Distribution and responses of the cardiac sympathetic receptors to mechanically induced circulatory changes

Silent myocardial ischemia

Although much progress has been made in reducing mortality from ischemic cardiovascular disease, this condition remains the leading cause of death throughout the world. This might in part be due to the fact that over half of patients have a catastrophic event (heart attack or sudden death) as their initial manifestation of coronary disease. Contributing to this statistic is the observation that the majority of myocardial ischemic episodes are silent, indicating an inability or failure to sense ischemic damage or stress on the heart. This review examines the clinical characteristics of silent myocardial ischemia, and explores mechanisms involved in the generation of angina pectoris. Possible mechanisms for the more common manifestation of injurious reductions in coronary flow; namely, silent ischemia, are also explored. A new theory for the mechanism of silent ischemia is proposed. Finally, the prognostic importance of silent ischemia and potential future directions for research are discussed.

Increased propensity for apnea in response to acute elevations in left atrial pressure during sleep in the dog

Periodic breathing is commonly observed in chronic heart failure (CHF) when pulmonary capillary wedge pressure is abnormally high and there is usually concomitant tachypneic hyperventilation. We hypothesized that acute pulmonary hypertension at pressures encountered in CHF and involving all of the lungs and pulmonary vessels would predispose to apnea/unstable breathing during sleep. We tested this in a chronically instrumented, unanesthetized dog model during non-rapid eye movement (NREM) sleep. Pulmonary hypertension was created by partial occlusion of the left atrium by means of an implanted balloon catheter in the atrial lumen. Raising mean left atrial pressure by 5.7 +/- 1.1 Torr resulted immediately in tachypneic hyperventilation [breathing frequency increased significantly from 13.8 to 19.9 breaths/min; end-tidal P(CO2) (P(ET(CO2))) fell significantly from 38.5 to 35.9 Torr]. This tachypneic hyperventilation was present during wakefulness, NREM sleep, and rapid eye movement sleep. In NREM sleep, this increase in left atrial pressure increased the gain of the ventilatory response to CO2 below eupnea (1.3 to 2.2 l.min(-1).Torr(-1)) and thereby narrowed the CO2 reserve [P(ET(CO2)) (apneic threshold) - P(ET(CO2)) (eupnea)], despite the decreased plant gain resulting from the hyperventilation. We conclude that acute pulmonary hypertension during sleep results in a narrowed CO2 reserve and thus predisposes toward apnea/unstable breathing and may, therefore, contribute to the breathing instability observed in CHF.

Cardiac afferents and neurohormonal activation in congestive heart failure

Cardiac chambers have afferent connections to the brainstem and to the spinal cord. Vagal afferents mediate depressor responses and become activated by volume expansion, increased myocardial contractility and atrial natriuretic factor. Sympathetic afferents, on the contrary, are activated by metabolic mediators, myocardial ischemia and cardiac enlargement. These opposite behaviors may lead to activation or suppression of the sympathetic nervous system and of the renin-angiotensin-aldosterone system. As cardiac diseases progress, the heart dilates, plasma norepinephrine increases, atrial natriuretic factor is released and the renin-angiotensin-aldosterone system is suppressed to maintain water and sodium excretion. This dissociation of the neurohormonal profile of cardiac patients, may be explained by coactivation of sympathetic afferents, by cardiac dilatation, and of vagal afferents by atrial natriuretic factor. In more advanced stages, atrial natriuretic factor suppression of the renin-angiotensin-aldosterone system is overridden by overt sympathetic activation and sodium and water retention ensues. Digitalis, angiotensin-converting enzyme inhibitors and beta-blockers selectively decrease cardiac adrenergic drive. A common mechanism of action, to all three groups of drugs, would be attenuation of sympathetic afferents and partial normalization of vagal afferents. Consequently, heart size and cardiac afferents emerge as the key factors to understand the pathophysiology and treatment of the syndrome of congestive heart failure.

Correlation of ventricular mechanosensory neurite activity with myocardial sensory field deformation

The mechanosensory activity generated by ventricular epicardial sensory neurites associated with afferent axons in thoracic sympathetic nerves was correlated with sensory field deformation (long axis, short axis, and transmural dimension changes), regional intramyocardial pressure, and ventricular chamber pressure in anesthetized dogs. Ventricular mechanosensory neurites generated activity that correlated best with strain developed along either the long or short axis of their epicardial sensory fields in most instances. Activity did not correlate normally to local wall thickness or to regional wall or chamber pressure development in most cases. During premature ventricular contractions, the activity generated by these sensory neurites correlated best with maximum strain developed along at least one sensory field epicardial vector. Identified sensory neurites were also activated by local application of the chemical bradykinin (10 microM) or by local ischemia. These data indicate that the activity generated by most ischemia-sensitive ventricular epicardial sensory neurites associated with afferent axons in sympathetic nerves is dependent on not only their local chemical milieu but on local mechanical deformation along at least one epicardial vector of their sensory fields.

