Parasympathetic Input Research Articles

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<i>Circulation</i> Editors’ Picks

The autonomic nervous system regulates postprandial hepatic lipid metabolism

The liver is a key organ in controlling glucose and lipid metabolism during feeding and fasting. In addition to hormones and nutrients, inputs from the autonomic nervous system are also involved in fine-tuning hepatic metabolic regulation. Previously, we have shown in rats that during fasting an intact sympathetic innervation of the liver is essential to maintain the secretion of triglycerides by the liver. In the current study, we hypothesized that in the postprandial condition the parasympathetic input to the liver inhibits hepatic VLDL-TG secretion. To test our hypothesis, we determined the effect of selective surgical hepatic denervations on triglyceride metabolism after a meal in male Wistar rats. We report that postprandial plasma triglyceride concentrations were significantly elevated in parasympathetically denervated rats compared with control rats (P = 0.008), and VLDL-TG production tended to be increased (P = 0.066). Sympathetically denervated rats also showed a small rise in postprandial triglyceride concentrations (P = 0.045). On the other hand, in rats fed on a six-meals-a-day schedule for several weeks, a parasympathetic denervation resulted in >70% higher plasma triglycerides during the day (P = 0.001), whereas a sympathetic denervation had no effect. Our results show that abolishing the parasympathetic input to the liver results in increased plasma triglyceride levels during postprandial conditions.

Psychophysiologic Remodeling of the Failing Human Heart

Autonomic imbalance is a therapeutic target in heart failure patients. Overactivation of the sympathetic branch of the autonomic nervous system has for years been treated with beta-adrenergic blocking drugs, although it cannot be long before invasive therapies such as renal sympathetic denervation, currently being tested for resistant hypertension, make their way to the heart failure arena. Insufficient parasympathetic input to the heart is already being augmented with implanted vagal nerve stimulators. It is our contention that biofeedback training could provide much of the same benefit as these more invasive therapies in improving autonomic imbalance, with the added advantage of increasing patient self-efficacy. This article describes a pilot study of biofeedback training in patients with end-stage heart failure awaiting heart transplantation. It was our goal to show not only that patients would appreciate this training and benefit from it but also that patient-controlled autonomic modulation could actually reverse the cellular and molecular markers of heart failure, or remodel the failing heart, similar to what we had previously shown for other types of heart failure therapy such as the left ventricular assist device. To our knowledge, this is the first study to demonstrate changes in the biology of the failing heart in response to a psychophysiologic intervention.

Nonlinear Methods to Assess Changes in Heart Rate Variability in Type 2 Diabetic Patients

BackgroundHeart rate variability (HRV) is an important indicator of autonomic modulation of cardiovascular function. Diabetes can alter cardiac autonomic modulation by damaging afferent inputs, thereby increasing the risk of cardiovascular disease. We applied nonlinear analytical methods to identify parameters associated with HRV that are indicative of changes in autonomic modulation of heart function in diabetic patients. ObjectiveWe analyzed differences in HRV patterns between diabetic and age-matched healthy control subjects using nonlinear methods. MethodsLagged Poincaré plot, autocorrelation, and detrended fluctuation analysis were applied to analyze HRV in electrocardiography (ECG) recordings. ResultsLagged Poincare plot analysis revealed significant changes in some parameters, suggestive of decreased parasympathetic modulation. The detrended fluctuation exponent derived from long-term fitting was higher than the short-term one in the diabetic population, which was also consistent with decreased parasympathetic input. The autocorrelation function of the deviation of inter-beat intervals exhibited a highly correlated pattern in the diabetic group compared with the control group. ConclusionThe HRV pattern significantly differs between diabetic patients and healthy subjects. All three statistical methods employed in the study may prove useful to detect the onset and extent of autonomic neuropathy in diabetic patients.

The Anatomical and Functional Relationship between Allergic Conjunctivitis and Allergic Rhinitis

There are numerous anatomic connections between the allergic conjunctivitis and allergic rhinitis. The most obvious reason is the physical connection via the nasolacrimal apparatus. However, a closer look at innervation, circulatory, lymphatic, and neurogenic systems reveals much more than a physical connection. The eye is richly innervated by parasympathetic nerves that enter the eyes after traveling in conjunction with the parasympathetic input to the nasal cavity. Parasympathetic innervation governing the tear film and nasal secretion can intersect at the pterygopalatine ganglion. Neurogenic inflammation affects both the eye and the nose as evidenced by the presence of the same neurogenic factors. Venous flow is in the SOV area connecting the eye and the nose, once thought to be without valves. In the past, this thinking is the basis for concern about the danger triangle of the face. Recent literature has shown otherwise. Although valves are present, there are still pathways where bidirectional flow exists and a venous connection is made. The most likely area for venous communication is the pterygoid plexus and cavernous sinus. The venous flow and connections also offers a pathway for allergic shiners. Understanding the mutual connections between the nasal mucosa and the ocular surface can also affect treatment strategies.

