Cardiovascular sympathetic afferent fibers

Phrenic afferent input excites C1-C2 spinal neurons in rats

Effects of electrical stimulation of the ipsilateral phrenic nerve above the heart were determined on cells in the C1-C2 spinal cord segments of 27 rats anesthetized with pentobarbital. Forty-five cells that responded to this stimulus were included in this study. These cells then were examined at the same stimulus parameters for effects of stimulating the ipsilateral phrenic nerve below the heart, the contralateral phrenic nerve above the heart, and/or the left (ipsilateral) cervical vagus nerve. Ipsilateral phrenic nerve stimulation below the heart had no effect on 20 of 20 cells tested. Seven of 16 cells tested for effects of contralateral phrenic nerve stimulation above the heart were excited and activity of 9 cells was unaffected. Activity changes and activation latencies were not significantly different in the 7 cells excited by both ipsilateral and contralateral phrenic nerve stimulation. Thirty-seven of 45 cells excited by ipsilateral phrenic nerve stimulation also were excited by stimulation of the left cervical (ipsilateral) vagus nerve. The mean increase in cell activity was significantly greater following vagal nerve stimulation compared to phrenic nerve stimulation, and mean activation latency was significantly longer for vagal stimulation. Excitatory receptive somatic fields were classified for 35 cells. Somatic field locations for most cells (80%) included the ipsilateral neck and jaw. Activity of 26 cells was increased by both noxious pinch and brushing the hair, activity of 6 cells was increased only by noxious pinch, and activity of 3 cells was increased only by brushing the hair. Results of this study indicate that there are afferent fibers in the phrenic nerve above the heart, but not below the heart, that excite cells in the C1-C2 segments of the spinal cord. Most cells also were excited by noxious stimuli applied to their somatic receptive fields. Thus, the phrenic nerve may provide a pathway for referral of pain to the neck and jaw from thoracic structures.

Adenosine activates cardiac sympathetic afferent fibers and potentiates the excitation induced by coronary occlusion

Adenosine is a possible mediator of cardiac pain during myocardial ischemia; however, little is known about the influence of adenosine on cardiac sympathetic afferent activity and thereby on its algogenic mechanism. In 20 anaesthetized, decerebrated, curarized and artificially ventilated cats, we studied the impulse activity of 20 single afferent sympathetic fibers with a left ventricular receptive field in relation to epicardial applications of adenosine, coronary artery occlusions and arterial pressure rises. All fibers increased their impulse activity (from 1.2 +/- 0.2 to 2.6 +/- 0.5 imp/s; P < 0.001) during slight (20 +/- 8%) rises in aortic pressure, thus exhibiting low-threshold receptor characteristics. In 10 cats, epicardial applications of three different doses of adenosine (0.1, 1 and 10 mg/ml) caused a brief increase in neural activity with dose-related responses. This response was abolished by aminophylline, a P1 purinergic inhibitor. In the other group of 10 cats, four subsequent 30-s occlusions of the coronary arterial vessel supplying the receptive fields of the fibers were performed, in control conditions and 30 s, 3 and 7 min, respectively, after the end of excitation induced by adenosine (1 mg/ml) application. During the control coronary occlusion the impulse activity increased from 1.1 +/- 0.1 to 5.5 +/- 0.7 imp/s (P < 0.0001). A similar activation was present during the second occlusion initiated 30 s after the end of adenosine-induced activation. In contrast, a significant potentiation of the response was observed (8.8 +/- 1.2 vs. 5.3 +/- 0.9 imp/s; P < 0.001) during the occlusion initiated 3 min after the end of excitation by adenosine. This effect was no longer present during the last occlusion performed after 7 min. When the protocol was repeated substituting adenosine with saline (n = 5) or after i.v. administration of aminophylline (n = 5), no potentiation was observed, even though the excitatory response to coronary occlusion was preserved. These data show that adenosine can activate cardiac sympathetic afferent fibers in a dose-related manner, and potentiate their responses to coronary occlusion, while leaving unaffected the responsiveness to a hemodynamic stimulus. The excitatory effects are likely to involve the P1 purinergic receptors. The potentiation phenomenon might play a role in the genesis of an algogenic code.

Cardiorespiratory control: an integrated view. H.-P. Koepchen in memoriam

H.-P. Koepchen's presence at this symposium on cardiocirculatory function during sleep would have given us an inspiring impetus. His death has meant a severe loss for all of us. Koepchen's integrated view on cardiorespiratory control and rhythmicity has been the content of his late work. His considerations as documented in various papers are used for this contribution. They are based on earlier experimental studies of spontaneous coordinations of rhythmic activities in anaesthetized dogs, conscious men, and microelectrode recordings and local cooling experiments in the brain stem of anaesthetized dogs and cats, which he had worked on together with his students.