Visceral pain: a review of experimental studies

This paper reviews clinical and basic science research reports and is directed toward an understanding of visceral pain, with emphasis on studies related to spinal processing. Four main types of visceral stimuli have been employed in experimental studies of visceral nociception: (1) electrical, (2) mechanical, (3) ischemic, and (4) chemical. Studies of visceral pain are discussed in relation to the use and 'adequacy' of these stimuli and the responses produced (e.g., behavioral, pseudoaffective, neuronal, etc.). We propose a definition of an adequate noxious visceral stimulus and speculate on spinal mechanisms of visceral pain.

Ganglionic distribution of afferent neurons innervating the canine heart and cardiopulmonary nerves

The ganglionic distribution of the perikarya of afferent axons in cardiopulmonary nerves or the heart was studied in 64 dogs by injecting horseradish peroxidase into physiologically identified cardiopulmonary nerves or different regions of the heart. In 6 additional dogs, horseradish peroxidase was injected into the aortic arch, pericardial sac, left ventricular cavity or the skin. After injections into cardiopulmonary nerves, retrogradely labeled perikarya were found in the ipsilateral nodose ganglion and the ipsilateral C7-T7 dorsal root ganglia. After injections into different regions of the heart, retrogradely labeled neurons were found in the nodose ganglia bilaterally and in the C6-T6 dorsal root ganglia bilaterally. Many more retrogradely labeled neurons were found in the nodose ganglia in comparison to the dorsal root ganglia. The largest numbers of retrogradely labeled perikarya in the dorsal root ganglia occurred in the T 2-4 ganglia following nerve or heart injections. Following injections into specific regions of the heart or individual physiologically identified cardiopulmonary nerves, regional distributions of labeled neurons could not be identified within or among ganglia with respect to the structures injected. Perikarya in dorsal root ganglia which were labeled after heart injections ranged in area from 436-3280 microns 2 (X = 1279 +/- 51 S.E.M.) while after skin injections labeled perikarya ranged in area from 224-5701 microns 2 (X = 1631 +/- 104 S.E.M.). The results show that the afferent innervation of the canine heart is provided by neurons located throughout the nodose ganglia and to a lesser degree in the C6-T6 dorsal root ganglia bilaterally. The bilateral distribution of cardiac afferent neurons raises questions regarding mechanisms underlying unilateral symptoms frequently associated with heart disease.

Activation of feline spinal neurones by potentiated ventricular contractions and other mechanical cardiac stimuli

1. Neurones in the spinal cord were tested for responses to premature ventricular contractions (PVCs), produced by controlled electrical extra stimuli, and other mechanical stimuli applied to the heart. Thirty-eight neurones were antidromically activated from the medial medullary reticular formation and/or the caudal thalamus, and twenty-four neurones did not project to these sites. 2. Only those neurones excited by electrical stimulation of the left stellate ganglion were tested for responses to PVCs. A total of twenty neurones (32%) responded to electrically induced PVCs. Three major patterns of responses occurred. Three neurones exhibited an early burst and a late burst (or bursts) during the arrhythmia, one neurone fired only an early burst, and sixteen neurones responded with only a late burst. The early bursts occurred shortly after the onset of the compensatory pause accompanying the PVC; the late bursts were usually associated with the subsequent potentiated contraction, although the stimulus eliciting the burst must often have occurred late in the compensatory pause. 3. Responses to PVCs were only seen in neurones receiving C fibre and A delta fibre input. However, there were some neurones with both A delta and C input that did not respond to PVCs. No neurones with only A delta input responded to PVCs. 4. Neurones projecting to thalamus were less likely to respond to PVCs than either spinoreticular neurones or neurones with unidentified projections. 5. Neurones responsive to PVCs were likely to exhibit a cardiac rhythmicity in their spontaneous or evoked activity. 6. A total of 42% of tested neurones responded to a rapid infusion of saline into the heart, 52% had a cardiac receptive field, and 74% responded to aortic occlusion. A given neurone might respond to one or more of these stimuli, without responding to every mechanical stimulus tested. 7. Cervical vagotomy never abolished a response to PVCs, although either the spontaneous discharge rate or magnitude of response was sometimes altered. 8. Neurones responsive to PVCs were also responsive to intracardiac bradykinin. In addition, 95% of the neurones received convergent somatic input. 9. We conclude that about a third of spinal neurones excited by electrical stimulation of the left stellate ganglion receive information regarding mechanical, presumably innocuous, events in the heart. Most responsive neurones also receive somatic input and noxious cardiac input, and this information is transmitted to the thalamus, reticular formation, and probably to other spinal segments.

