Cardiac, aortic, pericardial, and pulmonary receptors in the dog

Neurocardiology: Structure-Based Function

Cardiac control is mediated via a series of reflex control networks involving somata in the (i) intrinsic cardiac ganglia (heart), (ii) intrathoracic extracardiac ganglia (stellate, middle cervical), (iii) superior cervical ganglia, (iv) spinal cord, (v) brainstem, and (vi) higher centers. Each of these processing centers contains afferent, efferent, and local circuit neurons, which interact locally and in an interdependent fashion with the other levels to coordinate regional cardiac electrical and mechanical indices on a beat-to-beat basis. This control system is optimized to respond to normal physiological stressors (standing, exercise, and temperature); however, it can be catastrophically disrupted by pathological events such as myocardial ischemia. In fact, it is now recognized that autonomic dysregulation is central to the evolution of heart failure and arrhythmias. Autonomic regulation therapy is an emerging modality in the management of acute and chronic cardiac pathologies. Neuromodulation-based approaches that target select nexus points of this hierarchy for cardiac control offer unique opportunities to positively affect therapeutic outcomes via improved efficacy of cardiovascular reflex control. As such, understanding the anatomical and physiological basis for such control is necessary to implement effectively novel neuromodulation therapies. © 2016 American Physiological Society. Compr Physiol 6:1635-1653, 2016.

Effects of Lewis lung carcinoma and B16 melanoma on the innervation of the mouse trachea

Cancer patients often suffer from dyspnea the pathogenesis of which is incompletely understood. Both dyspnea and pulmonary diseases are closely linked to airway innervation. Recently, it was shown that Lewis lung carcinoma induces cardiac hypoinnervation in the mouse. We hypothesized that airway innervation undergoes similar changes as myocardial innervation and that this effect occurs in different mouse models of cancer. C57Bl6 mice were randomly assigned to subcutaneous injection of Lewis lung carcinoma cells (LLC, n=6), B16 melanoma cells (B16, n=6), or saline (control group, C, n=10). After 16 or 21 days, respectively, the trachea was processed for light and electron microscopic design-based stereology and the volume, surface area and length of axons ramifying in the tracheal wall were estimated. Body weight was reduced both in LLC and B16 vs. C. Hypoinnervation was present in both tumor groups compared to controls as volume and surface area of axons were significantly reduced in LLC and B16. However, the total length of tracheal axons and the mean number of axons per nerve fiber were reduced only in LLC but not in B16 compared to C indicating a differentially pronounced effect of cancer on tracheal innervation. In conclusion, reduced innervation of the trachea was observed in two different murine tumor models. These findings add to the pathophysiological concepts explaining cancer-related dyspnea and open new perspectives of treating this symptom.

Cancer induces cardiomyocyte remodeling and hypoinnervation in the left ventricle of the mouse heart

Cancer is often associated with cachexia, cardiovascular symptoms and autonomic dysregulation. We tested whether extracardiac cancer directly affects the innervation of left ventricular myocardium. Mice injected with Lewis lung carcinoma cells (tumor group, TG) or PBS (control group, CG) were analyzed after 21 days. Cardiac function (echocardiography), serum levels of TNF-α and Il-6 (ELISA), structural alterations of cardiomyocytes and their innervation (design-based stereology) and levels of innervation-related mRNA (quantitative RT-PCR) were analysed. The groups did not differ in various functional parameters. Serum levels of TNF-α and Il-6 were elevated in TG. The total length of axons in the left ventricle was reduced. The number of dense core vesicles per axon profile was reduced. Decreased myofibrillar volume, increased sarcoplasmic volume and increased volume of lipid droplets were indicative of metabolic alterations of TG cardiomyocytes. In the heart, the mRNA level of nerve growth factor was reduced whereas that of β1-adrenergic receptor was unchanged in TG. In the stellate ganglion of TG, mRNA levels of nerve growth factor and neuropeptide Y were decreased and that of tyrosine hydroxylase was increased. In summary, cancer induces a systemic pro-inflammatory state, a significant reduction in myocardial innervation and a catabolic phenotype of cardiomyocytes in the mouse. Reduced expression of nerve growth factor may account for the reduced myocardial innervation.

