Bradykinin causes hypotension by activating pulmonary sympathetic afferents in the rabbit

Research journey into multiple-sensor theory

In 1998, I was asked by the American Physiological Society to review a book written by Dr. Michael de Burgh Daly, Peripheral Arterial Chemoreceptors and Respiratory-Cardiovascular Integration. Inspired by this work, I came to appreciate how researchers in the later stages of their careers and who provide a detailed review of their experimental approach might effectively contribute to science, especially to the benefit of young scientists (Yu J. The Physiologist 41: 231, 1998.). This article is written in that vein. Over several decades of intensive investigation of cardiopulmonary reflexes, focused on the sensory receptors, my colleagues and I advanced a novel multiple-sensor theory (MST) to explain the role of the vagal mechanosensory system. Described here is our research journey through various stages of developing MST and the process of how the problem was identified, approached, and tackled. MST redefines conventional mechanosensor doctrines and is supported by new studies that clarify a century of research data. It entails reinterpretation of many established findings. Hopefully, this article will benefit young scientists, such as graduate and postdoctoral students in the cardiopulmonary sensory research field.

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.

A direct injection technique for investigation of lung sensory properties and reflex functions

NEW FINDINGS

This article reviews a unique direct injection technique that complements the more conventional right atrial injection and aerosol delivery methods to study sensory and reflex effects of the lung sensors. Used in combination with other methods, this technique should contribute to the pulmonary sensory research.

ABSTRACT

The lungs house sensory receptors (sensors) that mediate a variety of sensory and reflex responses to mechanical or chemical changes. These reflexes are mainly carried through pulmonary sympathetic and vagal afferent pathways. The chemosensors in the lung periphery are especially important in pulmonary diseases and their reflex responses have traditionally been studied either by aerosol delivery, which also activates receptors in the central airways, or by right atrial injection, which also activates receptors lying outside the lung. Thus, these techniques may confound the interpretation of sensory function. Our laboratory has developed a direct injection technique to deliver agents into the lung parenchyma, which complements the conventional techniques with some important advantages. This article reviews the technique.

Sympatho-excitatory response to pulmonary chemosensitive spinal afferent activation in anesthetized, vagotomized rats

The sensory innervation of the lung is well known to be innervated by nerve fibers of both vagal and sympathetic origin. Although the vagal afferent innervation of the lung has been well characterized, less is known about physiological effects mediated by spinal sympathetic afferent fibers. We hypothesized that activation of sympathetic spinal afferent nerve fibers of the lung would result in an excitatory pressor reflex, similar to that previously characterized in the heart. In this study, we evaluated changes in renal sympathetic nerve activity (RSNA) and hemodynamics in response to activation of TRPV1‐sensitive pulmonary spinal sensory fibers by agonist application to the visceral pleura of the lung and by administration into the primary bronchus in anesthetized, bilaterally vagotomized, adult Sprague‐Dawley rats. Application of bradykinin (BK) to the visceral pleura of the lung produced an increase in mean arterial pressure (MAP), heart rate (HR), and RSNA. This response was significantly greater when BK was applied to the ventral surface of the left lung compared to the dorsal surface. Conversely, topical application of capsaicin (Cap) onto the visceral pleura of the lung, produced a biphasic reflex change in MAP, coupled with increases in HR and RSNA which was very similar to the hemodynamic response to epicardial application of Cap. This reflex was also evoked in animals with intact pulmonary vagal innervation and when BK was applied to the distal airways of the lung via the left primary bronchus. In order to further confirm the origin of this reflex, epidural application of a selective afferent neurotoxin (resiniferatoxin, RTX) was used to chronically ablate thoracic TRPV1‐expressing afferent soma at the level of T1–T4 dorsal root ganglia pleura. This treatment abolished all sympatho‐excitatory responses to both cardiac and pulmonary application of BK and Cap in vagotomized rats 9–10 weeks post‐RTX. These data suggest the presence of an excitatory pulmonary chemosensitive sympathetic afferent reflex. This finding may have important clinical implications in pulmonary conditions inducing sensory nerve activation such as pulmonary inflammation and inhalation of chemical stimuli.

