A muscarinic inhibition of the noradrenaline release evoked by postganglionic sympathetic nerve stimulation

Autonomic control of ventricular function in health and disease: current state of the art

PURPOSE

Cardiac autonomic dysfunction is one of the main pillars of cardiovascular pathophysiology. The purpose of this review is to provide an overview of the current state of the art on the pathological remodeling that occurs within the autonomic nervous system with cardiac injury and available neuromodulatory therapies for autonomic dysfunction in heart failure.

METHODS

Data from peer-reviewed publications on autonomic function in health and after cardiac injury are reviewed. The role of and evidence behind various neuromodulatory therapies both in preclinical investigation and in-use in clinical practice are summarized.

RESULTS

A harmonic interplay between the heart and the autonomic nervous system exists at multiple levels of the neuraxis. This interplay becomes disrupted in the setting of cardiovascular disease, resulting in pathological changes at multiple levels, from subcellular cardiac signaling of neurotransmitters to extra-cardiac, extra-thoracic remodeling. The subsequent detrimental cycle of sympathovagal imbalance, characterized by sympathoexcitation and parasympathetic withdrawal, predisposes to ventricular arrhythmias, progression of heart failure, and cardiac mortality. Knowledge on the etiology and pathophysiology of this condition has increased exponentially over the past few decades, resulting in a number of different neuromodulatory approaches. However, significant knowledge gaps in both sympathetic and parasympathetic interactions and causal factors that mediate progressive sympathoexcitation and parasympathetic dysfunction remain.

CONCLUSIONS

Although our understanding of autonomic imbalance in cardiovascular diseases has significantly increased, specific, pivotal mediators of this imbalance and the recognition and implementation of available autonomic parameters and neuromodulatory therapies are still lagging.

Autonomic nervous system receptor-mediated regulation of mast cell degranulation modulates the inflammation after corneal epithelial abrasion

Mast cells (MCs) regulate wound healing and are influenced by the autonomic nervous system (ANS). However, the underlying mechanisms affecting wound healing outcomes remain elusive. Here, we explored the specific role of the ANS by regulating MC degranulation following corneal epithelium abrasion. A mouse model of corneal abrasion was established by mechanically removing a 2-mm central epithelium. Wound closure, neutrophil infiltration, and transcription of injured corneas were investigated using whole-mount immunostaining, flow cytometry, and RNA-sequencing analysis, respectively. Inhibition of MC degranulation by the MC stabilizers cromolyn sodium and lodoxamide tromethamine increased the infiltration of neutrophils and delayed healing of abraded corneas. Moreover, transcriptomic profiling analysis showed that purified MCs from the limbus expressed adrenergic and cholinergic receptors. Pharmacological manipulation and sympathectomy with 6-hydroxydopamine confirmed that sympathetic nervous system signaling inhibited MC degranulation after corneal abrasion, whereas parasympathetic nervous system signaling enhanced MC degranulation. We conclude that normal degranulation of MCs in the corneal limbus and crosstalk between the ANS and MCs are crucial for the appropriate control of inflammation and the repair progress of wounded corneas. This suggests a potential approach for improving defective corneal wound healing by the administration of clinically available autonomic activity-modulating agents.

Acute autonomic effects of rose oxide on cardiovascular parameters of Wistar and spontaneously hypertensive rats

AIMS

Anti-inflammatory molecules, such as rose oxide (RO), are likely to exert therapeutic effects in systemic arterial hypertension (SAH), a disease associated with abnormal immune responses. We aimed to investigate acute autonomic effects of RO on hemodynamic parameters of Wistar and spontaneously hypertensive rats (SHR).

METHODS

Rats were anesthetized and femoral artery and veins were cannulated. Next day, blood pressure (BP) and heart rate (HR) were recorded. Acute effects of RO (1.25, 2.5, or 5.0 mg/kg; iv) on BP, HR, and variability of systolic arterial pressure (SAP) and pulse interval (PI) were assessed. The effects of RO were also investigated in SHR, which received atropine (2 mg/kg), propranolol (4 mg/kg), or hexamethonium (20 mg/kg) 15 min before receiving RO. Vasorelaxant effects of RO (10-10 to 10-4 M) on aortic rings of rats were also assessed.

KEY FINDINGS

In Wistar rats, none of the RO doses evoked significant changes in BP, HR, and variability of SAP and PI. On the other hand, in SHR, RO elicited reduction in mean arterial pressure (MAP), and prevented the increase in the low frequency power (LF) of the SAP spectra. Pretreatment with atropine or propranolol did not alter hypotension, but attenuated RO-induced bradycardia. Hexamethonium prevented RO-induced hypotension and bradycardia. RO exerted vasorelaxant effects on aortic rings with (Wistar and SHR) or without functional endothelium (SHR only).

SIGNIFICANCE

Rose oxide, a monoterpene with anti-inflammatory properties, acts as an antihypertensive molecule due to its ability to acutely promote hypotension and bradycardia in spontaneously hypertensive rats.

Effects of sympatho-vagal interaction on ventricular electrophysiology and their modulation during beta-blockade

AIMS

The effects of sympatho-vagal interaction on heart rate (HR) changes are characterized by vagal dominance resulting in accentuated antagonism. Complex autonomic modulation of ventricular electrophysiology may exert prognostic arrhythmic impact. We examined the effects of concurrent sympathetic (SNS) and vagus (VNS) nerve stimulation on ventricular fibrillation threshold (VFT) and standard restitution (RT) in an isolated rabbit heart preparation with intact dual autonomic innervation, with and without beta-blockade.

METHODS AND RESULTS

Monophasic action potentials were recorded from left ventricular epicardial surface of dual-innervated isolated heart preparations from New Zealand white rabbits (n = 18). HR, VFT and RT were measured during different stimulation protocols (Protocol 1: VNS-SNS; Protocol 2: SNS-VNS) involving low- and high-frequency stimulations. A sub-study of Protocol 2 was performed in the presence of metoprolol tartrate. In both protocols, HR changes were characterized by vagal-dominant bradycardic component, affirming accentuated antagonism. During concurrent high-frequency VNS (HV), SNS prevails in lowering VFT in a frequency-sensitive manner during low (LS) or high (HS)-frequency stimulations (HV-LS: -2.8 ± 0.8 mA; HV-HS: -4.0 ± 0.9 mA, p < .05 vs. HV), with accompanying steepening of relative RT slope gradients (HV-LS: 223.54 ± 37.41%; HV-HS: 295.20 ± 60.86%, p < .05 vs. HV). In protocol 2, low (LV) and high (HV) vagal stimulations during concurrent HS raised VFT (HS-LV: 1.0 ± 0.4 mA; HS-HV: 3.0 ± 0.6 mA, p < .05 vs HS) with associated flattening of RT slopes (HS-LV: 32.40 ± 4.97%;HS-HV: 38.07 ± 6.37%; p < .05 vs HS). Metoprolol abolished accentuated antagonism in HR changes, reduced VFT and flattened RT globally during SNS-VNS.

CONCLUSIONS

Accentuated antagonism is absent in ventricular electrophysiological changes during sympatho-vagal interaction with sympathetic effect prevailing, suggesting a different mechanism at the ventricular level from heart rate effects. Metoprolol nullified accentuated antagonism with additional anti-fibrillatory effect beyond adrenergic blockade during sympatho-vagal stimulations.

