Expression of functional purinergic receptors in pulmonary neuroepithelial bodies and their role in hypoxia chemotransmission

Mechanosensitivity of Murine Lung Slowly Adapting Receptors: Minimal Impact of Chemosensory, Serotonergic, and Purinergic Signaling

Murine slowly adapting receptors (SARs) within airway smooth muscle provide volume-related feedback; however, their mechanosensitivity and morphology are incompletely characterized. We explored two aspects of SAR physiology: their inherent static mechanosensitivity and a potential link to pulmonary neuroepithelial bodies (NEBs). SAR mechanosensitivity displays a rate sensitivity linked to speed of inflation; however, to what extent static SAR mechanosensitivity is tuned for the very rapid breathing frequency (B f ) of small mammals (e.g., mouse) is unclear. NEB-associated, morphologically described smooth muscle-associated receptors (SMARs) may be a structural analog for functionally characterized SARs, suggesting functional linkages between SARs and NEBs. We addressed the hypotheses that: (1) rapid murine B f is associated with enhanced in vivo SAR static sensitivity; (2) if SARs and NEBs are functionally linked, stimuli reported to impact NEB function would alter SAR mechanosensitivity. We measured SAR action potential discharge frequency (AP f, action potentials/s) during quasi-static inflation [0-20 cmH2O trans-respiratory pressure (PTR)] in NEB-relevant conditions of hypoxia (FIO2 = 0.1), hypercarbia (FICO2 = 0.1), and pharmacologic intervention (serotonergic 5-HT3 receptor antagonist, Tropisetron, 4.5 mg/kg; P2 purinergic receptor antagonist, Suramin, 50 mg/kg). In all protocols, we obtained: (1) AP f vs. PTR; (2) PTR threshold; and (3) AP f onset at PTR threshold. The murine AP f vs. PTR response comprises high AP f (average maximum AP f: 236.1 ± 11.1 AP/s at 20 cmH2O), a low PTR threshold (mean 2.0 ± 0.1 cmH2O), and a plateau in AP f between 15 and 20 cmH2O. Murine SAR mechanosensitivity (AP f vs. PTR) is up to 60% greater than that reported for larger mammals. Even the maximum difference between intervention and control conditions was minimally impacted by NEB-related alterations: Tropisetron -7.6 ± 1.8% (p = 0.005); Suramin -10.6 ± 1.5% (p = 0.01); hypoxia +9.3 ± 1.9% (p < 0.001); and hypercarbia -6.2 ± 0.9% (p < 0.001). We conclude that the high sensitivity of murine SARs to inflation provides enhanced resolution of operating lung volume, which is aligned with the rapid B f of the mouse. We found minimal evidence supporting a functional link between SARs and NEBs and speculate that the <10% change in SAR mechanosensitivity during altered NEB-related stimuli is not consistent with a meaningful physiologic role.

Pulmonary neuroendocrine cells: physiology, tissue homeostasis and disease

Mammalian lungs have the ability to recognize external environments by sensing different compounds in inhaled air. Pulmonary neuroendocrine cells (PNECs) are rare, multi-functional epithelial cells currently garnering attention as intrapulmonary sensors; PNECs can detect hypoxic conditions through chemoreception. Because PNEC overactivation has been reported in patients suffering from respiratory diseases – such as asthma, chronic obstructive pulmonary disease, bronchopulmonary dysplasia and other congenital diseases – an improved understanding of the fundamental characteristics of PNECs is becoming crucial in pulmonary biology and pathology. During the past decade, murine genetics and disease models revealed the involvement of PNECs in lung ventilation dynamics, mechanosensing and the type 2 immune responses. Single-cell RNA sequencing further unveiled heterogeneous gene expression profiles in the PNEC population and revealed that a small number of PNECs undergo reprogramming during regeneration. Aberrant large clusters of PNECs have been observed in neuroendocrine tumors, including small-cell lung cancer (SCLC). Modern innovation of imaging analyses has enabled the discovery of dynamic migratory behaviors of PNECs during airway development, perhaps relating to SCLC malignancy. This Review summarizes the findings from research on PNECs, along with novel knowledge about their function. In addition, it thoroughly addresses the relevant questions concerning the molecular pathology of pulmonary diseases and related therapeutic approaches.