Heart rate variability during repeated arterial occlusion in coronary angioplasty

Acute coronary occlusion may cause severe autonomic reactions that can modify the clinical presentation of acute ischemic events. To evaluate whether adaptation in these autonomic reactions exists during repeated short coronary occlusions, heart rate (HR) and its variability in the time and frequency domains were analyzed in 70 patients with significant (50% to 95%) coronary artery stenosis immediately before and during 2 identical balloon occlusions of the vessel (mean 110 seconds). Reactions were compared with the range of nonspecific changes formed by analyzing a control group (n = 13) with no ischemia during balloon inflation in a totally occluded coronary artery. Thus, neither occlusion caused significant changes in HR or HR variability in 29 patients (41%). Vagal activation, as seen by an abnormal increase in HR variability or bradycardia, or both, was observed in 24 patients (34%). HR reactions in this group (p < 0.05) were significantly attenuated during the second occlusion. An opposite reaction (i.e., abnormal decrease in HR variability or tachycardia, or both) was observed in 17 patients (24%). A nonsignificant tendency for attenuation of the reactions was also seen in this group. Severity of chest pain, frequency of ST-segment shifts, or narrowing of pulse pressure were comparable during the 2 occlusions. Thus, a preceding short vessel occlusion-reperfusion cycle seems to attenuate autonomic HR reactions, especially vagal reactions, during subsequent coronary occlusion. Alleviation of extreme autonomic reactions may modify the clinical outcome of coronary occlusion in a beneficial way.

Neural regulation of sympathetic nerve activity in heart failure

One of the hallmarks of chronic congestive heart failure is an increase in sympathetic tone to the peripheral circulation and to the heart. A correlation between plasma norepinephrine and the severity of the heart failure state has been demonstrated. One mechanism that has been proposed to account for this sympathoexcitation is a depression in the baroreflex and, perhaps, cardiac reflex control of sympathetic nerve activity. This review summarizes work from several laboratories, including our own, that documents a depressed baroreflex control of heart rate and sympathetic nerve activity in both animals and humans with heart failure. The mechanism of the depressed baroreflex most likely is caused by reduced baroreceptor sensitivity as well as enhanced input to the central nervous system from cardiac receptors that are chemosensitive. Although sympathetic tone and arterial baroreflex sensitivity are altered in heart failure, there have been no studies showing a cause-and-effect relationship. Increases in plasma norepinephrine are similar in baroreceptor-denervated and intact dogs paced into heart failure. This latter observation cells into question the traditional concept of baroreceptor-mediated increases in sympathetic tone in heart failure.

Relation between myocardial infarction site and pain location in Q-wave acute myocardial infarction

Location, severity, duration, and time course of pain were assessed in 104 consecutive patients with either first or second, anterior or inferior Q-wave acute myocardial infarction (AMI). Pain severity was assessed using a visual analog scale. Pain location and radiation were similar in 48 patients with anterior and 56 patients with inferior wall AMI. Pain duration (6.1 +/- 6.4 vs 6.5 +/- 5.4 hours, p = NS) and severity (68 +/- 21 vs 61 +/- 21 mm, p = NS) were also similar. The pain was continuous in 34 patients with anterior (71%) and in 42 with inferior (75%) wall AMI. Among the 41 patients who did not receive thrombolytic therapy, 18 patients with continuous pain had a higher creatine kinase peak level than the remaining 23 patients with intermittent pain or preinfarction angina, or both (2,065 +/- 1,017 vs 1,162 +/- 994 IU, p < 0.01). The incidence of gastrointestinal symptoms was slightly higher in patients with inferior than anterior wall AMI (70% vs 48%, p < 0.05). Among 32 patients admitted with a second AMI, pain location was similar in 14 who had both infarcts in the same myocardial region, but was different in 12 of 18 (67%) who had a first and second infarct in different regions (p < 0.001). Thus, patients with anterior or inferior wall AMI experienced pain in similar body regions. However, in patients who presented with > 1 AMI, different locations of the infarction pain were highly predictive of ischemia occurring in different myocardial regions. Finally, patients with preinfarction angina or intermittent pain tended to have smaller infarcts.

