Effects of transient coronary artery occlusion on canine intrinsic cardiac neuronal activity

Spinal Anesthesia Reduces Myocardial Ischemia-triggered Ventricular Arrhythmias by Suppressing Spinal Cord Neuronal Network Interactions in Pigs

BACKGROUND

Cardiac sympathoexcitation leads to ventricular arrhythmias. Spinal anesthesia modulates sympathetic output and can be cardioprotective. However, its effect on the cardio-spinal reflexes and network interactions in the dorsal horn cardiac afferent neurons and the intermediolateral nucleus sympathetic neurons that regulate sympathetic output is not known. The authors hypothesize that spinal bupivacaine reduces cardiac neuronal firing and network interactions in the dorsal horn-dorsal horn and dorsal horn-intermediolateral nucleus that produce sympathoexcitation during myocardial ischemia, attenuating ventricular arrhythmogenesis.

METHODS

Extracellular neuronal signals from the dorsal horn and intermediolateral nucleus neurons were simultaneously recorded in Yorkshire pigs (n = 9) using a 64-channel high-density penetrating microarray electrode inserted at the T2 spinal cord. Dorsal horn and intermediolateral nucleus neural interactions and known markers of cardiac arrhythmogenesis were evaluated during myocardial ischemia and cardiac load-dependent perturbations with intrathecal bupivacaine.

RESULTS

Cardiac spinal neurons were identified based on their response to myocardial ischemia and cardiac load-dependent perturbations. Spinal bupivacaine did not change the basal activity of cardiac neurons in the dorsal horn or intermediolateral nucleus. After bupivacaine administration, the percentage of cardiac neurons that increased their activity in response to myocardial ischemia was decreased. Myocardial ischemia and cardiac load-dependent stress increased the short-term interactions between the dorsal horn and dorsal horn (324 to 931 correlated pairs out of 1,189 pairs, P < 0.0001), and dorsal horn and intermediolateral nucleus neurons (11 to 69 correlated pairs out of 1,135 pairs, P < 0.0001). Bupivacaine reduced this network response and augmentation in the interactions between dorsal horn-dorsal horn (931 to 38 correlated pairs out of 1,189 pairs, P < 0.0001) and intermediolateral nucleus-dorsal horn neurons (69 to 1 correlated pairs out of 1,135 pairs, P < 0.0001). Spinal bupivacaine reduced shortening of ventricular activation recovery interval and dispersion of repolarization, with decreased ventricular arrhythmogenesis during acute ischemia.

CONCLUSIONS

Spinal anesthesia reduces network interactions between dorsal horn-dorsal horn and dorsal horn-intermediolateral nucleus cardiac neurons in the spinal cord during myocardial ischemia. Blocking short-term coordination between local afferent-efferent cardiac neurons in the spinal cord contributes to a decrease in cardiac sympathoexcitation and reduction of ventricular arrhythmogenesis.

EDITOR’S PERSPECTIVE

Cardiac sympathectomy and spinal cord stimulation attenuate reflex-mediated norepinephrine release during ischemia preventing ventricular fibrillation

The purpose of this study was to define the mechanism by which cardiac neuraxial decentralization or spinal cord stimulation (SCS) reduces ischemia-induced ventricular fibrillation (VF). Direct measurements of norepinephrine (NE) levels in the left ventricular interstitial fluid (ISF) by microdialysis, in response to transient (15-minute) coronary artery occlusion (CAO), were performed in anesthetized canines. Responses were studied in animals with intact neuraxes and were compared with those in which the intrathoracic component of the cardiac neuraxes (stellate ganglia) or the intrinsic cardiac neuronal (ICN) system was surgically delinked from the central nervous system and those with intact neuraxes with preemptive SCS (T1-T3). With intact neuraxes, animals with exaggerated NE release due to CAO were at increased risk for VF. During CAO, there was a 152% increase in NE when the neuraxes were intact compared with 114% following stellate decentralization and 16% following ICN decentralization. During SCS, CAO NE levels increased by 59%. Risk for CAO-induced VF was 38% in controls, 8% following decentralization, and 11% following SCS. These data indicate that ischemia-related afferent neuronal transmission differentially engages central and intrathoracic sympathetic reflexes and amplifies sympathoexcitation. Differences in regional ventricular NE release are associated with increased risk for VF. Surgical decentralization or SCS reduced NE release and VF.

Electrophysiological effects of nicotinic and electrical stimulation of intrinsic cardiac ganglia in the absence of extrinsic autonomic nerves in the rabbit heart

Background

The intrinsic cardiac nervous system is a rich network of cardiac nerves that converge to form distinct ganglia and extend across the heart and is capable of influencing cardiac function.

Objective

The goals of this study were to provide a complete picture of the neurotransmitter/neuromodulator profile of the rabbit intrinsic cardiac nervous system and to determine the influence of spatially divergent ganglia on cardiac electrophysiology.

Methods

Nicotinic or electrical stimulation was applied at discrete sites of the intrinsic cardiac nerve plexus in the Langendorff-perfused rabbit heart. Functional effects on sinus rate and atrioventricular conduction were measured. Immunohistochemistry for choline acetyltransferase (ChAT), tyrosine hydroxylase, and/or neuronal nitric oxide synthase (nNOS) was performed using whole mount preparations.

