Autonomic control of the location and rate of the cardiac pacemaker in the sinoatrial fat pad of parasympathetically denervated dog hearts

INTRODUCTION

Parasympathetic activity predominates over sympathetic activity not only with respect to heart rate but also with respect to the pacemaker location in the dog heart. After we removed the parasympathetic neural elements in the sinoatrial (SA) fat pad in the right atrium, we observed that cervical vagus stimulation did not decrease the atrial rate, but it did suppress the increase in rate evoked by sympathetic stimulation. We determined whether the pacemaker rate and location were affected by presynaptic or postsynaptic mechanisms.

METHODS AND RESULTS

We determined the earliest activation site by means of isochronic activation mapping of the right atrium of open chest, anesthetized dog hearts. An electrode array, which consisted of 48 unipolar electrodes, was used to record atrial activation. This array covered the three main pacemaker regions, including the SA node region. After parasympathetic nerve fibers in the SA fat pad had been denervated, vagus stimulation at 10 and 30 Hz did not decrease the heart rate, but it attenuated the increase in heart rate evoked by sympathetic stimulation or isoproterenol. Vagus stimulation at 10 Hz during sympathetic stimulation did not shift the earliest activation site from the superior pacemaker region to the SA node region in 11 of 18 experiments. However, vagus stimulation at 10 Hz during isoproterenol infusion shifted the earliest activation site to the SA node region in 11 of 13 experiments. More intense vagus stimulation during combined sympathetic stimulation or isoproterenol infusion shifted the earliest activation site to the SA node or the inferior pacemaker region in 15 of 18 and in all experiments, respectively.

CONCLUSION

The results suggest that activation of parasympathetic elements not located in the SA fat pad attenuates the increase in heart rate and the shift in pacemaker location evoked by sympathetic activation. The sympathetic and parasympathetic effects interact at presynaptic and postsynaptic sites in the dog heart.

Arterial baroreflex function in conscious rats

Arterial baroreflex (ABR) is a very important mechanism in the regulation of cardiovascular activities. As ABR function is largely inhibited by anesthesia, its measurement in conscious animal becomes important. The present review summarizes the works concerning ABR function in conscious rats completed in our department in the last 10 years. Firstly, a new method was established to measure arterial baroreflex-blood pressure control (ABR-BP). ABR-BP and baroreflex sensitivity measured with classic method are two different parts of the ABR function. Secondly, it was proposed that ABR function predicted the end-organ damage in hypertension. Thirdly, interruption of ABR induced severe end-organ damages. Increased blood pressure variability (BPV) and activation of renin angiotensin system were involved in the mechanisms underlying organ damages in sinoaortic denervation (SAD) rats. Fourthly, we propose that amelioration of ABR function may serve as a new strategy for improving the prognosis of cardiovascular diseases. Ketanserin improved the impaired ABR function in SHR. Finally, the possibility to develop a strain of rats with spontaneous deficiency on ABR function is mentioned.

Candesartan inhibits sinoaortic denervation-induced cardiovascular hypertrophy in rats

AIM

To study the effect of candesartan cilexetil (candesartan), a new AT1 receptor antagonist, on sinoaortic denervation (SAD)-induced cardiovascular hypertrophy and its potential mechanisms in rats.

METHODS

For long-term treatment, candesartan (6 mg/kg/d) was given in rat food for 16 weeks after SAD surgery, and for acute treatment, a single dose of candesartan (3 mg/kg) was administrated intragastrically at 30 d after SAD.

RESULTS

The indexes of left ventricular and aortic hypertrophy in candesartan-treated SAD rats were decreased when compared with untreated SAD rats, and similar to or less than those in normal rats. SAD-induced cardiomyocyte hypertrophy, myocardial fibrosis, wall thickening of intramyocardial arterioles and aortae, and destruction of vascular internal elastin membrane were almost inhibited by candesartan. The plasma angiotensin II levels were markedly increased in treated SAD rats and negatively correlated with the indexes of hypertrophy. Both blood pressure and its variability were reduced by a single dose of candesartan during 3 h of observation period.

CONCLUSION

Candesartan can efficiently inhibit SAD-induced cardiovascular hypertrophy. In addition to known mechanisms, upregulation of circulating angiotensin II and stabilization of blood pressure may be involved in this cardiovascular protection of candesartan.

Parasympathetic effects on cardiac electrophysiology during exercise and recovery

