Recommendations for acting in elective and urgent thoracic surgery during SARS-CoV-2 pandemic. Spanish Society of Thoracic Surgery

La expansión de la pandemia producida por el nuevo coronavirus SARS-CoV-2 ha obligado a focalizar casi toda la asistencia sanitaria en pacientes con enfermedad COVID-19, obligando a suspender la mayoría de intervenciones quirúrgicas electivas programadas. La cirugía torácica es eminentemente oncológica, por lo que resulta obligada una adecuada priorización de los pacientes para ser intervenidos lo antes posible y en condiciones de máxima seguridad. Por otra parte, la afectación pulmonar grave COVID-19 presenta complicaciones que con frecuencia requieren procedimientos quirúrgicos torácicos urgentes en un contexto nuevo. La Sociedad Española de Cirugía Torácica (SECT) ha desarrollado este documento para establecer unas recomendaciones básicas para mantener la actividad quirúrgica electiva imprescindible y para orientar a los cirujanos que deban afrontar urgencias torácicas en este nuevo y desconocido entorno.

Bronchoscopic sclerosis of post-resectional bronchial fistulas

BACKGROUND

Pulmonary resection is, by far, the primary cause of bronchial fistula. This is a severe complication because of its morbidity and mortality and the related consumption of resources. Definitive closure continues to be a challenge with several therapeutic options, but none are optimal. We describe our experience in bronchoscopic application of ethanolamine and lauromacrogol 400 for the treatment of post-resection bronchial fistulas.

METHODS

Clinical records of 8 patients treated using this technique were collected prospectively. The diagnosis of a fistula was confirmed by flexible bronchoscopy. Sclerosis was indicated in the context of multimodal treatment. Sclerosant injection was performed under general anesthesia with a Wang 22G needle through a flexible bronchoscope. The procedure was repeated at 2-week intervals until definitive closure of the fistula was confirmed.

RESULTS

Fistula closure was achieved in 7 (87.5%) of the 8 patients, with persistence of the fistula in one patient who could not complete the treatment because of recurrence of his neoplastic pathology. No recurrence or complications related to the technique were registered.

CONCLUSIONS

Bronchoscopic sclerosis by means of submucosal injection of lauromacrogol 400 or ethanolamine should be part of the multimodal treatment of bronchopleural fistula after lung resection, pending further studies that contribute to the accurate establishment of optimal indications for this procedure.

Age protects from harmful effects produced by chronic intermittent hypoxia

Obstructive sleep apnoea (OSA) affects an estimated 3–7% of the adult population, the frequency doubling at ages >60–65 years. As it evolves, OSA becomes frequently associated with cardiovascular, metabolic and neuropsychiatric pathologies defining OSA syndrome (OSAS). Exposing experimental animals to chronic intermittent hypoxia (CIH) can be used as a model of the recurrent hypoxic and O2 desaturation patterns observed in OSA patients. CIH is an important OSA event triggering associated pathologies; CIH induces carotid body (CB)-driven exaggerated sympathetic tone and overproduction of reactive oxygen species, related to the pathogenic mechanisms of associated pathologies observed in OSAS. Aiming to discover why OSAS is clinically less conspicuous in aged patients, the present study compares CIH effects in young (3–4 months) and aged (22–24 months) rats. To define potential distinctive patterns of these pathogenic mechanisms, mean arterial blood pressure as the final CIH outcome was measured. In young rats, CIH augmented CB sensory responses to hypoxia, decreased hypoxic ventilation and augmented sympathetic activity (plasma catecholamine levels and renal artery content and synthesis rate). An increased brainstem integration of CB sensory input as a trigger of sympathetic activity is suggested. CIH also caused an oxidative status decreasing aconitase/fumarase ratio and superoxide dismutase activity. In aged animals, CIH minimally affected CB responses, ventilation and sympathetic-related parameters leaving redox status unaltered. In young animals, CIH caused hypertension and in aged animals, whose baseline blood pressure was augmented, CIH did not augment it further. Plausible mechanisms of the differences and potential significance of these findings for the diagnosis and therapy of OSAS are discussed.

Experimental Observations on the Biological Significance of Hydrogen Sulfide in Carotid Body Chemoreception

The cascade of transduction of hypoxia and hypercapnia, the natural stimuli to chemoreceptor cells, is incompletely understood. A particular gap in that knowledge is the role played by second messengers, or in a most ample term, of modulators. A recently described modulator of chemoreceptor cell responses is the gaseous transmitter hydrogen sulfide, which has been proposed as a specific activator of the hypoxic responses in the carotid body, both at the level of the chemoreceptor cell response or at the level of the global output of the organ. Since sulfide behaves in this regard as cAMP, we explored the possibility that sulfide effects were mediated by the more classical messenger. Data indicate that exogenous and endogenous sulfide inhibits adenyl cyclase finding additionally that inhibition of adenylyl cyclase does not modify chemoreceptor cell responses elicited by sulfide. We have also observed that transient receptor potential cation channels A1 (TRPA1) are not regulated by sulfide in chemoreceptor cells.

Hypoxic intensity: a determinant for the contribution of ATP and adenosine to the genesis of carotid body chemosensory activity

Excitatory effects of adenosine and ATP on carotid body (CB) chemoreception have been previously described. Our hypothesis is that both ATP and adenosine are the key neurotransmitters responsible for the hypoxic chemotransmission in the CB sensory synapse, their relative contribution depending on the intensity of hypoxic challenge. To test this hypothesis we measured carotid sinus nerve (CSN) activity in response to moderate and intense hypoxic stimuli (7 and 0% O(2)) in the absence and in the presence of adenosine and ATP receptor antagonists. Additionally, we quantified the release of adenosine and ATP in normoxia (21% O(2)) and in response to hypoxias of different intensities (10, 5, and 2% O(2)) to study the release pathways. We found that ZM241385, an A(2) antagonist, decreased the CSN discharges evoked by 0 and 7% O(2) by 30.8 and 72.5%, respectively. Suramin, a P(2)X antagonist, decreased the CSN discharges evoked by 0 and 7% O(2) by 64.3 and 17.1%, respectively. Simultaneous application of both antagonists strongly inhibited CSN discharges elicited by both hypoxic intensities. ATP release by CB increased in parallel to hypoxia intensity while adenosine release increased preferably in response to mild hypoxia. We have also found that the lower the O(2) levels are, the higher is the percentage of adenosine produced from extracellular catabolism of ATP. Our results demonstrate that ATP and adenosine are key neurotransmitters involved in hypoxic CB chemotransduction, with a more relevant contribution of adenosine during mild hypoxia, while vesicular ATP release constitutes the preferential origin of extracellular adenosine in high-intensity hypoxia.