Results

Stimulation within all ganglia produced either bradycardia, tachycardia, or a biphasic brady-tachycardia. Electrical stimulation of the right atrial and right neuronal cluster regions produced the largest chronotropic responses. Significant prolongation of atrioventricular conduction was predominant at the pulmonary vein-caudal vein region. Neurons immunoreactive (IR) only for ChAT, tyrosine hydroxylase, or nNOS were consistently located within the limits of the hilum and at the roots of the right cranial and right pulmonary veins. ChAT-IR neurons were most abundant (1946 ± 668 neurons). Neurons IR only for nNOS were distributed within ganglia.

Conclusion

Stimulation of intrinsic ganglia, shown to be of phenotypic complexity but predominantly of cholinergic nature, indicates that clusters of neurons are capable of independent selective effects on cardiac electrophysiology, therefore providing a potential therapeutic target for the prevention and treatment of cardiac disease.

Clinical neurocardiology defining the value of neuroscience-based cardiovascular therapeutics

The autonomic nervous system regulates all aspects of normal cardiac function, and is recognized to play a critical role in the pathophysiology of many cardiovascular diseases. As such, the value of neuroscience-based cardiovascular therapeutics is increasingly evident. This White Paper reviews the current state of understanding of human cardiac neuroanatomy, neurophysiology, pathophysiology in specific disease conditions, autonomic testing, risk stratification, and neuromodulatory strategies to mitigate the progression of cardiovascular diseases.

Translational neurocardiology: preclinical models and cardioneural integrative aspects

Neuronal elements distributed throughout the cardiac nervous system, from the level of the insular cortex to the intrinsic cardiac nervous system, are in constant communication with one another to ensure that cardiac output matches the dynamic process of regional blood flow demand. Neural elements in their various 'levels' become differentially recruited in the transduction of sensory inputs arising from the heart, major vessels, other visceral organs and somatic structures to optimize neuronal coordination of regional cardiac function. This White Paper will review the relevant aspects of the structural and functional organization for autonomic control of the heart in normal conditions, how these systems remodel/adapt during cardiac disease, and finally how such knowledge can be leveraged in the evolving realm of autonomic regulation therapy for cardiac therapeutics.

Myocardial infarction induces structural and functional remodelling of the intrinsic cardiac nervous system

KEY POINTS

Intrinsic cardiac (IC) neurons undergo differential morphological and phenotypic remodelling that reflects the site of myocardial infarction (MI). Afferent neural signals from the infarcted region to IC neurons are attenuated, while those from border and remote regions are preserved post-MI, giving rise to a 'neural sensory border zone'. Convergent IC local circuit (processing) neurons have enhanced transduction capacity following MI. Functional network connectivity within the intrinsic cardiac nervous system is reduced post-MI. MI reduces the response and alters the characteristics of IC neurons to ventricular pacing.

ABSTRACT

Autonomic dysregulation following myocardial infarction (MI) is an important pathogenic event. The intrinsic cardiac nervous system (ICNS) is a neural network located on the heart that is critically involved in autonomic regulation. The aims of this study were to characterize structural and functional remodelling of the ICNS post-MI in a porcine model (control (n = 16) vs. healed anteroapical MI (n = 16)). In vivo microelectrode recordings of basal activity, as well as responses to afferent and efferent stimuli, were recorded from intrinsic cardiac neurons. From control 118 neurons and from MI animals 102 neurons were functionally classified as afferent, efferent, or convergent (receiving both afferent and efferent inputs). In control and MI, convergent neurons represented the largest subpopulation (47% and 48%, respectively) and had enhanced transduction capacity following MI. Efferent inputs to neurons were maintained post-MI. Afferent inputs were attenuated from the infarcted region (19% in control vs. 7% in MI; P = 0.03), creating a 'neural sensory border zone', or heterogeneity in afferent information. MI reduced transduction of changes in preload (54% in control vs. 41% in MI; P = 0.05). The overall functional network connectivity, or the ability of neurons to respond to independent pairs of stimuli, within the ICNS was reduced following MI. The neuronal response was differentially decreased to ventricular vs. atrial pacing post-MI (63% in control vs. 44% in MI to ventricular pacing; P < 0.01). MI induced morphological and phenotypic changes within the ICNS. The alteration of afferent neural signals, and remodelling of convergent neurons, represents a 'neural signature' of ischaemic heart disease.

Influence of cardiac decentralization on cardioprotection

The role of cardiac nerves on development of myocardial tissue injury after acute coronary occlusion remains controversial. We investigated whether acute cardiac decentralization (surgical) modulates coronary flow reserve and myocardial protection in preconditioned dogs subject to ischemia-reperfusion. Experiments were conducted on four groups of anesthetised, open-chest dogs (n = 32): 1- controls (CTR, intact cardiac nerves), 2- ischemic preconditioning (PC; 4 cycles of 5-min IR), 3- cardiac decentralization (CD) and 4- CD+PC; all dogs underwent 60-min coronary occlusion and 180-min reperfusion. Coronary blood flow and reactive hyperemic responses were assessed using a blood volume flow probe. Infarct size (tetrazolium staining) was related to anatomic area at risk and coronary collateral blood flow (microspheres) in the anatomic area at risk. Post-ischemic reactive hyperemia and repayment-to-debt ratio responses were significantly reduced for all experimental groups; however, arterial perfusion pressure was not affected. Infarct size was reduced in CD dogs (18.6±4.3; p = 0.001, data are mean±1SD) compared to 25.2±5.5% in CTR dogs and was less in PC dogs as expected (13.5±3.2 vs. 25.2±5.5%; p = 0.001); after acute CD, PC protection was conserved (11.6±3.4 vs. 18.6±4.3%; p = 0.02). In conclusion, our findings provide strong evidence that myocardial protection against ischemic injury can be preserved independent of extrinsic cardiac nerve inputs.