Depressed parasympathetic tone is associated with an increased risk of sudden cardiac death. Exercise and the postexercise recovery period, which are associated with parasympathetic withdrawal, are high risk periods for sudden death. However, parasympathetic effects on cardiac electrophysiology during exercise and recovery have not been described. Electrophysiology studies were performed using noninvasive programmed stimulation (NIPS) in nine subjects (age 59 +/- 18 yr) with implanted dual-chamber devices and normal left ventricular function during multiple bicycle exercise sessions. NIPS was performed at rest, during exercise, and in the early recovery period both before and after parasympathetic blockade with atropine. Parasympathetic effect was defined as the value of the parameter of interest in the absence of atropine minus the value of the parameter in the presence of atropine. During exercise, sinus cycle length, atrioventricular (AV) block cycle length, AV interval, and ventricular effective refractory period shortened; in recovery, the values were intermediate between the rest and exercise values (P < 0.0001 by ANOVA). Parasympathetic effects on sinus cycle length, AV block cycle length, AV interval, and ventricular effective refractory period were 247 +/- 140, 58 +/- 20, 76 +/- 20, and 8.6 +/- 7.5 ms at rest, 106 +/- 20, 37 +/- 14, 24 +/- 13, and 2.6 +/- 7.8 ms during exercise, and 209 +/- 114, 50 +/- 23, 35 +/- 21, and 9.5 +/- 11.8 ms during recovery, respectively. There was poor correlation among the parasympathetic effects noted at the sinus node, AV node, and ventricle. Further work evaluating parasympathetic effects on cardiac electrophysiology during exercise and recovery in patients with heart disease is required to elucidate its role in modulating the risk of sudden cardiac death noted at these times.

ANP potentiates nonarterial baroreflex bradycardia: evidence from sinoaortic denervation in rats

Previous findings indicate that atrial natriuretic peptide (ANP) enhances the reflex bradycardia arising from stimulation of cardiac mechanoreceptors and chemoreceptors, but not that from arterial baroreceptors. The present study tests this proposal by examining the effect of ANP on these reflexes in six chronically sinoaortic-denervated (SAD), and eight sham-operated (sham), conscious rats. Arterial baroreceptor-heart rate (HR) reflex function was examined by constructing full-range steady-state blood pressure (BP)-HR curves using alternating doses of pressor (methoxamine, 2-100 microg/kg) and depressor (nitroprusside, 1-50 microg/kg) agents. Nonarterial baroreceptor reflex function was assessed by the 'ramp' bradycardic response to the rapid BP rise after i.v. methoxamine (100 microg/kg bolus dose). The cardiopulmonary chemoreflex was evoked by i.v. injections of serotonin (1-20 microg/kg). These three tests were performed on each rat during infusions, in random order, of rat ANP (150 ng/kg/min i.v.) and saline vehicle. The ability of ANP to significantly enhance ramp reflex bradycardia was not diminished in SAD compared with sham rats (+54 +/- 12% vs. +42 +/- 15%, respectively). ANP also significantly enhanced cardiopulmonary chemoreflex bradycardia in both groups (+60 +/- 15% in SAD, +40 +/- 8% in sham). Neither the normal steady-state BP-HR response in sham rats nor the small residual response in SAD rats was enhanced by ANP (-1 +/- 7% in sham, -11 +/- 8% in SAD). We conclude that ANP enhances reflex bradycardias of nonarterial, probably cardiac mechanoreceptor, origin.

[A quantitative analysis of peptidergic innervation in sinoatrial node in cases of sudden manhood death syndrome]

OBJECTIVE

To study the distribution and proportion of neuropeptide containing nervers in the sinus node in cases of sudden manhood death syndrome (SMDS) and to explore the mechanism of SMDS.

METHODS

Immunohistochemical staining and quantitative analysis of neuropeptide Y (NPY) and vasoactive intestinal peptide(VIP) in the sinus node in 6 cases of SMDS and in 12 cases of non-cardiac death(control group) were achieved by LSAB method and computerized image system.

RESULTS

As for NPY positive materials, VIP positive materials and the ratio of VIP/NPY in the sinus nodes, there were no significant difference between the control group and SMDS group.

CONCLUSION

The mechanism of SMDS and the abnormality of autonomic nervous innervation in the sinoatrial nodes maybe incorrelation.

QT & RR variability spots the earliest autonomic deregulation in diabetes. Fading of vagal sino-atrial drive but not of sympathetic ventricular responsiveness to life challenges

27 consecutive insulin-dependent diabetic patients (pts), under 50 years, with blood glucose controlled within normal limits and no significant or multiple cardiovascular/neurological complications in the lights of clinical tests, went through a protocol as follows: laiddown at relaxed rest for 10 min, then stood-up quietly for 7 min, and finally experienced a stress-interview for 10 min while supine. A thoracic ECG lead was digitized at I ms (Codas, Dataq Instr.), RR and QT intervals were software-detected, resampled at 500 ms, and Fourier-transformed over 3 min epochs to get auto-or cross-spectra. RR-by-QT mean square coherence detached the RR-independent fraction of QT low fequency (LF) spectral power, called idioventricular QT-LF. We detected autonomic impairment of three types (discriminant score = 92.31%), presumably differentiated upon the locus of lesion, using RR's basal variance and mean RR shortening when standing as follows: (I) RR shortening > 200 ms in 10 pts; (II) normal RR shortening but no RR variance in 4 pts; (III) stiff RR around 600 ms and no RR variance in 2 pts. The above pts have been excluded from further analysis. The remaining 11 pts with no such impairments (5M and 6F, 36.4 y +/- 4.4 SD, history of 6.0 y +/- 5.2) have been compared with 11 normal subjects in an age and gender-paired control group in two steps. Step 1: Preliminary MANOVA/ANOVA showed significant effects on the ensemble of spectral variables of every single factor (status: normal or patient group; intervention; gender) with no significant factor interactions. Significant effects of intervention or status on main RR spectral variables and on a few QT spectral variables were also documented. Step 2: Non-parametric tests showed that diabetics had (mildly to moderately) shorter mean RR, while their RR-LF was always significantly lower than those found in normals--a difference propagated to QT-LF but not to idioventricular QT-LF. In the intra-group there were similar responses to interventions except stress with respect to mean RR. Consistent reduction in RR-LF under moderate or no change in mean RR suggests vagal down- regulation that, judging by idioventricular QT-LF showing, goes perhaps before a similar process with sympathetic control of ventricles. This phase delay may introduce an early arrhythmic risk worth dealing with in secondary prevention.