Effects of cigarette smoke and hypoxia on pulmonary circulation in the guinea pig

Cigarette smoke (CS) and chronic hypoxia (CH) can produce pulmonary hypertension. Similarities and differences between both exposures and their interaction have not been explored. The aim of the present study was to investigate the effects of CS and CH, as single factors or in combination, on the pulmonary circulation in the guinea pig. 51 guinea pigs were exposed to CS for 12 weeks and 32 were sham-exposed. 50% of the animals in each group were additionally exposed to CH for the final 2 weeks. We measured pulmonary artery pressure (P(pa)), and the weight ratio between the right ventricle (RV) and left ventricle plus the septum. Pulmonary artery contractility in response to noradrenaline (NA), endothelium-dependent vasodilatation and distensibility were evaluated in organ bath chambers. The number of small intrapulmonary vessels showing immunoreactivity to smooth muscle (SM) α-actin and double elastic laminas was assessed microscopically. CS and CH induced similar increases of P(pa) and RV hypertrophy (p<0.05 for both), effects that were further enhanced when both factors were combined. CH increased the contractility to NA (p<0.01) and reduced the distensibility (p<0.05) of pulmonary arteries. Animals exposed to CS showed an increased number of small vessels with positive immunoreactivity to SM α-actin (p<0.01) and those exposed to CH a greater proportion of vessels with double elastic laminas (p<0.05). We conclude that CH amplifies the detrimental effects of CS on the pulmonary circulation by altering the mechanical properties of pulmonary arteries and enhancing the remodelling of pulmonary arterioles.

Carotid body function and ventilatory responses in intermittent hypoxia. Evidence for anomalous brainstem integration of arterial chemoreceptor input

Obstructive sleep apnea is a frequent medical condition consisting in repetitive sleep-related episodes of upper airways obstruction and concurrent events of arterial blood hypoxia. There is a frequent association of cardiovascular diseases and other pathologies to this condition conforming the obstructive sleep apnea syndrome (OSAS). Laboratory models of OSAS consist in animals exposed to repetitive episodes of intermittent hypoxia (IH) which also develop cardiovascular pathologies, mostly hypertension. The overall OSAS pathophysiology appears to be linked to the repetitive hypoxia, which would cause a sensitization of carotid body (CB) chemoreflex and chemoreflex-driven hyperreactivity of the sympathetic nervous system. However, this proposal is uncertain because hyperventilation, reflecting the CB sensitization, and increased plasma CA levels, reflecting sympathetic hyperreactivity, are not constant findings in patients with OSAS and IH animals. Aiming to solve these uncertainties we have studied the entire CB chemoreflex arch in a rat model of IH, including activity of chemoreceptor cells and CB generated afferent activity to brainstem. The efferent activity was measured as ventilation in normoxia, hypoxia, and hypercapnia. Norepinephrine turnover in renal artery sympathetic endings was also assessed. Findings indicate a sensitization of the CB function to hypoxia evidenced by exaggerated chemoreceptor cell and CB afferent activity. Yet, IH rats exhibited marked hypoventilation in all studied conditions and increased turnover of norepinephrine in sympathetic endings. We conclude that IH produces a bias in the integration of the input arising from the CB with a diminished drive of ventilation and an exaggerated activation of brainstem sympathetic neurons.

A revisit to O2 sensing and transduction in the carotid body chemoreceptors in the context of reactive oxygen species biology

Oxygen-sensing and transduction in purposeful responses in cells and organisms is of great physiological and medical interest. All animals, including humans, encounter in their lifespan many situations in which oxygen availability might be insufficient, whether acutely or chronically, physiologically or pathologically. Therefore to trace at the molecular level the sequence of events or steps connecting the oxygen deficit with the cell responses is of interest in itself as an achievement of science. In addition, it is also of great medical interest as such knowledge might facilitate the therapeutical approach to patients and to design strategies to minimize hypoxic damage. In our article we define the concepts of sensors and transducers, the steps of the hypoxic transduction cascade in the carotid body chemoreceptor cells and also discuss current models of oxygen- sensing (bioenergetic, biosynthetic and conformational) with their supportive and unsupportive data from updated literature. We envision oxygen-sensing in carotid body chemoreceptor cells as a process initiated at the level of plasma membrane and performed by a hemoprotein, which might be NOX4 or a hemoprotein not yet chemically identified. Upon oxygen-desaturation, the sensor would experience conformational changes allosterically transmitted to oxygen regulated K+ channels, the initial effectors in the transduction cascade. A decrease in their opening probability would produce cell depolarization, activation of voltage dependent calcium channels and release of neurotransmitters. Neurotransmitters would activate the nerve endings of the carotid body sensory nerve to convey the information of the hypoxic situation to the central nervous system that would command ventilation to fight hypoxia.

Effects of the polyamine spermine on arterial chemoreception

Polyamines modulate many biological functions. Here we report a novel inhibitory modulation by spermine of catecholamine release by the rat carotid body and have identified the molecular mechanism underpinning it. We used molecular (RT-PCR and confocal microscopy) and functional (i.e., neurotransmitter release, patch clamp recording and calcium imaging) approaches to test the involvement of: (i) voltage-dependent calcium channels, and; (ii) the extracellular calcium-sensing receptor, CaR, a G protein-coupled receptor which is also activated by polyamines. RT-PCR and immunohistochemistry of isolated carotid bodies revealed that only Ca(v)1.2 and Ca(v)2.2 were expressed in type 1 cells while Ca(v)1.3, Ca(v)1.4, Ca(v)2.1, Ca(v)2.3 and Ca(v)3.1, Ca(v)3.2 and Ca(v)3.3, could not be detected. CaR expression was detected exclusively in the nerve endings. In isolated carotid bodies, the hypoxia-dependent (7% O(2) for 10 minutes) and depolarization-evoked catecholamine release were partially suppressed by pre- (and co)-incubation with 500microM spermine. In dissociated type 1 glomus cells intracellular calcium concentration did not change following spermine treatment, but this polyamine did inhibit the depolarisation-evoked calcium influx. Whole-cell patch clamp recordings of HEK293 cells stably transfected with Ca(v)1.2 demonstrated that spermine inhibits this calcium channel. Interestingly, this inhibition was not apparent if the extracellular solution contained a concentration of Ba(2) above 2 mM as the charge carrier. In conclusion, spermine attenuates catecholamine release by the carotid body principally via inhibition of Ca(v)1.2. This mechanism may represent a negative feedback, which limits transmitter release during hypoxia.