Sensory nerves in lung and airways

Sensory nerves innervating the lung and airways play an important role in regulating various cardiopulmonary functions and maintaining homeostasis under both healthy and disease conditions. Their activities conducted by both vagal and sympathetic afferents are also responsible for eliciting important defense reflexes that protect the lung and body from potential health-hazardous effects of airborne particulates and chemical irritants. This article reviews the morphology, transduction properties, reflex functions, and respiratory sensations of these receptors, focusing primarily on recent findings derived from using new technologies such as neural immunochemistry, isolated airway-nerve preparation, cultured airway neurons, patch-clamp electrophysiology, transgenic mice, and other cellular and molecular approaches. Studies of the signal transduction of mechanosensitive afferents have revealed a new concept of sensory unit and cellular mechanism of activation, and identified additional types of sensory receptors in the lung. Chemosensitive properties of these lung afferents are further characterized by the expression of specific ligand-gated ion channels on nerve terminals, ganglion origin, and responses to the action of various inflammatory cells, mediators, and cytokines during acute and chronic airway inflammation and injuries. Increasing interest and extensive investigations have been focused on uncovering the mechanisms underlying hypersensitivity of these airway afferents, and their role in the manifestation of various symptoms under pathophysiological conditions. Several important and challenging questions regarding these sensory nerves are discussed. Searching for these answers will be a critical step in developing the translational research and effective treatments of airway diseases.

The hemodynamic response to blood loss in the conscious rat: contributions of cardiac vagal and cardiac spinal signals

The hemodynamic response to progressive blood loss passes through three distinct phases: an initial normotensive compensatory phase, a secondary hypotensive decompensatory phase, and a posthemorrhage recompensatory phase. The role of cardiac vagal and cardiac spinal signals in triggering the different phases of the response to hemorrhage was evaluated in the unanesthetized, freely moving rat by observing the effects on the response to 30% blood loss of prior cardiac vagal deafferentation (bilateral vagal rhizotomy) or prior cardiac spinal deafferentation (bilateral stellate ganglionectomy). In comparison to control animals, it was found that (i) cardiac spinal deafferentation significantly delayed the onset of the decompensatory phase, and (ii) cardiac vagal deafferentation slightly potentiated the decompensatory phase and impaired the recompensatory phase. These results indicate that it is cardiac spinal signals, rather than cardiac vagal signals, which in the conscious rat contribute to the triggering and progression of the decompensatory response to blood loss.

Gynura procumbens causes vasodilation by inhibiting angiotensin II and enhancing bradykinin actions

Previous studies showed that Gynura procumbens reduced blood pressure by blocking calcium channels and inhibiting the angiotensin-converting enzyme activity. The present experiments were to further explore the effects and mechanisms of a purer aqueous fraction (FA-I) of G. procumbens on angiotensin I (Ang I)-induced and angiotensin II (Ang II)-induced contraction of aortic rings and also on the bradykinin (BK) effect on cardiovascular system. Rat aortic rings suspended in organ chambers were used to investigate the vascular reactivity of FA-I. Effect of FA-I on BK was studied by in vitro and in vivo methods. Results show that FA-I significantly (P < 0.05) decreased the contraction evoked by Ang I and Ang II. In the presence of indomethacin (10 µM) or N-nitro-L-arginine methyl ester (0.1 µM), the inhibitory effect of FA-I on Ang II-induced contraction of aortic rings was reduced. Besides, FA-I potentiated the vasorelaxant effect and enhanced the blood pressure-lowering effect of BK. In conclusion, FA-I reduced the contraction evoked by Ang II probably via the endothelium-dependent pathways, which involve activation of the release of nitric oxide and prostaglandins. The inhibition of angiotensin-converting enzyme activity by FA-I may contribute to the potentiation of the effects of BK on cardiovascular system.

