine histofluorescence. Most cardiac ganglion neurons expressed NGF transcripts, and 40% contained mature and 47% proNGF immunoreactivity. Guanethidine treatment for 7 days decreased numbers of neurons expressing vesicular acetylcholine transporter, NGF transcripts and NGF immunoreactivity, but did not affect proNGF or vesicular monoamine transporter immunoreactivity. Stellate ganglionectomy had comparable effects on neurochemical phenotype and mature NGF immunoreactivity, but proNGF expression was additionally reduced. These findings show that individual cardiac ganglion neurons display markers of both cholinergic and noradrenergic transmission. Sympathetic noradrenergic innervation maintains levels of cholinergic but not noradrenergic marker protein. Sympathetic innervation also promotes cardiac ganglion neuronal NGF synthesis. Because chemical blockade of all noradrenergic transmission is no more effective than extrinsic sympathectomy, local intrinsic noradrenergic transmission is not a factor in regulating ganglion neuron phenotype.