Adenosine in peripheral chemoreception: new insights into a historically overlooked molecule--invited article

In the present article we review in a concise manner the literature on the general biology of adenosine signalling. In the first section we describe briefly the historical aspects of adenosine research. In the second section is presented the biochemical characteristics of this nucleoside, namely its metabolism and regulation, and its physiological actions. In the third section we have succinctly described the role of adenosine and its metabolism in hypoxia. The final section is devoted to the role of adenosine in chemoreception in the carotid body, providing a review of the literature on the presence of adenosine receptors in the carotid body; on the effects of adenosine at presynaptic level in carotid body chemoreceptor cells, as well as, its metabolism and regulation; and at postsynaptic level in carotid sinus nerve activity. Additionally, a review on the effects of adenosine in ventilation was done. This review discusses evidence for a key role of adenosine in the hypoxic response of carotid body and emphasizes new research likely to be important in the future.

Effects of mitochondrial poisons on glutathione redox potential and carotid body chemoreceptor activity

Low oxygen sensing in chemoreceptor cells involves the inhibition of specific plasma membrane K(+) channels, suggesting that mitochondria-derived reactive oxygen species (ROS) link hypoxia to K(+) channel inhibition, subsequent cell depolarization and activation of neurotransmitter release. We have used several mitochondrial poisons, alone and in combination with the antioxidant N-acetylcysteine (NAC), and quantify their capacity to alter GSH/GSSG levels and glutathione redox potential (E(GSH)) in rat diaphragm. Selected concentrations of mitochondrial poisons with or without NAC were tested for their capacity to activate neurotransmitter release in chemoreceptor cells and to alter ATP levels in intact rat carotid body (CB). We found that rotenone (1 microM), antimycin A (0.2 microg/ml) and sodium azide (5mM) decreased E(GSH); NAC restored E(GSH) to control values. At those concentrations mitochondrial poisons activated neurotransmitter release from CB chemoreceptor cells and decreased CB ATP levels, NAC being ineffective to modify these responses. Additional experiments with 3-nitroprionate (5mM), lower concentrations of rotenone and dinitrophenol revealed variable relationships between E(GSH) and chemoreceptor cell neurotransmitter release responses and ATP levels. These findings indicate a lack of correlation between mitochondrial-generated modifications of E(GSH) and chemoreceptor cells activity. This lack of correlation renders unlikely that alteration of mitochondrial production of ROS is the physiological pathway chemoreceptor cells use to signal hypoxia.

Low glucose effects on rat carotid body chemoreceptor cells' secretory responses and action potential frequency in the carotid sinus nerve

Glucose deprivation (hypoglycaemia) is counterbalanced by a neuroendocrine response in order to induce fast delivery of glucose to blood. Some central neurons can sense glucose, but nevertheless the most important glucose sensors/glycaemia regulators are located outside the brain. Some recent experimental evidence obtained in carotid body (CB) slices and isolated chemoreceptor cells in culture supports a role for the CB in glucose sensing and presumably glucose homeostasis, but this role has been questioned on the basis of a lack of effect of low glucose on the carotid sinus nerve activity. This work was performed in an attempt to clarify if low glucose is or is not a stimulus for the rat CB chemoreceptors. Using freshly isolated intact CB preparations we have monitored the release of catecholamines (CAs) and ATP from chemoreceptor cells in response to several concentrations of glucose, as indices of chemoreceptor cell sensitivity to glycaemia, and the electrical activity in the carotid sinus nerve (CSN), as an index of reflex-triggering output of the CB. We have observed that basal (20% O(2)) and hypoxia (7 and 10% O(2))-evoked release of CAs was identical in the presence of normal (5.55 mm) and low (3, 1 and 0 mm) glucose concentrations. 0 mm glucose did not activate the release of ATP from the CB, while hypoxia (5% O(2)) did. Basal and hypoxia (5% O(2))-induced CSN action potential frequency was identical with 5.55 and 1 mm glucose. Our results indicate that low glucose is not a direct stimulus for the rat carotid body chemoreceptors.

Chemoreception in the context of the general biology of ROS

Superoxide anion is the most important reactive oxygen species (ROS) primarily generated in cells. The main cellular constituents with capabilities to generate superoxide anion are NADPH oxidases and mitochondrial respiratory chain. The emphasis of our article is centered in critically examining hypotheses proposing that ROS generated by NADPH oxidase and mitochondria are key elements in O(2)-sensing and hypoxic responses generation in carotid body chemoreceptor cells. Available data indicate that chemoreceptor cells express a specific isoform of NADPH oxidase that is activated by hypoxia; generated ROS acting as negative modulators of the carotid body (CB) hypoxic responses. Literature is also consistent in supporting that poisoned respiratory chain can produce high amounts of ROS, making mitochondrial ROS potential triggers-modulators of the CB activation elicited by mitochondrial venoms. However, most data favour the notion that levels of hypoxia, capable of strongly activating chemoreceptor cells, would not increase the rate of ROS production in mitochondria, making mitochondrial ROS unlikely triggers of hypoxic responses in the CB. Finally, we review recent literature on heme oxygenases from two perspectives, as potential O(2)-sensors in chemoreceptor cells and as generators of bilirubin which is considered to be a ROS scavenger of major quantitative importance in mammalian cells.

The role of NADPH oxidase in carotid body arterial chemoreceptors

O(2)-sensing in the carotid body occurs in neuroectoderm-derived type I glomus cells where hypoxia elicits a complex chemotransduction cascade involving membrane depolarization, Ca(2+) entry and the release of excitatory neurotransmitters. Efforts to understand the exquisite O(2)-sensitivity of these cells currently focus on the coupling between local P(O2) and the open-closed state of K(+)-channels. Amongst multiple competing hypotheses is the notion that K(+)-channel activity is mediated by a phagocytic-like multisubunit enzyme, NADPH oxidase, which produces reactive oxygen species (ROS) in proportion to the prevailing P(O2). In O(2)-sensitive cells of lung neuroepithelial bodies (NEB), multiple studies confirm that ROS levels decrease in hypoxia, and that E(M) and K(+)-channel activity are indeed controlled by ROS produced by NADPH oxidase. However, recent studies in our laboratories suggest that ROS generated by a non-phagocyte isoform of the oxidase are important contributors to chemotransduction, but that their role in type I cells differs fundamentally from the mechanism utilized by NEB chemoreceptors. Data indicate that in response to hypoxia, NADPH oxidase activity is increased in type I cells, and further, that increased ROS levels generated in response to low-O(2) facilitate cell repolarization via specific subsets of K(+)-channels.