Changes in Heart Rate and Its Regulation by the Autonomic Nervous System Do Not Differ Between Forced and Voluntary Exercise in Mice

Most exercise studies in mice have relied on forced training which can introduce psychological stress. Consequently, the utility of mouse models for understanding exercise-mediated effects in humans, particularly autonomic nervous system (ANS) remodeling, have been challenged. We compared the effects of voluntary free-wheel running vs. non-voluntary swimming on heart function in mice with a focus on the regulation of heart rate (HR) by the ANS. Under conditions where the total excess O₂ consumption associated with exercise was comparable, the two exercise models led to similar improvements in ventricular function as well as comparable reductions in HR and its control by parasympathetic nervous activity (PNA) and sympathetic nervous activity (SNA), compared to sedentary mice. Both exercise models also increased HR variability (HRV) by similar amounts, independent of HR reductions. In all mice, HRV depended primarily on PNA, with SNA weakly affecting HRV at low frequencies. The differences in both HR and HRV between exercised vs. sedentary mice were eliminated by autonomic blockade, consistent with the similar intrinsic beating rates observed in atria isolated from exercised vs. sedentary mice. In conclusion, both forced and voluntary exercise induce comparable ventricular physiological remodeling as well as HR reductions and HR-independent enhancements of HRV which were both primarily dependent on increased PNA.

New and noteworthy

-No previous mouse studies have compared the effects of forced and voluntary exercise on the heart function and its modulation by the autonomic nervous system (ANS).-Both voluntary free-wheel running and forced swimming induced similar improvements in ventricular contractile function, reductions in heart rate (HR) and enhancements of HR variability (HRV).-HR regulation in exercised mice was linked to increased parasympathetic nerve activity and reduced sympathetic nerve activity.- HRV was independent of HR and depended primarily on PNA in both exercised and sedentary mice.- Complete cardiac autonomic blockade eliminated differences in both HR and HRV between exercised and sedentary mice.

Complex and interacting influences of the autonomic nervous system on cardiac electrophysiology in conscious mice

Mice may now be the preferred animal model for biomedical research due to its anatomical, physiological, and genetic similarity to humans. However, little is known about accentuated antagonism of chronotropic and dromotropic properties in conscious mice. Accordingly, we describe the complex and interacting influence of the autonomic nervous system on cardiac electrophysiology in conscious mice. Specifically, we report the effects of single and combined cardiac autonomic blockade on measurements of pulse interval (heart rate), atrio-ventricular interval, sinus node recovery time (SNRT), SNRT corrected for spontaneous sinus cycle, and Wenckebach cycle length in conscious mice free of the confounding influences of anesthetics and surgical trauma. Autonomic influences were quantified as the change in parameter induced by its selective blocker (Sympathetic or Parasympathetic Effect) or as the difference between the intrinsic value and the value after a selective blocker (Sympathetic or Parasympathetic Tonus). Sympatho-Vagal Balance (SVB) was assessed as the ratio of control interval to intrinsic interval. SVB suggests slight parasympathetic dominance in the control of cardiac electrophysiology intervals. Furthermore, results documents a complex interaction between the sympathetic and parasympathetic divisions of the autonomic nervous system in the control of cardiac electrophysiology parameters. Specifically, the parasympathetic effect was greater than the parasympathetic tonus in the control of cardiac electrophysiology parameters. In contrast, the sympathetic effect was smaller than the sympathetic tonus in the control of cardiac electrophysiology parameters. Results have important implications because actions of pharmacological agents that alter the autonomic control of cardiac electrophysiology are transformed by these interacting mechanisms.

Heart rate variability today

Heart rate variability (HRV) non-invasively assesses the activity of the autonomic nervous system. During the past 30 years, an increasing number of studies have related the imbalance of the autonomic nervous system (as assessed by HRV) to several pathophysiogical conditions, particularly in the setting of cardiovascular disease. Sudden death, coronary artery disease, heart failure, or merely cardiovascular risk factors (smoking, diabetes, hyperlipidemia, and hypertension) are the best-known clinical circumstances that can affect and/or be affected by the autonomic nervous system. Analyses of HRV variables have been proposed as a component of the clinical evaluation for patient risk stratification due to its independent prognostic information. Yet the potential for HRV to be used widely in clinical practice remains to be established.

Presynaptic autoreceptors regulating transmitter release

The discovery that the cytoplasmic membrane of presynaptic nerve terminals possess receptors that modulates release of neurotransmitters was made 35 years ago. This new concept represents a clear departure from the traditional view that neuronal communication was unidirectional, i.e. from the nerve terminal to the postsynaptic receptor, because the transfer of information via presynaptic receptors occurs in the opposite direction: from the synaptic cleft to the nerve terminals which release the neurotransmitter. Presynaptic release-modulating autoreceptors and heteroreceptors represent suitable targets for pharmacological intervention by exogenous compounds acting as agonists, partial agonists or antagonists. Such compounds may be of therapeutic value by influencing transmitter release presynaptically, and having fewer side effects than the well-established approach of using agonists or antagonist drugs to stimulate or block postsynaptic receptors.

Differential signalling pathways involved in cholinoceptor-dependent stimulation of nitric oxide isoforms in dental pulp

AIM

To investigate the role of muscarinic acetylcholine receptor (mAChR) subtype activity in the regulation of endothelial- (e) and neuronal- (n) nitric oxide synthase (NOS) expression and activity.

METHODOLOGY

Rat dental pulp tissue was used throughout the study. The e-nos and n-nos mRNA levels were specifically measured using reverse transcriptase polymerase chain reaction procedures that involve simultaneous co-amplification of both target cDNA and a reference template with a single set of primers. NOS activity was measured by the production of [U-(14)C]-citrulline from [U-(14)C]-arginine.

RESULTS

Stimulation of M(1)/M(2) and M(3)/M(4) mAChRs with pilocarpine caused an increase in e-nos and n-nos mRNA levels and NOS activity in the dental pulp. The specific mAChR subtype antagonists, L-NMMA, l-NIO and N(2)-propyl-L-arginine but not aminoguanidine attenuated all these effects. Inhibitors of phospholipase C (PLC), protein kinase C (PKC) and calcium/calmodulin (CaM) prevented the pilocarpine-dependent increase in n-nos and e-nos mRNA levels and NOS activity.

CONCLUSIONS

Activation of mAChR subtypes stimulated NOS activity by increasing the production of NO through e-nos and n-nos gene expression and NOS activity. The mechanism appears to occur secondarily to stimulation of CaM and PKC enzymatic activity.