Noninvasive in vivo model demonstrating the effects of autonomic innervation on pancreatic islet function

The autonomic nervous system is thought to modulate blood glucose homeostasis by regulating endocrine cell activity in the pancreatic islets of Langerhans. The role of islet innervation, however, has remained elusive because the direct effects of autonomic nervous input on islet cell physiology cannot be studied in the pancreas. Here, we used an in vivo model to study the role of islet nervous input in glucose homeostasis. We transplanted islets into the anterior chamber of the eye and found that islet grafts became densely innervated by the rich parasympathetic and sympathetic nervous supply of the iris. Parasympathetic innervation was imaged intravitally by using transgenic mice expressing GFP in cholinergic axons. To manipulate selectively the islet nervous input, we increased the ambient illumination to increase the parasympathetic input to the islet grafts via the pupillary light reflex. This reduced fasting glycemia and improved glucose tolerance. These effects could be blocked by topical application of the muscarinic antagonist atropine to the eye, indicating that local cholinergic innervation had a direct effect on islet function in vivo. By using this approach, we found that parasympathetic innervation influences islet function in C57BL/6 mice but not in 129X1 mice, which reflected differences in innervation densities and may explain major strain differences in glucose homeostasis. This study directly demonstrates that autonomic axons innervating the islet modulate glucose homeostasis.

Investigation of the mechanisms contributing to the compensatory increase in insulin secretion during dexamethasone-induced insulin resistance in rhesus macaques

Dexamethasone has well-described effects to induce insulin resistance and increase insulin secretion. Herein, we examined potential contributors to the effect of dexamethasone to increase insulin secretion in rhesus macaques. Six male rhesus macaques received daily injections of either saline or dexamethasone (0.25 mg/kg i.m. for 7 days) in random order with 3 weeks between treatments. At the end of the treatment period, animals were fasted overnight and underwent a feeding study the next day, during which blood samples were taken before and for 60 min after a meal in order to assess islet hormone and incretin secretion. Dexamethasone induced marked increases in fasting plasma insulin, glucagon, leptin, and adiponectin concentrations (P<0.05). Surprisingly, the glycemic response after meal ingestion was decreased twofold during dexamethasone treatment (P<0.05). Dexamethasone-treated animals exhibited a significant increase in both insulin and glucose-dependent insulinotropic polypeptide (GIP) secretion during the feeding study (P<0.05). However, glucagon-like peptide-1 secretion was significantly lower in dexamethasone-treated animals compared with controls (P<0.01). Fasting and meal-stimulated pancreatic polypeptide concentrations (an index of the parasympathetic input to the islet) did not differ between saline and dexamethasone treatments. However, the proinsulin:insulin ratio was decreased throughout the feeding study with dexamethasone treatment suggesting an improvement of β-cell function (P<0.05). In conclusion, the maintenance of euglycemia and reduction of postprandial glycemia with short-term dexamethasone treatment appears to be due to the marked elevations of fasting and meal-stimulated insulin secretion. Furthermore, increases in postprandial GIP secretion with dexamethasone treatment appear to contribute to the effect of dexamethasone treatment to increase insulin secretion.

Parkinson’s disease is not associated with gastrointestinal myenteric ganglion neuron loss