Participation of prostanoids in chemical activation of the pericardial pressor reflex in dogs

Experiments were performed on anaesthetized, open-chest dogs to determine the reflex effects on systemic blood pressure and heart rate produced by stimulation of the parietal pericardium with bradykinin, prostacyclin, prostaglandin E2 (PGE2), prostaglandin D2 (PGD2) and arachidonic acid. Pericardial application of bradykinin (1 microgram) consistently elicited reflex increases in blood pressure and heart rate, whereas application of prostanoids or arachidonic acid in doses up to 10 micrograms failed to produce any cardiovascular responses. Indomethacin, applied either directly to the parietal pericardium (1 microgram/ml) or given intravenously (5 mg/kg) caused a long lasting reduction of the reflex responses to bradykinin. The reflex effects of bradykinin could be temporarily restored by treatment of the pericardium with either prostacyclin (0.1 microgram/min) or PGE2 (0.1 microgram/min). PGD2 (0.1-1 microgram/min) did not influence the bradykinin induced pericardial reflex. Superfusion of arachidonic acid (3 micrograms/min) over the pericardium amplified the reflex effects of bradykinin when given before, but not when given after indomethacin treatment. The results indicate that locally formed prostanoids, specifically prostacyclin and PGE2, can facilitate activation of the pericardial pressor reflex by bradykinin. The findings may be relevant to the changes in cardiovascular activity occurring during pericardial inflammation.

Cardiac receptors: their function in health and disease

Cardiac receptors include both mechanically and chemically sensitive receptors located in atria and in ventricles. Atrial receptors innervated by myelinated vagal afferent fibers reflexly regulate heart rate and intravascular volume. On the other hand, stimulation of ventricular receptors can cause either reflex bradycardia and hypotension or, alternatively, excitation of the cardiovascular system. The former response is mediated by vagal afferents, whereas the latter is mediated by sympathetic (spinal) afferents. Under normal circumstances, cardiac receptors sense changes in wall motion or diastolic pressure and perhaps provide a fine tuning of the cardiovascular system. However, under certain pathological conditions such as coronary ischemia, which cause release of substances such as bradykinin and prostaglandins, there is an exaggerated response of the ventricular receptors. Because these receptors cause a reflex depression of the cardiovascular system and, in particular, induce renal vasodilation, they may protect the heart and kidney by lessening myocardial oxygen requirements and by increasing renal blood flow. In the situation of heart failure both atrial and ventricular receptors are reset and therefore provide for an exaggerated neurohumoral discharge. Finally, patients with aortic stenosis may demonstrate a paradoxical vasodilation and syncope during exercise when there likely is excessive stimulation of left ventricular receptors by the high transmural pressure.

Phenol topically applied to canine left ventricular epicardium interrupts sympathetic but not vagal afferents

The intracardiac pathways carrying the cardiovascular reflex responses mediated by cardiac sympathetic and vagal afferent fibers were examined in this study. We investigated the response to epicardial applications of bradykinin (5 micrograms) and nicotine (50 micrograms) before and after regional epicardial applications of 85% phenol in chloralose anesthetized open-chest dogs. Bradykinin stimulated sympathetic afferents, while nicotine stimulated vagal afferents. Topical applications of phenol were used to interrupt these pathways. Before phenol encircling, bradykinin significantly increased--whereas nicotine significantly decreased--mean arterial blood pressure when applied at the same sites. After phenol, nicotine applied to all sites within and outside the phenol circle continued to decrease mean arterial pressure, whereas bradykinin applied to sites within the circle no longer increased mean arterial pressure. Removal of aortic and carotid baroreceptors did not significantly affect these responses. Painting horizontal stripes of phenol on the anterior and posterior left ventricular free wall basal to the site of bradykinin application eliminated the elevation in mean arterial pressure produced by bradykinin. Reapplication of bradykinin basal to the stripe restored its response. Phenol stripes eliminated the nicotine vasodepressor response only when the stripe was painted in the atrioventricular groove. When bradykinin and nicotine were injected via a nonocclusive intracoronary catheter, both drugs elicited an early depressor response (interrupted by vagotomy) and, in some animals a late pressor response (interrupted by stellectomy). Epicardial phenol encircling the flow distribution of the cannulated coronary artery interrupted most or all of the sympathetic afferents mediating pressor responses to bradykinin or nicotine, while leaving the depressor responses intact. The depressor responses were eliminated by applying phenol to the atrioventricular groove or by transecting the cervical vagi. These data suggest that sympathetic afferent fibers travel in the superficial subepicardium in an apex-to-base direction. Vagal afferent fibers travel deeper in the myocardium until they approach the atrioventricular groove, where they ascend to the superficial subepicardium.

