Protein kinase R-like endoplasmatic reticulum kinase is a mediator of stretch in ventilator-induced lung injury.

BACKGROUND: Acute respiratory distress syndrome (ARDS) is a severe form of lung injury characterized by damage to the epithelial barrier with subsequent pulmonary edema and hypoxic respiratory failure. ARDS is a significant medical problem in intensive care units with associated high care costs. There are many potential causes of ARDS; however, alveolar injury associated with mechanical ventilation, termed ventilator-induced lung injury (VILI), remains a well-recognized contributor. It is thus critical to understand the mechanism of VILI. Based on our published preliminary data, we hypothesized that the endoplasmic reticulum (ER) stress response molecule Protein Kinase R-like Endoplasmic Reticulum Kinase (PERK) plays a role in transmitting mechanosensory signals the alveolar epithelium. METHODS: ER stress signal responses to mechanical stretch were studied in ex-vivo ventilated pig lungs. To explore the effect of PERK inhibition on VILI, we ventilated live rats and compared lung injury parameters to non-ventilated controls. The effect of stretch-induced epithelial ER Ca2+ signaling on PERK was studied in stretched alveolar epithelial monolayers. To confirm the activation of PERK in human disease, ER stress signaling was compared between ARDS and non-ARDS lungs. RESULTS: Our studies revealed increased PERK-specific ER stress signaling in response to overstretch. PERK inhibition resulted in dose-dependent improvement of alveolar inflammation and permeability. Our data indicate that stretch-induced epithelial ER Ca2+ release is an activator of PERK. Experiments with human lung tissue confirmed PERK activation by ARDS. CONCLUSION: Our study provides evidences that PERK is a mediator stretch signals in the alveolar epithelium.

The Splicing Factor hnRNPA1 Regulates Alternate Splicing of the MYLK Gene.

Profound lung vascular permeability is a cardinal feature of acute respiratory distress syndrome (ARDS) and ventilator-induced lung injury (VILI), two syndromes known to centrally involve the nonmuscle isoform of myosin light chain kinase (nmMLCK) in vascular barrier dysregulation. Two main splice variants, nmMLCK1 and nmMLCK2, are well represented in human lung endothelial cells and encoded by MYLK, and they differ only in the presence of exon 11 in nmMLCK1, which contains critical phosphorylation sites (Y464 and Y471) that influence nmMLCK enzymatic activity, cellular translocation, and localization in response to vascular agonists. We recently demonstrated the functional role of SNPs in altering MYLK splicing, and in the present study we sought to identify the role of splicing factors in the generation of nmMLCK1 and nmMLCK2 spliced variants. Using bioinformatic in silico approaches, we identified a putative binding site for heterogeneous nuclear ribonucleoprotein A1 (hnRNPA1), a recognized splicing factor. We verified hnRNPA1 binding to MYLK by gel shift analyses and that hnRNPA1 gene and protein expression is upregulated in mouse lungs obtained from preclinical models of ARDS and VILI and in human endothelial cells exposed to 18% cyclic stretch, a model that reproduces the excessive mechanical stress observed in VILI. Using an MYLK minigene approach, we established a direct role of hnRNPA1 in MYLK splicing and in the context of 18% cyclic stretch. In summary, these data indicate an important regulatory role for hnRNPA1 in MYLK splicing, and they increase understanding of MYLK splicing in the regulation of lung vascular integrity during acute lung inflammation and excessive mechanical stress, such as that observed in ARDS and VILI.

Lysyl Oxidase-Like 1 Protein Deficiency Protects Mice from Adenoviral Transforming Growth Factor-ß1-induced Pulmonary Fibrosis.