Dynamic remodeling of the guinea pig intrinsic cardiac plexus induced by chronic myocardial infarction

Myocardial infarction (MI) is associated with remodeling of the heart and neurohumoral control systems. The objective of this study was to define time-dependent changes in intrinsic cardiac (IC) neuronal excitability, synaptic efficacy, and neurochemical modulation following MI. MI was produced in guinea pigs by ligation of the coronary artery and associated vein on the dorsal surface of the heart. Animals were recovered for 4, 7, 14, or 50 days. Intracellular voltage recordings were obtained in whole mounts of the cardiac neuronal plexus to determine passive and active neuronal properties of IC neurons. Immunohistochemical analysis demonstrated an immediate and persistent increase in the percentage of IC neurons immunoreactive for neuronal nitric oxide synthase. Examination of individual neuronal properties demonstrated that after hyperpolarizing potentials were significantly decreased in both amplitude and time course of recovery at 7 days post-MI. These parameters returned to control values by 50 days post-MI. Synaptic efficacy, as determined by the stimulation of axonal inputs, was enhanced at 7 days post-MI only. Neuronal excitability in absence of agonist challenge was unchanged following MI. Norepinephrine increased IC excitability to intracellular current injections, a response that was augmented post-MI. Angiotensin II potentiation of norepinephrine and bethanechol-induced excitability, evident in controls, was abolished post-MI. This study demonstrates that MI induces both persistent and transient changes in IC neuronal functions immediately following injury. Alterations in the IC neuronal network, which persist for weeks after the initial insult, may lead to alterations in autonomic signaling and cardiac control.

Network interactions within the canine intrinsic cardiac nervous system: implications for reflex control of regional cardiac function

  The aims of the study were to determine how aggregates of intrinsic cardiac (IC) neurons transduce the cardiovascular milieu versus responding to changes in central neuronal drive and to determine IC network interactions subsequent to induced neural imbalances in the genesis of atrial fibrillation (AF). Activity from multiple IC neurons in the right atrial ganglionated plexus was recorded in eight anaesthetized canines using a 16-channel linear microelectrode array. Induced changes in IC neuronal activity were evaluated in response to: (1) focal cardiac mechanical distortion; (2) electrical activation of cervical vagi or stellate ganglia; (3) occlusion of the inferior vena cava or thoracic aorta; (4) transient ventricular ischaemia, and (5) neurally induced AF. Low level activity (ranging from 0 to 2.7 Hz) generated by 92 neurons was identified in basal states, activities that displayed functional interconnectivity. The majority (56%) of IC neurons so identified received indirect central inputs (vagus alone: 25%; stellate ganglion alone: 27%; both: 48%). Fifty per cent transduced the cardiac milieu responding to multimodal stressors applied to the great vessels or heart. Fifty per cent of IC neurons exhibited cardiac cycle periodicity, with activity occurring primarily in late diastole into isovolumetric contraction. Cardiac-related activity in IC neurons was primarily related to direct cardiac mechano-sensory inputs and indirect autonomic efferent inputs. In response to mediastinal nerve stimulation, most IC neurons became excessively activated; such network behaviour preceded and persisted throughout AF. It was concluded that stochastic interactions occur among IC local circuit neuronal populations in the control of regional cardiac function. Modulation of IC local circuit neuronal recruitment may represent a novel approach for the treatment of cardiac disease, including atrial arrhythmias.

Neural control hierarchy of the heart has not evolved to deal with myocardial ischemia

The consequences of myocardial ischemia are examined from the standpoint of the neural control system of the heart, a hierarchy of three neuronal centers residing in central command, intrathoracic ganglia, and intrinsic cardiac ganglia. The basis of the investigation is the premise that while this hierarchical control system has evolved to deal with "normal" physiological circumstances, its response in the event of myocardial ischemia is unpredictable because the singular circumstances of this event are as yet not part of its evolutionary repertoire. The results indicate that the harmonious relationship between the three levels of control breaks down, because of a conflict between the priorities that they have evolved to deal with. Essentially, while the main priority in central command is blood demand, the priority at the intrathoracic and cardiac levels is heart rate. As a result of this breakdown, heart rate becomes less predictable and therefore less reliable as a diagnostic guide as to the traumatic state of the heart, which it is commonly used as such following an ischemic event. On the basis of these results it is proposed that under the singular conditions of myocardial ischemia a determination of neural control indexes in addition to cardiovascular indexes has the potential of enhancing clinical outcome.

Physiology of spinal cord stimulation: review and update

Spinal cord stimulation (SCS) was an outgrowth of the well-known gate control theory presented by Melzack and Wall in 1965. Although the method has been used to treat chronic severe pain for more than three decades, very little was known about the physiological and biochemical mechanisms behind the beneficial effects until recently. We now know that SCS activates several different mechanisms to treat different types of pain such as neuropathic and ischemic. In general, these mechanisms seem most dependent on activation of only a few segments of the spinal cord. However, both animal studies and human observations have indicated that supraspinal circuits may contribute as well. In the treatment of neuropathic pain, intermittent SCS may give several hours of pain relief after cessation of the stimulation. This protracted effect indicates long-lasting modulation of neural activity involving changes in the local transmitter systems in the dorsal horns. In ischemic pain, animal experiments demonstrate that inhibition of afferent activity in the spinothalamic tracts, long-term suppression of sympathetic activity, and antidromic effects on peripheral reflex circuits may take part in the pain alleviation. Moderate SCS intensities seem to evoke sympathetic inhibition, but higher stimulation intensities may induce antidromically mediated release of vasoactive substances, eg, the calcitonin gene-related peptide (CGRP), resulting in peripheral vasodilation. The anti-ischemic effect of SCS in angina pectoris due to intermittent coronary ischemia probably occurs because application of SCS appears to result in a redistribution of cardiac blood supply, as well as a decrease in tissue oxygen demand. Recent studies indicate that SCS modulates the activity of cardiac intrinsic neurons thereby restricting the arrythmogenic consequences of intermittent local coronary ischemia. The present state of knowledge is briefly reviewed and recent research directions outlined.