Caffeine inhibition of rat carotid body chemoreceptors is mediated by A2A and A2B adenosine receptors

Caffeine, an unspecific antagonist of adenosine receptors, is commonly used to treat the apnea of prematurity. We have defined the effects of caffeine on the carotid body (CB) chemoreceptors, the main peripheral controllers of breathing, and identified the adenosine receptors involved. Caffeine inhibited basal (IC50, 210 microm) and low intensity (PO2 approximately 66 mm Hg/30 mm K+) stimulation-induced release of catecholamines from chemoreceptor cells in intact preparations of rat CB in vitro. Opposite to caffeine, 5'-(N-ethylcarboxamido)adenosine (NECA; an A2 agonist) augmented basal and low-intensity hypoxia-induced release. 2-p-(2-Carboxyethyl)phenethyl-amino-5'-N-ethylcaboxamido-adenosine hydrochloride (CGS21680), 2-hexynyl-NECA (HE-NECA) and SCH58621 (A2A receptors agents) neither affected catecholamine release nor altered the caffeine effects. The 8-cycle-1,3-dipropylxanthine (DPCPX; an A1/A2B antagonist) and 8-(4-{[(4-cyanophenyl)carbamoylmethyl]-oxy}phenyl)-1,3-di(n-propyl)xanthine (MRS1754; an A2B antagonist) mimicking of caffeine indicated that caffeine effects are mediated by A2B receptors. Immunocytochemical A2B receptors were located in tyrosine hydroxylase positive chemoreceptor cells. Caffeine reduced by 52% the chemosensory discharges elicited by hypoxia in the carotid sinus nerve. Inhibition had two components with pharmacological analysis indicating that A2A and A2B receptors mediate, respectively, the low (17 x 10(-9) m) and high (160 x 10(-6) m) IC50 effects. It is concluded that endogenous adenosine, via presynaptic A2B and postsynaptic A2A receptors, can exert excitatory effects on the overall output of the rat CB chemoreceptors.

Effect of p47phoxgene deletion on ROS production and oxygen sensing in mouse carotid body chemoreceptor cells

Membrane potential in oxygen-sensitive type I cells in carotid body is controlled by diverse sets of voltage-dependent and -independent K+channels. Coupling of Po2to the open-closed state of channels may involve production of reactive oxygen species (ROS) by NADPH oxidase. One hypothesis suggests that ROS are produced in proportion to the prevailing Po2and a subset of K+channels closes as ROS levels decrease. We evaluated ROS levels in normal and p47phoxgene-deleted [NADPH oxidase knockout (KO)] type I cells using the ROS-sensitive dye dihydroethidium (DHE). In normal cells, hypoxia elicited an increase in ROS, which was blocked by the specific NADPH oxidase inhibitor 4-(2-aminoethyl)-benzenesulfonyl fluoride (AEBSF, 3 mM). KO type I cells did not respond to hypoxia, but the mitochondrial uncoupler azide (5 μM) elicited increased fluorescence in both normal and KO cells. Hypoxia had no effect on ROS production in sensory and sympathetic neurons. Methodological control experiments showed that stimulation of neutrophils with a cocktail containing the chemotactic peptide N-formyl-Met-Leu-Phe (1 μM), arachidonic acid (10 μM), and cytochalasin B (5 μg/ml) elicited a rapid increase in DHE fluorescence. This response was blocked by the NADPH oxidase inhibitor diphenyleneiodonium (10 μM). KO neutrophils did not respond; however, azide (5 μM) elicited a rapid increase in fluorescence. Physiological studies in type I cells demonstrated that hypoxia evoked an enhanced depression of K+current and increased intracellular Ca2+levels in KO vs. normal cells. Moreover, AEBSF potentiated hypoxia-induced increases in intracellular Ca2+and enhanced the depression of K+current in low O2. Our findings suggest that local compartmental increases in oxidase activity and ROS production inhibit the activity of type I cells by facilitating K+channel activity in hypoxia.

Ventilatory responses and carotid body function in adult rats perinatally exposed to hyperoxia

Hypoxia increases the release of neurotransmitters from chemoreceptor cells of the carotid body (CB) and the activity in the carotid sinus nerve (CSN) sensory fibers, elevating ventilatory drive. According to previous reports, perinatal hyperoxia causes CSN hypotrophy and varied diminishment of CB function and the hypoxic ventilatory response. The present study aimed to characterize the presumptive hyperoxic damage. Hyperoxic rats were born and reared for 28 days in 55%-60% O2; subsequent growth (to 3.5-4.5 months) was in a normal atmosphere. Hyperoxic and control rats (born and reared in a normal atmosphere) responded with a similar increase in ventilatory frequency to hypoxia and hypercapnia. In comparison with the controls, hyperoxic CBs showed (1) half the size, but comparable percentage area positive to tyrosine hydroxylase (chemoreceptor cells) in histological sections; (2) a twofold increase in dopamine (DA) concentration, but a 50% reduction in DA synthesis rate; (3) a 75% reduction in hypoxia-evoked DA release, but normal high [K+]0-evoked release; (4) a 75% reduction in the number of hypoxia-sensitive CSN fibers (although responding units displayed a nearly normal hypoxic response); and (5) a smaller percentage of chemoreceptor cells that increased [Ca2+]1 in hypoxia, although responses were within the normal range. We conclude that perinatal hyperoxia causes atrophy of the CB-CSN complex, resulting in a smaller number of chemoreceptor cells and fibers. Additionally, hyperoxia damages O2-sensing, but not exocytotic, machinery in most surviving chemoreceptor cells. Although hyperoxic CBs contain substantially smaller numbers of chemoreceptor cells/sensory fibers responsive to hypoxia they appear sufficient to evoke normal increases in ventilatory frequency.