Idiopathic pulmonary fibrosis (IPF) is a progressive disease characterized by excessive deposition of extracellular matrix (ECM) in the lung parenchyma. The abnormal ECM deposition slowly overtakes normal lung tissue, disturbing gas exchange and leading to respiratory failure and death. ECM cross-linking and subsequent stiffening is thought to be a major contributor of disease progression and also promotes the activation of transforming growth factor (TGF)-ß1, one of the main profibrotic growth factors. Lysyl oxidase-like (LOXL) 1 belongs to the cross-linking enzyme family and has been shown to be up-regulated in active fibrotic regions of bleomycin-treated mice and patients with IPF. We demonstrate in this study that LOXL1-deficient mice are protected from experimental lung fibrosis induced by overexpression of TGF-ß1 using adenoviral (Ad) gene transfer (AdTGF-ß1). The lack of LOXL1 prevented accumulation of insoluble cross-linked collagen in the lungs, and therefore limited lung stiffness after AdTGF-ß1. In addition, we applied mechanical stretch to lung slices from LOXL1+/+ and LOXL1-/- mice treated with AdTGF-ß1. Lung stiffness (Young's modulus) of LOXL1-/- lung slices was significantly lower compared with LOXL1+/+ lung slices. Moreover, the release of activated TGF-ß1 after mechanical stretch was significantly lower in LOXL1-/- mice compared with LOXL1+/+ mice after AdTGF-ß1. These data support the concept that cross-linking enzyme inhibition represents an interesting therapeutic target for drug development in IPF.

The impact of low-frequency, low-force cyclic stretching of human bronchi on airway responsiveness.

BACKGROUND: In vivo, the airways are constantly subjected to oscillatory strain (due to tidal breathing during spontaneous respiration) and (in the event of mechanical ventilation) positive pressure. This exposure is especially problematic for the cartilage-free bronchial tree. The effects of cyclic stretching (other than high-force stretching) have not been extensively characterized. Hence, the objective of the present study was to investigate the functional and transcriptional response of human bronchi to repetitive mechanical stress caused by low-frequency, low-force cyclic stretching. METHODS: After preparation and equilibration in an organ bath, human bronchial rings from 66 thoracic surgery patients were stretched in 1-min cycles of elongation and relaxation over a 60-min period. For each segment, the maximal tension corresponded to 80% of the reference contraction (the response to 3 mM acetylcholine). The impact of cyclic stretching (relative to non-stretched controls) was examined by performing functional assessments (epithelium removal and incubation with sodium channel agonists/antagonists or inhibitors of intracellular pathways), biochemical assays of the organ bath fluid (for detecting the release of pro-inflammatory cytokines), and RT-PCR assays of RNA isolated from tissue samples. RESULTS: The application of low-force cyclic stretching to human bronchial rings for 60 min resulted in an immediate, significant increase in bronchial basal tone, relative to non-cyclic stretching (4.24 ± 0.16 g vs. 3.28 ± 0.12 g, respectively; p < 0.001). This cyclic stimulus also increased the affinity for acetylcholine (-log EC50: 5.67 ± 0.07 vs. 5.32 ± 0.07, respectively; p p < 0.001). Removal of airway epithelium and pretreatment with the Rho-kinase inhibitor Y27632 and inward-rectifier K+ or L-type Ca2+ channel inhibitors significantly modified the basal tone response. Exposure to L-NAME had opposing effects in all cases. Pro-inflammatory pathways were not involved in the response; cyclic stretching up-regulated the early mRNA expression of MMP9 only, and was not associated with changes in organ bath levels of pro-inflammatory mediators. CONCLUSION: Low-frequency, low-force cyclic stretching of whole human bronchi induced a myogenic response rather than activation of the pro-inflammatory signaling pathways mediated by mechanotransduction.

Deflation-activated receptors, not classical inflation-activated receptors, mediate the Hering-Breuer deflation reflex.

Many airway sensory units respond to both lung inflation and deflation. Whether those responses to opposite stimuli come from one sensor (one-sensor theory) or more than one sensor (multiple-sensor theory) is debatable. One-sensor theory is commonly presumed in the literature. This article proposes a multiple-sensor theory in which a sensory unit contains different sensors for sensing different forces. Two major types of mechanical sensors operate in the lung: inflation- and deflation-activated receptors (DARs). Inflation-activated sensors can be further divided into slowly adapting receptors (SARs) and rapidly adapting receptors (RARs). Many SAR and RAR units also respond to lung deflation because they contain DARs. Pure DARs, which respond to lung deflation only, are rare in large animals but are easily identified in small animals. Lung deflation-induced reflex effects previously attributed to RARs should be assigned to DARs (including pure DARs and DARs associated with SARs and RARs) if the multiple-sensor theory is accepted. Thus, based on the information, it is proposed that activation of DARs can attenuate lung deflation, shorten expiratory time, increase respiratory rate, evoke inspiration, and cause airway secretion and dyspnea.