Role of presynaptic nicotinic acetylcholine receptors in the regulation of gastrointestinal motility

Presynaptic nicotinic acetylcholine receptors (nAChRs) located on cholinergic terminals facilitate the release of acetylcholine (ACh), thereby constituting a fail-safe mechanism at strategic locations, such as the neuromuscular junction, where reliable transmission is vital. Accumulating data indicate that myenteric neurons in the enteric nervous system possess not only somatodendritic nAChRs, which mediate cholinergic transmission between neurons, but also presynaptic nAChRs. Functional evidence shows that these receptors mediate a positive feedback with respect to ACh release from myenteric motoneurons, and might therefore play an important role in the regulation of gastrointestinal motility. These presynaptic nAChRs were found to be more sensitive to nicotinic ligands than somatodendritic nAChRs and could therefore be primary targets of exogenous compounds, such as nicotine. This interaction might provide a neurochemical basis for the effect of smoking on gastrointestinal motility. Another important human pharmacological implication is based on our recent observation that monoamine uptake inhibitor-type antidepressant drugs are able to inhibit presynaptic nAChRs in the enteric nervous system. The disruption of the nAChR-mediated positive feedback modulation by antidepressants might explain the frequent occurrence of constipation, a common side effect, attributed to these drugs. Clarification of the role of presynaptic nAChRs in feedback mechanisms in the enteric nervous system might be instrumental in the development of new drugs affecting gastrointestinal motility.

M1 and M2 Muscarinic Acetylcholine Receptor Subtypes Mediate Ca2+ Channel Current Inhibition in Rat Sympathetic Stellate Ganglion Neurons

Muscarinic acetylcholine receptors (mAChRs) are known to mediate the acetylcholine inhibition of Ca2+ channels in central and peripheral neurons. Stellate ganglion (SG) neurons provide the main sympathetic input to the heart and contribute to the regulation of heart rate and myocardial contractility. Little information is available regarding mAChR regulation of Ca2+ channels in SG neurons. The purpose of this study was to identify the mAChR subtypes that modulate Ca2+ channel currents in rat SG neurons innervating heart muscle. Accordingly, the modulation of Ca2+ channel currents by the muscarinic cholinergic agonist, oxotremorine-methiodide (Oxo-M), and mAChR blockers was examined. Oxo-M–mediated mAChR stimulation led to inhibition of Ca2+ currents through voltage-dependent (VD) and voltage-independent (VI) pathways. Pre-exposure of SG neurons to the M1 receptor blocker, M1-toxin, resulted in VD inhibition of Ca2+ currents after Oxo-M application. On the other hand, VI modulation of Ca2+ currents was observed after pretreatment of cells with methoctramine (M2 mAChR blocker). The Oxo-M–mediated inhibition was nearly eliminated in the presence of both M1 and M2 mAChR blockers but was unaltered when SG neurons were exposed to the M4 mAChR toxin, M4-toxin. Finally, the results from single-cell RT-PCR and immunofluorescence assays indicated that M1 and M2 receptors are expressed and located on the surface of SG neurons. Overall, the results indicate that SG neurons that innervate cardiac muscle express M1 and M2 mAChR, and activation of these receptors leads to inhibition of Ca2+ channel currents through VI and VD pathways, respectively.

Possible neural mediation of the central effects of oxytocin on uterine motility

The central nervous system contains the nuclei at the origin of autonomic and neuroendocrine pathways to the uterus. Although the anatomical basis of these pathways is known, the conditions of their recruitment and their interactions in the context of copulation remain to be explored. We tested the hypothesis that some central mechanisms could simultaneously recruit both pathways to the uterus. In this aim, we recorded intrauterine pressure changes in anesthetized female rats at the estrus stage after intracerebroventricular (ICV) administration of oxytocin (OT). Doses of 0.3–300 ng elicited increases of frequency and amplitude of uterine contractions. These effects were partly mimicked by the OT agonist [Thr4,Gly7]OT but not by arginine vasopressin. They were blocked by the OT receptor antagonist atosiban delivered either ICV or intravenously. The latter suggests that ICV OT activated the systemic release of OT. The effects of OT were also blocked by hexamethonium, a ganglionic blocking agent, by atropine, a muscarinic receptor antagonist, and by Nω-nitro-l-arginine methyl ester, an inhibitor of nitric oxide synthesis. The results reveal that ICV OT recruits autonomic efferent pathways to the uterus. These results support our hypothesis that the activation of central nuclei can promote uterine contractility, and that OT may be a central coordinator of autonomic and neuroendocrine pathways. The hypothalamus, the source of direct OT-ergic projections to the pituitary, the brain stem, and the spinal cord, may be a target of central OT.

Interaction between direct sympathetic and vagus nerve stimulation on heart rate in the isolated rabbit heart

The interaction between the effects of vagus nerve stimulation (VS) and sympathetic stimulation (SS) on intrinsic heart rate was studied in the novel innervated isolated rabbit heart preparation. The effects of background VS, at different frequencies--2 Hz (low), 5 Hz (medium), 7 Hz (high)--on the chronotropic effects of different frequencies of SS--2 Hz (low), 5 Hz (medium), 10 Hz (high)--were studied. The experiments were repeated in the reverse direction studying the effects of different levels of background SS on the chronotropic effects of different levels of VS. Background VS reduced the overall positive chronotropic effect of SS at steady state in a frequency dependent manner and the rate of increase in heart rate during low and medium SS (but not high SS) was slowed in the presence of background VS. These results suggest that pre- and postjunctional mechanisms may be involved in the sympatho-vagal interaction on heart rate. On the other hand, the chronotropic effect of VS was enhanced in the presence of background SS. Vagal stimulation appears to play a dominant role over sympathetic stimulation in chronotropic effects on the isolated heart. The innervated isolated heart preparation is a valuable model to study the complex mechanisms underlying the interaction between sympathetic and parasympathetic stimulation on cardiac function.

Mortality, cardiac vagal control and physical training--what's the link?

There is little doubt that regular exercise results in increases in life expectancy and protects against adverse cardiac events in both healthy subjects and patients with cardiovascular disease. The mechanism of action of physical training remains unclear but a variety of evidence points towards an enhancement in cardiac vagal activity protecting against lethal arrhythmias. Just how physical training increases cardiac vagal activity is an area that is ill understood but plausible mechanisms include mediation via angiotensin II or NO. Further research is needed in this area. Exercise training is demanding and difficult, particularly for patients with cardiac disease. If the mechanism of increase in cardiac vagal activity with training can be determined it may be possible to use pharmacological approaches to mimic the effects of exercise with potentially beneficial effects.

Heart rate variability--a therapeutic target?

Reduced heart rate variability (HRV) is a powerful and independent predictor of an adverse prognosis in patients with heart disease and in the general population. The HRV is largely determined by vagally mediated beat to beat variability, conventionally known as respiratory sinus arrhythmia. Thus, HRV is primarily an indicator of cardiac vagal control. It is still unclear whether the relationship between measures of cardiac vagal control and mortality is causative or mere association. Possible mechanisms by which cardiac vagal activity might beneficially influence prognosis include a decrease in myocardial oxygen demand, a reduction in sympathetic activity and a decreased susceptibility of the ventricular myocardium to lethal arrhythmia. In animals, augmentation of cardiac vagal control by nerve stimulation or by drugs is associated with a reduction in sudden death in susceptible models. In humans a number of drugs which have been shown to reduce mortality and sudden death in large randomised trials can also be demonstrated to increase HRV. As a result of this evidence, it has been suggested that the effect of drugs or other therapeutic manoeuvres on HRV might be used to predict clinical efficacy. The use of HRV as a therapeutic target is discussed in this review.

Parasympathetic control of cardiac sympathetic activity: normal ventricular function versus congestive heart failure

BACKGROUND

Muscarinic receptors on adrenergic nerve terminals attenuate norepinephrine release. The role of these receptors in the modulation of cardiac norepinephrine release in humans remains uncertain.