Influence of morphine on the activity of low-threshold visceral mechanoreceptors in cats with acute pericarditis

The purpose of this investigation was to test whether morphine (morphinum hydrochloricum) applied to the receptive field of the thoracic visceral afferent fibres modifies their activity. Experiments were performed on chloralose-anaesthetised cats, paralysed and artificially ventilated, in a state of pericarditis that was induced by intrapericardial injection of lambda-carrageenan and kaolin. Resulting acute inflammation was proven histopathologically and documented electrocardiographically. Single afferent fibres with receptive fields in thoracic viscera were dissected from thoracic sympathetic chain (19 fibres), as well as the vagus nerve (9 fibres). All tested fibres transmitted sensory information from the low-threshold mechanoreceptors. As a final result, it was found that morphine (0.001-1.0 mg/ml) when applied locally activates, depending on the dose, afferent fibres as follows: 12 sympathetic afferents (out of 12 tested), and 7 vagal afferents (out of 9 tested). In examining the specificity of morphine action, the preliminary local application of naloxone (1.0 mg/ml) just before morphine, blocked all excitatory responses. The excitatory response was present whether the receptive field was located in the inflammatory area, or outside it, in group III or IV fibres.

Spinal cord lamina V and lamina VII interneuronal projections to sympathetic preganglionic neurons

This light and electron microscopic study sought to localize spinal cord interneurons that contribute to the normal and abnormal physiological regulation of spinal sympathetic preganglionic function. Sympathetic preganglionic neurons in caudal C8 through T4 of rat spinal cord were retrogradely labeled with wheat germ agglutinin (WGA) and/or cholera beta subunit (CT beta) following injections into the superior cervical ganglion (SCG). With two exceptions, the observed locations of retrogradely WGA- and CT beta-labeled sympathetic preganglionic neurons were as expected from previous studies. The exceptions were restricted populations of cells in caudal C8 and rostral T1 spinal segments. These neurons were classified as ventrolateral (vlSPN) and ventromedial (vmSPN) sympathetic preganglionic neurons; their somata and dendrites encircled dorsolateral lamina IX motoneurons. Only WGA was transported transneuronally following the retrograde labeling of sympathetic preganglionic neurons. Transneuronally WGA-labeled spinal interneurons were located principally in the reticulated division of lamina V and dorsolateral lamina VII. A strict segmental organization was observed. All transneuronally labeled interneurons were ipsilateral to, and coextensive with, retrogradely WGA-labeled sympathetic preganglionic neurons. Electron microscopic observations suggested that retrograde transsynaptic passage of WGA occurred within the sympathetic preganglionic neuropil and showed further that similar classes of organelles were WGA immunoreactive in retrogradely labeled sympathetic preganglionic neurons and in transneuronally labeled lamina V and lamina VII neurons: 1) cisternae and vesicles at the trans face of the Golgi apparatus, 2) large endosomes/dense bodies, and 3) multivesicular bodies. The data are consistent with two hypotheses: 1) Somatic and visceral primary afferent inputs to thoracic spinal cord modify segmental sympathetic preganglionic function through activation of a disynaptic pathway involving lamina V and/or lamina VII interneurons, and 2) long-loop propriospinal pathways access sympathetic preganglionic neurons through symmetrical, segmental interneuronal circuitry.

Torsades de pointes arrhythmia in a patient with left ventricular myxoma

A 38-year-old woman suffered from syncopes due to torsades de pointes arrhythmias. Echocardiography showed a left ventricular mass that was resected. By immunohistochemical investigations, an organized thrombus was excluded and the diagnosis of a rare ventricular myxoma was confirmed. Postoperatively the arrhythmias resolved without recurrence. We conclude that left ventricular myxoma may cause life-threatening arrhythmias, possibly by irritation of cardiac mechanoreceptors. To our knowledge, this is the first case of a left ventricular myxoma associated with a torsades de pointes arrhythmia.

Power spectral analysis of cardiovascular variability in patients at risk for sudden cardiac death