Gastrointestinal dysfunction is a prominent non-motor feature of Parkinson's disease (PD) that contributes directly to the morbidity of patients, complicates management of motor symptoms, and may herald incipient PD in patients without motor disability. Although PD has traditionally been considered a disease of dopaminergic neurons in the substantia nigra, analyses of gastrointestinal samples from PD patients have consistently revealed pathology in the enteric nervous system. The relationship of PD pathology to GI dysmotility is poorly understood, and this lack of understanding has led to limited success in developing treatments for PD-related GI symptoms. We have quantitatively compared myenteric neuron density and relative abundance of NO, VIP, and catecholamine neurons between patients with PD and control individuals along the length of the GI tract. In addition, we have examined the frequency of GI α-synuclein neuritic pathology and its co-localization with the same neuronal markers. We have included a comparison with a small population of patients with incidental Lewy bodies found at autopsy. These data indicate that there is no neuronal loss in the myenteric plexus in PD. Lewy body pathology parallels parasympathetic autonomic input from the dorsal motor nucleus of the vagus, not the distribution of extrinsic sympathetic input or intrinsic enteric neurons, and is only rarely co-localized with tyrosine hydroxylase. These data provide a critical background to which further analyses of the effect of PD on the GI tract may be compared and suggest that neuropathology in myenteric neurons is unlikely to be a causative factor in PD-related GI dysmotility.

Vagal Nerve Stimulation Blocks Peritoneal Macrophage Inflammatory Responsiveness After Severe Burn Injury

Large surface area burn injuries lead to activation of the innate immune system, which can be blocked by parasympathetic inputs mediated by the vagus nerve. We hypothesized that vagal nerve stimulation (VNS) would alter the inflammatory response of peritoneal macrophages after severe burn injury. Male BALB/c mice underwent right cervical VNS before 30% total body surface area steam burn and were compared with animals subjected to burn alone. Peritoneal macrophages were harvested at several time points following injury and exposed to lipopolysaccharide (LPS) in culture conditions. The inflammatory response of peritoneal macrophages was measured by analyzing changes in nuclear factor κB p65 phosphorylation using flow cytometry. We found that peritoneal macrophages isolated from mice subjected to burn injury were hyperresponsive to LPS challenge, suggesting burn-induced macrophage activation. We identified a protective role for VNS in blocking peritoneal macrophage activation. Analysis of the phosphorylation state of nuclear factor κB pathway mediator, p65 Rel A, revealed a VNS-mediated reduction in p65 phosphorylation levels after exposure to LPS compared with burn alone. In combination, these studies suggest VNS mediates the inflammatory response in peritoneal macrophages by affecting the set point of LPS responsiveness.

Autonomic boundary conditions for ventricular fibrillation and their implications for a novel defibrillation technique

The sympathetic and parasympathetic divisions of the autonomic nervous system modulate cardiac rhythm and the probability of arrhythmia occurrence. Both increased sympathetic drive and hypoxia increase the likelihood for ventricular fibrillation (VF). Vagus nerve stimulation (VNS) can protect from fatal arrhythmias via cholinergic and nitrergic action. We sought to determine boundary conditions for VF and defibrillation by autonomic manipulations accompanied or not by hypoxic changes in urethane-anesthetized rats. VF was induced with (1) vagotomy, (2) systemic high-dose (>15 mg/kg) isoproterenol, and (3) hypoxemia. When VNS (50 Hz) produced cardiac standstill, it converted every VF episode (59/59). A nitric oxide synthase inhibitor did not reduce VNS efficacy (13/14 episodes converted), but addition of atropine reduced VNS efficacy (11/27 episodes converted). VF can be induced by autonomic derangements only under constrained conditions, including sympathetic over-activation, reduced parasympathetic input, and hypoxemia. VNS can provide an alternative method to defibrillate via its cholinergic action.

Parasympathetic influences on emmetropization in chicks: Evidence for different mechanisms in form deprivation vs negative lens-induced myopia

