Atrial receptors in the cat

Interoceptive signals from the heart and coronary circulation in health and disease

This review considers interoceptive signalling from the heart and coronary circulation. Vagal and cardiac sympathetic afferent sensory nerve endings are distributed throughout the atria, ventricles (mainly left), and coronary artery. A small proportion of cardiac receptors attached to thick myelinated vagal afferents are tonically active during the cardiac cycle. Dependent upon location, these mechanoreceptors detect fluctuations in atrial volume and coronary arterial perfusion. Atrial volume and coronary arterial signals contribute to beat-to-beat feedback control and physiological homeostasis. Most cardiac receptors are attached to thinly myelinated or nonmyelinated C fibres, many of which are unresponsive to the cardiac cycle. Of these, there are many chemically sensitive cardiac receptors which are activated during myocardial stress by locally released endogenous substances. In contrast, some tonically inactive receptors become activated by irregular ventricular wall mechanics or by distortion of the ischaemic myocardium. Furthermore, some are excited both by chemical mediators of ischaemia and wall abnormalities. Reflex responses arising from cardiac receptors attached to thinly myelinated or nonmyelinated are complex. Impulses that project centrally through vagal afferents elicit sympathoinhibition and hypotension, whereas impulses travelling in cardiac sympathetic afferents and spinal pathways elicit sympathoexcitation and hypertension. Two opposing cardiac reflexes may provide a mechanism for fine-tuning a composite haemodynamic response during myocardial stress. Sympathetic afferents provide the primary pathway for transmission of cardiac nociception to the central nervous system. However, activation of sympathetic afferents may increase susceptibility to life-threatening arrhythmias. Notably, the cardiac sympathetic afferent reflex predominates in pathophysiological states including hypertension and heart failure.

Multiple sensor theory in cardiovascular mechanosensory units

Multiple sensor theory (MST) has advanced our understanding of how lung mechanosensors operate. That is, single lung units contain multiple homogeneous or heterogeneous sensors. Each detects sensor-specific mechanical information and interacts with other sensors lying within the unit sending integrated information to the brain to evoke reflexes. MST explains numerous controversial issues in the respiratory system. Recent studies in baroreceptors (BRs), along with reinterpretation of recordings appearing in the literature, indicate MST also operates in the cardiovascular (CV) system. This review outlines evidence supporting MST in the CV system and provides examples to apply the theory. Longstanding controversies surrounding the CV sensors are also considered.

Differential contributions of cardiac, coronary and pulmonary artery vagal mechanoreceptors to reflex control of the circulation

Distinct populations of stretch‐sensitive mechanoreceptors attached to myelinated vagal afferents are found in the heart and adjoining coronary and pulmonary circulations. Receptors at atrio‐venous junctions appear to be involved in control of intravascular volume. These atrial receptors influence sympathetic control of the heart and kidney, but contribute little to reflex control of systemic vascular resistance. Baroreceptors at the origins of the coronary circulation elicit reflex vasodilatation, like feedback control from systemic arterial baroreceptors, as well as having characteristics that could contribute to regulation of mean pressure. In contrast, feedback from baroreceptors in the pulmonary artery and bifurcation is excitatory and elicits a pressor response. Elevation of pulmonary arterial pressure resets the vasomotor limb of the systemic arterial baroreflex, which could be relevant for control of sympathetic vasoconstrictor outflow during exercise and other states associated with elevated pulmonary arterial pressure. Ventricular receptors, situated mainly in the inferior posterior wall of the left ventricle, and attached to unmyelinated vagal afferents, are relatively inactive under basal conditions. However, a change to the biochemical environment of cardiac tissue surrounding these receptors elicits a depressor response. Some ventricular receptors respond, modestly, to mechanical distortion. Probably, ventricular receptors contribute little to tonic feedback control; however, reflex bradycardia and hypotension in response to chemical activation may decrease the work of the heart during myocardial ischaemia. Overall, greater awareness of heterogeneous reflex effects originating from cardiac, coronary and pulmonary artery mechanoreceptors is required for a better understanding of integrated neural control of circulatory function and arterial blood pressure.