Viscerosomatic convergence onto T2-T4 spinoreticular, spinoreticular-spinothalamic, and spinothalamic tract neurons in the cat

Neurons of T2 to T4 spinal segments were antidromically activated from the medullary reticular formation and the contralateral region in or near the ventral posterior lateral nucleus of the thalamus. Sixty-seven percent of the spinoreticular neurons projected to the ipsilateral, contralateral, or both ipsilateral and contralateral medullary reticular formation. In addition, 21% of the cells projected to the reticular formation and thalamus and 12% projected only to the thalamus. These cells received both visceral and somatic inputs. Electrical stimulation of cardiopulmonary sympathetic afferent fibers elicited an early peak of cell activity in 30 cells, an early and late peak in 38 cells, and only a late peak in 1 cell. Seventy-one percent of the cells had simple somatic receptive fields; these fields were localized to the left forelimb and left upper thorax. The remaining 29% of the cells had complex receptive fields that often extended to the hind limb as well as bilaterally. Classification of all cells according to threshold for activation of the somatic receptive field showed that 86% of the cells were high-threshold, 10% were wide dynamic range, and 4% were high-threshold but, in addition, were inhibited during hair movement. Viscerosomatic convergence onto these cells support Ruch's convergence projection theory for referred pain. These projecting neurons may be involved with referred pain associated with angina pectoris. In addition, they may also be involved with cardiovascular adjustments.

Reflex effects evoked from the parietal pericardium in the dog: comparison with responses from the visceral pericardium

Experiments were performed on anaesthetized, open-chest dogs to determine the reflex effects on systemic blood pressure and heart rate produced by stimulation of the parietal pericardium with bradykinin and nicotine, and to compare these effects with those evoked by application of these substances to the visceral pericardium (epicardium) of the left ventricle. Bradykinin (0.01-6.0 micrograms) elicited reflex increases in blood pressure and heart rate when applied either to the parietal pericardium or to the ventricular epicardium; the responses evoked from both sites were dose-dependent from the threshold of 0.01 micrograms to a maximum at 1.0 micrograms of bradykinin. The reflex effects of bradykinin were not affected by either vagotomy or phrenic nerve section, but were suppressed by bilateral sectioning of the upper thoracic (T1-T4) white rami communicantes and stellectomy. In contrast to bradykinin, nicotine (20-100 micrograms) failed to produce any change in blood pressure and heart rate when applied to the parietal pericardium and evoked depressor responses when applied to the epicardium of the left ventricle; these depressor effects of nicotine were abolished by vagotomy. The results indicate that sympathetic, but not vagal, afferent endings innervating the parietal pericardium are susceptible to chemical stimulation. Bradykinin is a powerful algesic agent and is formed and released locally during inflammation. We suggest, therefore, that the pericardial sympathetic pressor reflex is nociceptive in nature and can be activated when kinin formation occurs during pericardial inflammation.

Response pattern of visceral afferent fibres, supplying the colon, upon chemical and mechanical stimuli

In the cat responses of afferent fibres in the inferior splanchnic nerves which innervate the colon and its mesentery were investigated upon intra-arterial bolus injection of bradykinin, of KCl- and of hypertonic NaCl-solution and upon ischaemia of the colon. One hundred and twenty-five units were analyzed. 1) Seventy-seven from 96 units with resting activity (80%) and 7 from 29 units without (24%) were excited by bradykinin. The exictation was present both when the colon wall contracted and when the contractions were prevented or when the colon was paralytic. 2) Fifty-two from 82 units with resting activity (67%) and 4 from 21 units without (19%) responded to KCl with short-lasting, high-frequency bursts. Injections of hypertonic NaCl-solutions had only small effects on the afferent units. 3) The responses of the units to distension of the colon (see [3]) and to the chemical stimuli were highly correlated. 4) Afferent units which responded to distension of the colon and to bradykinin were also excited by partial or complete ischaemia of the colon (produced by occlusion of both mesenteric arteries or of only the inferior mesenteric artery). During ischaemia the activity in the afferents increased and became burst-like. The response to distension of the colon increased in some of the afferent units. 5) Blood pressure responses to colon distension and to local bradykinin application also increased during ischaemia. 6) The results indicate that visceral afferent fibres from the colon in the inferior splanchnic nerves may be functionally homogeneous and may encode noxious stimuli applied to the colon.