Ciliary ganglionectomy inhibits the development of myopia in chicks (Schmid et al., 1999), but has no effect on the compensatory responses to spectacle lenses (Schmid and Wildsoet, 1996). This study was done to assess the potential influence of the other parasympathetic input to the choroid, the pterygopalatine ganglia, on the choroidal and axial responses to retinal defocus, and to form deprivation.4–5 week-old chicks had one of the following surgeries to one eye: (1) Section (X) of the autonomic part of cranial N VII (input to the pterygopalatine ganglia) (PPGX, n = 16), (2) PPGX plus ciliary ganglionectomy (PPG/CGX, n = 23) or (3) PPGX plus superior cervical ganglionectomy (PPG/SCGX, n = 10). Experimental eyes were fitted with positive or negative lenses, or diffusers, several days after surgery. In one group of PPG/CGX, eyes did not wear any devices (n = 8). Intact (no surgery) controls were done for all visual manipulations (lenses or diffusers). Sham surgeries were done for the PPG/CGX condition (n = 4). Ocular dimensions were measured using A-scan ultrasonography prior to the surgery, 5 days later when visual devices were placed on the eyes, at the end of lens- or diffuser-wear, and in the case of diffusers, 4 days after diffuser removal to look at “recovery”. Refractive errors were measured using a Hartinger's refractometer. IOP was measured in 7 PPG/CGX birds 7d after surgery.PPGX/CGX resulted in choroidal thickening (125 μm) and a decrease in IOP over one week post-surgery. It also prevented the development of myopia in response to form deprivation (X vs intact: 0.2 D vs −4.1 D; p < 0.005), by preventing the increase in axial elongation (250 μm vs 670 μm/5d; p < 0.005). In fact, growth rate slowed below normal (X vs fellow eyes: 250 μm vs 489 μm/5d; p = 0.002). By contrast, there were no effects of this lesion on the development of myopia in response to negative lenses (X vs intact: −5.4 D vs −5.3 D). All three lesions inhibited the compensatory choroidal thickening in response to myopic defocus (ANOVA, p = 0.0008), but had no effect on the thinning response to hyperopic defocus. These results argue for different underlying mechanisms for the growth responses to form deprivation vs negative lens wear. They also imply that choroidal thickening and thinning are not opposing elements of a single mechanism.

The role of self‐regulation in modulating the heart‐brain connection

Cardiovascular disease is characterized by autonomic imbalance. The sympathetic nervous system is activated in order to compensate for declining cardiac function, but the increased activation actually has deleterious effects over time. Mortality in end‐stage heart failure is predicted by elevated plasma norepinephrine levels and a decrease in overall heart rate variability. Beta adrenergic blocking drugs are increasingly used to prevent the cardiac remodeling which follows myocardial infarction or other stresses to the myocardium. Together with sympathetic overdrive, the parasympathetic branch of the autonomic nervous system is correspondingly down‐regulated, and it is now clear that decreased parasympathetic input to the cardiovascular system may be equally harmful, promoting a pro‐inflammatory state which further exacerbates disease. Biofeedback‐mediated stress management is a method for training individuals to control the balance of sympathetic and parasympathetic nervous system activation. Such training can be used to increase parasympathetic and decrease sympathetic activation, with corresponding effects on remodeling and disease progression. Work in our research program focuses on testing the hypothesis that patients with cardiovascular disease can be taught to improve autonomic balance, and that this improvement will result in decreased symptoms, improved quality of life, and possibly even disease reversal. Patient populations which are currently being studied and which will be discussed include those with end‐stage heart failure awaiting transplantation and those with documented coronary artery disease.

Highlights in basic autonomic neurosciences: Epileptic seizure and sudden death

People with epilepsy die unexpectedly for unknown pathological reasons at a rate 24 times greater than the general population. Such deaths are classified as sudden unexplained death in epilepsy (SUDEP). The etiology of the SUDEP is frequently unclear which prevents the development of a rational treatment of this problem. Mice lacking Kv1.1 Shaker-like potassium channels encoded by the Kcna1 gene exhibit severe seizures and die prematurely. In this article the authors postulated that Kv1.1-potassium deficiency may underlie primary neurogenic cardiac dysfunction and cause SUDEP. EEG–ECG recordings showed that Kcna1-null mice display atrioventricular (AV) conduction blocks, as well as bradycardia and premature ventricular contractions. During seizures the occurrence of AV conduction blocks increased, which might cause SUDEP. Then the authors used atropine and propranolol to block the autonomic nervous system and found that atropine ameliorated AV conduction block indicating excessive parasympathetic tone may cause a neurocardiac defect. There were no changes in Kv1.1-deficient cardiac structure, but extensive Kv1.1 expression in juxtaparanodes of the wild-type vagus nerve, the primary source of parasympathetic input to the heart. The authors concluded that Kv1.1 deficiency leads to impaired neural control of cardiac rhythmicity due in part to aberrant parasympathetic neurotransmission, making Kcna1 a strong candidate gene for causing human SUDEP.