Summary: This Review highlights the physiological relevance of pulmonary neuroendocrine cells, rare airway epithelial cells that form intrapulmonary sensory organs, abnormalities of which are associated with several pulmonary disorders, such as asthma and lung cancer.

Chemical warfare agent research in precision-cut tissue slices-a useful alternative approach

The use of chemical warfare agents (CWAs) in military conflicts and against civilians is a recurrent problem. Despite ongoing CWA research using in vitro or in vivo models, progress to elucidate mechanisms of toxicity and to develop effective therapies, decontamination procedures, and general countermeasures is still limited. Novel scientific approaches to address these questions are needed to expand perspectives on existing knowledge and gain new insights. To achieve this, the use of ex vivo techniques like precision-cut tissue slices (PCTSs) can be a valuable approach. Existing studies employing this economical and relatively easy to implement method show model suitability and comparability with the use of in vitro and in vivo models. In this article, we review research on CWAs in PCTSs to illustrate the advantages of the approach and to promote future applications.

Consider the lung as a sensory organ: A tip from pulmonary neuroendocrine cells

While the lung is commonly known for its gas exchange function, it is exposed to signals in the inhaled air and responds to them by collaborating with other systems including immune cells and the neural circuit. This important aspect of lung physiology led us to consider the lung as a sensory organ. Among different cell types within the lung that mediate this role, several recent studies have renewed attention on pulmonary neuroendocrine cells (PNECs). PNECs are a rare, innervated airway epithelial cell type that accounts for <1% of the lung epithelium population. They are enriched at airway branch points. Classical in vitro studies have shown that PNECs can respond to an array of aerosol stimuli such as hypoxia, hypercapnia and nicotine. Recent in vivo evidence suggests an essential role of PNECs at neuroimmunomodulatory sites of action, releasing neuropeptides, neurotransmitters and facilitating asthmatic responses to allergen. In addition, evidence supports that PNECs can function both as progenitor cells and progenitor niches following airway epithelial injury. Increases in PNECs have been documented in a large array of chronic lung diseases. They are also the cells-of-origin for small cell lung cancer. A better understanding of the specificity of their responses to distinct insults, their impact on normal lung function and their roles in the pathogenesis of pulmonary ailments will be the next challenge toward designing therapeutics targeting the neuroendocrine system in lung.

Sensory input to the central nervous system from the lungs and airways: A prominent role for purinergic signalling via P2X2/3 receptors

Specific subpopulations of lung-related primary afferent neurons in dorsal root and vagal sensory ganglia have been reported to express P2X2 and P2X3 receptors both in the neuronal cell bodies and in their peripheral terminals. The afferent innervation of airways and lungs is organised as sensory receptor structures, of which at least seven types with a vagal origin and two with a spinal origin have been reported. In view of the recently suggested therapeutic promise of ATP antagonism - specifically at P2X3 receptor expressing nociceptive fibres - in respiratory disorders, the present work focusses on four distinct populations of pulmonary sensory receptors that have so far been reported to express P2X2/3 receptors. Three of them originate from myelinated nerve fibres that display similar mechanosensor-like morphological and neurochemical characteristics. Two of the latter concern vagal nodose sensory fibres, either related to pulmonary neuroepithelial bodies (NEBs), or giving rise to smooth muscle-associated airway receptors (SMARs); the third gives rise to visceral pleura receptors (VPRs) and most likely arises from dorsal root ganglia. The fourth population concerns C-fibre receptors (CFRs) that also derive from neuronal cell bodies located in vagal nodose ganglia. Although the majority of the airway- and lung-related sensory receptors that express P2X2/3 receptors apparently do not belong to accepted nociceptive populations, these data definitely point out that ATP may be an important player in the physiological transduction of different lung-related afferent signals from the periphery to the CNS. The observed variety within the populations of pulmonary sensory receptors that express P2X2/3 receptors argues for a critical and careful interpretation of the functional data.