Functions of afferents in cardiovascular sympathetic nerves

This paper contains data and discussion of the role of afferents in the sympathetic trunks from the cardiovascular complex that evoke autonomic reflex action. A description is given of these cardiovascular reflexes induced by afferents of the sympathetic trunks in neurally intact as well as spinal animals. A positive feedback hypothesis is also proposed.

Neurophysiology and neuropharmacology of cardiovascular regulation and stress

Evidence has accumulated over the past several years indicating that environmental factors can have a substantial influence on cardiovascular dynamics. It has been hypothesized by many investigators that through these influence environmental stressors may be important to the etiology and maintenance of cardiovascular diseases. Since the nervous system is intimately involved in the regulation of cardiovascular function it may be assumed that environmental influences on cardiovascular dynamics are to a large extent mediated by the nervous system. This assumption is supported by the literature reviewed which indicates that there are many nervous system nuclei and neurotransmitter systems involved in the regulation of cardiovascular dynamics which are also involved in an organisms adjustment to environmental stressors. The conclusion is reached that further multidisciplinary research will reveal underlying neurophysiological and neuropharmacological mechanisms responsible for stress induced cardiovascular disease and lead to new methods of treatment.

Effects of intracoronary administration of bradykinin on the impulse activity of afferent sympathetic unmyelinated fibers with left ventricular endings in the cat

In anesthetized and artificially ventilated cats, we recorded the impulse activity of 23 afferent sympathetic unmyelinated fibers with left ventricular endings, dissected from the left sympathetic rami T3 and T4. All fibers displayed a spontaneous discharge at a rate of 0.79 +/- 0.2 (mean +/- SE) impulses/sec. During constriction of the thoracic aorta, the discharge increased to 1.92 +/- 0.2 impulses/sec. During myocardial ischemia, produced by interruption of left main coronary artery perfusion, supplied through an extracorporeal pump, the impulse activity increased to 1.73 +/- 0.3 impulses/sec. The mean latency for this excitation was 16.5 +/- 1.5 sec. The intracoronary administration of bradykinin (5 and 10 ng/kg) elicited a marked increase in impulse activity that, following 5 ng/kg, reached 2.06 +/- 0.2 impulses/sec, after a latency of 18 +/- 2 sec and in absence of significant hemodynamic changes. Myocardial ischemia and bradykinin never revealed the existence of silent afferent fibers included in the split nerve strand. The results obtained with this experimental model indicate that the ventricular endings of these afferent sympathetic unmyelinated fibers act as "polymodal" receptors. We hypothesize that the peripheral mechanism for cardiac nociception involves intensive excitation of fibers discharging spontaneously and not recruitment of silent fibers which are purely nociceptive in function.

Reflex cardiovascular and respiratory effects of serotonin in conscious and anesthetized dogs

Cardiovascular and respiratory effects of intra-left atrial or intra-left ventricular injection of serotonin were studied in conscious dogs (n = 8), anesthetized closed-chest dogs (n = 13) and anesthetized open-chest dogs (n = 9). Serotonin (50-200 microgram), injected as a bolus, resulted in an initial bradycardia and hypotension followed by a delayed tachycardia and hypertension in the conscious dogs. The hypertension was seen as an increase of 21.5 +/- 2.7 (mean +/ SE) mm Hg from a control pressure of 102.5 +/- 1.9 mm Hg, whereas the initial decrease in pressure was 22.6 +/- 1.9 mm Hg. The tachycardia was 23.3 +/- 3.9 beats/min above a control heart rate of 104.9 +/- 3.9 beats/min whereas the bradycardia was 58.5 +/- 3.7 beats/min below control. There was a significant attenuation of the hypotension in both groups of anesthetized dogs. In fact, no hypotension was elicited in the open-chest anesthetized dogs. Open-chest anesthetized dogs showed only a hypertensive response (mean increase 67.2 +/- 5.5 mm Hg). Stimulation of respiration was seen in all groups of dogs. In conscious dogs there was a 214.8 +/- 15.4% increase in respiratory depth and a 20.8 +/- 3.1 breaths/min increase in respiratory rate. Atropine significantly reduced the bradycardia and abolished the hypotension in conscious dogs. Bilateral cervical vagotomy did not abolish the response in open-chest anesthetized dogs. We conclude that the so-called "hypertensive coronary chemoreflex" is altered dramatically by the state of the preparation and by anesthesia.