The time series of successive heart periods present important variations around its mean value, determining the phenomenon of heart rate variability (HRV), assessed with both time and frequency domain approaches. A low standard deviation of the heart period (a time domain index of HRV) is a powerful prognostic indicator of sudden coronary death in patients recovering from acute myocardial infarction. Spectral analysis of HRV usually demonstrates two major components: indicated as LF (low frequency, approximately 0.1 Hz) and HF (high frequency, approximately 0.25 Hz). They are defined by center frequency and associated power, which is expressed in msec2 or normalized units. When assessed in normalized units, LF and HF provide quantitative indicators of neural control of the sinoatrial node. Numerous experimental and clinical studies have consistently indicated that the LF component is a marker of sympathetic modulation and HF a marker of vagal modulation; the LF/HF ratio is a synthetic index of sympathovagal balance. In the analysis of 24-hour Holter recordings of normal subjects, a circadian rhythmicity of spectral markers of sympathetic and vagal modulation is clearly present, with a sympathetic predominance during the day and a vagal predominance during the night. In patients recovering from an acute myocardial infarction, spectral analysis of HRV revealed an increased sympathetic and decreased vagal activity during early convalescence, and a return to their normal balance by 6 to 12 months. A clear increase of LF was also evident in patients studied within a few hours of the onset of symptoms related to an acute myocardial infarction, independent of its location. Similarly, LF increased during transient myocardial ischemia. An increase in markers of sympathetic activity has also been observed prior to episodes of malignant arrhythmias. Spectral analysis of HRV could help in the understanding of the role of abnormal neural mechanisms in sudden coronary death, thus contributing to its prevention.

Mechanisms of cardiac pain during coronary angioplasty

OBJECTIVES

This study was conducted to establish whether the cardiac pain patients experience during coronary angioplasty is modulated by 1) the stretching of the coronary artery wall, and 2) the mechanisms responsible for the ischemic preconditioning.

BACKGROUND

Anecdotal experimental observations indicate that stretching of the coronary artery wall is a stimulus adequate to cause cardiac pain. Furthermore, recent experimental studies indicate that adenosine, a mediator of the anginal pain, appears to play an important role in the genesis of ischemic preconditioning.

METHODS

We randomly allocated 48 consecutive patients undergoing coronary angioplasty into two groups. In Group A the second balloon inflation was performed at a higher level than the first; in Group B the first two inflations were performed at the same level of balloon pressure. The mean values (+/- 1 SD) of ST segment shift on the surface 12-lead electrocardiogram (ECG) and the intracoronary ECG were measured at the end of each inflation period. Severity of cardiac pain was also obtained at the same time by using a visual analog scale.

RESULTS

The mean ST segment shift during the second balloon inflation was significantly less than that during the first inflation in both groups of patients (12.8 +/- 9.3 vs. 18.5 +/- 11.9 mm, p < 0.001 and 13.7 +/- 10.1 vs. 21.3 +/- 13.9 mm, p < 0.001, respectively, in Groups A and B). Yet, the severity of cardiac pain during the second inflation was greater than that during the first inflation in Group A (40.8 +/- 32.7 vs. 26.9 +/- 27.2 mm, p < 0.01), whereas it was lesser in Group B (23.1 +/- 20.7 vs. 32.9 +/- 29.6 mm, p < 0.05). However, in the latter group, pain severity after normalization for the mean ST segment shift was similar during the first and second inflations (2.1 +/- 2.4 vs. 2.7 +/- 3.6, p = NS).

CONCLUSIONS

During coronary angioplasty, the cardiac pain experienced by patients is caused in part by stretching of the coronary artery wall. If the stretching is maintained at a constant level during repeated coronary occlusions, the cardiac pain is entirely predicted by the severity of myocardial ischemia and therefore does not appear to be directly modulated by the mechanisms responsible for the ischemic preconditioning.

Silent ischemia: a hypothetical mechanism

Ischemia of visceral organs, especially the heart, is often a painful and potentially life-threatening condition. However, in at least 75% of all cases myocardial ischemia may be "silent" (i.e., without pain or sensation). Yet, the mechanisms responsible for silent ischemia are not well understood. As such, many different theories have been advanced to explain silent ischemia; however, none have been able to adequately explain all of the experimental and clinical findings. This paper proposes a hypothetical mechanism that may help to understand mechanisms of silent ischemia.

Regional sympathetic innervation of the heart by means of metaiodobenzylguanidine imaging in silent ischemia

Metaiodobenzylguanidine was used to determine regional cardiac sympathetic innervation and whether it is reduced in silent ischemia. Patients with silent ischemia (group 1, n = 6) and angina (group 2, n = 5) and normal subjects (n = 6) underwent single-photon emission tomography at 4 hours. From base to midventricle, uptake increased in group 1 (p < 0.006), group 2 (p < 0.01), and the normal subjects (p < 0.004). In group 1 anterior ventricular uptake was greater than inferior uptake at the midventricle (p < 0.03) and apex (p < 0.05). In group 2 the same relationship was demonstrated at midventricle (p < 0.01) and apex (p < 0.05). Group 2 uptake was only significantly greater than group 1 at midventricle (p < 0.05). Innervation is greatest in midventricle and the apex of the left ventricle and greater in the anterior wall compared with the inferior wall. There is no evidence of reduced innervation in silent ischemia.