Reduced to oxidized glutathione ratios and oxygen sensing in calf and rabbit carotid body chemoreceptor cells

1. The aim of this work was to test the redox hypotheses of O(2) chemoreception in the carotid body (CB). They postulate that hypoxia alters the levels of reactive oxygen species (ROS) and the ratio of reduced to oxidized glutathione (GSH/GSSG), causing modifications to the sulfhydryl groups/disulfide bonds of K+ channel proteins, which leads to the activation of chemoreceptor cells. 2. We found that the GSH/GSSG ratio in normoxic calf CB (30.14 +/- 4.67; n = 12) and hypoxic organs (33.03 +/- 6.88; n = 10), and the absolute levels of total glutathione (0.71 +/- 0.07 nmol (mg tissue)(-1), normoxia vs. 0.76 +/- 0.07 nmol (mg tissue)(-1), hypoxia) were not statistically different. 3. N-Acetylcysteine (2 mM; NAC), a precursor of glutathione and ROS scavenger, increased normoxic glutathione levels to 1.03 +/- 0.06 nmol (mg tissue)(-1) (P < 0.02) and GSH/GSSG ratios to 59.05 +/- 5.05 (P < 0.001). 4. NAC (20 microM-10 mM) did not activate or inhibit chemoreceptor cells as it did not alter the normoxic or the hypoxic release of (3)H-catecholamines ((3)H-CAs) from rabbit and calf CBs whose CA deposits had been labelled by prior incubation with the natural CA precursor (3)H-tyrosine. 5. NAC (2 mM) was equally ineffective in altering the release of (3)H-CAs induced by stimuli (high external K+ and ionomycin) that bypass the initial steps of the hypoxic cascade of activation of chemoreceptor cells, thereby excluding the possibility that the lack of effect of NAC on normoxic and hypoxic release of (3)H-CAs results from a concomitant alteration of Ca(2+) channels or of the exocytotic machinery. 6. The present findings do not support the contention that O(2) chemoreception in the CB is linked to variations in the GSH/GSSG quotient as the redox models propose.

Intracellular Ca(2+) stores in chemoreceptor cells of the rabbit carotid body: significance for chemoreception

The notion that intracellular Ca(2+) (Ca(i)(2+)) stores play a significant role in the chemoreception process in chemoreceptor cells of the carotid body (CB) appears in the literature in a recurrent manner. However, the structural identity of the Ca(2+) stores and their real significance in the function of chemoreceptor cells are unknown. To assess the functional significance of Ca(i)(2+) stores in chemoreceptor cells, we have monitored 1) the release of catecholamines (CA) from the cells using an in vitro preparation of intact rabbit CB and 2) the intracellular Ca(2+) concentration ([Ca(2+)](i)) using isolated chemoreceptor cells; both parameters were measured in the absence or the presence of agents interfering with the storage of Ca(2+). We found that threshold [Ca(2+)](i) for high extracellular K(+) (K(e)(+)) to elicit a release response is approximately 250 nM. Caffeine (10-40 mM), ryanodine (0.5 microM), thapsigargin (0.05-1 microM), and cyclopiazonic acid (10 microM) did not alter the basal or the stimulus (hypoxia, high K(e)(+))-induced release of CA. The same agents produced Ca(i)(2+) transients of amplitude below secretory threshold; ryanodine (0.5 microM), thapsigargin (1 microM), and cyclopiazonic acid (10 microM) did not alter the magnitude or time course of the Ca(i)(2+) responses elicited by high K(e)(+). Several potential activators of the phospholipase C system (bethanechol, ATP, and bradykinin), and thereby of inositol 1,4,5-trisphosphate receptors, produced minimal or no changes in [Ca(2+)](i) and did not affect the basal release of CA. It is concluded that, in the rabbit CB chemoreceptor cells, Ca(i)(2+) stores do not play a significant role in the instant-to-instant chemoreception process.

Characterization of the synthesis and release of catecholamine in the rat carotid body in vitro

The aim of this work was to determine contents and turnover rates for dopamine (DA) and norepinephrine (NE) and to identify the catecholamine (CA) released during stimulation of the rat carotid body (CB). Turnover rates and the release of CA were measured in an in vitro preparation using a combination of HPLC and radioisotopic methods. Mean rat CB levels of DA and NE were 209 and 45 pmol/mg tissue, respectively. With [(3)H]tyrosine as precursor, rat CB synthesized [(3)H]CA in a time- and concentration-dependent manner; calculated turnover times for DA and NE were 5.77 and 11.4 h, respectively. Hypoxia and dibutyryl adenosine 3',5'-cyclic monophosphate significantly increased [(3)H]CA synthesis. In normoxia, rat CB released [(3)H]DA and [(3)H]NE in a ratio of 5:1, comparable to that of the endogenous tissue CA. Hypoxia and high K(+) preferentially released [(3)H]DA, nicotine preferentially released [(3)H]NE, and acidic stimuli released both amines in proportion to tissue content. Release of [(3)H]CA induced by hypoxia and high K(+) was nearly fully dependent on extracellular Ca(2+), whereas basal normoxic release was not altered by removal of Ca(2+) from the incubating solution. We conclude that the rat CB is an organ with higher levels of DA than NE that preferentially releases DA or NE in a stimulus-specific manner.

NADPH oxidase inhibition does not interfere with low PO2 transduction in rat and rabbit CB chemoreceptor cells

The aim of the present work was to elucidate the role of NADPH oxidase in hypoxia sensing and transduction in the carotid body (CB) chemoreceptor cells. We have studied the effects of several inhibitors of NADPH oxidase on the normoxic and hypoxia-induced release of [3H]catecholamines (CA) in an in vitro preparation of intact CB of the rat and rabbit whose CA deposits have been labeled by prior incubation with the natural precursor [3H]tyrosine. It was found that diphenyleneiodonium (DPI; 0.2-25 microM), an inhibitor of NADPH oxidase, caused a dose-dependent release of [3H]CA from normoxic CB chemoreceptor cells. Contrary to hypoxia, DPI-evoked release was only partially Ca2+ dependent. Concentrations of DPI reported to produce full inhibition of NADPH oxidase in the rat CB did not prevent the hypoxic release response in the rat and rabbit CB chemoreceptor cells, as stimulation with hypoxia in the presence of DPI elicited a response equaling the sum of that produced by DPI and hypoxia applied separately. Neopterin (3-300 microM) and phenylarsine oxide (0.5-2 microM), other inhibitors of NADPH oxidase, did not promote release of [3H]CA in normoxic conditions or affect the response elicited by hypoxia. On the basis of effects of neopterin and phenylarsine oxide, it is concluded that NADPH oxidase does not appear to play a role in oxygen sensing or transduction in the rat and rabbit CB chemoreceptor cells in vitro and, in the context of the present study, that DPI effects are not related to NADPH oxidase inhibition.