Activated cranial cervical cord neurons affect left ventricular infarct size and the potential for sudden cardiac death

To evaluate whether cervical spinal neurons can influence cardiac indices and myocyte viability in the acutely ischemic heart, the hearts of anesthetized rabbits subjected to 30 min of LAD coronary arterial occlusion (CAO) were studied 3h after reperfusion. Control animals were compared to those exposed to pre-emptive high cervical cord stimulation (SCS; the dorsal aspect of the C1-C2 spinal cord was stimulated electrically at 50 Hz; 0.2 ms; 90% of motor threshold, starting 15 min prior to and continuing throughout CAO). Four groups of animals were so tested: 1) neuroaxis intact; 2) prior cervical vagotomy; 3) prior transection of the dorsal spinal columns at C6; and 4) following pharmacological treatment [muscarinic (atropine) or adrenergic (atenolol, prazosin or yohimbine) receptor blockade]. Infarct size (IS) was measured by tetrazolium, expressed as percentage of risk zone. C1-C2 SCS reduced acute ischemia induced IS by 43%, without changing the incidence of sudden cardiac death (SCD). While SCS-induced reduction in IS was unaffected by vagotomy, it was no longer evident following transection of C6 dorsal columns or atropinization. Beta-adrenoceptor blockade eliminated ischemia induced SCD, while alpha-receptor blockade doubled its incidence. During SCS, myocardial ischemia induced SCD was eliminated following vagotomy while remaining unaffected by atropinization. These data indicate that, in contrast to thoracic spinal neurons, i) cranial cervical spinal neurons affect both adrenergic and cholinergic motor outflows to the heart such that ii) their activation modifies ventricular infarct size and lethal arrhythmogenesis.

Reactive oxygen species modulate neuronal excitability in rat intrinsic cardiac ganglia

Reactive oxygen species (ROS) are produced as by-products of oxidative metabolism and occur in the heart during ischemia and coronary artery reperfusion. The effects of ROS on the electrophysiological properties of intracardiac neurons were investigated in the intracardiac ganglion (ICG) plexus in situ and in dissociated neurons from neonatal and adult rat hearts using the whole-cell patch clamp recording configuration. Bath application of ROS donors, hydrogen peroxide (H(2)O(2)) and tert-butyl hydroperoxide (t-BHP) hyperpolarized, and increased the action potential duration of both neonatal and adult ICG neurons. This action was also recorded in ICG neurons in an adult in situ ganglion preparation. H(2)O(2) and t-BHP also inhibited voltage-gated calcium channel (VGCC) currents and shifted the current-voltage (I-V) relationship to more hyperpolarized potentials. In contrast, H(2)O(2) increased the amplitude of the delayed rectifier K(+) current in neonatal ICG neurons. In neonatal ICG neurons, bath application of either superoxide dismutase (SOD) or catalase, scavengers of ROS, prior to H(2)O(2) attenuated the hyperpolarizing shift but not the inhibition of VGCC by H(2)O(2). In contrast, in adult ICG neurons, application of SOD alone had no effect upon either VGCC current amplitude or the I-V relationship, whereas application of SOD prior to H(2)O(2) exposure abolished both the H(2)O(2)-mediated hyperpolarizing shift and inhibition. These data indicate that ROS alter depolarization-activated Ca(2+) and K(+) conductances which underlie neuronal excitability of ICG neurons. This affects action potential duration and therefore probably modifies autonomic control of the heart during ischemia/reperfusion.

Structural remodeling of nucleus ambiguus projections to cardiac ganglia following chronic intermittent hypoxia in C57BL/6J mice

The baroreflex control of heart rate (HR) is reduced following chronic intermittent hypoxia (CIH). Since the nucleus ambiguus (NA) plays a key role in baroreflex control of HR, we examined whether CIH remodels vagal efferent projections to cardiac ganglia. C57BL/6J mice (3-4 months of age) were exposed to either room air (RA) or CIH for 3 months. Confocal microscopy was used to examine NA axons and terminals in cardiac ganglia following Fluoro-Gold (FG) injections to label cardiac ganglia, and microinjections of tracer DiI into the left NA to anterogradely label vagal efferents. We found that: 1) Cardiac ganglia were widely distributed on the dorsal surface of the atria. Although the total number of cardiac ganglia did not differ between RA and CIH mice, the size of ganglia and the somatic area of cardiac principal neurons (PNs) were significantly decreased (P < 0.01), and the size of the PN nuclei was increased following CIH (P < 0.01). 2) NA axons entered cardiac ganglia and innervated PNs with dense basket endings in both RA and CIH mice, and the percentage of innervated PNs was similar (RA: 50 +/- 1.0%; CIH: 49 +/- 1.0%; P > 0.10). In CIH mice, however, swollen cardiac axons and terminals without close contacts to PNs were found. Furthermore, varicose endings around PNs appeared swollen and the axonal varicose area around PNs was almost doubled in size (CIH: 163.1 +/- 6.4 microm(2); RA: 88 +/- 3.9 microm(2), P < 0.01). Thus, CIH significantly altered the structure of cardiac ganglia and resulted in reorganized vagal efferent projections to cardiac ganglia. Such remodeling of cardiac ganglia and vagal efferent projections provides new insight into the effects of CIH on the brain-heart circuitry of C57BL/6J mice.