Nitric oxide facilitates vagal control of heart rate via actions in the cardiac parasympathetic ganglia of the anaesthetised dog

The effects of neuronal inhibition of nitric oxide (NO) production on the bradycardia resulting from stimulation of preganglionic and postganglionic parasympathetic fibres were investigated in an anaesthetised dog preparation following transection of the cervical vagi and in the presence of a beta-adrenoreceptor antagonist. Injection of 1-(2-trifluoromethylphenyl) imidazole (TRIM), an inhibitor of neuronally released NO, into the sinus node artery reduced the bradycardia evoked by right cervical vagal stimulation. In contrast, when the response to preganglionic stimulation had been abolished by hexamethonium (10 mg kg(-1)), the bradycardia following stimulation of postganglionic parasympathetic fibres on the atrial epicardium was unaffected by TRIM. First, these results confirm the facilitatory actions of neuronally released NO on vagal heart responses in the dog. Second, they indicate that this modulatory and facilitatory role of NO is likely to be exerted at vagal preganglionic-postganglionic synaptic mechanisms in the cardiac parasympathetic ganglia and not at the postganglionic-sinoatrial node synapse.

Peripheral vagal control of heart rate is impaired in neuronal NOS knockout mice

The role of nitric oxide (NO) in the vagal control of heart rate (HR) is controversial. We investigated the cholinergic regulation of HR in isolated atrial preparations with an intact right vagus nerve from wild-type (nNOS+/+, n = 81) and neuronal NO synthase (nNOS) knockout (nNOS-/-, n = 43) mice. nNOS was immunofluorescently colocalized within choline-acetyltransferase-positive neurons in nNOS+/+ atria. The rate of decline in HR during vagal nerve stimulation (VNS, 3 and 5 Hz) was slower in nNOS-/- compared with nNOS+/+ atria in vitro (P < 0.01). There was no difference between the HR responses to carbamylcholine in nNOS+/+ and nNOS-/- atria. Selective nNOS inhibitors, vinyl-L-niohydrochloride or 1-2-trifluoromethylphenyl imidazole, or the guanylyl cyclase inhibitor, 1H-[1,2,4]oxadiazolo[4,3-a]quinoxalin-1-one significantly (P < 0.05) attenuated the decrease in HR with VNS at 3 Hz in nNOS+/+ atria. NOS inhibition had no effect in nNOS-/- atria during VNS. In all atria, the NO donor sodium nitroprusside significantly enhanced the magnitude of the vagal-induced bradycardia, showing the downstream intracellular pathways activated by NO were intact. These results suggest that neuronal NO facilitates vagally induced bradycardia via a presynaptic modulation of neurotransmission.

Differential characteristics of neural circulatory control: early versus late after cardiac transplantation

BACKGROUND

Reappearance of low-frequency (LF) (+/-0.10 Hz) oscillations in RR interval (RR) after cardiac transplantation is indicative of sympathetic efferent reinnervation. We hypothesized that restored LF oscillations in RR in heart transplant recipients (HTRs) are linked to oscillations in muscle sympathetic nerve traffic (MSNA).

METHODS AND RESULTS

RR, RR variability, and MSNA were recorded 5+/-2 months (n=7, short-term HTRs) and 138+/-8 months (n=7, long-term HTRs) after heart transplantation and compared with matched hypertensive patients (n=7). A coherence function determined the coupling between LF oscillations in MSNA and RR. RR variance did not differ between short-term and long-term HTRs. However, LF variability was only 1+/-0.5 ms(2) in the short-term HTRs but was 15+/-8 ms(2) in the long-term HTRs (P<0.05). Normalized LF variability was also higher in the long-term HTRs (40+/-14 normalized unites) versus the short-term HTRs (6+/-3 normalized united, P<0.05) but did not differ from the LF variability of the hypertensive patients. Long-term HTRs were taking less cyclosporine (P<0.01) but had higher MSNA than the short-term HTRs (62+/-7 versus 31+/-7 burst/min, respectively, P<0.05). Coherence between LF oscillations in MSNA and RR was similar in the long-term HTRs (0.59+/-0.11) and the hypertensive patients (0.60+/-0.07) and was 3-fold greater than in the short-term HTRs (0.20+/-0.06, P<0.05).