Vagal and sympathetic reflexes of left ventricular origin on the efferent activity of cardiac and renal nerves on anaesthetized cats

The influence of reflexes of left ventricular origin on the postganglionic sympathetic activity and heart rate was investigated in anaesthetized cats. The experiments were to clarify 1. whether there are regionally different reflex adjustments due to an activation of ventricular receptors, 2. whether an increase of left ventricular diastolic pressure, which is known to activate afferent vagal fibres, causes an inhibition of sympathetic activity, 3. whether a coronary artery occlusion can activate a pressor reflex and a depressor reflex. Left ventricular receptors were stimulated by obstruction of the aortic root, coronary artery occlusion and mechanical stretch of the ventricular wall. In animals with intact CNS, all stimuli led to an inhibition of the activity of the inferior cardiac and renal sympathetic nerves and bradycardia. These reflex effects are initiated by mechanoreceptors and abolished by vagotomy. The inhibition of sympathetic activity was equally pronounced in the cardiac and renal nerves. After coronary artery occlusion and aortic obstruction, inhibition occurred as soon as the ventricular diastolic pressure had risen about 2 mmHg. In spinal animals both stimuli caused a sympathetic activation which was mainly restricted to the cardiac nerve. This activation is not due to mechanical changes, but rather a direct result of myocardial ischaemia. Coronary artery occlusion is able to produce both inhibition and activation of sympathetic fibres, but the activation is normally suppressed and thus seems not to be particularly important for circulatory control.

Search for a cardiac nociceptor: stimulation by bradykinin of sympathetic afferent nerve endings in the heart of the cat

1. We have examined the effect of bradykinin on impulse traffic in sympathetic afferent fibres from the heart, great vessels and pleura, and have attempted to identify cardiac nociceptors that on the basis of their functional characteristics might have a role in the initiation of cardiac pain. 2. In anaesthetized cats, we recorded afferent impulses from 'single-fibre' slips of the left 2nd--5th thoracic rami communicantes and associated chain, and selected fibres arising from endings in the heart, great vessels, pericardium and pleura. We applied bradykinin solution (0 . 1--1 . 0 microgram/ml.) locally to the site of the ending; we also injected bradykinin (0 . 3--1 . 0 microgram/kg) into the left atrium. 3. Afferent endings excited by bradykinin (159 of 191 tested) were of two types. The larger group (140) were primarily mechanoreceptors with A delta of C fibres (mean conduction velocity, 7 . 5 +/- 0 . 6 m/sec). They were very sensitive to light touch. Those located in the heart, great vessels or overlying pleura had a cardiac rhythm of discharge and were stimulated by an increase in blood pressure or cardiac volume. 4. Bradykinin increased mechanoreceptor firing from 0 . 7 +/- to 5 . 0 +/- 0 . 3 (mean +/- S.E. of mean) impulses/sec. Some endings appeared to be stimulated directly by bradykinin, others sensitized by it so that they responded more vigorously to the pulsatile mechanical stimulation associated with the cardiac cycle. 5. The smaller group of eighteen endings, of which ten were in the left ventricle, were primarily chemosensitive. Most had C fibres, a few had A delta fibres (mean conduction velocity, 2 . 3 +/- 0 . 7 m/sec). They were insensitive to light touch. With one exception they never fired with a cardiac rhythm, and even large increases in aortic or left ventricular pressure had little effect on impulse frequency. 6. Chemosensitive endings were stimulated by bradykinin, impulse activity increasing from 0 . 6 to 15 . 6 +/- 1 . 3 impulses/sec and remaining above the control level for 1-3 min. The evoked discharge, which was either continuous or occurred in irregular bursts, was not secondary to mechanical changes in the heart and great vessels. 7. Tachyphylaxis occurred when the interval between successive applications of bradykinin was 20 min or less. It was a feature of the response of both mechanosensitive and chemosensitive endings. 8. Because of their responsiveness to changes in pressure and their sensitivity to light touch, the mechanosensitive endings appear to be unlikely to subserve a primarily nociceptive function, although they may be responsible for evoking some of the components of the pseudoaffective response. By contrast, the chemosensitive endings appear well fitted to act as cardiac nociceptors.

Experimental pulmonary hypertension produced by surgical and chemical denervation of the pulmonary vasculature

The purpose of this study was to investigate the hypothesis that balloon distention of the main pulmonary artery (MPA) induces pulmonary hypertension that is produced by a neural reflex and to investigate the possible efferent components of its reflex arc. Using a specially designed triple-lumen balloon catheter, positioned under fluoroscopy in the MPA, the hemodynamic responses to MPA distention were studied before and after the following: surgical denervation of the bifurcation of the MPA, chemical sympathectomy (6-hydroxydopamine), 100 percent oxygen breathing, and vagotomy. Our findings suggest that the experimental pulmonary hypertension caused by distention of the MPA is produced by excitation of stretch receptors in or near the bifurcation of the MPA and that the efferent limb of this reflex is predominantly mediated via the adrenergic nervous system.