METHODS AND RESULTS

Twelve patients with normal left ventricular (LV) function and 18 with congestive heart failure (CHF) were studied. A radiotracer technique was used to measure cardiac norepinephrine spillover (CANESP) in response to intracoronary acetylcholine (ACh, 5x10(-5) Mol), and in response to intracoronary atropine (12 micrograms/min). ACh did not affect CANESP in the group of subjects with normal LV function, but it caused a significant reduction in those with CHF [197 (150 to 302) versus 168 (87 to 288) pmol/min, P<0.05]. Atropine caused a significant increase in CANESP in those with normal LV function [47 (27 to 51) versus 64 (38 to 139) pmol/min, P<0.05], but no change was observed in the CHF group.

CONCLUSIONS

Therefore, in the setting of heart failure and sympathetic activation, muscarinic receptor stimulation decreases CANESP, an effect not observed in patients with preserved LV function. Blockade of muscarinic receptors with atropine increased CANESP in patients with normal LV function, suggesting that cardiac parasympathetic tone has inhibitory effects on cardiac sympathetic activity. This basal inhibition was not observed in CHF patients in response to atropine. The lack of basal parasympathetic inhibition of cardiac sympathetic activity may play a role in the pathogenesis of cardiac sympathetic activation in heart failure.

Neurochemistry and pharmacology of the major hippocampal transmitter systems: synaptic and nonsynaptic interactions

Hippocampus plays a crucial role in important brain functions (e.g. memory, learning) thus in the past two decades this brain region became a major objective of neuroscience research. During this period large number of anatomical, neurochemical and electrophysiological data have been accumulated. While excellent reviews have been published on the anatomy and electrophysiology of hippocampal formation, the neurochemistry of this area has not been thoroughly surveyed. Therefore the aim of this review is to summarize the neurochemical and pharmacological data on the release of the major neurotransmitters found in the hippocampal region: glutamate (GLU), gamma-amino butyric acid (GABA), acetylcholine (ACh), noradrenaline (NA) and serotonin (5-HT). In addition, this review analyzes the synaptic and nonsynaptic interactions between hippocampal neuronal elements and overviews how auto- and heteroreceptors are involved in the presynaptic modulation of transmitter release. The presented data clearly show that transmitters released from axon terminals without synaptic contact play an important role in the fine tuning of communication between neurons within a neuronal circuit.

M4 muscarinic receptor activation modulates calcium channel currents in rat intracardiac neurons

Modulation of high-voltage-activated Ca2+ channels by muscarinic receptor agonists was investigated in isolated parasympathetic neurons of neonatal rat intracardiac ganglia using the amphotericin B perforated-patch whole cell recording configuration of the patch-clamp technique. Focal application of the muscarinic agonists acetylcholine (ACh), muscarine, and oxotremorine-M to the voltage-clamped soma membrane reversibly depressed peak Ca2+ channel current amplitude. The dose-response relationship obtained for ACh-induced inhibition of Ba2+ current (IBa) exhibited a half-maximal inhibition at 6 nM. Maximal inhibition of IBa amplitude obtained with 100 microM ACh was approximately 75% compared with control at +10 mV. Muscarinic agonist-induced attenuation of Ca2+ channel currents was inhibited by the muscarinic receptor antagonists pirenzepine (</=300 nM) and m4-toxin (</=100 nM), but not by AF-DX 116 (300 nM) or m1-toxin (60 nM). The dose-response relationship obtained for antagonism of muscarine-induced inhibition of IBa by m4-toxin gave an IC50 of 11 nM. These results suggest that muscarinic agonist-induced inhibition of high-voltage-activated Ca2+ channels in rat intracardiac neurons is mediated by the M4 muscarinic receptor. M4 receptor activation shifted the voltage dependence and depressed maximal activation of Ca2+ channels but had no effect on the steady-state inactivation of Ca2+ channels. Peak Ca2+ channel tail current amplitude was reduced >/=30% at +90 mV in the presence of ACh, indicating a voltage-independent component to the muscarinic receptor-mediated inhibition. Both dihydropyridine- and omega-conotoxin GVIA-sensitive and -insensitive Ca2+ channels were inhibited by ACh, suggesting that the M4 muscarinic receptor is coupled to multiple Ca2+ channel subtypes in these neurons. Inhibition of IBa amplitude by muscarinic agonists was also observed after cell dialysis using the conventional whole cell recording configuration. However, internal perfusion of the cell with 100 microM guanosine 5'-O-(2-thiodiphosphate) trilithium salt (GDP-beta-S) or incubation of the neurons in Pertussis toxin (PTX) abolished the modulation of IBa by muscarinic receptor agonists, suggesting the involvement of a PTX-sensitive G-protein in the signal transduction pathway. Given that ACh is the principal neurotransmitter mediating vagal innervation of the heart, the presence of this inhibitory mechanism in postganglionic intracardiac neurons suggests that it may serve for negative feedback regulation.

Different sympathetic-parasympathetic interactions on sinus rate and atrioventricular conduction in dog hearts

We investigated the sympathetic-parasympathetic interactions involved in SA nodal pacemaker activity and AV conductivity in the anesthetized dog heart. Stimulation of the intracardiac parasympathetic nerves to the SA nodal region (SAPS) and stimulation of the intracardiac parasympathetic nerves to the AV nodal region (AVPS) induced negative chronotropic and dromotropic responses, respectively. Cardiac sympathetic stimulation, aminophylline, 3-isobutyl-1-methylxanthine (IBMX, a relatively pure nonselective phosphodiesterase inhibitor) and methyl-1,4-dihydro-2,6-dimethyl-3-nitro-4-(2-trifluoromethylphenyl)-p iridine-5-carboxylate (Bay k 8644, a Ca2+ channel agonist) increased sinus rate and decreased AV conduction time. Sympathetic stimulation augmented the negative chronotropic response to SAPS but not the negative dromotropic response to AVPS, IBMX augmented both responses, Bay k 8644 augmented the chronotropic response and attenuated the dromotropic response, and aminophylline did not affect the chronotropic response to SAPS and inhibited the dromotropic response to AVPS. Additionally, when Bay k 8644 directly given via the AV node artery decreased AV conduction time, it attenuated the negative dromotropic response to AVPS and carbachol injected into the AV node artery. These results suggest that the differential sympathetic-parasympathetic interactions on sinus rate and AV conduction are at least partly induced by an interaction between changes in slow inward Ca2+ current or intracellular Ca2+ and the cardiac effects of acetylcholine in the heart in situ.