CONCLUSIONS

Cardiac reinnervation after long-term heart transplantation is characterized by a restoration of the coherence between LF oscillations in RR and MSNA. Higher MSNA in long-term than in short-term HTRs suggests that time elapsed after cardiac transplantation may be a major determinant of sympathetic excitation in heart transplant recipients.

Nitric oxide-cGMP pathway facilitates acetylcholine release and bradycardia during vagal nerve stimulation in the guinea-pig in vitro

1. We tested the hypothesis that nitric oxide (NO) augments vagal neurotransmission and bradycardia via phosphorylation of presynaptic calcium channels to increase vesicular release of acetylcholine. 2. The effects of enzyme inhibitors and calcium channel blockers on the actions of the NO donor sodium nitroprusside (SNP) were evaluated in isolated guinea-pig atrial-right vagal nerve preparations. 3. SNP (10 microM) augmented the heart rate response to vagal nerve stimulation but not to the acetylcholine analogue carbamylcholine (100 nM). SNP also increased the release of [3H]acetylcholine in response to field stimulation. No effect of SNP was observed on either the release of [3H] acetylcholine or the HR response to vagal nerve stimulation in the presence of the guanylyl cyclase inhibitor 1H-(1,2,4)-oxadiazolo-(4,3-a)-quinoxalin-1-one (ODQ, 10 microM). 4. The phosphodiesterase 3 (PDE 3) inhibitor milrinone (1 microM) increased the release of [3H] acetylcholine and the vagal bradycardia and prevented any further increase by SNP. SNP was still able to augment the vagal bradycardia in the presence of the protein kinase G inhibitor KT5823 (1 microM) but not after protein kinase A (PKA) inhibition with H-89 (0.5 microM) or KT5720 (1 microM) had reduced the HR response to vagal nerve stimulation. Neither milrinone nor H-89 changed the HR response to carbamylcholine. 5. SNP had no effect on the magnitude of the vagal bradycardia after inhibition of N-type calcium channels with omega-conotoxin GVIA (100 nM). 6. These results suggests that NO acts presynaptically to facilitate vagal neurotransmission via a cGMP-PDE 3-dependent pathway leading to an increase in cAMP-PKA-dependent phosphorylation of presynaptic N-type calcium channels. This pathway may augment the HR response to vagal nerve stimulation by increasing presynaptic calcium influx and vesicular release of acetylcholine.

[Autoregulation of renal blood flow and blood pressure variability in the conscious rat]

It is often proposed that autoregulatory mechanisms prevent acute changes in systemic blood pressure (BP) from being transmitted to the glomerular capillary circulation. However, it is not known whether renal blood flow (RBF) is still autoregulated when the kidney is exposed to exaggerated BP fluctuations, in particular hypertensive episodes. The aim of the present study was therefore to evaluate the efficacy of renal autoregulatory responses in an animal model of BP lability, the sinoaortic denervated (SAD) rat. BP and RBF were simultaneously recorded in 8 SAD (2 wks before study) and 8 baroreceptor intact (INT) Sprague-Dawley rats during approximately 3 h of spontaneous activity. The left kidney used for RBF recordings was denervated to prevent the interference of changes in renal sympathetic tone with autoregulatory responses. The SAD procedure modified neither the mean BP nor the mean RBF levels (111 +/- 1 mmHg and 11.3 +/- 1.3 mL/min in INT rats: 113 +/- 6 mmHg and 11.1 +/- 0.9 mL/min in SAD rats). However, SAD strongly increased the BP variability (coefficient of variation: 5.9 +/- 0.2% and 18.2 +/- 1.1% in INT and SAD rats, respectively). In spite of this marked BP lability, RBF variability was not significantly affected by the SAD procedure (9.1 +/- 0.8% and 12.4 +/- 1.6% in INT and SAD rats, respectively). In SAD rats, spontaneous hypertensive episodes (top 1% of BP values: 174 +/- 10 mmHg) did not induce increases in RBF (10.5 +/- 1.0 ml/min). Fast Fourier transform analysis revealed that in SAD rats, autoregulatory mechanisms attenuated approximately 80% of BP fluctuations in the 0.0015-0.01 Hz frequency range, suggesting a major involvement of the tubuloglomerular feedback. In conclusion, autoregulatory mechanisms have an ample capacity to protect the kidney against spontaneous BP fluctuations in the conscious rat. Consequently, BP variability per se is probably not detrimental to the kidney, as long as autoregulatory mechanisms are normally functioning.