The action of high K+ and aglycaemia on the electrical properties and synaptic transmission in rat intracardiac ganglion neurones in vitro

We have investigated the action of two elements of acute ischaemia, high potassium and aglycaemia, on the electrophysiological properties and ganglionic transmission of adult rat intracardiac ganglion (ICG) neurones. We used a whole-mount ganglion preparation of the right atrial ganglion plexus and sharp microelectrode recording techniques. Increasing extracellular K⁺ from its normal value of 4.7 mm to 10 mm decreased membrane potential and action potential after-hyperpolarization amplitude but otherwise had no effect on postganglionic membrane properties. It did, however, reduce the ability of synaptically evoked action potentials to follow high-frequency (100 Hz) repetitive stimulation. A further increase in K⁺ changed both the passive and the active membrane properties of the postganglionic neurone: time constant, membrane resistance and action potential overshoot were all decreased in high K⁺ (20 mm). The ICG neurones display a predominantly phasic discharge in response to prolonged depolarizing current pulses. High K⁺ had no impact on this behaviour but reduced the time-dependent rectification response to hyperpolarizing currents. At 20 mm, K⁺ practically blocked ganglionic transmission in most neurones at all frequencies tested. Aglycaemia, nominally glucose-free physiological saline solution (PSS), increased the time constant and membrane resistance of ICG neurones but otherwise had no action on their passive or active properties or ganglionic transmission. However, the combination of aglycaemia and 20 mm K⁺ displayed an improvement in passive properties and ganglionic transmission when compared with 20 mm K⁺ PSS. These data indicate that the presynaptic terminal is the primary target of high extracellular potassium and that aglycaemia may have protective actions against this challenge.

Epicardial fat: properties, function and relationship to obesity

Epicardial fat is a relatively neglected component of the heart. The purpose of this review was to examine the anatomic and biochemical data on epicardial fat; to examine the relationship of epicardial fat to obesity and to explore the potential role of epicardial fat in the relationship of obesity to coronary atherothrombotic disease. Epicardial fat covers 80% of the heart's surface and constitutes 20% of total heart weight. It is present along the distribution of the coronary arteries, over the right ventricle especially along the right border, anterior surface and at the apex. There is three- to fourfold more epicardial fat associated with the right than the left ventricle. Putative physiologic functions of epicardial fat are based on observational data and include: buffering coronary arteries against the torsion induced by the arterial pulse wave and cardiac contraction, facilitating coronary artery remodelling, regulating fatty acid homeostasis in the coronary microcirculation and providing fatty acids to cardiac muscle as a local energy source in times of high demand. A considerable amount of the data on epicardial fat originates from autopsy series that have the inherent problem that conditions leading to death may have altered body composition and adiposity. With this caveat, data indicate that epicardial fat mass increases age until age 20-40 years but thereafter the amount of epicardial fat is not dependent on age. The amount of epicardial fat correlates with heart weight but the presence of myocardial ischemia and hypertrophy does not alter the ratio of epicardial fat to cardiac muscle mass. A number of properties differentiate epicardial fat from other fat depots specifically its smaller adipocytes size; different fatty acid composition, high protein content; high rates of fatty acid incorporation, fatty acid synthesis, insulin-induced lipogenesis or fatty acid breakdown; low rates of glucose utilization, low expression (mRNA) of lipoprotein lipase, stearoyl-CoA desaturase and acetyl-CoA carboxylase-alpha, and slow regression during weight loss. There is a significant direct relationship between the amount of epicardial fat and general body adiposity. Clinical imaging studies have demonstrated a strong direct correlation between epicardial fat and abdominal visceral adiposity. Several lines of evidence support a role for epicardial fat in the pathogenesis of coronary artery disease, namely the close anatomic relationship between epicardial fat and coronary arteries; the positive correlation between the amount of epicardial fat and the presence of coronary atherosclerosis and the ability of adipose tissue to secrete hormones and cytokines that modulate coronary artery atherothrombosis. Thus, epicardial fat maybe an important factor responsible for cardiovascular disease in obesity.

Stochastic behavior of atrial and ventricular intrinsic cardiac neurons

To quantify the concurrent transduction capabilities of spatially distributed intrinsic cardiac neurons, the activities generated by atrial vs. ventricular intrinsic cardiac neurons were recorded simultaneously in 12 anesthetized dogs at baseline and during alterations in the cardiac milieu. Few (3%) identified atrial and ventricular neurons (2 of 72 characterized neurons) responded solely to regional mechanical deformation, doing so in a tightly coupled fashion (cross-correlation coefficient r = 0.63). The remaining (97%) atrial and ventricular neurons transduced multimodal stimuli to display stochastic behavior. Specifically, ventricular chemosensory inputs modified these populations such that they generated no short-term coherence among their activities (cross-correlation coefficient r = 0.21 +/- 0.07). Regional ventricular ischemia activated most atrial and ventricular neurons in a noncoupled fashion. Nicotinic activation of atrial neurons enhanced ventricular neuronal activity. Acute decentralization of the intrinsic cardiac nervous system obtunded its neuron responsiveness to cardiac sensory stimuli. Most atrial and ventricular intrinsic cardiac neurons generate concurrent stochastic activity that is predicated primarily upon their cardiac chemotransduction. As a consequence, they display relative independent short-term (beat-to-beat) control over regional cardiac indexes. Over longer time scales, their functional interdependence is manifest as the result of interganglionic interconnections and descending inputs.