Effects of Acetylcholine and Noradrenalin on Action Potentials of Isolated Rabbit Sinoatrial and Atrial Myocytes

The autonomic nervous system controls heart rate and contractility through sympathetic and parasympathetic inputs to the cardiac tissue, with acetylcholine (ACh) and noradrenalin (NA) as the chemical transmitters. In recent years, it has become clear that specific Regulators of G protein Signaling proteins (RGS proteins) suppress muscarinic sensitivity and parasympathetic tone, identifying RGS proteins as intriguing potential therapeutic targets. In the present study, we have identified the effects of 1 μM ACh and 1 μM NA on the intrinsic action potentials of sinoatrial (SA) nodal and atrial myocytes. Single cells were enzymatically isolated from the SA node or from the left atrium of rabbit hearts. Action potentials were recorded using the amphotericin-perforated patch-clamp technique in the absence and presence of ACh, NA, or a combination of both. In SA nodal myocytes, ACh increased cycle length and decreased diastolic depolarization rate, whereas NA decreased cycle length and increased diastolic depolarization rate. Both ACh and NA increased maximum upstroke velocity. Furthermore, ACh hyperpolarized the maximum diastolic potential. In atrial myocytes stimulated at 2 Hz, both ACh and NA hyperpolarized the maximum diastolic potential, increased the action potential amplitude, and increased the maximum upstroke velocity. Action potential duration at 50 and 90% repolarization was decreased by ACh, but increased by NA. The effects of both ACh and NA on action potential duration showed a dose dependence in the range of 1–1000 nM, while a clear-cut frequency dependence in the range of 1–4 Hz was absent. Intermediate results were obtained in the combined presence of ACh and NA in both SA nodal and atrial myocytes. Our data uncover the extent to which SA nodal and atrial action potentials are intrinsically dependent on ACh, NA, or a combination of both and may thus guide further experiments with RGS proteins.

PS16 - 79. The autonomic nervous system and lipid metabolism during feeding

We have previously shown that during fasting an intact hypothalamic NPY system and the sympathetic nervous system innervating the liver are important for maintaining VLDL-triglyceride secretion. Interestingly, when mimicking the fed state by injecting glucose centrally, VLDLtriglyceride secretion is inhibited which depends on an intact parasympathetic hepatic innervation. We thus hypothesize that during feeding the parasympathetic input to the liver is of physiological importance. We, therefore, determined the effect of a sympathetic or parasympathetic hepatic denervation on triglyceride metabolism during a regular feeding paradigm.

The Edinger‐Westphal nucleus: A historical, structural, and functional perspective on a dichotomous terminology

The eponymous term nucleus of Edinger-Westphal (EW) has come to be used to describe two juxtaposed and somewhat intermingled cell groups of the midbrain that differ dramatically in their connectivity and neurochemistry. On one hand, the classically defined EW is the part of the oculomotor complex that is the source of the parasympathetic preganglionic motoneuron input to the ciliary ganglion (CG), through which it controls pupil constriction and lens accommodation. On the other hand, EW is applied to a population of centrally projecting neurons involved in sympathetic, consumptive, and stress-related functions. This terminology problem arose because the name EW has historically been applied to the most prominent cell collection above or between the somatic oculomotor nuclei (III), an assumption based on the known location of the preganglionic motoneurons in monkeys. However, in many mammals, the nucleus designated as EW is not made up of cholinergic, preganglionic motoneurons supplying the CG and instead contains neurons using peptides, such as urocortin 1, with diverse central projections. As a result, the literature has become increasingly confusing. To resolve this problem, we suggest that the term EW be supplemented with terminology based on connectivity. Specifically, we recommend that 1) the cholinergic, preganglionic neurons supplying the CG be termed the Edinger-Westphal preganglionic (EWpg) population and 2) the centrally projecting, peptidergic neurons be termed the Edinger-Westphal centrally projecting (EWcp) population. The history of this nomenclature problem and the rationale for our solutions are discussed in this review.

Characterization of the fetal diaphragmatic magnetomyogram and the effect of breathing movements on cardiac metrics of rate and variability

Breathing movements are one of the earliest fetal motor behaviors to emerge and are a hallmark of fetal well-being. Fetal respiratory sinus arrhythmia (RSA) has been documented but efforts to quantify the influence of breathing on heart rate (HR) and heart rate variability (HRV) are difficult due to the episodic nature of fetal breathing activity. We used a dedicated fetal biomagnetometer to acquire the magnetocardiogram (MCG) between 36 and 38 weeks gestational age (GA). We identified and characterized a waveform observed in the raw data and independent component decomposition that we attribute to fetal diaphragmatic movements during breathing episodes. RSA and increased high frequency power in a time-frequency analysis of the IBI time-series was observed during fetal breathing periods. Using the diaphragmatic magnetomyogram (dMMG) as a marker, we compared time and frequency domain metrics of heart rate and heart rate variability between breathing and non-breathing epochs. Fetal breathing activity resulted in significantly lower HR, increased high frequency power, greater sympathovagal balance, increased short-term HRV and greater parasympathetic input relative to non-breathing episodes confirming the specificity of fetal breathing movements on parasympathetic cardiac influence. No significant differences between breathing and non-breathing epochs were found in two metrics reflecting total HRV or very low, low and intermediate frequency bands. Using the fetal dMMG as a marker, biomagnetometry can help to elucidate the electrophysiologic mechanisms associated with diaphragmatic motor function and may be used to study the longitudinal development of human fetal cardiac autonomic control and breathing activity.