Comparison of spontaneous vs. metronome-guided breathing on assessment of vagal modulation using RR variability

R-R interval variability (RR variability) is increasingly being used as an index of autonomic activity. High-frequency (HF) power reflects vagal modulation of the sinus node. Since vagal modulation occurs at the respiratory frequency, some investigators have suggested that HF power cannot be interpreted unless the breathing rate is controlled. We hypothesized that HF power during spontaneous breathing would not differ significantly from HF power during metronome-guided breathing. We measured HF power during spontaneous breathing in 20 healthy subjects and 19 patients with heart disease. Each subject's spontaneous breathing rate was determined, and the calculation of HF power was repeated with a metronome set to his or her average spontaneous breathing rate. There was no significant difference between the logarithm of HF power measured during spontaneous and metronome-guided breathing [4.88 +/- 0.29 vs. 5.29 +/- 0.30 ln(ms(2)), P = 0.32] in the group as a whole and when patients and healthy subjects were examined separately. We did observe a small (9.9%) decrease in HF power with increasing metronome-guided breathing rates (from 9 to 20 breaths/min). These data indicate that HF power during spontaneous and metronome-guided breathing differs at most by very small amounts. This variability is several logarithmic units less than the wide discrepancies observed between healthy subjects and cardiac patients with a heterogeneous group of cardiovascular disorders. In addition, HF power is relatively constant across the range of typical breathing rates. These data indicate that there is no need to control breathing rate to interpret HF power when RR variability (and specifically HF power) is used to identify high-risk cardiac patients.

Local opiate receptors in the sinoatrial node moderate vagal bradycardia

Met-enkephalin-arg-phe (MEAP) interrupts vagal bradycardia when infused into the systemic circulation. This study was designed to locate the opiate receptors functionally responsible for this inhibition. Previous observations suggested that the receptors were most likely located in either intracardiac parasympathetic ganglia or the pre-junctional nerve terminals innervating the sinoatrial node. In this study 10 dogs were instrumented with a microdialysis probe inserted into the sinoatrial node. The functional position of the probe was tested by briefly introducing norepinephrine into the probe producing an increase in heart rate of more than 30 beats/min. Vagal stimulations were conducted at 0.5, 1.2 and 4 Hz during vehicle infusion (saline ascorbate). Cardiovascular responses during vagal stimulation were recorded on-line. MEAP was infused directly into the sinoatrial node via the microdialysis probe. The evaluation of vagal bradycardia was repeated during the nodal application of MEAP, diprenorphine (opiate antagonist), and diprenorphine co-infused with MEAP. MEAP introduced into the sinoatrial node via the microdialysis probe reduced vagal bradycardia by more than half. Simultaneous local nodal blockade of these receptors with the opiate antagonist, diprenorphine, eliminated the effect of MEAP demonstrating the participation by opiate receptors. Systemic infusions of MEAP produced a reduction in vagal bradycardia nearly identical to that observed during nodal administration. When local nodal opiate receptors were blocked with diprenorphine, the systemic effect of MEAP was eliminated. These data lead us to suggest that the opiate receptors responsible for the inhibition of vagal bradycardia are located within the sinoatrial node with few, if any, participating extra-nodal or ganglionic receptors.

Inotropic, chronotropic, and dromotropic effects mediated via parasympathetic ganglia in the dog heart

Some parasympathetic ganglionic cells are located in the epicardial fat pad between the medial superior vena cava and the aortic root (SVC-Ao fat pad) of the dog. We investigated whether the ganglionic cells in the SVC-Ao fat pad control the right atrial contractile force, sinus cycle length (SCL), and atrioventricular (AV) conduction in the autonomically decentralized heart of the anesthetized dog. Stimulation of both sides of the cervical vagal complexes (CVS) decreased right atrial contractile force, increased SCL, and prolonged AV interval. Stimulation of the rate-related parasympathetic nerves to the sinoatrial (SA) node (SAPS) increased SCL and decreased atrial contractile force. Stimulation of the AV conduction-related parasympathetic nerves to the AV node prolonged AV interval. Trimethaphan, a ganglionic nicotinic receptor blocker, injected into the SVC-Ao fat pad attenuated the negative inotropic, chronotropic, and dromotropic responses to CVS by 33 approximately 37%. On the other hand, lidocaine, a sodium channel blocker, injected into the SVC-Ao fat pad almost totally inhibited the inotropic and chronotropic responses to CVS and partly inhibited the dromotropic one. Lidocaine or trimethaphan injected into the SAPS locus abolished the inotropic responses to SAPS, but it partly attenuated those to CVS, although these treatments abolished the chronotropic responses to SAPS or CVS. These results suggest that parasympathetic ganglionic cells in the SVC-Ao fat pad, differing from those in SA and AV fat pads, nonselectively control the atrial contractile force, SCL, and AV conduction partially in the dog heart.

Heart rate responses to a muscarinic agonist in rats with experimentally induced acute and subacute chagasic myocarditis