Stretch-induced Ca2+ signalling in vascular smooth muscle cells depends on Ca2+ store segregation.

AIM: Calcium is a key second messenger that can be mobilized from both the extracellular medium and intracellular calcium stores. Pulmonary arterial smooth muscle cells (PASMCs) respond to stretch by a calcium increase, a mechanism enhanced during pulmonary hypertension (PH). We investigated the role of the spatial organization between plasma membrane stretch-activated channels (SACs) and intracellular calcium stores [sarcoplasmic reticulum (SR), mitochondria, and lysosomes) in response to stretch. METHODS AND RESULTS: Studies were performed in freshly isolated PASMCs from both control and two different rat models of PH (chronically hypoxic and monocrotaline-treated rats). Co-immunolabellings revealed that the subcellular segregation between each subtype of SR ryanodine receptors (RyR1, RyR2, and RyR3), SERCA2 pumps (SERCA2a and SERCA2b), mitochondria, or lysosomes in freshly isolated PASMCs differs from control and PH PASMCs. In control PASMCs, stretching the membrane activates a Ca(2+) influx through SACs. This influx is amplified by cell hyperpolarization, a calcium release by subplasmalemmal RyR1 and is then buffered by mitochondria. In two different PH rat models, the calcium response to stretch is enhanced due to hyper-reactivity of SACs and a greater calcium amplification by all RyR subtypes. CONCLUSION: The spatial organization of RyR and calcium stores in PASMCs is important for cell signalling and plays a causal role in PH.

Interaction of cyclic mechanical stretch and toll-like receptor 4-mediated innate immunity in rat alveolar type II cells.

BACKGROUND AND OBJECTIVE: In cases of infection-induced acute lung injury, mechanical ventilation might be necessary to maintain oxygenation. Although low tidal volume ventilation is applied, alveolar over-distension may occur and result in ventilator-induced lung injury. In this study, we investigate (i) the influence of lipopolysaccharide (LPS) stimulation on high-amplitude stretching; and (ii) the effect of stretching on LPS-mediated immune response in isolated rat alveolar type II cells. METHODS: Type II cells were incubated with LPS and stretched for 24 h on elastic membranes. Initially we examined apoptosis and lactic acid dehydrogenase release in LPS-treated stretched cells. Furthermore we determined toll-like receptor (TLR) 4 expression, TLR4 signalling by analysis of nuclear factor κB (NF-κB) activation and the secretion of inflammatory cytokines (monocyte chemoattractant protein-1, macrophage inflammatory protein-2, interleukin-1 beta, tumour necrosis factor alpha). RESULTS: Our results show that LPS increases apoptosis and cytotoxicity in high amplitude stretched cells. Stretching and LPS activate NF-κB. The LPS influence is the prevailing one while no synergistic effects were observed by additional stretching. LPS stimulates an increased secretion of the inflammatory mediators only. Stretching had no influence on cytokines secretion. CONCLUSIONS: We conclude that activation of TLR4 mediated immunity intensifies cell damage caused by stretching whereas in return stretching had no influence on TLR4 mediated innate immunity.

Inhibition of augmented muscle vasoconstrictor drive following asphyxic apnoea in awake human subjects is not affected by relief of chemical drive.