Anatomical and physiological factors contributing to chronic muscle pain

Chronic muscle pain remains a significant source of suffering and disability despite the adoption of pharmacologic and physical therapies. Muscle pain is mediated by free nerve endings distributed through the muscle along arteries. These nerves project to the superficial dorsal horn and are transmitted primarily through the spinothalamic tract to several cortical and subcortical structures, some of which are more active during the processing of muscle pain than other painful conditions. Mechanical forces, ischemia, and inflammation are the primary stimuli for muscle pain, which is reflected in the array of peripheral receptors contributing to muscle pain-ASIC, P2X, and TRP channels. Sensitization of peripheral receptors and of central pain processing structures are both critical for the development and maintenance of chronic muscle pain. Further, variations in peripheral receptors and central structures contribute to the significantly greater prevalence of chronic muscle pain in females.

Recent advances and contraversies on the role of pulmonary neuroepithelial bodies as airway sensors

Pulmonary neuroepithelial bodies are polymodal sensors widely distributed within the airway mucosa of mammals and other species. Neuroepithelial body cells store and most likely release serotonin and peptides as transmitters. Neuroepithelial bodies have a complex innervation that includes vagal sensory afferent fibers and dorsal root ganglion fibers. Neuroepithelial body cells respond to a number of intraluminal airway stimuli, including hypoxia, hypercarbia, and mechanical stretch. This article reviews recent findings in the cellular and molecular biology of neuroepithelial body cells and their potential role as airway sensors involved in the control of respiration, particularly during the perinatal period. Alternate hypotheses and areas of controversy regarding potential function as mechanosensory receptors involved in pulmonary reflexes are discussed.

Purinergic signaling in the airways

Evidence for a significant role and impact of purinergic signaling in normal and diseased airways is now beyond dispute. The present review intends to provide the current state of knowledge of the involvement of purinergic pathways in the upper and lower airways and lungs, thereby differentiating the involvement of different tissues, such as the epithelial lining, immune cells, airway smooth muscle, vasculature, peripheral and central innervation, and neuroendocrine system. In addition to the vast number of well illustrated functions for purinergic signaling in the healthy respiratory tract, increasing data pointing to enhanced levels of ATP and/or adenosine in airway secretions of patients with airway damage and respiratory diseases corroborates the emerging view that purines act as clinically important mediators resulting in either proinflammatory or protective responses. Purinergic signaling has been implicated in lung injury and in the pathogenesis of a wide range of respiratory disorders and diseases, including asthma, chronic obstructive pulmonary disease, inflammation, cystic fibrosis, lung cancer, and pulmonary hypertension. These ostensibly enigmatic actions are based on widely different mechanisms, which are influenced by the cellular microenvironment, but especially the subtypes of purine receptors involved and the activity of distinct members of the ectonucleotidase family, the latter being potential protein targets for therapeutic implementation.

Dynamics of calcitonin gene-related peptide-like cells changes in the lungs of two-kidney, one-clip rats

Taking into consideration renal hypertension-induced homeostatic disorders and the key role of calcitonin gene-related peptide (CGRP) in many, systemic functions regulating systems, a question arises as to what an extent arterial hypertension affects the morphology and dynamics of pulmonary CGRP-immunopositive cell changes. The aim of the present study was to examine the distribution, morphology and dynamics of changes of CGRP-containing cells in the lungs of rats in the two-kidney, one-clip (2K1C) renovascular hypertension model. The studies were carried out on the lungs of rats after 3, 14, 28, 42, and 91 days long period from the renal artery clipping procedure. In order to identify neuroendocrine cells, immunohistochemical reaction was performed with the use of a specific antibody against CGRP. It was revealed that renovascular hypertension caused changes in the neuroendocrine, CGRP-containing cells in the lungs of rats. The changes, observed in the neuroendocrine cells, depended on time periods from experimentally induced hypertension. The highest intensity of changes in the neuroendocrine cells was observed in the lungs of rats after 14 days from the surgery.

Pulmonary neuroepithelial bodies as airway sensors: putative role in the generation of dyspnea

The neuroepithelial bodies (NEB) of the intrapulmonary airways (AW) are multimodal AW sensors responding to a variety of stimuli including hypoxia, hypercarbia, and mechanical stretch. NEBs are richly innervated by a diverse population of mostly vagal afferent nerve fibers and owing to their early developmental maturation may be especially important during the perinatal period. This article reviews recent findings of NEB functional morphology and innervation, and postulates a role in the generation of dyspnea. This is based on their potential for transduction of dyspneic stimuli and findings of NEB cell abnormalities in a number of pulmonary disorders presenting with this symptom.