Role of the alpha-adrenoceptor in regulating noradrenaline overflow by nerve stimulation. 1971

A study of the actions of phenoxybenzamine on transmitter overflow, neuronal and extraneuronal uptake of noradrenaline and in causing α-adrenoceptor blockade was carried out using the isolated cat nictitating membrane preparation. Phenoxybenzamine increased transmitter overflow elicited by nerve-stimulation at 10 Hz in a concentration dependent manner in the range 10−8 to 10−5 g/ml. Neuronal uptake of [3H]-noradrenaline was not inhibited by concentrations lower than 10−6 g/ml of phenoxybenzamine. With 10−7 g/ml of phenoxybenzamine a significant increase in transmitter overflow was obtained, although neuronal uptake of noradrenaline was not affected. Higher concentrations of phenoxybenzamine (10−6 and 10−5 g/ml) inhibited the neuronal uptake of noradrenaline and further increased transmitter overflow. Extraneuronal uptake of [3H]-noradrenaline was inhibited only with the highest concentration of phenoxybenzamine tested (10−5 g/ml) and therefore appears to be unrelated to the effects on transmitter overflow. There was a significant correlation between the degree of α-adrenoceptor block produced by phenoxybenzamine and the increase in transmitter overflow obtained by nerve stimulation. These results indicate that phenoxybenzamine, in addition to increasing overflow by preventing reuptake of noradrenaline, may increase transmitter release. The possibility that phenoxybenzamine acts on α-adrenoceptors in the adrenergic nerve terminal is discussed. These receptors would be involved in a negative feedback mechanism regulating transmitter release.

Effects of autonomic manipulation on ventricular fibrillation and internal cardiac defibrillation thresholds in pigs

Autonomic tone may contribute to cardiac arrhythmogenesis and influence the efficacy of implantable defibrillators. Fifty-two anesthetized pigs were randomized to: (1) methacholine (n = 12); (2) nitroprusside (n = 12); (3) phenylephrine (n = 12); (4) carbachol (n = 8); and (5) saline (n = 8). Ventricular fibrillation threshold (VFT) and triplicate defibrillation thresholds (DFT) were obtained before and during each intervention. Mean (+/- SE) VFT was increased with: methacholine (76 +/- 10.6 V vs 39 +/- 7.1 V, P < 0.001); phenylephrine (68 +/- 10.5 V vs 38 +/- 6.2 V, P < 0.001); and carbachol (106 +/- 11.5 V vs 30 +/- 6.5 V, P < 0.0001). Nitroprusside and saline failed to alter VFT. Mean (+/- SE) DFT was decreased with: methacholine (7.7 +/- 0.8) vs 9.7 +/- 0.8 J, P < 0.001); phenylephrine (9.8 +/- 0.9 J vs 11.3 +/- 1.0 J, P < 0.05); and carbachol (9.2 +/- 0.7 J vs 12.2 +/- 0.8 J, P < 0.0001), remaining unchanged following nitroprusside and saline infusion. Thus, modulation of autonomic tone modified arrhythmia susceptibility and the energy necessary for defibrillation, increased parasympathetic tone, increased VFT, and decreased DFT. Evaluation of autonomic balance, particularly parasympathetic tone, may be useful with the implantation of automatic defibrillators.

Autonomic interactions and chronotropic control of the heart: heart period versus heart rate

Autonomic control of the heart varies more linearly with heart period than with rate. Relative linearity confers a greater independence of basal autonomic activation and heart period changes. Thus, heart period appears to be more appropriate for characterizing cardiac phenomena such as autonomic interactions that involve significant baseline shifts. Simulated and published empirical data were used to demonstrate the importance of the chronotropic metric for characterizing autonomic interactions. Simulations revealed a significant autonomic interaction when heart rate, but not heart period, was the chronotropic metric. Published heart rate data also show a substantial autonomic interaction, whereas heart period data do not. These findings suggest that the choice of chronotropic metric can overstate the extent of autonomic interactions on cardiac chronotropic function.

In vivo effect of methyl-quinuclidinyl-benzylate on myocardial beta-adrenoceptor density

The muscarinic receptor antagonist methyl-quinuclidinyl-benzylate decreased myocardial beta-adrenoceptor density Bmax: 20.4 +/- 2.4 pmol/ml tissue versus 33.3 +/- 4 pmol/ml tissue in control dogs (P < 0.001), as assessed by using [11C]CGP-12177 (((2S)-4-(3-t-butyl-amino-2 hydroxypropoxy)-benzimidazol-2-one)) and positron emission tomography. In contrast, atropine did not induce any change in Bmax: 33.7 +/- 3.6 pmol/ml tissue. We hypothetized that methyl-quinuclidinyl-benzylate induced the release of norepinephrine from sympathetic nerve terminals, an effect which could be blocked by guanethidine. Guanethidine alone (10 mg/kg) did not change Bmax: 35.5 +/- 6 pmol/ml tissue. Guanethidine + methyl-quinuclidinyl-benzylate did not induce any significant change in Bmax: 31.5 +/- 5.1 pmol/ml tissue. Therefore, it seems likely that methyl-quinuclidinyl-benzylate acts at the presynaptic level, probably inducing the release of norepinephrine which then causes a down-regulation of beta-adrenoceptors.

Pharmacologic modulation of the autonomic nervous system in the prevention of sudden cardiac death. A study with propranolol, methacholine and oxotremorine in conscious dogs with a healed myocardial infarction

OBJECTIVES

The goal of the present study was to evaluate the antifibrillatory and hemodynamic effects of pharmacologic muscarinic activation and to compare them with those of beta-adrenergic blockade.

BACKGROUND

Recent studies suggest a correlation between increased vagal activity and a reduced incidence of sudden cardiac death. Electrical stimulation of the vagus nerve reduces the incidence of ventricular fibrillation in a conscious animal model of sudden cardiac death.

METHODS

Eleven dogs with healed anterior myocardial infarction, in which a 2-min left circumflex coronary artery occlusion during exercise caused ventricular fibrillation, were studied. They underwent subsequent tests with saline solution, propranolol (1 mg/kg body weight), methacholine (0.5 microgram/kg per min) and oxotremorine (8 micrograms/kg).

RESULTS

In the test with saline solution, 100% of the dogs developed ventricular fibrillation; this occurred in only 10% of the tests with propranolol (95% confidence interval 0.2% to 44%; p < 0.001), 60% of the tests with methacholine (95% confidence interval 26% to 88%, p = 0.05) and 37.5% of the tests with oxotremorine (95% confidence interval 8% to 75%, p = 0.005). Propranolol and oxotremorine significantly reduced heart rate compared with saline solution, whereas methacholine did not. Propranolol significantly reduced maximal first derivative of left ventricular pressure, (dP/dtmax), particularly during myocardial ischemia, compared with the other treatments (2,391 +/- 582 mm Hg/s [mean +/- 1 SD] with propranolol vs. 4,226 +/- 1,237, 4,922 +/- 584 and 4,358 +/- 1,109 mm Hg/s with saline solution, methacholine and oxotremorine, respectively, p < 0.005).

CONCLUSIONS

Propranolol was extremely effective against ventricular fibrillation. Methacholine and oxotremorine provided a significant, although less marked, protection and caused much less impairment of contractility compared with propranolol. Muscarinic receptor activation may represent a new approach to prevention of sudden cardiac death, particularly when beta-blockers are contraindicated and negative inotropic effects are to be avoided.