Progressive asphyxia, produced by a prolonged voluntary breath hold (end-expiratory apnoea), evokes large bursts of muscle sympathetic nerve activity (MSNA). These bursts increase in amplitude until the asphyxic break point is reached, at which point the bursts are inhibited. We tested the hypothesis that lung inflation, rather than relief from hypoxia and hypercapnia, is responsible for the inhibition of MSNA. Multiunit MSNA was recorded from motor fascicles of the common peroneal nerve in 11 subjects. Following a period of quiet breathing, subjects were instructed to behave as follows: (i) to hold their breath in expiration for as long as they could (mean duration 32.3 ± 1.9 s); (ii) to take a single breath of room air, 100% N(2) or 10% CO(2) + 90% N(2) at the asphyxic break point; (iii) to exhale and continue the apnoea until the next break point; and then (iv) to resume breathing. All subjects reported relief during inhalation of any gas, and could continue holding their breath for a further 30.7 ± 2.8 s with room air, 18.6 ± 1.7 s with N(2) and 11.8 ± 1.8 s with 10% CO(2) + 90% N(2). Despite the exaggerated chemoreceptor drive in the latter two conditions (hence the significantly shorter latencies to the subsequent asphyxic break point), the inhibition still occurred; moreover, there was no significant difference in duration of the inhibition of MSNA following the single breath of room air (7.6 ± 0.7 s), N(2) (6.2 ± 0.6 s) or 10% CO(2) + 90% N(2) (5.5 ± 0.4 s). Following the resumption of breathing, however, the duration of MSNA inhibition (11.0 ± 1.0 s) was significantly longer than that following a single breath. To investigate the involvement of chemoreceptors in the respiratory modulation of MSNA further, the same gases were used during an inspiratory-capacity apnoea, which causes a brief inhibition of MSNA during the inflation phase and a sustained increase during the hold phase. The duration of the apnoea was shortest after a breath of 10% O(2) + 90% N(2), but the latency until the bursts resumed after the inspiratory breath hold were similar for all gases, which suggests that there is no chemoreceptor involvement during the sympathetic silence seen during the inflation phase of inspiratory-capacity apnoeas. We conclude that neither peripheral nor central chemoreceptors are responsible for the inhibition of muscle vasoconstrictor drive following an end-expiratory apnoea or an end-inspiratory apnoea. Rather, we suggest that the inhibition is evoked by stretch receptors in the lungs and/or chest wall, which may also contribute to the longer inhibition associated with the hyperventilation following the subsequent resumption of rhythmic breathing.

Imaging and characterization of stretch-induced ATP release from alveolar A549 cells.

Abstract Mechano-transduction at cellular and tissue levels often involves ATP release and activation of the purinergic signalling cascade. In the lungs, stretch is an important physical stimulus but its impact on ATP release, the underlying release mechanisms and transduction pathways are poorly understood. Here, we investigated the effect of unidirectional stretch on ATP release from human alveolar A549 cells by real-time luciferin-luciferase bioluminescence imaging coupled with simultaneous infrared imaging, to monitor the extent of cell stretch and to identify ATP releasing cells. In subconfluent (<90%) cell cultures, single 1 s stretch (10-40%)-induced transient ATP release from a small fraction (1.5%) of cells that grew in number dose-dependently with increasing extent of stretch. ATP concentration in the proximity (150 µm) of releasing cells often exceeded 10 µm, sufficient for autocrine/paracrine purinoreceptor stimulation of neighbouring cells. ATP release responses were insensitive to the putative ATP channel blockers carbenoxolone and 5-nitro-2-(3-phenylpropyl-amino) benzoic acid, but were inhibited by N-ethylmaleimide and bafilomycin. In confluent cell cultures, the maximal fraction of responding cells dropped to <0.2%, but was enhanced several-fold in the wound/scratch area after it was repopulated by new cells during the healing process. Fluo8 fluorescence experiments revealed two types of stretch-induced intracellular Ca(2+) responses, rapid sustained Ca(2+) elevations in a limited number of cells and delayed secondary responses in neighbouring cells, seen as Ca(2+) waves whose propagation was consistent with extracellular diffusion of released ATP. Our experiments revealed that a single >10% stretch was sufficient to initiate intercellular purinergic signalling in alveolar cells, which may contribute to the regulation of surfactant secretion and wound healing.

Role of pulmonary stretch receptors and sympathetic system in the inhibition of reflex bradycardia produced by chemical stimulation of the periaqueductal gray matter of the rat.