Regulation of breathing by cardiopulmonary afferents

This chapter broadly reviews cardiopulmonary sympathetic and vagal sensors and their reflex functions during physiologic and pathophysiologic processes. Mechanosensory operating mechanisms, including their central projections, are described under multiple sensor theory. In addition, ways to interpret evidence surrounding several controversial issues are provided, with detailed reasoning on how conclusions are derived. Cardiopulmonary sensory roles in breathing control and the development of symptoms and signs and pathophysiologic processes in cardiopulmonary diseases (such as cough and neuroimmune interaction) also are discussed.

CENTRAL AND PERIPHERAL SYMPATHETIC ACTIVATION IN HEART FAILURE

The sympathetic nervous system overdrive occurring in heart failure has been reported since more than half a century. Refinements in the methodological approaches to assess human sympathetic neural function have allowed during recent years to better define various aspects related to the neuroadrenergic alteration. These include 1) the different participation of the individual regional sympathetic cardiovascular districts at the process, 2) the role of the central nervous system in determining the neuroadrenergic overdrive, 3) the involvement of baroreflex, cardiopulmonary reflex and chemoreflex mechanisms in the phoenomenon, which is also closely linked to inflammation and the immune reaction, 4) the relationships with the severity of the disease, its ischaemic or idiopathic nature and the preserved or reduced left ventricular ejection fraction and 5) the adverse functional and structural impact of the sympathetic activation on cardiovascular organs, such as the brain, the heart and the kidneys. Information have been also gained on the active role exerted by the sympathetic activation on the disease outcome and its potential relevance as target of the therapeutic interventions based on non-pharmacological, pharmacological and invasive approaches, including the renal denervation, the splanchnic sympathetic nerve ablation and the carotid baroreflex stimulation. The still undefined aspects of the neurogenic alterations and the unmet goals of the therapeutic approach having the sympathetic activation as a target of the intervention will be finally mentioned.

Electrophysiological characterisation of atrial volume receptors using ex vivo models of isolated rat cardiac atria

NEW FINDINGS

What is the central question of this study? What ex vivo preparation of the rat's cavoatrial junction is efficient for characterising atrial mechanoreceptors? What is the main finding and its importance? Of four different ex vivo preparations, static pressure, flow, open and euthermic, the optimal preparation was the euthermic one and involved direct recording from the right cardiac vagal branch with a Langendorff style perfusion at 37°C. Type A receptors were most common, and appeared insensitive to stretch and sensitive to atrial contraction. Type B and intermediate receptors were not isolated at 20°C but were observed closer to 37°C. The findings may suggest that type A and B receptors utilise different molecular transduction mechanisms.

ABSTRACT

Atrial volume receptors are a family of afferent neurons whose mechanically sensitive endings terminate in the atria, particularly at the cavoatrial junctions. These mechanosensors form the afferent limb of an atrial volume receptor reflex that regulates plasma volume. The prevailing functional classification of atrial receptors arose as a result of in vivo recordings in the cat and dog and were classified as type A, B or intermediate according to the timing of peak discharge during the cardiac cycle. In contrast, there have been far fewer studies of the common small laboratory mammals such as the rat. Using several ex vivo rat cavoatrial preparations, a total of 30 successful single cavoatrial mechanosensory recordings were obtained. These experiments show that the rat possesses type A, B and intermediate atrial mechanoreceptors as described for larger mammals. Recording these cavoatrial receptors proved challenging from the main vagus, but direct recording from the cardiac vagal branch greatly increased the yield of mechanically sensitive single units. In contrast to type A units, type B atrial mechanoreceptor activity was never observed at room temperature but required elevation of temperature to a more physiological range in order to be detected. The adequate stimulus for these receptors remains unclear; however, type A atrial receptors appear insensitive to direct atrial stretch when applied using a programmable positioner. The findings may suggest that type A and type B atrial receptors utilise different molecular transduction mechanisms.