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.

Reflex effects of stimulation of sympathetic afferents on the triangularis sterni

The purpose of this study was to determine whether contralateral inhibition of the triangularis sterni is produced by stimulation of intrathoracic sympathetic afferents. Dogs were anesthetized with sodium pentobarbital and placed on positive pressure ventilation. The chest was opened through a mid-sternal incision. Diaphragm and left and right triangularis sterni EMGs were recorded, post-vagotomy, before and during electrical stimulation of the left ventral ansa subclavia (VA), vagosympathetic trunk, ventrolateral and ventromedial cardiac nerves and, when present, the stellate cardiac nerve. Peak of the phasic diaphragm EMG and expiratory time were not significantly affected by stimulation of the VA. A significant decrease in inspiratory time was observed. Ipsilateral excitation and contralateral inhibition of the left and right triangularis sterni EMGs, respectively, were produced by stimulation of the VA. Stimulation of the other intrathoracic nerves produced a similar pattern of results. Conduction velocity determinations suggested that the afferents which produced the reflex responses are, at least in part, small A fibers.

Studies of reflexogenic effects of capsaicin and neuropeptides on neural afferents in the dog parietal pericardium

Stimulation of neural afferents in the parietal pericardium of anaesthetized, open-chest dogs by local application of capsaicin (0.1-100 micrograms) consistently induced dose-related pressor effects and tachycardia, whereas the application (0.1-1 microgram) of neuropeptides substance P (SP), neurokinin A (NKA), neurokinin B (NKB) or calcitonin gene-related peptide (CGRP) had no cardiovascular effect. Capsaicin-induced reflex responses were not affected by vagotomy, but were abolished by bilateral sectioning of the upper thoracic (T1-T4) white rami communicantes and stellectomy. Capsaicin-induced reflex tachycardia could also be abolished by a beta-adrenoceptor blockade with propranolol (0.5 mg/kg, IV), while ganglionic blockade with pentolinium (0.5 mg/kg, IV) eliminated both the tachycardia and pressor effects. Intravenous treatment with the cyclo-oxygenase inhibitors, indomethacin (5 mg/kg) or aspirin (100 mg/kg) had no effect on reflex pressor and heart rate responses to pericardial capsaicin. Also local treatment of the pericardium with either indomethacin (1 microgram/ml) or dual cyclooxygenase/lipoxygenase inhibitor, BW755C (10 micrograms/ml) failed to affect the responses to capsaicin. We conclude that (i) capsaicin-sensitive afferents which are present in the dog pericardium have a spinal origin and can initiate sympathetically-mediated reflex cardiovascular changes; (ii) the reflexogenic action of capsaicin on pericardial afferents does not depend on local production of eicosanoids; (iii) neuropeptides appear to be without reflexogenic effects on neural afferents in the dog parietal pericardium.

Regional myocardial glucose utilization by developing fetal and maternal hearts

Regional glucose utilization of the developing fetal feline heart was assessed during three stages of gestation and compared with the maternal heart and non-pregnant controls. The specific aims were to determine: 1) if glucose utilization by the whole heart changes from early to late gestation; 2) if there are differences in glucose utilization by specific regions of the heart; 3) if these regional differences in glucose utilization are consistent throughout gestation. Regional myocardial glucose utilization was measured using the [14C] 2-deoxyglucose high spatial resolution autoradiographic technique. Eleven fetal and 16 adult hearts were studied. Two of the fetuses were at 49 days of gestation, three were at 35 days, and six were at 25 days of gestation. This was the first study to assess regional myocardial glucose utilization in the developing fetus. Glucose utilization by the fetal heart was greater than that seen in the normal control adult or maternal heart, and was highest during early gestation. The posterior wall of the left ventricle had glucose utilization twice that measured for the anterior wall. Other regions were not significantly different. This information indicates that availability of glucose to the fetus is important for normal cardiac metabolism and development.

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.

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.