Evidence for two types of nicotinic receptors in the cat carotid body chemoreceptor cells

Current concepts on the location and functional significance of nicotinic receptors in the carotid body rest on alpha-bungarotoxin binding and autoradiographic studies. Using an in vitro preparation of the cat carotid body whose catecholamine deposits have been labeled by prior incubation with the tritiated natural precursor [3H]tyrosine, we have found that nicotine induces release of [3H]catecholamines in a dose-dependent manner (IC50 = 9.81 microM). We also found that mecamylamine (50 microM) completely abolished the nicotine-induced release, while alpha-bungarotoxin (100 nM; approximately 20 times its binding Kd) only reduced the release by 56%. These findings indicate that chemoreceptor cells, and perhaps other carotid body structures, contain nicotinic receptors that are not sensitive to alpha-bungarotoxin and force a revision of the current concepts on cholinergic mechanisms in the carotid body chemoreception.

Cellular mechanisms of oxygen chemoreception in the carotid body

The carotid bodies (CB) are arterial chemoreceptors that by sensing changes of arterial PO2, PCO2 and pH can initiate and modify ventilatory and cardiovascular reflexes in order to maintain PO2, PCO2 and pH within physiological levels. It is now generally accepted that the glomus or type I cells of the CB are the transducers of hypoxic stimuli, and relay chemosensory information to the brainstem via neurotransmitter release at synaptic contacts with afferent terminals of the carotid sinus nerve. This article reviews the mechanisms of the O2-sensing process at the cellular level. We consider first the transduction of the hypoxic stimulus, in which most of the experimental evidence currently favors a mechanism involving modulation of the electrical properties of type I cells. The last part of the article deals with the transmission of the stimulus between type I cells and afferent nerve terminals, and we present an overview on the issue of neurotransmission in the CB, summarizing the actions of the main neurotransmitters present in the organ.

Participation of Na+ channels in the response of carotid body chemoreceptor cells to hypoxia

The role played by Na+ channels of carotid body (CB) chemoreceptor cells was investigated by studying the effects of tetrodotoxin (TTX) on the release of 3H-labeled catecholamines ([3H]CA) by adult rabbit CBs previously incubated with the precursor [3H]tyrosine. TTX inhibited partially the release of [3H]CA elicited by mild hypoxia (10 or 7% O2) or by depolarizing incubation medium containing 20 or 30 mM KCl, but the response to more intense hypoxia (5 or 2% O2) or to higher KCl concentration (40 or 50 mM) was not significantly affected. The release of [3H]CA elicited by acidic stimuli, either 20% CO2 (pH 6.6) or the protonophore dinitrophenol (100 microM), although comparable in magnitude to that elicited by mild hypoxia, was not modified by TTX. These results provide evidence for the first time that Na+ channels of chemoreceptor cells participate in the transduction of hypoxic stimuli into the neurotransmitter release response of these cells and suggest that Na+ current operates as an amplifying device that enhances the initial cell depolarization mediated by the closure of the O2-sensitive K+ channels. Sympathetic denervation of CBs was followed by a marked reduction in the release of [3H]CA elicited by veratridine or by 20 mM KCl, suggesting that the number of Na+ channels in chemoreceptor cells decreases after denervation.

Effect of low O2 on glucose uptake in rabbit carotid body

Glucose consumption in the rabbit carotid body was studied in vitro by measuring phosphorylation rates of tracer concentrations of 2-[3H]deoxy-glucose. The rate of glucose consumption measured in 100% O2-equilibrated modified Tyrode medium was 61 nmol.g tissue-1 x min-1 and was linear for up to 30 min. Incubation of carotid bodies for 5 or 10 min in moderately hypoxic solution (20% O2-80% N2) resulted in a 44% increase in the rate of glucose consumption. The glucose consumption of the nodose ganglion was not affected during similar incubation with low-O2 medium. High-resolution autoradiography of freeze-dried tissues revealed that the type I parenchymal cells are the principal site of glucose consumption in both 100% O2- and 20% O2-incubated carotid bodies. This metabolic response of the carotid body to hypoxia was not secondary to neurotransmitter release, because similar elevations in glucose utilization were observed with low-O2 medium containing zero Ca2+, a condition in which the release of neurotransmitters from type I cells is inhibited. Lowering the pH of the incubation medium from 7.4 to 7 or 6.8 markedly reduced the rate of glucose utilization by both the carotid body and the nodose ganglion. Ouabain (2 x 10(-4) and 1 x 10(-3) M) reduced by 20% the glucose consumption of carotid bodies incubated in 100% O2-equilibrated solution and abolished the metabolic response produced by low-O2 medium. The results suggest that the utilization of metabolic energy is an integral component of the chemoreceptor response to hypoxia.

Characterization of cultured chemoreceptor cells dissociated from adult rabbit carotid body

Short-term cell cultures were obtained from enzymatically dissociated carotid bodies from adult rabbits, and morphological and functional characterization of the cultured chemoreceptor cells were carried out. Under phase contrast, freshly isolated type I cells are round, bright, and 10-14 microns in diameter and exhibit strong fluorescence when stained with the glyoxylic acid technique. The content of endogenous dopamine in the cultures increased from 80 pmol/10(5) cells 2 h after plating the cells to 200 pmol/10(5) cells on the 3rd day, and the rate of synthesis and storage of [3H]dopamine from the precursor [3H]tyrosine increased from 1.7 pmol.10(5) cells-1.h-1 in 1-day cultures to 4 pmol.10(5) cells-1.h-1 on the 3rd day; the later values represent 80-85% of the expected values for the intact carotid body. After labeling with [3H]tyrosine, cultured chemoreceptor cells release [3H]dopamine when challenged by hypoxia, high external K+, or the protonophore dinitrophenol, the pattern of response being similar to that of the intact carotid body. When studied by whole cell clamp recording, individual chemoreceptor cells exhibit a marked variability in the properties of some ionic currents; the data, however, do not support the existence of distinct subpopulations of type I cells.