* The effects of antimuscarinic agents on rectal compliance in patients with supraconal spinal cord injury

IntroductionConstipation affects up to 80% of spinal cord injury (SCI) patients and is cause of significant reduction of quality of life. Rectal compliance is determined by the rectal wall properties...

Effects of nitric oxide synthase inhibition on dorsal vagal stimulation‐induced pancreatic insulin secretion

Pancreatic islets receive parasympathetic input from the dorsal motor nucleus of the vagus (DMV). This study examined whether chemical activation of the DMV modulates pancreatic endocrine secretion and whether this process involves nitric oxide (NO). All experiments were conducted in isoflurane/urethane-anaesthetized Sprague Dawley rats after over-night fast. Stimulation of the DMV using bicuculline (Bic; GABAA antagonist, 100 pmol/25 nl) microinjections resulted in significant and rapid increases in glucose-mediated insulin secretion (9.2 ± 0.1 ng/ml peak) compared to vehicle (5.5 ± 2.0 ng/ml; P<0.05). A repeat microinjection of BIM made 40 min later did not increase insulin secretion significantly. Microinjection of Bic outside the DMV or microinjection of vehicle into the DMV did not affect insulin secretion. Blockade of muscarinic receptors (atropine methonitrate; 100μg/kg min, i.v. for 20 minutes) reduced the activation of glucose-mediated insulin secretion produced by stimulation of the DMV from 9.2 ± 0.9 ng/ml to 1.6 ± 0.1 ng/ml (P<0.05). On the other hand, L-nitroarginine (NO synthase inhibitor, 30 mg/kg, i.v.) increased the excitatory effect of DMV stimulation on glucose-mediated insulin secretion (from 8.9 ± 1.2 ng/ml to 15.3 ± 3.0 ng/ml; P<0.05). This suggests that NO may play an inhibitory role in the vagal regulation of pancreatic endocrine secretion. Supported by AHMRF.

Spinal cord stimulators for treatment of ventricular arrhythmias: Antiarrhythmic in a can

e Sympathetic activation and its dysregulation in the seting of left ventricular systolic dysfunction have been learly documented and are thought to play an important ole in arrhythmogenesis. The paradigm of modulation of ympathetic activation by ganglionectomy has proved to be eneficial in a variety of syndromes in which ventricular rrhythmias lead to mortality. The finding of parasymathetic withdrawal occurs in parallel with cardiac sympahetic activation, a phenomenon that can be documented at he level of the sinoatrial node as well as in the myocardium. Modulation of parasympathetic withdrawal as an ntiarrhythmic strategy has been evaluated by Schwartz t al, who directly stimulated the vagus and demontrated successful prevention of ventricular fibrillation in n ischemic canine model. As such, the arrhythmic risk f left ventricular dysfunction appears to occur as a unction of both sympathetic activation and parasympahetic withdrawal. Most importantly, a complex interaction exists between ympathetic and parasympathetic inputs to a variety of vasular beds, whereby parasympathetic tone is a potent modlator of norepinephrine release. This important complex nteraction of muscarinic receptors and pathways, which lays an important role in “net” sympathetic activation, has een demonstrated in the heart and other vascular territoies. In humans with heart failure in contrast to control ubjects, stimulation of myocardial muscarinic receptors ecreased cardiac norepinephrine spillover, a reflection of fferent myocardial sympathetic outflow. The inhibitory effect of muscarinic stimulation on norepinephrine release displays the functional characteristic of accentuated antagonism whereby the inhibitory effect of parasympathetic input is greatest in the setting of sympathetic activation but has little impact in the setting of unstimulated, basal sympathetic outflow. This concept has been repeatedly documented in animal models and has been directly documented in humans among whom clear differences in modulation of