Reflex sympathoexcitation by cardiac sympathetic afferents during myocardial ischemia. Role of adenosine

BACKGROUND

Stimulation of cardiac sympathetic afferents during myocardial ischemia has been attributed to bradykinin released from the ischemic myocardium. Recent data from human studies suggest that adenosine may serve as this mediator. Our experiments were done to determine whether reflex sympathoexcitatory responses to activation of cardiac sympathetic afferents during myocardial ischemia could be inhibited by blockade of adenosine receptors and augmented by increasing the local concentrations of adenosine in the ischemic myocardium.

METHODS AND RESULTS

Experiments were done in 29 anesthetized dogs with bilateral vagotomy and carotid sinus denervation. Activation of cardiac sympathetic afferent fibers was induced by occlusion of the left anterior descending coronary artery (LAD) combined with rapid atrial pacing (pacing rate, 200 beats per minute). LAD occlusion plus rapid atrial pacing increased renal sympathetic nerve activity by 20 +/- 4% before but by only 7 +/- 1% after administration of aminophylline (100 mg i.v.), an adenosine receptor antagonist. In contrast, LAD occlusion during rapid atrial pacing increased renal sympathetic nerve activity by 18 +/- 8% before and 61 +/- 15% after treatment with dipyridamole (0.57 mg/kg i.v.), an inhibitor of adenosine reuptake. In a separate group of dogs, LAD occlusion during rapid atrial pacing increased renal sympathetic nerve activity similarly before and after sham treatment.

CONCLUSIONS

These data suggest that adenosine released during myocardial ischemia activates cardiac sympathetic afferents to cause reflex sympathoexcitation. These findings are consistent with observations made in humans that suggest that adenosine is the mediator of the sensation of angina pectoris, which also results from stimulation of cardiac sympathetic afferents.

Silent myocardial ischemia

The documentation of abnormalities related to myocardial ischemia, whether symptomatic or silent, is of central importance. Whenever this information is available, it should be used in the overall assessment of the patient at risk for adverse outcome. The level of concern for treatment of CAD should be based on the risk implications associated with the ischemia-related abnormalities detected during objective testing rather than on the presence or absence of pain. The exercise stress test is still the single most useful test to begin the evaluation of a patient with an analyzable ST segment. In persons suspected of having CAD, the detection of ischemic-type ST-segment depression, at a low workload (e.g., < 120 beats/min or < 6.5 METS) of > 2 mm magnitude or persisting for more than 6 min implies high risk for adverse outcome. Asymptomatic ischemia during everyday activities, detected by Holter monitoring, in the high-risk patient, most probably adds additional risk beyond the risk of an abnormal stress test alone. Left ventricular imaging by two-dimensional echocardiography, RNA, angiogram, vest, etc, showing an ejection fraction > or = 40%, reversible wall motion abnormalities in multiple regions and redistribution defects or a failure to increase ejection fraction during exercise even if the patient remains asymptomatic, also imply high risk. The presence of any of these abnormal findings, regardless of symptoms, should therefore prompt as high a degree of concern as with ischemia-related signals associated with pain. Thus any therapy chosen should be directed toward elimination of transient ischemia, not just relief of symptoms that may or may not be ischemia related. If this course is chosen, the efficacy of the therapeutic regimen and possible progression of CAD should be assessed with follow-up testing for ischemia. We believe that risk factor modification and aspirin should be considered for most, if not all, patients in whom ischemia, silent or symptomatic, is suspected or detected. If symptoms or ischemia suggesting low risk is present, anti-ischemic medical therapy may be considered, but follow-up is advised. If a high-risk ischemic signal, even without symptoms, is detected, medical therapy should be used to attempt to modify the signal. If the ischemic signal suggesting high risk persists despite medical therapy, revascularization should be considered. Until additional data from large clinical trials are available, this approach appears to have the greatest likelihood of modifying the adverse outcome of CAD.

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

In order to further elucidate the regulatory function of canine epicardial neurons, the effects of transient coronary artery occlusion on their spontaneous activity was studied. Fifty-eight individual, spontaneously active units were identified by means of their action potential configurations in specific loci of atrial and ventricular epicardial fat of 10 anesthetized dogs. The activity of 49 of the units was modified by one minute of coronary artery occlusion. Twenty-four of the 49 responding units exhibited increased activity and 37 decreased activity during coronary artery occlusions. Activity changes were sometimes, but not always, associated with decreased left ventricular intramyocardial systolic pressure. During reperfusion, the activity of 6 units was increased compared to control levels, even though ventricular pressures remained the same. Following acute decentralization, 48% of previously active units generated spontaneous activity; the activity of 89% of these was altered during coronary artery occlusion despite the fact that overall cardiodynamics were unchanged. Following hexamethonium administration, the activity generated by 9 of 10 spontaneously active units was modified by coronary artery occlusion. It is concluded that transient coronary artery occlusion can modify the activity generated by intrinsic cardiac neurons, such modification involving central and peripheral neuronal interactions.