Effects of almitrine on the release of catecholamines from the rabbit carotid body in vitro

1. Almitrine increases ventilation by stimulating the carotid body (CB) arterial chemoreceptors but neither its intraglomic target nor its mechanism of action have been elucidated. 2. We have tested the hypothesis that chemoreceptor cells are targets for almitrine by studying its effects on the release of 3H-catecholamines in an in vitro rabbit CB preparation. 3. It was found that almitrine (0.3 and 1.5 x 10(-6) M; i.e. 0.2 and 1 mg ml-1) increases the resting release of 3H-catecholamines from CBs (previously loaded with [3H]-tyrosine) incubated in a balanced 95% O2/5% CO2-equilibrated solution. 4. Almitrine at a concentration of 3 x 10(-6) M (2 mg l-1) also augmented the release of 3H-catecholamines elicited by incubating the CBs in a hypoxic solution (equilibrated with 7% O2/5% CO2 in N2), by high external K+ (35 mM) and by veratridine (2 x 10(-5) M), but did not modify release induced by dinitrophenol (7.5 x 10(-5) M). 5. At the same concentration (3 x 10(-6) M), almitrine increased the rate of dopamine synthesis and was ineffective in modifying the cyclic AMP levels in either normoxic or hypoxic CBs. 6. It is concluded that chemoreceptor cells are the intraglomic targets for almitrine. The mechanisms of action of almitrine on chemoreceptor cells are discussed.

Oxygen and acid chemoreception in the carotid body chemoreceptors

The carotid bodies are arterial chemoreceptors that are sensitive to blood PO2, PCO2 and pH. They are the origin of reflexes that are crucial for maintaining PCO2 and pH in the internal milieu and for adjusting the O2 supply according to the metabolic needs of the organism in situations of increased demand, such as exercise and while breathing at decreased O2 partial pressures during ascent or when living at high altitude. Chemoreceptor cells of the carotid body transduce the blood-borne stimuli into a neurosecretory response that is dependent on external Ca2+. These cells have an O2-sensitive K+ current that is reversibly inhibited by low PO2. It is proposed that the depolarization produced by inhibition of this K+ current activates Ca2+ channels; Ca2+ influx and neurosecretion follow. The cells have also a potent Na(+)-Ca2+ antiporter that could be responsible for the intracellular Ca2+ rise required to trigger the release of neurotransmitters during high PCO2 or low pH stimulation.

The role of dihydropyridine-sensitive Ca2+ channels in stimulus-evoked catecholamine release from chemoreceptor cells of the carotid body

The present study utilized an in vitro preparation of the rabbit carotid body, with tissue catecholamine stores labeled by incubation with 3H-tyrosine. The goal was to characterize pharmacologically the voltage-dependent Ca2+ channels present in the type I (glomus) cells of this arterial chemoreceptor organ, and to elucidate their role as pathways for Ca2+ entry. We found that release of 3H-dopamine induced by high external potassium was over 95% dependent on external calcium concentration and that this release was 90-100% inhibited by the dihydropyridine antagonists, nisoldipine and nitrendipine, and was potentiated by the dihydropyridine agonist, BayK 8644. Therefore, any stimulus-induced, calcium-dependent release of 3H-dopamine that was inhibited by nisoldipine and potentiated by BayK 8644, was considered to be supported by Ca2+ entry into the cells via voltage-dependent Ca2+ channels. Significant differences were observed in the release of 3H-dopamine induced by 75 vs 25 mM K+. On prolonged stimulation, release induced by 75 mM K+ was large and transient, whilst that induced by 25 mM K+, although more moderate, was sustained. The release elicited by 75 mM K+ was inhibited approximately 90% by 1.5 mM Co2+ or 625 nM nisoldipine, while release by 25 mM K+ was completely blocked by 0.6 mM Co2+ or 125 nM nisoldipine. Low PO2-induced release of 3H-dopamine was 95% dependent on Ca2+, and was inhibited by nisoldipine (625 nM) in a manner inversely proportional to the intensity of hypoxic stimulation, i.e. 79% inhibition at a PO2 of 49 Torr, and 20% inhibition at PO2 of 0 Torr. BayK 8644 potentiated the release induced by moderate hypoxic stimuli. Release elicited by high PCO2/low pH, or by Na(+)-propionate or dinitrophenol-containing solutions, was approximately 80% Ca(2+)-dependent, and the dihydropyridines failed to modify this release. It is concluded that type I cells possess voltage-dependent Ca2+ channels sensitive to the dihydropyridines, which in agreement with previous electrophysiological data should be defined as L-type Ca2+ channels. Calcium entry which supports the release of 3H-dopamine elicited by moderate hypoxia should occur mainly through these channels while the release induced by strong hypoxic stimuli will be served by Ca2+ entry which occurs in part via voltage-dependent Ca2+ channels, and in part through an additional pathway, probably a Na+/Ca2+ exchanger.(ABSTRACT TRUNCATED AT 400 WORDS)

Ionic mechanisms for the transduction of acidic stimuli in rabbit carotid body glomus cells

1. The release of [3H]dopamine (DA) in response to inhibition of the Na+ pump or to intracellular acid load was studied in rabbit carotid bodies (CB) previously incubated with the precursor [3H]tyrosine. The ionic requirements of the release response and the involvement of specific ion transport systems were investigated. 2. Inhibition of the Na+ pump, by incubating the CB with ouabain or in K(+)-free medium, evokes a DA release response which requires the presence of Na+ and Ca2+ in the medium and is insensitive to nisoldipine. This suggests that the response is triggered by entry of external Ca2+ through Na(+)-Ca2+ exchange, a consequence of the increase in intracellular Na+ resulting from inhibition of the pump. 3. Incubation of the CB in medium equilibrated with 20% CO2 at pH 6.6, or in medium containing the protonophore dinitrophenol (DNP) or the weak acid propionate, elicits a DA release response which requires also the presence of Na+ and Ca2+ in the medium and is insensitive to dihydropyridines. 4. Ethylisopropylamiloride (EIPA), an inhibitor of the Na(+)-H+ exchanger, markedly decreases the release response elicited by DNP or propionate in bicarbonate-free medium, but has not any effect in bicarbonate-buffered medium. In the latter condition, the EIPA-insensitive release of DA is inhibited by reducing the HCO3- concentration in the medium to 2 mM or by removal of Cl-, suggesting that in bicarbonate-buffered medium a Na(+)-dependent HCO3(-)-Cl- exchanger is involved in the release response. 5. It is concluded that the release of DA by the chemoreceptor cells in response to acidic stimulation is triggered by entry of external Ca2+ through Na(+)-Ca2+ exchange. This exchange is promoted by the increase of intracellular Na+ that results from the operation of Na(+)-coupled H(+)-extruding mechanisms activated by the acid load.