Neuroimmune interaction in inflammatory diseases

The inflammatory response is modulated through interactions among the nervous, endocrine, and immune systems. Intercommunication between immune cells and the autonomic nervous system is a growing area of interest. Spatial and temporal information about inflammatory processes is relayed to the central nervous system (CNS) where neuroimmune modulation serves to control the extent and intensity of the inflammation. Over the past few decades, research has revealed various routes by which the nervous system and the immune system communicate. The CNS regulates the immune system via hormonal and neuronal pathways, including the sympathetic and parasympathetic nerves. The immune system signals the CNS through cytokines that act both centrally and peripherally. This review aims to introduce the concept of neuroimmune interaction and discuss its potential clinical application, in an attempt to broaden the awareness of this rapidly evolving area and open up new avenues that may aid in the treatment of inflammatory diseases.

Characterization of thoracic spinal neurons with noxious convergent inputs from heart and lower airways in rats

Respiratory symptoms experienced in some patients with cardiac diseases may be due to convergence of noxious cardiac and pulmonary inputs onto neurons of the central nervous system. For example, convergence of cardiac and respiratory inputs onto single solitary tract neurons may be in part responsible for integration of regulatory and defensive reflex control. However, it is unknown whether inputs from the lungs and heart converge onto single neurons of the spinal cord. The present aim was to characterize upper thoracic spinal neurons responding to both noxious stimuli of the heart and lungs in rats. Extracellular potentials of single thoracic (T3) spinal neurons were recorded in pentobarbital anesthetized, paralyzed, and ventilated male rats. A catheter was placed in the pericardial sac to administer bradykinin (BK, 10 microg/ml, 0.2 ml, 1 min) as a noxious cardiac stimulus. The lung irritant, ammonia, obtained as vapor over a 30% solution of NH(4)OH was injected into the inspiratory line of the ventilator (0.5-1.0 ml over 20 s). Intrapericardial bradykinin (IB) altered activity of 58/65 (89%) spinal neurons that responded to inhaled ammonia (IA). Among those cardiopulmonary convergent neurons, 81% (47/58) were excited by both IA and IB, and the remainder had complex response patterns. Bilateral cervical vagotomy revealed that vagal afferents modulated but did not eliminate responses of individual spinal neurons to IB and IA. The convergence of pulmonary and cardiac nociceptive signaling in the spinal cord may be relevant to situations where a disease process in one organ influences the behavior of the other.

Afferent pathway and neuromodulation of superficial and deeper thoracic spinal neurons receiving noxious pulmonary inputs in rats

The occurrence of vagally mediated afferent signaling by lung irritants is well known. However, spinal visceral afferent pathways also might be relevant to pulmonary irritation. In the present study, responses and modulation of superficial and deep T3 spinal neurons were examined using inhaled ammonia, and the peripheral afferent fibers were also characterized in part. Extracellular potentials of single thoracic (T3) spinal neurons were recorded in pentobarbital anesthetized, paralyzed, and ventilated male rats. Ammonia vapor (0.5, 1.0, 2.0 ml) was injected into the inspiratory line of the ventilator for 20 s. Inhaled ammonia (IA, 1.0 ml) excited 5/6 neurons and inhibited one spinal neuron recorded in superficial laminae, whereas deeper neurons responded with excitatory (E, n = 20), inhibitory (I, n = 4) or biphasic patterns (6 E-I, 3 I-E). Electrical and chemical stimulation of C1-C2 spinal neurons primarily suppressed T3 neuronal responses to IA. Resiniferatoxin (2 microg/kg, i.v.), which desensitizes afferent fibers containing transient receptor potential vanilloid receptor-1 (TRPV-1), abolished excitatory responses of 8/8 neurons to IA. Bilateral cervical vagotomy did not affect IA responses in 5 superficial neurons while 7 deeper neurons showed variable responses. 82% (32/39) of the spinal neurons responding to IA also received convergent noxious inputs from somatic fields in the chest and back areas. These results suggested that superficial and deeper spinal neuronal activation by inhaled ammonia mainly depended upon pulmonary sympathetic afferent fibers expressing TRPV-1. Additionally, C1-C2 spinal neurons, supraspinal sites and vagal afferents modulated the thoracic spinal neuronal responses to lower airway irritation.