The present study examined the role of the sympathetic system and pulmonary afferent feedback in the baroreflex inhibition by chemical stimulation of the dorsal periaqueductal gray matter (DPAG) of the anesthetized rat. The baroreflex bradycardia was induced by phenylephrine infusions (PHE, 50 µg/ml/min, i.v.) given either alone or combined with glutamate microinjections (GLU, 10 nmol/100 nl) into the DPAG. GLU microinjections alone produced marked increases in respiratory amplitude (67±19%), but barely changed the respiratory frequency (15±3 cpm) and blood pressure (14±2 mm Hg), and did not affect the heart rate. In contrast, the same injections produced a 92% inhibition of PHE-induced bradycardia (from -62 to -5 bpm). Because GLU microinjections per se had little effects on blood pressure, the baroreflex inhibition should be credited to the deactivation of both the vagal and sympathetic reflex pathways at the medulla. Indeed, the baroreflex was inhibited in only 47% following the DPAG stimulation of atenolol-treated rats. The GLU-evoked inhibition of baroreflex was also correlated with concomitant increases in respiratory amplitude. The role of pulmonary feedback in baroreflex inhibition was thus examined before and after the neuromuscular blockade of atenolol-treated rats. In spontaneously breathing rats, GLU microinjections reversed PHE-induced bradycardia to tachycardia, thereby producing a 153% inhibition of reflex bradycardia (from -38 bpm to +20 bpm). In contrast, the baroreflex inhibition was attenuated in only 53% after neuromuscular blockade (from -34 to -16 bpm). Data are the first evidence of the contribution of pulmonary stretch receptor feedback in DPAG-evoked inhibition of reflex bradycardia.

High altitude simulation, substance P and airway rapidly adapting receptor activity in rabbits.

To investigate whether there is a change in airway rapidly adapting receptor (RAR) activity during high altitude exposure, rabbits were placed in a high altitude simulation chamber (barometric pressure, 429 mm Hg). With 12 h exposure, when there was pulmonary congestion, an increase in basal RAR activity was observed. With 36 h exposure, when there was alveolar edema, there was a further increase in basal RAR activity. In these backgrounds, there was an increase in the sensitivity of the RARs to substance P (SP). To assess whether there was an increase in lung SP level, neutral endopeptidase activity was determined which showed a decrease in low barometric pressure exposed groups. It is concluded that along with the SP released, pulmonary congestion and edema produced, respectively by different durations of low barometric pressure exposure cause a progressive increase in RAR activity which may account for the respiratory symptoms reported in climbers who are unacclimatized.

Voltage-gated sodium channels in nociceptive versus non-nociceptive nodose vagal sensory neurons innervating guinea pig lungs.

Lung vagal sensory fibres are broadly categorized as C fibres (nociceptors) and A fibres (non-nociceptive; rapidly and slowly adapting low-threshold stretch receptors). These afferent fibre types differ in degree of myelination, conduction velocity, neuropeptide content, sensitivity to chemical and mechanical stimuli, as well as evoked reflex responses. Recent studies in nociceptive fibres of the somatosensory system indicated that the tetrodotoxin-resistant (TTX-R) voltage-gated sodium channels (VGSC) are preferentially expressed in the nociceptive fibres of the somatosensory system (dorsal root ganglia). Whereas TTX-R sodium currents have been documented in lung vagal sensory nerves fibres, a rigorous comparison of their expression in nociceptive versus non-nociceptive vagal sensory neurons has not been carried out. Using multiple approaches including patch clamp electrophysiology, immunohistochemistry, and single-cell gene expression analysis in the guinea pig, we obtained data supporting the hypothesis that the TTX-R sodium currents are similarly distributed between nodose ganglion A-fibres and C-fibres innervating the lung. Moreover, mRNA and immunoreactivity for the TTX-R VGSC molecules Na(V)1.8 and Na(V)1.9 were present in nearly all neurons. We conclude that contrary to findings in the somatosensory neurons, TTX-R VGSCs are not preferentially expressed in the nociceptive C-fibre population innervating the lungs.

Stretch-activated signaling pathways responsible for early response gene expression in fetal lung epithelial cells.