Sequence of excitation as a factor in sympathetic-parasympathetic interactions in the heart

We determined the influence of differences in the time of initiation of sympathetic and vagal stimulation (both at 10 Hz) on the cardiac autonomic interactions in 16 open-chest anesthetized dogs. We always ended the concurrent sympathetic and vagal stimulations simultaneously. Sympathetic stimulation alone for 1 minute increased heart rate by 90 +/- 7 (mean +/- SEM) beats per minute, and vagal stimulation alone for 1 minute decreased heart rate by 67 +/- 5 beats per minute; i.e., the algebraic sum of these responses was an increase of 23 beats per minute. However, combined sympathetic and vagal stimulation for 1 minute actually decreased heart rate by 35 beats per minute; i.e., the vagal effects predominated. When vagal stimulation was initiated first, the chronotropic responses to combined stimulation were not significantly affected by the duration of antecedent vagal stimulation. However, when sympathetic stimulation was initiated first, the vagal predominance (disparity between the summated individual responses and the combined response) progressively diminished as we increased the duration of antecedent sympathetic stimulation. The vagal predominance diminished from a value of 67 +/- 21 beats per minute when the stimulations were initiated simultaneously to a value of 37 +/- 21 beats per minute when the duration of antecedent sympathetic stimulation was 10 minutes. Sympathetic stimulation releases not only norepinephrine but also neuropeptide Y, and this neuropeptide inhibits vagal neurotransmission. Our data suggest, therefore, that the longer the antecedent sympathetic stimulation, the greater the inhibition of vagal neurotransmission (presumably by the neuropeptide Y) and, therefore, the less pronounced the vagal predominance.

Deficiency of sympathetic nervous system function in myasthenia

The norepinephrine (NE) and epinephrine (E) responses to forearm ischemia were studied in 24 myasthenic patients and 22 subjects with lumbar disc disease (control group). In some of these myasthenic (11 cases) and control (11 cases) subjects the NE and E responses to orthostasis were also investigated. In controls both stimuli induced a rise in NE urinary excretion without significantly changing the E excretion. On the other hand, in myasthenic patients forearm ischemia and orthostasis were followed by an augmentation in E excretion, the NE excretion remaining unchanged or even undergoing a depression. In other myasthenic patients (10 cases) and subjects with lumbar disc disease (10 cases) NE and E responses to exercise were tested. The results were similar to those presented above: controls reacted to exercise by a rise in NE excretion, while myasthenic patients, by an augmentation in E excretion; the NE excretion underwent no change or even decreased after exercise in myasthenia. The obliteration of NE response to all three testing stimuli found in myasthenic patients may be considered as a sign of their sympathetic deficiency. In such patients the normal E discharge produced by forearm ischemia, orthostasis or exercise may be interpreted as a compensatory reaction, being probably the consequence of sympathetic deficiency. The noradrenergic deficit of myasthenic patients is ascribable to their cholinergic hyperactivity.

The role of enhanced vagal activity on ischemic ventricular tachycardia: pharmacologic basis of inefficiency

The effects of pharmacologic modulation of vagal activity on ischemic ventricular tachycardia were evaluated in 21 conscious dogs after permanent left anterior descending coronary artery (LAD) occlusion. Studies were done on spontaneous ventricular tachycardia (cycle length 383 +/- 100 msec, n = 21), 24 to 72 hours after LAD occlusion, and on inducible sustained monomorphic ventricular tachycardia (cycle length 251 +/- 30 msec, n = 6), 4 to 7 days after LAD occlusion. Edrophonium (1 mg/kg intravenously), a cholinesterase inhibitor, and methacholine (0.1 to 1 mg intravenously), a muscarinic agonist, had no significant effect on the rate or QRS morphology of either type of tachycardia, despite severe slowing of the sinoatrial rate. Similarly, atropine (up to 60 micrograms/kg intravenously) had no effect on the rate and QRS morphology of either type of tachycardia. In an attempt to enhance myocardial drug delivery to the ischemic and infarcted left ventricle, edrophonium (1 mg/kg) and methacholine (0.1 to 0.2 mg) were injected retrogradely through the great cardiac vein. This did not impart any significant therapeutic advantage over the systemic intravenous route. Sympathetic beta blockade did not affect the therapeutic outcome (n = 5) with either edrophonium or methacholine. It is concluded that direct or indirect enhancement of cardiac vagal activity has no effect on ischemic ventricular tachycardia in this model of subacute myocardial infarction. The lack of efficacy appears to be independent of myocardial drug delivery to ischemic ventricular site(s) and background sympathetic activity. Such a lack of efficacy may be caused by ischemia-mediated degeneration of vagal nerve terminals, by altered responsiveness of muscarinic receptors at infarcted arrhythmogenic myocardial sites, or both.

Vagal stimulation and prevention of sudden death in conscious dogs with a healed myocardial infarction

The interest for the antifibrillatory effect of vagal stimulation has been largely limited by the fact that this concept seemed restricted to acute experiments in anesthetized animals. To explore the potentially protective role of vagal stimulation in conscious animals we developed a chronically implantable device to be placed around the cervical right vagus. An anterior myocardial infarction was produced in 161 dogs; 1 month later an exercise stress test was performed on the 105 survivors. Toward the end of the test the circumflex coronary artery was occluded for 2 minutes. Fifty-nine (56%) dogs developed ventricular fibrillation and, before this test was repeated, were assigned either to a control group (n = 24) or to be instrumented with the vagal device (n = 35). Five dogs were excluded because of electrode malfunction. Compared with the heart rate level attained after 30 seconds of occlusion during exercise in the control test, vagal stimulation led to a decrease of approximately 75 beats/min (from 255 +/- 33 to 170 +/- 36 beats/min, p less than 0.001). In the control group 22 (92%) of 24 dogs developed ventricular fibrillation during the second exercise and ischemia test. By contrast, during vagal stimulation ventricular fibrillation occurred in only 3 (10%) of the 30 dogs tested and recurred in 26 (87%) during an additional exercise and ischemia test in the control condition (p less than 0.001 versus the vagal stimulation test; internal control analysis). Combined analysis of the tests performed in the control condition showed that ventricular fibrillation was reproducible in 48 (89%) of the 54 dogs tested. The protective effect of vagal stimulation was also significant in the group comparison analysis and even after exclusion of those four dogs in which ventricular fibrillation was not reproducible (92% versus 11.5%, control versus vagal stimulation, p less than 0.001). When heart rate was kept constant by atrial pacing, the vagally mediated protection was still significant (p = 0.015) as five (55%) of nine dogs survived the test. This study shows that vagal stimulation, performed shortly after the onset of an acute ischemic episode in conscious animals with a healed myocardial infarction, can effectively prevent ventricular fibrillation. This striking result seems to depend on multiple mechanisms having a synergistic action. The decrease in heart rate is an important but not always essential protective mechanism. The electrophysiological effects secondary to the vagally mediated antagonism of the sympathetic activity on the heart are likely to play a major role.