Evidence for a role of neuroepithelial bodies as complex airway sensors: comparison with smooth muscle-associated airway receptors

The epithelium of intrapulmonary airways in many species harbors diffusely spread innervated groups of neuroendocrine cells, called neuroepithelial bodies (NEBs). Data on the location, morphology, and chemical coding of NEBs in mammalian lungs are abundant, but none of the proposed functions has so far been fully established. Besides C-fiber afferents, slowly adapting stretch receptors, and rapidly adapting stretch receptors, recent reviews have added NEBs to the list of presumed sensory receptors in intrapulmonary airways. Physiologically, the innervation of NEBs, however, remains enigmatic. This short overview summarizes our present understanding of the chemical coding and exact location of the receptor end organs of myelinated vagal airway afferents in intrapulmonary airways. The profuse populations that selectively contact complex pulmonary NEB receptors are compared with the much smaller group of smooth muscle-associated airway receptors. The main objective of our contribution was to stimulate the idea that the different populations of myelinated vagal afferents that selectively innervate intraepithelial pulmonary NEBs may represent subpopulations of the extensive group of known electrophysiologically characterized myelinated vagal airway receptors. Future efforts should be directed toward finding out which airway receptor groups are selectively coupled to the complex NEB receptors.

Reflex regulation of airway sympathetic nerves in guinea-pigs

Sympathetic nerves innervate the airways of most species but their reflex regulation has been essentially unstudied. Here we demonstrate sympathetic nerve-mediated reflex relaxation of airway smooth muscle measured in situ in the guinea-pig trachea. Retrograde tracing, immunohistochemistry and electrophysiological analysis identified a population of substance P-containing capsaicin-sensitive spinal afferent neurones in the upper thoracic (T1-T4) dorsal root ganglia (DRG) that innervate the airways and lung. After bilateral vagotomy, atropine pretreatment and pre-contraction of the trachealis with histamine, nebulized capsaicin (10-60 microm) evoked a 63+/-7% reversal of the histamine-induced contraction of the trachealis. Either the beta-adrenoceptor antagonist propranolol (2 microm, administered directly to the trachea) or bilateral sympathetic nerve denervation of the trachea essentially abolished these reflexes (10+/-9% and 6+/-4% relaxations, respectively), suggesting that they were mediated primarily, if not exclusively, by sympathetic adrenergic nerve activation. Cutting the upper thoracic dorsal roots carrying the central processes of airway spinal afferents also markedly blocked the relaxations (9+/-5% relaxation). Comparable inhibitory effects were observed following intravenous pretreatment with neurokinin receptor antagonists (3+/-7% relaxations). These reflexes were not accompanied by consistent changes in heart rate or blood pressure. By contrast, stimulating the rostral cut ends of the cervical vagus nerves also evoked a sympathetic adrenergic nerve-mediated relaxation that were accompanied by marked alterations in blood pressure. The results indicate that the capsaicin-induced reflex-mediated relaxation of airway smooth muscle following vagotomy is mediated by sequential activation of tachykinin-containing spinal afferent and sympathetic efferent nerves innervating airways. This sympathetic nerve-mediated response may serve to oppose airway contraction induced by parasympathetic nerve activation in the airways.