Coronary pressor reflexes

The heart differs from other cardiovascular reflexogenic structures because it has two prominent inputs to the central nervous system. On input is spinal and is mediated by afferent cardiac sympathetic nerve fibers. The other is medullary and is mediated by afferent vagal fibers. The number of fibers projecting centrally appears to be similar in the two systems. The reflex effects produced by excitation of the two inputs are complicated and can be either pressor or depressor. However, reflex pressor effects seem to be more prominant for the spinal input and reflex depressor effects more prominent for the medullary input.

Anatomical and physiological characterization of avian sympathetic cardiac efferents

In the preceding paper6 describing the right sympathetic cardiac nerve of the pigeon we reported a fast compound action potential component with no chronotropic effect, and single unit recordings from sympathetic postganglionic neurons demonstrated that this action potential component was not reflected in the antidromic postganglionic latencies. Electron microscopy then indicated the presence of numerous myelinated fibers in the nerve, and together these findings suggested the existence of a substantial number of sympathetic cardiac afferents in the right cardiac nerve. The present paper confirms this by demonstrating with retrograde transport of horseradish peroxidase that some fibers of the right sympathetic cardiac nerve have their cells of origin in dorsal root ganglia. This was also shown to be the case for the left sympathetic cardiac nerve. Selective activation of the myelinated fiber contingent was then shown to elicit a short latency decrease in arterial blood pressure that could be further augmented by the activation of the smaller unmyelinated fibers. This reflex depressor response to activation of sympathetic cardiac afferents survived bilateral vagotomy but was blocked by atropine. It is therefore concluded that both the left and right sympathetic cardiac nerves of the pigeon contain afferent fibers, both myelinated and unmyelinated, and that the reflex effect of activating these fibers is a sympathetically mediated vasodilation that is atropine-sensitive.

Afferent sympathetic nerve fibres with aortic endings

1. We recorded the electrical impulse activity of thirty-five single afferent fibres with aortic endings isolated from the third to the sixth left thoracic sympathetic rami communicantes of anaesthetized cats. The endings of each fibre were localized by mechanical probing of the opened aorta at the end of each experiment. 2. Twenty-four fibres had a single aortic receptive field. Eleven fibres had several and distinct receptive fields (from two to four): they were usually located in nearby aortic areas or, in addition, in other proximal portions of the arterial tree or in the adjacent pleura and connective tissue. 3. Twenty-nine fibres had conduction velocities ranging between 5 and 27 m/sec (Group Adelta), while six fibres had conduction velocities between 0-2 and 1-2m/sec (Group C). 4. The spontaneous impulse activity was in phase with the aortic pressure pulse and consisted of not more than one impulse per pressure pulse. It was increased during increases in aortic pressure and, conversely, decreased during decreases in aortic pressure. In vivo and post mortem studies showed that these mechanoreceptors had an impulse activity which rapidly adapted during sustained stimuli. They thus seem to signal pulsatile aortic stretch. 5. These aortic sympathetic afferents are likely to be part of a nervous pathway through which pressor reflexes, exhibiting positive feed-back characteristics, can elicited.

Central neural regulation of heart and blood vessels in mammals

The study of the central regulation of the circulation in the past has been directed primarily at observing reflex responses to stimulation of peripheral receptors and at producing changes in cardiovascular parameters during electrical stimulation of central sites. These studies have demonstrated that the nervous system can regulate the circulation to different vascular beds with a high degree of specificity and that it has the ability to provide a range of coordinated responses which are appropriate to the metabolic needs of a particular behavioural pattern. In addition, it has become firmly established that the nervous system is capable of coupling cardiovascular changes with other autonomic and somatic activities to produce an integrated response. In the last decade it has become apparent that although the mode of operation of central cardiovascular regulation has been described in general terms, very little is known about the accurate anatomical localization of neuronal circuits and pathways and of impulse traffic corresponding to the changes in cardiovascular parameters that have been observed. This essay reviews recent information on discrete neuronal circuits and pathways and their mode of operation in electrophysiological terms. One of the most serious difficulties in this endeavour is the problem of demonstrating specificity of pathways and circuits because patterns of firing of afferent and efferent peripheral nerves can be usually identified, but the demonstration of specificity of central structures is a conceptual and technical challenge to the most skilled investigator. Several studies have been made in the last decade in an attempt to trace anatomically and functionally pathways involved in central cardiovascular regulation. Progress has been made especially with regard to the precise sites of termination of cardiovascular afferent fibres and the pattern of discharge of efferent cardiovascular neurons; some work has also been done to trace discrete pathways between the hypothalamus and the medulla and the medulla and the spinal cord. However, in view of the difficulties of establishing the specificity of cardiovascular pathways, progress will depend on the acquisition of a wiring diagram of simple cardiovascular reflex arcs before attempts are made to study the functional interactions of regions in the brain that have been traditionally associated with central regulation of the circulation. Future experiments should concentrate less on the demonstration of cardiovascular responses to stimulation or lesions in the central nervous system and more on the connections of discrete regions.(ABSTRACT TRUNCATED AT 400 WORDS)