Evolutionary origins of the purinergic signalling system

Purines appear to be the most primitive and widespread chemical messengers in the animal and plant kingdoms. The evidence for purinergic signalling in plants, invertebrates and lower vertebrates is reviewed. Much is based on pharmacological studies, but important recent studies have utilized the techniques of molecular biology and receptors have been cloned and characterized in primitive invertebrates, including the social amoeba Dictyostelium and the platyhelminth Schistosoma, as well as the green algae Ostreococcus, which resemble P2X receptors identified in mammals. This suggests that contrary to earlier speculations, P2X ion channel receptors appeared early in evolution, while G protein-coupled P1 and P2Y receptors were introduced either at the same time or perhaps even later. The absence of gene coding for P2X receptors in some animal groups [e.g. in some insects, roundworms (Caenorhabditis elegans) and the plant Arabidopsis] in contrast to the potent pharmacological actions of nucleotides in the same species, suggests that novel receptors are still to be discovered.

Membrane compartments and purinergic signalling: occurrence and function of P2X receptors in lung

P2X receptors are cation-selective ion channels activated by extracellular ATP. They form homo- and heterotrimeric complexes that differ in their functional properties and subcellular localization. These membrane ion channels are also expressed in pulmonary epithelial cells. Recent work indicates that alveolar epithelial type I cells selectively express P2X(4) and P2X(7) receptor subtypes in addition to a large number of other ion channels present in the alveolar epithelium. Up- or downregulation of their expression is associated with several disease states. This minireview analyses the role of P2X receptors and of extracellular ATP and adenosine in lung disease, the relationship of P2X receptors to other ion channels in the alveolar epithelium and their distribution in lipid rafts/caveolae.

Purinergic signaling in the pulmonary neuroepithelial body microenvironment unraveled by live cell imaging

Pulmonary neuroepithelial bodies (NEBs) are densely innervated groups of complex sensory airway receptors involved in the regulation of breathing. Together with their surrounding Clara-like cells, they exhibit stem cell potential through their capacity to regenerate depopulated areas of the epithelium following lung injury. We have employed confocal live cell imaging microscopy and novel electrophysiological techniques in a new ex vivo lung slice model to unravel potential purinergic signaling pathways within the NEB microenvironment. Quinacrine histochemistry indicated high amounts of vesicular ATP in NEB cells. Using a "reporter-patching" method adapted to create a uniquely sensitive and selective biosensor for the direct detection of ATP release from NEBs ex vivo, we demonstrated quantal ATP release from NEBs following their depolarization. Enhancing enzymatic extracellular ATP hydrolysis or inhibiting P2 receptors confirmed the central role of ATP in paracrine interactions between NEB cells and Clara-like cells. Combined calcium imaging, pharmacology, and immunohistochemistry showed that ligand-binding to functional P2Y(2) receptors underpins the activation of Clara-like cells. Hence, NEB cells communicate with their cellular neighbors in the NEB microenvironment by releasing ATP, which rapidly evokes purinergic activation of surrounding Clara-like cells. Besides ATP acting on the P2X(3) receptor expressing vagal sensory nerve terminals between NEB cells, local paracrine purinergic signaling within this potential stem cell niche may be important to both normal airway function, airway epithelial regeneration after injury, and/or the pathogenesis of small cell lung carcinomas.

Neurotransmitter localization in the neuroepithelial cells and unipolar neurons of the respiratory tract in the bichir, Polypterus bichir bichir G. ST-HIL

Immunohistochemical localisation of neurotransmitters was used to determine the distribution of unipolar neurons and neuroepithelial cells (NECs) in the respiratory tract of the bichir, Polypterus bichir bichir. NECs were commonly encountered in the mucociliated epithelium of the lung. Unipolar neurons were located in the submucosal and muscle layers of the glottis. The results suggest the presence of tyrosine hydroxylase (TH) and nNOS immunoreactivities in NECs. In addition, ACh-E/nNOS and TH/nNOS nerve fibers were also found associated with these cells. Unipolar neuronal cells showed a chemical code including the presence of 5-HT, ACh-E, peptides and P2x2 receptors. The present findings indicate that nitric oxide (NO) is a primitive transmitter of neuroepithelial oxygen-sensitive chemoreceptor cells together with acetylcholine. The coexistence of ACh-E with other substances in the unipolar neurons, but not with NO, may be a property of vagal postganglionic neurons since the emergence of the cranial autonomic pathways in the earliest vertebrates. It would be interesting to know about the provenance of the nerves in contact with NECs, which appear to have a complex innervation pattern.