Expression of transient receptor potential channels TRPC1 and TRPV4 in venoatrial endocardium of the rat heart

The atrial volume receptor reflex arc serves to regulate plasma volume. Atrial volume receptors located in the endocardium of the atrial wall undergo mechanical deformation as blood is returned to the atria of the heart. The mechanosensitive channel(s) responsible for regulating plasma volume remain to be determined. Here we report that the TRP channel family members TRPC1 and TRPV4 were expressed in sensory nerve endings in the atrial endocardium. Furthermore, TRPC1 and TRPV4 were coincident with the nerve ending vesicle marker synaptophysin. Calcitonin gene-related peptide was exclusively confined to the myo- and epicardium of the atria. The small conductance Ca(2+)-activated K(+) channels (SK2 and SK4) were also present, however there was no relationship between SK and TRP channels. SK2 channels were expressed in nerves in the epicardium, while SK4 channels were in some regions of the endocardium but appeared to be present in epithelial cells rather than sensory endings. In conclusion, we have provided the first evidence for TRPC1 and TRPV4 channels as potential contributors to mechanosensation in the atrial volume receptors.

Neurally mediated syncope: a review of cardiac and arterial receptors

This article reviews the basic physiology of the reflexogenic areas of the heart and the mechano- and baroreceptors that regulate cardiovascular and autonomic homeostasis, all of which contribute to our understanding of the pathogenesis of neurally mediated syncope. The mechanisms of neurally mediated syncope may involve excessive activation of ventricular receptors that trigger severe hypotension and bradycardia. Thus, neurally mediated syncope may be the clinical expression of the Bezold-Jarisch reflex, which occurs in situations of increased sympathetic activity, perhaps as a result of heightened cardiac receptor sensitivity. The arterial baroreceptors exert a ubiquitous influence on the heart and circulation, and serve primarily to buffer transient changes in arterial pressure by transmitting sensory information regarding their stretch to the central nervous system. This information, in conjunction with cardiac receptor input, elicits alterations in neural efferent output from sympathetic and parasympathetic fibers to provide subtle, continuous regulation of beat-to-beat cardiovascular hemodynamics to an array of physiologic and psychological stressors.

Atrial receptor discharge in dogs with chronically induced difference in blood volume

1. Urinary responses to stimulation of atrial receptors have been found to be greater in dogs with a high blood volume than in dogs with a low blood volume. Two groups of dogs with different blood volume were examined by distending a large balloon in the left atrium, to stimulate atrial receptors and find out whether the increase in atrial receptor discharge was different in the two groups of dogs. 2. The relationship between atrial receptor discharge and left atrial pressure was determined in twenty-six fibres studied in four dogs with a high blood volume and sixteen fibres studied in three dogs with a low blood volume. 3. The slope of the relationship representing the increase in atrial receptor activity during increases in left atrial pressure, and the activity at each left atrial pressure, were significantly greater in dogs with a high blood volume than in dogs with a low blood volume. 4. This study has shown that the atrial receptor discharge and their responses to increases in left atrial pressure are greater in dogs with a high blood volume than in those with a low blood volume.

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.

Left atrial type B receptor response during hypothermia in dogs

1. The response of left atrial type B receptors to hypothermia and its effect on the V-wave pressure-receptor activity relationship was studied in ten anaesthetized and thoracotomized dogs. Hypothermia was produced by surface cooling of the animal. 2. With the drop in the body temperature there was a corresponding fall in the peak frequency of discharge. Reduction in the number of impulses per cardiac cycle and the average resting activity of these receptors became significant only when body temperature fell below 31 degrees C. The Q10 for these receptors is about 2.5, i.e., similar to that for the other mechanoreceptors and consistent with the view that these receptors are in parellel with the muscle fibres. 3. The V-wave pressure-receptor activity relationships were not altered by acute hypothermia.