High-tidal volume ventilation has been shown to increase the expression of several inflammation-associated genes prior to overt physiologic lung injury. Herein, using an in vitro stretch system, we investigated the mechanotransduction pathways involved in ventilation-induced expression of these early response genes (i.e., early growth response gene (Egr)1, heat-shock protein (HSP)70, and the pro-inflammatory cytokines interleukin (IL)-1beta, IL-6, and MIP-2). Mechanical stretch of fetal lung epithelial cells activated various signaling pathways, resulting in transient or progressive increases in gene expression of the early response genes. The transient increase in Egr1 and IL-6 expression was mediated via p44/42 mitogen-activated protein kinase (p44/42 MAPK), while nuclear factor-kappaB (NF-kappaB) was responsible for the sustained and progressive increase in expression of HSP70 and MIP-2. Blockage of Egr-1 expression did not affect the upregulation of IL-6, HSP70, MIP-2, and itself by stretch. Inhibition of calcium mobilization abolished stretch-induced p44/42 MAPK activation and NF-kappaB nuclear translocation as well as increased expression of all early response genes. Similar results were obtained with an inhibitor of Ras. These results suggest that mechanical stretch of fetal lung epithelial cells evokes a complex network of signaling molecules, which diverge downstream to regulate the temporal expression of a unique set of early response genes, but upstream converge at calcium. Thus, calcium mobilization may be a point of hierarchical integration of mechanotransduction in lung epithelial cells.

[Relationship of stress index with lung recruitment and gas exchange in dogs with acute respiratory distress syndrome].

OBJECTIVE: To determine the relationship of stress index with lung recruitment and gas exchange in dogs with acute respiratory distress syndrome (ARDS). METHODS: The ARDS model was induced by infusion of oleic acid intravenously in anesthetized dogs. During volume control ventilation with constant inspiratory flow, the pressure-time (P-t) curve was fitted to a power equation: P = a.time(b)+c, where coefficient b (stress index) describes the shape of the curve: b = 1, straight curve; b < 1, progressive increase in slope; and b > 1, progressive decrease in slope. Tidal volume (V(T)) was 6 ml/kg, and positive end-expiratory pressure (PEEP) was set to obtain a b value between 0.9 and 1.1 before (b = 1) and after (b = 1 after recruiting maneuver) application of a recruiting maneuver (RM). PEEP was changed to obtain 0.6 < b < 0.8 and 1.1 < b < 1.3. Experimental condition sequence was random. Recruited volume (RV) was measured by static pressure-volume curve method. Hemodynamics, pulmonary mechanics and gas exchange were observed at the same time. RESULTS: At b = 1 without RM, the PEEP was (5.0 +/- 3.0) cm H2O, the RV was (27 +/- 15) ml, and the RV increased to (166 +/- 84) ml significantly at b = 1 after RM [PEEP (10.8 +/- 2.3) cm H2O (1 cm H2O = 0.098 kPa), q = 3.18, P < 0.01]. At 1.1 < b < 1.3 after RM, the PEEP was (16.8 +/- 1.1) cm H2O and the RV was (262 +/- 57) ml, which was higher than that at b = 1 after RM (q = 2.54, P = 0.023). At 0.6 < b < 0.8 after RM, the PEEP was (5.6 +/- 2.2) cm H2O and the RV was lower than that at b = 1 after RM (q = 2.85, P = 0.013). The partial pressure of oxygen in arterial blood (PaO2) in b = 1, 0.6 < b < 0.8 and 1.1 < b < 1.3 after RM were (319 +/- 49) mm Hg (1 mm Hg = 0.133 kPa), (246 +/- 57) mm Hg and (314 +/- 27) mm Hg respectively, which was higher than the PaO2 at b = 1 without RM [(153 +/- 64) mm Hg, all q = 2.81, all P < 0.05]. The PaO2 at 0.6 < b < 0.8 was lower than that at b = 1 after RM (q = 2.81, P = 0.005), while there was no significant difference between the PaO2 at 1.1 < b < 1.3 and that at b = 1 after RM. The peak airway pressure and plateau pressure at 1.1 < b < 1.3 were higher than those at b = 1 after RM (q = 6.02, 5.72, all P < 0.05). CONCLUSION: In the b = 1 after RM, there were better PaO2 and lower airway pressure, suggesting that b = 1 after RM may be a good indicator for PEEP titration.

Cough: what's in a name?