Pulmonary neuroendocrine cell system in pediatric lung disease-recent advances

The airway epithelium of human and animal lungs contains highly specialized pulmonary neuroendocrine cells (PNEC), distributed as solitary cells and as innervated clusters, neuroepithelial bodies (NEB). The designation "PNEC system" stems from the expression of both neural and endocrine cell phenotypes, including the synthesis and release of amine (serotonin, 5-HT) and a variety of neuropeptides (that is, bombesin). The role and function of PNEC in the lung have remained a subject of speculation for many years. During the last decade, studies using modern techniques of cellular and molecular biology revealed a complex functional role for PNEC, beginning during the early stages of lung development as modulators of fetal lung growth and differentiation and at the time of birth as airway O2 sensors involved in neonatal adaptation. Postnatally and beyond, PNEC/NEB are providers of a lung stem cell niche that is important in airway epithelial regeneration and lung carcinogenesis. The focus of this review is to present and discuss recent findings pertaining to the responses of PNEC to intrauterine environmental stimuli, ontogeny and molecular regulation of PNEC differentiation, innervation of NEB, and their role as airway chemoreceptors, including mechanisms of O2 sensing and chemotransmission of hypoxia stimulus. Abnormalities of PNEC/NEB have been reported in a variety of pediatric pulmonary disorders but the clinical significance or the mechanisms involved are unknown. The discussion on the possible role of PNEC/NEB in the pathogenesis and pathobiology of pediatric lung diseases includes congenital lung disorders, bronchopulmonary dysplasia, disorders of respiratory control, neuroendocrine hyperplasia of infancy, cystic fibrosis, bronchial asthma, and pulmonary hypertension.

Physiology and pathophysiology of purinergic neurotransmission

This review is focused on purinergic neurotransmission, i.e., ATP released from nerves as a transmitter or cotransmitter to act as an extracellular signaling molecule on both pre- and postjunctional membranes at neuroeffector junctions and synapses, as well as acting as a trophic factor during development and regeneration. Emphasis is placed on the physiology and pathophysiology of ATP, but extracellular roles of its breakdown product, adenosine, are also considered because of their intimate interactions. The early history of the involvement of ATP in autonomic and skeletal neuromuscular transmission and in activities in the central nervous system and ganglia is reviewed. Brief background information is given about the identification of receptor subtypes for purines and pyrimidines and about ATP storage, release, and ectoenzymatic breakdown. Evidence that ATP is a cotransmitter in most, if not all, peripheral and central neurons is presented, as well as full accounts of neurotransmission and neuromodulation in autonomic and sensory ganglia and in the brain and spinal cord. There is coverage of neuron-glia interactions and of purinergic neuroeffector transmission to nonmuscular cells. To establish the primitive and widespread nature of purinergic neurotransmission, both the ontogeny and phylogeny of purinergic signaling are considered. Finally, the pathophysiology of purinergic neurotransmission in both peripheral and central nervous systems is reviewed, and speculations are made about future developments.

Effects of combined cholinergic-purinergic block upon cat carotid body chemoreceptors in vitro

Since acetylcholine (ACh) and ATP have been proposed as excitatory co-transmitters at synapses between glomus cells and sensory nerve endings of the carotid body (CB), we tested such hypothesis by studying the effects of combined cholinergic-purinergic block on the chemosensory activity recorded from cat's carotid bodies perfused and/or superfused in vitro. The preparations were bathed with Tyrode's solution, either normoxic (PO2=98.5+/-13.5 Torr) or hypoxic (PO2=31.8+/-5.2 Torr), and the frequency of chemosensory impulses (fchi) was recorded from the carotid (sinus) nerve. Dose-response curves for fchi increases evoked by intra-stream boluses of acetylcholine, nicotine and ATP were studied. A combination of mecamylamine 2 microM and suramin 50 microM, applied through the perfusate or superfusate, suppressed nicotine- and ATP-induced increases in fchi, but the basal chemosensory activity in normoxia and the chemosensory excitation elicited by hypoxic superfusion were preserved, although variably reduced in most preparations. Thus, in spite of the excitatory effects provoked by applying ACh and ATP to the perfused/superfused CB in vitro, a co-release of these substances cannot account entirely for the chemosensory excitation induced by hypoxic stimulation of the CB.