The nature of the atrial receptors responsible for a reflex increase in heart rate in the dog

1. In dogs anaesthetized with chloralose, small latex balloons were positioned at the left pulmonary vein-atrial junctions so as to stretch this region. By recording action potentials from slips of the cervical vagi it was established that distension of these balloons stimulated receptor endings in the atrial endocardium which discharged into the myelinated branches of the vagi i.e. Paintal type A, type B and Intermediate type receptors. 2. In other dogs, cooling the cervical vagus in steps of 2 degrees C reduced this response in vagal myelinated fibres. With twelve receptors the response to distension was reduced by 30% when the vagus was cooled to 16 degrees C, by 70% when cooled to 12 degrees C and abolished at 8--12 degrees C. 3. In a third group of dogs, distension of balloons at the pulmonary vein-atrial junctions resulted in a reflex increase in heart rate. Cooling the cervical vagi in these dogs in stages to 8 degrees C reduced this increase in heart rate. In nine dogs the response was reduced by 20% when the vagi was cooled to 16 degrees C, by 70% when cooled to 12 degrees C and abolished between 12 and 8 degrees C. 4. In a fourth group of dogs, distension of balloons at the pulmonary vein-atrial junctions was shown also to activate receptor endings in the atria which discharged into non-myelinated branches of the Vagi. In twelve receptors, cooling the cervical vagus in steps of 2 degrees C reduced this evoked increase in activity in non-myelinated fibres. These responses were abolished over a wide range of temperature unlike the responses observed above. 5. It is concluded that the increase in heart rate which follows distension of balloons at the pulmonary vein-atrial junctions is mediated solely by the Paintal-type receptors which discharge into the myelinated fibres in the vagi.

Type A atrial receptors in the cat: effects of changes in atrial volume and contractility

1. Action potentials were recorded from filaments of the right cervical vagus in anaesthetized, paralysed cats. Right atrial receptors with type A (twelve units) and Intermediate type (two units) patterns of spontaneous discharge were selected and their responses to changes in atrial volume were analysed. 2. Changes in atrial volume of similar magnitude were produced under four different conditions: a, innervated hearts; b, denervated hearts; c, depression of atrial muscle contractility induced after cardiac denervation and d, non-beating hearts. 3. In innervated hearts the systolic discharge of each receptor showed a characteristic response to changes in atrial volume. Cardiac denervation and depression of atrial contractility markedly altered this response in terms of frequency of discharge threshold and 'sensitivity'. 4. During increments in atrial volume all the receptors but one assumed an Intermediate pattern of discharge. The diastolic firing rate was, however, higher for any given atrial pressure, in innervated hearts than under conditions b, c and d. 5. In innervated hearts the response of the receptors to atrial systole was characterized by a higher frequency of discharge and a lower threshold with respect to the responses of the same receptors to atrial filling. These differences were minimized at high atrial volumes and during depression of atrial contractility. 6. The results indicate that the responses of the receptors to atrial systole are mainly dependent upon the state of contraction of atrial muscle and that the differences between systolic and diastolic discharge are mainly due to the high dynamic component of the stretch during atrial contraction.