Thoracic spinal cord stimulation reduces the risk of ischemic ventricular arrhythmias in a postinfarction heart failure canine model

BACKGROUND

Thoracic spinal cord stimulation (SCS) is a promising therapy in treating refractory angina. This study was designed to investigate SCS with regard to the risk of arrhythmias during myocardial ischemia and its cardiac electrophysiological effects.

METHODS AND RESULTS

We studied 22 dogs with healed anterior myocardial infarction (MI) and superimposed heart failure (HF) induced by rapid ventricular pacing. SCS was applied at the dorsal T1-T2 segments of the spinal cord (at 50 Hz, 0.2 ms) for 15 minutes. Transient (2-minute) myocardial ischemia was induced on 2 separate occasions (no SCS and SCS) to provoke ventricular arrhythmias (ventricular tachycardia/ventricular fibrillation; VT/VF). Ischemic episodes were separated by 90 minutes, and dogs were randomly assigned to receive SCS or no SCS before the first or second ischemic episode. SCS reduced the occurrence of VT/VF from 59% to 23% when SCS was applied during transient myocardial ischemia (odds ratio, 0.36; 95% confidence interval, 0.1626 to 0.5646; P=0.0009). SCS also decreased sinus rate by 7.5+/-14 bpm (P=0.048), increased the PR interval by 11.1+/-14.7 ms (P=0.009), and reduced systolic blood pressure by 9.8+/-13.6 mm Hg (P=0.02).

CONCLUSIONS

Thoracic SCS appears to protect against ischemic VT/VF in a canine model of healed MI and HF. SCS reduced sinus rate and systolic blood pressure, changes consistent with the previously known antisympathetic effect of SCS, which may have contributed to the antiarrhythmic benefits.

Intrinsic cardiac nervous system in tachycardia induced heart failure

The purpose of this study was to test the hypothesis that early-stage heart failure differentially affects the intrinsic cardiac nervous system's capacity to regulate cardiac function. After 2 wk of rapid ventricular pacing in nine anesthetized canines, cardiac and right atrial neuronal function were evaluated in situ in response to enhanced cardiac sensory inputs, stimulation of extracardiac autonomic efferent neuronal inputs, and close coronary arterial administration of neurochemicals that included nicotine. Right atrial neuronal intracellular electrophysiological properties were then evaluated in vitro in response to synaptic activation and nicotine. Intrinsic cardiac nicotine-sensitive, neuronally induced cardiac responses were also evaluated in eight sham-operated, unpaced animals. Two weeks of rapid ventricular pacing reduced the cardiac index by 54%. Intrinsic cardiac neurons of paced hearts maintained their cardiac mechano- and chemosensory transduction properties in vivo. They also responded normally to sympathetic and parasympathetic preganglionic efferent neuronal inputs, as well as to locally administered alpha-or beta-adrenergic agonists or angiotensin II. The dose of nicotine needed to modify intrinsic cardiac neurons was 50 times greater in failure compared with normal preparations. That dose failed to alter monitored cardiovascular indexes in failing preparations. Phasic and accommodating neurons identified in vitro displayed altered intracellular membrane properties compared with control, including decreased membrane resistance, indicative of reduced excitability. Early-stage heart failure differentially affects the intrinsic cardiac nervous system's capacity to regulate cardiodynamics. While maintaining its capacity to transduce cardiac mechano- and chemosensory inputs, as well as inputs from extracardiac autonomic efferent neurons, intrinsic cardiac nicotine-sensitive, local-circuit neurons differentially remodel such that their capacity to influence cardiodynamics becomes obtunded.

Adenosine A1 receptor activation reduces myocardial reperfusion effects on intrinsic cardiac nervous system

The intrinsic cardiac nervous system is the final common integrator of regional cardiac function. The ischemic myocardium modifies this nervous system. We sought to determine the role that intrinsic cardiac neuronal P(1) purinergic receptors play in transducing myocardial ischemia and the subsequent reperfusion. The activity generated by ventricular neurons was recorded concomitant with cardiac hemodynamic variables in 44 anesthetized pigs. Regional ventricular ischemia was induced by briefly occluding (30 s) the ventral interventricular coronary artery distal to the arterial blood supply of identified ventricular neurons. Adenosine (100 microM) was administered to these neurons via their local arterial blood supply during or immediately after transient coronary artery occlusion. Occlusion was also performed following local administration of adenosine A(1) [8-cyclopentyl-1,3-dipropylxanthine (DPCPX)] or A(2) [3,7-dimethyl-1-propargylxanthine (DMPX)] receptor blocking agents. The activity generated by ventricular neurons was modified by transient coronary artery occlusion and the subsequent reperfusion (||Delta|| 112 +/- 14 and 168 +/- 34 impulses/min, respectively; P < 0.01 vs. preischemic states). Locally administered adenosine attenuated neuronal responses to reperfusion (-75%; P < 0.01 compared with normal reperfusion) but not ischemia. The neuronal stabilizing effects that adenosine elicited during reperfusion persisted in the presence of DMPX but not DPCPX. It is concluded that activation of neuronal adenosine A(1) receptors stabilizes the intrinsic cardiac nervous system during reperfusion.