Metabolic activation of carotid body glomus cells by hypoxia

The effects of low O2 on glucose consumption in the rabbit carotid body were studied using the in vitro 2-deoxyglucose technique. Metabolically active structures within the tissue were localized autoradiographically after freeze-drying and vacuum fixation/embedding of selected incubated tissue samples. In 100% O2-equilibrated media, the mean basal glucose consumption calculated from the rate of 2-[1,2-3H]deoxy-D-glucose phosphorylation and its specific activity in the incubation media was 61 nmol.g tissue-1.min-1 in the carotid body and 42 nmol.g tissue-1.min-1 in parallel experiments with nodose ganglia. Low PO2 (20% O2-equilibrated media in vitro) increased glucose consumption in the carotid body by 44% but did not alter glucose metabolism of nodose ganglia. Autoradiographic data showed that preneural type I parenchymal cells are the principal site of glucose consumption in carotid chemosensory tissue. The mechanisms responsible for the hypoxia-induced increase in glucose consumption by the type I cells are discussed in relation to sensory transduction by the carotid body chemoreceptors.

Effects of cyanide and uncouplers on chemoreceptor activity and ATP content of the cat carotid body

In cat carotid bodies (c.b.'s) incubated in vitro with [3H]tyrosine to label the stores of catecholamines, it was found that CN promotes dose- and Ca2+-dependent release of [3H]dopamine (DA) from c.b. tissues in parallel to the increased electrical activity recorded from the carotid sinus nerve (c.s.n.). Two different uncouplers, dinitrophenol (DNP) and carbonyl-cyanide-m-chlorophenyl-hydrazone (CCCP), both activate also in a dose-dependent fashion, release of DA and electrical activity in the c.s.n. However, while cyanide (CN) (10(-4) M) applied during 5 min reduced the adenosine triphosphate (ATP) content of the c.b. by 45%, DNP (2.5 x 10(-4) M) and CCCP (10(-6) M) applied for the same period of time did not modify the ATP levels of the organ. At the above concentrations, the 3 agents increased about 8-fold the electrical activity recorded from the c.s.n. Thus, contrary to the postulates of the metabolic hypotheses, our findings indicate that the decrease in the ATP content in the c.b. is not a prerequisite for the activation of the chemoreceptors. We propose alternative mechanisms to explain the chemostimulant action of the metabolic poisons.

Activation of the release of dopamine in the carotid body by veratridine. Evidence for the presence of voltage-dependent Na+ channels in type I cells

Veratridine (50 microM), an agent known to activate voltage-dependent Na+ channels, induced a strong release of [3H]dopamine from the rabbit carotid body in vitro. The effect of veratridine was dependent on the presence of both Na+ and Ca2+ in the extracellular medium and was abolished by 1 microM tetrodotoxin. These results suggest that chemoreceptor type I cells have voltage-dependent Na+ channels, which could be involved in the depolarization of the cell membrane and activation of voltage-dependent Ca2+ channels.

Effects of high potassium on the release of [3H]dopamine from the cat carotid body in vitro

Using an in vitro preparation of the cat carotid body, we have characterized the release of [3H]dopamine (DA) induced by high extracellular K+. Pulse superfusion (3 min) with high K+ Tyrode solution (20-80 mM) evoked a concentration-dependent release of [3H]DA from type I cells with a threshold at about 20 mM-extracellular K+ and a plateau at about 60 mM-extracellular K+. Equivalent low extracellular Na+ concentration ([Na+]o) solutions osmotically balanced with sucrose did not induce release. The high extracellular K+ concentration ([K+]o)-evoked release of [3H]DA by type I cells was dependent on the presence of Ca2+ in the superfusion media. On prolonged (10-14 min) superfusion with high K+ Tyrode solution, the [3H]DA release evoked by 60 mM-extracellular K+ was transient, while that evoked by 30 mM-extracellular K+ was sustained. In preparations superfused for 6 min with 60 mM-extracellular K+ and zero extracellular Ca2+ concentration ([Ca2+]o) Tyrode solution, reintroduction of Ca2+ did not elicit a secretory response. Ba2+ was a potent secretagogue of [3H]DA in preparations superfused with normal and zero [Ca2+]o Tyrode solution. Additionally, Ba2+ was capable of eliciting a secretory response from type I cells in preparations previously exposed (6 min) to 60 mM-extracellular K+, whether or not [Ca2+]o was present. With regards to stimulus-secretion coupling, our results indicate that high [K+]o probably depolarizes type I cells. This effect would, in turn, activate voltage-dependent Ca2+ channels, allowing the entrance of this ion to activate the neurosecretory response.

Effects of 2-deoxy-D-glucose on in vitro cat carotid body

The process of chemosensory transduction in the arterial chemoreceptors is not well understood. According to the metabolic hypothesis of chemoreception, a decrease in arterial pO2 will produce a decrease in the adenosine triphosphate (ATP) content in the chemosensory type I cells, leading to release of a neurotransmitter and increased sensory neural activity. There is a paucity of direct experimental support for this hypothesis, and in the present work, we have tested the postulates of the metabolic hypothesis in an in vitro preparation of cat carotid body using 2-deoxy-D-glucose as an ATP-depleting agent. This preparation, when superfused with Tyrode containing 5 mM Na+-pyruvate instead of glucose, responds normally to hypoxia, low pH and acetylcholine, and maintains normal ATP levels. Under these conditions, 2-deoxy-D-glucose is a chemostimulant, i.e. electrical activity in the carotid sinus nerve is increased, with a threshold concentration of 0.25 mM and a maximum response at about 2-4 mM. These concentrations of 2-deoxyglucose evoke a dose-dependent release of [3H]dopamine (synthesized from [3H]tyrosine) from the carotid bodies which parallels the electrical activity. The 2-deoxy-D-glucose-evoked release and electrical activity is dependent on the presence of extracellular Ca2+. These same concentrations of 2-deoxy-D-glucose significantly reduce the ATP content of the carotid bodies. The events postulated by the metabolic hypothesis, i.e. decrease in ATP content, release of a putative neurotransmitter and activation of the sensory nerve endings are found to occur simultaneously. A possible cause-effect relationship between these three events is discussed.