Spectral analysis of sympathetic discharge, R-R interval and systolic arterial pressure in decerebrate cats

In 19 decerebrate and artificially ventilated cats, we analyzed, with a power spectral methodology, the variability simultaneously present in R-R interval and in thoracic preganglionic sympathetic outflow. R-R interval was characterized, as already described in humans and other experimental preparations, by two rhythmic components occurring at a frequency of about 0.1 Hz (low-frequency, LF) and at one corresponding to respiratory rate (high-frequency, HF) which, in these experiments, was set at 0.32 Hz. Two similar rhythmic components were also present in the sympathetic discharge. Arterial pressure changes were produced by aorta or vena cava flow obstruction in order to produce reflex responses in sympathetic activity. Reflex sympathetic excitations induced an increase in the LF component of both R-R interval and sympathetic discharge variabilities, while the HF components were simultaneously reduced. In contrast, reflex sympathetic inhibitions were accompanied by a decrease in LF components of both variability signals, while the HF components were simultaneously increased. A significant and positive correlation was found between changes in impulse activity and the amplitude of LF component of either R-R interval or sympathetic discharge variabilities. These data support the hypothesis that the low-frequency component of R-R variability can be used as a marker of sympathetic modulation.

A critical review of the afferent pathways and the potential chemical mediators involved in cardiac pain

There is considerable evidence that on the anterior surface of the heart (which is usually supplied by the left anterior descending and the proximal part of the left circumflex coronary arteries), sympathetic efferent reflexes characterized by tachycardia and/or hypertension predominate following experimental or pathological perturbations. These cardiovascular reflexes are accompanied by an increase in presumed nociceptive afferent traffic and, in pathological condition, by pain. In these experiments, there is generally no effect of vagotomy on afferent nerve traffic, and lower cervical and upper thoracic sympathectomies help provide relief from angina. On the other hand, experimental or pathological perturbations involving the inferior-posterior surface of the heart (supplied by the right and distal parts of the left circumflex coronary arteries), are characterized by vagal efferent reflexes, resulting in bradycardia and/or hypotension. These reflexes are accompanied by an increase in vagal afferent nerve traffic and, in pathological conditions, by pain. In these experiments, vagotomy generally abolishes such cardiovascular reflexes, and lower cervical and upper thoracic sympathectomies are not effective in the relief from angina. Although cardiac sympathetic afferents are unquestionably involved in the central transmission of nociceptive information from the heart, it is also likely that there is a contributing role from the vagus in cardiac pain. It is important experimentally to understand the natural stimulus that gives rise to angina. In the clinical situation, a decrease in coronary blood flow or an increase in the metabolic demands of the myocardium due to increased work are obvious precipitating factors which lead to myocardial ischemia. In the experimental situation, occlusion of the coronary arteries is often used as a stimulus which mimics myocardial ischemia. As people who frequently experience angina have varying degrees of coronary artery disease, it is difficult to accept that the state of the coronary arteries of the normal experimental animal bear any resemblance to the state of the coronary arteries under pathological conditions. That is, the gain of homeostatic reflexes, the basal concentrations of neuroactive substances in the plasma, the myocardium and the afferent terminals, the excitability of the afferents, access of chemical mediators (e.g. bradykinin, 5-HT, adenosine, histamine, prostaglandins, potassium, lactate), to afferents, and the overall function of the animal are all significantly different. We have no idea how control mechanisms have been altered in the person with severe coronary artery disease compared to the normal patient or the "normal" experimental animal.(ABSTRACT TRUNCATED AT 400 WORDS)

Intracardiac phenylbiguanide causes excitation of spinal neurons by activation of cardiac sympathetic afferents

The responses of spinothalamic, spinoreticular, and unidentified spinal neurons to intracardiac administration of phenylbiguanide, a 5-HT3 receptor agonist, were examined in anesthetized cats and monkeys. Eighteen neurons were excited, 5 were inhibited, and 12 were unresponsive to this stimulus. Results suggest that cardiac sympathetic afferents mediate the excitatory responses produced by phenylbiguanide, because bilateral cervical vagotomy failed to block these responses, and aortic injections of phenylbiguanide had little effect on cell activity.