Neural control of the heart: developmental changes in ionic conductances in mammalian intrinsic cardiac neurons

The expression and properties of ionic channels were investigated in dissociated neurons from neonatal and adult rat intracardiac ganglia. Changes in the hyperpolarization-activated and ATP-sensitive K+ conductances during postnatal development and their role in neuronal excitability were examined. The hyperpolarization-activated nonselective cation current, Ih, was observed in all neurons studied and displayed slow time-dependent rectification. An inwardly rectifying K+ current, IK(IR), was present in a population of neurons from adult but not neonatal rats and was sensitive to block by extracellular Ba2+ Using the perforated-patch recording configuration, an ATP-sensitive K+ (KATP) conductance was identified in > or = 50% of intracardiac neurons from adult rats. Levcromakalim evoked membrane hyperpolarization, which was inhibited by the sulphonylurea drugs, glibenclamide and tolbutamide. Exposure to hypoxic conditions also activated a membrane current similar to that induced by levcromakalim and was inhibited by glibenclamide. Changes in the complement of ion channels during postnatal development may underlie observed differences in the function of intracardiac ganglion neurons during maturation. Furthermore, activation of hyperpolarization-activated and KATP channels in mammalian intracardiac neurons may play a role in neural regulation of the mature heart and cardiac function during ischaemia-reperfusion.

Long-term modulation of the intrinsic cardiac nervous system by spinal cord neurons in normal and ischaemic hearts

Electrical excitation of the dorsal aspect of the rostral thoracic spinal cord imparts long-term therapeutic benefits to patients with angina pectoris. Such spinal cord stimulation also induces short-term suppressor effects on the intrinsic cardiac nervous system. The purpose of this study was to determine whether spinal cord stimulation (SCS) induces long-term effects on the intrinsic nervous system, particularly in the presence of myocardial ischaemia. The activity generated by right atrial neurons was recorded in 10 anesthetized dogs during basal states, during prolonged (15 min) occlusion of the left anterior descending coronary artery, and during the subsequent reperfusion phase. Neuronal activity and cardiovascular indices were also monitored when the dorsal T1-T4 segments of the spinal cord were stimulated electrically (50 Hz; 0.2 ms) at an intensity 90% of motor threshold (mean 0.32 mA) for 17 min. SCS was performed before, during and after 15-min periods of regional ventricular ischaemia. Occlusion of a major coronary artery, one that did not perfuse investigated neurons, resulted in their excitation. Ischaemia-induced neuronal excitatory effects were suppressed (-76% from baseline) by SCS. SCS suppression of intrinsic cardiac neuronal activity persisted during the subsequent reperfusion period; after terminating 17 min of SCS, at least 20 min elapsed before intrinsic cardiac neuronal activity returned to baseline values. It is concluded that populations of intrinsic cardiac neurons are activated by inputs arising from the ischaemic myocardium. Ischaemia-induced activation of these neurons is nullified by SCS. The neuronal suppressor effects that SCS induces persist not only during reperfusion, but also for an extended period of time thereafter. These long-term effects may account, in part, for the fact that SCS imparts clinical benefit to patients with angina of cardiac origin not only during its application, but also for a time thereafter.

Effects of chronic cardiac decentralization on functional properties of canine intracardiac neurons in vitro

Although intrinsic cardiac neurons display ongoing activity after chronic interruption of extrinsic autonomic inputs to the heart, the effects of decentralization on individual neurons remain unknown. The objective of this study was to determine the effects of chronic (3-4 wk) surgical decentralization on intracellular properties of, and neurotransmission among, neurons contained within the canine intrinsic right atrial ganglionated plexus in vitro. Properties of neurons from decentralized hearts were compared with those of neurons from sham-operated hearts (controls). Two populations of neurons were identified by their firing behavior in response to intracellular current injection. Fifty-nine percent of control neurons and 72% of decentralized neurons were phasic (discharged one action potential on excitation). Forty-one percent of control neurons and 27% of decentralized neurons were accommodating (multiple discharge with decrementing frequency). After chronic decentralization, input resistance of phasic neurons increased, whereas the duration of afterhyperpolarization of accommodating neurons decreased. Postsynaptic responses to interganglionic nerve stimulation were evoked in 89% of control neurons and 83% of decentralized neurons; the majority of these responses involved nicotinic receptors. These results show that, after chronic decentralization, intrinsic cardiac neurons 1) undergo changes in membrane properties that may lead to increased excitability while 2) maintaining synaptic neurotransmission within the intrinsic cardiac ganglionated plexus.

An ATP-sensitive K(+) conductance in dissociated neurones from adult rat intracardiac ganglia