We administered arecoline to rats, with experimentally induced chagasic myocarditis, in order to study the sinus node sensitivity to a muscarinic agonist. Sixteen month old rats were inoculated with 200,000 T. cruzi parasites ("Y" strain). Between days 18 and 21 (acute stage), 8 infected rats and 8 age-matched controls received intravenous arecoline as a bolus injection at the following doses: 5. 0, 10.0, 20.0, 40.0, and 80.0 microg/kg. Heart rate was recorded before, during and after each dose of arecoline. The remaining 8 infected animals and 8 controls were subjected to the same experimental procedure during the subacute stage, i.e., days 60 to 70 after inoculation. The baseline heart rate, of the animals studied during the acute stage (349 +/- 68 bpm, mean +/- SD), was higher than that of the controls (250 +/- 50 bpm, p < 0.005). The heart rate changes were expressed as percentage changes over baseline values. A dose-response curve was constructed for each group of animals. Log scales were used to plot the systematically doubled doses of arecoline and the induced-heart rate changes. The slope of the regression line for the acutely infected animals (r = - 0.99, b =1.78) was not different from that for the control animals (r = - 0.97, b = 1.61). The infected animals studied during the subacute stage (r = - 0.99, b = 1.81) were also not different from the age-matched controls (r = - 0.99, b = 1.26, NS). Consequently, our results show no pharmacological evidence of postjunctional hypersensitivity to the muscarinic agonist arecoline. Therefore, these results indirectly suggest that the postganglionic parasympathetic innervation, of the sinus node of rats with autopsy proved chagasic myocarditis, is not irreversibly damaged by Trypanosoma cruzi.

Sympathetic reinnervation of the sinus node and exercise hemodynamics after cardiac transplantation

BACKGROUND

Sympathetic cardiac reinnervation occurs variably after cardiac transplantation (CT) in humans. We hypothesized that sinus node reinnervation would partially restore normal chronotropic response to exercise.

METHODS AND RESULTS

Thirteen recent CT recipients, 28 late CT recipients (> or =1 year after CT), and 20 control subjects were studied. Sinus node sympathetic reinnervation was determined by heart rate (HR) change after tyramine injection into the artery that perfused the sinus node. HR changes of <5 and > or =15 bpm were defined, respectively, as denervation and marked reinnervation. During treadmill exercise, HR, blood pressure, and expired O(2) and CO(2) were measured. All early transplant recipients exhibited features typical of denervation (basal HR, 88+/-2 bpm; peak HR, 132+/-4 bpm, peaking 1.8+/-0.3 minutes after exercise cessation and slowly declining after exercise). A similar pattern was found in the 12 late transplant recipients with persistent sinus node denervation. However, in patients with marked reinnervation, exercise HR rose more (peak HR, 142+/-4 and 141+/-2 bpm), peaked earlier after cessation of exercise (0.7+/-0.4 and 0. 3+/-0.1 minute), and fell more rapidly. Exercise duration and maximal oxygen consumption were not related significantly to reinnervation status, but a trend existed for longer exercise time in markedly reinnervated patients.

CONCLUSIONS

The present studies suggest that sympathetic reinnervation of the sinus node is accompanied by partial restoration of normal HR response to exercise. Both maximal oxygen consumption and exercise duration were markedly shorter in CT patients than in control subjects, and most of the difference was not related to innervation status.

Sympathetic reinnervation of sinus node and left ventricle after heart transplantation in humans: regional differences assessed by heart rate variability and positron emission tomography

BACKGROUND

Orthotopic heart transplantation (HTx) results in complete cardiac denervation. Reestablished partial sympathetic nerve function has been found in patients some years after HTx. However, the atrial and ventricular regional patterns of reinnervation have not been established.

METHODS

Two parallel methods were used to evaluate the regional restoration of sympathetic nerves in the myocardium. Patients were investigated with respect to ventricular reinnervation (VI) using positron emission tomography (PET) and the norepinephrine analogue C-11-hydroxyephedrine (HED). Tracer uptake was quantified using dynamic imaging protocols, yielding regional HED retention fraction. A regional value above 7%/minute ( +/- 2.5 SD above the mean value of denervated hearts) was considered evidence of sympathetic reinnervation. Spectral analysis of heart rate variability (HRV) served as a quantitative marker for reinnervation at the sinus node (SI). Spectra of HRV during positive head-up tilt were calculated. The low frequency (LF) power spectral density (0.05 to 0.18 Hz) was evaluated.

RESULTS

After HTx (4. 6 +/- 3.9 years; range, 0.2 to 13.6 years), 38 patients (aged 50.9 +/- 7.6 years; range, 37 to 65 years) were investigated by PET imaging and HRV. Twenty-two patients with a mean HED retention of 10. 7 +/- 2.6%/minute were classified as left ventricular reinnervated. Sixteen patients with a mean HED retention of 4.8 +/- 0.8%/minute did not reach the threshold. The time difference after HTx was significant for these 2 groups, 5.3 +/- 3.0 years vs 3.8 +/- 4.7 years ( p < 0.05 ). The LF power spectral density of the ventricular reinnervated patients was 5.9 +/- 8.6 ms(2), and 1.8 +/- 4.4 ms(2) (p <0.005 ) for those not reinnervated. Low frequency showed small values and narrow distribution for the patients not reinnervated, assuming sinus node denervation, and showed extended distribution for the reinnervated, suggesting a heterogeneous reinnervation pattern.

CONCLUSIONS

Two non-invasive parallel methods were used to investigate regional reestablishment of cardiac nerves in the myocardium in HTx patients. Left VI assessed by PET imaging and SI by HRV was congruent in 60% of HTx patients. Lack of SI paralleled absence of VI. Our results suggest that partial VI occurs prior to SI.