Single cardiac vagal fiber activity, acute myocardial ischemia, and risk for sudden death

Experimental and clinical evidence indicates that high risk for sudden death is significantly correlated with post-myocardial infarction depression in two "markers" of vagal activity, heart rate variability and baroreflex sensitivity. The present experiments were designed to answer some of the questions generated by those findings. In 33 anesthetized cats, the neural activity of single cardiac vagal efferent fibers was recorded in control conditions and after injection of phenylephrine (n = 33), before and during a 1-hour coronary artery occlusion (CAO) (n = 17), and before and after removal of the left stellate ganglion (n = 16). In the first minute after CAO, vagal activity increased by 35% from 1.66 +/- 0.37 to 2.57 +/- 0.62 impulses/sec (p less than 0.01); despite a slight decline, it remained for the entire CAO above the control values, to which it returned after CAO release. Of 17 cats, ventricular fibrillation occurred in nine (susceptible) and eight survived (resistant). Resistant and susceptible cats had different reflex vagal responses to CAO. Whereas the resistant cats had a 48% (p less than 0.01) increase by the second minute of CAO, susceptible cats had no change (-18%, p = NS) in vagal activity. These differences were independent of blood pressure changes. The increase in vagal efferent activity in response to the blood pressure rise induced by phenylephrine (baroreceptive reflex) was more marked in the resistant cats compared with the susceptible cats (+246 +/- 66% versus +80 +/- 14%, p less than 0.025). Just before the injection of phenylephrine, vagal activity was not different between resistant and susceptible cats (1.58 +/- 0.35 versus 1.48 +/- 0.30 impulses/sec, p = NS). In 16 cats, left stellectomy increased cardiac vagal efferent activity by 75% (p less than 0.01), and the reflex vagal activation secondary to phenylephrine was further enhanced (from 2.2 +/- 0.4 to 4.7 +/- 0.7 impulses/sec, p less than 0.001). These data demonstrate that 1) cardiac vagal efferent activity increases in response to acute myocardial ischemia--much more so among the animals destined to survive, 2) before CAO, susceptible and resistant animals can be identified by the vagal response to blood pressure increase (assessed clinically by baroreflex sensitivity) and not by tonic vagal activity (assessed clinically by heart rate variability), and 3) the findings with left stellectomy support the hypothesis that vagal activity decreases after myocardial infarction because of an increase, secondary to abnormal stretch of the cardiac mechanoreceptors, in cardiac sympathetic afferent traffic, which exerts a tonic restraint on vagal outflow.

Cardiovascular neural regulation explored in the frequency domain

A consistent link appears to exist between predominance of vagal or sympathetic activity and predominance of HF or LF oscillations, respectively: RR variability contains both of these rhythms, and their relative powers appear to subserve a reciprocal relation like that commonly found in sympathovagal balance. In this respect, it is our opinion that rhythms and neural components always interact, just like flexor and extensor tones or excitatory and inhibitory cardiovascular reflexes, and that it is misleading to separately consider vagal and sympathetic modulations of heart rate. In humans and experimental animals, functional states likely to be accompanied by an increased sympathetic activity are characterized by a shift of the LF-HF balance in favor of the LF component; the opposite occurs during presumed increases in vagal activity. In addition, LF oscillation evaluated from SAP variability appears to be a convenient marker of the sympathetic modulation of vasomotor activity. Although based on indirect markers, the exploration in the frequency domain of cardiovascular neural regulation might disclose a unitary vision hard to reach through the assemblage of more specific but fragmented pieces of information.

Cardiac chemoreceptors: pharmacological curiosities or physiological tools?

1. We have tested in unanaesthetized rabbits two hypotheses regarding a physiological role for cardiogenic chemoreflexes in acute central hypovolaemia. 2. In rabbits, the sympathoinhibitory phase of acute central hypovolaemia depends on the activation of a brain-stem delta-opioid receptor mechanism by a signal from the heart. Blockade of this by fourth ventricular injection of the delta-receptor antagonist ICI 174864 had no effect on the reflex haemodynamic responses to left atrial phenylbiguanide or intrapericardial nicotine. 3. Intravenous administration of the 5-HT3 receptor antagonist MDL 72222, or intrapericardial administration of the nicotinic ganglionic cholinoceptor antagonist mecamylamine HCl, had no effect on the haemodynamic response to acute central hypovolaemia. 4. We conclude that phenylbiguanide-sensitive myocardial afferents and nicotine-sensitive epicardial afferents play no part in the response to acute hypovolaemia in rabbits, and that the reflex effects evoked by chemically exciting these afferents do not depend on a brain-stem delta-opioid mechanism.