1. An ATP-sensitive K(+) (K(ATP)) conductance has been identified using the perforated patch recording configuration in a population (52%) of dissociated neurones from adult rat intracardiac ganglia. The presence of the sulphonylurea receptor in approximately half of the intracardiac neurones was confirmed by labelling with fluorescent glibenclamide-BODIPY. 2. Under current clamp conditions in physiological solutions, levcromakalim (10 microM) evoked a hyperpolarization, which was inhibited by the sulphonylurea drugs glibenclamide and tolbutamide. 3. Under voltage clamp conditions in symmetrical (140 mM) K(+) solutions, bath application of levcromakalim evoked an inward current with a density of 8 pA pF(-1) at -50 mV and a slope conductance of approximately 9 nS, which reversed close to the potassium equilibrium potential (E(K)). Cell dialysis with an ATP-free intracellular solution also evoked an inward current, which was inhibited by tolbutamide. 4. Bath application of either glibenclamide (10 microM) or tolbutamide (100 microM) depolarized adult intracardiac neurones by 3-5 mV, suggesting that a K(ATP) conductance is activated under resting conditions and contributes to the resting membrane potential. 5. Activation of a membrane current by levcromakalim was concentration dependent with an EC(50) of 1.6 microM. Inhibition of the levcromakalim-activated current by glibenclamide was also concentration dependent with an IC(50) of 55 nM. 6. Metabolic inhibition with 2,4-dinitrophenol and iodoacetic acid or superfusion with hypoxic solution (P(O2) approximately 16 mmHg) also activated a membrane current. These currents exhibited similar I-V characteristics to the levcromakalim-induced current and were inhibited by glibenclamide. 7. Activation of K(ATP) channels in mammalian intracardiac neurones may contribute to changes in neural regulation of the mature heart and cardiac function during ischaemia-reperfusion.

Pathology of intrinsic cardiac neurons from ischemic human hearts

Various populations of intrinsic cardiac neurons influence regional cardiac function tonically. It is not known whether such neurons are affected by disease states and, if so, in what manner. Therefore, the morphology of intrinsic cardiac ganglia obtained from patients with angiographic evidence of compromised regional coronary blood supply was studied. Posterior atrial ganglia and surrounding fat, removed at the time of cardiac surgery, were placed immediately in saline and within 15-120 min (average of about 40 min) in 0.5% paraformaldehyde/2.5% glutaraldehyde. In 32 studied ganglia, 35% of 473 intrinsic cardiac neurons displayed striking pathological changes at the light and ultrastructural level. The other cells displayed normal morphology. The cytoplasm of 74% of the abnormal cells had one or more of three types of inclusions: (1) darkly stained lamellated inclusions (Type I), (2) membrane-bound whorls and parallel arrays of lightly stained membranes, as well as fine granular material (Type II), or (3) concentric layers of lightly stained membranes with a darker, granular core (Type III). Neurons with inclusions were markedly enlarged (66 x 54 microm vs. 40 x 34 microm for normal neurons) and displayed fewer dendrites. Some neurons contained electron lucent vacuoles indicative of degeneration while others showed frank degeneration, being fragmented, shrunken, and misshapen. Phagocytic cells containing lamellated inclusions and cellular debris were found in ganglia with abnormal neurons. Some axon terminals also displayed degenerative changes. The identification of pathological changes in the human intrinsic cardiac nervous system has implications with respect to the functional integrity of this final common regulator of cardiac function in disease states.

Sensitivity of canine intrinsic cardiac neurons to H2O2 and hydroxyl radical

To determine whether intrinsic cardiac neurons are sensitive to oxygen-derived free radicals in situ, studies were performed in 44 open-chest anesthetized dogs. 1) When H2O2 (600 microM) was administered to right atrial neurons of 36 dogs via their local arterial blood supply, neuronal activity either increased (+92% in 16 dogs) or decreased (-61% in 20 dogs), depending on the population of neurons studied. H2O2 (600 microM) administered into the systemic circulation did not affect neuronal activity, measured cardiac indexes, or aortic pressure. 2) The iron-chelating agent deferoxamine (20 mg/kg iv), a chemical that prevents the formation of oxygen-derived free radicals, reduced the activity generated by neurons (-57%) in 8 of 10 dogs. 3) H2O2 did not affect neuronal activity when administered in the presence of deferoxamine in these 10 dogs. 4) When the ATP-sensitive potassium (KATP) channel opener cromakalim (20 microM) was administered to intrinsic cardiac neurons in another 21 animals via their regional arterial blood supply, ongoing neuronal activity in 15 of these dogs decreased by 54%. 5) Neuronal activity was not affected by H2O2 when administered in the presence of cromakalim in 16 dogs. These data indicate that 1) some intrinsic cardiac neurons are sensitive to exogenous H2O2, 2) such neurons are tonically influenced by locally produced oxygen-derived free radicals in situ, and 3) intrinsic cardiac neurons possess KATP channels that are functionally important during oxidative challenge.

Differential selectivity of cardiac neurons in separate intrathoracic autonomic ganglia

Analyses of activity generated by neurons in middle cervical or stellate ganglia versus intrinsic cardiac ganglia were performed to determine how neurons in different intrathoracic ganglia, which are involved in cardiac regulation, interact. Discharges of 19% of intrathoracic extracardiac neurons and 32% of intrinsic cardiac neurons were related to cardiodynamics. Epicardial touch increased the activity generated by approximately 80% of intrinsic cardiac neurons and approximately 60% of extracardiac neurons. Both populations responded similarly to epicardial chemical stimuli. Activity generated by neurons in intrinsic cardiac ganglia demonstrated no consistent short-term relationships to neurons in extracardiac ganglia. Myocardial ischemia influenced extracardiac and intrinsic cardiac neurons similarly. Carotid artery baroreceptors influenced neurons in ipsilateral extracardiac ganglia. After decentralization from the central nervous system, intrinsic cardiac neurons received afferent inputs primarily from cardiac chemosensitive neurites, whereas middle cervical ganglion neurons received afferent inputs primarily from cardiac mechanosensory neurites. It is concluded that the populations of neurons in different intrathoracic ganglia can display differential reflex control of cardiac function. Their redundancy in function and noncoupled behavior minimizes cardiac dependency on a single population of intrathoracic neurons.