The cough reflex (CR) and the expiration reflex (ER) are two defensive reflexes from the respiratory tract, the latter mainly from the larynx. Both are elicited by mechanical and chemical irritation of the airway mucosa, and are a characteristic of airway diseases, but they have different functions. The CR first draws air into the lungs, to accentuate the subsequent expulsive phase; the ER consists of a strong expiration, to prevent aspiration of material into the lungs. They have different sensory pathways, central nervous circuits, and physiological and pharmacological modulations. In practice, coughing often consists of a combination of the two reflexes, a cough bout, epoch or attack. Articles on cough usually do not distinguish between the two reflexes, or whether the coughs are single events or epochs; they usually only measure frequency of expiratory efforts, and neglect other aspects. Current methods for measuring and assessing cough are described, with indications of when the use of these methods may be important.

Respiration-related afferents to parabrachial pontine regions.

The dorsolateral pons around the parabrachial nucleus is an important participant in respiratory control. This area involves various respiration-related neurons, and their respiratory modulation is thought to arise from afferents from medullary respiratory neurons. Today, however, only a limited number of afferent sources have been identified. First, relatively well-characterized afferents to the pons are those originating from two types of the lung stretch receptors, slowly adapting and rapidly adapting receptors. That is, the majority of the second-order relay neurons of these receptors in the nucleus tractus solitarii project to the pons. Second, certain types of respiratory neurons of the medullary respiratory groups are either known to or presumed to project to the pons. For instance, major inhibitory neurons of the Botzinger complex, augmenting and decrementing expiratory neurons, send afferents to the pons. This article overviews such afferents and discusses their connectivity with pontine neurons.

Cough and other reflexes on irritation of airway mucosa in man.

Both human and animal studies show that irritation of airway mucosa elicits a variety of reflex responses such as coughing, apnoea, and laryngeal closure. Most of the information concerning these reflex responses were obtained in anesthetized conditions with little applicability to awake conditions. Various aspects of cough and other reflexes on irritation of the airway mucosa are discussed. Studies on awake humans showed that stimulation of the laryngeal mucosa with a small amount of distilled water during wakefulness causes elicitation of the expiration reflex, cough reflex, and swallowing reflex while other types of responses are scarcely observed. In addition, the duration of these responses is remarkably short. In contrast, the same stimulation causes more variant, prolonged, and exaggerated responses during a light depth of anesthesia. An increase in depth of anesthesia abolishes expiratory efforts such as coughing and the expiration reflex whereas the apnoeic reflex and laryngeal closure reflex are resistant to the depressant effect of anesthesia. Also, the respiratory reflex responses to airway irritation varied, depending on the site of stimulation: both laryngeal and tracheal stimulation cause vigorous respiratory responses whereas bronchial stimulation causes little or no respiratory responses. These results indicate not only that the types and magnitude of reflex responses is greatly modified by the central nervous state but also that the site of stimulation is crucial for determining the pattern of respiratory responses elicited by airway stimulation in humans.

Flecainide blocks the stimulatory effect of veratridine on slowly adapting pulmonary stretch receptors in anaesthetized rabbits without changing lung mechanics.

We examined the responses of slowly adapting pulmonary stretch receptors (PSRs), total lung resistance (RL) and dynamic lung compliance (Cdyn) to administered veratridine before and after pretreatment with atropine or flecainide in anaesthetized, artificially ventilated rabbits with bilateral vagotomy. Administration of veratridine (10 and 30 micrograms kg-1) caused vigorous stimulation of PSRs, resulting in a tonic discharge of receptors during both inflation and deflation, but did not significantly alter either RL or Cdyn. The veratridine-induced PSR stimulation became more prominent, as the dose of this alkaloid was increased. Pretreatment with atropine (1 or 2 mg kg-1) had no significant effect on the excitatory response of PSRs to veratridine. The veratridine-induced PSR stimulation was inhibited by treatment with flecainide (1, 2 and 3 mg kg-1), a sodium channel blocker, and this inhibition was dose-dependent. These results suggest that activation of PSRs following veratridine administration probably related to the increased influx of sodium ions into the receptive terminals but does not depend upon bronchoconstriction.