Afferent fibres from pulmonary arterial baroreceptors in the left cardiac sympathetic nerve of the cat

1. Afferent discharges were recorded from the left cardiac sympathetic nerve or the third sympathetic ramus communicans of anaesthetized cats. Twenty-one single units with baroreceptor activity were obtained.2. The receptors of each unit were localized to the extrapulmonary part of the pulmonary artery, determined by direct mechanical probing of the wall of the pulmonary artery after death of the animals. Conduction velocity of the fibres ranged from 2.5 to 15.7 m/sec.3. Afferent discharges occurred irregularly under artificial ventilation. The impulse activity was increased when pulmonary arterial pressure was raised by an intravenous infusion of Locke solution, or by occlusion of lung roots, and decreased by bleeding the animal from the femoral artery.4. Above a threshold pressure, discharges occurred synchronously with the systolic pressure pulse in the pulmonary artery. A progressive further rise in pressure did not produce an increase in the number of impulses per heart beat. Occlusion of lung roots initially elicited a burst of discharges but the number of impulses for each cardiac cycle gradually decreased.5. The receptors responded to repetitive mechanical stimuli up to a frequency of 10/sec, but failed to respond to stimuli delivered at 20/sec.6. The results provide further evidence for the presence of afferent fibres in the cardiac sympathetic nerve. These afferent fibres are likely to provide the spinal cord with specific information only on transient changes in pulmonary arterial pressure.

Nervous activity of afferent cardiac sympathetic fibres with atrial and ventricular endings

1. We recorded the electrical activity of single afferent cardiac fibres isolated from the third and fourth left thoracic sympathetic rami communicantes of anaesthetized cats. Their conduction velocities ranged from 12 to 32 m/sec.2. The endings of each fibre were localized to one cardiac chamber by mechanical probing of the opened heart performed at the end of the experiment.3. The impulse activity was spontaneous and, in fibres with atrial or ventricular endings, it was in phase with a particular atrial or ventricular event.4. This nervous activity increased during increases in pressure occurring in the chamber where the endings were located. Conversely, decreases in pressure were accompanied by decreased nervous discharge.5. In some experiments the left coronary artery was perfused at different flows and pressures. Brief decreases or increases in coronary flow and pressure decreased or increased, respectively, the discharge of fibres with atrial or ventricular endings. Fibres were excited by intracoronary injections of veratridine.6. Cessation of coronary pump flow increased the discharge of fibres with atrial or ventricular endings only when myocardial ischaemia was accompanied by signs of heart failure.7. These afferent cardiac sympathetic fibres which provide the spinal cord with continuous specific information on cardiac events are likely to contribute to the neural control of circulation.

Spinal sympathetic reflexes initiated by coronary receptors

1. The main left coronary artery of vagotomized spinal cats was perfused at different flows and pressures. The changes in pressure were limited to the coronary bed.2. Increased coronary flow which increased coronary arterial pressure provoked a reflex increase in sympathetic discharge in the white ramus of the third thoracic spinal nerve and the inferior cardiac nerve. Reflex reductions in activity were not observed.3. Occlusion of the coronary sinus and myocardial ischaemia, due to cessation of pump inflow, evoked similar reflex increases of sympathetic activity. The effect of myocardial ischaemia was apparent before systemic arterial blood pressure fell or left ventricular end-diastolic pressure rose.4. Increased coronary arterial pressure, myocardial ischaemia and coronary sinus occlusion could activate the same preganglionic neurone.5. The afferent limb of the excitatory coronary-sympathetic reflex was in the cardiac sympathetic nerves, mainly on the left. Afferent nerve fibres running in these nerves and in the third left thoracic sympathetic ramus communicans were excited by increased coronary arterial pressure, myocardial ischaemia, and occlusion of the coronary sinus. Inhibition was not observed. Many of the receptors were further localized by direct probing over the coronary vessels and adjacent myocardium.6. Some receptors were excited by increased coronary arterial pressure alone, others by coronary sinus occlusion, and still others by myocardial ischaemia. In addition, some receptors were excited by all three stimuli.