Cardiocardiac excitatory reflexes during myocardial ischemia

Myocardial ischemia represents a stimulus capable of exciting the receptive endings of both the vagal and sympathetic sensory fibers which innervate the heart. As a consequence, multiple and simultaneous reflexes can occur, mediated by the vagal and the sympathetic afferent limbs which, in turn, can modify both the vagal and the sympathetic outflows. Reflexes mediated by vagal afferent fibers are predominantly inhibitory, while those mediated by sympathetic afferent fibers are largely excitatory. In the experimental laboratory the various reflexes can be artificially isolated and analyzed, while in the clinical setting this is not feasible. However, in recent years a computer analysis of cardiovascular short-term variability seems to furnish new indices of vagal and sympathetic modulatory activities. Such analysis in the frequency domain thus seems to provide a tool for both experimental and clinical evaluation of the neural mechanisms accompanying various physiological and pathophysiological events, including acute myocardial ischemia.

Characteristics of cardiovascular reflexes originating from 5-HT3 receptors in the heart and lungs of unanaesthetized rabbits

1. When phenylbiguanide (1-PBG) (6.25-400 micrograms) was injected into the left atrium, right atrium or pulmonary artery of unanesthetized rabbits it caused dose-dependent falls of heart rate and arterial pressure, and short-lived hypopnoea or apnoea. The threshold dose was 50-100 micrograms. Maximal falls of heart rate (86-108 beats/min) and arterial pressure (33-35 mmHg) occurred at a dose of 200 micrograms. The latency between injection and onset of the bradycardia was 2.2-2.6 s and did not depend on the route. Cardiac output fell transiently with heart rate, but at the time of the maximal fall of arterial pressure it had returned to normal. All effects were abolished by intrapericardial procaine. The haemodynamic effects were exaggerated by sino-aortic barodenervation. Intrapericardial 1-PBG (200-400 micrograms) was without effect. Injection of 1-PBG (greater than 50-100 micrograms) into the aortic arch caused a variable increase in heart rate and arterial pressure. 2. When both cervical vagus nerves were crushed the depressor effects of atrial 1-PBG were reduced by only 76-84%. 3. The dose-response curves for left atrial and pulmonary artery injection of 1-PBG were shifted successively to the right by intravenous infusion of the 5-HT3 antagonist MDL72222 (0.1 and 1.0 mg/kg). 4. We conclude that in unanesthetized rabbits left atrial 1-PBG selectively excites myocardial afferents, whereas right atrial or pulmonary artery 1-PBG excites afferents that originate close to the pulmonary vasculature. In each case 1-PBG acts through pharmacologically specific 5-HT3 receptors. The afferents run mainly, but not exclusively, in the vagus nerves. The reflex fall of arterial pressure is accounted for almost entirely by a decrease in peripheral resistance.

Cardiovascular reflexes from cardiac sensory receptors

The mammalian heart, especially its left ventricle, is densely innervated by sensory nerves. One set of these travels to the brainstem in the vagus nerves; the other to the spinal cord in sympathetic nerves. Excitation of vagal cardiac afferents, especially unmyelinated afferents from the left ventricle, cause a reflex bradycardia and fall in blood pressure and, under some conditions, a massive release of AVP. The sympathetic afferents convey the sensation of cardiac pain, but innocuous stimuli may cause a reflex tachycardia and rise in blood pressure. Both sympathetic and vagal cardiac afferents can be excited by mechanical distension of the heart (mechanoreceptors), and by a variety of foreign and endogenous chemical substances (chemosensitive receptors). It is not yet clear whether the effective natural stimulus to these receptors is mechanical, or through the chemical products of myocardial metabolism. Neither is it clear whether information from the heart exerts a minute-to-minute regulatory effect on the circulation, or whether it has a purely defensive role in the face of extreme disturbances of cardiac function. Cardiogenic reflexes are also thought to be the cause of haemodynamic and humoral disturbances that occur in clinical conditions such as myocardial ischaemia or infarction, left ventricular outflow obstruction, and acute reduction in central blood volume as well as during coronary angiography.

The autonomic nervous system is not a purely efferent system

The concept of the autonomic nervous system as purely efferent does not seem to describe satisfactorily the patterns of its actions and mobilizing mechanisms. It is herein suggested that the autonomic nervous system should be rather considered as composed of functional modules comprising the visceral afferents, the integrating centers and the visceral efferents (sympathetic and parasympathetic).