The Cox maze iii procedure: parallel normalization of sinus node dysfunction, improvement of atrial function, and recovery of the cardiac autonomic nervous system

OBJECTIVE

The Cox maze III procedure includes isolation of the pulmonary veins and multiple incisions in both atria in what corresponds to partial autotransplantation and partial denervation of the heart. The aim of this prospective longitudinal study was to identify physiologic effects of reinnervation on changes in heart rate at rest and in response to various stimulations and on atrial function after the Cox maze III procedure.

PATIENTS AND METHODS

Power spectral analysis of heart rate variability, exercise testing, 24-hour Holter monitoring, electrocardiography, and transthoracic and transesophageal echocardiography were performed in 30 adult patients after the combined Cox maze III procedure and mitral valve surgery (maze group). They were prospectively followed up at 1, 3, 6, and 12 months after the operation. The results were compared with those of 15 heart transplant recipients (transplant group) and normal probands (healthy adults, n = 12).

RESULTS

The physiologic effects of denervation with no differences in cardiac autonomic activity between the groups were seen early after the operation. Later, evidence of autonomic reinnervation was observed only in the maze group but not in the transplant group. Inappropriate heart rate responses during physical exercise were clearly evident in both groups after 1 and 3 months, with progressive improvement seen between 6 and 12 months only in the maze group. Left atrial function after the Cox maze procedure improved parallel to the recovery of sinus node function.

CONCLUSION

Progressive improvement of sinus node function and atrial contractions with significant functional normalization 1 year after the Cox maze procedure corresponded to functional reinnervation and recovery of the autonomic nervous system.

Characterization of sinoatrial parasympathetic innervation in humans

INTRODUCTION

The response to sinoatrial parasympathetic nerve stimulation (shortened atrial refractoriness) was used to determine the atrial distribution of these nerve fibers in humans. We hypothesized that, in humans, parasympathetic nerves that innervate the sinoatrial node also innervate the right atrium and that the greatest density of innervation is near the sinoatrial nodal fat pad.

METHODS AND RESULTS

Temporary epicardial wire electrodes were sutured in pairs in the sinoatrial nodal fat pad, high right atrium, and right ventricle by direct visualization during coronary artery bypass surgery in nine patients. Appropriate electrode placement was confirmed by electrically stimulating the fat pad in the operating room to prolong sinus cycle length by 50%. Experiments were performed in the electrophysiology laboratory 1 to 5 days after surgery. Programmed atrial stimulation was performed via an endocardial electrode catheter advanced to the right atrium. The catheter tip electrode was moved in 1-cm concentric zones around the epicardial wires by fluoroscopic guidance. Atrial refractoriness was determined in the presence and absence of sinoatrial parasympathetic nerve stimulation at each catheter site. In 8 of 9 patients, parasympathetic nerve stimulation reproducibly prolonged sinus cycle length by 50%. There was no effect on AV nodal conduction (no prolongation of PR interval) and no change in AV nodal refractoriness. Atrial effective refractory periods reproducibly shortened in response to parasympathetic nerve stimulation in 1-cm zones up to 3 cm surrounding the fat pad, by a mean (+/- SEM) of 26.6+/-4.3 msec (zone 1), 11.4+/-1.8 msec (zone 2), and 10.0+/-2.5 msec (zone 3), respectively (P = 0.0001). At distances > 3 cm from the fat pad, the effective refractory period did not shorten.

CONCLUSION

Stimulation of parasympathetic nerves that innervate the sinoatrial node shortened atrial refractoriness in humans.

Parasympathetic inhibition of sympathetic effects on pacemaker location and rate in hearts of anesthetized dogs

INTRODUCTION

The site of impulse origin in the right atrium generally is considered to be a single static locus within the sinoatrial (SA) node. Previous investigators showed that the pacemaker site may shift due to changes in sympathetic or parasympathetic neural activity. We investigated the interactions between sympathetic and parasympathetic influences on the site of impulse initiation in the right atrium in anesthetized dogs.

METHODS AND RESULTS

We determined the site of impulse initiation and the spread of excitation over the anterior and posterior regions of the right atrium by a matrix of 48 unipolar recording electrodes. We assessed the spread of excitation at 3-msec intervals by constructing isochronal activation sequence maps. Sympathetic stimulation increased the frequency of atrial excitation (i.e., the heart rate), but also shifted the earliest activation region (EAR) from a locus in the SA node to a locus in the superior vena cava (the superior pacemaker site). Vagus stimulation decreased the heart rate and shifted the EAR to a lower site in the SA node or a site in the inferior right atrium along the sulcus terminalis (the inferior pacemaker site). A short period of vagus stimulation during a more prolonged sympathetic stimulation elicited a larger decrease in rate than did vagus stimulation alone and shifted the EAR from the superior site to the SA node or to the inferior site. After atropine, combined stimulation shifted the EAR to the superior site, but propranolol did not change EAR location.

CONCLUSION

Our results suggest that parasympathetic activity predominates over sympathetic activity not only on heart rate, but also on the location of the EAR in the anesthetized dog.