Responses of atrial mechanoreceptors to pulsation of atrial volume

1. Firing patterns of single atrial fibres were monitored in order to study their sensitivity to rate of change of atrial volume and their dependence on receptor location in the atria.2. To that effect, sinusoidal fluctuations of volume (+/-1 ml.; 60/min) were superimposed on the natural pulsations of the heart of spontaneously breathing anaesthetized cats.3. Stimulus-response histograms revealed that maximal firing frequency of B-impulses preceded maximal volume change by 10-80 degrees (average 48+/-7 degrees (S.E.), thirteen right and left atrial receptors) indicating that sensitivity to rate of change of atrial volume at least equalled volume sensitivity.4. The rate sensitive component of B-firing increased with respect to the volume sensitive component when atrial filling diminished and was larger for high threshold receptors than for low threshold receptors (P<0.001).5. The A-bursts were not affected by atrial pulsing in eight out of nine anatomically localized receptors; inspiration prolonged the A-bursts of three receptors and advanced the timing of A-bursts with respect to the a-wave (10-50 msec) of all nine receptors.6. About half of all anatomically localized receptors were traced to the interatrial septum. These septal receptors differed from right and left atrial receptors as follows: (a) they were exclusively of the AB-type (fired during atrial a- and v-wave); (b) their B-firing rates were significantly lower (P<0.05); (c) A-firing patterns remained largely unaffected by respiration or by atrial pulsing; (d) they signalled volume changes of either atria; (e) no association of rate sensitivity with threshold was detected (P>0.95). The physiological role of septal receptors remains to be elucidated.7. It was concluded that B-fibres were sensitive to both change and rate of change of atrial volume; their role as sensors of venous return is suggested strongly. Results further suggested that A- and B-firing are determined largely by the location of atrial stretch receptors.

Atrial receptors in the dog and rabbit

1. Action potentials were recorded from slips of the cervical vagi in anaesthetized dogs and rabbits. Single functional units with atrial patterns of discharge (Paintal Type A, B and intermediate) were obtained and then attempts were made to alter (i.e. convert) their patterns of discharge. Finally the points of origin of these action potentials were located.2. Thirty unselected units were investigated in thirty dogs. Twenty-seven of these were located in the endocardium of the vein-atrial system and the ratio of the type A, type B and intermediate type receptors was 1:16:10; three units were located elsewhere in the chest. Conversion of the pattern of discharge was achieved in twenty of the twenty-seven units; conversion was achieved in the single type A unit.3. In a second series of experiments in dogs, eight Paintal Type A units were selectively studied in fifteen animals. Four of these were located in the endocardium and all were converted. The remaining four were located outside the endocardium and conversion could not be achieved in two of these. Thus in the entire investigation, the ;type A' units which could not be converted were all located at sites other than the atrial endocardium.4. In the corresponding unselected study in the rabbit, eleven units were studied in eleven animals. Nine of these units were located in the atrial endocardium and the ratio of the type A, type B and intermediate type receptors was 2:1:6. Conversion was achieved in both type A units, the sole type B unit and two of the intermediate units. One of the two units found outside the atrial endocardium was a ;type A' unit and could not be converted.5. The present investigation has shown that the atrial receptors with a Paintal Type A pattern of discharge are relatively rare in both dogs and rabbits. Conversion of the pattern of discharge is a relatively common phenomenon. Evidence for the proposition that there is one basic type of atrial receptor whose pattern of discharge is determined by its precise location in the vein-atrial system is discussed.

Cardiopulmonary vagal afferents in the monkey: a survey of receptor activity

A survey was made of vagal afferents whose endings originated in cardiopulmonary areas of the Rhesus monkey. Recordings of action potentials from single fiber preparations of the left cervical vagus were made in both open and closed chest monkeys. A total of 425 receptors were identified in sixteen animals. These consisted of 347 pulmonary stretch receptors (one of which increased its discharge during expiration), 42 aortic baroreceptors, 4 ventricular pressure receptors, 1 epicardial ventricular receptor, 7 type A atrial receptors and 24 type B atrial receptors. The response of each cardiovascular receptor was tested by altering the stimulus for their discharge. Aortic and ventricular baroreceptors increased their discharge in response to an increase in blood pressure induced by intravenous norepinephrine. Type A atrial receptors did not increase their discharge in response to an increase in atrial pressure during intravenous administration of isotonic saline, while type B atrial receptors did. The discharge of the latter became continuous following the intravenous administration of veratridine sulphate (20 microgram). It is concluded that the basic types of cardiopulmonary afferents exist in the non-human primate and that they respond similarly to those which have been demonstrated in lower species.