Bradykinin B2 receptors mediate pulmonary sympathetic afferents induced reflexes in rabbits

Endogenous bradykinin (BK) is an established mediator of pulmonary inflammation, yet its role in lung disease is unclear. In the rabbit, injecting BK into the lung parenchyma elicits reflex hyperpnea, tachypnea, hypotension, and bradycardia by stimulating pulmonary sympathetic afferents. To further explore bradykinin effects, breathing pattern (phrenic nerve and abdominal muscle activities) and hemodynamics (blood pressure and heart rate) were examined in anesthetized, open-chest, and mechanically ventilated rabbits. Three receptor agonists [bradykinin, selective B(1) (des-Arg(9)-BK), and selective B(2) (Tyr(8)-BK)], as well as three B(2) receptor antagonists, B6029 (N alpha-Adamantaneacetyl)-Bradykinin, B(1)650 (D-Arg-[Hyp(3), Thi(5,8), D-Phe(7)]-Bradykinin, or Hoe-140 (D-Arg-[Hyp(3), Thi(5), D-Tic(7), Oic(8)] bradykinin), were used to identify the responsible receptor subtype. In both intact and vagotomized rabbits, injecting BK or a selective B(2) agonist into the lung elicited similar cardiopulmonary responses. These reflex responses were greatly attenuated or blocked by pre-injecting B(2) antagonists into the right atrium or into the lung parenchyma. In contrast, the B(1) agonist elicited fewer cardiopulmonary effects in intact rabbits and had no effect in vagotomized rabbits. We conclude that BK stimulates pulmonary sympathetic afferents [Soukhova, G., Wang, Y., Ahmed, M., Walker, J., Yu, J., 2003. Bradykinin stimulates respiratory drive by activating pulmonary sympathetic afferents in the rabbit. J. Appl. Physiol. 95, 241-249.; Wang, Y., Soukhova, G., Proctor, M., Walker, J., Yu, J., 2003. Bradykinin causes hypotension by activating pulmonary sympathetic afferents in the rabbit. J. Appl. Physiol. 95, 233-240.], eliciting a characteristic cardiopulmonary reflex via B(2) receptors.

Airway mechanosensors

A mechanosensory unit is a functional unit that contains multiple receptors (or encoders) with different characteristics, including rapidly adapting receptors, slowly adapting receptors, and deflation-activated-receptors. Each is capable of sensing different aspects of lung mechanics. The sensory unit is both a transducer and a processor. Significant information integration occurs at the intra-encoder and inter-encoder levels. Within an encoder, the information is encoded as analog signals and integrated by amplitude modulation. Information from each single stretch-activated channel is processed through several levels of temporal and spatial summation, producing a generator potential that encodes averaged overall information within the encoder. This analog signal is transformed into a digital signal in the form of action potentials that are encoded as frequency (frequency modulation). These all-or-none propagated action potentials from different encoders interact through a competitive selection mechanism. Such inter-encoder interaction may occur at several levels, because of the fractal nature of the sensory unit. Inter-encoder interaction retains representative information but eliminates redundant information, resulting in the final output to the central nervous system, where multiple decoders specific for different variables decipher the encoded information for further processing.

Bradykinin stimulates respiratory drive by activating pulmonary sympathetic afferents in the rabbit

We recently identified a vagally mediated excitatory lung reflex by injecting hypertonic saline into the lung parenchyma (Yu J, Zhang JF, and Fletcher EC. J Appl Physiol 85: 1485-1492, 1998). This reflex increased amplitude and burst rate of phrenic (inspiratory) nerve activity and suppressed external oblique abdominal (expiratory) muscle activity. In the present study, we tested the hypothesis that bradykinin may activate extravagal pathways to stimulate breathing by assessing its reflex effects on respiratory drive. Bradykinin (1 microg/kg in 0.1 ml) was injected into the lung parenchyma of anesthetized, open-chest and artificially ventilated rabbits. In most cases, bradykinin increased phrenic amplitude, phrenic burst rate, and expiratory muscle activity. However, a variety of breathing patterns resulted, ranging from hyperpnea and tachypnea to rapid shallow breathing and apnea. Bradykinin acts like hypertonic saline in producing hyperpnea and tachypnea, yet the two agents clearly differ. Bradykinin produced a higher ratio of phrenic amplitude to inspiratory time and had longer latency than hypertonic saline. Although attenuated, bradykinin-induced respiratory responses persisted after vagotomy. We conclude that bradykinin activates multiple afferent pathways in the lung; portions of its respiratory reflexes are extravagal and arise from sympathetic afferents.