Anatomical study of the neural ganglionated plexus in the canine right atrium: implications for selective denervation and electrophysiology of the sinoatrial node in dog

The aim of the present study was to elucidate the topography and architecture of the intrinsic neural plexus (INP) in the canine right atrium because of its importance for selective denervation of the sinoatrial node (SAN). The morphology of the intrinsic INP was revealed by a histochemical method for acetylcholinesterase in whole hearts of 36 mongrel dogs and examined by stereoscopic, contact, and electron microscopes. At the hilum of the heart, nerves forming a right atrial INP were detected in five sites adjacent to the right superior pulmonary veins and superior vena cava (SVC). Nerves entered the epicardium and formed a INP, the ganglia of which, as a wide ganglionated field, were continuously distributed on the sides of the root of the SVC (RSVC). The epicardiac ganglia located on the RSVC were differentially involved in the innervation of the sinoatrial node, as revealed by epicardiac nerves emanating from its lower ganglia that proceed also into the atrial walls and right auricle. The INP on the RSVC (INP-RSVC) varied from animal to animal and in relation to the age of the animal. The INP-RSVC of juvenile dogs contained more small ganglia than that of adult animals. Generally, the canine INP-RSVC included 434+/-29 small, 17+/-4 medium-sized, and 3+/-1 large epicardiac ganglia that contained an estimated 44,700, 6,400, and 2,800 neurons, respectively. Therefore, the canine right atrium, including the SAN, may be innervated by more than 54,000 intracardiac neurons residing mostly in the INP-RSVC. In conclusion, the present study indicates that epicardiac ganglia that project to the SA-node are distributed more widely and are more abundant than was previously thought. Therefore, both selective and total denervation of the canine SAN should involve the whole region of the RSVC containing the INP-RSVC.

Nonpharmacologic validation of the intrinsic heart rate in cardiac transplant recipients

BACKGROUND

The maximum sinus rate during exertion in humans is inversely related to age. However, the sinus rate at rest is quite variable. The intrinsic heart (IHR) following pharmacologic blockade of autonomic tone with propranolol and atropine has been proposed as a test of sinus node function and is related to age by the linear regression equation: IHR = 118.1 - (0.57 x age). Whether this relationship exists for transplanted hearts for which the donor sinus node is denervated has not been determined.

METHODS

The relationship between the resting heart rate and the age of the donor heart was examined in 103 patients 1 year following orthotopic cardiac transplantation in the absence of rejection or intercurrent illness. Patients receiving beta-blockers, calcium blockers, antiarrhythmic drugs, digitalis, theophylline, or with biopsy evidence of rejection or abnormal coronary arteriograms were excluded from analysis.

RESULTS

The recipient age, left ventricular ejection fraction, pulmonary capillary pressure, cardiac index, donor heart ischemic time and cardiopulmonary bypass time did not correlate with the rate of the resting donor sinus node. The resting heart rate was inversely related to age of the donor heart by the linear regression equation: HR = 112.0 - (046 x age).

CONCLUSION

The resting rate of the denervated sinus node is related to donor age with a regression equation that is similar, though slightly slower, than that predicted after pharmacologic autonomic blockade.

Organization of the sympathetic innervation of the forelimb resistance vessels in the cat

Detailed information on the outflow pathway of sympathetic vasoconstrictor fibers to the upper extremity is lacking. We studied the organization of the sympathetic innervation of the forelimb resistance vessels and of the sinoatrial (SA) node in the decerebrated, artificially respirated cat. The distal portion of sectioned individual rami T1-8 and the sympathetic chain immediately caudal to T8 on the right side were electrically stimulated while the right forelimb perfusion pressure (forelimb perfused at constant flow) and heart rate were recorded. Increases in perfusion pressure were evoked by stimulation of T2-8 (maximal response T7: 55 +/- 2.3 mm Hg). Responses were still evoked by stimulation of the sympathetic chain immediately caudal to T8 (44 +/- 15 mm Hg). Increases in heart rate were evoked by the stimulation of more rostral rami (T1-5; maximal response T3: 55.2 +/- 8 bpm). These vasoconstrictor and cardioacceleratory responses were blocked by the cholinergic antagonists hexamethonium and scopolamine. Sectioning of the vertebral nerve and the T1 ramus abolished the vasoconstrictor response. Stimulation of the vertebral nerve and of the proximal portion of the sectioned T1 ramus increased perfusion pressure (69 +/- 9 and 34 +/- 14 mm Hg, respectively), which was unaffected by ganglionic cholinergic block. These data suggest that forelimb resistance vessel control is subserved by sympathetic preganglionic neurons located mainly in the middle to caudal thoracic spinal segments. Some of the postganglionic axons subserving vasomotor function course through the T1 ramus, in addition to the vertebral nerve.

IMPLICATIONS

Forelimb vasculature is controlled by sympathetic preganglionic neurons located in middle to caudal thoracic spinal segments and by postganglionic axons carried in the T1 ramus and vertebral nerve. This helps to provide the anatomical substrate of interruption of sympathetic outflow to the upper extremity produced by major conduction anesthesia of the stellate ganglion or spinal cord.