Release of endothelial mediators and sympathetic transmitters at different coronary flow rates in rabbit hearts

1. The release of three endothelial mediators, namely, endothelial-derived relaxing factor (EDRF), prostacyclin (PGI2) and endothelin, and of two sympathetic neurotransmitters, noradrenaline and neuropeptide Y (NPY), from resting or sympathetically stimulated rabbit Langendorff hearts was investigated at normal or elevated coronary flow. The sympathetic nerves to the hearts were stimulated at 5 Hz for 30 s and the cardiac effluent was analysed for nitrite (metabolite of EDRF) with electron paramagnetic resonance spectrometry, for 6-keto-PGF1 alpha (metabolite of PGI2) with gas chromatography/mass spectrometry, for endothelin- and NPY-like immunoreactivity with radioimmunoassay, and for noradrenaline and purines with liquid chromatography. 2. During perfusion of the hearts at normal flow (35 +/- 1.4 ml min-1) the effluent concentration of nitrite was 0.15 +/- 0.02 microM, that of 6-keto-PGF1 alpha 0.74 +/- 0.08 nM, and that of endothelin-like immunoreactivity 0.18 +/- 0.01 pM. Nerve stimulation augmented the release of 6-keto-PGF1 alpha from 76 +/- 8 to 99 +/- 10 pmol (3 min)-1 (P less than 0.05), but did not affect the release of nitrite or endothelin-like immunoreactivity. Nerve stimulation also facilitated the outflow of noradrenaline and of NPY-like immunoreactivity by 52 +/- 11 pmol (3 min)-1 and 19 +/- 7 fmol (3 min)-1, respectively. 3. Elevation of the coronary flow to 79 +/- 3.2 ml min-1 did not affect the effluent concentrations of nitrite, 6-keto-PGF1 alpha and endothelin-like immunoreactivity, implying that their outflows were augmented. Sympathetic stimulation at elevated coronary flow did not further augment the outflow of endothelial mediators or of NPY-like immunoreactivity, but increased the outflow of noradrenaline by 62 +/- 12%, in comparison to stimulation at normal flow. Perfusion of the heart with the noradrenaline uptake blocker desipramine (5 microM) completely abolished the promoting effecting of elevated coronary flow on noradrenaline outflow during sympathetic stimulation. 4. These data indicate that an increase in coronary flow in perfused rabbit hearts is paralleled by a corresponding facilitation of the formation of the endothelial mediators, EDRF, prostacyclin and endothelin. Such an elevation of mediator formation does not affect nerve stimulation-induced release of sympathetic transmitters in the heart.

Evidence for structural sympathetic reinnervation after orthotopic cardiac transplantation in humans

BACKGROUND

Cardiac transplantation (CT) causes total cardiac denervation.

METHODS AND RESULTS

To test directly for sympathetic reinnervation in humans, we measured the cardiac release of norepinephrine (NE) in response to tyramine (an agent that causes NE release from intact sympathetic nerve terminals) and sustained handgrip exercise (a reflex sympathetic stimulus) in 12 patients less than 5 months after CT, in 50 patients 1 year or more after CT, and in eight patients without CT. Plasma [NE] was measured in the aorta [( NE]Ao) and coronary sinus [( NE]CS) at rest, after tyramine administration (55 micrograms/kg, i.v.), and during sustained handgrip exercise. Cardiac NE release was determined by subtracting [NE]Ao from [NE]CS [( NE]CS-Ao). NE release was defined as [NE]CS-Ao during the intervention-[NE]CS-Ao at rest (delta [NE]CS-Ao). In patients studied within 5 months of CT, no significant NE release occurred after tyramine administration (delta [NE]CS-Ao, 33 +/- 18 pg/ml; range, -98 to 117 pg/ml) or handgrip exercise (delta [NE]CS-Ao, -34 +/- 10 pg/ml; range, -46 to 8 pg/ml; n = 10). Conversely, in 39 of 50 patients studied 1 year or more after CT, tyramine administration caused a significant cardiac NE release (delta [NE]CS-Ao, 500 +/- 59 pg/ml; range, -11 to 1,918 pg/ml), and handgrip exercise caused a significant NE release in 17 of 41 patients (delta [NE]CS-Ao, 189 +/- 34 pg/ml; range, -211 to 949 pg/ml). In normally innervated patients, tyramine caused an even larger NE release (delta [NE]Ao-CS, 1,943 +/- 210 pg/ml; range, 1,152 to 2,977 pg/ml), and handgrip exercise caused a significant NE release in two of seven patients (delta [NE]CS-Ao, 143 +/- 51 pg/ml; range, -15 to 338 pg/ml).

CONCLUSIONS

Early after CT, neither tyramine nor handgrip exercise caused a significant cardiac release of NE, suggesting sympathetic denervation. Late after CT, most patients had a significant, but subnormal, NE release in response to pharmacological or reflex stimuli, suggesting that limited sympathetic reinnervation occurs in most patients after orthotopic CT.

Presynaptic interaction between vagal and sympathetic innervation in the heart: modulation of acetylcholine and noradrenaline release

It is generally accepted that there is a functional antagonism between the sympathetic and parasympathetic (vagal) effects on the heart. In this study guinea-pig right atria loaded either with [3H]noradrenaline or [3H]choline were used and the release of [3H]noradrenaline or [3H]acetylcholine in response to field stimulation was measured under conditions when the negative feedback modulation was excluded. Strong evidence was obtained for a one-sided interaction between the sympathetic and vagal nerves at the level of the prejunctional axon terminals that send the final chemical message to the heart muscle affecting heart rate and force. Acetylcholine released from the vagal nerve inhibited its own release and also decreased the release of noradrenaline from the sympathetic axon terminals through muscarinic receptor stimulation. But muscarinic receptors located on cholinergic axon terminals are different from those present on the noradrenergic axon terminals. There is a significant difference in the dissociation constants (Kd) of different antimuscarinic drugs: The Kd values of pancuronium on vagal and sympathetic axon terminals were 5.68 +/- 0.41 and 7.20 +/- 0.25, respectively. By contrast, noradrenaline released from the sympathetic nerves or exogenous noradrenaline were not able to modulate the release of acetylcholine from the cholinergic axon terminals even under condition when the negative feedback modulation of acetylcholine release was excluded. These findings indicate that vagal axon terminals are not equipped with alpha 2- or alpha 1-adrenoceptors. However, noradrenaline released from the sympathetic axon terminals was able to inhibit its own release via alpha 2-adrenoceptor stimulation.

Effects of Tityus serrulatus scorpion venom and one of its purified toxins (toxin gamma) on the isolated guinea-pig heart

1. The effects of Tityus serrulatus scorpion venom and its most important toxin (toxin gamma) were investigated on isolated guinea-pig hearts, perfused with Locke solution, by the Langendorff's method. 2. The cardiac contraction, the coronary flow and the electrocardiogram (ECG) were simultaneously recorded. 3. Bolus injections of 25, 50 or 100 micrograms of scorpion venom and 2.5, 5 or 10 micrograms of toxin gamma in the heart evoked complex effects which were divided into 3 phases: an initial phase (tachycardia or bradycardia associated with an increase in contractile force), an intermediate phase (oscillations of cardiac rate, contractile force and coronary flow, due to wandering pacemakers) and a third phase (sinus tachycardia). 4. The bradycardia and the oscillations of rhythm were prevented by atropine, whereas the tachycardia and the increase in contractile force were prevented either by reserpine or propranolol. 5. Scorpion venom or toxin gamma induced a ST segment displacement in the ECG, explained by a transitory myocardial hypoxia, due to an increase in the contractile force and a simultaneous decrease of the coronary flow. 6. Perfusion of the heart with Locke solution containing 2% scorpion antivenom prevented almost totally the effects elicited by the venom. 7. It is concluded that the complex effects induced by scorpion venom and toxin gamma are due to the simultaneous release of acetylcholine and catecholamines from postganglionic nerve fibers in the heart.