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.

Purinergic P2 receptors as targets for novel analgesics

Following hints in the early literature about adenosine 5'-triphosphate (ATP) injections producing pain, an ion-channel nucleotide receptor was cloned in 1995, P2X3 subtype, which was shown to be localized predominantly on small nociceptive sensory nerves. Since then, there has been an increasing number of papers exploring the role of P2X3 homomultimer and P2X2/3 heteromultimer receptors on sensory nerves in a wide range of organs, including skin, tongue, tooth pulp, intestine, bladder, and ureter that mediate the initiation of pain. Purinergic mechanosensory transduction has been proposed for visceral pain, where ATP released from epithelial cells lining the bladder, ureter, and intestine during distension acts on P2X3 and P2X2/3, and possibly P2Y, receptors on subepithelial sensory nerve fibers to send messages to the pain centers in the brain as well as initiating local reflexes. P1, P2X, and P2Y receptors also appear to be involved in nociceptive neural pathways in the spinal cord. P2X4 receptors on spinal microglia have been implicated in allodynia. The involvement of purinergic signaling in long-term neuropathic pain and inflammation as well as acute pain is discussed as well as the development of P2 receptor antagonists as novel analgesics.

Functional facets of the pulmonary neuroendocrine system

Pulmonary neuroendocrine cells (PNECs) have been around for 60 years in the scientific literature, although phylogenetically they are ancient. Their traditionally ascribed functions include chemoreception and regulation of lung maturation and growth. There is recent evidence that neuroendocrine (NE) differentiation in the lung is regulated by genes and pathways that are conserved in the development of the nervous system from Drosophila to humans (such as achaete-scute homolog-1), or implicated in the carcinogenesis of the nervous or NE system (such as the retinoblastoma tumor suppressor gene). In addition, complex neural networks are in place to regulate chemosensory and other functions. Even solitary PNECs appear to be innervated. For the first time ever, we have mouse models for lung NE carcinomas, including the most common and virulent small cell lung carcinoma. Moreover, PNECs may be important for inflammatory responses, and pivotal for lung stem cell niches. These discoveries signify an exciting new era for PNECs and are likely to have therapeutic and diagnostic applications.

Mechanical stretch-induced serotonin release from pulmonary neuroendocrine cells: implications for lung development

Pulmonary neuroendocrine cells (PNEC) produce amine (serotonin, 5-HT) and peptides (e.g., bombesin, calcitonin) with growth factor-like properties and are thought to play an important role in lung development. Because physical forces are essential for lung growth and development, we investigated the effects of mechanical strain on 5-HT release in PNEC freshly isolated from rabbit fetal lung and in the PNEC-related tumor H727 cell line. Cultures exposed to sinusoidal cyclic stretch showed a significant 5-HT release inhibitable with gadolinium chloride (10 nM), a blocker of mechanosensitive channels. In contrast to hypoxia (Po2 approximately 20 mmHg), stretch-induced 5-HT release was not affected by Ca2+-free medium or nifedipine (50 microM), excluding the exocytic pathway. In H727 cells, stretch failed to release calcitonin, a peptide stored within dense core vesicles (DCV), whereas hypoxia caused massive calcitonin release. 5-HT released by mechanical stretch is derived predominantly from the cytoplasmic pool, because it is rapid ( approximately 5 min) and is releasable from early (20 days of gestation) fetal PNEC containing few DCV. Both mechanical stretch and hypoxia upregulated expression of tryptophan hydroxylase, the rate-limiting enzyme of 5-HT synthesis. We conclude that mechanical strain is an important physiological stimulus for the release of 5-HT from PNEC via mechanosensitive channels with potential effects on lung development and resorption of lung fluid at the time of birth.