Characterization of presynaptic vascular muscarinic receptors inhibiting endogenous noradrenaline overflow in the portal vein of the freely moving rat

1. In the portal vein of permanently cannulated, freely moving, unanaesthetized rats, methacholine (MCh) is able to inhibit the electrically-evoked endogenous noradrenaline (NA) overflow. This inhibition is mediated by presynaptic inhibitory muscarinic heteroreceptors. 2. By use of pirenzepine, 4-diphenylacetoxy-N-methylpiperidine methobromide (4-DAMP) and AF-DX 116 as M1-, M3-, and M2-selective antagonists respectively, the MCh (0.1 microM)-induced inhibition of the electrically-evoked NA overflow could be reversed to the control stimulation value dose-dependently. 3. The potency order of the antagonists was: 4-DAMP greater than AF-DX 116 greater than pirenzepine, pIC50 values being 8.50, 7.96 and 7.01, respectively. 4. From these results it was concluded that the inhibitory presynaptic heteroreceptors in the portal vein of conscious unrestrained rats are of the cardiac M2-subtype.

Autonomic nervous stimulation affects left ventricular relaxation more than left ventricular contraction

We studied the effects of stimulation of the vagal and also the sympathetic efferent cardiac nerves on left ventricular (LV) relaxation and contraction in 11 anesthetized, open-chest dogs. In each dog, we paced the ventricles at a fixed rate of 120 beats.min-1 and kept the systemic arterial pressure constant. The maximum rate of LV pressure decline, (dP/dt)min, and the time constant of LV isovolumic pressure decline, tau, were used as our indexes of LV relaxation. The maximum rate of LV pressure rise, (dP/dt)max, was used as our measure of LV contractility. Vagal stimulation decreased (dP/dt)min more than (dP/dt)max (P less than 0.01) when examined at frequencies that ranged from 2 to 12 Hz. Vagal stimulation at 12 Hz reduced (dP/dt)min by 26% (P less than 0.001) and increased tau by 57% (P less than 0.0001) but decreased (dP/dt)max by only 20%. Sympathetic stimulation, at frequencies that ranged from 2 to 12 Hz, increased (dP/dt)min more than (dP/dt)max (P less than 0.001). Sympathetic stimulation at 12 Hz increased (dP/dt)min by 130% (P less than 0.0001) whereas it increased (dP/dt)max by 60% (P less than 0.0001). Sympathetic stimulation at 12 Hz decreased tau by 74% (P less than 0.0001). Our studies suggest that cardiac autonomic nerve stimulation affects left ventricular relaxation more than left ventricular contraction.

Presynaptic muscarinic receptors inhibiting endogenous noradrenaline release in the portal vein of the freely moving rat

1. In the portal vein of the freely moving unanaesthetized rat, the existence of presynaptically located inhibitory muscarinic receptors was investigated by use of the muscarinic agonist methacholine (MCh) 2. Infusion of MCh (0.3 micrograms min-1) did not significantly inhibit the endogenous noradrenaline (NA) overflow in portal plasma. However, after inducing high intra-synaptic concentrations of NA by blocking the presynaptic alpha 2-adrenoceptors with yohimbine (1 mg kg-1), MCh (0.3 microgram min-1) was able to reduce the yohimbine-induced enhanced NA overflow by 38%. 3. The MCh-induced inhibition was almost completely abolished after blockade of the presynaptic muscarinic receptors with atropine (0.6 mg kg-1). 4. During electrical stimulation of the portal vein nervous plexus the evoked NA overflow was strongly inhibited (95%) during MCh-infusion (0.3 microgram min-1). Again atropine (0.6 mg kg-1) was able to reverse this inhibition. 5. These results show the existence of presynaptic muscarinic receptors inhibiting endogenous NA overflow from the portal vein nervous plexus under conditions of enhanced sympathetic activity in the freely moving rat.

Different receptor subtypes mediate the dual presynaptic effects of 5-hydroxytryptamine on peripheral sympathetic neurones

1. The aim of the present work was to characterize the presynaptic 5-HT receptors that mediate either the facilitation of the responses to nerve stimulation in the nictitating membrane of the cat or the inhibition of the responses to nerve stimulation in the guinea-pig atria. 2. In the nictitating membrane of the cat, the shift to the left in the frequency-response curves produced by 5-HT (0.1 microM) was prevented by the 5-HT3 receptor antagonists, metoclopramide (1 microM) and MDL 72222 (0.01 microM). 3. The facilitatory effect of 5-HT is also prevented by the 5-HT2 receptor antagonist, 0.01 microM ketanserin. Nevertheless, this drug reduced by itself the responses to both nerve stimulation and exogenous NA in the nictitating membrane. 4. In the guinea-pig isolated atria, the inhibitory effect of 5-HT on the chronotropic responses to cardioaccelerans nerve stimulation was mimicked by the mixed 5-HT1A + 5-HT1B + 5-HT1D receptor agonist 5-carboxamidotryptamine (5-CT 0.1 and 1 microM). The 5-HT1A receptor agonist 8-OH-DPAT (0.1 and 1 microM) did not modify the responses of the atria to the nerve stimulation. 5. The 5-HT2 receptor antagonists, ketanserin (0.01 and 0.1 microM) and cyproheptadine (1 microM), did not prevent the inhibitory effect of 5-HT in the guinea-pig atria. 6. The present results suggest that the facilitatory effects of 5-HT in the nictitating membrane of the cat are linked to the activation of 5-HT3 receptors whereas the inhibitory effects observed in the guinea-pig atria are mediated by 5-HT1-like receptors.

Demonstration of adrenergic and dopaminergic receptors in cultured sympathetic neurons--their coupling to cAMP but not to the transmitter release process

Experiments were carried out on cultured sympathetic neurons of the chick embryo; first, to demonstrate the presence of adrenergic and dopaminergic receptors, and then to see if these receptors are involved in regulation of transmitter release. We show that alpha 2-agonists, norepinephrine, epinephrine and clonidine, had no effect on neuronal cyclic 3',5'-adenosine monophosphate content. Forskolin enhanced neuronal cyclic 3',5'-adenosine monophosphate from a control value of about 20 pmoles/mg protein to 150 pmoles/mg protein. In the presence of alpha 2-agonists and forskolin the cyclic 3,5'-adenosine monophosphate content increased between 340 and 430 pmoles/mg protein. The alpha 1-agonist, phenylephrine, had no such effect. The facilitatory effect of alpha 2-agonist on forskolin-stimulated cyclic 3',5'-adenosine monophosphate production was blocked by the alpha 2-antagonist, yohimbine, but not the alpha 1-agonist, prazosin. Dopamine did not affect neuronal cyclic 3',5'-adenosine monophosphate content, but forskolin-stimulated increase in cyclic 3',5'-adenosine monophosphate was further facilitated by dopamine, and this effect was blocked by haloperidol. Activation of neuronal alpha 2-receptors by norepinephrine, epinephrine and clonidine did not interfere with electrically induced release of tritium from [3H]-norepinephrine-loaded sympathetic neurons. However, if sympathetic neurons were co-cultured with heart cells, clonidine, norepinephrine and epinephrine markedly inhibited the stimulation-induced release. Yohimbine or phentolamine partially reversed the inhibitory effects of alpha 2-agonists. alpha 2-Agonists and -antagonists also modified stimulation-induced release of tritium from [3H]norepinephrine-loaded hearts of the chick embryo.(ABSTRACT TRUNCATED AT 250 WORDS)